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Potential Applicability of Earth to Air Heat Exchanger for Cooling in a Colombian Tropical Weather

Potential Applicability of Earth to Air Heat Exchanger for Cooling in a Colombian Tropical Weather buildings Article Potential Applicability of Earth to Air Heat Exchanger for Cooling in a Colombian Tropical Weather Sergio Alexander Peñaloza Peña and Julián Ernesto Jaramillo Ibarra * Mechanical Engineering School, Universidad Industrial de Santander, Bucaramanga 680002, Colombia; sergio.pennaloza@gmail.com * Correspondence: jejarami@uis.edu.co Abstract: Buildings exhibit a high energy consumption compared with other economic sectors. While percentages vary from country to country, buildings are responsible for approximately 40% of the global energy demand. Most of this is consumed for achieving human thermal comfort. In Colombia, the government promotes policies for the adoption of efficient energy strategies in this sector. The earth to air heat exchanger (EAHE) can be used to reduce the cooling load of a building. Therefore, this study aims to evaluate the energy savings that can be obtained by installing an EAHE in a tropical climate in Colombia. To do so, a mathematical model is implemented in TRNSYS (Transient System Simulation Tool) to predict the thermal performance and the cooling capacity of the EAHE. The system is modeled as a function of pipe length, diameter, material, thickness and air mass flow. Moreover, soil, local atmospheric conditions and building features are taken into account. It is found that the air leaves the EAHE at temperatures between 20.9 C and 24.1 C, which are approximately 3 C below ambient temperature. Furthermore, the economic feasibility of the project is verified. Thereby, it is demonstrated that the EAHE can be a competitive alternative to current HVAC systems. Keywords: passive cooling; EAHE; temperature potential; ventilation; TRNSYS; soil temperature Citation: Peñaloza Peña, S.A.; Jaramillo Ibarra, J.E. Potential Applicability of Earth to Air Heat Exchanger for Cooling in a Colombian Tropical Weather. 1. Introduction Buildings 2021, 11, 219. https:// Energy supply is fundamental for the economic development and the well-being of the doi.org/10.3390/buildings11060219 population of any country. Furthermore, energy consumption has increased considerably due to growing needs to ensure thermal comfort conditions inside buildings [1].For this Academic Editor: Ambrose Dodoo reason, the achievement of indoor thermal comfort while minimizing energy consumption in buildings is a crucial aim around the world. Therefore, during the last decade, there Received: 20 March 2021 has been a rising interest in implementing alternative sources to replace conventional Accepted: 18 May 2021 cooling of buildings [2–4]. In Colombia, about 70% of energy generation is obtained Published: 21 May 2021 from hydropower. Consequently, the Colombian electricity sector is highly vulnerable to sufficient water availability [5]. For instance, during 2015 and 2016, the droughts suffered in Publisher’s Note: MDPI stays neutral this country as a result of the natural phenomenon called “El Niño” were close to causing with regard to jurisdictional claims in a blackout due to the drop in the electricity generation of the hydroelectric plants [6]. published maps and institutional affil- For this reason, governmental and non-governmental entities are promoting policies to iations. diversify energy sources. They are taking measures oriented to new generation systems and management of demand, especially in those areas with a high level of energy consumption, such as cooling of buildings [7]. One of the promising passive cooling techniques is Earth to Air Heat Exchanger Copyright: © 2021 by the authors. (EAHE). It takes advantage of the fact that ground temperature, at a certain depth, is Licensee MDPI, Basel, Switzerland. almost constant throughout the year. This allows its use as a heat sink. EAHE consists This article is an open access article of a simple system of buried pipes through which ambient air circulates. Therefore, part distributed under the terms and of the thermal air energy is transferred to the ground during this movement. Then, the conditions of the Creative Commons outlet air from the EAHE can be directly used for space cooling or as pre-cooled air in an Attribution (CC BY) license (https:// HVAC system [2,8,9]. The degree of success of the EAHE depends on many parameters, for creativecommons.org/licenses/by/ instance, local atmospheric conditions such as solar radiation, ambient air temperature and 4.0/). Buildings 2021, 11, 219. https://doi.org/10.3390/buildings11060219 https://www.mdpi.com/journal/buildings Buildings 2021, 11, 219 2 of 18 relative humidity, as well as design aspects of the heat exchanger: Configuration, depth of burial, air flow rate, pipes diameter and length [10–12]. In the last decade, many studies have been done on EAHE configurations to analyze its energy performance. Results showed that EAHE is an energy-efficient system that can be used to achieve thermal comfort inside buildings. Lee and Strand [13] evaluated the cooling and heating potential of earth tubes in four representative locations in the U.S. They created a mathematical model for EAHE in an Energy Plus software environment. A detailed algorithm was used to calculate the soil temperature variation for each pipe for every time step of the simulation. Moreover, Ascione et al. [14] studied the energy performances achievable using an EAHE in different Italian climates. This work showed that the best performance of the EAHE system can be obtained with a pipe length of 50 m, buried 3 m deep in wet soil. Benhmammou and Draoui [15] developed a one-dimensional transient model to study the thermal performance of EAHEs for air cooling in summer in the Algerian Sahara Desert. Their results revealed that the daily mean efficiency increases when the length of the pipe does, but it decreases when the cross-section area of the pipe or air velocity is reduced. Shojaee and Malek [16] evaluated the potential energy savings of a four-pipe EAHE for different climates of Iran. Their findings indicated that the average energy savings changes according to the weather in the analyzed cities. Furthermore, the EAHE shows better behavior when the soil is silt, in comparison with loam and clay soil. In this sense, several researches have evaluated the EAHE performance in different locations. These works have demonstrated that the EAHE can provide excellent ther- mal comfort and indoor air quality, with low energy consumption. Despite their general good performance, EAHE efficiency depends on local atmospheric conditions and soil characteristics. For this reason, it is important to evaluate the EAHE considering local features [8,13–15,17]. Moreover, there is a lack of this kind of research in tropical cli- mates, where the amplitude of the soil temperature wave is smaller. As a consequence, in tropical regions the attenuation of the temperature also occurs in the most superficial layers [18,19]. Therefore, the purpose of this study is to evaluate the performance of an EAHE in a Colombian tropical weather in terms of ambient air-cooling capacity. This is done by means of the modelling of the system in TRNSYS. Furthermore, the present study focuses on the simulation of an EAHE system designed to improve thermal comfort in a laboratory room. The space is also simulated using TRNSYS. The laboratory is located in the Mechanical Engineering School building at Universidad Industrial de Santander (UIS), Bucaramanga, Colombia. While experimental validation is required to ensure the effectiveness of possible enhancements, the use of simulation tools allows a better understanding of the system and its performance under different operation conditions. Nowadays, detailed simulation models are becoming more important in the design phase. TRNSYS software is commonly used to study this kind of system. Libraries of this numerical tool have been verified in various studies. Moreover, it has demonstrated to be a very useful tool for analyzing and optimizing the performance of EAHE in different types of constructions, including offices, hospitals and residential buildings [20–25]. This document starts by describing the local weather, soil characteristics and building construction details considered for the EAHE design and evaluation. Afterwards, the process carried out for the parametric analysis, thermal loads estimate, coefficient of performance evaluation and financial analysis is explained in the methodology section. Then, results and discussion are presented to show the relevant findings of this work. Finally, the conclusions are outlined. 2. Description of Study Case 2.1. Local Weather Conditions and Soil Characteristics Weather conditions are measured by a Vantage Pro2 weather station installed at the Mechanical Engineering School building in the Universidad Industrial de Santander (UIS), 0 00  0 00 Bucaramanga, Colombia (7 8 23.604 N; 73 7 15.204 W). For the location, there are two Buildings 2021, 11, x FOR PEER REVIEW 3 of 18 2. Description of Study Case Buildings 2021, 11, 219 3 of 18 2.1. Local Weather Conditions and Soil Characteristics Weather conditions are measured by a Vantage Pro2 weather station installed at the Mechanical Engineering School building in the Universidad Industrial de Santander (UIS), Bucaramanga, Colombia (7°8′23.604″ N; 73°7′15.204″ W). For the location, there are rainy seasons, from March to May and September to November. Moreover, there are two two rainy seasons, from March to May and September to November. Moreover, there are dry seasons, from December to February and June to August. The annual air temperature two dry seasons, from December to February and June to August. The annual air temper- was atmeasur ure was mea ed during sured duri 2017; ng 2017 it varied ; it vafried from rom 20.9 20 C .9 °C to 31.3 to 31.3 C.°C. The The mean mean annual annualair air relative relative humidity was 80%, with a minimum of 49%. Figure 1 shows an example of the humidity was 80%, with a minimum of 49%. Figure 1 shows an example of the measured measured data corresponding to the second week of January, right in the middle of the data corresponding to the second week of January, right in the middle of the dry season dry season from December to February. Furthermore, the mean annual solar radiation 2 from December to February. Furthermore, the mean annual solar radiation was 700 W/m . was 700 W/m . The rainfall varied from 1 mm to 3 mm per day during rainy and dry The rainfall varied from 1 mm to 3 mm per day during rainy and dry seasons, respectively. seasons, respectively. 18 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time [min] Ambient temperature Relative humidity Figure 1. Example of measured relative humidity and air temperature for a given week. Figure 1. Example of measured relative humidity and air temperature for a given week. Buildings 2021, 11, x FOR PEER REVIEW 4 of 18 Because soil characteristics and its temperature variation are some of the main con- Because soil characteristics and its temperature variation are some of the main concerns cerns in EAHEs, an electronic tool is designed to measure the temperature and moisture in EAHEs, an electronic tool is designed to measure the temperature and moisture content content of soil for the location (see Figure 2). It has seven sensors for measuring under- of soil for the location (see Figure 2). It has seven sensors for measuring underground about 28% VWC. The fitting is performed for each sensor with R greater than 99% and it ground soil temperatures and moisture. The sensors are located 0.5 m from each other soil temperatures and moisture. The sensors are located 0.5 m from each other between is represented by this equation: between 0 and 3 m depth. The device is situated in the place where the EAHE will be 𝐶 339,314,287 315,950,014 ℎ 0.013089055 ℎ 0 and locat3 ed (7 m °0 depth. 8′21.6″ N; The 73device °07′12.8″is W) situated . All tempin erat the ure s place ensors wher are ce alithe bratEAHE ed againwill st a Cbe ole located 0.0000282115481 ℎ 3,28761590 10 ℎ 0 00  0 00 (1) Parmer Polystat Standard 1-C6 bath, which has an accuracy of ±0.01 °C according to its (7 08 21.6 N; 73 07 12.8 W). All temperature sensors are calibrated against a Cole Parmer 195,409,715 10 ℎ 460,914,550 10 ℎ calibration certificate. Each sensor is tested 20 times for six temperatures: 15, 17, 19, 21, 23 Polystat Standard 1-C6 bath, which has an accuracy of0.01 C according to its calibration where h means data read by the DAQ, i.e., an integer number from 0 to 1024 due to the and 25 °C. Observed accuracy is below 1%. certificate. Each sensor is tested 20 times for six temperatures: 15, 17, 19, 21, 23 and 25 C. DAQ number of bits. The test performance after calibration for each sensor is shown in Furthermore, to calibrate the volumetric water content (VWC) sensors, eighty sam- Observed accuracy is below 1%. Appendix A. ples of ground are taken from the ground. This ground is dried for 24 hours in an oven at 120 °C. Then, each sample is prepared with a VWC from 1% to 40% with a step of 1%. Each sensor is tested three times for each VWC value. All sensors show a saturation point at Figure 2. Temperature and humidity measurement device. Figure 2. Temperature and humidity measurement device. Temperature and soil moisture content data are measured with an interval of 15 minutes during the most representative weeks for rainy and dry seasons, respectively. Additionally, it is known that soil temperature is influenced by the physical properties of the ground, i.e., porosity, permeability and texture. Furthermore, each soil type has a dif- ferent thermal conductivity depending on its VWC. Sand with high VWC has a high ther- mal conductivity. Nonetheless, soils that have a high content of clay or organic materials like shale or coal have low thermal conductivity. For this reason, a sample for each depth is taken and a laboratory texture test is performed (see Figure 3). Figure 3. Samples taken for a laboratory texture test. 2.2. Building Details Generally, the main source of energy consumption in a building is the cooling de- mand in a hot climate. This principally depends on the construction materials, the enve- lope and the glazed surfaces. Other significant factors that influence the building loads Temperature [°C] Relative humidity [%] 𝑉𝑊 Buildings 2021, 11, x FOR PEER REVIEW 4 of 18 about 28% VWC. The fitting is performed for each sensor with R greater than 99% and it is represented by this equation: 𝐶 339,314,287 315,950,014 ℎ 0.013089055 ℎ 0.0000282115481 ℎ 3,28761590 10 ℎ (1) 195,409,715 10 ℎ 460,914,550 10 ℎ where h means data read by the DAQ, i.e., an integer number from 0 to 1024 due to the Buildings 2021, 11, 219 4 of 18 DAQ number of bits. The test performance after calibration for each sensor is shown in