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Performance comparison of the solar-driven supercritical organic Rankine cycle coupled with the vapour-compression refrigeration cycle

Performance comparison of the solar-driven supercritical organic Rankine cycle coupled with the... Performance comparison of solar driven supercritical organic Rankine cycle coupled with vapor compression refrigeration cycle Results: Key trade off: Highest exergy efficiency 34.21% Thermal and exergy efficiency Highest thermal efficiency 68.58% COP of system COP of system 2.27 Working fluid selection Best working fluid R227ea 0.7 0.35 SPT-SORC-VCC system η (R227ea) th 0.6 η (R236fa) 0.30 th Parametric 1 η (R245fa) SORC pump PTCs fields analysis th 0.5 0.25 3 η (R1234ze) th Solar η (R134a) pump th Flow regulators 0.4 2 0.20 HX η (R227ea) ex 8 η (R236fa) ex Turbine 0.3 Condenser 4 0.15 Expansion η (R245fa) ex valve 10 Computational SORC VCC η (R1234ze) 7 0.10 0.2 ex technique (EES) η (R134a) ex Recuperator Mixer 0.05 0.1 Compressor 0.5 0.6 0.7 0.8 0.9 1.0 Evaporator G (kW/m ) 17 16 Keywords: parametric analysis; solar parabolic trough collector; vapour-compression refrigeration cycle super - critical organic Rankine cycle; cogeneration; cooling; power generation heat of the Sun, which is used to drive the different types Introduction of thermodynamic cycles [5]. Energy consumption used for cooling and power gener - Nowadays, solar parabolic trough collectors (PTCs) ation has increased drastically in recent years [1]. The are being widely used as heat sources to run combined energy consumed in air-conditioning and cooling equip- as well as simple cycle electricity-generation systems. In ment is large [2].The vapour-compression refrigeration this direction, a few studies were conducted using PTCs system is a widely used cooling system in which the com- for trigeneration and cogeneration applications, e.g. pressor is the highest energy-consuming component be- Al-Sulaiman et al. [6] carried out a study on the PTC-driven cause this energy is used to increase the coolant pressure. ORC for utilizing waste heat for a cogeneration process. However, more energy use in the cooling system increases They found that during trigeneration, the energy efficiency carbon-dioxide emissions, leading to higher global tem- was increased from 15% to 94%. Al-Sulaiman [7] carried out perature and greenhouse effects. Therefore, renewable- a thermodynamic analysis of the PTC-integrated steam energy technology is now used for refrigeration purposes. Rankine cycle with ORC and R134a as the best working fluid Such renewable energy sources are geothermal, solar, bio- among the other selected working fluids because it pro- mass, etc. [3]. The justification for using renewable energy vided the highest electrical efficiency of 26%. The exergy for cooling is to reduce emissions of carbon dioxide and and energy analysis of the PTC-integrated supercritical power consumption. Moreover, the cogeneration system organic Rankine cycle (SORC) system was examined by of the organic Rankine cycle and vapour-compression Singh and Mishra [8]. Exergetic metrics including the fuel- cycle (ORC–VCC) is a heat-driven power and refrigeration depletion ratio, improvement potential and irreversibility system. The heat-driven cooling system is a clean tech- ratio were found to be 0.579, 11 859 kW and 0.9296, respect- nology and free from pollution. Solar energy is one of the ively. Among the other working fluids tested, R600a was the most promising renewable heat sources because of its low best. Singh and Mishra [9] carried out a study on the PTC- costs, noise-free operation and abundance in nature [4]. driven supercritical carbon-dioxide cycle (sCO )–ORC. They Solar energy is the most suitable for cooling, heating and discovered that solar irradiation increased the thermal and power generating, among other renewable energy sources. exergy efficiency of the combined cycle. R407c was chosen Solar collectors are being used to harvest solar energy as the best working fluid, with a combined system thermal where heat-transfer fluid (HTF) circulates for absorbing the Thermal efficiency (SORC-VCC-PTC) Exergy efficiency (SORC-VCC-PTC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 478 | Clean Energy, 2021, Vol. 5, No. 3 and exergy efficiency of 43.49% and 78.07%, respectively, at ORC–VCC system operated by solar power. They concluded 0.95 kW/m . A PTC-driven partial heating sCO cycle com- that the ice production and the cooling power depended on bined with ORC was studied by Khan and Mishra [10]. They the condensation and generation temperature. R245fa was discovered that solar irradiation enhanced the thermal selected as an appropriate working fluid for this purpose. and exergy efficiency, while the solar incidence angle re- Using R245fa, cooling power and ice production per unit duced performance. Al-Zahrani and Dincer [11] performed metre square collector area were 126.44 W/m and 7.61 kg/ exergy and energy analysis of the PTC-driven sCO cycle to m -day, respectively. Moles et  al. [ 20] proposed a model of convert solar heat into power. They considered the param- the low-temperature heat-activated combined ORC–VCC eters of the PTC as design parameters such as solar irradi- system. They concluded that the computed thermal and ation, receiver emittance and solar-beam incidence angle. electrical COP of the ORC–VCC system varied between 0.30– The exergy and energy efficiency of the PTC were observed 1.10 and 15–110, respectively. Additionally, R1234ze(E) was as 66.35% and 38.51%, respectively. considered an acceptable working fluid for enhanced effi- The SORC is better technology for the conversion of ciencies. Li et  al. [21] performed a working-fluid selection waste heat to power. For example, Song et al. [12] analysed study for the combined ORC–VCC system. They concluded the SORC to recover waste heat at low temperature. They that butane was chosen as the better working fluid for the concluded that the SORC was an appropriate method for ORC–VCC system, with temperature variations ranging industrial waste-heat recovery and the R152a was ob- from 60°C to 90°C, −15°C to 15°C and 30°C to 55°C at boiler served as the best-performing working fluid and produced exit, evaporation and condensation temperatures varying, the maximum net power and thermal efficiency. Kalra respectively. The COP of the complete system was obtained et  al. [13] demonstrated that the operating fluid pressure as 0.47 based on butane at a boiler exit temperature of 90°C. was above its critical pressure in the SORC and found that It has been observed from the above-mentioned litera- the SORC had various benefits over its subcritical ORC be- ture survey that several studies have been conducted on cause the heat-source cooling curve in the SORC matched the ORC–VCC cogeneration system. However, no study was the working-fluid heating curve. A parametric analysis of conducted on the SORC–VCC cogeneration system driven by the SORC system for the medium-temperature geothermal PTCs. The impacts of the PTC design parameters on the co- heat source was undertaken by Moloney et  al. [14]. They generation system have also not been observed in previous came to the conclusion that the SORC was more efficient research. This demonstrates the novelty of the current study. than the ORC for low-temperature heat sources. Saadon The main objective of the present study is to examine the and Islam [15] performed a thermal analysis of the SORC exergetic and energetic performance of the cogeneration and discovered that it had a higher thermal efficiency and SORC–VCC system simultaneously for the cooling and power power output than the conventional ORC with a preheater. generation driven by PTCs. The system-performance param- Also several studies were conducted on the combined eters were considered to be exergy and thermal efficiency, ORC–VCC system in recent years. Pektezel and Acar [16] the COP of the cogeneration system and exergy destruction. analysed the combined ORC and single and double evap- The effects on the system performance of different param- orator VCC systems for exergy and energy. They discovered eters such as solar irradiation, the solar-beam incidence that R600a was the most suitable fluid for the combined angle and the HTF velocity in the absorber tube, condenser system. They also discovered that, with a single evaporator, temperature and evaporator temperature, and turbine inlet the coefficient of performance (COP) and exergy efficiency pressure were examined. Finally, the performance of the co- of the combined cycle were higher than with a dual evap- generation system was compared with and without PTCs. orator. The ORC-integrated VCC system was subjected to an exergy and energy study by Saleh [17]. He came to the 1 System description conclusion that R602 was a good working fluid for system performance and environmental concerns. Finally, he con- Fig. 1 shows the schematic diagram of the SORC–VCC cluded that the highest COP, exergy efficiency and pressure system driven by the PTC. A PTC field, a regenerating SORC ratio of the turbine of the system and the corresponding and a VCC are all part of the system. Because there is a total mass flow rate of the working fluid using R602 were lot of heat energy available at the turbine outlet, regen- 0.99, 53.8%, 12.2 and 0.005  kg/s-kW, respectively, at 25°C eration of the SORC is used in this study. With the help of condenser temperature and maintaining other parameters a recuperator, this heat is utilized to warm the incoming as constant. Javanshir et  al. [ 18] performed thermal and stream to the heat exchanger. Also, the SORC is beneficial exergoeconomic analyses on the regenerative ORC–VCC for maximum power production [13]. This system uses two system. Their results indicated that among other selected fluids: a single HTF in the solar field and another fluid in working fluids, R134a showed the lowest exergy and thermal both the SORC and VCC subsystems. However, the SORC efficiencies while R143a and R22 showed the highest exergy and VCC subsystems have different mass flow rates. There and energy efficiency. They also observed that the total unit is a common condenser for both subsystems. For control- cost of the product was found to be $60.7/GJ. The maximum ling the mass flow rate to both of the subsystems, two flow exergy destruction was found in the boiler followed by the regulators and a common condenser for both cycles are turbine. Hu et  al. [19] investigated the performance of an used. To control the mass flow rate to both cycles, two flow Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 479 PTCs fields SORC pump Solar pump Flow regulators HX 4 Turbine Condenser Expansion valve SORC VCC Recuperator 5 Mixer Compressor Evaporator 13 17 16 Fig. 1: Schematic of the PTC-driven SORC–VCC cogeneration system system has already been conducted by Al-Sulaiman [7]. Table 1: Input conditions for simulation of the proposed Modelling of the system was performed using the exergy model and energy balance in each component. Results are calcu- Parabolic trough collector LS-3 [22] lated with help of the Engineering Equation Solver (EES) Maximum exit temperature of HTF 390°C [7] [23] software. Maximum exit pressure of HTF 10 MPa HTF mass flow rate 0.575 (kg/s) Atmospheric pressure 1.013 bar Atmospheric temperature 25°C 2.1 Thermal