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More than half of the world’s population currently lives in cities. An essential constituent of future sustainable cities is energy efficient and ecologically sound buildings which ensure high levels of comfort and convenience without reducing the standards of living. At present, a significant part of primary fossil fuels is spent for heating/cooling of buildings, thus, greatly contributing to total GHG emissions. In this paper, typical heat losses in dwellings are considered taking the United Kingdom and the Russian Federation as examples. The role of adsorption-based technologies for more rational use of heat in buildings is discussed. Fundamentals of inter-seasonal adsorptive heat storage (AHS) are briefly considered. A tentative upper limit of the AHS storage density is estimated. Current practice of inter-seasonal AHS and novel smart adsorbents promising for this emerging technology are overviewed. Since a portion of the heat losses in ventilation system significantly increases in modern buildings, a new approach to regenerating heat and moisture in this system is discussed. Finally, optimization trends of the AHS in buildings are briefly considered. Keywords: Sustainable city, Low-energy building, Adsorptive transformation of heat, Waste heat, Energy efficiency, Heat storage, Heat regeneration, Nanotailoring new adsorbents, Moisture recovery Review heat consumption in buildings (heating and hot water Introduction production, HHWP) is responsible for 24 % of the total Past centuries are characterized by persistent growth of energy consumption of the EU-27 (2008) [7]. Forty five urban population. At present, more than half of the percent of total UK final energy consumption for 2012 world’s population currently lives in cities [1]. It is pre- was for heating purposes; it being known that more than dicted that by 2050 the urban population will reach 70 % half (24 %) was used for domestic heating [8]. A one [2]. As the major part of population live in cities, future fourth of this amount is additionally used for domestic cities must meet severe demands imposed by a sustain- hot water [8, 9]. According to the Parliament Committee ability concept, which includes several interrelated areas, [10], residential GHG emissions account for 66 % of such as, energy, ecology, economics, politics and culture buildings emissions. The building sector in Canada [3–5]. In this review, the first domain is mainly accounts for about 30 % of total GHG emissions [11]. In addressed and energy efficient buildings, which are an Germany, the share of households in total final energy essential constituent of future sustainable cities, are con- consumption in 2011 reaches 25 % or 52 Mtoe (million sidered. In order such buildings to ensure high levels of tonnes of oil equivalent), space heating being dominated comfort and convenience without necessarily sacrificing (72 % of the final households energy consumption [12]). the standards of living, an input of emerging low-carbon More data for Germany together with similar infor- technologies aimed at rational use of thermal energy in mation for China and the USA can be found else- dwellings must be decidedly increased [5, 6]. where [13, 14]. In the Russian Federation (RF), 43 % Indeed, at present, a significant part of primary fossil of the produced centralized heat or 138 Mtoe is used fuels is spent for heating/cooling of buildings, thus, for buildings [15]. This agrees well with the data of greatly contributing to GHG emissions. In Europe, the ref. [16] that Russian residential, public, and commer- cial buildings in 2005 were responsible for 144.5 Mtoe of final energy use (Table 1), at that 75 % of Correspondence: aristov@catalysis.ru Boreskov Institute of Catalysis, Ac. Lavrentiev av. 5, Novosibirsk, Russia Novosibirsk State University, Pirogova str. 2, Novosibirsk, Russia © 2015 Aristov. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Aristov Future Cities and Environment (2015) 1:10 Page 2 of 13 Table 1 Evaluation of the energy efficiency potential in buildings (Mtoe) (the data are taken from [16]) Type Consumption in 2005 Technical potential Economic potential Market potential, 2010 prices Total buildings 144.54 68.61 58.59 49.70 Residential 108.24 53.42 44.78 37.98 Public and commercial 36.31 15.20 13.81 11.72 this heat is consumed in dwellings. Space heating is [20]: a technical GHG reduction potential for the the leading heat end-user (73.7 % in 2011), while the Russian building stock by 2030 ranges between 26 and rest (26.3 %) is used for HHWP [17]. 47 % of the national baseline. Despite all systematic and active actions that have gone on, heat losses in buildings still remain enormous, The structure of heat losses in dwellings hence, there is considerable potential for improving en- The UK has around 26.7 million domestic houses and ergy efficiency in residential buildings and the related in- flats [21], and this building stock is one of the oldest in frastructure. According to the International Energy Europe. Old houses have significantly higher heat con- Agency (IEA), the building sector can reduce energy sumption: e.g. a typical Victorian house demands more consumption with an estimated global energy savings of than twice as much energy for space heating than a 1509 Mtoe which can possibly mitigate 12.6 Gt (giga- modern house [22]. Much of the heat supplied to UK tonnes) of CO emissions by 2050 [18]. The UK Govern- buildings is wasted, that is mainly due to poor buildings ment has set an ambitious target to