Appendix A. Furthermore, to calibrate the volumetric water content (VWC) sensors, eighty samples of ground are taken from the ground. This ground is dried for 24 hours in an oven at 120 C. Then, each sample is prepared with a VWC from 1% to 40% with a step of 1%. Each sensor is tested three times for each VWC value. All sensors show a saturation point at about 28% VWC. The fitting is performed for each sensor with R greater than 99% and it is represented by this equation: VW C = 339, 314, 287 315, 950, 014 h + 0.013089055 h 3 8 4 0.0000282115481 h + 328, 761, 590 10  h (1) 11 5 15 6 195, 409, 715 10  h + 460, 914, 550 10  h where h means data read by the DAQ, i.e., an integer number from 0 to 1024 due to the DAQ Figure 2. number Temperature and humidity measurement devi of bits. The test performance after calibration ce. for each sensor is shown in Appendix A. Temperature and soil moisture content data are measured with an interval of 15 Temperature and soil moisture content data are measured with an interval of 15 min minutes during the most representative weeks for rainy and dry seasons, respectively. during the most representative weeks for rainy and dry seasons, respectively. Additionally, Addit it is known ionallythat , it is soil known temperatur that soil t e is emp influenced erature is by influenced b the physical y th pr e physical pr operties of the operties of ground, the ground, i.e., porosity,i.permeability e., porosity, p and ermeabi textur lity e. Furthermor and texture. F e, each urthermore, e soil type a has ch soil type a different has thermal a dif- ferent thermal conduct conductivity depending ivity on depending its VWC. Sand on its with VW high C. Sand with high VWC has a high VWC h thermal as conductivity a high ther-. ma Nonetheless, l conductivi soils ty. Nonethel that have ess, soil a highs tha content t have a of clay high content of or organic materials clay or orga like ni shale c maor teria coal ls like have sh low ale or thermal coal have conductivity low thermal . For cond this uct reason, ivity. For a sample this rea for son, each a sample depth fo isr e taken ach dept and h a laboratory texture test is performed (see Figure 3). is taken and a laboratory texture test is performed (see Figure 3). Figure 3. Samples taken for a laboratory texture test. Figure 3. Samples taken for a laboratory texture test. 2 2.2. .2. Build Building ing Details Details General Generally ly,, t the he main main sour sour ce ce of of ener energygy c consumption onsumption i in an building a building is the is t cooling he cool demand ing de- in a hot climate. This principally depends on the construction materials, the envelope mand in a hot climate. This principally depends on the construction materials, the enve- lope and the and t glazed he glasurfaces. zed surfac Other es. Otsignificant her significant factors factthat ors tinfluence hat influence t the building he buildiloads ng loads are the occupancy and the internal sources of heat such as lights or electrical appliances. Because of the space available to install the EAHE, only one space inside the building is considered: The Design Laboratory in the Mechanical Engineering School building at Universidad Industrial de Santander (UIS). For this room, all the heat sources and architectural characteristics are taken into account (see Figures 4 and 5). 𝑉𝑊 Buildings 2021, 11, x FOR PEER REVIEW 5 of 18 Buildings 2021, 11, x FOR PEER REVIEW 5 of 18 are the occupancy and the internal sources of heat such as lights or electrical appliances. are the occupancy and the internal sources of heat such as lights or electrical appliances. Because of the space available to install the EAHE, only one space inside the building is Because of the space available to install the EAHE, only one space inside the building is considered: The Design Laboratory in the Mechanical Engineering School building at Uni- considered: The Design Laboratory in the Mechanical Engineering School building at Uni- Buildings 2021, 11, 219 5 of 18 versidad Industrial de Santander (UIS). For this room, all the heat sources and architec- versidad Industrial de Santander (UIS). For this room, all the heat sources and architec- tural characteristics are taken into account (see Figures 4 and 5). tural characteristics are taken into account (see Figures 4 and 5). Figure 4. Building front view, Design Laboratory highlighted. Figure 4. Building front view, Design Laboratory highlighted. Figure 4. Building front view, Design Laboratory highlighted. Figure 5. Building top view, Design Laboratory highlighted. Figure 5. Building top view, Design Laboratory highlighted. Figure 5. Building top view, Design Laboratory highlighted. Geometrical modeling was done on TRNSYS 3d plug-in for google SketchUp to draw Geometrical modeling was done on TRNSYS 3d plug-in for google SketchUp to draw Geometrical modeling was done on TRNSYS 3d plug-in for google SketchUp to draw the Design Laboratory with the following dimensions: 7.82  21.55  3.075 m . The model the Design Laboratory with the following dimensions: 7.82 × 21.55 × 3.075 m . The model the Design Laboratory with the following dimensions: 7.82 × 21.55 × 3.075 m . The model includes the geometry in conjunction with architectural parameters such as windows, includes the geometry in conjunction with architectural parameters such as windows, includes the geometry in conjunction with architectural parameters such as windows, shelters, doors and walls. Then, the energetic model was generated on TRNSYS Building shelters, doors and walls. Then, the energetic model was generated on TRNSYS Building shelters, doors and walls. Then, the energetic model was generated on TRNSYS Building frontend (TRNBuild). The building model was imported into TRNBuild to calculate the frontend (TRNBuild). The building model was imported into TRNBuild to calculate the frontend (TRNBuild). The building model was imported into TRNBuild to calculate the cooling load (see Figure 6). cooling load (see Figure 6). cooling load (see Figure 6). Buildings 2021, 11, x FOR PEER REVIEW 6 of 18 For this purpose, the masonry, structural elements, envelope materials and architec- Buildings 2021, 11, 219 6 of 18 tural details were considered. The thermo-physical properties of each one of the consid- ered components were included. Figure 6. Building model in TRNSYS. Figure 6. Building model in TRNSYS. The Design Laboratory has occupant capacity for 26 people. The scheduling occu- For this purpose, the masonry, structural elements, envelope materials and architec- pancy, lights and appliances operation of the Design Laboratory were estimated accord- tural details were considered. The thermo-physical properties of each one of the considered ing to ASHRAE Fundamentals [26] and ASHRAE standard 90.1 [27]. Furthermore, infil- components were included. trations and ventilation requirements were set according to the same source. Table 1 pre- The Design Laboratory has occupant capacity for 26 people. The scheduling occupancy, sents a summary of the main parameters used in the energetic modelling for cooling load lights and appliances operation of the Design Laboratory were estimated according to ASHRAE calculation. Fundamentals Furthermore, [26build ] anding ASHRAE charactst eri andar stics d rega 90.1 rdi [27 ng constructi ]. Furthermor on ma e, infiltrations terials and and envelope a ventilation re summa requir riz ements ed in Appendix B. were set according to the same source. Table 1 presents a summary of the main parameters used in the energetic modelling for cooling load Table 1. Space parameters for cooling load calculation. calculation. Furthermore, building characteristics regarding construction materials and envelope are summarized in Appendix B. Parameter Value The second floor of Mechanical Engineering Table 1. Space parameters for cooling load calculation. Location of building School Parameter Value Application Design laboratory Building area 169 m Location of building The second floor of Mechanical Engineering School Application Design laboratory T° Setpoint 24 °C Building area 169 m Number of people 26 T Setpoint 24 C Light 3.69 W/m Number of people 26 Equipment 1080.58 W Light 3.69 W/m Infiltration 0.2 m /s Equipment 1080.58 W Infiltration 0.2 m /s HR (low-level limit) 50% HR (low-level limit) 50% Moreover, to verify the results observed from the simulation carried out in TRNSYS, air temperature was measured in the modelled space. An illustrative comparison between simulated and experimental data is presented in Figure 7. This time segment was chosen Buildings 2021, 11, x FOR PEER REVIEW 7 of 18 Buildings 2021, 11, 219 7 of 18 Moreover, to verify the results observed from the simulation carried out in TRNSYS, air temperature was measured in the modelled space. An illustrative comparison between simulated and experimental data is presented in Figure 7. This time segment was chosen to illustrate the comparison between experimental and predicted temperature. As can be to illustrate the comparison between experimental and predicted temperature. As can be seen, a difference of about 8% was achieved. seen, a difference of about 8% was achieved. 563 573 583 593 603 613 623 633 643 653 Time [h] Measured temperature Simulated temperature Figure 7. Illustrative comparison between experimental (green solid line) and predicted air temperature inside the studied Figure 7. Illustrative comparison between experimental (green solid line) and predicted air temperature inside the studied space (black solid line). space (black solid line). 3. Methodology 3. Methodology 3.1. EAHE Cooling Potential: Parametric Analysis 3.1. EAHE Cooling Potential: Parametric Analysis A TRNSYS model was set up to simulate the heat transfer process. The EAHE sub- A TRNSYS model was set up to simulate the heat transfer process. The EAHE routine, Type 997, simulated the thermal interaction between the buried heat exchanger subroutine, Type 997, simulated the thermal interaction between the buried heat exchanger and the ground (see Figure 8). Boundary conditions were established according to the and the ground (see Figure 8). Boundary conditions were established according to the measured soil properties. Furthermore, operating parameters were set to obtain the outlet measured soil properties. Furthermore, operating parameters were set to obtain the outlet air temperature [18]. Additionally, weather data from Vantage Pro2 station were acquired air temperature [18]. Additionally, weather data from Vantage Pro2 station were acquired through Type 99. Then, this Type provided them to Type 997 (see Figure 8). through Type 99. Then, this Type provided them to Type 997 (see Figure 8). A full description of the model is reported in TESS documentation. The main type, A full description of the model is reported in TESS documentation. The main type, Type 997, was fine-tuned to capture the physical, geographical and climatic characteristics Type 997, was fine-tuned to capture the physical, geographical and climatic characteristics of the site. The simulations were performed with a PVC pipe (thermal conductivity equal of the site. The simulations were performed with a PVC pipe (thermal conductivity equal to 0.185 W/m·K) buried at 1.5 m from the surface to evaluate the performance of EAHE. to 0.185 W/mK) buried at 1.5 m from the surface to evaluate the performance of EAHE. The simulation tests were conducted using a pipe length of 60 m, a diameter of 8 in (0.2 The simulation tests were conducted using a pipe length of 60 m, a diameter of 8 in (0.2 m) m) and a mass flow of 1240 kg/h as reference. However, one of these three design param- and a mass flow of 1240 kg/h as reference. However, one of these three design parameters eters was varied for each of the simulation tests while the others remained constant. The was varied for each of the simulation tests while the others remained constant. The design design parameters examined were mass flow rates of 620, 1240 and 1860 kg/s. Pipe diam- parameters examined were mass flow rates of 620, 1240 and 1860 kg/s. Pipe diameters eters were 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m). Pipe lengths were 90, 150, and 200 m, were 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m). Pipe lengths were 90, 150, and 200 m, varying each parameter independently. varying each parameter independently. The influence of grid parameters, such as the node size, node growth multiplier or The influence of grid parameters, such as the node size, node growth multiplier or the the relationship factor between the cylindrical node and square node on the EAHE outlet relationship factor between the cylindrical node and square node on the EAHE outlet air air temperature, was verified. temperature, was verified. Temperature [°C] Buildings 2021, 11, x FOR PEER REVIEW 8 of 18 The EAHE's operating mode was in continuous mode. Therefore, Type 112 also op- erated in continuous mode. It has an ON/OFF control which is described as follows: If the Buildings 2021, 11, 219 8 of 18 input control signal to the fan is less than 0.5, then the fan is OFF. On the other hand, if it is equal to or greater than 0.5, the fan is ON. Figure 8. TRNSYS soil performance model. Figure 8. TRNSYS soil performance model. The EAHE’s operating mode was in continuous mode. Therefore, Type 112 also 3.2. Coefficient of Performance operated in continuous mode. It has an ON/OFF control which is described as follows: If the input control signal to the fan is less than 0.5, then the fan is OFF. On the other hand, if The coefficient of performance (COP) of the system was evaluated using Equation (2) it is equal to or greater than 0.5, the fan is ON. within the TRANSYS environment. In this equation, COP is the coefficient of performance, 𝑚 is the air mass flow rate through the pipe, 𝐶 is the pipe discharge coefficient and 𝑐 3.2. Coefficient of Performance is the air specific heat capacity. Furthermore, 𝑄 is the work done by the blower and The coefficient of performance (COP) of the system was evaluated using Equation (2) 𝑇 and 𝑇 are the air temperatures at the EAHE inlet and outlet, respectively. The within the TRANSYS environment. In this equation, COP is the coefficient of performance, COP indicates the energy efficiency of the system [28]. The EAHE cooling potential is m is the air mass flow rate through the pipe, C is the pipe discharge coefficient and c is d p evaluated based on the analysis of the daily COP. The cooling load is calculated by TRN- the air specific heat capacity. Furthermore, Q is the work done by the blower and T and i inlet SYS subroutine, Type 56. This is carried out as reported in Multizone Building modeling T are the air temperatures at the EAHE inlet and outlet, respectively. The COP indicates exit [29]. the energy efficiency of the system [28]. The EAHE cooling potential is evaluated based 𝑚 𝐶 𝑐 𝑇 𝑇 on the analysis of the daily COP. The cooling load is calculated by TRNSYS subroutine, (2) 𝐶𝑂𝑃 Type 56. This is carried out as reported in Multizone Building modeling [29]. mC c (T T ) d inlet exit COP = (2) 3.3. Financial Analysis There are several economic exploration techniques. Two of them were used in this 3.3. Financial Analysis study: Net present value (NPV) and internal rate of return (IRR) [30]. These analyses con- sider the initial capital, operations, savings and associated costs. The initial capital or in- There are several economic exploration techniques. Two of them were used in this vest study: ment Net incl pr udes esent : He value at trans (NPV) fer subs andys internal tem, air d rate istribution of return su (IRR) bsystem, [30]. centrif Theseuanalyses gal fans, consider the initial capital, operations, savings and associated costs. The initial capital or investment includes: Heat transfer subsystem, air distribution subsystem, centrifugal fans, ductworks and borehole excavation. The operation costs involve electricity consumption and maintenance of the EAHE. The savings include the air conditioning system (AC) and its inherent electricity consumption. The EAHE and AC investment cost are obtained by a Buildings 2021, 11, x FOR PEER REVIEW 9 of 18 ductworks and borehole excavation. The operation costs involve electricity consumption and maintenance of the EAHE. The savings include the air conditioning system (AC) and its inherent electricity consumption. The EAHE and AC investment cost are obtained by a detailed unit price analysis. The equipment, material and labor prices are provided by the local industries. Furthermore, an economic analysis was completed based on some necessary assump- tions: Inflation rates are ignored, as well as the annual increase of cost by electricity con- Buildings 2021, 11, 219 9 of 18 sumption. Moreover, a discount rate of 8% which applies in Colombia [31] and the lifespan of the EAHE equal to 25 years were evaluated. 