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Adsorbent working pairs for solar thermal energy storage in buildings
Renewable Energy xxx (2016) 1e8
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Adsorbent working pairs for solar thermal energy storage in buildings Andrea Frazzica*, Angelo Freni CNR e ITAE, Istituto di Tecnologie Avanzate per l'Energia “Nicola Giordano”, Via Salita S. Lucia sopra Contesse 5, I-98126, Santa Lucia, Messina, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2016 Received in revised form 15 September 2016 Accepted 20 September 2016 Available online xxx
In this study, the thermodynamic analysis of several adsorption working pairs for adsorption heat storage applications at domestic level is presented. The selected working pairs employ different working fluids (i.e. water, ethanol, ammonia, methanol) and different adsorbent materials such as classical zeolites, silica gels, alumino-phosphates, composite sorbents and activated carbons. The simulations have been performed taking into account desorption temperatures in the range between 80 C and 120 C, compatible with non-concentrating solar thermal collectors, under seasonal heat storage working conditions. The composite sorbent MWCNT-LiCl with both water and methanol as working fluid showed the highest heat storage density under practical working boundary conditions. Among the standard adsorbents, the zeotype AQSOA Z02 showed promising achievable heat storage densities. Classical working pairs, such as zeolite 13X/water, commonly employed for heat storage applications, are not suitable for this working range. Finally, also the influence of the metal to sorbent mass ratio, due to the heat exchanger, was investigated, demonstrating that it can reduce the achievable amount of heat released to the user up to 30%. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Adsorption Thermal energy storage Solar thermal Thermochemical heat storage
1. Introduction Renewable heating and cooling (RHC) sector, which mainly comprises solar thermal, geothermal and biomass energy, has been defined as the “sleeping giant” in 2011 by the European Renewable Energy Council (EREC) in the “Re-thinking 2050” document [1]. Indeed, it is expected that RHC will represent approximately 21% and 45% of the total final energy consumption in 2030 and 2050 respectively [1]. Among the RHC technologies, solar thermal seems the most promising thanks to its wide availability as well as to the reached technological maturity, which is making it as a valid alternative not only for standard applications (e.g. domestic hot water production, space heating), but also for more advanced applications (e.g. industrial heat, solar cooling) [2]. Nevertheless, there are still open research and development topics that may increase efficiency as well as cost effectiveness of solar thermal energy technology in order to promote and stimulate its deployment at large market scale. At the components level, the main fields of research are oriented towards solar thermal collectors, control and performance assessment and thermal energy storage [2]. Particularly, thermal energy storage (TES) is a crucial component in a solar
* Corresponding author. E-mail address: [email protected] (A. Frazzica).
thermal system, in order to cover the mismatch between energy supply and demand, thus increasing the achievable solar exploitation [3]. For this reason, it needs to be further investigated and optimized, in order to increase energy storage density and to limit energy losses through the environment during its operation [2]. Considering its working principle, thermochemical TES could be considered as the most promising solution to fulfil these technological needs [4]. Indeed, in a thermochemical TES, heat is stored as chemical potential deriving from the breaking of bonds between the sorbent material and the working fluid. This guarantees highenergy storage density, since the enthalpy of reaction can be order of magnitudes higher than specific heat, as well as no degradation of stored energy during time, since heat can remain stored as long as the sorbate and the working fluid are kept separated. Accordingly, thermochemical TES represents a viable way to enhance solar thermal systems performance both through high energy density daily heat storage [5,6] as well as for seasonal heat storage applications [7,8]. Main features of different thermochemical heat storage technologies have been recently discussed in a review [9]. Among the wider class of thermochemical TES, the adsorption one seems the most suitable for residential building applications. Indeed, it is characterized by good energy storage density, possibility to store energy at low-medium temperature (i.e. below 100 C), typical of non-concentrating solar thermal collector technologies usually employed in this field, and ability to be
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Nomenclature A cp E k M n p Q r R T V Y w DH Ds Dw
adsorption potential (kJ kg1) specific heat (kJ kg1 K1) characteristic energy of the working pair (kJ kg1) pre-exponential Dubinin factor () mass (kg) Dubinin exponent () pressure (Pa) heat (kJ kg1) metal to sorbent mass ratio (kg kg1) universal gas constant (J mol1 K1) temperature (K) volume (m3) slope of the saturated adsorbate line on the Clapeyron diagram (K) uptake (kg kg1) enthalpy (kJ kg1) entropy variation (kJ kg1 K1) uptake variation (kg kg1)
operated both as daily and seasonal storage [10]. In order to develop an efficient adsorption TES, the first step is represented by the selection of the proper working pair. In the literature the performance of several adsorbent materials (e.g. zeolites, silica gels, composite sorbents) and working fluids (e.g. water, ammonia, alcohols) have been investigated, in the past, mainly for refrigeration and heat pumping applications [11,12]. More recently, Aristov [13] has reported an interesting review of available systems and adsorption working pairs for heat storage application in buildings. Nevertheless, a comprehensive thermodynamic analysis of different adsorption working pairs for heat storage applications has not been reported so far. Accordingly, the present paper aims at a thermodynamic comparison of the main adsorption working pairs available in the literature for adsorption heat transformer technologies. The analysis is carried out as a function of working boundary conditions.
2. Performance evaluation of the adsorption heat storage Generally, there are two system configurations for adsorption TES: closed and open cycle. The present paper will mainly focus on closed adsorption systems. Deep analysis of open adsorption TES can be found elsewhere [14]. In the following, the closed adsorption TES working principle will be shortly introduced. More detailed analysis can be found elsewhere [10]. Fig. 1 reports the working cycle for a closed adsorption TES on the Clapeyron diagram. Fig. 2 schematically represents the typical adsorption TES architecture, made up of a closed reactor containing the adsorbent material put in contact with a heat exchanger (i.e. adsorber) and a closed reactor containing the liquid adsorbate, which acts as condenser/evaporator depending on the working phase. Furthermore, in Fig. 2, heat and working fluid (adsorbate) fluxes occurring during each working phase are highlighted. During charging phase (i.e. desorption), the adsorber, in which the adsorbent material is saturated of adsorbate, is regenerated exploiting heat coming from the solar heat source, Qdes. This amount of energy can be distinguished, following the cycle in Fig. 1, in two different quantities: Q1, usually known as isosteric heat, which represents the amount of sensible thermal energy spent to
Greek symbols density (kg m3)
r
Subscripts 1, 2, 3, 4 cycle phases ads adsorption amb ambient ave average disch discharged eff effective ev evaporation L liquid max maximum met metal min minimum s sorbent sat saturation sens sensible st storage v vapour
heat up the adsorbent material and the adsorbate under isosteric conditions, during which no desorption happens. This phase is needed in order to increase the pressure inside the adsorber to the same level of the condenser pressure. Q2, which is the energy spent to desorb the adsorbate from the material, plus a small amount of specific heat to increase the adsorbent material temperature up to the final temperature, Tmax. The desorbed vapour is continuously condensed in the condenser. The heat of condensation, Qc, is usually dissipated in the ambient. Nevertheless, if needed, it can be also exploited by the user in daily heat storage applications, for instance by storing it in a buffer heat storage. Once the charging process is completed, the connection between condenser and adsorber is closed. In this condition, the system can keep the stored energy for indefinite time, since the thermal energy is stored as adsorption potential between adsorbate and adsorbent material. In order to get back the stored thermal energy, the connection between liquid adsorbate reservoir, which in this phase acts as evaporator, and adsorber is again opened. During this discharging phase (i.e. adsorption), the adsorbate is evaporated adsorbing heat from the ambient, Qev, then the vapour fluxes to the adsorber, since the adsorption process is exothermic, heat is released to the user, Qads. Also in this case, Qads comprises two different components, Q3 which represents the energy delivered during isosteric cooling down process, which brings the pressure down to the evaporator pressure and Q4 which mainly represents the energy associated to the enthalpy of adsorption. In the evaluation of an adsorption heat storage, two main quantities need to be taken into account, namely, the temperature lift, DTlift, and the energy storage density, Es (both per unit of mass and volume) [15]. The temperature lift represents the temperature difference between adsorber and evaporator during the adsorption phase and between adsorber and condenser during the desorption phase. Since the adsorption phase represents the discharging phase of the heat storage, it can be regarded as the achievable temperature upgrade from the low temperature heat source (evaporator) to the user. During the desorption phase, it can be considered as the temperature difference needed to drive the thermodynamic cycle. The energy storage density represents the amount of stored energy per unit of volume/mass.