reduce national insulation, heating of unoccupied spaces or inappropri- greenhouse gas emissions by at least 80 % by 2050, and ate occupant behaviour [21]. As seen from Fig. 1b, for about a half of this reduction is to be due to improved average dwelling, the main losses due to building fabric operation of buildings. Similarly, the declared national are related to heat fluxes through the walls (37 %) and German policy goal is to reduce nation-wide CO emis- windows (19 %) together with those associated with air sions by 80 % till 2050 (against 1990). According to a re- infiltration in ventilation system (18 %). Heating water cent study, about 60 % of that reduction has to come for showers, baths and sinks demands around 87 TWh a from the building sector [19]. year, and this heat is completely lost [21]. Technical potential of energy saving in the Russian By the end of 2012, the total floor area of dwellings in buildings sector exceeds 68 Mtoe or 47 % of the total Russia reached 3.3 billion m , 72.2 % being related to consumption, the largest potential being evaluated for urban development [17]. In Russia, the space heating is re- residential buildings (Table 1) [16]. The major part sponsible of 58 % of overall energy consumption in resi- (67 %) of saving can be implemented through the reduc- dential buildings with district heating systems which cover tion of district heating use for space heating and hot c.a. 75 % of all dwellings. A small part of dwellings built water. About 85 % of the technical potential is econom- after 2000 meet acceptable heat efficiency requirements, ically viable, while 72 % is market attractive with the whereas the majority of older buildings have significantly 2010 energy prices [16]. Essential saving can be also lower space heating efficiency. The value of Specific reached by improving the infrastructure of centralized Energy Consumption (SEC) for space heating strongly de- heat supply [15]. Similar estimations were reported in pends on the year of dwelling construction: for buildings Fig. 1 (a) Schematics of heat losses of building, (b) share of heat losses in the average UK building and the whole building stock (adapted from [21]) Aristov Future Cities and Environment (2015) 1:10 Page 3 of 13 erected before 1990, in 1991–2000 and after 2000, the lower heat losses through the building constructions average SEC is estimated as 0.97, 0.55 and 0.38 GJ/m / (Fig. 2). It is interesting to mention that the structure year, respectively [16]. New, and much more severe, stan- of heat losses in the Russian dwellings erected before dards for thermal protection of buildings were set in 2003 2000 (the left half of Fig. 2) is close to that reported by a new Russian building code (SNiP 23–02–2003 “Ther- for the average building in the UK (Fig. 1b). mal Performance of Buildings” [23]). These standards are Accordingly, the current strategic tendency is a de- based on the total number of heating degree-days (HDD) crease in heat consumption in dwellings by reducing for particular climatic zone/city. The heating period H is heat losses. The heat consumption can also be re- fixed as a cold season with the mean daily outdoor duced by applying inter-seasonal heat storage. During temperature T ≤8°С. For Moscow, H = 214 days and the summer period there is an excess of solar heat ind HDD(T ≤8°С) = 4,943 (°C⋅day)/year, for Novosibirsk – available that meets little or no heat demand which is ind 230 days and 6,601 (°C⋅day)/year, and the average HDDs shifted to the winter months. The inter-seasonal ad- in Russia is 5,140 (°С⋅day)/year. For comparison, in sorptive heat storage (AHS) has been considered as a London the HDD(T ≤ 10 °C) is only 1,860 (°С⋅day)/year tool to harmonize the heat availability and demand, ind (in 2006) [http://ukclimateprojections.metoffice.gov.uk/ and, hence, to cover/reduce heating peaks in winter media.jsp?mediaid=87928&filetype=pdf]. Thus, in the RF period (see 3.1). the demand for thermal energy needed for space heating An important tendency clearly seen from Fig. 2 is is significantly larger than in the UK. that, for modern dwellings with better fabric Another important factor affecting the heating stan- insulation, a portion of the heat losses due to air in- dards is the dwelling size or the number of storeys. filtration significantly increases and becomes domin- Thermal performance of individual low-rise buildings ant (almost 50 %) in high-rise buildings. Therefore, a (1–3 storeys) and multi-storied apartment buildings further improvement of the dwellings thermal effi- (4–25 storeys) differs essentially: in smaller dwellings, ciency is associated, first of all, with a decrease in 1m of heated rooms area is accompanied by 1.7– heat losses due to air infiltration. This can be 3.3 m of building fabric area, whereas for larger achieved by implementation of new technologies of buildings this ratio is considerably less and varies indoor climate control, including emerging adsorption from 0.6 to 1.3 m [17], which leads to radically methods (see e.g. [24–27] and 3.2 below). Fig. 2 The share of heat losses in individual, two-storied (top), and apartment, nine-storied (bottom) buildings under climatic conditions of Moscow. Left - buildings erected before 2000, right – buildings erected according to the new standards of 2003 (re-plotted from [17]). Numbers denote the specific heat losses Aristov Future Cities and Environment (2015) 1:10 Page 4 of 13 Possible directions of future R&D directed at further It is worthy to make an estimation of tentative upper improving AHS technologies