# A reality check on long-term thermochemical heat storage for household applications

Kokouvi Edem N'TSOUKPOEª,*, Frédéric KUZNIKᵇ

## Abstract

Long-term thermochemical and sorption heat storage processes for household applications are struggling to fulfill promises, probably because of an initial misunderstanding about the nature of these processes and the definition of their performance criteria. This perspective paper focuses on long-term thermochemical heat storage for space heating and domestic hot water production, which are the main applications targeted by the vast majority of studies. It provides a documented view on the technology, while questioning the relevance of the idea of long-term thermochemical heat storage for household applications and the performance criteria used to evaluate the corresponding systems. The authors present a fundamental analysis of this storage technology, examining the nature of the processes and the most realistic potential availabilities on the application side. Additionally, the criteria used to assess the performance of the materials, processes or systems are discussed. Finally, based on data collected on many prototypes available in the

<footnote>ªLaboratoire Énergies Renouvelables et Efficacité Énergétique (LabEREE), Département Génie Électrique, Énergétique et Industriel, Institut International d'Ingénierie de l'Eau et de l'Environnement (2iE), 01 BP 594 Ouagadougou 01, Burkina Faso</footnote>

<footnote>ᵇUniversité de Lyon, INSA de Lyon, CETHIL UMR5008, F-69622, Villeurbanne, France</footnote>

<footnote>*Corresponding author. Phone: +22664538850</footnote>

<footnote>Email address: edem.ntsoukpoe@2ie-edu.org (Kokouvi Edem N'TSOUKPOE)</footnote>
literature, some of the limitations of these processes have been highlighted. It appears that seasonal thermochemical heat storage is subjected to significant thermal losses. The volumetric energy storage density exhibited by the processes based on solid hydrates or aqueous solutions is prohibitive for long-term thermal energy storage for domestic hot water and, in most cases, for house heating. So far, the performance or advantages of thermochemical energy storage have been probably overestimated in the literature. The authors of this paper suggest that, before undertaking new research activities on thermochemical heat storage, researchers should carefully consider the challenges pointed out here.

**Keywords:** thermochemical heat storage, sorption heat storage, seasonal storage, long-term thermal storage, energy storage density, solar energy management
# 1. Introduction

Thermal energy storage is a promising solution to enhancing energy efficiency and the widespread adoption of solar energy [1]. There are three methods to store thermal energy: sensible heat storage, latent heat storage and thermal storage in the form of chemical potential (sorption and thermo-chemical energy storage) (Figure 1). In sensible heat storage, the technique consists in supplying heat to a material, which remains in the same physical phase state, mostly a liquid or a solid phase. This results in the increase of the temperature of the material and the amount of heat stored is the product of the thermal capacity of the material by the temperature change. A thermal insulation is usually required to reduce the thermal losses to the surroundings. The main techniques used for sensible heat storage, especially at low temperature, are presented in Figure 1. Latent heat storage is based on the heat released or absorbed by a material, then called phase change material (PCM), while changing phase. During the charging phase, the material absorbs heat while changing phase at or above its melting (or vaporization) temperature. During the discharging phase, the material releases the latent heat while changing phase back to its initial state at or below its solidification (or condensation) temperature. As specific latent heat of materials are usually large compared to the specific heat capacity of materials used in a limited temperature change range, latent heat storage offers higher energy storage density than sensible heat storage (Figure 1), about 5 to 14 times [2].

[Figure 1 about here.]

Among the methods used for thermal energy storage, heat storage in
the form of chemical potential has the lowest technology readiness but also
the highest potential, provided the possibility for research funding [3]. It was
Goldstein [4, 5], in 1961, who probably first brought forward the idea of using
reversible chemical reactions to store solar thermal energy. Because of the
first oil crisis of 1973, research teams were mobilized to work on thermochemical solar energy storage for buildings [6–8] until the end of the 1980s [6–10].
Thermochemical heat storage for buildings was reactivated in the early 2000
thanks to the Kyoto protocole.