4. Results and Discussion detailed unit price analysis. The equipment, material and labor prices are provided by the 4.1. Soil Temperature and Volume Water Content local industries. Results in Figure 9 show that the lowest temperature of 19.6 °C was obtained just Furthermore, an economic analysis was completed based on some necessary assump- below ground surface (0 m depth). However, it is strongly affected by the ambient tem- tions: Inflation rates are ignored, as well as the annual increase of cost by electricity perature and humidity [32]. The heat exchange of the ground surface with the ambient is consumption. Moreover, a discount rate of 8% which applies in Colombia [31] and the due to convection with the air and radiation with the sky. It also receives solar irradiation lifespan of the EAHE equal to 25 years were evaluated. during daytime. Deeper ground layers experience fewer variations in temperature and the changes 4. Results and Discussion exhibit a larger time delay than those of shallower soils. This is caused by the high thermal 4.1. Soil Temperature and Volume Water Content inertia of the soil under the surface of the earth [33]. While the temperature increases with Results in Figure 9 show that the lowest temperature of 19.6 C was obtained just below depth, its fluctuation between day and night reduces. Deeper samples can be considered ground surface (0 m depth). However, it is strongly affected by the ambient temperature as a heated basin [19]. and humidity [32]. The heat exchange of the ground surface with the ambient is due Therefore, for the present study, the EAHE would be buried at 1.5 m depth. Here, to convection with the air and radiation with the sky. It also receives solar irradiation temperature and its amplitude remained almost constant at 22.3 °C during the testing pe- during daytime. riod (see Figure 9). 0 1000 2000 3000 4000 5000 6000 7000 8000 Time [min] Ambient temperature 0 m 0.5 1 m 1.5 m 2 m 2.5 m 3 m Figure 9. Soil temperature measured at different depths. Figure 9. Soil temperature measured at different depths. Regarding the VWC, measurements show that near the surface (0 and 0.5 m) it is Deeper ground layers experience fewer variations in temperature and the changes affected by atmospheric conditions. However, when depth increases, the VWC is more exhibit a larger time delay than those of shallower soils. This is caused by the high thermal constant. On average, it is 25% at 1.5 m depth (see Figure 10). inertia of the soil under the surface of the earth [33]. While the temperature increases with depth, its fluctuation between day and night reduces. Deeper samples can be considered as a heated basin [19]. Therefore, for the present study, the EAHE would be buried at 1.5 m depth. Here, temperature and its amplitude remained almost constant at 22.3 C during the testing period (see Figure 9). Regarding the VWC, measurements show that near the surface (0 and 0.5 m) it is affected by atmospheric conditions. However, when depth increases, the VWC is more constant. On average, it is 25% at 1.5 m depth (see Figure 10). Temperature [°C] Buildings 2021, 11, 219 10 of 18 Buildings 2021, 11, x FOR PEER REVIEW 10 of 18 0 10002000300040005000600070008000 Time [min] 0 m 0.5 m 1 m 1.5 m 2 m 2.5 3 m Figure 10. Measured VWC at different depths. Figure 10. Measured VWC at different depths. Results indicate that the soil in the EAHE location is composed of 57.6% sand, 12.4% Results indicate that the soil in the EAHE location is composed of 57.6% sand, limestone and 30% clay. Soil thermophysical properties were estimated according to tex- 12.4% limestone and 30% clay. Soil thermophysical properties were estimated accord- tural properties as follows: Thermal conductivity 2.42 W/m·K, volumetric heat capacity ing to textural properties as follows: Thermal conductivity 2.42 W/mK, volumetric 3 −6 2 2.9 MJ/m ·K, specific heat capacity 1.45 kJ/kg·K and thermal diffusivity 0.83 × 10 m /s heat capacity 2.9 MJ/m K, specific heat capacity 1.45 kJ/kgK and thermal diffusivity [34–36]. 6 2 0.83  10 m /s [34–36]. 4.2. EAHE Cooling Potential: Parametric Analysis 4.2. EAHE Cooling Potential: Parametric Analysis Different simulations are conducted using fixed design parameters while one of them Different simulations are conducted using fixed design parameters while one of them is varied for each of the simulation tests. The first simulation evaluates the influence of is varied for each of the simulation tests. The first simulation evaluates the influence of the the mass flow rates of 620, 1240 and 1860 kg/h. These values correspond approximately to mass flow rates of 620, 1240 and 1860 kg/h. These values correspond approximately to 1, 2 and 3 ACH (Air Changes per Hour), respectively, for a design laboratory of 510 m 1, 2 and 3 ACH (Air Changes per Hour), respectively, for a design laboratory of 510 m according to ASHRAE-62.1 [30]. according to ASHRAE-62.1 [30]. As can be seen in Figure 11, the simulated EAHE air outlet temperature followed the As can be seen in Figure 11, the simulated EAHE air outlet temperature followed the variations of the ambient outdoor air temperatures with a minor amplitude. The ambient variations of the ambient outdoor air temperatures with a minor amplitude. The ambient air temperature range was 19.1 °C to 27.8 °C in January, while the simulated outlet air air temperature range was 19.1 C to 27.8 C in January, while the simulated outlet air temperature was 22.1 °C to 23.7 °C for 620 kg/h, 21.5 °C to 24.3 °C for 1240 kg/h and 21.1 temperature was 22.1 C to 23.7 C for 620 kg/h, 21.5 C to 24.3 C for 1240 kg/h and °C to 24.8 °C for 1860 kg/h. 21.1 C to 24.8 C for 1860 kg/h. Furthermore, the outlet air temperature increased when the airflow rate inside the Furthermore, the outlet air temperature increased when the airflow rate inside the EAHE increased. Similar results were obtained by other authors [10,34]. This behavior can EAHE increased. Similar results were obtained by other authors [10,34]. This behavior can be explained because a higher mass flow with the same diameter implies a higher velocity be explained because a higher mass flow with the same diameter implies a higher velocity of the air inside the tube. Consequently, the overall heat exchange was reduced owing to of the air inside the tube. Consequently, the overall heat exchange was reduced owing to the minor residence time of the air inside the pipe [14]. the minor residence time of the air inside the pipe [14]. Once the influence of the mass flow was stated, the performance of EAHE was eval- Once the influence of the mass flow was stated, the performance of EAHE was uated with a constant tube diameter of 8 in (0.2 m) and operated with an air mass flow evaluated with a constant tube diameter of 8 in (0.2 m) and operated with an air mass rate of 1240 kg/h for three different pipe lengths (90, 150 and 200 m). Again, the EAHE flow rate of 1240 kg/h for three different pipe lengths (90, 150 and 200 m). Again, the outlet air temperature varied according to the ambient temperature. However, while am- EAHE bient temperature fluctuate outlet air temperatur d between 19.5 °C e varied according and 27 °C, the EAHE to the ambient temperatur air outlet tem e.pera However ture , while ambient temperature fluctuated between 19.5 C and 27 C, the EAHE air outlet for 90 m pipe length varied from 21.6 °C to 24 °C. For 150 m pipe length, EAHE air outlet temperature for 90 m pipe length varied from 21.6 C to 24 C. For 150 m pipe length, EAHE air outlet temperature varied from 22 C to 23.3 C, and for 200 m pipe length, it varied from 22 C to 23 C (see Figure 12). VWC [%] Buildings 2021, 11, x FOR PEER REVIEW 11 of 18 Buildings 2021, 11, 219 11 of 18 Buildings 2021, 11, x FOR PEER REVIEW 11 of 18 temperature varied from 22 °C to 23.3 °C, and for 200 m pipe length, it varied from 22 °C to 23 °C (see Figure 12). temperature varied from 22 °C to 23.3 °C, and for 200 m pipe length, it varied from 22 °C to 23 °C (see Figure 12). 21 22 168 188 208 228 248 268 288 308 328 Time [h] 168 188 208 228 248 268 288 308 328 Ambient temperature 620 kg/h 1240 kg/h 1860 kg/h Time [h] Ambient temperature 620 kg/h 1240 kg/h 1860 kg/h Figure 11. EAHE outlet air temperature vs air mass flow rate. Figure 11. EAHE outlet air temperature vs air mass flow rate. Figure 11. EAHE outlet air temperature vs air mass flow rate. 168 188 208 228 248 268 288 308 328 168 188 208 228 248 268 288 308 328 Time [h] Time [h] Ambient temperature 90 m 150 m 200 m Ambient temperature 90 m 150 m 200 m Figure 12. Figure 12. EA EA HE outlet HE outlet air air te temperature vs pipe length. mperature vs pipe length. Figure 12. EAHE outlet air temperature vs pipe length. Thereby, it can be observed that an increase in the pipe length results in a decrease Thereby, it can be observed that an increase in the pipe length results in a decrease Thereby, it can be observed that an increase in the pipe length results in a decrease in the variations of outlet air temperatures. Thus, peaks of the air temperature during the in the variations of outlet air temperatures. Thus, peaks of the air temperature during the in the variations of outlet air temperatures. Thus, peaks of the air temperature during day are reduced. These results are similar to those reported by [10,12,34]. Finally, the in- day are reduced. These results are similar to those reported by [10,12,34]. Finally, the in- the day are reduced. These results are similar to those reported by [10,12,34]. Finally, the fluence of the tube diameter was evaluated, i.e., 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m) fluence of the tube diameter was evaluated, i.e., 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m) influence of the tube diameter was evaluated, i.e., 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m) are considered. are con are consider sidere ed. d. For these simulations, a pipe length of 60 m, buried at 1.5 m depth with an air mass flow rate of 1240 kg/s, was imposed. As Figure 13 shows, the outlet air temperature for Temperature [°C] Temperature [°C] Temperature [°C] Temperature [°C] Buildings 2021, 11, x FOR PEER REVIEW 12 of 18 Buildings 2021, 11, 219 12 of 18 For these simulations, a pipe length of 60 m, buried at 1.5 m depth with an air mass flow rate of 1240 kg/s, was imposed. As Figure 13 shows, the outlet air temperature for a 4 in (0.1 m) pipe diameter varied from 21.1 °C during the night to 24.8 °C in the day, whereas it changed from 21.5 °C to 24.3 °C for 8 in (0.2 m), and from 21.6 °C to 24.3 °C for a 4 in (0.1 m) pipe diameter varied from 21.1 C during the night to 24.8 C in the day, 12 in (0.3 m). whereas it changed from 21.5 C to 24.3 C for 8 in (0.2 m), and from 21.6 C to 24.3 C for Therefore, an increase in the pipe diameter from 8 in to 12 in had a rather limited 12 in (0.3 m). influence on the outlet air temperature fluctuations. 168 188 208 228 248 268 288 308 328 Time [h] Ambient temperature 4 in 8 in 12 in Figure 13. EAHE outlet air temperature vs pipe diameter. Figure 13. EAHE outlet air temperature vs pipe diameter. In summary, it is considered that the best combination of EAHE design parameters Therefore, an increase in the pipe diameter from 8 in to 12 in had a rather limited to get an outlet air temperature varying between 21.1 °C and 24.3 °C is: 8 in (0.2 m) diam- influence on the outlet air temperature fluctuations. eter, 150 m length and air mass flow of 1240 kg/h. In summary, it is considered that the best combination of EAHE design parameters to However, for the Design Laboratory, an air mass flow of 1240 kg/h would allow only get an outlet air temperature varying between 21.1 C and 24.3 C is: 8 in (0.2 m) diameter, 2 air changes per hour (ACH), but according to ASHRAE [30], 2.6 ACH are required. 