for dwellings are briefly limit of the AHS storage density. Water has a large heat considered in 3.3. of evaporation ΔH , and is considered as the most ev promising and ecologically clean working fluid for AHS applications. For making this estimation, let’s assume Adsorption technologies for rational use of heat in that the heat storage process is simply water evapor- buildings ation. During this endothermic transition of water mole- Although there are many applications of adsorption tech- cules from liquid to vapour state, the amount of heat nologies for rational use of energy [29, 30, 31, 32, 33], we consumed (stored) is A(H O) = ΔH = 2.25 MJ/kg = 2.25 2 ev consider here those which focus mainly on the building 3 3 GJ/m = 630 kWh/m (at 100 °C). For AHS, the amount sector. of heat needed for complete water desorption (or max- imal heat storage density A )is max Adsorptive inter-seasonal heat storage Inter-seasonal heat storage can be an effective tool to mas A ¼ ΔH w ðper 1kg dry adsorbentÞð1Þ des max max reduce heating peaks in dwellings in winter period [28–34]. Indeed, solar heating potential is high in or summer and low in winter, what is just opposite to heat demand (see e.g. Fig. 1 in ref. [35]), therefore a vol 3 long-term (several months) storage of the excess heat A ¼ ΔH w ρðper 1m adsorbentÞ; ð2Þ des max max in simmer is needed. The inter-seasonal AHS can op- erate as closed (evacuated) [36] and open (coupled to where ΔH is the average heat of water desorption [in des the ambient) [37] cycles. Useful comparison of these kJ/kg], w is the maximal mass of water adsorbed per max two AHS variations was performed in [38] by using 1 kg adsorbent, and ρ is the adsorbent apparent density. energy and exergy balance analysis. The authors For adsorbents with high affinity to water vapour, like clearly demonstrated that the adsorptive systems may common zeolites, the ΔH -value can be 1.5–2 times des be as efficient as, and more compact than other types larger than the heat of water evaporation ΔH , w ev max of the thermal energy storage systems, however, there = (0.2–0.4) kg/kg and ρ =600–800 kg/m , hence, A (- max is a much room for efficiency improvement for both zeolites) = (0.67–1.80) MJ/kg = (0.35–1.45) GJ/m that is closed and open AHS systems. (0.2–0.7) A(H O). Several new adsorbents (SWSs, FAMs, MOFs – see below) have an advanced water sorption The heat storage density The density A of adsorption capacity w = 0.7–1.8 g/g and the maximal heat stor- max mas heat storage is considered to lie between 100 and 500 age density A can reach or even somewhat exceed the max 3 3 kWh/m or (0.36–1.8) GJ/m being larger than that of estimated storage density A(H O) (Table 2). The heat 3 3 vol sensible (c.a. 50 kWh/m ) and latent (c.a. 100 kWh/m ) A stored per unit adsorbent volume is always lower max heat storage [29, 30, 39–41]. If the annual heating de- than A(H O). Therefore, the above A(H O)-estimation 2 2 mand is 25 kWh/m /year that is typical for standards of can be used as a reference value that delineates an upper low energy buildings in Europe a 100 m house con- limit of the heat storage density via adsorption process. sumes for heating about 9 GJ per year. These heat needs Eqs. (1) and (2) take into account only the latent heat can be, in principle, covered by solar energy, because of material. A useful method of experimental evaluation even in the UK the amount of solar radiation incident of both latent and sensible heat was suggested in [41] on a correctly orientated roof of a typical house exceeds and then used in [35] and [43]. The sensible heat was its energy consumption during a year [42]. Accordingly, found to be 4–7 % of the latent one (in the temperature efficient inter-seasonal heat storage is necessary in this range 50–200 °C) and can be used either for immediate case. The volume of a heat storage material is reasonable consumption (e.g. for HHWP) or short-term storage. 3 3 (2–10 m ) only if the storage density is above 1 GJ/m Under real conditions the storage density is expected to (270 kWh/m ). In actual practice, the volume of the heat be lower than the maximal one calculated by eqs. (1) and storage unit is larger than the material volume itself by a (2), because only a part of the adsorbed water is involved factor of 2–4 due to volumes of heat exchangers, evapor- in the storage process. To realize how much water can be ator/condenser, pumps/fans, pipes, etc. involved, let’s consider a simple three temperature (3T) For countries with colder climate, means, a larger closed cycle of AHS unit. Although the thermodynamics HDD-value, like Russia, the adsorption storage alone is similar, an open AHS cycle is considered on a psychro- can hardly cover the overall heat demand during winter. metric chart of humid air (not presented) as described in In this case, the heat storage systems need to work [32]. Practical implementations of the AHS open and alongside a complementary space heating unit. closed systems are quite different as well (see 3.1.4). Aristov Future Cities and Environment (2015) 1:10 Page 5 of 13 mas vol Table 2 Maximal mass of water adsorbed w , average desorption heat ΔH , maximal heat storage densities A and A max des max max (calculated by eqs. (1) and (2)) mas vol Adsorbent w , g/g ΔH , MJ/kg A , MJ/kg A , GJ/m Reference max des max max Silica gel Fuji RD 0.4 2.40 0.96 0.77 [95] Zeolite 13X 0.34 3.8 1.29 0.83 [96] Zeolite 4A 0.22 3.05 0.67 0.49 [97] Zeolite MgX 0.45 2.68 1.21 - [53] SWS-1 L = CaCl /silica gel 0.65 2.65 1.72 1.55 [98] SWS-9 V = LiNO /Vermiculite 1.80 2.30 4.15 