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Fig. 1. Working cycle of an adsorption heat storage: the red lines represent the charging phase (desorption) while the blue lines the discharging phase (adsorption). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.1. Heat storage density The heat storage density represents the main parameter to compare achievable performance of different working pairs. Accordingly, the heat storage density for a given working pair can be calculated as follows:
Qst ¼ DHads Dw
(1)
where DHads [kJ/kg] represents the adsorption enthalpy and Dw [kg/kg] the uptake variation of the working pair under the investigated boundary conditions. The heat storage density can be calculated either per unit of mass, Ms, or per unit of volume, Vs. It has to be pointed out that, in case of daily heat storage application, the heat of condensation can be also exploited, if a small sensible heat storage buffer is coupled to the adsorption heat storage. In this case, the achievable heat storage density is usually almost double respect to the seasonal heat storage density under the same conditions.
2.2. Heat released to the user The heat released to the user during discharging (adsorption) phase represents another useful parameter. Indeed, this parameter
differs from the heat storage density as defined above, since, when the time lag between charging and discharging is sufficiently longer (i.e. seasonal heat storage conditions), a portion of the stored energy is spent during adsorption (i.e. discharging) to heat up the adsorbent material, which, after long time, can be considered in thermal equilibrium with the ambient. Accordingly, the heat available during discharge phase for a given working pair could be calculated as follows:
Qdisch ¼ Qst Qsens
(2)
where
Qsens ¼ ðcps þ cpL wmin þ cpv DwÞðTads Tamb Þ
(3)
This sensible energy is spent to heat up the adsorbent material, the adsorbed adsorbate and the vapour coming from the evaporator. It has to be pointed out that, in case of daily heat storage application, the amount of sensible heat spent to heat up the system during adsorption phase is lower, since it does not reach the equilibrium with the surrounding environment. In this framework, a parameter affecting the heat released to the user is represented by the heat exchanger mass. It has to be taken into account in order to give an idea of the heat that can be released at component level (i.e. adsorber), which represents a useful
Fig. 2. Closed adsorption heat storage cycle: charging and discharging phase.
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A. Frazzica, A. Freni / Renewable Energy xxx (2016) 1e8
information for first evaluation of heat storage reactor dimensions for a given application. Accordingly, a metal to sorbent mass ratio, r [kg/kg], was defined. The presence of metal mass affects the released energy, as defined by equation (3). In particular, it has been taken into account by substituting the specific heat of adsorbent, cps, with an effective specific heat, cpeff, defined as:
psat A ¼ R T ln p
cpeff ¼ cps þ r cpmet
n T w ¼ w0 exp k 1 Tsat
(4)
Furthermore, also an effective density has been introduced, to evaluate the volumetric heat released to the user, which is reduced due to the presence of the metallic heat exchanger:
reff ¼
rs
(5)
1 þ rrs r met
3. Investigated working pairs The literature reports several working pairs available for heat storage applications [4]. The most common working fluid for this application is water, since it has high thermodynamic performance (i.e. enthalpy of evaporation, specific heat) and it has no polluting effect. Nevertheless, it has limitation in terms of employable low temperature heat source, since it cannot be operated below 0 C, due to freezing effect. To overcome this limit, other working fluids can be used for adsorption TES applications, namely, ammonia, methanol and ethanol. Particularly, ammonia and methanol are characterized by good thermodynamic performance, but they need to be carefully handled due to their toxicity. Differently, ethanol, whose thermodynamic features are slightly lower than the others fluids, is not toxic. Accordingly, all these working fluids will be investigated in the present paper. Table 1 summarizes main features of the identified working fluids: A deep review about available adsorbent materials can be found elsewhere [16,17]. It is well known that several theories and models can describe the adsorption equilibrium curves. In Ref. [18] a deep review of the available approaches is reported, discussing advantages and fields of application. Among these, the empirical equations deriving from the Polanyi potential theory [19] represent a good compromise between the experimental data fitting accuracy and the possibility of correlating the equilibrium data with a unique variable, defined as adsorption potential, which is function of both temperature and pressure. Particularly, the Dubinin-Astakhov (DA) equation has been widely employed in literature to describe equilibrium data of several working pairs employing different adsorbent materials and different working fluids [20e22].
n A w ¼ wmax exp E
(6)
where the adsorption potential, A, is defined as:
(7)
Similar to the DA approach there is also the DubininRadushkevich (DR), whose modified version has been widely used in the literature especially for activated carbon/ammonia working pairs:
(8)
Sometimes, as reported in Ref. [18] it is not possible to accurately describe equilibrium data of a working pair over the full range of adsorption potential with only one set of parameters (i.e. E, n). In this case, a polynomial equation, employing different fitting parameters can be used. Following this approach, for the investigated working pairs that showed equilibrium curves that cannot be properly represented by only one set of parameters, the temperature-independent equilibrium curves have been divided in different segments. Each segment has been then properly fitted by varying two parameters, E and n, in order to have a fitting accuracy within 5% of deviation for the whole adsorption potential range of the temperature-independent curve. Furthermore, one more advantage of the potential theory is the possibility of deriving the isosteric enthalpy of adsorption again as function of the adsorption potential, which represents a powerful tool to make thermodynamic calculations of the working pairs achievable performance. Actually, for the DA approach it can be calculated as follow:
DHads ðwÞ ¼ DHev þ AðwÞ T DsðwÞ
(9)
While for the DR approach, it is in the form:
DHads ðwÞ ¼ R Y
T Tsat
(10)
where Y represents the slope of the saturated adsorbate line on the Clapeyron diagram that for ammonia, is 2823.4 K. Accordingly, all the selected working pairs investigated in the present paper have been analysed employing either the DubininAstakhov or the Dubinin-Radushkevich formalism to describe their equilibrium data. Some data have been taken from the literature while others from experimental data measured at the CNR ITAE lab as reported in Ref. [23]. In Table 2, fitting parameters for DA and DR equations are reported for all the selected and investigated working pairs. Particularly, both adsorption and desorption equilibrium curves have been separately fitted for the working pairs showing a hysteresis effect between adsorption and desorption branches. The selection of the adsorbent materials for each working fluid has been carried out, trying to represent all the main classes of adsorbents reported in literature [16]. Accordingly, for water as refrigerant, the zeolite 13X [22] has been selected as the most widely used “classical” zeolite for adsorption heat storage applications. Zeolite DDZ70, produced by UOP [31], has been selected since it represents the first commercial example of adsorbent
Table 1 Main features of the investigated working fluids. Working fluid
Freezing temperature [ C]
Enthalpy of evaporation @ 25 C [kJ/kg]
Specific heat liquid phase [kJ/kg K]
Density [kg/m3]
Water Ammonia Methanol Ethanol
0 78 97 114
2435.1 1166.1 1186.3 922.8
4.19 4.59 2.54 2.45
1000 681 791 789
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Table 2 Equilibrium data of the investigated working pairs. Working pair
Adsorption/desorption
A [kJ/kg]
w0 [g/g]
E [kJ/kg]
n
k
Ref.