150 m length and air mass flow of 1240 kg/h. Moreover, if the air mass flow increases so does outlet air temperature. Alternatively, the However, for the Design Laboratory, an air mass flow of 1240 kg/h would allow air mass flow and pipe length can be increased simultaneously without affecting the only 2 air changes per hour (ACH), but according to ASHRAE [30], 2.6 ACH are required. EAHE outlet air temperature, while ACH is improved. Moreover, if the air mass flow increases so does outlet air temperature. Alternatively, the For this reason, the EAHE is designed as a network of buried pipes that has seven air mass flow and pipe length can be increased simultaneously without affecting the EAHE tubes of 36 m length (a fan for each one), a diameter of 8 in (0.2 m) and an air mass flow outlet air temperature, while ACH is improved. rate of 1560 kg/h in continuous operation mode. The simulated results for the last 1000 For this reason, the EAHE is designed as a network of buried pipes that has seven hours of one year are presented in Figure 14. As the figure shows, the continuous opera- tubes of 36 m length (a fan for each one), a diameter of 8 in (0.2 m) and an air mass flow rate tion strategy takes advantage of night cooling. of 1560 kg/h in continuous operation mode. The simulated results for the last 1000 h of During night hours, ambient air temperature goes down and cools the soil, which one year are presented in Figure 14. As the figure shows, the continuous operation strategy allows it to recover its cooling capacity [2,37]. The thermal performance analysis reveals takes advantage of night cooling. that the outlet air mean temperature is 22.5 °C and varies between 20.9 °C and 24.1 °C (see During night hours, ambient air temperature goes down and cools the soil, which Figure 14). allows it to recover its cooling capacity [2,37]. The thermal performance analysis reveals Hot air peaks are suppressed in this way. The results obtained in this work agree that the outlet air mean temperature is 22.5 C and varies between 20.9 C and 24.1 C (see with the study reported by Díaz-Hernández [37] for an EAHE under similar weather con- Figure 14). ditions. Hot air peaks are suppressed in this way. The results obtained in this work agree with the study reported by Díaz-Hernández [37] for an EAHE under similar weather conditions. Temperature [°C] Buildings 2021, 11, 219 13 of 18 Buildings 2021, 11, x FOR PEER REVIEW 13 of 18 7760 7860 7960 8060 8160 8260 8360 8460 8560 8660 8760 Time [h] Ambient temperature EAHE outlet temperature Figure 14. Ambient temperature vs EAHE outlet air temperature. Figure 14. Ambient temperature vs EAHE outlet air temperature. 4.3. Coefficient of Performance (COP) 4.3. Coefficient of Performance (COP) The COP was calculated using TRNSYS software. The model incorporated the cli- The COP was calculated using TRNSYS software. The model incorporated the climatic matic data (Type 99), the behavior of soil (Type 997), operating conditions (Types 56 and data (Type 99), the behavior of soil (Type 997), operating conditions (Types 56 and 997) and 997) and building details (Type 56). Variations in COP depended on soil temperature. building details (Type 56). Variations in COP depended on soil temperature. When the When the ambient air temperature as higher than the soil temperature, the air released ambient air temperature as higher than the soil temperature, the air released heat to the heat to the soil. In this manner, the EAHE air cooled down space. This cooling effect took soil. In this manner, the EAHE air cooled down space. This cooling effect took place from place from 9 am to 8 pm. 9 am to 8 pm. On the other hand, at night, from 8 pm to 9 am, the soil temperature was higher than On the other hand, at night, from 8 pm to 9 am, the soil temperature was higher than the ambient air temperature and heat flowed from soil to air. The weekly maximum cool- the ambient air temperature and heat flowed from soil to air. The weekly maximum cooling ing COP was around 160 and the daily variations of COP were found in a range between COP was around 160 and the daily variations of COP were found in a range between 0.91 0.91 and 160. and 160. The high value of the COP and the parallel distribution of the EAHE allow energy The high value of the COP and the parallel distribution of the EAHE allow energy savings compared with a standard HVAC system. Low power consumption fans (0.12 kW savings compared with a standard HVAC system. Low power consumption fans (0.12 kW for the motor) installed in each pipe could be used to vary the operation mode for future for the motor) installed in each pipe could be used to vary the operation mode for future research on wet soil conditions [33]. When a low cooling potential is required, some fans research on wet soil conditions [33]. When a low cooling potential is required, some could be turned off to increase the COP. The magnitude of calculated COP values is sim- fans could be turned off to increase the COP. The magnitude of calculated COP values is ilar to typical values found in the literature and reflect the high efficiency of the heat ex- similar to typical values found in the literature and reflect the high efficiency of the heat changer [38]. exchanger [38]. 4.4. Financial Analysis 4.4. Financial Analysis Figure 15 illustrates the instantaneous energy load calculated. The heat was transmit- Figure 15 illustrates the instantaneous energy load calculated. The heat was transmit- ted in or out of the laboratory mainly through external walls, windows, doors and ceiling. ted in or out of the laboratory mainly through external walls, windows, doors and ceiling. Moreover, some heat was transmitted through the floor. Total heat load is the summation Moreover, some heat was transmitted through the floor. Total heat load is the summation of the sensible heat with the total latent heat. of the sensible heat with the total latent heat. The energetic model allows the calculation of air conditioning capacity. According to the analysis, 95% of occupancy hours can be satisfied with a cooling capacity of 10 refrig- eration tons RT (35.168 kW) which satisfies the requirements of 300 unmet load hours of the ASHRAE standard 90.1 [27]. Temperature [°C] Buildings 2021, 11, 219 14 of 18 Buildings 2021, 11, x FOR PEER REVIEW 14 of 18 0 1000 2000 3000 4000 5000 6000 7000 8000 Time [h] Figure 15. Total heat transfer obtained from TRANSYS model. Figure 15. Total heat transfer obtained from TRANSYS model. In this section, the HVAC system costs associated with maintenance, energy con- The energetic model allows the calculation of air conditioning capacity. According to sumption and the initial investment are compared to the costs of the EAHE construction the analysis, 95% of occupancy hours can be satisfied with a cooling capacity of 10 refriger- and operation. The EAHE and HVAC investment were calculated using a detailed unit ation tons RT (35.168 kW) which satisfies the requirements of 300 unmet load hours of the price analysis during the year 2019. Furthermore, equipment, material and labor prices ASHRAE standard 90.1 [27]. were provide In this section, d by the local industr the HVAC system y. Table 2 summarizes the cost costs associated with maintenance, of the EAHE energy and the consump- HVAC systems. The maintenance and energy consumption costs are evaluated annually. tion and the initial investment are compared to the costs of the EAHE construction and However, the investment of the project is only considered for year zero. operation. The EAHE and HVAC investment were calculated using a detailed unit price analysis during the year 2019. Furthermore, equipment, material and labor prices were Table 2. EAHE vs HVAC costs. provided by the local industry. Table 2 summarizes the cost of the EAHE and the HVAC systems. The maintenance and energy consumption costs are evaluated annually. However, Cash Flow EAHE HVAC the investment of the project is only considered for year zero. Investment cost ($) US$ 23,825.40 US$ 6338.02 Maintenance cost ($) US$ 524.62 US$ 139.75 Table 2. EAHE vs. HVAC costs. Electricity consumption cost US$ 1111.92 US$ 5678.76 ($/annual) Cash Flow EAHE HVAC Investment cost ($) US$23,825.40 US$6338.02 The cash inflows are all costs of HVAC; they are incomes. The cash outflows are all Maintenance cost ($) US$524.62 US$139.75 costs of EA Electricity H consumption E; they are lo cost sses. The en ($/annual) ergy consumption of the HVAC represent US$1111.92 US$5678.76 s the larg- est portion of the total operational cost. Cash flow was calculated in TRNSYS and consid- ered: Cooling schedule setup point and occupancy schedule, cooling capacity of the air The cash inflows are all costs of HVAC; they are incomes. The cash outflows are conditioner, operation mode, frequency of use, the electricity tariff in Bucaramanga and all costs of EAHE; they are losses. The energy consumption of the HVAC represents the its weather conditions. largest portion of the total operational cost. Cash flow was calculated in TRNSYS and Moreover, the maintenance cost for the mechanical system and civil construction is considered: Cooling schedule setup point and occupancy schedule, cooling capacity of the about 3% of the total investment cost for each one [39]. These cash flows are added in each air conditioner, operation mode, frequency of use, the electricity tariff in Bucaramanga and year of the overall analysis period, resulting in an NPV calculated equal to US$ 25,662.3. its weather conditions. Then, applying the criteria of NPV > 0, a positive value means that this project is a rentable Moreover, the maintenance cost for the mechanical system and civil construction is way to invest the money. On the other hand, the internal rate of return is calculated as about 3% of the total investment cost for each one [39]. These cash flows are added in each 23%. The interest rate obtained is higher than bank interest rates, and payback time is year of the overall analysis period, resulting in an NPV calculated equal to US$25,662.3. Then, applying the criteria of NPV > 0, a positive value means that this project is a rentable Qt [kW] Buildings 2021, 11, 219 15 of 18 way to invest the money. On the other hand, the internal rate of return is calculated as 23%. The interest rate obtained is higher than bank interest rates, and payback time is archived in approximately six years, which, from a basic financial point of view, makes the project profitable. 5. Conclusions The technical and financial feasibility of an EAHE installed in tropical weather in Colombia to acclimatize a laboratory inside the Mechanical Engineering School (UIS) build- ing at Bucaramanga was analyzed. To do so, the EAHE and the laboratory were simulated using TRNSYS. Moreover, to provide inputs for the model, the temperature, the volume of the water content and thermophysical properties of the soil were experimentally obtained. Furthermore, local weather conditions were measured. Regarding technical aspects, an EAHE with a length of 252 m (7 pipes in parallel) and a pipe diameter of 8 in (0.2 m) produced the best results. Furthermore, an air mass flow rate of 1560 kg/h provided enough air exchanges per hour without reducing the EAHE performance while satisfying calculated thermal loads. With these dimensions and operating conditions, the EAHE delivered air at temperatures between 20.9 C and 24.1 C, which are within the established conditions of thermal comfort. For the calculated specifications, the project had an IRR of 23% and a payback period of six years. Therefore, the project is considered financially viable. The EAHE could also be implemented in tropical areas with similar atmospheric and soil conditions to increase the thermal inertia of the buildings and/or reduce thermal loads. For instance, cities like Medellin or Cali. It is left for future work to carry out a prototype, which allows corroborating the data obtained from simulations in the design stage of the EAHE. Likewise, it is recommended to carry out prolonged tests of thermal saturation of the ground. Author Contributions: Conceptualization, S.A.P.P. and J.E.J.I.; methodology, S.A.P.P.; software, S.A.P.P.; valida-tion, S.A.P.P.; formal analysis, S.A.P.P.; investigation, S.A.P.P.; resources, J.E.J.I.; data curation, S.A.P.P.; writing—origi-nal draft preparation, S.A.P.P.; writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ministry of Science, Technology, and Innovation (Ministerio de Ciencia, Tecnología e Innovación)—MINCIENCIAS (Project Contract No. 80740-798-2019). Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors wish to thank the Vice-Rectorate for Research and Extension (Vicer- rectoría de Investigación y Extensión) from the Universidad Industrial de Santander (Project 8600), and the Energy Mining Planning Unit (Unidad de Planeación Minero Energética—UPME). Conflicts of Interest: The authors declare no conflict of interest. Appendix A Table A1. VWC Measurements from each sensor once calibrated. Sample Sensor 1 Sensor 2 Sensor 3 Sensor 4 Sensor 5 Sensor 6 Sensor 7 %VWC 1.0 1.1 1.2 1.0 1.4 0.9 1.5 1.4 2.0 2.3 1.9 1.7 2.3 2.1 2.2 2.3 3.0 3.1 2.9 3.1 2.9 3.2 3.1 3.0 4.0 3.9 4.2 4.2 4.3 4.1 3.8 3.9 5.0 4.9 4.8 5.1 5.3 5.2 5.0 4.9 Buildings 2021, 11, 219 16 of 18 Table A1. Cont. Sample Sensor 1 Sensor 2 Sensor 3 Sensor 4 Sensor 5 Sensor 6 Sensor 7 %VWC 6.0 6.2 5.9 5.8 5.9 5.9 6.2 6.1 7.0 6.9 7.2 7.3 6.9 6.9 6.9 7.0 8.0 7.8 7.9 8.0 8.1 8.3 7.8 8.1 9.0 9.1 9.3 8.8 8.9 8.9 9.0 8.7 10.0 10.3 10.1 9.8 10.2 10.3 10.1 10.0 11.0 11.0 10.8 10.9 11.3 10.9 10.8 11.1 12.0 12.2 12.1 12.2 11.9 12.4 11.8 11.9 13.0 13.0 13.2 13.3 12.8 13.1 13.3 12.9 14.0 14.2 13.9 14.1 14.3 14.4 14.3 14.1 15.0 15.4 15.2 15.0 15.3 15.2 15.3 14.9 16.0 15.9 16.5 16.3 16.2 15.8 15.9 15.0 17.0 16.7 16.3 16.2 17.3 17.1 17.4 16.9 18.0 18.5 18.6 18.3 18.0 18.2 18.1 17.8 19.0 18.9 19.0 19.2 19.1 18.9 19.4 19.2 20.0 20.1 20.4 20.2 20.1 19.8 19.3 20.2 21.0 21.0 21.2 21.5 21.2 21.4 20.8 20.8 22.0 22.1 21.8 21.7 22.3 22.0 22.5 22.2 23.0 23.0 22.9 23.1 22.9 23.4 22.9 22.7 24.0 23.7 23.9 24.0 24.0 23.6 23.8 24.1 25.0 25.1 25.0 25.5 25.1 25.1 25.4 24.9 26.0 26.1 25.9 26.1 26.1 25.8 26.2 26.5 27.0 26.9 26.9 27.2 27.1 27.4 26.8 27.3 28.0 28.1 27.9 28.5 27.9 28.3 28.4 28.1 29.0 29.2 28.7 28.6 29.0 28.8 29.3 28.7 30.0 29.9 30.2 30.4 30.2 29.7 29.8 29.7 Appendix B Table A2. Building construction materials and envelope characteristics. Construction Elements Description Buffer plates: 3000 PSI concrete, rough finish and thickness 7 cm. Leveling mortar: Thickness 5 cm, concrete 2500 PSI with -inch aggregate, mesh 4 mm electro welded, crosslinked of 20  20 cm. Floors Polished concrete: 3 cm thick, with application surface of hardener with dosage of heavy traffic of 5 kg/m . Dilations of 1.5 cm, 2  2 m. Polished finish. Partition walls: Bricks type H15. Masonry Mortar: 1:3 cement: sand, 1 cm thick, alternating joints between courses. Sun shades 3.075  0.55  0.1-m concrete supported on tie beams and masonry H12 of 90 cm. 24  24-inch natural fiber panels with thicknesses between 4 and 16 millimeters, Ceiling attached to 5/16-inch die-cut frame. 1.5 cm thick frieze, 1: 3 ratio cement: sand, # 6 sieve. Friezes, stuccoes and paintings Stucco: A mixture of plaster, lime and cement. Sanded twice. Painting: Two coats of armor-type paint applied with rollers. Windows in colorless natural glass of 5 mm thickness, a single sheet. Windows and doors Gasket in the frame. Main doors made of glass colorless tempered 10 mm and veneer aluminum. Buildings 2021, 11, 219 17 of 18 References 1. Huo, T.; Ren, H.; Cai, W. Estimating urban residential building-related energy consumption and energy intensity in China based on improved building stock turnover model. Sci. Total Environ. 2019, 650, 427–437. [CrossRef] 2. Ahmed, S.F.; Khan, M.M.K.; Amanullah, M.T.O.; Rasul, M.G.; Hassan, N.M.S. Performance assessment of earth pipe cooling system for low energy buildings in a subtropical climate. Energy Convers. Manag. 2015, 106, 815–825. [CrossRef] 3. Al-Ajmi, F.; Loveday, D.L.; Hanby, V.I. The cooling potential of earth-air heat exchangers for domestic buildings in a desert climate. Build. Environ. 2006, 41, 235–244. [CrossRef] 4. Santamouris, M.; Kolokotsa, D. Passive cooling dissipation techniques for buildings and other structures: The state of the art. 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Díaz-Hernández, H.P.; Macias-Melo, E.V.; Aguilar-Castro, K.M.; Hernández-Pérez, I.; Xamán, J.; Serrano-Arellano, J.; López- Manrique, L.M. Experimental study of an earth to air heat exchanger (EAHE) for warm humid climatic conditions. Geothermics 2020, 84, 101741. [CrossRef] 38. Pfafferott, J. Evaluation of earth-to-air heat exchangers with a standardised method to calculate energy efficiency. Energy Build. 2003, 35, 971–983. [CrossRef] 39. Liu, Y.; Qin, X.S.; Chiew, Y.M. Investigation on potential applicability of subsurface cooling in Singapore. Appl. Energy 2013, 103, 197–206. [CrossRef] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Buildings Multidisciplinary Digital Publishing Institute

