A review of salt hydrates for seasonal heat storage in domestic applications

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Currently society is moving from carbon-based to more renewable energy sources in order to become less dependent on fossil fuels.A dominant part of the energy consumption of European residential sector (around 70% of the total consumption [1]) is related to domestic space heating and hot tap water generation.A cleaner sourcing of this part of the energy sector will have a large impact on the carbon production.For generation of carbon-free heat, new energy production techniques must be implemented, the majority of which are based on capturing solar radiation.However, solar radiation fluctuates on different time scales, i.e., hourly, daily and seasonally The power generated fluctuates, resulting in a variable and unpredictable supply of heat.For matching heat demand and supply, heat storage systems that account for the timescale of radiation fluctuations are required.
In this article, the focus is on seasonal storage in the built environment in the future, requiring a storage capacity of about 7-12 GJ in a typical West European dwelling based on the passive house standard (http://www.passivehouse.com/).This storage capacity is based on an average dwelling in the Netherlands with a floor area of 120 m 2 , with the passive house standard of 15 kWh/m 2 for newly built houses and 28 kWh/m 2 in renovated houses [2,3].A promising heat storage concept is based on a thermochemical reaction, which was suggested by Goldstein [4] in the sixties and gained interest in the last decade [5][6][7].The solid materials involved in these reactions are called thermochemical materials (TCMs).Key advantages with respect to techniques like sensible heat storage and phase change materials (PCM) include nearly loss-free storage period and high energy density.In general, a gas-solid equilibrium reaction can be represented by: MX Á nLðsÞ MX Á mLðsÞ þ ðn À mÞ Á LðgÞ; ð1Þ wherein MXÁnL(s) is a solid salt complex consisting of a salt MXÁmL (s) and (n-m) mol of reactive gas L. In the current literature reactive gas L is considered to be H 2 O, NH 3 or CH 3 OH.As the targeted heat storage system should be used in residential areas, NH 3 and CH 3 OH are not considered because of currently strict Dutch safety regulation [8].As a result, H 2 O is considered a reactive gas in this article.
The amount of reactive gas L inside salt complex MX is called the loading of the salt.The formation reaction of MXÁnL is exothermic, i.e. it produces energy what can be used when for heating purposes.The enthalpy of this formation reaction is D r H m!n P reactant DH i À P products DH i < 0. The reverse decomposition reaction of MXÁnL is endothermic, D r H m!n ¼ ÀDH n!m thus costs energy.This happens during summer heat storage periods.The equilibrium reaction in Eq. (1) implies that the maximum loading of a salt MX at a temperature T is determined by the vapor pressure of L(g).

Aim
During the past decade, many researchers have investigated TCM's as heat storage materials.The first generation of salt hydrates based on TCMs have already been developed, varying from labscale [9][10][11] to field demonstrations [12][13][14].A complete overview of the systems constructed in the last decade is given by Scapino et al. [6].A great body of research is also available on high potential salts for temperature storage below 100 °C, such as MgSO 4 [15][16][17][18][19], MgCl 2 [19][20][21][22], SrBr 2 [7,23], Na 2 S [12,24] and CaCl 2 [22,25,26] which have been studied in detail.Storage of heat for temperature applications between 100 and 300 °C already shows some promising results with salts based on CaO/Ca(OH) 2 [27] and CaC 2 O 4 /CaC 2 O 4 ÁH 2 O [28].Furthermore, some reviews have been published on TCM's [5,[29][30][31][32][33][34], that use the energy density as selection criterion, with one exception focusing on applied working conditions [5] during hydration/dehydration.In the latter study three salts were selected: MgSO 4 , LaCl 3 and SrBr 2 based on dehydration below 105 °C and rehydration at 20 mbar vapor pressure at 25 °C, which corresponds to the saturated vapor pressure in equilibrium with a water reservoir at 17 °C.However, the missing parameter for selection in this review is the generated temperature T h during the hydration reaction, since this temperature is the highest output temperature the heat battery can deliver.
For introduction of TCMs on the market, it is important that TCMs are able to match the demands of the customers.As a first indication it is therefore necessary to determine if TCMs can theoretically match such demands.In the present work, we attempted to analyze and extend the search for pressure-temperature (pT) data on the basis of demanded working conditions of a TCM reactor in the built environment, i.e. a system that can store 10 GJ, deliver hot tap water at 65 °C and can be charged in summer with the help of solar panels.The goal is to select TCM's which fulfill the temperature demand and energy density of the consumer, with the help of available pT data, and analyse the plausibility of using these TCM's.Firstly, the process of data collection will be summarized, secondly the selection criteria are explained.Based on these criteria a short list of the most promising salts will be generated and discussed in view of the target application.