modelling of the SORC–VCC system Effectiveness heat exchanger 0.95 [7] While solving equations, the following assumptions were SORC turbine’s isentropic efficiency 0.87 [18] considered: (i) steady-state conditions are assumed for all Compressor’s isentropic efficiency 0.8 [18] thermodynamic processes; (ii) friction and pressure losses SORC pump’s isentropic efficiency 0.8 [18] are negligible in each component and pipes; (iii) heat loss SORC’s mass flow rate 1.5 kg/s to the surroundings is negligible in each component ex- VCC’s mass flow rate 1 kg/s cept the condenser unit; (iv) the working-fluid conditions Turbine inlet pressure 50 bar Outlet pressure to the compressor/turbine 6.018 bar at the entry to the compressor and exit to the condenser Condenser temperature 35°C [17] in the VCC subsystem are saturated vapour and saturated Effectiveness of recuperator 0.95 [7] liquid, respectively; (v) the PTCs have 50 collectors in series Evaporator temperature 0°C [17] in a single row and there are a total of seven rows and the length of each collector is 12.27 m [7]. Modelling equations for the cogeneration (SORC–VCC) regulators are used. The fluid flow directions are shown by system are adapted from the previous studies [13, 18]. the arrows. The main advantage of this combined SORC– The total solar heat available in the SORC–VCC cogener - VCC system is that it produces cooling when power is not ation system is determined as: needed or vice versa. The advantage of using a combined SORC–VCC system produces cooling when refrigeration is Q = m · (h − h ) (1) HX SORC 4 10 not needed. Therefore, this system is beneficial throughout m ˙ where is the working-fluid mass flow rate in the SORC the year [17]. A solar field with a few hundred PTCs is used. SORC subsystem. The type of collector and other input data are given in The work done by the turbine is given as: Table 1. The T-s diagram has been displayed in Fig. 2. W = m · (h − h ) · η SORC 4 5s (2) Turbine turbine The heat recovered in the in the recuperator is given by the 2 Thermodynamic analysis heat balance equation: The mathematical modelling of the SORC–VCC system ˙ ˙ has been conducted in this section. Modelling of the PTC m · (h − h )= m · (h − h )(3) SORC 6 5 SORC 10 9 Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 480 | Clean Energy, 2021, Vol. 5, No. 3 PTC heat source SORC Tc VCC Te Entropy Fig. 2: T-s diagram for the PTC-driven SORC–VCC cogeneration system If the same working fluid is circulating having the same Furthermore, when the system is brought to dead condi- mass flow rate on the hot and cold sides of the recuperator, tions (298.15 K temperature, 1.013 bar pressure), the max- then the effectiveness of the recuperator is given as: imum theoretical work obtained is termed as exergy. On (T − T ) Actual heat transfer the basis of the control volume, the exergy balance equa- 5 6 ε = = (4) (T − T ) Maximum heat transfer tion is determined as: 5 9 Å ã The rejection of heat by the condenser unit is given as: 0 1 − Q − W − (m Ex ) − (m Ex ) − Ed = 0 Q c.v i i e Q =(m ˙ + m ˙ ) · (h − h ) (5) cond SORC VCC 8 7 (13) where subscript Q denotes the thermal property numeric- where m is the mass flow rate of the working fluid in VCC ally available at a given stage and Ed refers to the exergy- the VCC subsystem. destruction rate. Furthermore, Ex signifies the specific Applying the energy-balance equation in the mixer: system exergy. Without considering the chemical exergy, ˙ ˙ ˙ ˙ (m + m ) · h = m · h + m · h(6) SORC VCC 7 SORC 6 VCC 14 potential energy and kinetic energy, the physical exergy The compressor work is given by: can then be assessed as [7, 25]: ˙ Ex =(h − h ) − T (s − s )(14) m · (h − h ) ph 0 0 0 VCC 14s 13 W = (7) comp comp The exergy inlet to the system and obtained from solar ra- diation by Petela’s formula [26] is described as: where h is the enthalpy at state 14 when the isentropic 14s work is to be done. ñ ô Å ã Å ã 1 T 4 T 0 0 Further, the heat absorbed by the evaporation or cooling Ex = A · G · 1 + − (15) s p b 3 T 3 T su su effect can be given as: where T is the temperature of the Sun, i.e. 5800 K [7]. su Q = m · (h − h ) (8) evap VCC 13 15 Ex evap can be defined as [18]: The SORC pump work can be written as: Å ã Ex =Q · − 1(16) evap evap m · (h − h ) T SORC 9s 8 e W = (9) SORC pump SORC pump where T is the evaporator temperature. The exergy efficiency of the SORC–VCC congregation The thermal efficiency of the cogeneration system (SORC– system is defined as [18]: VCC) is determined as [18]: ˙ ˙ W + Q W + Ex net evap net evap η = η = (17) ex th (10) Ex HX S The exergy efficiency of the overall system (SORC–VCC– ˙ ˙ ˙ ˙ W = W − W − W net turbine compressor SORC pump(11) PTC) is shown as [24]: The overall thermal efficiency of the system is calculated as [24]: Ed Total η = 1 − (18) ex,overall η = η · η th,overall ex,overall Carnot(12) Ex Temperature Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 481 where Ed is the total exergy destruction of the in the study of Mwesigye et  al. [27]. The working fluids in Total overall system. the SORC system should be non-toxic, environmentally Another performance parameter to be defined is the friendly, economical, able to use heat energy from the COP of the SORC–VCC cogeneration system (COP ).This source of waste heat and also, at optimum pressure and can be defined as [17]: temperature, the thermophysical and thermodynamic properties should be optimal [28]. Due to these reasons, evap fluids such as R227ea, R236fa, R245fa, R1234ze and R134a COP = η · COP = S SORC vcc (19) ˙ ˙ Q + W HX SORC pump were selected. The thermophysical properties of the con- sidered working fluids are shown in Table 2. η COP where and vcc are the efficiency of the SORC SORC and the COP of the vapour-compression subsystem, respectively. 2.3 Verification of the model The current model was validated using previous studies. 2.2 Working-fluid selection The PTC as well as the cogeneration SORC–VCC system has Working-fluid selection is challenging because it affects been separately verified. Verification with the change in the performance and economic feasibility of the system. heat loss in the absorber tube with the temperature change It is good if the critical temperatures of the working fluid above the ambient temperature and exit temperature of are near the temperature of the heat source and the tem- the PTC have been shown in Tables 3 and 4, respectively, at perature of the waste [17]. The HTF selected in the current the same baseline situations. There is <1% deviation from the study is Therminol-VP1 in the absorber tube of the solar experimental results. It is better agreement than in the collector to absorb the heat of the Sun because this syn- previous theoretical study by Al-Sulaiman [7] and the ex- thetic oil has excellent heat-transfer properties as well perimental study by Dudley et al. [22]. Apart from this, the as good temperature stability compared with other HTFs thermal efficiency of the SORC system has been validated such as molten salt and a maximum working temperature by the study by Le et al. [29] under the same baseline condi- of 400°C [27]. Its thermophysical properties were discussed tions as given in Table 5. Table 6 shows the COP variation of Table 2: Thermophysical properties of the working fluids [16, 17] Working fluids Weights (g/mole) Critical temperature (°C) Critical pressure (MPa) ODP GWP R227ea 170.03 101.8 2.925 0 3580 R236fa 152.04 124.92 3.2 0 680 R245fa 134.05 154.1 3.65 0 1050 R1234ze 104.04 109.36 3.635 0 <1 R134a 102.03 101 4.059 0 1430 Table 3: Heat loss (W/m) variation with temperature difference Temperature difference (°C) Current model Dudley et al. [22] Al-Sulaiman [7] 100.6 9.78 10.6 8.719 149.1 16.24 19.3 19.3 196.7 31.15 30.6 34.2 245.8 53.48 45.4 53.0 293.3 66.69 62.9 75.5 Table 4: Verification of the exit temperature of the PTC with experimental results T (K) 2 3 Cases G (W/m ) Discharge (m /s) T (K) T (K) Dudley et al. [22] Current model Error (%) b 0 i 1 933.7 47.7 294.35 375.35 397.15 397.56 0.10 2 968.2 47.8 295.55 424.15 446.45 446.72 0.06 3 982.3 49.1 297.45 470.65 492.65 492.02 0.12 4 909.5 54.7 299.45 523.85 542.55 541.99 0.10 5 937.9 55.5 299.35 570.95 589.55 590.11 0.09 6 880.6 55.6 301.95 572.15 590.35 590.26 0.01 7 903.2 56.3 300.65 529.05 647.15 647.35 0.03 8 920.9 56.8 304.25 652.65 671.15 671.51 0.05 Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 482 | Clean Energy, 2021, Vol. 5, No. 3 Table 5: Verification of the SORC system Thermal efficiency (%) Baseline conditions Literature Literature Current model Error (%) R134a, heat-source temperature = 139°C Le et al. [29] 12 11.99 –1 other selected working fluids. Near the critical conditions, Table 6: COP variation of the VCC system with the evaporator the working fluid performs better. The exergy efficiency of average temperature the cogeneration system also improved with the solar ir - COP radiation. The reason behind this is that, as the irradiation increases, the inlet exergy to the cogeneration system in- Evaporator average Krishnan temperature (°C) Current model et al. [30] creases according to Equation (20) and simultaneously the exergy destruction decreases. R227ea has the highest 13 2.85 2.99 exergy efficiency ranging from 86.38% to 92.9% at 0.5–0.95 13.5 3.22 3.12 kW/m , respectively. 14 3.33 3.45 The overall system (PTC-SORC–VCC) exergy and 14.5 3.82 3.95 thermal efficiency were also examined. It is observed 15 3.98 4.02 from Fig. 4 that the overall system thermal efficiency in- 15.5 4.10 4.21 16 4.42 4.38 creases with the solar irradiation due to the same reason as above. R227ea and R134a, among the other selected working fluids, gave respectively the highest and lowest the VCC system with the evaporator average temperature thermal efficiency. Using R227ea, the solar irradiation in- and validated by the experimental study by Krishnan et al. creases from 0.5 to 0.95 kW/m, which results in a nearly [30]. Table 7 shows that the thermal efficiency of the SORC– 58.88% increase in the thermal efficiency. The rise in solar VCC cogeneration system matches that from the study by irradiation results in an increase in the overall system Javanshir et  al. [18]. Therefore, the current model is ready exergy efficiency. As the values of the solar irradiation for analysis. vary from 0.5 to 0.95 kW/m, the exergy efficiency based on R227ea improves by almost 58.90%, as illustrated in Fig. 4. 