1.16 [75] SWS-1 V = CaCl /Vermiculite 1.80 2.35 4.2 1.25 [99] SIM-3b = MgSO /Vermiculite 1.94 - 0.41 0.14 [35] (MgSO + MgCl )/Attapulgite - - 1.59 -[54] 4 2 AQSOA-Z02 0.33 3.25 1.07 0.55 [100] AlPO-Tric 0.31 3.17 0.98 - [56] MIL-101 1.40 1.83 2.57 - [62] MIL-125NH 0.47 2.85 1.33 0.39 [66] MOF-841 0.48 3.05 1.47 - [67] the value was obtained by integrating DSC-thermogram of the sample within a temperature range of 30 to 100 °C; it is likely to be underestimated the isothermal heat directly measured during water sorption at 30 °C and a relative humidity of 85 % on the sample preliminary dried at 130 °C A 3T cycle of adsorptive heat storage This cycle con- charging (2–3) and discharging (4–1) stages the heat sists of two isosters and two isobars and is commonly can be stored as long as needed without any heat presented on the Clapeyron diagram ln(P)vs. (−1/T) insulation. (Fig. 3). During isobaric desorption (2–3) the amount Three temperatures (those of evaporator T ,condenser of water adsorbed reduces from w to w ,and T = T , and external heat source T ) uniquely define the max min c 1 g the heat Q consumed for the water desorption cycle [44]. The heat Q is released for consumers at d a is stored. The stored heat can be release during temperature ranging between T and T = T ,therefore T 4 1 c c isobaric adsorption (4–1) when the dewatered should be at least 30–35 °C (the lowest input temperature adsorbent sucks water vapour from an evaporator for floor heating). The heat of condensation Q can also (for closed AHS cycles) or from the ambient air (for be used for heating at temperature T . For simple and rela- open cycles). The heat of evaporation Q is deemed tively cheap flat receivers of solar energy, the temperature to be taken from the ambient for free. Between the of heat carrier T available for water desorption commonly is 70–110 °C. Hydration of adsorbent up to the maximal uptake w occurs at point 1 by absorbing water vapour max saturated at temperature T =5 – 15 °C. For a model AHS cycle, we have fixed the following set of temperatures: T = 10 °C, T = 35 °C, and T = 90 °C, and calculated the c g mass of water exchanged along such cycle Δw = w – max vol w and appropriate cycle heat storage density A that min max appears to be lower than the maximal one displayed in Table 2 byafactor of 2–12 (Table 3). Significant reduction of the storage density is especially typical for zeolites which need the regeneration temperature much higher than the set temperature T = 90 °C. This confirms that a high affinity of adsorbent and adsorbate to each other or/ and a large total sorption capacity are not important of themselves. The heat storage density is defined by the amount of adsorbate exchanged within the particular AHS cycle, means, under certain conditions of heat storage Fig. 3 Common Clapeyron diagram of a 3T AHS closed cycle process [45]. Aristov Future Cities and Environment (2015) 1:10 Page 6 of 13 Table 3 Mass of water exchanged Δw and appropriate heat in [49] and applied for analyzing adsorptive transform- storage densities A for a model AHS cycle (T = 10 °C, T =35 max e c ation of heat in [45]. °C, and T = 90 °C) mas vol 3 Adsorbent Δw, g/g A , MJ/kg A , GJ/m Reference max max New adsorbents promising for AHS For advanced Silica gel Fuji RD 0.12 0.29 0.23 [101] AHS performance, it is necessary to have an adsorb- Zeolite 13X 0.03 0.11 0.07 [96] vol ent with high heat storage density A ,large max SWS-1 L 0.17 0.45 0.32 [98] temperature lift L, and low regeneration temperature SWS-9V 0.43 1.15 0.45 [75] T .Itis evident,thattwolast requirements areina SWS-1V 0.33 0.76 0.21 [99] certain conflict. Indeed, the regeneration process is AQSOA-Z02 0.19 0.62 0.31 [102] easier for adsorbents with low affinity to water vapour, whereas a large temperature lift, on the con- AQSOA-Z02 0.26 0.84 0.42 [43] trary, needs high affinity. Therefore, intelligent com- MIL-125NH 0.21 0.64 0.19 [67] promise between these tendencies is necessary. We T = 10 °C, T = 30 °C and T =70 °C e c des do not consider here dynamic parameters of the T = 15 °C, T = 28 °C and T =90 °C e c des promising adsorbents, because the overall dynamics in real AHS units is more dependent on the Thus, the inter-seasonal heat storage can cover whole organization of heat and mass transfer in the overall winter heating needs for low-energy buildings in a rela- unit “adsorber – heat exchanger”,Ad-HEx(see3.1.4 tively mild climate (reasonable HDD-value). For less per- and 3.5). fect buildings and colder climates, the heat stored vol The largest overall storage density A (up to 1.55 during summer can yet be used in winter to reduce fossil max fuel consumption, smooth heating peaks and, hence, de- GJ/m ) was documented for composites “salt in porous crease bills for heating. In this case, the AHS can be matrix” that are known as Selective Water Sorbents used as an auxiliary tool for common space heating (SWSs) [50]. They present a family of solid sorbents spe- system. cifically developed in the Boreskov Institute of Catalysis The 3T cycle on Fig. 3 allows introducing other two (Novosibirsk, Russia) for adsorptive transformation and parameters important for AHS: storage of low temperature heat [50, 51]. The SWSs a) a minimal desorption temperature T = T . If the were shown to exhibit an intermediate behaviour be- min 2 two boundary temperatures T and T are fixed, this tween solid adsorbents, salts hydrates and liquid absor- e c temperature can be estimated from simple Trutoun’s bents. Combination of high storage density of the salts rule [44, 46, 47]. and good