System description
Since a heat storage of approximate 10 GJ stored heat is needed to overcome seasonal fluctuations [35], a system level energy density of 1 GJ/m 3 is considered in this paper, meaning that 10 m 3 of reactor should be placed in housing.The system energy density (considering the entire installation, including TCM material, piping, valves, control devices) is based on the energy density of 5 times the energy density of a water tank by a DT of 50 °C.Ten cubic meters of reactor is probably still an overestimation of the available space for such a system as the space is limited i.e. apartments.
In Fig. 1 the concept of a heat storage system with help of a TCM is schematically given.On the top, the reactor system is shown, where two compartments are drawn, one filled with a TCM and the other with water, in between these compartments a valve is located.
For heat storage, two main concepts are considered, closed and open systems [7].In the case of a closed system both compartments are part of the system and all water necessary for the hydration/dehydration reactions is stored within the system.In the case of an open system, the water is not stored in the system itself, but externally released/supplied to the system dependent on TCM dehydration/hydration.
The working conditions of TCM systems are determined by the phase diagram of the TCM in question.A phase diagram indicates the conditions under which a certain TCM undergoes hydration or dehydration.In the left bottom corner of Fig. 1, a schematic phase diagram of reaction from Eq. ( 1) is shown.Applying a condition (combination of water vapor pressure and temperature) below the solid line results in a hydrate MXÁmH 2 O.For hydrating this material, a condition should be applied above the solid line.In a system, the temperature of the TCM can be varied with the help of a heat exchanger.Vapor pressure inside the TCM heat storage system is determined by the temperature of the water compartment.Consequently the applied temperature is directly related to the vapor pressure according to the equilibrium line between liquid water and water vapor (the dotted line).The conditions of hydration and dehydration can be found by combining both equilibrium lines in one diagram.
In the case of hydration (producing heat), the initial material in the TCM reactor is MXÁmH 2 O and the temperature in the system is T w1 , meaning that the vapor pressure inside the system is equal to p h .The applied condition around the TCM in that case is above the equilibrium line between MXÁmH 2 O and MXÁnH 2 O. Consequently MXÁmH 2 O hydrates into MXÁnH 2 O.During this reaction the temperature of the TCM reactor will increase as the hydration reaction is an exothermic reaction.As long as the temperature of the TCM reactor is below T h the hydration reaction will continue.At temperature T h , both phases (MXÁmH 2 O and MXÁnH 2 O) can exist as that is the equilibrium temperature corresponding to vapor pressure p h .The vapor pressure in the system can only remain constant as the temperature of the water in the system is kept constant.This

TCM
Water source means in the case of TCM hydration, the evaporation heat of the water should be compensated by heating the water vessel, otherwise the temperature of the water vessel decreases and the vapor pressure in the reactor will decrease.With help of a heat exchanger the reactor can release heat in a controlled way.Note: we assume that both the temperature of the TCM reactor and water vessel can vary independently from each other.
In the case of dehydration (regenerating the TCM) a certain temperature is applied to the TCM T d with a heat exchanger and the reactor is filled with MXÁmH 2 O.In that case, the vapor pressure applied to the system by the TCM is equal to p d .As long as this vapor pressure is higher than the vapor pressure of the water vessel, the TCM will dehydrate.Since that will cost energy, the TCM should be heated in order to maintain the same vapor pressure.The added water in the gas phase will condensate in the water vessel, which will increase the temperature in the water vessel and the equilibrium vapor pressure in the heat storage system.These losses can be compensated by cooling the water vessel.
In this case, it is assumed that cooling and heating of the water vessel has the same source: a ground pump.In that case the temperature in the vessel T w2 will be higher in the summer than in the winter T w1 as a ground pump delivers a higher temperature in the summer.