3 Results and discussion The variation in solar irradiation also has a significant In the present study, a thermal performance analysis of effect on the COP , as denoted in Fig. 5. Fig. 5 shows that the PTC-driven SORC–VCC cogeneration system was con- the COP is a function of the solar irradiation. As the solar ducted. The effects of solar irradiation (G ), solar-beam b irradiation increases, this leads to an increase in the COP . incidence angle, HTF velocity in the absorber tube, tur - Increased solar irradiation has no effect on the VCC sub- bine inlet pressure (TIP), condenser temperature () and T c system, which means no effect on the COP . However, an VCC evaporator temperature (T ) on thermal performance were e increase in solar irradiation leads to an increase in η [8]. SORC examined. While investigating the effects of the param- The COP increases with solar irradiation based on Equation eters, other parameters were kept constant as listed in (19). R134a has the highest COP and R227ea the lowest among Table 1. A computer program was made using EES software the considered working fluids. This means that if this system to solve the modelling equations. only works for cooling purposes, R134a is more effective than R277ea. Using the working fluid R134a, as the solar irradiation varied from 0.5 to 0.95 kW/m, the COP increased by nearly 3.1 Performance evaluation with solar 7.69%. It is also shown that the exergy destruction decreases irradiation with the solar irradiation. It has the reverse trend to that of the exergy efficiency, as explained above. R134a and R227ea gave Solar irradiation, also termed as the direct normal irradi- the highest and lowest exergy destruction as shown in Fig. 5. ation, is a primary parameter to be examined. The vari- R134a gave exergy-destruction decreases from 3966 to 2060 ation in solar irradiation was taken as 0.5–0.95 kW/m as kW when the solar irradiation varied from 0.5 to 0.95 kW/m , per the climate of Mumbai, India. Variation in solar irradi- respectively. Furthermore, without the solar SORC–VCC, the ation has a large impact on the system performance ran- system is more thermodynamically efficient than with solar. ging from 0.5 to 0.95 kW/m. The thermal performance of the SORC–VCC cogeneration system increases with solar irradiation due to the effective utilization of collector rows 3.2 Performance evaluation with solar as shown in Fig. 3. Among the selected working fluids, incidence angle R227ea gives the highest thermal efficiency that varies from 47.55% to 51.13% at 0.5–0.95 kW/m, respectively, be- Fig. 6 shows that the thermal and exergy efficiency of the cause R227ea has the lowest critical pressure compared to cogeneration system decreases with the increase in the Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 483 Table 7: Verification of the cogeneration SORC–VCC system Thermal efficiency (%) Baseline conditions Literature Literature Current model Error (%) R143a, TIP = 3600 kPa, heat-source temperature = 150°C Javanshir et al. [18] 27.21 27.98 2.49 0.52 0.94 η (R227ea) th 0.92 η (R236fa) th 0.50 η (R245fa) 0.90 th η (R1234ze) th 0.88 η (R134a) th 0.48 0.86 η (R227ea) ex η (R236fa) 0.84 ex 0.46 η (R245fa) ex 0.82 η (R1234ze) ex 0.80 η (R134a) ex 0.44 0.78 0.5 0.6 0.7 0.8 0.9 1.0 G (kW/m ) Fig. 3: Variation in the efficiency of the SORC–VCC cogeneration system with solar irradiation 0.7 0.35 η (R227ea) th 0.6 η (R236fa) 0.30 th η (R245fa) th 0.5 0.25 η (R1234ze) th η (R134a) th 0.4 0.20 η (R227ea) ex η (R236fa) ex 0.3 0.15 η (R245fa) ex η (R1234ze) 0.2 ex 0.10 η (R134a) ex 0.1 0.05 0.5 0.6 0.7 0.8 0.9 1.0 G (kW/m ) Fig. 4: Variation in the efficiency of the overall system (SORC–VCC–PTC) with solar irradiation solar incidence angle. An increase in the incidence angle (19), the COP decreases with the solar incidence angle. reduces the exergetic, energetic and optical efficiencies The highest and lowest COP were found with R134a and of the PTCs [11] and leads to system-performance reduc- R227ea, respectively. The COP decreased from 2.6 to 1.3 tion. The maximum and minimum thermal efficiency as the incidence angle increased from 3° to 30°, respect- and exergy efficiency were found for R227ea and R134a, ively, whereas the exergy destruction increased with the respectively. The thermal and exergy efficiency de- solar incidence angle, as shown in Fig. 7. It has been dis- creased by 51.02–50.57% and 92.70–91.88% with increases cussed above that the exergy efficiency decreased with in the solar incidence angle from 3° to 30°, respectively, the solar incidence angle, which is why the exergy de- based on R227ea. Therefore, it is important to minimize struction showed a reverse trend. The exergy destruction the solar incidence angle to obtain better performance for all other selected working fluids was found to be be- of the system. Also, the COP decreased with the solar tween these of the fluids R227ea and R134a. As the solar incidence angle as shown in Fig. 7. The solar incidence incidence angle increased from 3° to 30°, the exergy de- angle does not affect the COP . However, it does affect struction increased from 3966 to 2060 kW on the basis of vcc the efficiency of the SORC. Thus, according to Equation R134a, as seen in Fig. 7. Thermal efficiency (SORC-VCC) Thermal efficiency (SORC-VCC-PTC) Exergy efficiency (SORC-VCC-PTC) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 484 | Clean Energy, 2021, Vol. 5, No. 3 2.3 4500 COP (R227ea) 2.2 COP (R236ea) COP (R245fa) COP (R1234ze) 2.1 COP (R134a) Ed (R227ea) Ed (R236fa) 2.0 Ed (R245fa) Ed (R1234ze) Ed (R134a) 1.9 0.5 0.6 0.7 0.8 0.9 1.0 G (kW/m ) Fig. 5: Variation in the COP with solar irradiation 0.52 0.93 η (R227ea) 0.51 th 0.92 η (R236fa) th η (R245fa) th 0.51 η (R1234ze) th 0.91 η (R134a) th 0.50 η (R227ea) ex 0.90 η (R236fa) ex 0.50 η (R245fa) ex η (R1234ze) 0.89 ex 0.49 η (R134a) ex 0.49 0.88 05 10 15 20 25 30 Angle of incidence (degree) Fig. 6: Variation in the efficiency of the SORC–VCC cogeneration system with solar incidence angle 2.8 2400 2.6 COP (R227ea) 2.4 COP (R236fa) 2.2 COP (R245fa) COP (R1234ze) 2.0 s COP (R134a) 1.8 Ed (R227ea) Ed (R236fa) 1.6 Ed (R245fa) Ed (R1234ze) 1.4 Ed (R134a) 1.2 1.0 1400 Angle of incidence (degree) Fig. 7: Variation in the COP with the solar incidence angle 3.3 Effects of HTF velocity in the absorber tube the HTF velocity in the absorber tube. The increase in the on system performance efficiency with the velocity is due to the increase in the velocity of the fluid. The Reynolds number is increased Fig. 8 displays that the thermal and exergy efficiency of as a result of the increase in the convective heat-transfer the SORC–VCC cogeneration system has increased with COP COP Thermal efficiency (SORC-VCC) Exergy destruction (SORC-VCC) (kW) Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 485 coefficient; therefore, much heat is carried by the HTF. 3.4 Effects of the TIP on system performance This leads to an increase in efficiency. Among the The system performance also depends on the TIP. As operating fluids considered, R227ea and R134a were re- shown in Fig. 10, the efficiency of the cogeneration system ported to have the greatest and smallest thermal and en- increased slightly with the TIP. The TIP variation had no ergy efficiency. Thermal and exergy efficiency increased impact on the VCC subsystem performance. The VCC sub- from 47.55% to 51.13% and from 86.38% to 92.9% when system does not contribute to the efficiency of the SORC– the velocity increased from 0.01 to 0.1 m/s on the basis VCC cogeneration system. Therefore, the efficiency of the of R227ea. The COP also increased with the velocity of cogeneration system depends only on the performance of the HTF. As the velocity increased, the efficiency of the the SORC subsystem. It is due only to the η because the SORC SORC was enhanced. However, there was no effect on the η increases with the TIP [8]. It is observed that R227ea SORC COP . Therefore, according to Equation (19), the COP vcc s and R134a provided the maximum and minimum thermal increased with the HTF velocity as seen in Fig. 9. The efficiency of the cogeneration system. In the case of the highest value of the COP was found with R134a followed R227ea, the TIP increase from 40 to 55 bar improved the by R245fa, R236fa, R1234ze and R227ea. The exergy de- cogeneration system thermal efficiency by almost 0.85%. struction of the SORC–VCC cogeneration system was also From Fig. 10, it can be seen that as the TIP increased, the investigated with the HTF velocity in the absorber tube. exergy efficiency of the cogeneration system also in- The exergy destruction decreased with the HTF velocity. creased. The destruction of exergy decreases with the TIP The highest exergy destruction was identified with R134a led to exergy-efficiency improvement [8]. Using R227ea, the followed by R1234ze, R245fa, R236fa and R227ea with increase in the TIP from 40 to 55 bar improved the exergy values of 3966, 3673, 3515, 3241 and 2757 kW, respectively, efficiency from 91.82% to 92.60%. at an HTF velocity of 0.01 m/s, as shown in Fig. 9. 0.52 0.94 0.92 η (R227ea) th η (R236fa) th 0.50 0.90 η (R245fa) th η (R1234ze) 0.88 th η (R134a) th 0.48 0.86 η (R227ea) ex η (R236fa) ex 0.84 η (R245fa) ex 0.46 0.82 η (R1234ze) ex η (R134a) ex 0.80 0.44 0.78 0.00 0.02 0.04 0.06 0.08 0.10 HTF velocity in absorber tube (m/s) Fig. 8: Variation in the efficiency of the SORC–VCC cogeneration system with HTF velocity 2.3 4500 2.2 COP (R227ea) COP (R236fa) COP (R245fa) 2.1 COP (R1234ze) COP (R134a) Ed(R227ea) Ed(R236fa) 2.0 2000 Ed(R245fa) Ed(R1234ze) Ed(R134a) 1.9 0.02 0.04 0.06 0.08 0.10 HTF velocity in absorber tube (m/s) Fig. 9: Variation in the COP with the HTF velocity in the absorber tube COP Thermal efficiency (SORC-VCC) Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 486 | Clean Energy, 2021, Vol. 5, No. 3 In addition to the cogeneration system performance, efficiency increased with the TIP and, consequently, the the performance of the overall system was also investi- exergy destruction decreased with the TIP. The highest gated and the overall system (SORC–VCC–PTC) thermal and lowest exergy were obtained with the working fluids efficiency was found to increase with the TIP, as seen in R134a and R227ea, respectively, although the exergy de- Fig. 11. The exergy efficiency of the overall system also struction for fluids R236fa, R245fa and R1234ze was found depended on the TIP. The exergy efficiency of the overall between these two. Further, comparing Fig. 10 with Fig. 11, system improved with the TIP. The TIP did not affect the the performance of the PTCs combined system perform- PTC and VCC subsystems. As a result, the exergy efficiency ance was reduced. It does not mean that the solar system was only enhanced by the SORC subsystem. In the case of is worse overall, but only from the efficiency point of view. the working fluid R227ea, the TIP increases from 40 to 55 bar improved the exergy efficiency by 5.5%, as shown in 3.5 Performance evaluation with condenser the right axis of Fig. 11. temperature The COP is also a function of the TIP. The COP slightly S S increased with the TIP. The reason behind this is that the The condenser temperature also has an impact on both variation in the TIP had no effect on the COP , although subsystems. As the condenser temperature increased, VCC η increased with the TIP [8 , 18], from Equation (19), this led to a slight decrease in the exergy and thermal ef- SORC the COP increased. R134a and R227ea show the highest ficiency of the cogeneration system (SORC–VCC), as shown and lowest COP , respectively. If the TIP increased from 40 in Fig. 13. Improvement in the condenser temperature re- to 55 bars, the COP increased by nearly 1.43%, as shown duced the enthalpy drop in the turbine leading to the low er in Fig. 12. Fig. 12 also shows that the exergy destruction output work. Therefore, the thermal efficiency of the SORC decreased with the TIP. As already discussed, the exergy and COP system was reduced. Consequently, the thermal vcc 0.52 0.93 η (R227ea) th 0.51 0.92 η (R236fa) th η (R245fa) th 0.51 0.91 η (R1234ze) th η (R134a) th 0.50 0.90 η (R227ea) ex η (R236fa) ex 0.50 0.89 η (R245fa) ex η (R1234ze) ex 0.49 0.88 η (R134a) ex 0.49 0.87 40 42 44 46 48 50 52 54 56 TIP (bar) Fig. 10: Variation in the efficiency of the SORC–VCC cogeneration system with the TIP 0.62 0.33 η (R227ea) 