mass transfer of the porous matrix allows the design of advanced materials and AHS units with high T ¼ðÞ T =T ð3Þ efficiency and specific power. Very important advantage min c e of SWSs is an opportunity for nano-tailoring their sorp- within an accuracy of ±1 °C [44]. Evidently, if the tion properties by varying the salt chemical nature and temperature of external heat source T is lower than content, porous structure of the host matrix and synthe- T , it is not sufficient for adsorbent regeneration and min sis conditions [51, 52]. It is especially valuable for inter- the AHS cycle cannot be realized at all. A low T -value seasonal AHS, because allows adaptation of a heat helps in easier adsorbent regeneration that allows stor- storing adsorbent to particular climatic conditions at the age of heat with appropriately low temperature location of AHS unit. At present, these materials are potential; comprehensively tested in many laboratories all over the b) a temperature lift L =(T - T ). A large temperature c e world [28, 29, 35, 53]. lift permits higher temperature level of heat released Loose grains of clay mineral attapulgite impregnated during heat rejection (discharging) stage. For working with mixture of MgSO and MgCl hydrates was investi- 4 2 pairs that follow Trutoun’s rule, the temperature lift can gated for suitability as a heat storage material [54]. It be estimated as [48]. was shown that the partial substitution of MgSO by MgCl resulted in higher sorption heat. This larger heat L ¼ðT −T Þ¼½T ðT Þ =T ¼ T ð1T =T Þ: c e c c g c c g release was accompanied with higher temperature lift. The energy density of the composite containing a mix- ð4Þ ture of 20 wt.% MgSO and 80 wt.% MgCl and satu- 4 2 Thus, the temperature lift increases if adsorbent has rated at T = 30 °C and RH = 85 % was 1590 kJ/kg. The high regeneration temperature, means, strong affinity to desorption temperature of 130 °C was sufficient for the water vapour. This affinity can be quantitatively charac- composite regeneration, that makes the new material terized by the Dubinin adsorption potential as suggested promising for solar thermal energy storage. Aristov Future Cities and Environment (2015) 1:10 Page 7 of 13 Systematic study of the SWS-type composites for or at least lessen, this shortcoming were considered in AHS was performed in refs. [35, 55]. The authors [52]. tested various hygroscopic salts (CaCl ,MgSO , Quite interesting storage parameters were obtained 2 4 Ca(NO ) ,LiNO , and LiBr) and host matrices (silica for new zeolite-like materials that do not contain sili- 3 2 3 gel, zeolite, activated carbon and mineral vermiculite) con, namely, pure and substituted aluminophosphates to obtain their combinations promising for open AHS [56–59]. E.g. an APO-Tric material was suggested for systems and studied them by a variety of physico- use in low temperature solar energy storage [56], be- chemical methods (BET, BJH, TG,DSC,SEM,EDX, cause it exchanges 0.25 g H O/g in an extremely nar- etc). It was found that the sorption capacity of all row range of the relative pressure P/P =0.12–0.15 composite materials is significantly higher (up to and can be regenerated at 80–90 °C. Quite promising 1.9 g/g) compared to their raw host matrices alone storage properties were revealed for SAPO-34, AlPO- suggesting the addition of the salt is beneficial for 18, AlPO-17, AlPO-5, etc [58]. The disadvantages of moisture and, hence, heat storage. It was demon- this class of materials are a relatively small pore vol- strated that the pore structure damage may occur for ume (e.g. 0.33 cm /g for AQSOA-Z02, Table 2) and the non-vermiculite matrices, however no damage was high cost. The main practical strength is an availabil- observed in the vermiculite as a host. Vermiculite ity of several commercial adsorbents (AQSOA-Z01, with either lithium bromide or calcium chloride ap- AQSOA-Z02 and AQSOA-Z05) in the form of coated pears to have significantly larger AHS potential when heat exchangers produced by the Mitsubishi Plastics compared to both the raw matrices and the other Ltd. (see refs. [59–61] and Fig. 4d). composites [35]. Among new AHS adsorbents, metal organic frame- A recent survey of the current-state-of-the art of the works (MOFs) are attracting an increasing attention (e.g. SWS materials can be found elsewhere [52]. In particu- MIL-101, MIL-125NH and MOF-841 in Table 2) due to lar, the authors considered potential troubles that could their unique combination of properties such as high sur- impede an actual reduction of these new composites to face area, crystalline open structure, tunable pore size practice. This is, first of all, a leakage of the salt solution and functionality [62–64]. These materials can exhibit formed during sorption out of the matrix pores, which very large water adsorption capacity (e.g. 1.4 g/g for can cause a corrosion of metal parts of AHT unit and MIL-101, Table 2) and low regeneration temperature, emission of non-condensable gas. Several ways to avoid, however, their affinity to water is, in general, too low to Fig. 4 Various configurations of “adsorber – heat exchanger” unit: open systems - a honeycomb ceramic element made of WSS [71] (a); closed systems - overall (b) and fragment (c) views of the Ad-HEx with loose grains [76], (d) - fragment of the fin-and-tube heat exchanger coated with AQSOA-Z01 [60] Aristov Future Cities and Environment (2015) 1:10 Page 8 of 13 ensure sufficient temperature lift. In our opinion, the po- lift, however, after the initial period (1–2 h), the mois- tential of MOFs for AHS has