Zeolite 13X/water Silica gel Siogel/water AQSOA Z02/water
Ads þ Des Ads þ Des Ads
Full range Full range <450 >450 <200 >200 < 305 >305 < 410 >410 <180 >180 < 260 >260 <175 >175 < 245 >245 <600 >600 <300 >300 < 321 >321 < 585 >585 <330 >330 < 335 >3335 < 420 >420 <415 >415 Full range Full range Full range Full range Full range <130 >130 < 145 >145
0.341 0.32 0.31
1192.3 245 388.8 265 400 810 410 410 300 222.2 55 275 215 71.5 550 500 49 181 34.5 463 295 326.7 236.6 23 310 394 e e 190.35 149.59 365.8 130 130 62.2
1.55 1.1 3 0.8 3.5 1.8 6 1.2 2 5 0.5 2.3 4.7 0.5 2 0.95 0.41 1.52 0.39 4.7 1.6 18 1.3 0.5 1.5 8.5 0.8529 1.0554 1.5 1.8 1.76 1.7 14 1.5
e e e e e e e e e e e e e e e e e e e e e e e e e e 5.0775 4.613 e e e e e e
[24] ITAE [22] ITAE ITAE ITAE ITAE ITAE ITAE [22] ITAE ITAE ITAE ITAE ITAE ITAE [25] [25] [25] [25] [26] [26] [26] [26] [26] [26] [27] [27] [24] [24] [21] [26] [26] [26]
Des
AQSOA Z01/water
Ads
Des
Zeolite DDZ70/water
Ads þ Des
SWS 1L/water
Ads þ Des
MWCNT-LiCl/water
Ads
Des SRD 1352/2/ammonia KOH-AC/ammonia SRD 1352/2/ethanol SG-LiBr/ethanol Carbotech A35/1/methanol MWCNT-LiCl/methanol
Ads Ads Ads Ads Ads Ads
þ þ þ þ þ þ
Des Des Des Des Des Des
material properly modified for adsorption heat transformation purposes. AQSOA Z02 and Z01, produced by Mitsubishi [32], have been selected as representative of the zeotype structure, which are considered as the most promising option for the development of adsorption heat transformers. Silica gel Siogel, produced by Oker Chemie [33], represents the wide class of microporous silica gels, which are commonly employed thanks to their cost effectiveness. Finally, SWS 1L (i.e. mesoporous silica gel impregnated by CaCl2) [34] and the recently developed Multi Wall Carbon NanoTube impregnated by LiCl (MWCNT-LiCl) [26] have been chosen to represent the available composite sorbents reported in literature. Concerning the other working fluids, in general, the most commonly employed adsorbent materials are the activated carbons. For this reason, the most performing activated carbons reported in literature have been chosen. Furthermore, as composite sorbents, silica gel impregnated by LiBr, has been identified for ethanol adsorption applications [28] and MWCNT impregnated by LiCl has been identified for methanol adsorption applications [26]. New adsorbent materials, considered promising for heat storage applications such as Metal Organic Frameworks (MOFs) [29], have not been taken into account, since are still at an early stage of development and too far from practical applicability. From adsorption point of view, the adsorption capacity of standard adsorbent materials is limited by their available pore volume (usually limited to 0.3e0.35 cm3/g). On the contrary, for sorbent composites the adsorption capacity depends on the interaction between working fluid and embedded inorganic salt, which can guarantee the possibility of properly tailor adsorption capacity and thus its achievable heat storage capacity. As highlighted by Table 2, equilibrium curves of classical adsorbent materials such as zeolite 13X, silica gel and activated
0.3
0.21
0.23 1.9
1.3
0.8392 0.6245 0.647 0.537 0.621 2.2
carbons can be satisfactorily represented by only one set of fitting parameters for the entire adsorption potential range. On the contrary, as expected, zeotypes (i.e. AQSOA Z02 and AQSOA Z01) and composite sorbents equilibrium curves need to be subdivided in different segments, in order to increase the fitting accuracy. Indeed, the zeotypes show a partially hydrophilic/hydrophobic behaviour, which causes the well-known S-shaped equilibrium curve evolution [30] that cannot be efficiently represented by only one set of parameters. Similarly, for composite materials, since the adsorption capacity highly depends on the reaction between the working fluid and the inorganic salt, the equilibrium curve has a non-regular evolution, due to the different chemical reaction steps between the salt and the working fluid [26]. 4. Simulation results 4.1. Working pairs heat storage density In the present paragraph, simulation results for adsorption heat storage application under typical seasonal working conditions are reported. Indeed, since adsorption heat storage technology belongs to the wider class of thermochemical heat storage, so far, its main application has been for seasonal heat storage, thanks to the advantages above reported. In the following, a comparison among the identified working pairs is carried out. In Fig. 3 seasonal heat storage density of the investigated working pairs as a function of temperature lift is reported. It has to be pointed out that, in this calculation, the condensation temperature is different from the adsorption temperature, and kept constant at 30 C, since heat of condensation released during desorption phase is not recovered to increase heat storage density.
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Fig. 3. Seasonal energy storage densities of the identified working pairs as a function of the temperature lift: a) water as working fluid; b) other working fluids. Desorption temperature fixed at 100 C, condensation temperature fixed at 30 C, evaporation temperature at 10 C.
The working pairs employing water as working fluids usually show higher energy storage density compared to the other working fluids. This is clearly related to the higher evaporation enthalpy of water compared to the others. The only exception is represented by the composite MWCNT-LiCl operated with methanol, which shows heat storage density higher than most of the water-based adsorbent materials, thanks to its extremely high adsorption capacity. Particularly, among the water adsorbents, the composites MWCNTLiCl and SWS 1L showed the highest storage density, which are usually more than double compared to other adsorbent materials like AQSOA Z02, Z01 and Silica gel Siogel. The discontinuity highlighted in the heat storage density of MWCNT-LiCl with water demonstrates how this highly depends on the reaction step between water and salt, which can cause a sudden increase of adsorption capacity and relative heat storage density. Particularly, for low temperature lifts (i.e. up to 30 C), MWCNTLiCl both with water and methanol, are the best options, with heat storage density ranging between 250 and 100 kWh/m3. Overcoming a certain threshold (about 35 C for water and 30 C for methanol) this composite strongly reduces the achievable heat storage density, due to limited interaction between working fluids and salt. Above 35 C, also SWS 1L and AQSOA Z02 with water are competitive options. Looking at the achievable heat storage density of the other working pairs, the most performing, KOH-AC/ammonia, showed about three times lower heat storage density than MWCNT-LiCl/ water. Another factor affecting the heat storage density is also the
different adsorbent material density. Since no data are available in literature for MWCNT-LiCl composite, a density of 300 kg/m3 has been guessed, considering the possibility of compressing the composite itself in order to increase its density, while, for instance, 500 kg/m3 have been considered for the KOH-AC as reported in Ref. [27]. Fig. 4 reports the seasonal energy storage density of the investigated working pairs as a function of the desorption temperature, which corresponds to different solar thermal collectors technologies and solar irradiation levels. In this case, adsorption, condensation and evaporation temperatures have been fixed at 50 C, 30 C and 10 C respectively. The resulting heat storage densities are highly affected by the regeneration temperature. Indeed, MWCNT-LiCl/water shows the highest heat storage density in a range between 90 and 105 C of regeneration temperature. Above 105 C the highest heat storage density is achieved by SWS 1L, thanks to the most efficient regeneration phase achievable at higher temperatures. At high temperature, also AQSOA Z02 and Zeo DDZ70 are competitive options. Interestingly, under these conditions, AQSOA Z01 is not able to store thermal energy, since temperature difference between evaporator and adsorber is too large. Among the other working pairs, as already highlighted before, due to the low evaporation temperature coupled to the high adsorption temperature, the MWCNT-LiCl/methanol shows the lower achievable heat storage density, even at high regeneration temperature. Furthermore, it is evident that the ones employing ammonia and methanol as
Fig. 4. Seasonal energy storage densities of the identified working pairs as a function of the desorption temperature: a) water as working fluid; b) other working fluids. Condensation temperature fixed at 30 C, adsorption temperature at 50 C, evaporation temperature at 10 C.
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Fig. 5. Seasonal energy storage densities of the identified working pairs for three different evaporation temperatures. Desorption temperature fixed at 100 C, condensation temperature at 30 C, adsorption temperature at 50 C, ambient temperature at 20 C.
working fluid take more advantage than the ones based on ethanol as working fluid from the increasing in desorption temperature. However, their heat storage density is far from the best working pairs employing water as working fluid. In Fig. 5, the effect of evaporation temperature on the achievable heat storage density is analysed. Three different evaporator temperatures, namely, 0 C, 10 C and 20 C, which can be achieved by different low temperature heat sources usually employed, such as ambient air, solar thermal collectors and ground heat exchangers, depending on the climatic conditions have been investigated. Clearly, working pairs employing water as working fluid cannot be operated for low temperature heat sources at 0 C, due to freezing issues. This parameter is very important for seasonal heat storage applications, since during winter season, this temperature can vary a lot. It is evident that the application at very low temperature 0 C (e.g. air source evaporators), cannot guarantee enough energy storage density, usually lower than 10 kWh/m3. This confirms the need to employ either solar thermal collectors or ground heat exchangers as low temperature heat source to get satisfactory heat storage densities. Interestingly, under these working boundary conditions, also the composite MWCNT-LiCl/methanol does not reach good performance. Indeed, it suffers a lot the low evaporation pressure, which hinders the reaction between methanol and LiCl, limiting the working fluid exchange and, thus, the achievable heat storage density. MWCNT-LiCl/water demonstrates again the highest storage density, but also SWS 1L and AQSOA Z02 with water and MWCNT-LiCl/methanol represent good alternative when higher evaporation temperature is available. 4.2. Heat released to the user As reported above, the heat released to the user is always reduced by the energy spent to heat up inert masses. This effect has been then analysed in the present paragraph. All the calculations have been performed considering an ambient temperature, Tamb in equation (3), at 20 C. Compared to the heat storage density of each working pair, the effect of the inert masses reduces the heat available to the user in a range between 2 and 20%, depending on the analysed working boundary condition. More relevant is the effect of the heat exchanger mass. For instance, Fig. 6 summarizes the influence of metal masses for three water-based working pairs (i.e. AQSOA Z02, Silica gel Siogel and MWCNT-LiCl) and one ammonia-based working pair (i.e. KOH-AC/ ammonia). Stainless steel has been supposed as heat exchanger
7
Fig. 6. Effect of the metal to sorbent mass ratio on the heat released to the user in seasonal heat storage application for three working pairs employing water as working fluid and one working pair employing ammonia. Desorption temperature fixed at 100 C. Adsorption temperature at 50 C. Condensation temperature at 30 C, evaporation temperature at 10 C.