0.60 th η (R236fa) 0.32 th 0.58 η (R245fa) th 0.31 η (R1234ze) 0.56 th η (R134a) th 0.30 0.54 η (R227ea) ex 0.29 η (R236fa) ex 0.52 η (R245fa) ex 0.28 0.50 η (R1234ze) ex η (R134a) ex 0.27 0.48 0.26 0.46 40 42 44 46 48 50 52 54 56 TIP (bar) Fig. 11: Variation in the efficiency of the overall system (SORC–VCC–PTC) with the TIP Thermal efficiency (SORC-VCC-PTC) Thermal efficiency (SORC-VCC) Exergy efficiency (SORC-VCC-PTC) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 487 performance of the cogeneration system was reduced. The Finally, in this section, the impact of the condenser tem- exergy efficiency of the cogeneration system also decreased perature variation on the system COP was also discussed, as with the condenser temperature because, according to shown in Fig. 15. In the case of R134a, as the condenser tem- Equation (12), the exergy efficiency is directly related to perature varied from 303 K to 328 K, the COP decreased by the thermal efficiency, as revealed in Fig. 13. Furthermore, 49.8%. It is already known that the η decreases with the SORC the thermal efficiency of the overall system also depends condenser temperature. However, the temperature, pressure on the condenser temperature. As the temperature of the and enthalpy increase with the condenser temperature at condenser increased from 303 K to 328 K, the thermal ef- the outlet of the compressor. This results in a reduction in the ficiency decreased by almost 14.07%, as shown in Fig. 14. COP . As a result, the COP decreases according to Equation vcc This was due only to the performance of the SORC–VCC (19). R134a and R227ea show the highest and lowest COP , re- system, not to that of the PTC system. The effect of the spectively. It can also be seen in Fig. 15 that if this system only condenser temperature on the exergy efficiency of the works for the purpose of cooling, then R134a is selected as overall system (SORC–VCC–PTC) was discussed. The exergy the appropriate working fluid among other selected working efficiency also decreased with the condenser temperature. fluids. The exergy destruction increases with the condenser It is already known that thermal efficiency decreases with temperature. The exergy efficiency slightly increases with the temperature of the condenser. Hence the exergy effi- condenser temperature, therefore the exergy decreases with ciency also decreased with the condenser temperature by the condenser temperature. The highest exergy destruction Equation (12). The efficiencies of the other fluids are be- was found with R134a, followed by R1234ze, R245fa, R236fa tween those of the R227ea and R134a fluids, as shown in and R227ea of 2347, 2176, 2061, 1932 and 1644 kW, respect- Fig. 14. ively, as displayed in right axis of Fig. 15. 2.30 2600 2.25 COP (R227ea) COP (R236fa) 2.20 COP (R245fa) COP (R1234ze) 2.15 s COP (R134a) 2000 s Ed (R227ea) 2.10 Ed (R236fa) Ed (R245fa) 2.05 Ed (R1234ze) Ed (R134a) 2.00 1.95 1400 40 42 44 46 48 50 52 54 56 TIP (bar) Fig. 12: Variation in the COP with the TIP 0.52 0.93 η (R227ea) th 0.51 0.92 η (R236fa) th η (R245fa) th 0.51 η (R1234ze) th 0.91 η (R134a) th 0.50 η (R227ea) ex 0.90 η (R236fa) ex 0.50 η (R245fa) ex η (R1234ze) ex 0.89 0.49 η (R134a) ex 0.88 0.49 300 305 310 315 320 325 330 T (K) Fig. 13: Variation in the efficiency of the SORC–VCC cogeneration system with the condenser temperature Thermal efficiency (SORC-VCC) COP Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 488 | Clean Energy, 2021, Vol. 5, No. 3 0.38 0.66 η (R227ea) 0.64 th 0.36 η (R236fa) th 0.62 η (R245fa) th 0.34 0.60 η (R1234ze) th 0.58 η (R134a) 0.32 th 0.56 η (R227ea) ex 0.30 η (R236fa) 0.54 ex η (R245fa) ex 0.52 0.28 η (R1234ze) ex 0.50 η (R134a) ex 0.26 0.48 0.24 0.46 300 305 310 315 320 325 330 T (K) Fig. 14: Variation in the efficiency of the overall system (SORC–VCC–PTC) with the condenser temperature 2.8 2400 2.6 COP (227ea) 2200 s 2.4 COP (236fa) 2.2 COP (245fa) COP (R1234ze) 2.0 COP (R134a) 1.8 Ed (227ea) Ed (R236fa) 1.6 Ed (R245fa) Ed (R1234ze) 1.4 Ed (R134a) 1.2 1.0 1400 305 310 315 320 325 330 T (K) Fig. 15: Variation in the COP with the condenser temperature behind this is that the COP increases with the evapor - 3.6 Performance evaluation with evaporator vcc temperature ator temperature [17]. Also, the saturation pressure of the evaporator improves with the improvement of the evap- Furthermore, the effect of the evaporator temperature orator temperature for all working fluids leading to a de- was also examined. The cooling capacity is directly af- cline in the compressor work with constant condenser fected by the evaporator temperature. During examining temperature. Conversely, the refrigeration effect improves the evaporator-temperature effect on the system perform- with the evaporator temperature. Both impacts enhance ance, all other assumed parameters are kept constant, as the COP . However, simultaneously, the η is not af- SORC vcc listed in Table 1 such as the condenser temperature and fected by the evaporator temperature. As per Equation (19), pressure, evaporator pressure, etc. F ig. 16 reveals that the this resulted in improvement in the COP , as shown in Fig. efficiency is not significantly affected by the evaporator 17. Also, R134a and R227ea showed the highest and lowest temperature because there was no impact of the evapor - values, respectively. Regarding the exergy destruction of ator temperature on the SORC subsystem performance. the cogeneration cycle, the exergy destruction also was not The working fluids R227ea and R134a gave the highest affected much by the evaporator temperature like the effi- and lowest efficiency, respectively. Further to Fig. 16, it is ciencies, as discussed above. At a constant evaporator tem- noticed that the evaporator temperature also did not sig- perature, the highest and lowest exergy destruction was nificantly affect the exergy efficiency of the cogeneration for R134a and R227ea, respectively, whereas the exergy de- system because the exergy efficiency has as direct relation struction for the R245fa was significantly affected due to COP with the thermal efficiency [31]. However, the was different thermophysical properties of R245fa, as shown in affected by the evaporator temperature significantly and the right axis of Fig. 17. it increased with the evaporator temperature. The reason COP Thermal efficiency (SORC-VCC-PTC) Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC-PTC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 489 0.51 0.93 η (R227ea) th 0.51 0.92 η (R236fa) th η (R245fa) th η (R1234ze) th 0.50 0.91 η (R134a) th η (R227ea) ex 0.50 0.90 η (R236fa) ex η (R245fa) ex η (R1234ze) ex 0.49 0.89 η (R134a) ex 0.49 0.88 255 260 265 270 275 280 285 290 T (K) Fig. 16: Variation in the efficiency of the SORC–VCC cogeneration system with the evaporator temperature 4.0 2400 3.5 COP (R227ea) COP (R236fa) 3.0 COP (R245fa) COP (R1234ze) COP (R134a) 2.5 Ed (R227ea) Ed (R236fa) 2.0 Ed (R245fa) Ed (R1234ze) Ed (R134a) 1.5 1.0 1400 255 260 265 270 275 280 285 290 T (K) Fig. 17: Variation in the COP with the evaporator temperature Table 8: Exergy-destruction rate (kW) for each component 3.7 Exergy destruction in each component In addition to the above parameters, the exergy destruc- Components R227ea R236fa R245fa R1234ze R134a tion for each component and for each working fluid has Solar field 6894 7265 7253 8006 8067 been also evaluated, as revealed in Table 8. On the basis HX 218.3 309.1 311.8 288.5 312.8 of R134a, a major part of the energy destruction rate was Turbine 1092 1216 1361 1596 1787 found in the PTCs followed by the turbine and heat ex- Recuperator 46.3 52.62 26.99 24.21 17.24 changer with values of 8067, 1787 and 312.8 kW, respect- Condenser 23.03 64.64 130.5 15.51 22.01 ively. It can be seen that PTCs alone accounted for 76.32% Mixer 181.7 191.9 172.5 175.9 167.6 of the total exergy destruction of the overall system. The Compressor 170.3 186.1 153.9 159.5 147.2 total exergy input into the cogeneration system was cal- EX valve 2.937 3.088 3.109 3.509 3.695 culated at ~20250 kW. It is therefore shown that 39.83% Evaporator 15.36 21.41 7.783 24.9 27.54 ORC pump 16.47 10.87 18.31 19.19 17.61 of the total inlet exergy was destroyed in the solar field only. This is due to the high exergy losses in the PTCs as well as the high outlet and inlet temperature difference of was ~3.695 kW only. Among the other considered working the HTF stream of the collector. Therefore, the PTC is the fluids, R227ea showed the lowest exergy-destruction rate. worst subsystem in terms of exergetic performance in the R227ea is superior because this has a more supercritical current model. Therefore, it is necessary to design the PTC operating range due to its low critical values compared system carefully. It can be also observed from Table 8 that with other selected fluids, as listed in Table 2. The optimal the expansion valve was responsible for the lowest exergy- parameters are summarized in Table 9. destruction rate among the other components and that it COP s Thermal efficiency (SORC-VCC) Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 490 | Clean Energy, 2021, Vol. 5, No. 3 Table 9: Summary of values of optimal parameters at 0.95 kW/ Nomenclature m  solar irradiation Q Heat transfer (kW) T Evaporator Parameters Values temperature (K) Optimum thermal efficiency 51.13% s Specific entropy η Thermal th Optimum exergy efficiency 92.9% (kJ/kg-K) efficiency Optimum COP of system 2.27 Ed Exergy-destruction m Mass flow rate HTF Best working fluid for power generation R227ea rate (kW) of HTF (kg/s) Best working fluid for cooling R134a G Solar irradiation T Condenser b c (kW/m ) temperature (K) 4 Conclusions A Aperture area (m) H Specific en- In the present study, an exergy and energy analysis of the thalpy (kJ/ PTC-driven SORC–VCC cogeneration system was conducted. kg) The following conclusions were made from the results: T Atmospheric η Exergy ex temperatures efficiency • In Mumbai, India, the exergy efficiency, thermal effi- T Temperature (K) T Temperature su ciency and COP of the SORC–VCC cogeneration system of the Sun increased with solar irradiation and HTF velocity in the (K) absorber tube, but declined with the solar-beam inci- W Work (kW) dence angle. As a result, in order to increase the per - Subscripts formance of the cogeneration system, the PTCs must be am ambient i Inlet carefully designed. cond condenser o Outside • The maximum values of exergy efficiency, thermal effi- comp compressor rec Recuperator evap evaporator e Exit ciency and COP of the SORC–VCC cogeneration system s solar 0 environmental were 92.9%, 51.13% and 2.27, respectively, using R227ea 2 conditions and R134a at 0.95 kW/m of solar irradiation. At 0.95 kW/ m , the overall system exergy efficiency and thermal ef- VCC vapour compres- ficiency were 62.78% and 34.5%, respectively. sion cycle • The exergy efficiency, thermal efficiency and COP de- creased with the condenser temperature and increased with the TIP. The COP also increased continuously with Acknowledgements the evaporator temperature and reached a maximum Y.K. acknowledges the support of Department of Mechanical, value of 3.764 at 328 K for R134a. 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Performance comparison of the solar-driven supercritical organic Rankine cycle coupled with the vapour-compression refrigeration cycle

Clean Energy , Volume 5 (3) – Sep 1, 2021