In 2012, the European Technology Platform on Renewable Heating and Cooling gave a very ambitious target of "novel thermochemical storage technologies with an effective storage density of more than 300 kWh m⁻³" [11]. From a standpoint of view, thermochemical energy storage was recognized as a key technology for energy-efficient buildings and districts [12, 13]. Note that thermochemical heat storage is also being investigated under the framework of the International Energy Agency [14]. Ideas for thermochemical or sorption thermal energy storage applications have been suggested in such diverse fields as co-generation efficiency improvement [15], industrial waste heat recovery [16], dispatchability of concentrating solar power plants [17] as well as dispatchability of solar photovoltaics [18], long-distance transportation of thermal energy [19] and exhaust gas reduction in automobile applications [20]. The emphasis, however, is mainly put on the development of a compact long-term solar thermal energy storage for space heating and domestic hot water (DHW) preparation as pointed out by several reviews articles [21–25]. There are three main motivations behind this option:

i) 36% of global final energy consumption is for buildings which represents
about 40% of total direct and indirect CO₂ emissions [26]; most of this energy is consumed in the form of heat (e.g. in 2017, 79% of final energy consumption by households in the European Union was for space heating (64%) and DHW (15%)) [27]).

ii) Thermal energy storage in the form of chemical potential offers high storage densities, nearly 8–10 times higher than sensible heat storage in the case of thermochemical storage [28–30].

iii) Negligible heat losses occur during the storage duration [31], i.e. between the charging and discharging phases.

The commercialization<fnref n="1" /> of the technology of thermal storage in the form of chemical potential was found as long overdue [32]. This delay is not related to the technical complexity and economic implications of the considered thermal storage processes alone. There is also the fact that the technology is struggling to fulfill promises in terms of technical performance, probably because of an initial misunderstanding about the nature of these processes and the definition of their performance criteria such as the energy storage density. Many review articles on thermochemical heat storage have been published [21, 22, 24, 28, 33, 34] but they have mainly focused on the technological development and materials, with no close look at the actual limitations of the technology with regards to the applications, disregarding the most re-

<footnote n="1">¹According to our survey, two products have been introduced to the market. They are rather devoted to solar heating and cooling (Climatewell TM changes to SaltX, with another focus of its activities: saltxtechnology.com) or mobile cold boxes applications than to thermal energy storage.</footnote>
alistic potential availabilities of the various resources that are necessary for their implementation.

This perspective paper focuses on long-term thermochemical heat storage for space heating and DHW production, which are the main applications targeted by the vast majority of studies [21, 22, 24, 25, 28, 33]. In this paper, the authors intend to provide a documented view on the technology after more than fifteen years of personal intense activity and experience on the technology, while questioning the relevance of the idea of long-term thermochemical heat storage for household applications and the performance criteria used to evaluate the corresponding systems. For that purpose, the following targets have been set for the present study:

i) to examine the various definitions of performance criteria used in the analyses and comparisons of applications of long-term thermal energy storage in the form of chemical energy, with an emphasis on the common related confusions.

ii) to provide an insight of the actual limitations of the technology with regards to the requirements of long-term solar heat storage for household applications, taking into account the most realistic potential availabilities of the various resources that are necessary for their implementation, based on data collected on several prototypes available in the literature.

iii) to identify the crucial issues to consider before starting any new work on thermochemical/sorption energy storage for household applications.

In the above set targets resides the novelty of this paper, which is not a descriptive review of works on long-term thermochemical heat storage for
household applications but rather a perspective paper, with focus on out-
standing issues, especially the ones that are likely to prevent actual imple-
mentation or adoption of the technology.

The rest of this paper is structured as presented in Figure 2. In Section 2, the operation principle, the different concepts and the basic terminology used in thermal energy storage in the form of chemical energy are given. This section also presents a preliminary fundamental analysis of this storage technology, examining the nature of the processes and the most realistic potential availabilities on the application side. Section 3 discusses the criteria used to assess the performance of the materials, processes or systems, the common confusions and suggestions for harmonization; the order of magnitude of reported performances in the litterature are roughly provided. Section 4 is devoted to the performance and real potential of thermochemical storage based on data collected on several prototypes available in the literature. The final section gathers the main conclusions of the present discussion.

[Figure 2 about here.]