material. The presence of the metal plays two different detrimental roles on the volumetric heat released to the user. On one hand, more inert masses are present, which causes an increasing of energy spent to heat up the system. On the other hand, the lower effective density tends to reduce the overall released heat. Particularly, for MWCNT-LiCl/water and AQSOA Z02/water this reduction ranges between 10% and 35% when r ¼ 3. This confirms that this parameter cannot be neglected during preliminary sizing of a seasonal heat storage. As reported in Ref. [35] the typical thermal energy consumption for heating and domestic hot water for a German single-family house, built between 1995 and 2014, is around 60 kWh/(m2 year). Accordingly, it is possible to make a rough estimation of the heat storage volume of the adsorber necessary to cover the entire energy demand of a 100 m2 single-family house. Considering the best identified working pair, MWCNT-LiCl/water with a metal to sorbent mass ratio of 3, regenerated at 100 C with a condensation temperature of 30 C and able to deliver 50 C during winter, exploiting a low temperature heat source at 10 C (see Fig. 6), about 115 m3 of heat storage are needed. It has to be pointed out that, in order to estimate the volume of the whole heat storage, also other components, like evaporator and hydraulics should be taken into account. This heavily depends on the design phase of the system.
5. Conclusions The thermodynamic analysis carried out for adsorption TES applications in the domestic sector demonstrated that this technology can be considered promising both for daily and seasonal operating conditions. One of the key issue for proper heat storage development is the identification of the most performing working pair under practical working boundary conditions. Among the working pairs available in literature, the composite based on MWCNT and LiCl, with both water and methanol as working fluid, demonstrated the highest achievable heat storage density being 130 kWh/m3 under practical working conditions. Another valid option is represented by the zeotype AQSOA Z02 and the composite sorbent SWS 1L and water, whose achievable heat storage density is sometimes comparable to the one obtained with MWCNT-LiCl. Other working pairs employing water as working fluid, such as classical zeolite 13X and silica gels are characterized by lower heat storage density, which make them less attractive. Especially, zeolite
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13X cannot be efficiently operated when non-concentrating solar thermal collectors, usually employed in domestic applications, represent the primary heat source. The others working pairs, employing different working fluids (i.e. ethanol, ammonia, methanol) can exploit lower evaporator temperatures, but their performance are still too low if compared to the most efficient working pairs. This confirms how, the composite sorbents, allowing a tailoring of the adsorption properties, can represent the most effective way to quickly reach a marketability of this technology. Other classes of adsorbents, like MOFs, are still in an early stage of development, but already showed promising features for a further improving of achievable thermal energy storage performance. Finally, the analysed influence of the heat exchanger metal mass demonstrated the importance of this parameter in the evaluation of the heat that can actually be exploited by the user during the discharging phase. Indeed, for instance, the dischargeable energy in adsorbers employing MWCNT-LiCl and AQSOA Z02/water can be reduced of about 10% and 35% respectively when a metal (stainless steel) to sorbent mass ratio of 3 is considered. Acknowledgments The authors would like to acknowledge Dr. Larisa Gordeeva and Dr. Alexandra Grekova for having provided the experimental data for MWCNT-LiCl composite sorbent. References [1] Re-thinking 2050-A 100% Renewable Energy Vision for the European Union, EREC e European Renewable Energy Council, 2010. [2] Strategic Research Priorities for Solar Thermal Technology, European Technology Platform on Renewable Heating and Cooling, 2012. [3] T. Khadiran, M.Z. Hussein, Z. Zainal, R. Rusli, Advanced energy storage materials for building applications and their thermal performance characterization: a review, Renew. Sustain. Energy Rev. 57 (2016) 916e928. [4] N. Yu, R.Z. Wang, L.W. Wang, Sorption thermal storage for solar energy, Prog. Energy Combust. Sci. 39 (2013) 489e514. [5] D. Jaehnig, R. Hausner, W. Wagner, C. Isaksson, Thermo-chemical storage for solar space heating in single-family house, in: Proceedings of 10th International Conference on Thermal Energy Storage, New Jersey, USA, 2006. [6] G. Li, S. Qian, H. Lee, Y. Hwang, R. Radermacher, Experimental investigation of energy and exergy performance of short term adsorption heat storage for residential application, Energy 65 (2014) 675e691. [7] B. Michel, N. Mazet, S. Mauran, D. Stitou, J. Xu, Thermochemical process for seasonal storage of solar energy: characterization and modeling of a high density reactive bed, Energy 47 (2012) 553e563. [8] D. Dicaire, F.H. Tezel, Regeneration and efficiency characterization of hybrid adsorbent for thermal energy storage of excess and solar heat, Renew. Energy 36 (2011) 986e992. [9] D. Aydin, S.P. Casey, S. Riffat, The latest advancements on thermochemical heat storage systems, Renew. Sustain. Energy Rev. 41 (2015) 356e367. [10] H.A. Zondag, Sorption heat storage (Chapter 6), Sol. Energy Storage (2015) 135e154. [11] L.W. Wang, R.Z. Wang, R.G. Oliveira, A review on adsorption working pairs for refrigeration, Renew. Sustain. Energy Rev. 13 (2009) 518e534. [12] M. Pons, F. Meunier, G. Cacciola, R.E. Critoph, M. Groll, L. Puigjaner, B. Spinner, F. Ziegler, Thermodynamic based comparison of sorption systems for cooling
and heat pumping, Int. J. Refrig. 22 (1999) 5e17. [13] Yu I. Aristov, Current progress in adsorption technologies for low-energy buildings, Future Cities Environ. (2015) 1e10. [14] A. Hauer, Evaluation of adsorbent materials for heat pump and thermal energy storage applications in open systems, Adsorption 13 (2007) 399e405. [15] T. Nunez, H.-M. Henning, W. Mittelbach, Adsorption cycle modeling: characterization and comparison of material, in: Proceedings of International Sorption Heat Pump Conference, 1999, pp. 209e217. Munich, Germany. [16] Y.I. Aristov, Challenging offers of material science for adsorption heat transformation: a review, Appl. Therm. Eng. 50 (2013) 1610e1618. [17] Freni A, Maggio G, Sapienza A, Frazzica A, Restuccia G, Vasta S. Comparative analysis of promising adsorbent/adsorbate pairs for adsorptive heat pumping, air conditioning and refrigeration, Applied Thermal Engineering, accepted. [18] Y.I. Aristov, Adsorptive transformation of heat: principles of construction of adsorbents database, Appl. Therm. Eng. 42 (2012) 18e24. [19] M. Polanyi, Theories of the adsorption of gases, Trans. Faraday Soc. 28 (1932) 316e333. [20] I.I. El-Sharkawy, K. Kuwahara, B.B. Saha, S. Koyama, K.C. Ng, Experimental investigation on adsorption of ethanol onto activated carbon fibers for possible application in adsorption cooling system, Appl. Therm. Eng. 26 (2006) 859e865. [21] S.K. Henninger, M. Schicktanz, P.P.C. Hügenell, H. Sievers, H.-M. Henning, Evaluation of methanol adsorption on activated carbons for thermally driven chillers part I: thermophysical characterization, Int. J. Refrig. 35 (2012) 543e553. [22] S. Kayal, S. Baichuan, B.B. Saha, Adsorption characteristics of AQSOA zeolites and water for adsorption chillers, Int. J. Heat Mass Transf. 92 (2016) 1120e1127. [23] L. Gordeeva, A. Freni, T.A. Krieger, G. Restuccia, Y.I. Aristov, Composites “lithium halides in silica gel pores”: methanol sorption equilibrium, Microporous Mesoporous Mater. 112 (2008) 254e261. [24] B. Mette, H. Kerskes, H. Drück, H. Müller-Steinhagen, Experimental and numerical investigations on the water vapor adsorption isotherms and kinetics of binderless zeolite 13X, Int. J. Heat Mass Transf. 71 (2014) 555e561. [25] M.M. Tokarev, B.N. Okunev, M.S. Safonov, L.I. Kheifets, Y.I. Aristov, Approximation equations for describing the sorption equilibrium between water vapor and a CaCl2-in-silica gel composite sorbent, Russ. J. Phys. Chem. 79 (2005) 1490e1493. [26] A. Grekova, L. Gordeeva, Y.I. Aristov, Composite sorbents “Li/Ca halogenides inside multi-wall carbon nano-tubes” for thermal energy storage, Sol. Energy Mater. Sol. Cells 155 (2016) 176e183. [27] Z. Tamainot-Telto, S.J. Metcalf, R.E. Critoph, Y. Zhong, R. Thorpe, Carboneammonia pairs for adsorption refrigeration applications: ice making, air conditioning and heat pumping, Int. J. Refrig. 32 (2009) 1212e1229. [28] V. Brancato, A. Frazzica, A. Sapienza, L. Gordeeva, A. Freni, Ethanol adsorption onto carbonaceous and composite adsorbents for adsorptive cooling system, Energy 84 (2015) 177e185. [29] M.F. De Lange, K. Verouden, T. Vlugt, J. Gascon, F. Kapteijn, Adsorption-Driven heat pumps: the potential of metaleorganic frameworks, Chem. Rev. 115 (2015) 12205e12250. [30] A. Frazzica, A. Sapienza, A. Freni, Novel experimental methodology for the characterization of thermodynamic performance of advanced working pairs for adsorptive heat transformers, Appl. Therm. Eng. 72 (2014) 229e236. [31] S.R. Dunne, S.M. Taqvi, Adsorption Cooling Using Adsorbent-coated Surfaces, Elsevier Science Publishers Ltd, Amsterdam, 1998, pp. 1101e1106. [32] K. Okamoto, M. Teduka, T. Nakano, S. Kubokawa, H. Kakiuchi, The development of Aqsoa water vapour adsorbent and Aqsoa coated heat exchanger, in: Proceeding of Innovative Materials for Processes in Energy Systems (IMPRES), Singapore, 2010. [33] SIOGEL WHITE, Oker-Chemie GmbH, May 2008 (Product Information). [34] Y.I. Aristov, G. Restuccia, G. Cacciola, V.N. Parmon, A family of new working materials for solid sorption air conditioning systems, Appl. Therm. Eng. 22 (2002) 191e204. [35] Renovating Germany's Building Stock, Buildings Performance Institute Europe (BPIE), 2015. ISBN: 9789491143137.