Collection of data
The thermodynamic data on salts are comprised of the following parameters: pT-data, crystal densities of the considered hydrates, reaction enthalpies, entropies and melting points.A total of 262 salts (563 reactions) were considered (see Table 1), of which the majority of the data originates from The International Critical Tables [36] and Glasser [37].In case pT data were not available, they were deduced from the reaction energies [38].
The basic thermodynamic equation for equilibrium between a condensed phase (solid or liquid) and the vapor phase of a pure substance, under conditions of low pressure, is used for this investigation [38]: are all thermodynamic quantities at p 0 = 1 bar and T 0 = 298.15K.Note that the reaction enthalpy is defined for a certain reaction from an initial (m) to a final (n) hydration state (n > m).For one salt hydrate, different enthalpies of reactions and thus reaction conditions may exist depending on the considered reactions.The uncertainty of the calculated TCM temperature by a given vapor pressure is calculated with help of a 95% interval.Hereby it is assumed that the error in T; p 0 and R are negligible in Eq. ( 2).In the error calculations based on enthalpy/entropy data, the error in enthalpy and entropy is taken from the literature source or if the source does not mention the error the maximum error is assumed to be five times the unity of the most significant digit i.e. in case the enthalpy is given by 40.3 kJ/mol, the error is assumed to be 0.5 kJ/mol.
An example of the error calculation is given in Fig. 2. The pTdata measured by Polyachenok et al. [39] is plotted, including the calculated data with the help of Eq. ( 2) and the enthalpy/ entropy values of Glasser [38].Both data sources fit with each other.By small variations in equilibrium vapor pressure observed for CuCl 2 based on the calculation with Eq. ( 2), the temperature error of the condenser/evaporator of 1-4 °C is found in the temperature range of 0-100 °C to match with a certain hydration/dehydration temperature.In the case the hydration/dehydration temperature is known, the error in pressure is on the order of 10%.This means that the calculated condenser temperature can have an error on the order of 5 °C.
With help of the collected data, various parameters are calculated which are necessary to know for selecting an appropriate hydrate reaction for the foreseen application.The energy density of a system is calculated for an open system (no water storage included in the system) according to the following equation: For the closed system (water is stored inside the system), the volume of water molecules involved in the reaction is considered and the energy density is also calculated by: wherein M w [kg/mol] is the molar mass of the liquid water and q w [kg/m 3 ] the density of liquid water.The energy density of both open and closed systems depend strongly on the accuracy of the crystal density and the reaction enthalpy and can be calculated similarly as for the T TCM .Although extreme accuracy is taken into account, variations of 2% on the energy densities can be feasible.
In case the output temperature of the TCM reaction is not sufficient for the targeted heating application, a second heating step is possible.On the basis of the first reaction, water is heated with DT, generating a vapor pressure necessary for inducing the second reaction wherein the TCM produces the demanded temperature.In that case, the energy density of the system with a double hydration step ðE=V openÀII Þ will drop for an open system according to: The volume variation during hydration/dehydration is calculated on the basis of the crystal structure density according to: The price is calculated based on price per kg P kg;i , deducted from the loading i of the salt hydrate indicated by the supplier and neglecting the cost of water.The price per MJ P MJ is calculated according to: By referring to the current database it is possible to select an appropriate hydrate reaction for any application.As the enthalpy and entropy are known for all reactions in the database, an equilibrium vapor pressure can be found for each TCM temperature and vice versa with the help of Eq. ( 2).This database will guide the material and system developers during initial screening of suitable hydration/dehydration reactions for particular applications based on crucial demands: output temperature (T hydration ), regeneration temperature (T dehydration ) and energy density E=V.

Selection procedure
An appropriate TCM for seasonal heat storage should meet a certain set of thermodynamic conditions.These conditions are listed in Table 2.We used two different set of thermodynamic conditions (filters) for selecting suitable candidates.These filters reflect a strict and flexible selection criteria, as explained hereafter.In addition, an analysis is made on non-thermodynamic conditions such as price, chemical stability and safety.
Filter 1 selects a TCM which can fulfill the demands of a particular heat storage system.This heat storage system should be able to store 10 GJ of heat, in order to overcome seasonal fluctuations [35,40], within 10 m 3 .Therefore, an energy density of 2 GJ/m 3 on material level (without considering water storage, open system configuration) is targeted as the reactor system (piping, valves, tubing) and material porosity will decreasing the energy density on system level.Beside volumetric considerations, the TCM should provide temperatures of domestic hot water (DHW) (T > 65 °C) and space heating (HW) (T > 40 °C) within one heating step.This temperature should be reached with a corresponding vapor pressure of 12 mbar, which is equal to an equilibrium vapor pressure of a water source at 10 °C.This 10 °C is based on keeping the temperature of the evaporator constant with help of a borehole based on the ground temperature and the ground temperature at 7 m below surface is approximately 12 °C over the year in the Netherlands [41].
Loading of the heat storage system is foreseen to be performed with heat from solar panels.Different panels are available for domestic applications; flat plate collectors and evacuated tubular collectors.Depending on the specific collector design, different temperatures can be reached, where the power output of the collector depends on the output temperature.In case higher output temperatures are required the total collector energy output in a year will decrease.In this case a dehydration temperature of 100 °C is considered, where a low-cost flat plate reactor can generate 1.8 GJ/m 2 per year [42] with given output temperature.During  [39] and pT-data calculated with help of thermodynamic data and Eq. ( 2) [38].The numbers indicate the water loading in the different regions.Between the water vapor line and the CuCl 2 Á2H 2 O-CuCl 2 line, the solid dihydrate will deliquescence, but the exact conditions are unknown.dehydration explained as before, the condenser temperature is a significant variable, which will rise during condensation.In order for the dehydration process to continue, the condenser should be cooled.A temperature of 17 °C inside the water drain is considered as reasonable, provided there is a borehole or air cooling.As a consequence, the TCM should dehydrate at a temperature below 100 °C and with a water vapor pressure of 20 mbar.In consideration of mechanical stability, the melting point should be above the dehydration temperature.
The second filter is introduced as a compromise between the strict boundary conditions for an ideal salt hydrate and achievable boundary conditions acknowledged to available salt hydrates.The energy density is lowered to a value of 1.3 GJ/m 3 on material level, which results in approximately 1 GJ/m 3 in a closed system.The hydration/dehydration temperatures are increased/decreased respectively.This filter will definitely impact system level design, e.g., higher dehydration temperatures and additional heating to reach hot tap water temperatures.