The proposed analysis is not necessarily relevant for other applications
such as short term or high temperature industrial applications, whose con-
straints are different from the ones of household applications. For instance,
there are currently research activities on concentrated solar power heat stor-
age using thermochemical energy storage technology [17] and they are out of
the scope of the present work.
## 2. Basic thermodynamics analysis of thermal energy storage in form of chemical potential

### 2.1. Terminology

For convenience, the expressions "sorption" and "thermochemical" heat storage are commonly used as synonyms although they are different. Here is a short overview on the terminology used in the field of thermal energy storage as extracted from our previous work [34]. It is worth mentioning that this terminology is in agreement with the definitions given in [35]. Sorption is a phenomenon of fixation of a gas (sorbate) by a solid or liquid (sorbent). The term sorption covers absorption and adsorption. In the context of thermal energy storage, a phenomenon of penetration of a gas into a liquid is called absorption. The term "adsorption" is understood as a phenomenon of uptake of a gas on a surface of a solid. Physical absorption and physical adsorption (physisorption) involve physical bonds, mainly van der Waals forces, and are characterized by divariant equilibriums. Thermochemical heat storage includes chemisorption (chemical adsorption) or any reversible chemical reaction used to store thermal energy; it involves valency forces and monovariant equilibriums: the energy is stored in chemical bonds, offering larger heat of reaction than physisorption. For simplicity, in this paper, the expressions "thermochemical storage" and "sorption storage" will be used interchangeably unless explicitly stated otherwise. It is worth mentioning that the aforementioned abuse of language is common in the scientific literature.
## 2.2. Operation principle

The operating principle of thermal energy storage in form of physical or chemical energy relies on a reversible physico-chemical reaction, which is used to store energy. Basically, thermochemical heat storage systems may be classified as closed or open system [33, 36]. A comparison of both types of systems, extracted from [37], is provided in Table 3.

[Table 1 about here.]

The operating principle of a closed thermochemical heat storage system is presented in Figure 3. During the charging phase, heat $Q_c$ is transferred to the material in the reactor. When absorbing the heat, the material decomposes and releases the gas, which flows to the condenser. Then, the condensation of the gas occurs while the heat of condensation $Q_{cond}$ is removed. The condensate is stored separately in a tank. During the discharging phase, the condensate (liquid) is evaporated at a temperature $T_{evap}$, while absorbing the heat of vaporisation $Q_{evap}$ supplied by the low temperature heat source. The generated vapor in the evaporator flows to the reactor where it reacts with the sorbent. This results in the release of the discharging heat $Q_d$. In closed systems, there is a need of heat exchangers to transfer/recover the heat to / from the material in the reactor and in the condenser/evaporator. Closed system operates under a pressure different from the atmospheric pressure, usually low pressure because of the common use of water as sorbate; maintaining low pressure or vacuum conditions for long-term, especially in large vessels may be technically challenging.

[Figure 3 about here.]
The use of an open system, Figure 4, is an alternative to get away from low pressure related constraints. Open systems operate under atmospheric pressure, using a mixture of a carrier inert gas and the sorbate, typically dry air and water vapor (i.e., moist air). During the charging phase, the carrier (or the mixture) is heated by at the heat source and flows through the storage material. The material receives the charging heat $Q_c$ and release the sorbate, which is carried away by the carrier inert gas. During the discharging phase, the mixture of gas is passed through the dehydrated material. The gas is captured by the sorbent. The released heat $Q_d$ of this exothermic phenomenon is recovered by the carrier gas (or the mixture). The carrier gas is the heat transfer fluid. Open systems are usually fixed bed configurations [23] and more rarely moving bed systems, for instance, a revolving drum [38].

[Figure 4 about here.]