not been unveiled yet, and ture adsorption rate and thus heat generation decreased it is quite possible that a target-oriented modification or sharply. A sharp drop in performance over the four cy- screening of MOFs will reveal novel working pairs highly cles was found for zeolite 13X which requires a higher efficient for AHS. In particular, very promising results regeneration temperature (>180 °C) than that used have been reported for UiO-66NH2 [65], MIL-125NH2 (100 °C) for the research. The composite SIM-3a (ver- [66], MOF-801 and MOF-841 [67]. miculite doped with CaCl ) was found to be the most The above analysis is mainly based on considering the promising candidate for open TES systems due to its lar- storage density, temperature lift and regeneration ger heat generated (120–160 kWh/m ) and rapid mass temperature, and do not take into account other import- uptake at higher RH levels. Moreover, SIM-3a is non- ant practical requirements to adsorbent optimal for toxic material, has a low regeneration temperature, good AHS. These are minimal or no adsorption/desorption cyclic ability, and low cost (~0.48 $/kg) [55]. It is worthy hysteresis, low costs and easy availability, high cyclic sta- to note that for inter-seasonal heat storage the material bility, free from poison, non-combustibility, no environ- cost is far more important than for short-term heat mental effects, low maintenance requirements, etc. storage. Some of these issues have become clear during practical The authors of ref. [71] used as a host porous matrix testing of the advanced adsorbents as reported below. another cheap mineral material, that was a Wakkanai si- liceous shale (WSS). The WSS is a type of siliceous mudstone that is widely distributed in Wakkanai, the Current practice of inter-seasonal adsorptive heat northern part of Hokkaido (Japan). It is a natural meso- storage Since a large variety of adsorbents have already porous material composed of silicon dioxide (SiO ) with been tested for AHS, we consider here only several se- pores of 5–40 nm size. A new AHS composite was lected, mostly recent, examples of both open and closed formed by inserting LiCl into the WSS pores. To over- AHS systems, and refer a Reader to appropriate review come the problems of low heat and mass transfer and papers (e.g. [28–40, 68–70]) and references therein. high hydrodynamic resistance in an open AHS system, Among open AHS systems the most famous and well the WSS was formed by adding a binder into a honey- tested is an open heat storage system installed in a comb ceramic element of 10 cm x 10 cm x 20 cm with school building in Munich (Germany) and connected to 36 cells/cm and a cell wall thickness of 0.28 mm the local district heating network [32]. It contained (Fig. 4a). The composite containing 9.6 wt.% LiCl 7,000 kg of zeolite 13X as beads. The unit was tested showed a good potential for storing heat with a volumet- during the heating period 1997/98 under the following ric heat storage density of 0.180 GJ/m and could supply conditions: the air flow rate−6,000 m /h, the desorption air heated up to 53 °C. No material degradation was ob- temperature−130 °C, the pressure of water vapour at the served during at least 250 cycles at a quite low desorp- heat release stage−32 mbar that corresponds to a dew- tion temperature of 60 °C. point of 25 °C. For a large temperature lift of 40 °C, a An open-type AHS setup equipped with 40 kg of com- maximal energy density is estimated to be 0.49 GJ/m posite sorbent CaCl /(mesoporous silica gel) was tested (or 0.55 GJ/m if the sensible heat can be utilized), as a batch adsorber in [72, 73]. The concentration of the whereas in the experiments a lower value of 0.446 GJ/ CaCl impregnating solution was found to be an import- m was recorded, which is 81 % of the theoretical one. ant factor affecting the storage properties of the com- Such a large storage density is due to a relatively high re- posites. The composite prepared by impregnating silica generation temperature of 130 °C. It is worthy to remind gel with a 30 wt.% CaCl solution showed a high storage that for the above model AHS cycle with lower regener- capacity of 1.02 MJ/kg (0.81 GJ/m ) that remained stable vol ation temperature of 90 °C, A ¼ 0:07 GJ=m after 500 consequent sorption/desorption cycles. The max (Table 3). heat discharging temperature varied from 47 °C to 30 °C The authors of [55] analysed the performance of sev- and the sorbent can be charged at temperature 90 °C or eral candidate materials (1.5 dm of adsorbent) under below. Mathematical modelling of the AHS process pre- experimental hygrothermal cycling tests together with dicted that the specific heat storage capacity can be in- energy and exergy analysis of the results obtained. They creased up to 1.35 MJ/kg at a regeneration temperature tested eight composites “salt in porous matrix” identified of 100 °C [74]. before in [35] as well as raw zeolite 13X and mesoporous SWS-type materials were successfully used also in silica gel (Geejay Chemicals, UK) under typical condi- closed AHS systems [75]. For instance, lithium nitrate tions of open AHS cycle. The studied adsorbents were was introduced into silica gel [76] and vermiculite [75] located in a test tray as a bed of loose grains. It was matrices and tested in typical AHS cycle (Fig. 5). LiNO found that the zeolite provided the highest temperature is a salt which bounds water less strongly than calcium Aristov Future Cities and Environment (2015) 1:10 Page 9 of 13 The distance between the fins was about 2 mm, so that 2–4 grains could be