Database
The database consists of 563 entries with complementary thermodynamic data from multiple sources.The full list is given in Appendix A. As mentioned before, different dehydration reactions may occur with one salt.For that reason, multiple hydrate transitions are considered in this database.
A histogram of the energy densities of the available TCM reactions (only 397 records within our database contain information about energy densities) is plotted in Fig. 3.As can be seen, the histogram peaks around 1 GJ/m 3 .The number of hydrate reactions with an energy density above 2 GJ/m 3 (filter 1) is only 114.
Analyzing the maximum hydration and minimum dehydration temperature at the conditions stated in Table 2, two histograms are generated, plotted in Fig. 4A and B, respectively.The shape of the histogram plotting the hydration temperature peaks around 30 °C, implying that most salts have a too low output temperature for generating domestic hot tap water under the applied conditions.Here we stress that hydration reactions of MgSO 4 (monohydrate to hepta-or hexahydrate at 24 and 21 °C, respectively) and CaCl 2 (anhydrous to hexahydrate at 31 °C), often considered as promising salts and extensively investigated [15][16][17][18][19]22,25,26], fail in this respect.Reactions with a TCM temperature below 10 °C are suspicious and indicated as such in Appendix A.
The minimum temperatures necessary for the different dehydration reactions are given in Fig. 4B.This figure shows that most salts lose water readily at 40 °C to a condenser at 20 mbar of water vapor pressure.If the temperature window of filter 1 is applied on this dataset, 165 salts fit the hydration conditions and 415 salts fit the dehydration conditions, respectively.Reactions with a TCM temperature below the 17 °C are suspicious and are indicated as such in Appendix A.
By applying all conditions of filter 1 at once on the dataset, the number of possible TCM candidates reduces to four: Na 2 S, LiCl, EuCl 3 and GdCl 3 .The last two salts are rare earth metals and cannot be used on a large scale with reasonable prices.Furthermore GdCl 3 is toxic (MSDS safety sheet) and LiCl is strongly corrosive [43] and expensive [44], which makes both candidates unfavorable for application.The last material is the most promising of these four materials.It has a high theoretical energy density and the dehydration temperature is relatively low.On the other hand, Na 2 S is corrosive [43,45], reactive [46] with the risk of outgassing of H 2 S [47,48]) and Na 2 S is mentioned as a dual use material by the Australia Group [49].Beside Na 2 S, the US government considers H 2 S (a potential outgas of Na 2 S) as a high priority chemical threat, as well as being used as a potential weapon of mass destruction by terrorists [48].Experience with Na 2 S can be found in several heat storage projects like TEPIDIUS [12], SWEAT [24] and MERITS [50,51].These projects all suffered initially from corrosion and faced variable results by overcoming this issue i.e. by coating all surfaces inside  the reactor [51].Until now, it is unknown how the performance of the coatings will be on time scales of years.Also concerns are raised regarding the release of the last 1.5 water molecules Na 2 -SÁ5H 2 O, whereby the melting temperature and dehydration temperature are equal at the current vapor pressure, thereby limiting the loading power [24].In case no complete dehydration is reached, the energy density will drop from 2.79 GJ/m 3 to 1.58 GJ/ m 3 .In addition, local variation in the temperature during dehydration can cause melting of the TCM, which challenges the stability of the performance of the heat storage system.
If filter 2 is considered, a shortlist of 25 hydration reactions remains.A summary of thermodynamic conditions is given in Fig. 5.The hydration and dehydration temperatures are plotted against the energy density for the hydration reactions, as well as the known melting temperatures of the hydrates.The initial and final hydration states are indicated below the salt labels.Table 3 gives a more detailed overview of the thermodynamic data and application considerations of these 25 TCM's.In the next section a detailed evaluation of this long list will be performed.

Evaluation of top 25 reactions
Table 3 lists the hydrate reactions, selected on the basis of filter 2 working conditions.In this section all factors (thermodynamic and non-thermodynamic) are evaluated to arrive at a short-list of TCMs mostly suitable for seasonal heat storage.

Energy density
Heat storage system performance only partly depends on the material energy density.The effective system energy density is dependent on the choice of an open or closed system.
In Fig. 6, the energy density of a pure TCM is plotted against the energy density of a reactor at different porosities.In all cases an open system leads to a higher energy density, but the difference between the open and closed system decreases with increasing porosity.For example, based on Fig. 6, a TCM of 3.0 GJ/m 3 and a porosity of 30% in a closed system has a system energy density similar to a TCM material with an energy density of 1.8 GJ/m 3 and equal porosity in an open system.Practically this means, that in case the 1 GJ/m 3 at system level is demanded, the material should be at least above 1.4 GJ/m 3 and 2.0 GJ/m 3 for an open and closed system, respectively.Hereby, we did not even consider volume for the reactor itself, piping, insulation, valves, control systems, etc.
Besides different energy densities on system level between open and closed system, another important difference exists between these systems.In a closed system, the pressure is minimized (low-pressure/vacuum), and as a result, the vapor transport is extremely fast, however the heat transfer from salt to heat exchanger determines the power output.Conversely, in an open system, the water transport is slow, but the air blown through the system can be used as heat conductive medium.As a result, for both systems, different issues should be considered for a sufficient output power [62].
At this point it is stressed that not all salt hydrates can be used in an open system due to unwanted side effects.For example, Na 2 S should to be used in a closed system to avoid i.e. the transition of Na 2 S into Na 2 CO 3 as Na 2 is reactive with CO 2 from the atmosphere [63].The best performing reactions solely based on the energy density criterion are Na 2 S (0.5-5), GdCl 3 (0-6) and EuCl 3 (0-6) in an open as well as a closed system.