Potential Applicability of Earth to Air Heat Exchanger for Cooling in a Colombian Tropical Weather

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buildings Article Potential Applicability of Earth to Air Heat Exchanger for Cooling in a Colombian Tropical Weather Sergio Alexander Peñaloza Peña and Julián Ernesto Jaramillo Ibarra * Mechanical Engineering School, Universidad Industrial de Santander, Bucaramanga 680002, Colombia; sergio.pennaloza@gmail.com * Correspondence: jejarami@uis.edu.co Abstract: Buildings exhibit a high energy consumption compared with other economic sectors. While percentages vary from country to country, buildings are responsible for approximately 40% of the global energy demand. Most of this is consumed for achieving human thermal comfort. In Colombia, the government promotes policies for the adoption of efficient energy strategies in this sector. The earth to air heat exchanger (EAHE) can be used to reduce the cooling load of a building. Therefore, this study aims to evaluate the energy savings that can be obtained by installing an EAHE in a tropical climate in Colombia. To do so, a mathematical model is implemented in TRNSYS (Transient System Simulation Tool) to predict the thermal performance and the cooling capacity of the EAHE. The system is modeled as a function of pipe length, diameter, material, thickness and air mass flow. Moreover, soil, local atmospheric conditions and building features are taken into account. It is found that the air leaves the EAHE at temperatures between 20.9 C and 24.1 C, which are approximately 3 C below ambient temperature. Furthermore, the economic feasibility of the project is verified. Thereby, it is demonstrated that the EAHE can be a competitive alternative to current HVAC systems. Keywords: passive cooling; EAHE; temperature potential; ventilation; TRNSYS; soil temperature Citation: Peñaloza Peña, S.A.; Jaramillo Ibarra, J.E. Potential Applicability of Earth to Air Heat Exchanger for Cooling in a Colombian Tropical Weather. 1. Introduction Buildings 2021, 11, 219. https:// Energy supply is fundamental for the economic development and the well-being of the doi.org/10.3390/buildings11060219 population of any country. Furthermore, energy consumption has increased considerably due to growing needs to ensure thermal comfort conditions inside buildings [1].For this Academic Editor: Ambrose Dodoo reason, the achievement of indoor thermal comfort while minimizing energy consumption in buildings is a crucial aim around the world. Therefore, during the last decade, there Received: 20 March 2021 has been a rising interest in implementing alternative sources to replace conventional Accepted: 18 May 2021 cooling of buildings [2–4]. In Colombia, about 70% of energy generation is obtained Published: 21 May 2021 from hydropower. Consequently, the Colombian electricity sector is highly vulnerable to sufficient water availability [5]. For instance, during 2015 and 2016, the droughts suffered in Publisher’s Note: MDPI stays neutral this country as a result of the natural phenomenon called “El Niño” were close to causing with regard to jurisdictional claims in a blackout due to the drop in the electricity generation of the hydroelectric plants [6]. published maps and institutional affil- For this reason, governmental and non-governmental entities are promoting policies to iations. diversify energy sources. They are taking measures oriented to new generation systems and management of demand, especially in those areas with a high level of energy consumption, such as cooling of buildings [7]. One of the promising passive cooling techniques is Earth to Air Heat Exchanger Copyright: © 2021 by the authors. (EAHE). It takes advantage of the fact that ground temperature, at a certain depth, is Licensee MDPI, Basel, Switzerland. almost constant throughout the year. This allows its use as a heat sink. EAHE consists This article is an open access article of a simple system of buried pipes through which ambient air circulates. Therefore, part distributed under the terms and of the thermal air energy is transferred to the ground during this movement. Then, the conditions of the Creative Commons outlet air from the EAHE can be directly used for space cooling or as pre-cooled air in an Attribution (CC BY) license (https:// HVAC system [2,8,9]. The degree of success of the EAHE depends on many parameters, for creativecommons.org/licenses/by/ instance, local atmospheric conditions such as solar radiation, ambient air temperature and 4.0/). Buildings 2021, 11, 219. https://doi.org/10.3390/buildings11060219 https://www.mdpi.com/journal/buildings Buildings 2021, 11, 219 2 of 18 relative humidity, as well as design aspects of the heat exchanger: Configuration, depth of burial, air flow rate, pipes diameter and length [10–12]. In the last decade, many studies have been done on EAHE configurations to analyze its energy performance. Results showed that EAHE is an energy-efficient system that can be used to achieve thermal comfort inside buildings. Lee and Strand [13] evaluated the cooling and heating potential of earth tubes in four representative locations in the U.S. They created a mathematical model for EAHE in an Energy Plus software environment. A detailed algorithm was used to calculate the soil temperature variation for each pipe for every time step of the simulation. Moreover, Ascione et al. [14] studied the energy performances achievable using an EAHE in different Italian climates. This work showed that the best performance of the EAHE system can be obtained with a pipe length of 50 m, buried 3 m deep in wet soil. Benhmammou and Draoui [15] developed a one-dimensional transient model to study the thermal performance of EAHEs for air cooling in summer in the Algerian Sahara Desert. Their results revealed that the daily mean efficiency increases when the length of the pipe does, but it decreases when the cross-section area of the pipe or air velocity is reduced. Shojaee and Malek [16] evaluated the potential energy savings of a four-pipe EAHE for different climates of Iran. Their findings indicated that the average energy savings changes according to the weather in the analyzed cities. Furthermore, the EAHE shows better behavior when the soil is silt, in comparison with loam and clay soil. In this sense, several researches have evaluated the EAHE performance in different locations. These works have demonstrated that the EAHE can provide excellent ther- mal comfort and indoor air quality, with low energy consumption. Despite their general good performance, EAHE efficiency depends on local atmospheric conditions and soil characteristics. For this reason, it is important to evaluate the EAHE considering local features [8,13–15,17]. Moreover, there is a lack of this kind of research in tropical cli- mates, where the amplitude of the soil temperature wave is smaller. As a consequence, in tropical regions the attenuation of the temperature also occurs in the most superficial layers [18,19]. Therefore, the purpose of this study is to evaluate the performance of an EAHE in a Colombian tropical weather in terms of ambient air-cooling capacity. This is done by means of the modelling of the system in TRNSYS. Furthermore, the present study focuses on the simulation of an EAHE system designed to improve thermal comfort in a laboratory room. The space is also simulated using TRNSYS. The laboratory is located in the Mechanical Engineering School building at Universidad Industrial de Santander (UIS), Bucaramanga, Colombia. While experimental validation is required to ensure the effectiveness of possible enhancements, the use of simulation tools allows a better understanding of the system and its performance under different operation conditions. Nowadays, detailed simulation models are becoming more important in the design phase. TRNSYS software is commonly used to study this kind of system. Libraries of this numerical tool have been verified in various studies. Moreover, it has demonstrated to be a very useful tool for analyzing and optimizing the performance of EAHE in different types of constructions, including offices, hospitals and residential buildings [20–25]. This document starts by describing the local weather, soil characteristics and building construction details considered for the EAHE design and evaluation. Afterwards, the process carried out for the parametric analysis, thermal loads estimate, coefficient of performance evaluation and financial analysis is explained in the methodology section. Then, results and discussion are presented to show the relevant findings of this work. Finally, the conclusions are outlined. 2. Description of Study Case 2.1. Local Weather Conditions and Soil Characteristics Weather conditions are measured by a Vantage Pro2 weather station installed at the Mechanical Engineering School building in the Universidad Industrial de Santander (UIS), 0 00  0 00 Bucaramanga, Colombia (7 8 23.604 N; 73 7 15.204 W). For the location, there are two Buildings 2021, 11, x FOR PEER REVIEW 3 of 18 2. Description of Study Case Buildings 2021, 11, 219 3 of 18 2.1. Local Weather Conditions and Soil Characteristics Weather conditions are measured by a Vantage Pro2 weather station installed at the Mechanical Engineering School building in the Universidad Industrial de Santander (UIS), Bucaramanga, Colombia (7°8′23.604″ N; 73°7′15.204″ W). For the location, there are rainy seasons, from March to May and September to November. Moreover, there are two two rainy seasons, from March to May and September to November. Moreover, there are dry seasons, from December to February and June to August. The annual air temperature two dry seasons, from December to February and June to August. The annual air temper- was atmeasur ure was mea ed during sured duri 2017; ng 2017 it varied ; it vafried from rom 20.9 20 C .9 °C to 31.3 to 31.3 C.°C. The The mean mean annual annualair air relative relative humidity was 80%, with a minimum of 49%. Figure 1 shows an example of the humidity was 80%, with a minimum of 49%. Figure 1 shows an example of the measured measured data corresponding to the second week of January, right in the middle of the data corresponding to the second week of January, right in the middle of the dry season dry season from December to February. Furthermore, the mean annual solar radiation 2 from December to February. Furthermore, the mean annual solar radiation was 700 W/m . was 700 W/m . The rainfall varied from 1 mm to 3 mm per day during rainy and dry The rainfall varied from 1 mm to 3 mm per day during rainy and dry seasons, respectively. seasons, respectively. 18 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time [min] Ambient temperature Relative humidity Figure 1. Example of measured relative humidity and air temperature for a given week. Figure 1. Example of measured relative humidity and air temperature for a given week. Buildings 2021, 11, x FOR PEER REVIEW 4 of 18 Because soil characteristics and its temperature variation are some of the main con- Because soil characteristics and its temperature variation are some of the main concerns cerns in EAHEs, an electronic tool is designed to measure the temperature and moisture in EAHEs, an electronic tool is designed to measure the temperature and moisture content content of soil for the location (see Figure 2). It has seven sensors for measuring under- of soil for the location (see Figure 2). It has seven sensors for measuring underground about 28% VWC. The fitting is performed for each sensor with R greater than 99% and it ground soil temperatures and moisture. The sensors are located 0.5 m from each other soil temperatures and moisture. The sensors are located 0.5 m from each other between is represented by this equation: between 0 and 3 m depth. The device is situated in the place where the EAHE will be 𝐶 339,314,287 315,950,014 ℎ 0.013089055 ℎ 0 and locat3 ed (7 m °0 depth. 8′21.6″ N; The 73device °07′12.8″is W) situated . All tempin erat the ure s place ensors wher are ce alithe bratEAHE ed againwill st a Cbe ole located 0.0000282115481 ℎ 3,28761590 10 ℎ 0 00  0 00 (1) Parmer Polystat Standard 1-C6 bath, which has an accuracy of ±0.01 °C according to its (7 08 21.6 N; 73 07 12.8 W). All temperature sensors are calibrated against a Cole Parmer 195,409,715 10 ℎ 460,914,550 10 ℎ calibration certificate. Each sensor is tested 20 times for six temperatures: 15, 17, 19, 21, 23 Polystat Standard 1-C6 bath, which has an accuracy of0.01 C according to its calibration where h means data read by the DAQ, i.e., an integer number from 0 to 1024 due to the and 25 °C. Observed accuracy is below 1%. certificate. Each sensor is tested 20 times for six temperatures: 15, 17, 19, 21, 23 and 25 C. DAQ number of bits. The test performance after calibration for each sensor is shown in Furthermore, to calibrate the volumetric water content (VWC) sensors, eighty sam- Observed accuracy is below 1%. Appendix A. ples of ground are taken from the ground. This ground is dried for 24 hours in an oven at 120 °C. Then, each sample is prepared with a VWC from 1% to 40% with a step of 1%. Each sensor is tested three times for each VWC value. All sensors show a saturation point at Figure 2. Temperature and humidity measurement device. Figure 2. Temperature and humidity measurement device. Temperature and soil moisture content data are measured with an interval of 15 minutes during the most representative weeks for rainy and dry seasons, respectively. Additionally, it is known that soil temperature is influenced by the physical properties of the ground, i.e., porosity, permeability and texture. Furthermore, each soil type has a dif- ferent thermal conductivity depending on its VWC. Sand with high VWC has a high ther- mal conductivity. Nonetheless, soils that have a high content of clay or organic materials like shale or coal have low thermal conductivity. For this reason, a sample for each depth is taken and a laboratory texture test is performed (see Figure 3). Figure 3. Samples taken for a laboratory texture test. 2.2. Building Details Generally, the main source of energy consumption in a building is the cooling de- mand in a hot