## 2.3. Thermodynamic nature of thermal energy storage in form of chemical potential systems

Usually, the charging temperature $T_c$ of a thermochemical energy storage system is not equal to the discharging temperature $T_d$. The charging heat of a thermochemical energy storage process may be divided into three parts [39, 40] (Table 1): the binding energy $Q_{bind}$, the heat of vaporization of the sorbate $Q_{vap}$ and the sensible heat $Q_{sen}$. The binding energy is the energy of linkage between the sorbent and the sorbate (or the reactants) due to physical forces (mostly Van der Waals forces) or valence forces (chemical reactions). It is lower in the case of physical sorption than in the case of
chemical reaction. For chemisorption reaction using hydrated salts<fnref n="2" />, it is
possible to show that the enthalpy of reaction of one mole of water ranges
from 54 kJ mol⁻¹ to 73 kJ mol⁻¹ [15, 41]. Since the enthalpy of condensation
of water at low pressure is about 44 kJ mol⁻¹, the direct conclusion is that
the heat of condensation represents from 65% up to 80% of the reaction en-
ergy [15, 42, 43]. Then, the heat actually stored in the process represents
between 20% and 35% of the total heat required for decomposition for salt
hydrates. Typically, this energy represents between 20% and 35% of the to-
tal decomposition heat when using salt hydrates [41] and even less in the
case of physisorption for processes described by various authors [43–48]. In
the latter case, the share of the binding energy depends on the concentra-
tion range involved and is defined by the operation temperatures. The heat
of vaporization is the amount of heat required for the vaporization of the
sorbate. It depends on both the temperature during charging T_c and the
temperature in the condenser T_cond. It also involves the latent heat of con-
densation Q_latency, the superheating between T_cond and T_c called Q_surch and
the heat taken away by the condensate to the storage tank Q_w,stock. The
sum of the first two heats corresponds to the condensation heat Q_cond in the
case of closed systems. The sensible heat Q_sen corresponds to heat required
to raise the temperature of the material from its initial temperature (usually
ambient temperature) to the charging temperature. It is a part of the overall
sensible heat involved in the reactor, which is discussed in Section 3.2. In
the discharging mode, the required heat for evaporation may be larger than

<footnote n="2">²Almost all the studies devoted to low-temperature chemical energy storage (i.e., building applications) use salt hydrates.</footnote>
the heat released by the reactor, especially in the case of absorption heat
storage. From this table, the heat actually stored in the process represents
between 6% and 32% of the total heat required for desorption.

As explained in section 2, the energy effectively transferred to the material
during storage is $Q_c - Q_{cond}$ and $Q_{cond}$ is the energy of condensation, that can
be recovered or not. Similarly, during discharging phase, the energy recovered
by the material is $Q_d$. The reaction requires an energy of vaporization $Q_{vap}$,
that can be free or not. Of course, the global efficiency of the system is
highly dependent on the possibility to recover $Q_{cond}$ and produce zero-energy
or energy-free $Q_{vap}$. So, on the whole, the maximum energy stored in the
thermochemical material is only 35% of the total energy required during the
charging phase.

As previously discussed, a minimum of 65% of the heat supplied by the
high-temperature source during charging is rejected at the condenser. Of
course, the recovery of this heat into the global process should be realized
either by direct recovery of this heat or by heat storage for a use during
discharging. This is an aspect that is important to emphasize because ther-
mochemical storage processes behave more like a heat pump with, depending
on the case, a small thermal storage or practically no actual storage. Indeed,
thermochemical heat storage is mainly used to raise the temperature of the
heat supplied to the evaporator: the proportion of the heat actually stored is
very small (ideally, a maximum of 35% of the heat provided during charging)
compared to the heat to be upgraded or to be delivered to the user. Such
process is also called thermochemical heat transformer (THT). For various
proposed absorption systems [42, 44, 50], the evaporation heat is even greater
than the heat to be released at the absorber (discharging heat), therein al-
ready considering ideal cases. It means that in winter, the system is actually
operating like a heat pump while the charging in summer is simply the re-
generation of the potential of the heat pump: there is no heat recovery in
winter and instead only heat upgrading.

In contrast to sensible or latent heat storage methods, which are direct
thermal energy storage techniques, thermochemical energy storage is an in-
direct heat storage technique [51]: heat is stored in the form of a bond
energy. This nature of thermochemical energy storage needs to be kept in
mind while comparing thermochemical heat storage with the other thermal
storage modes to avoid easily biased or questionable comparisons.