Volume variation
The relative variation of crystal volume during a hydration/ dehydration reaction is shown in Table 3.This number is calculated based on the crystal density of highest and lowest hydrates involved in the studied reaction (both listed in Table 3).Fig. 7 shows the crystal volume variation of all considered dehydration reactions in the database against the energy density involved in this reaction.The figure shows a linear trend of 22% volume variation per GJ, meaning that in general the crystal volume varies more in case the energy density of the hydrate increases.Grains are in Fig. 5.A selection of the database, which fits the drafted working conditions of filter 1 and/or 2, where the gray shaded area fits the working conditions of filter 1.The maximum hydration and the minimum dehydration temperature of the different hydrate couples are plotted against the reaction energy density on material level (open system).The vapor pressure is equal to 20 mbar and 12 mbar during dehydration and hydration, respectively.In addition the lowest melting temperature of the involved hydrates within the reaction is plotted.

Table 3
List of the most suitable 25 hydrate reactions based on the working conditions stated in Table 2 with the chosen parameters.The salts are sorted in descending order based on the energy density of an open system.'Energy densities open system' of salts whereby an open system is unsafe as a result of toxic gas formation are given in italic and the energy density of the closed system is used for the ranking.The deliquescence point is based on the vapor pressure whereby the hydrate will form a deliquescence and can no longer transform to a higher hydrate at 25 °C.As a reference, the price is given in euro per kg, which is the price of 1 kg of the stable hydrate under ambient conditions.R: rare earth metal; Tx: acute toxic category 1-3; T: acute toxic category, ⁄: based on the price of LiCl, ⁄⁄: not exactly known, -: unknown.Safety/Instable generally dehydrated heterogeneously over the grain and so a grain will not shrink in a regular fashion.As a consequence, void spaces will be formed inside the initial grain as locally pieces of the grain contract.The new formed voids will decreases the mechanically stability of the original grain.This will results in pulverization as the grains are multiple times loaded/unloaded [27].
The pulverization of the grains changes the permeability of the bed and thus the performance of the entire reactor also changes [14].In addition, swelling and shrinkage of the grains may induce stresses within the heat exchanger.Another implication of volume changes is related to the heat conductivity of the materials.As for many hydrates the thermal conductivity is on the order of 0.5 W m À1 K À1 [64] and that of air is on the order of 0.024 W m À1 K À1 , where the empty space in between the grains will act as an insulator.Based on the effective medium theory, the heat conductivity of a grain with 20% or 40% void space within the grain will be reduced by 40% and 70%, respectively [65].In order to minimize the void space, it is favorable to choose a material with a small volume variation.However, such smaller volume variation is accompanied with a lower energy density (see in Fig. 7).
A final note on void spaces within grains should be made regarding reaction kinetics: cracking may open internal pathways in grains for water, which improves water transport and thus heat generation [66].Thus, a compromise between heat conductivity, water transport and energy density is inevitable.

Hydration temperature
The hydration temperature indicates the maximum temperature that can be generated in a certain reaction at the given vapor pressure of 12 mbar.This is important in view of the application, as the output temperature of the TCM will determine if a salt is only feasible for domestic heating or also for hot tap water as well.In case the TCM is insufficient for generating the required temperature, additional after-heating is necessary.This can be achieved with electrical heating, but post-heating with a second TCM reactor is an option as well.This can be accomplished by generation of higher vapor pressure at the evaporator.
The temperatures in Table 3 are calculated for a system with a water vapor pressure of 12 mbar.In case the vapor pressure increases, i.e. the evaporator temperature increases, the maximum reaction temperature will increase as well.So, if part of the heat generated with the first reactor is added to the evaporator, a higher reaction temperature can be reached, being of interest to reactions with output temperatures between 50 and 65 °C.The drawback of this approach is the reduction of the effective energy density (see Eqs. ( 6) and ( 7)).For example, in case of a 2-step system on CuCl 2 , wherein the temperature of the evaporator is increased 13 °C to reach an output temperature of 65 °C, the energy density drops from 1.74 GJ/m 3 to 1.21 GJ/m 3 and from 1.13 GJ/m 3 to 0.72 GJ/m 3  for an open and closed system, respectively.This drop in energy density is significant, but if only part of the volume of TCM is used for domestic hot water, where the drop in energy density of the entire system is less significant.