climate. This principally depends on the construction materials, the enve- lope and the glazed surfaces. Other significant factors that influence the building loads Temperature [°C] Relative humidity [%] 𝑉𝑊 Buildings 2021, 11, x FOR PEER REVIEW 4 of 18 about 28% VWC. The fitting is performed for each sensor with R greater than 99% and it is represented by this equation: 𝐶 339,314,287 315,950,014 ℎ 0.013089055 ℎ 0.0000282115481 ℎ 3,28761590 10 ℎ (1) 195,409,715 10 ℎ 460,914,550 10 ℎ where h means data read by the DAQ, i.e., an integer number from 0 to 1024 due to the Buildings 2021, 11, 219 4 of 18 DAQ number of bits. The test performance after calibration for each sensor is shown in Appendix A. Furthermore, to calibrate the volumetric water content (VWC) sensors, eighty samples of ground are taken from the ground. This ground is dried for 24 hours in an oven at 120 C. Then, each sample is prepared with a VWC from 1% to 40% with a step of 1%. Each sensor is tested three times for each VWC value. All sensors show a saturation point at about 28% VWC. The fitting is performed for each sensor with R greater than 99% and it is represented by this equation: VW C = 339, 314, 287 315, 950, 014 h + 0.013089055 h 3 8 4 0.0000282115481 h + 328, 761, 590 10  h (1) 11 5 15 6 195, 409, 715 10  h + 460, 914, 550 10  h where h means data read by the DAQ, i.e., an integer number from 0 to 1024 due to the DAQ Figure 2. number Temperature and humidity measurement devi of bits. The test performance after calibration ce. for each sensor is shown in Appendix A. Temperature and soil moisture content data are measured with an interval of 15 Temperature and soil moisture content data are measured with an interval of 15 min minutes during the most representative weeks for rainy and dry seasons, respectively. during the most representative weeks for rainy and dry seasons, respectively. Additionally, Addit it is known ionallythat , it is soil known temperatur that soil t e is emp influenced erature is by influenced b the physical y th pr e physical pr operties of the operties of ground, the ground, i.e., porosity,i.permeability e., porosity, p and ermeabi textur lity e. Furthermor and texture. F e, each urthermore, e soil type a has ch soil type a different has thermal a dif- ferent thermal conduct conductivity depending ivity on depending its VWC. Sand on its with VW high C. Sand with high VWC has a high VWC h thermal as conductivity a high ther-. ma Nonetheless, l conductivi soils ty. Nonethel that have ess, soil a highs tha content t have a of clay high content of or organic materials clay or orga like ni shale c maor teria coal ls like have sh low ale or thermal coal have conductivity low thermal . For cond this uct reason, ivity. For a sample this rea for son, each a sample depth fo isr e taken ach dept and h a laboratory texture test is performed (see Figure 3). is taken and a laboratory texture test is performed (see Figure 3). Figure 3. Samples taken for a laboratory texture test. Figure 3. Samples taken for a laboratory texture test. 2 2.2. .2. Build Building ing Details Details General Generally ly,, t the he main main sour sour ce ce of of ener energygy c consumption onsumption i in an building a building is the is t cooling he cool demand ing de- in a hot climate. This principally depends on the construction materials, the envelope mand in a hot climate. This principally depends on the construction materials, the enve- lope and the and t glazed he glasurfaces. zed surfac Other es. Otsignificant her significant factors factthat ors tinfluence hat influence t the building he buildiloads ng loads are the occupancy and the internal sources of heat such as lights or electrical appliances. Because of the space available to install the EAHE, only one space inside the building is considered: The Design Laboratory in the Mechanical Engineering School building at Universidad Industrial de Santander (UIS). For this room, all the heat sources and architectural characteristics are taken into account (see Figures 4 and 5). 𝑉𝑊 Buildings 2021, 11, x FOR PEER REVIEW 5 of 18 Buildings 2021, 11, x FOR PEER REVIEW 5 of 18 are the occupancy and the internal sources of heat such as lights or electrical appliances. are the occupancy and the internal sources of heat such as lights or electrical appliances. Because of the space available to install the EAHE, only one space inside the building is Because of the space available to install the EAHE, only one space inside the building is considered: The Design Laboratory in the Mechanical Engineering School building at Uni- considered: The Design Laboratory in the Mechanical Engineering School building at Uni- Buildings 2021, 11, 219 5 of 18 versidad Industrial de Santander (UIS). For this room, all the heat sources and architec- versidad Industrial de Santander (UIS). For this room, all the heat sources and architec- tural characteristics are taken into account (see Figures 4 and 5). tural characteristics are taken into account (see Figures 4 and 5). Figure 4. Building front view, Design Laboratory highlighted. Figure 4. Building front view, Design Laboratory highlighted. Figure 4. Building front view, Design Laboratory highlighted. Figure 5. Building top view, Design Laboratory highlighted. Figure 5. Building top view, Design Laboratory highlighted. Figure 5. Building top view, Design Laboratory highlighted. Geometrical modeling was done on TRNSYS 3d plug-in for google SketchUp to draw Geometrical modeling was done on TRNSYS 3d plug-in for google SketchUp to draw Geometrical modeling was done on TRNSYS 3d plug-in for google SketchUp to draw the Design Laboratory with the following dimensions: 7.82  21.55  3.075 m . The model the Design Laboratory with the following dimensions: 7.82 × 21.55 × 3.075 m . The model the Design Laboratory with the following dimensions: 7.82 × 21.55 × 3.075 m . The model includes the geometry in conjunction with architectural parameters such as windows, includes the geometry in conjunction with architectural parameters such as windows, includes the geometry in conjunction with architectural parameters such as windows, shelters, doors and walls. Then, the energetic model was generated on TRNSYS Building shelters, doors and walls. Then, the energetic model was generated on TRNSYS Building shelters, doors and walls. Then, the energetic model was generated on TRNSYS Building frontend (TRNBuild). The building model was imported into TRNBuild to calculate the frontend (TRNBuild). The building model was imported into TRNBuild to calculate the frontend (TRNBuild). The building model was imported into TRNBuild to calculate the cooling load (see Figure 6). cooling load (see Figure 6). cooling load (see Figure 6). Buildings 2021, 11, x FOR PEER REVIEW 6 of 18 For this purpose, the masonry, structural elements, envelope materials and architec- Buildings 2021, 11, 219 6 of 18 tural details were considered. The thermo-physical properties of each one of the consid- ered components were included. Figure 6. Building model in TRNSYS. Figure 6. Building model in TRNSYS. The Design Laboratory has occupant capacity for 26 people. The scheduling occu- For this purpose, the masonry, structural elements, envelope materials and architec- pancy, lights and appliances operation of the Design Laboratory were estimated accord- tural details were considered. The thermo-physical properties of each one of the considered ing to ASHRAE Fundamentals [26] and ASHRAE standard 90.1 [27]. Furthermore, infil- components were included. trations and ventilation requirements were set according to the same source. Table 1 pre- The Design Laboratory has occupant capacity for 26 people. The scheduling occupancy, sents a summary of the main parameters used in the energetic modelling for cooling load lights and appliances operation of the Design Laboratory were estimated according to ASHRAE calculation. Fundamentals Furthermore, [26build ] anding ASHRAE charactst eri andar stics d rega 90.1 rdi [27 ng constructi ]. Furthermor on ma e, infiltrations terials and and envelope a ventilation re summa requir riz ements ed in Appendix B. were set according to the same source. Table 1 presents a summary of the main parameters used in the energetic modelling for cooling load Table 1. Space parameters for cooling load calculation. calculation. Furthermore, building characteristics regarding construction materials and envelope are summarized in Appendix B. Parameter Value The second floor of Mechanical Engineering Table 1. Space parameters for cooling load calculation. Location of building School Parameter Value Application Design laboratory Building area 169 m Location of building The second floor of Mechanical Engineering School Application Design laboratory T° Setpoint 24 °C Building area 169 m Number of people 26 T Setpoint 24 C Light 3.69 W/m Number of people 26 Equipment 1080.58 W Light 3.69 W/m Infiltration 0.2 m /s Equipment 1080.58 W Infiltration 0.2 m /s HR (low-level limit) 50% HR (low-level limit) 50% Moreover, to verify the results observed from the simulation carried out in TRNSYS, air temperature was measured in the modelled space. An illustrative comparison between simulated and experimental data is presented in Figure 7. This time segment was chosen Buildings 2021, 11, x FOR PEER REVIEW 7 of 18 Buildings 2021, 11, 219 7 of 18 Moreover, to verify the results observed from the simulation carried out in TRNSYS, air temperature was measured in the modelled space. An illustrative comparison between simulated and experimental data is presented in Figure 7. This time segment was chosen to illustrate the comparison between experimental and predicted temperature. As can be to illustrate the comparison between experimental and predicted temperature. As can be seen, a difference of about 8% was achieved. seen, a difference of about 8% was achieved. 563 573 583 593 603 613 623 633 643 653 Time [h] Measured temperature Simulated temperature Figure 7. Illustrative comparison between experimental (green solid line) and predicted air temperature inside the studied Figure 7. Illustrative comparison between experimental (green solid line) and predicted air temperature inside the studied space (black solid line). space (black solid line). 3. Methodology 3. Methodology 3.1. EAHE Cooling Potential: Parametric Analysis 3.1. EAHE Cooling Potential: Parametric Analysis A TRNSYS model was set up to simulate the heat transfer process. The EAHE sub- A TRNSYS model was set up to simulate the heat transfer process. The EAHE routine, Type 997, simulated the thermal interaction between the buried heat exchanger subroutine, Type 997, simulated the thermal interaction between the buried heat exchanger and the ground (see Figure 8). Boundary conditions were established according to the and the ground (see Figure 8). Boundary conditions were established according to the measured soil properties. Furthermore, operating parameters were set to obtain the outlet measured soil properties. Furthermore, operating parameters were set to obtain the outlet air temperature [18]. Additionally, weather data from Vantage Pro2 station were acquired air temperature [18]. Additionally, weather data from Vantage Pro2 station were acquired through Type 99. Then, this Type provided them to Type 997 (see Figure 8). through Type 99. Then, this Type provided them to Type 997 (see Figure 8). A full description of the model is reported in TESS documentation. The main type, A full description of the model is reported in TESS documentation. The main type, Type 997, was fine-tuned to capture the physical, geographical and climatic characteristics Type 997, was fine-tuned to capture the physical, geographical and climatic characteristics of the site. The simulations were performed with a PVC pipe (thermal conductivity equal of the site. The simulations were performed with a PVC pipe (thermal conductivity equal to 0.185 W/m·K) buried at 1.5 m from the surface to evaluate the performance of EAHE. to 0.185 W/mK) buried at 1.5 m from the surface to evaluate the performance of EAHE. The simulation tests were conducted using a pipe length of 60 m, a diameter of 8 in (0.2 The simulation tests were conducted using a pipe length of 60 m, a diameter of 8 in (0.2 m) m) and a mass flow of 1240 kg/h as reference. However, one of these three design param- and a mass flow of 1240 kg/h as reference. However, one of these three design parameters eters was varied for each of the simulation tests while the others remained constant. The was varied for each of the simulation tests while the others remained constant. The design design parameters examined were mass flow rates of 620, 1240 and 1860 kg/s. Pipe diam- parameters examined were mass flow rates of 620, 1240 and 1860 kg/s. Pipe diameters eters were 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m). Pipe lengths were 90, 150, and 200 m, were 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m). Pipe lengths were 90, 150, and 200 m, varying each parameter independently. varying each parameter independently. The influence of grid parameters, such as the node size, node growth multiplier or The influence of grid parameters, such as the node size, node growth multiplier or the the relationship factor between the cylindrical node and square node on the EAHE outlet relationship factor between the cylindrical node and square node on the EAHE outlet air air temperature, was verified. temperature, was verified. Temperature [°C] Buildings 2021, 11, x FOR PEER REVIEW 8 of 18 The EAHE's operating mode was in continuous mode. Therefore, Type 112 also op- erated in continuous mode. It has an ON/OFF control which is described as follows: If the Buildings 2021, 11, 219 8 of 18 input control signal to the fan is less than 0.5, then the fan is OFF. On the other hand, if it is equal to or greater than 0.5, the fan is ON. Figure 8. TRNSYS soil performance model. Figure 8. TRNSYS soil performance model. The EAHE’s operating mode was in continuous mode. Therefore, Type 112 also 3.2. Coefficient of Performance operated in continuous mode. It has an ON/OFF control which is described as follows: If the input control signal to the fan is less than 0.5, then the fan is OFF. On the other hand, if The coefficient of performance (COP) of the system was evaluated using Equation (2) it is equal to or greater than 0.5, the fan is ON. within the TRANSYS environment. In this equation, COP is the coefficient of performance, 𝑚 is the air mass flow rate through the pipe, 𝐶 is the pipe discharge coefficient and 𝑐 3.2. Coefficient of Performance is the air specific heat capacity. Furthermore, 𝑄 is the work done by the blower and The coefficient of performance (COP) of the system was evaluated using Equation (2) 𝑇 and 𝑇 are the air temperatures at the EAHE inlet and outlet, respectively. The within the TRANSYS environment. In this equation, COP is the coefficient of performance, COP indicates the energy efficiency of the system [28]. The EAHE