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Adsorbent working pairs for solar thermal energy storage in buildings Andrea Frazzica*, Angelo Freni CNR e ITAE, Istituto di Tecnologie Avanzate per l'Energia “Nicola Giordano”, Via Salita S. Lucia sopra Contesse 5, I-98126, Santa Lucia, Messina, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2016 Received in revised form 15 September 2016 Accepted 20 September 2016 Available online xxx
In this study, the thermodynamic analysis of several adsorption working pairs for adsorption heat storage applications at domestic level is presented. The selected working pairs employ different working fluids (i.e. water, ethanol, ammonia, methanol) and different adsorbent materials such as classical zeolites, silica gels, alumino-phosphates, composite sorbents and activated carbons. The simulations have been performed taking into account desorption temperatures in the range between 80 C and 120 C, compatible with non-concentrating solar thermal collectors, under seasonal heat storage working conditions. The composite sorbent MWCNT-LiCl with both water and methanol as working fluid showed the highest heat storage density under practical working boundary conditions. Among the standard adsorbents, the zeotype AQSOA Z02 showed promising achievable heat storage densities. Classical working pairs, such as zeolite 13X/water, commonly employed for heat storage applications, are not suitable for this working range. Finally, also the influence of the metal to sorbent mass ratio, due to the heat exchanger, was investigated, demonstrating that it can reduce the achievable amount of heat released to the user up to 30%. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Adsorption Thermal energy storage Solar thermal Thermochemical heat storage
1. Introduction Renewable heating and cooling (RHC) sector, which mainly comprises solar thermal, geothermal and biomass energy, has been defined as the “sleeping giant” in 2011 by the European Renewable Energy Council (EREC) in the “Re-thinking 2050” document [1]. Indeed, it is expected that RHC will represent approximately 21% and 45% of the total final energy consumption in 2030 and 2050 respectively [1]. Among the RHC technologies, solar thermal seems the most promising thanks to its wide availability as well as to the reached technological maturity, which is making it as a valid alternative not only for standard applications (e.g. domestic hot water production, space heating), but also for more advanced applications (e.g. industrial heat, solar cooling) [2]. Nevertheless, there are still open research and development topics that may increase efficiency as well as cost effectiveness of solar thermal energy technology in order to promote and stimulate its deployment at large market scale. At the components level, the main fields of research are oriented towards solar thermal collectors, control and performance assessment and thermal energy storage [2]. Particularly, thermal energy storage (TES) is a crucial component in a solar
* Corresponding author. E-mail address: [email protected] (A. Frazzica).
thermal system, in order to cover the mismatch between energy supply and demand, thus increasing the achievable solar exploitation [3]. For this reason, it needs to be further investigated and optimized, in order to increase energy storage density and to limit energy losses through the environment during its operation [2]. Considering its working principle, thermochemical TES could be considered as the most promising solution to fulfil these technological needs [4]. Indeed, in a thermochemical TES, heat is stored as chemical potential deriving from the breaking of bonds between the sorbent material and the working fluid. This guarantees highenergy storage density, since the enthalpy of reaction can be order of magnitudes higher than specific heat, as well as no degradation of stored energy during time, since heat can remain stored as long as the sorbate and the working fluid are kept separated. Accordingly, thermochemical TES represents a viable way to enhance solar thermal systems performance both through high energy density daily heat storage [5,6] as well as for seasonal heat storage applications [7,8]. Main features of different thermochemical heat storage technologies have been recently discussed in a review [9]. Among the wider class of thermochemical TES, the adsorption one seems the most suitable for residential building applications. Indeed, it is characterized by good energy storage density, possibility to store energy at low-medium temperature (i.e. below 100 C), typical of non-concentrating solar thermal collector technologies usually employed in this field, and ability to be
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Nomenclature A cp E k M n p Q r R T V Y w DH Ds Dw
adsorption potential (kJ kg1) specific heat (kJ kg1 K1) characteristic energy of the working pair (kJ kg1) pre-exponential Dubinin factor () mass (kg) Dubinin exponent () pressure (Pa) heat (kJ kg1) metal to sorbent mass ratio (kg kg1) universal gas constant (J mol1 K1) temperature (K) volume (m3) slope of the saturated adsorbate line on the Clapeyron diagram (K) uptake (kg kg1) enthalpy (kJ kg1) entropy variation (kJ kg1 K1) uptake variation (kg kg1)
operated both as daily and seasonal storage [10]. In order to develop an efficient adsorption TES, the first step is represented by the selection of the proper working pair. In the literature the performance of several adsorbent materials (e.g. zeolites, silica gels, composite sorbents) and working fluids (e.g. water, ammonia, alcohols) have been investigated, in the past, mainly for refrigeration and heat pumping applications [11,12]. More recently, Aristov [13] has reported an interesting review of available systems and adsorption working pairs for heat storage application in buildings. Nevertheless, a comprehensive thermodynamic analysis of different adsorption working pairs for heat storage applications has not been reported so far. Accordingly, the present paper aims at a thermodynamic comparison of the main adsorption working pairs available in the literature for adsorption heat transformer technologies. The analysis is carried out as a function of working boundary conditions.