Dehydration temperature
The dehydration temperature is the minimum dehydration temperature at which the TCM can dehydrate to the desired loading with a given water vapor pressure of 20 mbar, based on the condenser temperature in the summer.The temperature inside the TCM can be accomplished in several ways e.g., solar collectors, electrical heating, waste heat.The output temperatures of solar collectors strongly depend on the strength of the solar radiation.This radiation intensity fluctuates by strength, depending on the hour of the day, cloudiness and season.Generally a lower dehydration temperature results in a reduction of solar collector surface for the production of the desired temperature and power output [42].Ideally, the dehydration temperature should be kept as low as possible, to be able to charge the battery multiple times a year.Besides the appropriate salt selection, this also depends on the condenser temperature.In practice, the condenser temperature is the temperature of the water reservoir in summer, i.e. 17 °C in Western Europe climate.

Melting point
During dehydration of pure hydrates, the melting point determines the maximum dehydration temperature.If the temperature  exceeds the melting point, the grains will clog together, which results in a change in porosity of the reactor, affecting the power in-and output.Because of this, CaCl 2 hexahydrate and tetrahydrate cannot be used as TCM above 40 °C as they have melting points of 30 and 45 °C, respectively.As a consequence the transition of CaCl 2 (0-2) should be handled with care as during hydration at lower temperatures, the higher hydrates (hexahydrate and tetrahydrate) can be formed.If this is the case, clogging of the grains is still possible in case the grains are heated above the melting points, which makes it challenging to work with CaCl 2 as a TCM.

Deliquescence vapor pressure
Deliquescence is a key parameter for grain stability both in view of hydration at low temperatures and TCM storage.In case of deliquescence, the grains can clog to each other, changing the local porosity of the TCM.This affects the power which can be delivered/absorbed by the reactor as the vapor transport will be hampered.
The known deliquescence vapor pressures at 25 °C are listed in Table 3, which is only an indication.For specific applications a different temperature should be selected.As can be seen, some hydrates will tend to deliquescence in case the hydration vapor pressure is applied when the TCM is still cold (T < 25 °C).For example MgCl 2 and CaCl 2 can deliquescence during the initial hydration process as the TCM is still cold and the vapor pressure of 12 mbar is applied, which is unwanted.
Different strategies for anticipating deliquescence effects can be used.For example a preheating step can avoid deliquescence or start at a lower condenser temperature.Furthermore, deliquescence as well as melting in a TCM reactor might be accepted in case a stabilization technique is used to avoid clogging, such as matrix impregnation [70] or microencapsulation by polymers [71].A constant grain structure is desired, to secure a steady heating/dehydration rate of the reactor.

Price
The price of the material is a crucial boundary condition affecting the economic feasibility of any heat storage system.For that reason, rare earth metals, like EuCl 3 and GdCl 3 , are thus not considered.The prices mentioned in this article are based on industrial scale produced materials where for the real application bulk material will be used as the base material.We tried to estimate the prices of the material as accurately as possible.Hereby, we cooperated with a distributor and manufacturer within the chemical and food market [44] to come up with realistic market prices.The price is given in euro per kg, as a reference, which is the price of 1 kg of the stable hydrate under ambient conditions.The effective price, expressed in euro per joule, determines the actual price per energy.
Based on price as an indicator, MgCl 2 is currently the most promising candidate, with reservation of bulk price fluctuations for the listed salts in the future.Other good candidates are Na 2 S (0.5-5; 2-5) and CaCl 2 (0-2).

Safety
Safety is an important aspect in the salt selection procedure, as the TCM will be applied in the domestic environment involving all stages of the heat storage system life cycle, from installation, main-tenance and revision, operation, dismantlement and accidents.Safety impact refers to health effects or environmental damage that may be produced by a chemical.
1.The LD50-values (i.e.Lethal Dosis for 50% of subjects) of the considered salts is listed in Table 3, in which we categorize an LD50 below 25 mg/kg as highly toxic; between 25 and 200 as toxic and between 200 and 2000 as harmful [72].Based on this classification, GdCl 3 and NiCl 2 are toxic, and Na 2 S is harmful, but a borderline case.Note that from the point of view of the customer, the LD50 value is merely an indication of health risks, as the customer will not directly interact with the materials.2. Special attention is devoted to the following hydrates that are explicitly mentioned to be toxic or acute toxic in the MSDS safety sheets: MnI 2 , VOSO 4 and CuCl 2 , although this is not shown based on the LD50 values.3. Possible side-reactions due to outgassing or catalytic effect of salts should be addressed case-specifically.Examples are H 2 S and HCl-formation in the case of Na 2 S and MgCl 2 , respectively.Ca(ClO 4 ) 2 is a strong oxidizing agent, which is of major concern in fires and relevant for working conditions.