2.4. The low temperature heat source during discharging and related chal-
lenges

Basically, the heat of vaporization Qvap must be provided via the evap-
orator (closed system) or a humidifier (open system) by a heat source at
low temperature, typically 5°C to 20°C in the case of seasonal heat storage.
Potential sources in winter of this low temperature heat are often given as
a geothermal well or a solar collector field. A priori, the surface of an or-
dinary domestic solar thermal collector field would require an extension to
provide all this heat in winter conditions, especially during periods when the
heating demand is at peak. This necessary heat at low temperature is usu-
ally assumed to be available at extremely low cost or for free [52]. However,
the provisioning of this heat involves the installation of units that cannot
be neglected in comparison with other heat storage methods, including from
the point of view of bulk. When extracting energy from the environment,
it is not always possible to justify that the heat delivered by a geothermal
well or a field of solar thermal collectors is free because these installations
imply investments and operating costs (electrical consumptions of circula-
tors and maintenance) whose payback time is often closely analyzed by an
investor like conventional sources of heat. In addition, these units cannot
always provide the required temperature level for this evaporation because
the heat requirements and the availability of the solar resource are often in
phase opposition. Additional heat storage may then be necessary for the
night, for example, and this storage volume would need to be considered. In
some cases, the recovery of a portion of the condensation heat released by the
thermochemical storage is considered to ensure evaporation [53, 54], which
would mean recovering up to 80% - or even more in the case of absorption-
of the stored heat for this purpose.

[Figure 5 about here.]

The aforementioned remarks remain globally valid in the case of open
systems because, contrary to what is often implied, it is not possible to rely
on the humidity present in the ambient air, especially in winter, when the
air is cold and possesses a low water vapor pressure (Figure 5): the vapor
pressures usually considered in these studies (≈ 12 mbar to 25 mbar) are well
above the vapor pressures available in winter in the locations for which the
storage system is envisioned. The recovery of moisture from the building,
which is another possible strategy, would probably be largely inadequate
(significantly increasing the load of the building), and its feasibility has not
yet been demonstrated.
Referring to real conditions of the ambient air in winter, the storage
densities would be significantly reduced, or even impossible, knowing that the
values presented in Figure 5 are means and that the water vapor pressures
and outdoor temperatures would be more unfavorable when heat needs would
be at peak.

3. Main performance criteria used for thermochemical storage ma-
terials and processes

A quick examination of the literature on thermochemical storage systems and processes shows that the performance criteria used and presented do not correspond to the same realities from one author to another, although the terms are exactly the same. This makes it difficult to compare one thermochemical material, process, or storage system to another; some reported comparisons seem even objectively misleading. The main performance criteria are the thermal energy storage density, the energy efficiency of the storage and the cost of the material or system. The authors of the paper therefore believe that it is important to briefly comment on these criteria without much pretense to resolve the difficulties related to their relevance, which may depend on the context of the proposed storage project.

3.1. *Volumetric energy storage density*

The volumetric energy storage density is probably the most prominent
advantage for thermal storage in the form of chemical potential, which ex-
plains the enormous interest for seasonal heat storage applications. The
energy storage density of the material is not a property of the material but
rather depends on the operating conditions, in particular, the charging tem-
perature, the discharging temperature (user side), the temperature of the
low-temperature heat source and the power density. Any relevant compari-
son of the energy storage densities of material or systems must therefore be
made without losing sight of these aspects. The volumetric energy storage
density can be simply defined as the amount of energy (heat in this case)
that can be extracted from the considered system divided by the volume of
the system. By referring to the material, it is possible to distinguish in par-
ticular the absolute energy storage density, whose reference volume is that
of the solid grains and the bulk energy storage density, which considers the
total volume occupied by the bed of particles considered and thus the pores
and void volumes between particles (Figure 6) [42].

[Figure 6 about here.]

For absorption storage, these distinctions are irrelevant because the stor-
age density is related to the volume of liquid. The question of the density
of storage goes beyond the material because the end user is definitely inter-
ested in the congestion of the system as a whole. The boundary, in this case,
goes to the process or system level. At the process level, considering the
volume of the condensed fluid (water in almost all studies aimed at house-
hold applications) is a minimum for closed processes, although this volume
is not always considered. The energy storage density considering the volume
of the condensed fluid can be called the reduced energy storage density [15].
This reduction does not apply to open systems, as water is released into the
atmosphere and therefore not stored.
The volumes of the different heat exchangers as well as those of other com-
ponents, for example, those of possible vapor diffusers or thermal insulators,
should also be considered but are often neglected and considered negligible,
rightly or wrongly, compared to the involved storage volumes. When com-
pared with other storage methods, it might be more accurate and relevant
to consider the bulk of the related process components, e.g., possible cooling
towers (for condensation) and geothermal wells (for evaporation or conden-
sation). This all depends on the context. For example, when using water
from a watercourse as heat sink for condensation during summer, it may be
legitimate to disregard the watercourse but less so when using a 10 m³ tank
of rainwater as a heat sink, as was in the case of the HYDES prototype [55].