2. Performance evaluation of the adsorption heat storage Generally, there are two system configurations for adsorption TES: closed and open cycle. The present paper will mainly focus on closed adsorption systems. Deep analysis of open adsorption TES can be found elsewhere [14]. In the following, the closed adsorption TES working principle will be shortly introduced. More detailed analysis can be found elsewhere [10]. Fig. 1 reports the working cycle for a closed adsorption TES on the Clapeyron diagram. Fig. 2 schematically represents the typical adsorption TES architecture, made up of a closed reactor containing the adsorbent material put in contact with a heat exchanger (i.e. adsorber) and a closed reactor containing the liquid adsorbate, which acts as condenser/evaporator depending on the working phase. Furthermore, in Fig. 2, heat and working fluid (adsorbate) fluxes occurring during each working phase are highlighted. During charging phase (i.e. desorption), the adsorber, in which the adsorbent material is saturated of adsorbate, is regenerated exploiting heat coming from the solar heat source, Qdes. This amount of energy can be distinguished, following the cycle in Fig. 1, in two different quantities: Q1, usually known as isosteric heat, which represents the amount of sensible thermal energy spent to
Greek symbols density (kg m3)
r
Subscripts 1, 2, 3, 4 cycle phases ads adsorption amb ambient ave average disch discharged eff effective ev evaporation L liquid max maximum met metal min minimum s sorbent sat saturation sens sensible st storage v vapour
heat up the adsorbent material and the adsorbate under isosteric conditions, during which no desorption happens. This phase is needed in order to increase the pressure inside the adsorber to the same level of the condenser pressure. Q2, which is the energy spent to desorb the adsorbate from the material, plus a small amount of specific heat to increase the adsorbent material temperature up to the final temperature, Tmax. The desorbed vapour is continuously condensed in the condenser. The heat of condensation, Qc, is usually dissipated in the ambient. Nevertheless, if needed, it can be also exploited by the user in daily heat storage applications, for instance by storing it in a buffer heat storage. Once the charging process is completed, the connection between condenser and adsorber is closed. In this condition, the system can keep the stored energy for indefinite time, since the thermal energy is stored as adsorption potential between adsorbate and adsorbent material. In order to get back the stored thermal energy, the connection between liquid adsorbate reservoir, which in this phase acts as evaporator, and adsorber is again opened. During this discharging phase (i.e. adsorption), the adsorbate is evaporated adsorbing heat from the ambient, Qev, then the vapour fluxes to the adsorber, since the adsorption process is exothermic, heat is released to the user, Qads. Also in this case, Qads comprises two different components, Q3 which represents the energy delivered during isosteric cooling down process, which brings the pressure down to the evaporator pressure and Q4 which mainly represents the energy associated to the enthalpy of adsorption. In the evaluation of an adsorption heat storage, two main quantities need to be taken into account, namely, the temperature lift, DTlift, and the energy storage density, Es (both per unit of mass and volume) [15]. The temperature lift represents the temperature difference between adsorber and evaporator during the adsorption phase and between adsorber and condenser during the desorption phase. Since the adsorption phase represents the discharging phase of the heat storage, it can be regarded as the achievable temperature upgrade from the low temperature heat source (evaporator) to the user. During the desorption phase, it can be considered as the temperature difference needed to drive the thermodynamic cycle. The energy storage density represents the amount of stored energy per unit of volume/mass.
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Fig. 1. Working cycle of an adsorption heat storage: the red lines represent the charging phase (desorption) while the blue lines the discharging phase (adsorption). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.1. Heat storage density The heat storage density represents the main parameter to compare achievable performance of different working pairs. Accordingly, the heat storage density for a given working pair can be calculated as follows:
Qst ¼ DHads Dw
(1)
where DHads [kJ/kg] represents the adsorption enthalpy and Dw [kg/kg] the uptake variation of the working pair under the investigated boundary conditions. The heat storage density can be calculated either per unit of mass, Ms, or per unit of volume, Vs. It has to be pointed out that, in case of daily heat storage application, the heat of condensation can be also exploited, if a small sensible heat storage buffer is coupled to the adsorption heat storage. In this case, the achievable heat storage density is usually almost double respect to the seasonal heat storage density under the same conditions.
2.2. Heat released to the user The heat released to the user during discharging (adsorption) phase represents another useful parameter. Indeed, this parameter
differs from the heat storage density as defined above, since, when the time lag between charging and discharging is sufficiently longer (i.e. seasonal heat storage conditions), a portion of the stored energy is spent during adsorption (i.e. discharging) to heat up the adsorbent material, which, after long time, can be considered in thermal equilibrium with the ambient. Accordingly, the heat available during discharge phase for a given working pair could be calculated as follows:
Qdisch ¼ Qst Qsens
(2)
where
Qsens ¼ ðcps þ cpL wmin þ cpv DwÞðTads Tamb Þ
(3)
This sensible energy is spent to heat up the adsorbent material, the adsorbed adsorbate and the vapour coming from the evaporator. It has to be pointed out that, in case of daily heat storage application, the amount of sensible heat spent to heat up the system during adsorption phase is lower, since it does not reach the equilibrium with the surrounding environment. In this framework, a parameter affecting the heat released to the user is represented by the heat exchanger mass. It has to be taken into account in order to give an idea of the heat that can be released at component level (i.e. adsorber), which represents a useful
Fig. 2. Closed adsorption heat storage cycle: charging and discharging phase.
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information for first evaluation of heat storage reactor dimensions for a given application. Accordingly, a metal to sorbent mass ratio, r [kg/kg], was defined. The presence of metal mass affects the released energy, as defined by equation (3). In particular, it has been taken into account by substituting the specific heat of adsorbent, cps, with an effective specific heat, cpeff, defined as:
psat A ¼ R T ln p
cpeff ¼ cps þ r cpmet
n T w ¼ w0 exp k 1 Tsat
(4)
Furthermore, also an effective density has been introduced, to evaluate the volumetric heat released to the user, which is reduced due to the presence of the metallic heat exchanger:
reff ¼
rs
(5)
1 þ rrs r met
3. Investigated working pairs The literature reports several working pairs available for heat storage applications [4]. The most common working fluid for this application is water, since it has high thermodynamic performance (i.e. enthalpy of evaporation, specific heat) and it has no polluting effect. Nevertheless, it has limitation in terms of employable low temperature heat source, since it cannot be operated below 0 C, due to freezing effect. To overcome this limit, other working fluids can be used for adsorption TES applications, namely, ammonia, methanol and ethanol. Particularly, ammonia and methanol are characterized by good thermodynamic performance, but they need to be carefully handled due to their toxicity. Differently, ethanol, whose thermodynamic features are slightly lower than the others fluids, is not toxic. Accordingly, all these working fluids will be investigated in the present paper. Table 1 summarizes main features of the identified working fluids: A deep review about available adsorbent materials can be found elsewhere [16,17]. It is well known that several theories and models can describe the adsorption equilibrium curves. In Ref. [18] a deep review of the available approaches is reported, discussing advantages and fields of application. Among these, the empirical equations deriving from the Polanyi potential theory [19] represent a good compromise between the experimental data fitting accuracy and the possibility of correlating the equilibrium data with a unique variable, defined as adsorption potential, which is function of both temperature and pressure. Particularly, the Dubinin-Astakhov (DA) equation has been widely employed in literature to describe equilibrium data of several working pairs employing different adsorbent materials and different working fluids [20e22].
n A w ¼ wmax exp E
(6)
where the adsorption potential, A, is defined as:
(7)
Similar to the DA approach there is also the DubininRadushkevich (DR), whose modified version has been widely used in the literature especially for activated carbon/ammonia working pairs:
(8)
Sometimes, as reported in Ref. [18] it is not possible to accurately describe equilibrium data of a working pair over the full range of adsorption potential with only one set of parameters (i.e. E, n). In this case, a polynomial equation, employing different fitting parameters can be used. Following this approach, for the investigated working pairs that showed equilibrium curves that cannot be properly represented by only one set of parameters, the temperature-independent equilibrium curves have been divided in different segments. Each segment has been then properly fitted by varying two parameters, E and n, in order to have a fitting accuracy within 5% of deviation for the whole adsorption potential range of the temperature-independent curve. Furthermore, one more advantage of the potential theory is the possibility of deriving the isosteric enthalpy of adsorption again as function of the adsorption potential, which represents a powerful tool to make thermodynamic calculations of the working pairs achievable performance. Actually, for the DA approach it can be calculated as follow:
DHads ðwÞ ¼ DHev þ AðwÞ T DsðwÞ
(9)
While for the DR approach, it is in the form:
DHads ðwÞ ¼ R Y
T Tsat
(10)
where Y represents the slope of the saturated adsorbate line on the Clapeyron diagram that for ammonia, is 2823.4 K. Accordingly, all the selected working pairs investigated in the present paper have been analysed employing either the DubininAstakhov or the Dubinin-Radushkevich formalism to describe their equilibrium data. Some data have been taken from the literature while others from experimental data measured at the CNR ITAE lab as reported in Ref. [23]. In Table 2, fitting parameters for DA and DR equations are reported for all the selected and investigated working pairs. Particularly, both adsorption and desorption equilibrium curves have been separately fitted for the working pairs showing a hysteresis effect between adsorption and desorption branches. The selection of the adsorbent materials for each working fluid has been carried out, trying to represent all the main classes of adsorbents reported in literature [16]. Accordingly, for water as refrigerant, the zeolite 13X [22] has been selected as the most widely used “classical” zeolite for adsorption heat storage applications. Zeolite DDZ70, produced by UOP [31], has been selected since it represents the first commercial example of adsorbent
Table 1 Main features of the investigated working fluids. Working fluid
Freezing temperature [ C]
Enthalpy of evaporation @ 25 C [kJ/kg]
Specific heat liquid phase [kJ/kg K]
Density [kg/m3]
Water Ammonia Methanol Ethanol
0 78 97 114
2435.1 1166.1 1186.3 922.8
4.19 4.59 2.54 2.45
1000 681 791 789
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Table 2 Equilibrium data of the investigated working pairs. Working pair
Adsorption/desorption
A [kJ/kg]
w0 [g/g]
E [kJ/kg]
n
k
Ref.