cooling potential is m is the air mass flow rate through the pipe, C is the pipe discharge coefficient and c is d p evaluated based on the analysis of the daily COP. The cooling load is calculated by TRN- the air specific heat capacity. Furthermore, Q is the work done by the blower and T and i inlet SYS subroutine, Type 56. This is carried out as reported in Multizone Building modeling T are the air temperatures at the EAHE inlet and outlet, respectively. The COP indicates exit [29]. the energy efficiency of the system [28]. The EAHE cooling potential is evaluated based 𝑚 𝐶 𝑐 𝑇 𝑇 on the analysis of the daily COP. The cooling load is calculated by TRNSYS subroutine, (2) 𝐶𝑂𝑃 Type 56. This is carried out as reported in Multizone Building modeling [29]. mC c (T T ) d inlet exit COP = (2) 3.3. Financial Analysis There are several economic exploration techniques. Two of them were used in this 3.3. Financial Analysis study: Net present value (NPV) and internal rate of return (IRR) [30]. These analyses con- sider the initial capital, operations, savings and associated costs. The initial capital or in- There are several economic exploration techniques. Two of them were used in this vest study: ment Net incl pr udes esent : He value at trans (NPV) fer subs andys internal tem, air d rate istribution of return su (IRR) bsystem, [30]. centrif Theseuanalyses gal fans, consider the initial capital, operations, savings and associated costs. The initial capital or investment includes: Heat transfer subsystem, air distribution subsystem, centrifugal fans, ductworks and borehole excavation. The operation costs involve electricity consumption and maintenance of the EAHE. The savings include the air conditioning system (AC) and its inherent electricity consumption. The EAHE and AC investment cost are obtained by a Buildings 2021, 11, x FOR PEER REVIEW 9 of 18 ductworks and borehole excavation. The operation costs involve electricity consumption and maintenance of the EAHE. The savings include the air conditioning system (AC) and its inherent electricity consumption. The EAHE and AC investment cost are obtained by a detailed unit price analysis. The equipment, material and labor prices are provided by the local industries. Furthermore, an economic analysis was completed based on some necessary assump- tions: Inflation rates are ignored, as well as the annual increase of cost by electricity con- Buildings 2021, 11, 219 9 of 18 sumption. Moreover, a discount rate of 8% which applies in Colombia [31] and the lifespan of the EAHE equal to 25 years were evaluated. 4. Results and Discussion detailed unit price analysis. The equipment, material and labor prices are provided by the 4.1. Soil Temperature and Volume Water Content local industries. Results in Figure 9 show that the lowest temperature of 19.6 °C was obtained just Furthermore, an economic analysis was completed based on some necessary assump- below ground surface (0 m depth). However, it is strongly affected by the ambient tem- tions: Inflation rates are ignored, as well as the annual increase of cost by electricity perature and humidity [32]. The heat exchange of the ground surface with the ambient is consumption. Moreover, a discount rate of 8% which applies in Colombia [31] and the due to convection with the air and radiation with the sky. It also receives solar irradiation lifespan of the EAHE equal to 25 years were evaluated. during daytime. Deeper ground layers experience fewer variations in temperature and the changes 4. Results and Discussion exhibit a larger time delay than those of shallower soils. This is caused by the high thermal 4.1. Soil Temperature and Volume Water Content inertia of the soil under the surface of the earth [33]. While the temperature increases with Results in Figure 9 show that the lowest temperature of 19.6 C was obtained just below depth, its fluctuation between day and night reduces. Deeper samples can be considered ground surface (0 m depth). However, it is strongly affected by the ambient temperature as a heated basin [19]. and humidity [32]. The heat exchange of the ground surface with the ambient is due Therefore, for the present study, the EAHE would be buried at 1.5 m depth. Here, to convection with the air and radiation with the sky. It also receives solar irradiation temperature and its amplitude remained almost constant at 22.3 °C during the testing pe- during daytime. riod (see Figure 9). 0 1000 2000 3000 4000 5000 6000 7000 8000 Time [min] Ambient temperature 0 m 0.5 1 m 1.5 m 2 m 2.5 m 3 m Figure 9. Soil temperature measured at different depths. Figure 9. Soil temperature measured at different depths. Regarding the VWC, measurements show that near the surface (0 and 0.5 m) it is Deeper ground layers experience fewer variations in temperature and the changes affected by atmospheric conditions. However, when depth increases, the VWC is more exhibit a larger time delay than those of shallower soils. This is caused by the high thermal constant. On average, it is 25% at 1.5 m depth (see Figure 10). inertia of the soil under the surface of the earth [33]. While the temperature increases with depth, its fluctuation between day and night reduces. Deeper samples can be considered as a heated basin [19]. Therefore, for the present study, the EAHE would be buried at 1.5 m depth. Here, temperature and its amplitude remained almost constant at 22.3 C during the testing period (see Figure 9). Regarding the VWC, measurements show that near the surface (0 and 0.5 m) it is affected by atmospheric conditions. However, when depth increases, the VWC is more constant. On average, it is 25% at 1.5 m depth (see Figure 10). Temperature [°C] Buildings 2021, 11, 219 10 of 18 Buildings 2021, 11, x FOR PEER REVIEW 10 of 18 0 10002000300040005000600070008000 Time [min] 0 m 0.5 m 1 m 1.5 m 2 m 2.5 3 m Figure 10. Measured VWC at different depths. Figure 10. Measured VWC at different depths. Results indicate that the soil in the EAHE location is composed of 57.6% sand, 12.4% Results indicate that the soil in the EAHE location is composed of 57.6% sand, limestone and 30% clay. Soil thermophysical properties were estimated according to tex- 12.4% limestone and 30% clay. Soil thermophysical properties were estimated accord- tural properties as follows: Thermal conductivity 2.42 W/m·K, volumetric heat capacity ing to textural properties as follows: Thermal conductivity 2.42 W/mK, volumetric 3 −6 2 2.9 MJ/m ·K, specific heat capacity 1.45 kJ/kg·K and thermal diffusivity 0.83 × 10 m /s heat capacity 2.9 MJ/m K, specific heat capacity 1.45 kJ/kgK and thermal diffusivity [34–36]. 6 2 0.83  10 m /s [34–36]. 4.2. EAHE Cooling Potential: Parametric Analysis 4.2. EAHE Cooling Potential: Parametric Analysis Different simulations are conducted using fixed design parameters while one of them Different simulations are conducted using fixed design parameters while one of them is varied for each of the simulation tests. The first simulation evaluates the influence of is varied for each of the simulation tests. The first simulation evaluates the influence of the the mass flow rates of 620, 1240 and 1860 kg/h. These values correspond approximately to mass flow rates of 620, 1240 and 1860 kg/h. These values correspond approximately to 1, 2 and 3 ACH (Air Changes per Hour), respectively, for a design laboratory of 510 m 1, 2 and 3 ACH (Air Changes per Hour), respectively, for a design laboratory of 510 m according to ASHRAE-62.1 [30]. according to ASHRAE-62.1 [30]. As can be seen in Figure 11, the simulated EAHE air outlet temperature followed the As can be seen in Figure 11, the simulated EAHE air outlet temperature followed the variations of the ambient outdoor air temperatures with a minor amplitude. The ambient variations of the ambient outdoor air temperatures with a minor amplitude. The ambient air temperature range was 19.1 °C to 27.8 °C in January, while the simulated outlet air air temperature range was 19.1 C to 27.8 C in January, while the simulated outlet air temperature was 22.1 °C to 23.7 °C for 620 kg/h, 21.5 °C to 24.3 °C for 1240 kg/h and 21.1 temperature was 22.1 C to 23.7 C for 620 kg/h, 21.5 C to 24.3 C for 1240 kg/h and °C to 24.8 °C for 1860 kg/h. 21.1 C to 24.8 C for 1860 kg/h. Furthermore, the outlet air temperature increased when the airflow rate inside the Furthermore, the outlet air temperature increased when the airflow rate inside the EAHE increased. Similar results were obtained by other authors [10,34]. This behavior can EAHE increased. Similar results were obtained by other authors [10,34]. This behavior can be explained because a higher mass flow with the same diameter implies a higher velocity be explained because a higher mass flow with the same diameter implies a higher velocity of the air inside the tube. Consequently, the overall heat exchange was reduced owing to of the air inside the tube. Consequently, the overall heat exchange was reduced owing to the minor residence time of the air inside the pipe [14]. the minor residence time of the air inside the pipe [14]. Once the influence of the mass flow was stated, the performance of EAHE was eval- Once the influence of the mass flow was stated, the performance of EAHE was uated with a constant tube diameter of 8 in (0.2 m) and operated with an air mass flow evaluated with a constant tube diameter of 8 in (0.2 m) and operated with an air mass rate of 1240 kg/h for three different pipe lengths (90, 150 and 200 m). Again, the EAHE flow rate of 1240 kg/h for three different pipe lengths (90, 150 and 200 m). Again, the outlet air temperature varied according to the ambient temperature. However, while am- EAHE bient temperature fluctuate outlet air temperatur d between 19.5 °C e varied according and 27 °C, the EAHE to the ambient temperatur air outlet tem e.pera However ture , while ambient temperature fluctuated between 19.5 C and 27 C, the EAHE air outlet for 90 m pipe length varied from 21.6 °C to 24 °C. For 150 m pipe length, EAHE air outlet temperature for 90 m pipe length varied from 21.6 C to 24 C. For 150 m pipe length, EAHE air outlet temperature varied from 22 C to 23.3 C, and for 200 m pipe length, it varied from 22 C to 23 C (see Figure 12). VWC [%] Buildings 2021, 11, x FOR PEER REVIEW 11 of 18 Buildings 2021, 11, 219 11 of 18 Buildings 2021, 11, x FOR PEER REVIEW 11 of 18 temperature varied from 22 °C to 23.3 °C, and for 200 m pipe length, it varied from 22 °C to 23 °C (see Figure 12). temperature varied from 22 °C to 23.3 °C, and for 200 m pipe length, it varied from 22 °C to 23 °C (see Figure 12). 21 22 168 188 208 228 248 268 288 308 328 Time [h] 168 188 208 228 248 268 288 308 328 Ambient temperature 620 kg/h 1240 kg/h 1860 kg/h Time [h] Ambient temperature 620 kg/h 1240 kg/h 1860 kg/h Figure 11. EAHE outlet air temperature vs air mass flow rate. Figure 11. EAHE outlet air temperature vs air mass flow rate. Figure 11. EAHE outlet air temperature vs air mass flow rate. 168 188 208 228 248 268 288 308 328 168 188 208 228 248 268 288 308 328 Time [h] Time [h] Ambient temperature 90 m 150 m 200 m Ambient temperature 90 m 150 m 200 m Figure 12. Figure 12. EA EA HE outlet HE outlet air air te temperature vs pipe length. mperature vs pipe length. Figure 12. EAHE outlet air temperature vs pipe length. Thereby, it can be observed that an increase in the pipe length results in a decrease Thereby, it can be observed that an increase in the pipe length results in a decrease Thereby, it can be observed that an increase in the pipe length results in a decrease in the variations of outlet air temperatures. Thus, peaks of the air temperature during the in the variations of outlet air temperatures. Thus, peaks of the air temperature during the in the variations of outlet air temperatures. Thus, peaks of the air temperature during day are reduced. These results are similar to those reported by [10,12,34]. Finally, the in- day are reduced. These results are similar to those reported by [10,12,34]. Finally, the in- the day are reduced. These results are similar to those reported by [10,12,34]. Finally, the fluence of the tube diameter was evaluated, i.e., 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m) fluence of the tube diameter was evaluated, i.e., 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m) influence of the tube diameter was evaluated, i.e., 4 in (0.1 m), 8 in (0.2 m) and 12 in (0.3 m) are considered. are con are consider sidere ed. d. For these simulations, a pipe length of 60 m, buried at 1.5 m depth with an air mass flow rate of 1240 kg/s, was imposed. As Figure 13 shows, the outlet air temperature for Temperature [°C] Temperature [°C] Temperature [°C] Temperature [°C] Buildings 2021, 11, x FOR PEER REVIEW 12 of 18 Buildings 2021, 11, 219 12 of 18 For these simulations, a pipe length of 60 m, buried at 1.5 m depth with an air mass flow rate of 1240 kg/s, was imposed. As Figure 13 shows, the outlet air temperature for a 4 in (0.1 m) pipe diameter varied from 21.1 °C during the night to 24.8 °C in the day, whereas it changed from 21.5 °C to 24.3 °C for 8 in (0.2 m), and from 21.6 °C to 24.3 °C for a 4 in (0.1 m) pipe diameter varied from 21.1 C during the night to 24.8 C in the day, 12 in (0.3 m). whereas it changed from 21.5 C to 24.3 C for 8 in (0.2 m), and from 21.6 C to 24.3 C for Therefore, an increase in the pipe diameter from 8 in to 12 in had a rather limited 12 in (0.3 m). influence on the outlet air temperature fluctuations. 168 188 208 228 248 268 288 308 328 Time [h] Ambient temperature 4 in 8 in 12 in Figure 13. EAHE outlet air temperature vs pipe diameter. Figure 13. EAHE outlet air temperature vs pipe diameter. In summary, it is considered that the best combination of EAHE design parameters Therefore, an increase in the pipe diameter from 8 in to 12 in had a rather limited to get an outlet air temperature varying between 21.1 °C and 24.3 °C is: 8 in (0.2 m) diam- influence on the outlet air temperature fluctuations. eter, 150 m length and air mass flow of 1240 kg/h. In summary, it is considered that the best combination of EAHE design parameters to However, for the Design Laboratory, an air mass flow of 1240 kg/h would allow only get an outlet air temperature varying between 21.1 C and 24.3 C is: 8 in (0.2 m) diameter, 2 air changes per hour (ACH), but according to ASHRAE [30], 2.6 ACH are required. 