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Copyright © 2021 National Institute of Clean-and-Low-Carbon Energy
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10.1093/ce/zkab028
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Abstract

Performance comparison of solar driven supercritical organic Rankine cycle coupled with vapor compression refrigeration cycle Results: Key trade off: Highest exergy efficiency 34.21% Thermal and exergy efficiency Highest thermal efficiency 68.58% COP of system COP of system 2.27 Working fluid selection Best working fluid R227ea 0.7 0.35 SPT-SORC-VCC system η (R227ea) th 0.6 η (R236fa) 0.30 th Parametric 1 η (R245fa) SORC pump PTCs fields analysis th 0.5 0.25 3 η (R1234ze) th Solar η (R134a) pump th Flow regulators 0.4 2 0.20 HX η (R227ea) ex 8 η (R236fa) ex Turbine 0.3 Condenser 4 0.15 Expansion η (R245fa) ex valve 10 Computational SORC VCC η (R1234ze) 7 0.10 0.2 ex technique (EES) η (R134a) ex Recuperator Mixer 0.05 0.1 Compressor 0.5 0.6 0.7 0.8 0.9 1.0 Evaporator G (kW/m ) 17 16 Keywords: parametric analysis; solar parabolic trough collector; vapour-compression refrigeration cycle super - critical organic Rankine cycle; cogeneration; cooling; power generation heat of the Sun, which is used to drive the different types Introduction of thermodynamic cycles [5]. Energy consumption used for cooling and power gener - Nowadays, solar parabolic trough collectors (PTCs) ation has increased drastically in recent years [1]. The are being widely used as heat sources to run combined energy consumed in air-conditioning and cooling equip- as well as simple cycle electricity-generation systems. In ment is large [2].The vapour-compression refrigeration this direction, a few studies were conducted using PTCs system is a widely used cooling system in which the com- for trigeneration and cogeneration applications, e.g. pressor is the highest energy-consuming component be- Al-Sulaiman et al. [6] carried out a study on the PTC-driven cause this energy is used to increase the coolant pressure. ORC for utilizing waste heat for a cogeneration process. However, more energy use in the cooling system increases They found that during trigeneration, the energy efficiency carbon-dioxide emissions, leading to higher global tem- was increased from 15% to 94%. Al-Sulaiman [7] carried out perature and greenhouse effects. Therefore, renewable- a thermodynamic analysis of the PTC-integrated steam energy technology is now used for refrigeration purposes. Rankine cycle with ORC and R134a as the best working fluid Such renewable energy sources are geothermal, solar, bio- among the other selected working fluids because it pro- mass, etc. [3]. The justification for using renewable energy vided the highest electrical efficiency of 26%. The exergy for cooling is to reduce emissions of carbon dioxide and and energy analysis of the PTC-integrated supercritical power consumption. Moreover, the cogeneration system organic Rankine cycle (SORC) system was examined by of the organic Rankine cycle and vapour-compression Singh and Mishra [8]. Exergetic metrics including the fuel- cycle (ORC–VCC) is a heat-driven power and refrigeration depletion ratio, improvement potential and irreversibility system. The heat-driven cooling system is a clean tech- ratio were found to be 0.579, 11 859 kW and 0.9296, respect- nology and free from pollution. Solar energy is one of the ively. Among the other working fluids tested, R600a was the most promising renewable heat sources because of its low best. Singh and Mishra [9] carried out a study on the PTC- costs, noise-free operation and abundance in nature [4]. driven supercritical carbon-dioxide cycle (sCO )–ORC. They Solar energy is the most suitable for cooling, heating and discovered that solar irradiation increased the thermal and power generating, among other renewable energy sources. exergy efficiency of the combined cycle. R407c was chosen Solar collectors are being used to harvest solar energy as the best working fluid, with a combined system thermal where heat-transfer fluid (HTF) circulates for absorbing the Thermal efficiency (SORC-VCC-PTC) Exergy efficiency (SORC-VCC-PTC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 478 | Clean Energy, 2021, Vol. 5, No. 3 and exergy efficiency of 43.49% and 78.07%, respectively, at ORC–VCC system operated by solar power. They concluded 0.95 kW/m . A PTC-driven partial heating sCO cycle com- that the ice production and the cooling power depended on bined with ORC was studied by Khan and Mishra [10]. They the condensation and generation temperature. R245fa was discovered that solar irradiation enhanced the thermal selected as an appropriate working fluid for this purpose. and exergy efficiency, while the solar incidence angle re- Using R245fa, cooling power and ice production per unit duced performance. Al-Zahrani and Dincer [11] performed metre square collector area were 126.44 W/m and 7.61 kg/ exergy and energy analysis of the PTC-driven sCO cycle to m -day, respectively. Moles et  al. [ 20] proposed a model of convert solar heat into power. They considered the param- the low-temperature heat-activated combined ORC–VCC eters of the PTC as design parameters such as solar irradi- system. They concluded that the computed thermal and ation, receiver emittance and solar-beam incidence angle. electrical COP of the ORC–VCC system varied between 0.30– The exergy and energy efficiency of the PTC were observed 1.10 and 15–110, respectively. Additionally, R1234ze(E) was as 66.35% and 38.51%, respectively. considered an acceptable working fluid for enhanced effi- The SORC is better technology for the conversion of ciencies. Li et  al. [21] performed a working-fluid selection waste heat to power. For example, Song et al. [12] analysed study for the combined ORC–VCC system. They concluded the SORC to recover waste heat at low temperature. They that butane was chosen as the better working fluid for the concluded that the SORC was an appropriate method for ORC–VCC system, with temperature variations ranging industrial waste-heat recovery and the R152a was ob- from 60°C to 90°C, −15°C to 15°C and 30°C to 55°C at boiler served as the best-performing working fluid and produced exit, evaporation and condensation temperatures varying, the maximum net power and thermal efficiency. Kalra respectively. The COP of the complete system was obtained et  al. [13] demonstrated that the operating fluid pressure as 0.47 based on butane at a boiler exit temperature of 90°C. was above its critical pressure in the SORC and found that It has been observed from the above-mentioned litera- the SORC had various benefits over its subcritical ORC be- ture survey that several studies have been conducted on cause the heat-source cooling curve in the SORC matched the ORC–VCC cogeneration system. However, no study was the working-fluid heating curve. A parametric analysis of conducted on the SORC–VCC cogeneration system driven by the SORC system for the medium-temperature geothermal PTCs. The impacts of the PTC design parameters on the co- heat source was undertaken by Moloney et  al. [14]. They generation system have also not been observed in previous came to the conclusion that the SORC was more efficient research. This demonstrates the novelty of the current study. than the ORC for low-temperature heat sources. Saadon The main objective of the present study is to examine the and Islam [15] performed a thermal analysis of the SORC exergetic and energetic performance of the cogeneration and discovered that it had a higher thermal efficiency and SORC–VCC system simultaneously for the cooling and power power output than the conventional ORC with a preheater. generation driven by PTCs. The system-performance param- Also several studies were conducted on the combined eters were considered to be exergy and thermal efficiency, ORC–VCC system in recent years. Pektezel and Acar [16] the COP of the cogeneration system and exergy destruction. analysed the combined ORC and single and double evap- The effects on the system performance of different param- orator VCC systems for exergy and energy. They discovered eters such as solar irradiation, the solar-beam incidence that R600a was the most suitable fluid for the combined angle and the HTF velocity in the absorber tube, condenser system. They also discovered that, with a single evaporator, temperature and evaporator temperature, and turbine inlet the coefficient of performance (COP) and exergy efficiency pressure were examined. Finally, the performance of the co- of the combined cycle were higher than with a dual evap- generation system was compared with and without PTCs. orator. The ORC-integrated VCC system was subjected to an exergy and energy study by Saleh [17]. He came to the 1 System description conclusion that R602 was a good working fluid for system performance and environmental concerns. Finally, he con- Fig. 1 shows the schematic diagram of the SORC–VCC cluded that the highest COP, exergy efficiency and pressure system driven by the PTC. A PTC field, a regenerating SORC ratio of the turbine of the system and the corresponding and a VCC are all part of the system. Because there is a total mass flow rate of the working fluid using R602 were lot of heat energy available at the turbine outlet, regen- 0.99, 53.8%, 12.2 and 0.005  kg/s-kW, respectively, at 25°C eration of the SORC is used in this study. With the help of condenser temperature and maintaining other parameters a recuperator, this heat is utilized to warm the incoming as constant. Javanshir et  al. [ 18] performed thermal and stream to the heat exchanger. Also, the SORC is beneficial exergoeconomic analyses on the regenerative ORC–VCC for maximum power production [13]. This system uses two system. Their results indicated that among other selected fluids: a single HTF in the solar field and another fluid in working fluids, R134a showed the lowest exergy and thermal both the SORC and VCC subsystems. However, the SORC efficiencies while R143a and R22 showed the highest exergy and VCC subsystems have different mass flow rates. There and energy efficiency. They also observed that the total unit is a common condenser for both subsystems. For control- cost of the product was found to be $60.7/GJ. The maximum ling the mass flow rate to both of the subsystems, two flow exergy destruction was found in the boiler followed by the regulators and a common condenser for both cycles are turbine. Hu et  al. [19] investigated the performance of an used. To control the mass flow rate to both cycles, two flow Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 479 PTCs fields SORC pump Solar pump Flow regulators HX 4 Turbine Condenser Expansion valve SORC VCC Recuperator 5 Mixer Compressor Evaporator 13 17 16 Fig. 1: Schematic of the PTC-driven SORC–VCC cogeneration system system has already been conducted by Al-Sulaiman [7]. Table 1: Input conditions for simulation of the proposed Modelling of the system was performed using the exergy model and energy balance in each component. Results are calcu- Parabolic trough collector LS-3 [22] lated with help of the Engineering Equation Solver (EES) Maximum exit temperature of HTF 390°C [7] [23] software. Maximum exit pressure of HTF 10 MPa HTF mass flow rate 0.575 (kg/s) Atmospheric pressure 1.013 bar Atmospheric temperature 25°C 2.1 Thermal modelling of the SORC–VCC system