Zeolite 13X/water Silica gel Siogel/water AQSOA Z02/water
Ads þ Des Ads þ Des Ads
Full range Full range <450 >450 <200 >200 < 305 >305 < 410 >410 <180 >180 < 260 >260 <175 >175 < 245 >245 <600 >600 <300 >300 < 321 >321 < 585 >585 <330 >330 < 335 >3335 < 420 >420 <415 >415 Full range Full range Full range Full range Full range <130 >130 < 145 >145
0.341 0.32 0.31
1192.3 245 388.8 265 400 810 410 410 300 222.2 55 275 215 71.5 550 500 49 181 34.5 463 295 326.7 236.6 23 310 394 e e 190.35 149.59 365.8 130 130 62.2
1.55 1.1 3 0.8 3.5 1.8 6 1.2 2 5 0.5 2.3 4.7 0.5 2 0.95 0.41 1.52 0.39 4.7 1.6 18 1.3 0.5 1.5 8.5 0.8529 1.0554 1.5 1.8 1.76 1.7 14 1.5
e e e e e e e e e e e e e e e e e e e e e e e e e e 5.0775 4.613 e e e e e e
[24] ITAE [22] ITAE ITAE ITAE ITAE ITAE ITAE [22] ITAE ITAE ITAE ITAE ITAE ITAE [25] [25] [25] [25] [26] [26] [26] [26] [26] [26] [27] [27] [24] [24] [21] [26] [26] [26]
Des
AQSOA Z01/water
Ads
Des
Zeolite DDZ70/water
Ads þ Des
SWS 1L/water
Ads þ Des
MWCNT-LiCl/water
Ads
Des SRD 1352/2/ammonia KOH-AC/ammonia SRD 1352/2/ethanol SG-LiBr/ethanol Carbotech A35/1/methanol MWCNT-LiCl/methanol
Ads Ads Ads Ads Ads Ads
þ þ þ þ þ þ
Des Des Des Des Des Des
material properly modified for adsorption heat transformation purposes. AQSOA Z02 and Z01, produced by Mitsubishi [32], have been selected as representative of the zeotype structure, which are considered as the most promising option for the development of adsorption heat transformers. Silica gel Siogel, produced by Oker Chemie [33], represents the wide class of microporous silica gels, which are commonly employed thanks to their cost effectiveness. Finally, SWS 1L (i.e. mesoporous silica gel impregnated by CaCl2) [34] and the recently developed Multi Wall Carbon NanoTube impregnated by LiCl (MWCNT-LiCl) [26] have been chosen to represent the available composite sorbents reported in literature. Concerning the other working fluids, in general, the most commonly employed adsorbent materials are the activated carbons. For this reason, the most performing activated carbons reported in literature have been chosen. Furthermore, as composite sorbents, silica gel impregnated by LiBr, has been identified for ethanol adsorption applications [28] and MWCNT impregnated by LiCl has been identified for methanol adsorption applications [26]. New adsorbent materials, considered promising for heat storage applications such as Metal Organic Frameworks (MOFs) [29], have not been taken into account, since are still at an early stage of development and too far from practical applicability. From adsorption point of view, the adsorption capacity of standard adsorbent materials is limited by their available pore volume (usually limited to 0.3e0.35 cm3/g). On the contrary, for sorbent composites the adsorption capacity depends on the interaction between working fluid and embedded inorganic salt, which can guarantee the possibility of properly tailor adsorption capacity and thus its achievable heat storage capacity. As highlighted by Table 2, equilibrium curves of classical adsorbent materials such as zeolite 13X, silica gel and activated
0.3
0.21
0.23 1.9
1.3
0.8392 0.6245 0.647 0.537 0.621 2.2
carbons can be satisfactorily represented by only one set of fitting parameters for the entire adsorption potential range. On the contrary, as expected, zeotypes (i.e. AQSOA Z02 and AQSOA Z01) and composite sorbents equilibrium curves need to be subdivided in different segments, in order to increase the fitting accuracy. Indeed, the zeotypes show a partially hydrophilic/hydrophobic behaviour, which causes the well-known S-shaped equilibrium curve evolution [30] that cannot be efficiently represented by only one set of parameters. Similarly, for composite materials, since the adsorption capacity highly depends on the reaction between the working fluid and the inorganic salt, the equilibrium curve has a non-regular evolution, due to the different chemical reaction steps between the salt and the working fluid [26]. 4. Simulation results 4.1. Working pairs heat storage density In the present paragraph, simulation results for adsorption heat storage application under typical seasonal working conditions are reported. Indeed, since adsorption heat storage technology belongs to the wider class of thermochemical heat storage, so far, its main application has been for seasonal heat storage, thanks to the advantages above reported. In the following, a comparison among the identified working pairs is carried out. In Fig. 3 seasonal heat storage density of the investigated working pairs as a function of temperature lift is reported. It has to be pointed out that, in this calculation, the condensation temperature is different from the adsorption temperature, and kept constant at 30 C, since heat of condensation released during desorption phase is not recovered to increase heat storage density.
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Fig. 3. Seasonal energy storage densities of the identified working pairs as a function of the temperature lift: a) water as working fluid; b) other working fluids. Desorption temperature fixed at 100 C, condensation temperature fixed at 30 C, evaporation temperature at 10 C.
The working pairs employing water as working fluids usually show higher energy storage density compared to the other working fluids. This is clearly related to the higher evaporation enthalpy of water compared to the others. The only exception is represented by the composite MWCNT-LiCl operated with methanol, which shows heat storage density higher than most of the water-based adsorbent materials, thanks to its extremely high adsorption capacity. Particularly, among the water adsorbents, the composites MWCNTLiCl and SWS 1L showed the highest storage density, which are usually more than double compared to other adsorbent materials like AQSOA Z02, Z01 and Silica gel Siogel. The discontinuity highlighted in the heat storage density of MWCNT-LiCl with water demonstrates how this highly depends on the reaction step between water and salt, which can cause a sudden increase of adsorption capacity and relative heat storage density. Particularly, for low temperature lifts (i.e. up to 30 C), MWCNTLiCl both with water and methanol, are the best options, with heat storage density ranging between 250 and 100 kWh/m3. Overcoming a certain threshold (about 35 C for water and 30 C for methanol) this composite strongly reduces the achievable heat storage density, due to limited interaction between working fluids and salt. Above 35 C, also SWS 1L and AQSOA Z02 with water are competitive options. Looking at the achievable heat storage density of the other working pairs, the most performing, KOH-AC/ammonia, showed about three times lower heat storage density than MWCNT-LiCl/ water. Another factor affecting the heat storage density is also the
different adsorbent material density. Since no data are available in literature for MWCNT-LiCl composite, a density of 300 kg/m3 has been guessed, considering the possibility of compressing the composite itself in order to increase its density, while, for instance, 500 kg/m3 have been considered for the KOH-AC as reported in Ref. [27]. Fig. 4 reports the seasonal energy storage density of the investigated working pairs as a function of the desorption temperature, which corresponds to different solar thermal collectors technologies and solar irradiation levels. In this case, adsorption, condensation and evaporation temperatures have been fixed at 50 C, 30 C and 10 C respectively. The resulting heat storage densities are highly affected by the regeneration temperature. Indeed, MWCNT-LiCl/water shows the highest heat storage density in a range between 90 and 105 C of regeneration temperature. Above 105 C the highest heat storage density is achieved by SWS 1L, thanks to the most efficient regeneration phase achievable at higher temperatures. At high temperature, also AQSOA Z02 and Zeo DDZ70 are competitive options. Interestingly, under these conditions, AQSOA Z01 is not able to store thermal energy, since temperature difference between evaporator and adsorber is too large. Among the other working pairs, as already highlighted before, due to the low evaporation temperature coupled to the high adsorption temperature, the MWCNT-LiCl/methanol shows the lower achievable heat storage density, even at high regeneration temperature. Furthermore, it is evident that the ones employing ammonia and methanol as
Fig. 4. Seasonal energy storage densities of the identified working pairs as a function of the desorption temperature: a) water as working fluid; b) other working fluids. Condensation temperature fixed at 30 C, adsorption temperature at 50 C, evaporation temperature at 10 C.