Chemical stability of the complexes
A heat battery based on TCMs should reasonably have a service life of about 15-20 years, preferably without replacing the TCM material.A stable material performance is desired, i.e. the TCM should not decompose or transform into another material.Based on this criterion, CrCl 2 an FeCl 2 are eliminated from Table 3, since Cr 2+ and Fe 2+ are prone to oxidation in a humid environment [73].A different oxidation state will affect the pT characteristics, thus being unwanted.
A second point of concern is the decomposition by outgassing.For example, MgCl 2 is known to produce HCl gas at high temperatures above 140 °C [60], consequently reducing the amount of TCM in the reactor.Recent studies indicate that at dehydration temperatures below 100 °C during cyclic hydration/dehydration HCl gas is produced [74].This reduces the amount of TCM and the HCl gas may induce corrosion and increases the pressure inside a closed system.
Although outgassing of Na 2 S is not studied as thoroughly as MgCl 2 , it is mentioned by several scientists that Na 2 S can emit H 2 S [47,63], which is strongly corrosive.Another hydrate in the list that may decompose under the considered working conditions is Mg(NO 3 ) 2 [75].
For all these hydrates, the stability of the material should be considered in view of the service life duration.The rate of chemical decomposition will determine whether a hydrate should be rejected or not.Current data on decomposition rates are insufficient to exclude the above mentioned hydrates, although these salts are unfavorable as a first choice from this perspective.

Hydration/dehydration kinetics
The reaction kinetics of a TCM have a strong effect on the power in-and output of a heat battery.Generally increasing the DT between the applied temperature and the minimum dehydration temperature will increase the dehydration rate [76].This also holds for increasing the vapor pressure: increasing the difference between the applied vapor pressure and the minimum hydration vapor pressure will increase the hydration rate [77].
For most hydrates, multiple dehydration data can be found in literature and in most cases the dehydration data deviate from each other as i.e. the grain size [69], temperature profile [78], sample holder [79] strongly affect the observed characteristics.In general, for each material a single study can be conducted to summarize all available data whereby the history of the grains is considered.In addition, a similar study can be performed on hydration experiments although the amount of available literature is limited, as controlling the vapor pressure during hydration in a TGA/DSC is complicated.
As we did not perform a study on the hydration/dehydration kinetics for each material, it is difficult to distinguish based on kinetics.One exception we made, based on our own lab experiments and literature [5,59], KAl(SO 4 ) 2 shows very slow (i.e.too slow) hydration reaction kinetics, which we see as problematic for our application.

Short list
The final column of Table 3 summarizes the critical points of concern i.e. potential showstopper for using salt hydrates to store heat under the considered boundary conditions.For the majority of the listed salts, concerns arise regarding the material price, stability of the TCM or safety of the system.K 2 CO 3 is the only remaining candidate without points of concern, despite the fact it is the lowest-ranked salt in our shortlist in terms of energy density.Note that K 2 CO 3 can operate in an open system without safety or stability issues, contrary to high potential candidates like Na 2 S and MgCl 2 .The difference in energy density will be strongly reduced in that case.In addition, K 2 CO 3 performs sufficiently well considering dehydration temperature, melting point, deliquescence point and price.As K 2 CO 3 is a relatively new salt in view of thermal heat storage, a short overview of the literature is given in Appendix B. Other salts with potential are MgCl 2 , assuming HCl production can be minimized and deliquescence can be avoided or CuCl 2 in case of multiple hydration/dehydration cycles a year.