Effectiveness heat exchanger 0.95 [7] While solving equations, the following assumptions were SORC turbine’s isentropic efficiency 0.87 [18] considered: (i) steady-state conditions are assumed for all Compressor’s isentropic efficiency 0.8 [18] thermodynamic processes; (ii) friction and pressure losses SORC pump’s isentropic efficiency 0.8 [18] are negligible in each component and pipes; (iii) heat loss SORC’s mass flow rate 1.5 kg/s to the surroundings is negligible in each component ex- VCC’s mass flow rate 1 kg/s cept the condenser unit; (iv) the working-fluid conditions Turbine inlet pressure 50 bar Outlet pressure to the compressor/turbine 6.018 bar at the entry to the compressor and exit to the condenser Condenser temperature 35°C [17] in the VCC subsystem are saturated vapour and saturated Effectiveness of recuperator 0.95 [7] liquid, respectively; (v) the PTCs have 50 collectors in series Evaporator temperature 0°C [17] in a single row and there are a total of seven rows and the length of each collector is 12.27 m [7]. Modelling equations for the cogeneration (SORC–VCC) regulators are used. The fluid flow directions are shown by system are adapted from the previous studies [13, 18]. the arrows. The main advantage of this combined SORC– The total solar heat available in the SORC–VCC cogener - VCC system is that it produces cooling when power is not ation system is determined as: needed or vice versa. The advantage of using a combined SORC–VCC system produces cooling when refrigeration is Q = m · (h − h ) (1) HX SORC 4 10 not needed. Therefore, this system is beneficial throughout m ˙ where is the working-fluid mass flow rate in the SORC the year [17]. A solar field with a few hundred PTCs is used. SORC subsystem. The type of collector and other input data are given in The work done by the turbine is given as: Table 1. The T-s diagram has been displayed in Fig. 2. W = m · (h − h ) · η SORC 4 5s (2) Turbine turbine The heat recovered in the in the recuperator is given by the 2 Thermodynamic analysis heat balance equation: The mathematical modelling of the SORC–VCC system ˙ ˙ has been conducted in this section. Modelling of the PTC m · (h − h )= m · (h − h )(3) SORC 6 5 SORC 10 9 Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 480 | Clean Energy, 2021, Vol. 5, No. 3 PTC heat source SORC Tc VCC Te Entropy Fig. 2: T-s diagram for the PTC-driven SORC–VCC cogeneration system If the same working fluid is circulating having the same Furthermore, when the system is brought to dead condi- mass flow rate on the hot and cold sides of the recuperator, tions (298.15 K temperature, 1.013 bar pressure), the max- then the effectiveness of the recuperator is given as: imum theoretical work obtained is termed as exergy. On (T − T ) Actual heat transfer the basis of the control volume, the exergy balance equa- 5 6 ε = = (4) (T − T ) Maximum heat transfer tion is determined as: 5 9 Å ã The rejection of heat by the condenser unit is given as: 0 1 − Q − W − (m Ex ) − (m Ex ) − Ed = 0 Q c.v i i e Q =(m ˙ + m ˙ ) · (h − h ) (5) cond SORC VCC 8 7 (13) where subscript Q denotes the thermal property numeric- where m is the mass flow rate of the working fluid in VCC ally available at a given stage and Ed refers to the exergy- the VCC subsystem. destruction rate. Furthermore, Ex signifies the specific Applying the energy-balance equation in the mixer: system exergy. Without considering the chemical exergy, ˙ ˙ ˙ ˙ (m + m ) · h = m · h + m · h(6) SORC VCC 7 SORC 6 VCC 14 potential energy and kinetic energy, the physical exergy The compressor work is given by: can then be assessed as [7, 25]: ˙ Ex =(h − h ) − T (s − s )(14) m · (h − h ) ph 0 0 0 VCC 14s 13 W = (7) comp comp The exergy inlet to the system and obtained from solar ra- diation by Petela’s formula [26] is described as: where h is the enthalpy at state 14 when the isentropic 14s work is to be done. ñ ô Å ã Å ã 1 T 4 T 0 0 Further, the heat absorbed by the evaporation or cooling Ex = A · G · 1 + − (15) s p b 3 T 3 T su su effect can be given as: where T is the temperature of the Sun, i.e. 5800 K [7]. su Q = m · (h − h ) (8) evap VCC 13 15 Ex evap can be defined as [18]: The SORC pump work can be written as: Å ã Ex =Q · − 1(16) evap evap m · (h − h ) T SORC 9s 8 e W = (9) SORC pump SORC pump where T is the evaporator temperature. The exergy efficiency of the SORC–VCC congregation The thermal efficiency of the cogeneration system (SORC– system is defined as [18]: VCC) is determined as [18]: ˙ ˙ W + Q W + Ex net evap net evap η = η = (17) ex th (10) Ex HX S The exergy efficiency of the overall system (SORC–VCC– ˙ ˙ ˙ ˙ W = W − W − W net turbine compressor SORC pump(11) PTC) is shown as [24]: The overall thermal efficiency of the system is calculated as [24]: Ed Total η = 1 − (18) ex,overall η = η · η th,overall ex,overall Carnot(12) Ex Temperature Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 481 where Ed is the total exergy destruction of the in the study of Mwesigye et  al. [27]. The working fluids in Total overall system. the SORC system should be non-toxic, environmentally Another performance parameter to be defined is the friendly, economical, able to use heat energy from the COP of the SORC–VCC cogeneration system (COP ).This source of waste heat and also, at optimum pressure and can be defined as [17]: temperature, the thermophysical and thermodynamic properties should be optimal [28]. Due to these reasons, evap fluids such as R227ea, R236fa, R245fa, R1234ze and R134a COP = η · COP = S SORC vcc (19) ˙ ˙ Q + W HX SORC pump were selected. The thermophysical properties of the con- sidered working fluids are shown in Table 2. η COP where and vcc are the efficiency of the SORC SORC and the COP of the vapour-compression subsystem, respectively. 2.3 Verification of the model The current model was validated using previous studies. 2.2 Working-fluid selection The PTC as well as the cogeneration SORC–VCC system has Working-fluid selection is challenging because it affects been separately verified. Verification with the change in the performance and economic feasibility of the system. heat loss in the absorber tube with the temperature change It is good if the critical temperatures of the working fluid above the ambient temperature and exit temperature of are near the temperature of the heat source and the tem- the PTC have been shown in Tables 3 and 4, respectively, at perature of the waste [17]. The HTF selected in the current the same baseline situations. There is <1% deviation from the study is Therminol-VP1 in the absorber tube of the solar experimental results. It is better agreement than in the collector to absorb the heat of the Sun because this syn- previous theoretical study by Al-Sulaiman [7] and the ex- thetic oil has excellent heat-transfer properties as well perimental study by Dudley et al. [22]. Apart from this, the as good temperature stability compared with other HTFs thermal efficiency of the SORC system has been validated such as molten salt and a maximum working temperature by the study by Le et al. [29] under the same baseline condi- of 400°C [27]. Its thermophysical properties were discussed tions as given in Table 5. Table 6 shows the COP variation of Table 2: Thermophysical properties of the working fluids [16, 17] Working fluids Weights (g/mole) Critical temperature (°C) Critical pressure (MPa) ODP GWP R227ea 170.03 101.8 2.925 0 3580 R236fa 152.04 124.92 3.2 0 680 R245fa 134.05 154.1 3.65 0 1050 R1234ze 104.04 109.36 3.635 0 <1 R134a 102.03 101 4.059 0 1430 Table 3: Heat loss (W/m) variation with temperature difference Temperature difference (°C) Current model Dudley et al. [22] Al-Sulaiman [7] 100.6 9.78 10.6 8.719 149.1 16.24 19.3 19.3 196.7 31.15 30.6 34.2 245.8 53.48 45.4 53.0 293.3 66.69 62.9 75.5 Table 4: Verification of the exit temperature of the PTC with experimental results T (K) 2 3 Cases G (W/m ) Discharge (m /s) T (K) T (K) Dudley et al. [22] Current model Error (%) b 0 i 1 933.7 47.7 294.35 375.35 397.15 397.56 0.10 2 968.2 47.8 295.55 424.15 446.45 446.72 0.06 3 982.3 49.1 297.45 470.65 492.65 492.02 0.12 4 909.5 54.7 299.45 523.85 542.55 541.99 0.10 5 937.9 55.5 299.35 570.95 589.55 590.11 0.09 6 880.6 55.6 301.95 572.15 590.35 590.26 0.01 7 903.2 56.3 300.65 529.05 647.15 647.35 0.03 8 920.9 56.8 304.25 652.65 671.15 671.51 0.05 Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 482 | Clean Energy, 2021, Vol. 5, No. 3 Table 5: Verification of the SORC system Thermal efficiency (%) Baseline conditions Literature Literature Current model Error (%) R134a, heat-source temperature = 139°C Le et al. [29] 12 11.99 –1 other selected working fluids. Near the critical conditions, Table 6: COP variation of the VCC system with the evaporator the working fluid performs better. The exergy efficiency of average temperature the cogeneration system also improved with the solar ir - COP radiation. The reason behind this is that, as the irradiation increases, the inlet exergy to the cogeneration system in- Evaporator average Krishnan temperature (°C) Current model et al. [30] creases according to Equation (20) and simultaneously the exergy destruction decreases. R227ea has the highest 13 2.85 2.99 exergy efficiency ranging from 86.38% to 92.9% at 0.5–0.95 13.5 3.22 3.12 kW/m , respectively. 14 3.33 3.45 The overall system (PTC-SORC–VCC) exergy and 14.5 3.82 3.95 thermal efficiency were also examined. It is observed 15 3.98 4.02 from Fig. 4 that the overall system thermal efficiency in- 15.5 4.10 4.21 16 4.42 4.38 creases with the solar irradiation due to the same reason as above. R227ea and R134a, among the other selected working fluids, gave respectively the highest and lowest the VCC system with the evaporator average temperature thermal efficiency. Using R227ea, the solar irradiation in- and validated by the experimental study by Krishnan et al. creases from 0.5 to 0.95 kW/m, which results in a nearly [30]. Table 7 shows that the thermal efficiency of the SORC– 58.88% increase in the thermal efficiency. The rise in solar VCC cogeneration system matches that from the study by irradiation results in an increase in the overall system Javanshir et  al. [18]. Therefore, the current model is ready exergy efficiency. As the values of the solar irradiation for analysis. vary from 0.5 to 0.95 kW/m, the exergy efficiency based on R227ea improves by almost 58.90%, as illustrated in Fig. 4. 