To illustrate the importance of the definition of common references in
the comparison of energy storage density, we consider, for instance, the per-
formance of the absorption thermal energy storage system investigated by
Zhang et al. [56]. The authors compared the performance of their reported
prototype to the performance of other sorption thermal energy storage pro-
totypes in the literature (see Table 2 extracted from their article). They
indicated an energy storage density based on prototype of 110 kWh m⁻³ and
concluded that "*The performance [of their prototype] is better than those of
previous thermal energy storage systems, which proves that the absorption
thermal energy storage system using LiBr-H₂O may be a good option for
thermal energy storage*".

Let now consider the data provided in the paper of Zhang et al. [56]:

• Weak solution and water total volume: 0.24 m³,

• Solution barrel and the refrigerant barrel total volume: 0.62 m³,
* Total space occupied by the prototype (length × width × high): 3.97 m³,

* Storage capacity of the prototype (heating water at 43 °C): 25.8 kWh.

It appears that the energy storage density based on prototype of 110 kWh m⁻³ given by the authors is calculated using the volume occupied by weak solution and water. In our opinion, the prototype energy storage density would be 6.5 kWh m⁻³ using the total space occupied by the prototype. It is probably true that the prototype was not already optimized as the report was delivered; thus, comparison with other prototypes would have been at least avoided and a storage density based on the total the solution barrel plus the refrigerant barrel volume could have been sufficient, i.e. 41.6 kWh m⁻³. Still regarding the comparison, it should be pointed out that the close three phase absorption based on LiCl-H₂O is a commercial machine under the product name Climatewell 10. Now, the 85 kWh m⁻³ used in the comparison in Table 2 is the short-term energy storage density of the prototype of Climatewell 10, i.e., due to the physical design of the unit, with the necessary heat exchangers, pipes and stores for solution and water [59]. When considering the energy storage density of the material, Climatewell specifications indicate 253 kWh m⁻³ for LiCl. Given the volume of water of 117 kg and the capacity of the machine of 35 kWh, the energy storage density based on the volumes of LiCl and water would be in the range of 137 kWh m⁻³.

Another aspect is the fact that in some reported energy storage densities, the sensible heat recovery is included. For example, in reference [60], the discharging of the prototype immediately follows the charging so that the sensible heat available in the mass of the prototype (active and inert materials) has been recovered, amounting to approximately 40% of the discharging
heat. While this may be legitimate in the case of short-term thermal energy
storage, it is not representative of the seasonal storage process; thus, the
reported energy storage density is largely overestimated in this regard.

Participants in IEA-SHC Task 42/24 on Compact Thermal Energy Stor-
age agreed that for thermochemical storage to be competitive, the system
energy storage density must be 4 to 8 times higher than that of water [61],
which the authors estimate as approximately 360±120 kWh m⁻³. In the pre-
vious remark, the energy density of water is calculated using a temperature
difference ΔT = 50 °C.

Sometimes, the energy storage density is also reported as the mass energy
storage density [62, 63]. Depending on the application, this performance
indicator may be relevant. It is important to consider that classification based
on mass energy storage density does not lead to the same result when using
volumetric energy storage density as a performance indicator. Therefore,
energy storage density improvements based on mass (e.g., [31, 64]) do not
necessarily imply an improvement in volumetric energy storage density.

3.2. *Energy efficiency*

The energy efficiency of a thermochemical storage process is usually de-
fined as the ratio of the amount of heat energy released by the process during
discharge to the amount of heat energy required during the charging of the
process [45, 65–68]. For a perfect chemisorption heat storage using hydrated
salts, it is possible to assess the maximum energy efficiency in buildings [41]:

* equal to 100% if the heat of evaporation during discharging phase is assumed freely available and there is no condensation recovery during charging phase,
* about 60% if the heat of evaporation is taken into account and there is no condensation recovery during charging phase,

* about 180% if the heat of evaporation is assumed freely available during discharging phase and there is condensation recovery during charging phase [69].