housed in this gap. The composite LiNO /vermiculite (SWS-9 V) ex- changes 0.43 (g H O)/(g sorbent) in an exceptionally narrow temperature range, 33–36 °C (heat rejection) and 62–65 °C (adsorbent regeneration), corresponding to a remarkable heat storage capacity of 1.15 MJ/kg. It is much larger than for similar composite based on mesoporous silica gel, SWS-9 L (Fig. 5). If relate the storage capacity to the total volume of the Ad-HEx unit (Fig.4b),it isequal to 0.45 GJ/m . Thisissuper- ior to common adsorbents and Ad-HEx units suggested for storage of heat with temperature poten- tial of 60–70 °C. Another solution for enhancing heat and mass transfer was investigated for the residential applica- Fig. 5 Equilibrium isobars of water adsorption at 12.6 mbar (●) and tion in [60]. A commercial heat exchanger was desorption at 56.2 mbar (○) and appropriate AHS cycles for composites SWS-9 V (A-B-C-D) and SWS-9 L (A’-B’-C’-D’) [75] adopted for an adsorption bed to quicken the char- ging process by improving heat and mass transfer. It is a copper tube and aluminium plain fin type heat exchanger with 12 parallel four-pass circuits, 1.8 mm chloride, lithium bromide or calcium nitrate already re- fin spacing and a 0.25 mm thickness adsorbent coat- ported for AHS applications. Therefore, this composite ing on all fin and tube surfaces (Fig. 4d). The com- sorbent was specifically developed to operate at low re- mercial material AQSOA-Z01 (Mitsubishi Plastics) generation temperature (<65–70 °C). The sorbent parti- [77] was used as the adsorbent bed material. When cles were embedded inside a compact heat exchanger of the regeneration and heat rejection temperatures were a finned flat-tube type (Fig. 4b, c). This Ad-HEx config- 70 °C and 30 °C, respectively, the heat storage density uration has the following advantages: a) compactness was found to be 0.805 MJ/kg or 0.15 GJ/(m of the and low weight, as the heat exchanger is made of alu- Ad-HEx). The loading difference was 0.164 kg water/ minium; b) good heat transfer properties due to the high kg adsorbent. The authors suggested several possible heat transfer area and a high thermal conductivity of solutions for the system performance improvement, aluminium; c) high vapour permeability as provided by e.g. adding more adsorbents into the fin spacing of granular packing. Fig. 4b, c shows the overall and the de- the Ad-HEx or onto heat transfer tubes to form con- tailed view of the Ad-HEx with loose silica gel grains. solidated layer. Fig. 6 a Scheme of the regeneration process: the temperature and moisture profiles at various times t <t <t . Reprinted from [26]; (b) view of 1 2 3 the VENTIREG unit (prototype IV) Aristov Future Cities and Environment (2015) 1:10 Page 10 of 13 This brief overview of the current practice in inter- application of porous and polymer membranes partially seasonal AHS shows that permeable for moisture as suggested in [83–85]. A new approach (the so called VENTIREG) was suggested in a) for several AHS systems tested the storage density [26] for simultaneous regeneration of both heat and are approaching a desired level of 1 GJ/m ; moisture in ventilation systems for cold climate coun- b) the composite sorbents “salt in porous matrix” tries. This approach was deemed to ensure an efficient demonstrated great potential to obtain a high AHS moisturizing of the supplied air to approach indoor con- density and to reduce an AHS system size as ditions of human thermal comfort which is extremely compared to the other heat storage materials. The important because people spend more than 90 % of their main advantages of these composites are as follows: time in buildings [86]. i) low charging temperature (solar energy absorbed The main principal of the VENTIREG method is as by flat receivers can be stored), ii) large storage follows (Fig. 6a). To exchange the sensible heat between capacity, and iii) possibility to controllably modify the inlet (fresh) and outlet (exhausted) air fluxes, a gran- their sorption properties to match conditions of ulated layer (1) of heat storing material is placed closer particular heat storage cycles (e.g. variation of the to the unit exit. Before this layer, layer 2 of water salt and matrix nature, etc). The main shortcoming adsorbing material is located closer to the room side. It is a relatively low temperature lift. It can be serves as a water buffer. The unit operates in two enhanced by applying salts with higher affinity to modes: water vapour at the expense of appropriate increase in the charging temperature. outflow mode: a warm and humid indoor air is blown by an extract fan through the relatively dry adsorbent, To summarize, despite the significant progress in ad- which captures and retains the indoor moisture. Dried sorbent synthesis and applications in various AHS cy- and warm air enters layer 1 and heats it up. After that, cles, still there is a big room for their further the air flux switches; optimization and development of new adsorbents with inflow mode: a dry and cold outdoor air is blown by a advanced and pre-requested properties. supply fan through the warm layer 1 and is heated up to the temperature close to that in the room T , thus, in Regeneration of heat and moisture in ventilation system of recovers the stored heat. Passing through the layer of dwellings in cold climates the humid adsorbent, warm and dry air causes the As manifested above, an efficient way of further im- retained water to be desorbed and come back to the provement of the dwellings thermal efficiency is associ- room, thus, maintaining the