Please cite this article in press as: A. Frazzica, A. Freni, Adsorbent working pairs for solar thermal energy storage in buildings, Renewable Energy (2016), http://dx.doi.org/10.1016/j.renene.2016.09.047
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Fig. 5. Seasonal energy storage densities of the identified working pairs for three different evaporation temperatures. Desorption temperature fixed at 100 C, condensation temperature at 30 C, adsorption temperature at 50 C, ambient temperature at 20 C.
working fluid take more advantage than the ones based on ethanol as working fluid from the increasing in desorption temperature. However, their heat storage density is far from the best working pairs employing water as working fluid. In Fig. 5, the effect of evaporation temperature on the achievable heat storage density is analysed. Three different evaporator temperatures, namely, 0 C, 10 C and 20 C, which can be achieved by different low temperature heat sources usually employed, such as ambient air, solar thermal collectors and ground heat exchangers, depending on the climatic conditions have been investigated. Clearly, working pairs employing water as working fluid cannot be operated for low temperature heat sources at 0 C, due to freezing issues. This parameter is very important for seasonal heat storage applications, since during winter season, this temperature can vary a lot. It is evident that the application at very low temperature 0 C (e.g. air source evaporators), cannot guarantee enough energy storage density, usually lower than 10 kWh/m3. This confirms the need to employ either solar thermal collectors or ground heat exchangers as low temperature heat source to get satisfactory heat storage densities. Interestingly, under these working boundary conditions, also the composite MWCNT-LiCl/methanol does not reach good performance. Indeed, it suffers a lot the low evaporation pressure, which hinders the reaction between methanol and LiCl, limiting the working fluid exchange and, thus, the achievable heat storage density. MWCNT-LiCl/water demonstrates again the highest storage density, but also SWS 1L and AQSOA Z02 with water and MWCNT-LiCl/methanol represent good alternative when higher evaporation temperature is available. 4.2. Heat released to the user As reported above, the heat released to the user is always reduced by the energy spent to heat up inert masses. This effect has been then analysed in the present paragraph. All the calculations have been performed considering an ambient temperature, Tamb in equation (3), at 20 C. Compared to the heat storage density of each working pair, the effect of the inert masses reduces the heat available to the user in a range between 2 and 20%, depending on the analysed working boundary condition. More relevant is the effect of the heat exchanger mass. For instance, Fig. 6 summarizes the influence of metal masses for three water-based working pairs (i.e. AQSOA Z02, Silica gel Siogel and MWCNT-LiCl) and one ammonia-based working pair (i.e. KOH-AC/ ammonia). Stainless steel has been supposed as heat exchanger
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Fig. 6. Effect of the metal to sorbent mass ratio on the heat released to the user in seasonal heat storage application for three working pairs employing water as working fluid and one working pair employing ammonia. Desorption temperature fixed at 100 C. Adsorption temperature at 50 C. Condensation temperature at 30 C, evaporation temperature at 10 C.
material. The presence of the metal plays two different detrimental roles on the volumetric heat released to the user. On one hand, more inert masses are present, which causes an increasing of energy spent to heat up the system. On the other hand, the lower effective density tends to reduce the overall released heat. Particularly, for MWCNT-LiCl/water and AQSOA Z02/water this reduction ranges between 10% and 35% when r ¼ 3. This confirms that this parameter cannot be neglected during preliminary sizing of a seasonal heat storage. As reported in Ref. [35] the typical thermal energy consumption for heating and domestic hot water for a German single-family house, built between 1995 and 2014, is around 60 kWh/(m2 year). Accordingly, it is possible to make a rough estimation of the heat storage volume of the adsorber necessary to cover the entire energy demand of a 100 m2 single-family house. Considering the best identified working pair, MWCNT-LiCl/water with a metal to sorbent mass ratio of 3, regenerated at 100 C with a condensation temperature of 30 C and able to deliver 50 C during winter, exploiting a low temperature heat source at 10 C (see Fig. 6), about 115 m3 of heat storage are needed. It has to be pointed out that, in order to estimate the volume of the whole heat storage, also other components, like evaporator and hydraulics should be taken into account. This heavily depends on the design phase of the system.
5. Conclusions The thermodynamic analysis carried out for adsorption TES applications in the domestic sector demonstrated that this technology can be considered promising both for daily and seasonal operating conditions. One of the key issue for proper heat storage development is the identification of the most performing working pair under practical working boundary conditions. Among the working pairs available in literature, the composite based on MWCNT and LiCl, with both water and methanol as working fluid, demonstrated the highest achievable heat storage density being 130 kWh/m3 under practical working conditions. Another valid option is represented by the zeotype AQSOA Z02 and the composite sorbent SWS 1L and water, whose achievable heat storage density is sometimes comparable to the one obtained with MWCNT-LiCl. Other working pairs employing water as working fluid, such as classical zeolite 13X and silica gels are characterized by lower heat storage density, which make them less attractive. Especially, zeolite
Please cite this article in press as: A. Frazzica, A. Freni, Adsorbent working pairs for solar thermal energy storage in buildings, Renewable Energy (2016), http://dx.doi.org/10.1016/j.renene.2016.09.047
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13X cannot be efficiently operated when non-concentrating solar thermal collectors, usually employed in domestic applications, represent the primary heat source. The others working pairs, employing different working fluids (i.e. ethanol, ammonia, methanol) can exploit lower evaporator temperatures, but their performance are still too low if compared to the most efficient working pairs. This confirms how, the composite sorbents, allowing a tailoring of the adsorption properties, can represent the most effective way to quickly reach a marketability of this technology. Other classes of adsorbents, like MOFs, are still in an early stage of development, but already showed promising features for a further improving of achievable thermal energy storage performance. Finally, the analysed influence of the heat exchanger metal mass demonstrated the importance of this parameter in the evaluation of the heat that can actually be exploited by the user during the discharging phase. Indeed, for instance, the dischargeable energy in adsorbers employing MWCNT-LiCl and AQSOA Z02/water can be reduced of about 10% and 35% respectively when a metal (stainless steel) to sorbent mass ratio of 3 is considered. Acknowledgments The authors would like to acknowledge Dr. Larisa Gordeeva and Dr. Alexandra Grekova for having provided the experimental data for MWCNT-LiCl composite sorbent. References [1] Re-thinking 2050-A 100% Renewable Energy Vision for the European Union, EREC e European Renewable Energy Council, 2010. [2] Strategic Research Priorities for Solar Thermal Technology, European Technology Platform on Renewable Heating and Cooling, 2012. [3] T. Khadiran, M.Z. Hussein, Z. Zainal, R. Rusli, Advanced energy storage materials for building applications and their thermal performance characterization: a review, Renew. Sustain. Energy Rev. 57 (2016) 916e928. [4] N. Yu, R.Z. Wang, L.W. Wang, Sorption thermal storage for solar energy, Prog. Energy Combust. Sci. 39 (2013) 489e514. [5] D. Jaehnig, R. Hausner, W. Wagner, C. Isaksson, Thermo-chemical storage for solar space heating in single-family house, in: Proceedings of 10th International Conference on Thermal Energy Storage, New Jersey, USA, 2006. [6] G. Li, S. Qian, H. Lee, Y. Hwang, R. Radermacher, Experimental investigation of energy and exergy performance of short term adsorption heat storage for residential application, Energy 65 (2014) 675e691. [7] B. Michel, N. Mazet, S. Mauran, D. Stitou, J. Xu, Thermochemical process for seasonal storage of solar energy: characterization and modeling of a high density reactive bed, Energy 47 (2012) 553e563. [8] D. Dicaire, F.H. Tezel, Regeneration and efficiency characterization of hybrid adsorbent for thermal energy storage of excess and solar heat, Renew. Energy 36 (2011) 986e992. [9] D. Aydin, S.P. Casey, S. Riffat, The latest advancements on thermochemical heat storage systems, Renew. Sustain. Energy Rev. 41 (2015) 356e367. [10] H.A. Zondag, Sorption heat storage (Chapter 6), Sol. Energy Storage (2015) 135e154. [11] L.W. Wang, R.Z. Wang, R.G. Oliveira, A review on adsorption working pairs for refrigeration, Renew. Sustain. Energy Rev. 13 (2009) 518e534. [12] M. Pons, F. Meunier, G. Cacciola, R.E. Critoph, M. Groll, L. Puigjaner, B. Spinner, F. Ziegler, Thermodynamic based comparison of sorption systems for cooling
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Please cite this article in press as: A. Frazzica, A. Freni, Adsorbent working pairs for solar thermal energy storage in buildings, Renewable Energy (2016), http://dx.doi.org/10.1016/j.renene.2016.09.047