150 m length and air mass flow of 1240 kg/h. Moreover, if the air mass flow increases so does outlet air temperature. Alternatively, the However, for the Design Laboratory, an air mass flow of 1240 kg/h would allow air mass flow and pipe length can be increased simultaneously without affecting the only 2 air changes per hour (ACH), but according to ASHRAE [30], 2.6 ACH are required. EAHE outlet air temperature, while ACH is improved. Moreover, if the air mass flow increases so does outlet air temperature. Alternatively, the For this reason, the EAHE is designed as a network of buried pipes that has seven air mass flow and pipe length can be increased simultaneously without affecting the EAHE tubes of 36 m length (a fan for each one), a diameter of 8 in (0.2 m) and an air mass flow outlet air temperature, while ACH is improved. rate of 1560 kg/h in continuous operation mode. The simulated results for the last 1000 For this reason, the EAHE is designed as a network of buried pipes that has seven hours of one year are presented in Figure 14. As the figure shows, the continuous opera- tubes of 36 m length (a fan for each one), a diameter of 8 in (0.2 m) and an air mass flow rate tion strategy takes advantage of night cooling. of 1560 kg/h in continuous operation mode. The simulated results for the last 1000 h of During night hours, ambient air temperature goes down and cools the soil, which one year are presented in Figure 14. As the figure shows, the continuous operation strategy allows it to recover its cooling capacity [2,37]. The thermal performance analysis reveals takes advantage of night cooling. that the outlet air mean temperature is 22.5 °C and varies between 20.9 °C and 24.1 °C (see During night hours, ambient air temperature goes down and cools the soil, which Figure 14). allows it to recover its cooling capacity [2,37]. The thermal performance analysis reveals Hot air peaks are suppressed in this way. The results obtained in this work agree that the outlet air mean temperature is 22.5 C and varies between 20.9 C and 24.1 C (see with the study reported by Díaz-Hernández [37] for an EAHE under similar weather con- Figure 14). ditions. Hot air peaks are suppressed in this way. The results obtained in this work agree with the study reported by Díaz-Hernández [37] for an EAHE under similar weather conditions. Temperature [°C] Buildings 2021, 11, 219 13 of 18 Buildings 2021, 11, x FOR PEER REVIEW 13 of 18 7760 7860 7960 8060 8160 8260 8360 8460 8560 8660 8760 Time [h] Ambient temperature EAHE outlet temperature Figure 14. Ambient temperature vs EAHE outlet air temperature. Figure 14. Ambient temperature vs EAHE outlet air temperature. 4.3. Coefficient of Performance (COP) 4.3. Coefficient of Performance (COP) The COP was calculated using TRNSYS software. The model incorporated the cli- The COP was calculated using TRNSYS software. The model incorporated the climatic matic data (Type 99), the behavior of soil (Type 997), operating conditions (Types 56 and data (Type 99), the behavior of soil (Type 997), operating conditions (Types 56 and 997) and 997) and building details (Type 56). Variations in COP depended on soil temperature. building details (Type 56). Variations in COP depended on soil temperature. When the When the ambient air temperature as higher than the soil temperature, the air released ambient air temperature as higher than the soil temperature, the air released heat to the heat to the soil. In this manner, the EAHE air cooled down space. This cooling effect took soil. In this manner, the EAHE air cooled down space. This cooling effect took place from place from 9 am to 8 pm. 9 am to 8 pm. On the other hand, at night, from 8 pm to 9 am, the soil temperature was higher than On the other hand, at night, from 8 pm to 9 am, the soil temperature was higher than the ambient air temperature and heat flowed from soil to air. The weekly maximum cool- the ambient air temperature and heat flowed from soil to air. The weekly maximum cooling ing COP was around 160 and the daily variations of COP were found in a range between COP was around 160 and the daily variations of COP were found in a range between 0.91 0.91 and 160. and 160. The high value of the COP and the parallel distribution of the EAHE allow energy The high value of the COP and the parallel distribution of the EAHE allow energy savings compared with a standard HVAC system. Low power consumption fans (0.12 kW savings compared with a standard HVAC system. Low power consumption fans (0.12 kW for the motor) installed in each pipe could be used to vary the operation mode for future for the motor) installed in each pipe could be used to vary the operation mode for future research on wet soil conditions [33]. When a low cooling potential is required, some fans research on wet soil conditions [33]. When a low cooling potential is required, some could be turned off to increase the COP. The magnitude of calculated COP values is sim- fans could be turned off to increase the COP. The magnitude of calculated COP values is ilar to typical values found in the literature and reflect the high efficiency of the heat ex- similar to typical values found in the literature and reflect the high efficiency of the heat changer [38]. exchanger [38]. 4.4. Financial Analysis 4.4. Financial Analysis Figure 15 illustrates the instantaneous energy load calculated. The heat was transmit- Figure 15 illustrates the instantaneous energy load calculated. The heat was transmit- ted in or out of the laboratory mainly through external walls, windows, doors and ceiling. ted in or out of the laboratory mainly through external walls, windows, doors and ceiling. Moreover, some heat was transmitted through the floor. Total heat load is the summation Moreover, some heat was transmitted through the floor. Total heat load is the summation of the sensible heat with the total latent heat. of the sensible heat with the total latent heat. The energetic model allows the calculation of air conditioning capacity. According to the analysis, 95% of occupancy hours can be satisfied with a cooling capacity of 10 refrig- eration tons RT (35.168 kW) which satisfies the requirements of 300 unmet load hours of the ASHRAE standard 90.1 [27]. Temperature [°C] Buildings 2021, 11, 219 14 of 18 Buildings 2021, 11, x FOR PEER REVIEW 14 of 18 0 1000 2000 3000 4000 5000 6000 7000 8000 Time [h] Figure 15. Total heat transfer obtained from TRANSYS model. Figure 15. Total heat transfer obtained from TRANSYS model. In this section, the HVAC system costs associated with maintenance, energy con- The energetic model allows the calculation of air conditioning capacity. According to sumption and the initial investment are compared to the costs of the EAHE construction the analysis, 95% of occupancy hours can be satisfied with a cooling capacity of 10 refriger- and operation. The EAHE and HVAC investment were calculated using a detailed unit ation tons RT (35.168 kW) which satisfies the requirements of 300 unmet load hours of the price analysis during the year 2019. Furthermore, equipment, material and labor prices ASHRAE standard 90.1 [27]. were provide In this section, d by the local industr the HVAC system y. Table 2 summarizes the cost costs associated with maintenance, of the EAHE energy and the consump- HVAC systems. The maintenance and energy consumption costs are evaluated annually. tion and the initial investment are compared to the costs of the EAHE construction and However, the investment of the project is only considered for year zero. operation. The EAHE and HVAC investment were calculated using a detailed unit price analysis during the year 2019. Furthermore, equipment, material and labor prices were Table 2. EAHE vs HVAC costs. provided by the local industry. Table 2 summarizes the cost of the EAHE and the HVAC systems. The maintenance and energy consumption costs are evaluated annually. However, Cash Flow EAHE HVAC the investment of the project is only considered for year zero. Investment cost ($) US$ 23,825.40 US$ 6338.02 Maintenance cost ($) US$ 524.62 US$ 139.75 Table 2. EAHE vs. HVAC costs. Electricity consumption cost US$ 1111.92 US$ 5678.76 ($/annual) Cash Flow EAHE HVAC Investment cost ($) US$23,825.40 US$6338.02 The cash inflows are all costs of HVAC; they are incomes. The cash outflows are all Maintenance cost ($) US$524.62 US$139.75 costs of EA Electricity H consumption E; they are lo cost sses. The en ($/annual) ergy consumption of the HVAC represent US$1111.92 US$5678.76 s the larg- est portion of the total operational cost. Cash flow was calculated in TRNSYS and consid- ered: Cooling schedule setup point and occupancy schedule, cooling capacity of the air The cash inflows are all costs of HVAC; they are incomes. The cash outflows are conditioner, operation mode, frequency of use, the electricity tariff in Bucaramanga and all costs of EAHE; they are losses. The energy consumption of the HVAC represents the its weather conditions. largest portion of the total operational cost. Cash flow was calculated in TRNSYS and Moreover, the maintenance cost for the mechanical system and civil construction is considered: Cooling schedule setup point and occupancy schedule, cooling capacity of the about 3% of the total investment cost for each one [39]. These cash flows are added in each air conditioner, operation mode, frequency of use, the electricity tariff in Bucaramanga and year of the overall analysis period, resulting in an NPV calculated equal to US$ 25,662.3. its weather conditions. Then, applying the criteria of NPV > 0, a positive value means that this project is a rentable Moreover, the maintenance cost for the mechanical system and civil construction is way to invest the money. On the other hand, the internal rate of return is calculated as about 3% of the total investment cost for each one [39]. These cash flows are added in each 23%. The interest rate obtained is higher than bank interest rates, and payback time is year of the overall analysis period, resulting in an NPV calculated equal to US$25,662.3. Then, applying the criteria of NPV > 0, a positive value means that this project is a rentable Qt [kW] Buildings 2021, 11, 219 15 of 18 way to invest the money. On the other hand, the internal rate of return is calculated as 23%. The interest rate obtained is higher than bank interest rates, and payback time is archived in approximately six years, which, from a basic financial point of view, makes the project profitable. 5. Conclusions The technical and financial feasibility of an EAHE installed in tropical weather in Colombia to acclimatize a laboratory inside the Mechanical Engineering School (UIS) build- ing at Bucaramanga was analyzed. To do so, the EAHE and the laboratory were simulated using TRNSYS. Moreover, to provide inputs for the model, the temperature, the volume of the water content and thermophysical properties of the soil were experimentally obtained. Furthermore, local weather conditions were measured. Regarding technical aspects, an EAHE with a length of 252 m (7 pipes in parallel) and a pipe diameter of 8 in (0.2 m) produced the best results. Furthermore, an air mass flow rate of 1560 kg/h provided enough air exchanges per hour without reducing the EAHE performance while satisfying calculated thermal loads. With these dimensions and operating conditions, the EAHE delivered air at temperatures between 20.9 C and 24.1 C, which are within the established conditions of thermal comfort. For the calculated specifications, the project had an IRR of 23% and a payback period of six years. Therefore, the project is considered financially viable. The EAHE could also be implemented in tropical areas with similar atmospheric and soil conditions to increase the thermal inertia of the buildings and/or reduce thermal loads. For instance, cities like Medellin or Cali. It is left for future work to carry out a prototype, which allows corroborating the data obtained from simulations in the design stage of the EAHE. Likewise, it is recommended to carry out prolonged tests of thermal saturation of the ground. Author Contributions: Conceptualization, S.A.P.P. and J.E.J.I.; methodology, S.A.P.P.; software, S.A.P.P.; valida-tion, S.A.P.P.; formal analysis, S.A.P.P.; investigation, S.A.P.P.; resources, J.E.J.I.; data curation, S.A.P.P.; writing—origi-nal draft preparation, S.A.P.P.; writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ministry of Science, Technology, and Innovation (Ministerio de Ciencia, Tecnología e Innovación)—MINCIENCIAS (Project Contract No. 80740-798-2019). Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors wish to thank the Vice-Rectorate for Research and Extension (Vicer- rectoría de Investigación y Extensión) from the Universidad Industrial de Santander (Project 8600), and the Energy Mining Planning Unit (Unidad de Planeación Minero Energética—UPME). Conflicts of Interest: The authors declare no conflict of interest. Appendix A Table A1. VWC Measurements from each sensor once calibrated. Sample Sensor 1 Sensor 2 Sensor 3 Sensor 4 Sensor 5 Sensor 6 Sensor 7 %VWC 1.0 1.1 1.2 1.0 1.4 0.9 1.5 1.4 2.0 2.3 1.9 1.7 2.3 2.1 2.2 2.3 3.0 3.1 2.9 3.1 2.9 3.2 3.1 3.0 4.0 3.9 4.2 4.2 4.3 4.1 3.8 3.9 5.0 4.9 4.8 5.1 5.3 5.2 5.0 4.9 Buildings 2021, 11, 219 16 of 18 Table A1. Cont. Sample Sensor 1 Sensor 2 Sensor 3 Sensor 4 Sensor 5 Sensor 6 Sensor 7 %VWC 6.0 6.2 5.9 5.8 5.9 5.9 6.2 6.1 7.0 6.9 7.2 7.3 6.9 6.9 6.9 7.0 8.0 7.8 7.9 8.0 8.1 8.3 7.8 8.1 9.0 9.1 9.3 8.8 8.9 8.9 9.0 8.7 10.0 10.3 10.1 9.8 10.2 10.3 10.1 10.0 11.0 11.0 10.8 10.9 11.3 10.9 10.8 11.1 12.0 12.2 12.1 12.2 11.9 12.4 11.8 11.9 13.0 13.0 13.2 13.3 12.8 13.1 13.3 12.9 14.0 14.2 13.9 14.1 14.3 14.4 14.3 14.1 15.0 15.4 15.2 15.0 15.3 15.2 15.3 14.9 16.0 15.9 16.5 16.3 16.2 15.8 15.9 15.0 17.0 16.7 16.3 16.2 17.3 17.1 17.4 16.9 18.0 18.5 18.6 18.3 18.0 18.2 18.1 17.8 19.0 18.9 19.0 19.2 19.1 18.9 19.4 19.2 20.0 20.1 20.4 20.2 20.1 19.8 19.3 20.2 21.0 21.0 21.2 21.5 21.2 21.4 20.8 20.8 22.0 22.1 21.8 21.7 22.3 22.0 22.5 22.2 23.0 23.0 22.9 23.1 22.9 23.4 22.9 22.7 24.0 23.7 23.9 24.0 24.0 23.6 23.8 24.1 25.0 25.1 25.0 25.5 25.1 25.1 25.4 24.9 26.0 26.1 25.9 26.1 26.1 25.8 26.2 26.5 27.0 26.9 26.9 27.2 27.1 27.4 26.8 27.3 28.0 28.1 27.9 28.5 27.9 28.3 28.4 28.1 29.0 29.2 28.7 28.6 29.0 28.8 29.3 28.7 30.0 29.9 30.2 30.4 30.2 29.7 29.8 29.7 Appendix B Table A2. Building construction materials and envelope characteristics. Construction Elements Description Buffer plates: 3000 PSI concrete, rough finish and thickness 7 cm. Leveling mortar: Thickness 5 cm, concrete 2500 PSI with -inch aggregate, mesh 4 mm electro welded, crosslinked of 20  20 cm. Floors Polished concrete: 3 cm thick, with application surface of hardener with dosage of heavy traffic of 5 kg/m . Dilations of 1.5 cm, 2  2 m. Polished finish. Partition walls: Bricks type H15. Masonry Mortar: 1:3 cement: sand, 1 cm thick, alternating joints between courses. Sun shades 3.075  0.55  0.1-m concrete supported on tie beams and masonry H12 of 90 cm. 24  24-inch natural fiber panels with thicknesses between 4 and 16 millimeters, Ceiling attached to 5/16-inch die-cut frame. 1.5 cm thick frieze, 1: 3 ratio cement: sand, # 6 sieve. Friezes, stuccoes and paintings Stucco: A mixture of plaster, lime and cement. Sanded twice. Painting: Two coats of armor-type paint applied with rollers. 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Journal

Buildings – Multidisciplinary Digital Publishing Institute

Published: May 21, 2021

Keywords: passive cooling; EAHE; temperature potential; ventilation; TRNSYS; soil temperature

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