3 Results and discussion The variation in solar irradiation also has a significant In the present study, a thermal performance analysis of effect on the COP , as denoted in Fig. 5. Fig. 5 shows that the PTC-driven SORC–VCC cogeneration system was con- the COP is a function of the solar irradiation. As the solar ducted. The effects of solar irradiation (G ), solar-beam b irradiation increases, this leads to an increase in the COP . incidence angle, HTF velocity in the absorber tube, tur - Increased solar irradiation has no effect on the VCC sub- bine inlet pressure (TIP), condenser temperature () and T c system, which means no effect on the COP . However, an VCC evaporator temperature (T ) on thermal performance were e increase in solar irradiation leads to an increase in η [8]. SORC examined. While investigating the effects of the param- The COP increases with solar irradiation based on Equation eters, other parameters were kept constant as listed in (19). R134a has the highest COP and R227ea the lowest among Table 1. A computer program was made using EES software the considered working fluids. This means that if this system to solve the modelling equations. only works for cooling purposes, R134a is more effective than R277ea. Using the working fluid R134a, as the solar irradiation varied from 0.5 to 0.95 kW/m, the COP increased by nearly 3.1 Performance evaluation with solar 7.69%. It is also shown that the exergy destruction decreases irradiation with the solar irradiation. It has the reverse trend to that of the exergy efficiency, as explained above. R134a and R227ea gave Solar irradiation, also termed as the direct normal irradi- the highest and lowest exergy destruction as shown in Fig. 5. ation, is a primary parameter to be examined. The vari- R134a gave exergy-destruction decreases from 3966 to 2060 ation in solar irradiation was taken as 0.5–0.95 kW/m as kW when the solar irradiation varied from 0.5 to 0.95 kW/m , per the climate of Mumbai, India. Variation in solar irradi- respectively. Furthermore, without the solar SORC–VCC, the ation has a large impact on the system performance ran- system is more thermodynamically efficient than with solar. ging from 0.5 to 0.95 kW/m. The thermal performance of the SORC–VCC cogeneration system increases with solar irradiation due to the effective utilization of collector rows 3.2 Performance evaluation with solar as shown in Fig. 3. Among the selected working fluids, incidence angle R227ea gives the highest thermal efficiency that varies from 47.55% to 51.13% at 0.5–0.95 kW/m, respectively, be- Fig. 6 shows that the thermal and exergy efficiency of the cause R227ea has the lowest critical pressure compared to cogeneration system decreases with the increase in the Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 483 Table 7: Verification of the cogeneration SORC–VCC system Thermal efficiency (%) Baseline conditions Literature Literature Current model Error (%) R143a, TIP = 3600 kPa, heat-source temperature = 150°C Javanshir et al. [18] 27.21 27.98 2.49 0.52 0.94 η (R227ea) th 0.92 η (R236fa) th 0.50 η (R245fa) 0.90 th η (R1234ze) th 0.88 η (R134a) th 0.48 0.86 η (R227ea) ex η (R236fa) 0.84 ex 0.46 η (R245fa) ex 0.82 η (R1234ze) ex 0.80 η (R134a) ex 0.44 0.78 0.5 0.6 0.7 0.8 0.9 1.0 G (kW/m ) Fig. 3: Variation in the efficiency of the SORC–VCC cogeneration system with solar irradiation 0.7 0.35 η (R227ea) th 0.6 η (R236fa) 0.30 th η (R245fa) th 0.5 0.25 η (R1234ze) th η (R134a) th 0.4 0.20 η (R227ea) ex η (R236fa) ex 0.3 0.15 η (R245fa) ex η (R1234ze) 0.2 ex 0.10 η (R134a) ex 0.1 0.05 0.5 0.6 0.7 0.8 0.9 1.0 G (kW/m ) Fig. 4: Variation in the efficiency of the overall system (SORC–VCC–PTC) with solar irradiation solar incidence angle. An increase in the incidence angle (19), the COP decreases with the solar incidence angle. reduces the exergetic, energetic and optical efficiencies The highest and lowest COP were found with R134a and of the PTCs [11] and leads to system-performance reduc- R227ea, respectively. The COP decreased from 2.6 to 1.3 tion. The maximum and minimum thermal efficiency as the incidence angle increased from 3° to 30°, respect- and exergy efficiency were found for R227ea and R134a, ively, whereas the exergy destruction increased with the respectively. The thermal and exergy efficiency de- solar incidence angle, as shown in Fig. 7. It has been dis- creased by 51.02–50.57% and 92.70–91.88% with increases cussed above that the exergy efficiency decreased with in the solar incidence angle from 3° to 30°, respectively, the solar incidence angle, which is why the exergy de- based on R227ea. Therefore, it is important to minimize struction showed a reverse trend. The exergy destruction the solar incidence angle to obtain better performance for all other selected working fluids was found to be be- of the system. Also, the COP decreased with the solar tween these of the fluids R227ea and R134a. As the solar incidence angle as shown in Fig. 7. The solar incidence incidence angle increased from 3° to 30°, the exergy de- angle does not affect the COP . However, it does affect struction increased from 3966 to 2060 kW on the basis of vcc the efficiency of the SORC. Thus, according to Equation R134a, as seen in Fig. 7. Thermal efficiency (SORC-VCC) Thermal efficiency (SORC-VCC-PTC) Exergy efficiency (SORC-VCC-PTC) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 484 | Clean Energy, 2021, Vol. 5, No. 3 2.3 4500 COP (R227ea) 2.2 COP (R236ea) COP (R245fa) COP (R1234ze) 2.1 COP (R134a) Ed (R227ea) Ed (R236fa) 2.0 Ed (R245fa) Ed (R1234ze) Ed (R134a) 1.9 0.5 0.6 0.7 0.8 0.9 1.0 G (kW/m ) Fig. 5: Variation in the COP with solar irradiation 0.52 0.93 η (R227ea) 0.51 th 0.92 η (R236fa) th η (R245fa) th 0.51 η (R1234ze) th 0.91 η (R134a) th 0.50 η (R227ea) ex 0.90 η (R236fa) ex 0.50 η (R245fa) ex η (R1234ze) 0.89 ex 0.49 η (R134a) ex 0.49 0.88 05 10 15 20 25 30 Angle of incidence (degree) Fig. 6: Variation in the efficiency of the SORC–VCC cogeneration system with solar incidence angle 2.8 2400 2.6 COP (R227ea) 2.4 COP (R236fa) 2.2 COP (R245fa) COP (R1234ze) 2.0 s COP (R134a) 1.8 Ed (R227ea) Ed (R236fa) 1.6 Ed (R245fa) Ed (R1234ze) 1.4 Ed (R134a) 1.2 1.0 1400 Angle of incidence (degree) Fig. 7: Variation in the COP with the solar incidence angle 3.3 Effects of HTF velocity in the absorber tube the HTF velocity in the absorber tube. The increase in the on system performance efficiency with the velocity is due to the increase in the velocity of the fluid. The Reynolds number is increased Fig. 8 displays that the thermal and exergy efficiency of as a result of the increase in the convective heat-transfer the SORC–VCC cogeneration system has increased with COP COP Thermal efficiency (SORC-VCC) Exergy destruction (SORC-VCC) (kW) Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 485 coefficient; therefore, much heat is carried by the HTF. 3.4 Effects of the TIP on system performance This leads to an increase in efficiency. Among the The system performance also depends on the TIP. As operating fluids considered, R227ea and R134a were re- shown in Fig. 10, the efficiency of the cogeneration system ported to have the greatest and smallest thermal and en- increased slightly with the TIP. The TIP variation had no ergy efficiency. Thermal and exergy efficiency increased impact on the VCC subsystem performance. The VCC sub- from 47.55% to 51.13% and from 86.38% to 92.9% when system does not contribute to the efficiency of the SORC– the velocity increased from 0.01 to 0.1 m/s on the basis VCC cogeneration system. Therefore, the efficiency of the of R227ea. The COP also increased with the velocity of cogeneration system depends only on the performance of the HTF. As the velocity increased, the efficiency of the the SORC subsystem. It is due only to the η because the SORC SORC was enhanced. However, there was no effect on the η increases with the TIP [8]. It is observed that R227ea SORC COP . Therefore, according to Equation (19), the COP vcc s and R134a provided the maximum and minimum thermal increased with the HTF velocity as seen in Fig. 9. The efficiency of the cogeneration system. In the case of the highest value of the COP was found with R134a followed R227ea, the TIP increase from 40 to 55 bar improved the by R245fa, R236fa, R1234ze and R227ea. The exergy de- cogeneration system thermal efficiency by almost 0.85%. struction of the SORC–VCC cogeneration system was also From Fig. 10, it can be seen that as the TIP increased, the investigated with the HTF velocity in the absorber tube. exergy efficiency of the cogeneration system also in- The exergy destruction decreased with the HTF velocity. creased. The destruction of exergy decreases with the TIP The highest exergy destruction was identified with R134a led to exergy-efficiency improvement [8]. Using R227ea, the followed by R1234ze, R245fa, R236fa and R227ea with increase in the TIP from 40 to 55 bar improved the exergy values of 3966, 3673, 3515, 3241 and 2757 kW, respectively, efficiency from 91.82% to 92.60%. at an HTF velocity of 0.01 m/s, as shown in Fig. 9. 0.52 0.94 0.92 η (R227ea) th η (R236fa) th 0.50 0.90 η (R245fa) th η (R1234ze) 0.88 th η (R134a) th 0.48 0.86 η (R227ea) ex η (R236fa) ex 0.84 η (R245fa) ex 0.46 0.82 η (R1234ze) ex η (R134a) ex 0.80 0.44 0.78 0.00 0.02 0.04 0.06 0.08 0.10 HTF velocity in absorber tube (m/s) Fig. 8: Variation in the efficiency of the SORC–VCC cogeneration system with HTF velocity 2.3 4500 2.2 COP (R227ea) COP (R236fa) COP (R245fa) 2.1 COP (R1234ze) COP (R134a) Ed(R227ea) Ed(R236fa) 2.0 2000 Ed(R245fa) Ed(R1234ze) Ed(R134a) 1.9 0.02 0.04 0.06 0.08 0.10 HTF velocity in absorber tube (m/s) Fig. 9: Variation in the COP with the HTF velocity in the absorber tube COP Thermal efficiency (SORC-VCC) Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 486 | Clean Energy, 2021, Vol. 5, No. 3 In addition to the cogeneration system performance, efficiency increased with the TIP and, consequently, the the performance of the overall system was also investi- exergy destruction decreased with the TIP. The highest gated and the overall system (SORC–VCC–PTC) thermal and lowest exergy were obtained with the working fluids efficiency was found to increase with the TIP, as seen in R134a and R227ea, respectively, although the exergy de- Fig. 11. The exergy efficiency of the overall system also struction for fluids R236fa, R245fa and R1234ze was found depended on the TIP. The exergy efficiency of the overall between these two. Further, comparing Fig. 10 with Fig. 11, system improved with the TIP. The TIP did not affect the the performance of the PTCs combined system perform- PTC and VCC subsystems. As a result, the exergy efficiency ance was reduced. It does not mean that the solar system was only enhanced by the SORC subsystem. In the case of is worse overall, but only from the efficiency point of view. the working fluid R227ea, the TIP increases from 40 to 55 bar improved the exergy efficiency by 5.5%, as shown in 3.5 Performance evaluation with condenser the right axis of Fig. 11. temperature The COP is also a function of the TIP. The COP slightly S S increased with the TIP. The reason behind this is that the The condenser temperature also has an impact on both variation in the TIP had no effect on the COP , although subsystems. As the condenser temperature increased, VCC η increased with the TIP [8 , 18], from Equation (19), this led to a slight decrease in the exergy and thermal ef- SORC the COP increased. R134a and R227ea show the highest ficiency of the cogeneration system (SORC–VCC), as shown and lowest COP , respectively. If the TIP increased from 40 in Fig. 13. Improvement in the condenser temperature re- to 55 bars, the COP increased by nearly 1.43%, as shown duced the enthalpy drop in the turbine leading to the low er in Fig. 12. Fig. 12 also shows that the exergy destruction output work. Therefore, the thermal efficiency of the SORC decreased with the TIP. As already discussed, the exergy and COP system was reduced. Consequently, the thermal vcc 0.52 0.93 η (R227ea) th 0.51 0.92 η (R236fa) th η (R245fa) th 0.51 0.91 η (R1234ze) th η (R134a) th 0.50 0.90 η (R227ea) ex η (R236fa) ex 0.50 0.89 η (R245fa) ex η (R1234ze) ex 0.49 0.88 η (R134a) ex 0.49 0.87 40 42 44 46 48 50 52 54 56 TIP (bar) Fig. 10: Variation in the efficiency of the SORC–VCC cogeneration system with the TIP 0.62 0.33 η (R227ea) 0.60 th η (R236fa) 0.32 th 0.58 η (R245fa) th 0.31 η (R1234ze) 0.56 th η (R134a) th 0.30 0.54 η (R227ea) ex 0.29 η (R236fa) ex 0.52 η (R245fa) ex 0.28 0.50 η (R1234ze) ex η (R134a) ex 0.27 0.48 0.26 0.46 40 42 44 46 48 50 52 54 56 TIP (bar) Fig. 11: Variation in the efficiency of the overall system (SORC–VCC–PTC) with the TIP Thermal efficiency (SORC-VCC-PTC) Thermal efficiency (SORC-VCC) Exergy efficiency (SORC-VCC-PTC) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 487 performance of the cogeneration system was reduced. The Finally, in this section, the impact of the condenser tem- exergy efficiency of the cogeneration system also decreased perature variation on the system COP was also discussed, as with the condenser temperature because, according to shown in Fig. 15. In the case of R134a, as the condenser tem- Equation (12), the exergy efficiency is directly related to perature varied from 303 K to 328 K, the COP decreased by the thermal efficiency, as revealed in Fig. 13. Furthermore, 49.8%. It is already known that the η decreases with the SORC the thermal efficiency of the overall system also depends condenser temperature. However, the temperature, pressure on the condenser temperature. As the temperature of the and enthalpy increase with the condenser temperature at condenser increased from 303 K to 328 K, the thermal ef- the outlet of the compressor. This results in a reduction in the ficiency decreased by almost 14.07%, as shown in Fig. 14. COP . As a result, the COP decreases according to Equation vcc This was due only to the performance of the SORC–VCC (19). R134a and R227ea show the highest and lowest COP , re- system, not to that of the PTC system. The effect of the spectively. It can also be seen in Fig. 15 that if this system only condenser temperature on the exergy efficiency of the works for the purpose of cooling, then R134a is selected as overall system (SORC–VCC–PTC) was discussed. The exergy the appropriate working fluid among other selected working efficiency also decreased with the condenser temperature. fluids. The exergy destruction increases with the condenser It is already known that thermal efficiency decreases with temperature. The exergy efficiency slightly increases with the temperature of the condenser. Hence the exergy effi- condenser temperature, therefore the exergy decreases with ciency also decreased with the condenser temperature by the condenser temperature. The highest exergy destruction Equation (12). The efficiencies of the other fluids are be- was found with R134a, followed by R1234ze, R245fa, R236fa tween those of the R227ea and R134a fluids, as shown in and R227ea of 2347, 2176, 2061, 1932 and 1644 kW, respect- Fig. 14. ively, as displayed in right axis of Fig. 15. 2.30 2600 2.25 COP (R227ea) COP (R236fa) 2.20 COP (R245fa) COP (R1234ze) 2.15 s COP (R134a) 2000 s Ed (R227ea) 2.10 Ed (R236fa) Ed (R245fa) 2.05 Ed (R1234ze) Ed (R134a) 2.00 1.95 1400 40 42 44 46 48 50 52 54 56 TIP (bar) Fig. 12: Variation in the COP with the TIP 0.52 0.93 η (R227ea) th 0.51 0.92 η (R236fa) th η (R245fa) th 0.51 η (R1234ze) th 0.91 η (R134a) th 0.50 η (R227ea) ex 0.90 η (R236fa) ex 0.50 η (R245fa) ex η (R1234ze) ex 0.89 0.49 η (R134a) ex 0.88 0.49 300 305 310 315 320 325 330 T (K) Fig. 13: Variation in the efficiency of the SORC–VCC cogeneration system with the condenser temperature Thermal efficiency (SORC-VCC) COP Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 488 | Clean Energy, 2021, Vol. 5, No. 3 0.38 0.66 η (R227ea) 0.64 th 0.36 η (R236fa) th 0.62 η (R245fa) th 0.34 0.60 η (R1234ze) th 0.58 η (R134a) 0.32 th 0.56 η (R227ea) ex 0.30 η (R236fa) 0.54 ex η (R245fa) ex 0.52 0.28 η (R1234ze) ex 0.50 η (R134a) ex 0.26 0.48 0.24 0.46 300 305 310 315 320 325 330 T (K) Fig. 14: Variation in the efficiency of the overall system (SORC–VCC–PTC) with the condenser temperature 2.8 2400 2.6 COP (227ea) 2200 s 2.4 COP (236fa) 2.2 COP (245fa) COP (R1234ze) 2.0 COP (R134a) 1.8 Ed (227ea) Ed (R236fa) 1.6 Ed (R245fa) Ed (R1234ze) 1.4 Ed (R134a) 1.2 1.0 1400 305 310 315 320 325 330 T (K) Fig. 15: Variation in the COP with the condenser temperature behind this is that the COP increases with the evapor - 3.6 Performance evaluation with evaporator vcc temperature ator temperature [17]. Also, the saturation pressure of the evaporator improves with the improvement of the evap- Furthermore, the effect of the evaporator temperature orator temperature for all working fluids leading to a de- was also examined. The cooling capacity is directly af- cline in the compressor work with constant condenser fected by the evaporator temperature. During examining temperature. Conversely, the refrigeration effect improves the evaporator-temperature effect on the system perform- with the evaporator temperature. Both impacts enhance ance, all other assumed parameters are kept constant, as the COP . However, simultaneously, the η is not af- SORC vcc listed in Table 1 such as the condenser temperature and fected by the evaporator temperature. As per Equation (19), pressure, evaporator pressure, etc. F ig. 16 reveals that the this resulted in improvement in the COP , as shown in Fig. efficiency is not significantly affected by the evaporator 17. Also, R134a and R227ea showed the highest and lowest temperature because there was no impact of the evapor - values, respectively. Regarding the exergy destruction of ator temperature on the SORC subsystem performance. the cogeneration cycle, the exergy destruction also was not The working fluids R227ea and R134a gave the highest affected much by the evaporator temperature like the effi- and lowest efficiency, respectively. Further to Fig. 16, it is ciencies, as discussed above. At a constant evaporator tem- noticed that the evaporator temperature also did not sig- perature, the highest and lowest exergy destruction was nificantly affect the exergy efficiency of the cogeneration for R134a and R227ea, respectively, whereas the exergy de- system because the exergy efficiency has as direct relation struction for the R245fa was significantly affected due to COP with the thermal efficiency [31]. However, the was different thermophysical properties of R245fa, as shown in affected by the evaporator temperature significantly and the right axis of Fig. 17. it increased with the evaporator temperature. The reason COP Thermal efficiency (SORC-VCC-PTC) Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC-PTC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 Khan and Mishra | 489 0.51 0.93 η (R227ea) th 0.51 0.92 η (R236fa) th η (R245fa) th η (R1234ze) th 0.50 0.91 η (R134a) th η (R227ea) ex 0.50 0.90 η (R236fa) ex η (R245fa) ex η (R1234ze) ex 0.49 0.89 η (R134a) ex 0.49 0.88 255 260 265 270 275 280 285 290 T (K) Fig. 16: Variation in the efficiency of the SORC–VCC cogeneration system with the evaporator temperature 4.0 2400 3.5 COP (R227ea) COP (R236fa) 3.0 COP (R245fa) COP (R1234ze) COP (R134a) 2.5 Ed (R227ea) Ed (R236fa) 2.0 Ed (R245fa) Ed (R1234ze) Ed (R134a) 1.5 1.0 1400 255 260 265 270 275 280 285 290 T (K) Fig. 17: Variation in the COP with the evaporator temperature Table 8: Exergy-destruction rate (kW) for each component 3.7 Exergy destruction in each component In addition to the above parameters, the exergy destruc- Components R227ea R236fa R245fa R1234ze R134a tion for each component and for each working fluid has Solar field 6894 7265 7253 8006 8067 been also evaluated, as revealed in Table 8. On the basis HX 218.3 309.1 311.8 288.5 312.8 of R134a, a major part of the energy destruction rate was Turbine 1092 1216 1361 1596 1787 found in the PTCs followed by the turbine and heat ex- Recuperator 46.3 52.62 26.99 24.21 17.24 changer with values of 8067, 1787 and 312.8 kW, respect- Condenser 23.03 64.64 130.5 15.51 22.01 ively. It can be seen that PTCs alone accounted for 76.32% Mixer 181.7 191.9 172.5 175.9 167.6 of the total exergy destruction of the overall system. The Compressor 170.3 186.1 153.9 159.5 147.2 total exergy input into the cogeneration system was cal- EX valve 2.937 3.088 3.109 3.509 3.695 culated at ~20250 kW. It is therefore shown that 39.83% Evaporator 15.36 21.41 7.783 24.9 27.54 ORC pump 16.47 10.87 18.31 19.19 17.61 of the total inlet exergy was destroyed in the solar field only. This is due to the high exergy losses in the PTCs as well as the high outlet and inlet temperature difference of was ~3.695 kW only. Among the other considered working the HTF stream of the collector. Therefore, the PTC is the fluids, R227ea showed the lowest exergy-destruction rate. worst subsystem in terms of exergetic performance in the R227ea is superior because this has a more supercritical current model. Therefore, it is necessary to design the PTC operating range due to its low critical values compared system carefully. It can be also observed from Table 8 that with other selected fluids, as listed in Table 2. The optimal the expansion valve was responsible for the lowest exergy- parameters are summarized in Table 9. destruction rate among the other components and that it COP s Thermal efficiency (SORC-VCC) Exergy destruction (SORC-VCC) (kW) Exergy efficiency (SORC-VCC) Downloaded from https://academic.oup.com/ce/article/5/3/476/6375889 by DeepDyve user on 28 September 2021 490 | Clean Energy, 2021, Vol. 5, No. 3 Table 9: Summary of values of optimal parameters at 0.95 kW/ Nomenclature m  solar irradiation Q Heat transfer (kW) T Evaporator Parameters Values temperature (K) Optimum thermal efficiency 51.13% s Specific entropy η Thermal th Optimum exergy efficiency 92.9% (kJ/kg-K) efficiency Optimum COP of system 2.27 Ed Exergy-destruction m Mass flow rate HTF Best working fluid for power generation R227ea rate (kW) of HTF (kg/s) Best working fluid for cooling R134a G Solar irradiation T Condenser b c (kW/m ) temperature (K) 4 Conclusions A Aperture area (m) H Specific en- In the present study, an exergy and energy analysis of the thalpy (kJ/ PTC-driven SORC–VCC cogeneration system was conducted. kg) The following conclusions were made from the results: T Atmospheric η Exergy ex temperatures efficiency • In Mumbai, India, the exergy efficiency, thermal effi- T Temperature (K) T Temperature su ciency and COP of the SORC–VCC cogeneration system of the Sun increased with solar irradiation and HTF velocity in the (K) absorber tube, but declined with the solar-beam inci- W Work (kW) dence angle. As a result, in order to increase the per - Subscripts formance of the cogeneration system, the PTCs must be am ambient i Inlet carefully designed. cond condenser o Outside • The maximum values of exergy efficiency, thermal effi- comp compressor rec Recuperator evap evaporator e Exit ciency and COP of the SORC–VCC cogeneration system s solar 0 environmental were 92.9%, 51.13% and 2.27, respectively, using R227ea 2 conditions and R134a at 0.95 kW/m of solar irradiation. At 0.95 kW/ m , the overall system exergy efficiency and thermal ef- VCC vapour compres- ficiency were 62.78% and 34.5%, respectively. sion cycle • The exergy efficiency, thermal efficiency and COP de- creased with the condenser temperature and increased with the TIP. The COP also increased continuously with Acknowledgements the evaporator temperature and reached a maximum Y.K. acknowledges the support of Department of Mechanical, value of 3.764 at 328 K for R134a. 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Journal

Clean EnergyOxford University Press

Published: Sep 1, 2021

References