Conclusion and outlook
An extensive review of 563 hydrate reactions is performed, resulting in a database, wherein the thermodynamic data of these reactions are summarized (see Appendix A).With help of the current database it is possible to select an appropriate hydrate reaction for any application.For example an industry application will have different working temperatures, number of cycles and other safety regulations than applications in households.As the enthalpy and entropy data of 563 hydrate reactions are listed in this database, for each reaction an equilibrium vapor pressure can be found for a given temperature and vice versa.This database will help the material and system developers for performing initial screening on suitable hydration/dehydration reactions for particular applications based on crucial demands: output temperature (T hydration ), regeneration temperature (T dehydration ) and energy density ðE=VÞ.
In this study the database is used to evaluate the theoretical potentials and limitations of salt hydrates as thermochemical materials (TCM) for heat storage in the built environment under boundary conditions relevant to (seasonal) use in domestic environments.A list of 25 candidate salts is composed from this database based on four criteria, i.e. energy density on material level above 1.3 GJ/m 3 , hydration temperature above 50 °C, dehydration temperature below 120 °C and a melting point above the dehydration temperature.
Considering these conditions, commonly suggested salt hydrates like CaCl 2 (hexa-and tetrahydrate) and MgSO 4 (heptaand hexahydrate) did not fit the demands for seasonal heat storage (for domestic heating and hot tap water using the hydration reaction).According to the database, it is impossible to reach the required temperatures during hydration with a reasonable energy storage density.
As the goal of this review is to find a TCM for domestic application over a period of 20-30 years, the list additionally analyzes other critical parameters: safety, chemical stability, kinetics and price.It turns out that almost each material in this top 25 candidate list has its own challenges or foreseen problems.One of the largest problems is the price: eleven selected materials are disregarded after price investigations.
Finally, a single candidate -K 2 CO 3 -remains from the list of 25 candidate salt hydrates under the boundary conditions considered.K 2 CO 3 has an energy density on material level that is still 6 times larger than water with a DT of 50 °C and has no heat loss during storage.By taking into account that the heat storage system also consumes volume, the energy density will drop significantly.Relatively small challenges on system level are expected concerning maintenance, as K 2 CO 3 is not strongly corrosive, no higher hydrates are known and it has no known unsafe side-reactions.As it does not reveal any decomposition reactions, an open system is an option with K 2 CO 3 .MgCl 2 can be considered as a candidate as well provided a solution for HCl-outgassing and deliquescence is found.
The 1.3 GJ/m 3 energy density of K 2 CO 3 makes it rarely unlikely that 10 GJ of heat will be stored in domestic applications, as this means that the material consumes almost 8 m 3 , without considering porosity, volume of reactor vessel and heat exchangers.As a consequence, a heat storage system on the basis of K 2 CO 3 with a single hydration/dehydration cycle a year will be unlikely based on the storage volume.In a multicyclic application, however, the amount of stored heat decreases per cycle, likewise decreasing the amount of material, storage volume and price of the system.Moreover, an increased number of cycles per year appears more feasible when the dehydration temperature is lower.At lower dehydration temperatures, a solar system or waste heat can more easily deliver the required temperature.
A lower dehydration temperature however, means in general a lower hydration (i.e.output) temperature as well.This may be anticipated by increasing the evaporator temperature or by serial application of TCM systems.K 2 CO 3 has a relatively low dehydration temperature, which may facilitate multiple cycles per year.To further assess its potential as a heat storage material additional TGA/DSC experiments were performed, which will be published subsequently as full papers.
Based on an extensive review of 563 hydrate reactions, no ideal salt exists for seasonal heat storage under the boundary conditions considered.With the current concept of seasonal heat storage, including concepts of closed and open systems, whereby only one dehydration cycle per year is performed and a system energy density of 1 GJ/m 3 , it is not realistic for large scale implementation to rely on pure salt hydrates as heat storage materials.Although the present view of seasonal heat storage seems unprofitable, multiple usages per year, utilization in peak shaving [80] or storing waste heat [81] are all promising prospects.In conclusion: changing the concepts behind seasonal heat storage is in our view necessary to overcome energy density and price issues, which will provide TCM with an opportunity to be a valid technique for heat storage in the future.

Appendix A
Although care is taken to minimize errors in the following table, some mistakes are found during detailed analysis of the table.The data which did not fit lab experiments or seems unrealistic are still included, but indicated with an asterisk Ã .As not all materials could be studied in detail, some mistakes will still be unmarked in the table.
Salt gives the basis on the hydrate in the reaction; H is the highest loading in the reaction; L the lowest loading in the reaction; E=V is the energy density of the reaction in an open system without porosity; DH is the enthalpy of the reaction; DS the entropy of the reaction; T hyd the maximum hydration temperature by 12 mbar water vapor pressure and T deh the minimum dehydration temperature by 20 mbar water vapor pressure.The used type of thermodynamic data and source is given in column source and in case of the pT data the minimum and maximum temperature of the used pT data is given in columns T min and T max .For the combined transitions, DH and DS are not given, but can be calculated with help of the data of the involved hydrates, which are also given in this table.The error of T deh is on the order of 1-3 °C, the error in E=V is on the order of 2%.Fig. 8.The phase diagram of K 2 CO 3 in equilibrium with water.The data are a combination of experimental and calculated data from the literature, i.e., Lescouer [36], Glasser [38], Foote [96] and Greenspan [54].

Salt
and the number of cycles of the material.Their results indicate that the composed phase diagram is correct within the expected errors.
Appendix C. Supplementary material

Fig. 3 .
Fig. 3.A histogram of the energy density of 361 studied hydration reactions.The reaction energy density is calculated on the basis of the molecular volume of the highest hydrate in the reaction and the enthalpy change.

Fig. 4 .
Fig. 4. A histogram of 361 studied hydration reactions showing the maximum hydration temperature with a vapor pressure of 12 mbar (A) and the minimum dehydration temperature with a vapor pressure of 20 mbar (B).

Fig. 7 .
Fig.7.The volume variation of dehydration reactions plotted against the energy density of the TCM.The linear fit indicates that on average each stored GJ results in a 22% volume variation of the hydrate.

Fig. 6 .
Fig. 6.The TCM energy density plotted against the reactor energy density.Different porosities for open and closed systems are considered.

Table 1
Overview of the data inside the database.

Table 2
Thermodynamic criteria for selecting hydrates suitable for seasonal heat storage.E=V in this table refers the energy density of the TCM, whereby only the volume of the TCM is considered.A vapor pressure of 12 and 20 mbar corresponds to respectively a water temperature of 10 and 17 °C.