indoor moisture balance. ated with heat recovery from the ventilation systems. Because of the finite heat capacities of layers 1 and 2, This is especially important for countries with a cold cli- the temperature of incoming air is slowly decreasing, mate (typical of Russia, Canada, the North Europe and and the air flux switches when the temperature USA), where the difference between indoor and outdoor difference (T – T) reaches a predetermined value ΔT , in 0 temperatures can reach in winter up to 60 °C or even and so on. more. This leads to enormous heat losses, therefore effi- cient exchange of heat between the exhaust and supply To study and optimize the heat and moisture recovery, air fluxes is absolutely necessary for reducing the heat four experimental units with the air flux up to 25 (I), 40 losses. Numerous experimental and modelling studies (II) and 135 (III and IV) m /h were built and tested. Both have clearly demonstrated that up to a 90 % heat recov- common (silica, alumina) and novel (SWS-1A = alumina ery can be reached by using modern plate [78, 79] and impregnated with CaCl ) adsorbents were used as buffers regenerative [80, 81] air-to-air heat exchangers. Another of water. The composite sorbent SWS-1A was found to problem that appears during cold winter is continuous demonstrate better performance than the common com- loosing of moisture through ventilation system [82, 83]. mercial adsorbents. Owing to higher adsorption capacity Indeed, the absolute humidity of supplied (outdoor) air of this composite, the adsorbent loading is less by a factor is extremely low (e.g. ≤ 0.29 g/m at -30 °C), that results of 2–3 that leads to smaller unit size and much lower in dramatic reducing the indoor relative humidity (down hydrodynamic resistance of the unit. This could allow to 10–20 % or even below) in winter season that greatly the using of cheap blade-type fans instead of centrifu- disbalances the indoor thermal comfort. Therefore, in gal ones and give a reduction of the electricity con- cold countries it is extremely important to organize not sumption. Units III and IV consume for air blowing only heat exchange, but moisture exchange between the 20–40 W of electric power and give the heating load fluxes as well to maintain the indoor humidity within of about 600–1400 W that corresponds to a Coeffi- the reasonable range. One of the solution is the cient of Performance of 25–35. So little electricity Aristov Future Cities and Environment (2015) 1:10 Page 11 of 13 consumption is a very important advantage of the mature technologies, like PCM or sensible heat storage, VENTIREG approach. Prototype IV (Fig. 6b) was con- further R&D is required in the field of adsorbents, heat structed in a way to ensure a continuous operation, and mass transfer, and advanced components. not intermittent as for units I-III [26]. We hope that this review will give new impulses to fur- Thus, the tested VENTIREG units exchange stale, con- ther consolidating international R&D activities in materials taminated room air with fresh outdoor air, recovering up science and applied thermal engineering to obtain novel to 95 % of heat and 70–90 % of moisture from the ex- revolutionary adsorbents and technologies for improving haust air. Moreover, it prevents the formation of ice at energy efficiency and thermal comfort in buildings. the unit exit that is one more severe problem to be Abbreviations solved under real winter conditions [87]. According to Ad-HEx: Adsorber – heat exchanger; AHS: Adsorptive heat storage; the data of Table 3 of the recent review paper [82], the EU: European union; GHG: Green house gases; HDD: Heating degree-days; VENTIREG unit demonstrates the best properties HHWP: Heating and hot water production; IEA: International energy agency; MOF: Metal organic frameworks; Mtoe: Million tonnes of oil equivalent; among different heat and mass recovery systems tested PCM: Phase change materials; RF: Russian Federation; RH: Relative humidity; under various cold climate conditions. SEC: Specific energy consumption; SWS: Selective water sorbents; UK: United Kingdom; USA: United States of America; WSS: Wakkanai siliceous shale. Future R&D for AHS optimization Competing interests Despite a significant progress in AHS achieved, still The authors declare that they have no competing interests. there is a much room for further developing this emer- ging technology [48, 88, 89]. To reach this ambitious Acknowledgments goal, systematic R&D must be made at least, but not last, The author thanks the Russian Foundation for Basic Researches (project 14- 08-01186a) for partial financial support. in the following directions: Received: 24 August 2015 Accepted: 7 November 2015 – development of new efficient adsorbents with the cycle heat storage density larger that 1.0 GJ/m . References Their adsorption properties have to be harmonized 1. United Nations Population Fund. (2012). State of the World population 2011 with boundary temperatures of appropriate heat 2. UN-HABITAT (2008). State of the world’s cities report 2008–9: Harmonious storage cycles [45]; cities: http://www.fastcodesign.com/1669244/by-2050-70-of-the-worlds- population-will-be-urban-is-that-a-goodthing (accessed November 13, 2015) – improvement of heat and mass transfer in the unit 3. Dresner S (2002) The Principles of Sustainability. Earth Scan Publications, London “adsorbent – heat exchanger”. 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Future Cities and Environment – Springer Journals
Published: Dec 1, 2015
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