However, the energy efficiency values of thermochemical storage prototypes are much lower, typically 15% to 50%, according to [47, 52, 65, 78, 70, 76], for both closed and open systems, for at least two reasons. The first reason is related to the difficulty of recovering all the heat stored in these systems with a reasonable power [53, 65, 72]. The second reason is that these processes are subject to significant sensible heat losses, mainly because the process is not continuous.

First, sensible heat is required to raise the temperature of the material in the reactor from its initial temperature (usually ambient temperature) to the charging temperature. In the ideal case, if this sensible heat is small for salt hydrates (approximately 3% to 10%, Table 1), it may reach up to 40% of the charging heat or more in the case of absorption storage, not only because of the large difference in temperature between the thermal storage tank in winter and the desorption temperature but also because of the low heat of mixing or binding exhibited by physisorption phenomena [42]. In the case of long-term solar heat storage, during the charging period in summer, the reactor temperature increases during the daytime and cools down at night, resulting in the lost of this sensible heat. The operation is repeated every day such that the share of sensible heat losses becomes greater than in the ideal case, where the whole material decomposes during a single rise in
temperature. It is with the aim to approach this ideal case that the reactor
is often imagined as an association of several modules or compartments to be
charged/discharged separately [73]. During the discharging phase, a similar
reasoning may be applied: a portion of heat released by the reaction is used
to raise the temperature of the material.

Sensible heat losses concern not only the storage material but also the entire inert mass of the reactor (metal envelopes, heat exchangers, diffusers...) and heat losses to the ambient environment (which can be very high, including in the case of open systems e.g., [48, 65]), for instance, through the reactor walls during charging or discharging.

It is therefore important to consider these heat losses in the evaluation of
storage volumes because the given energy storage densities do not consider
the thermal efficiency of the system. The definition of thermal efficiency
that have been used here - and it is the one usually used in the literature
- considers that the heat necessary for evaporation is freely extracted from
the environment. However, as already mentioned, thermochemical storage
processes, at least those based on salt hydrates and physisorption, operate
rather as heat transformers [15, 43]. This seems equally important in the case
of short- and medium-term thermal energy storage (three days to approximately a week), where thermal storage is used for economic reasons, such as
the case of micro-CHP plants [15] or CSPs, where no free low-temperature
source is available. This calls for the definition of new energy efficiency indicators [15, 42] to consider this important heat of evaporation. The use
of exergy efficiency, although relevant [42, 74], does not seem completely
satisfactory.
### 3.3. Coefficient of performance

The coefficient of performance (COP) has long been considered high, especially for closed systems. For closed solid-gas processes, there are no moving parts, and there is practically no intrinsic (or internal) electrical consumption related to the process; electricity would simply be needed for the regulation and control of the process. In the case of absorption heat storage processes, there may only be a solution circulator to consider. Climatewell reported, for example, an electric COP of 96 for heating with the TCA Climatewell 10 system [75]. For open systems, the power consumption of the fan and any humidifier are more significant. This leads to COPs ranging from 1.2 to up to 30 [48, 76, 77] on different prototypes, which are, however, not optimized. In comparison, conventional heat pumps may offer real-world COPs of 1.8 to 6.5 [78] for a temperature lift of 15 °C to 60 °C. Thermochemical heat storage units hence need to offer significant electrical COP advantages compared to conventional heat pumps to balance their drawbacks. Indeed, they are demanding in comparison with a conventional heat pump because they require charging at high temperature and a relatively high temperature at the low-temperature heat source, a temperature level that cannot be provided by ambient air in the discharging period (Figure 5).

### 3.4. Cost of material and energy storage cost

The cost of the material is often analyzed in terms of price per unit mass (€·kg⁻¹, e.g., [62, 79, 80]) and per unit volume (€·m⁻³, e.g., [59]). For instance, the material cost of silica gel is about 4300€·m⁻³ while zeolite 13X is 2000€·m⁻³ to 3000€·m⁻³ [81]. Chemisorption materials can reach up to 42300€·m⁻³ for SrBr₂ [74]. It is worth mentioning that researchers are
also working on low-cost materials like, for instance, ettringite, a compound
of cement having a price around 650 €·m⁻³ [82, 83]. These indicators are
objectively not very consistent: it would be just as legitimate to reason with
€·mol⁻¹. Since the common reference to the processes that are compared
is the amount of heat stored, it seems more judicious to reason at least
in terms of price per stored energy amount (€·kWh⁻¹) when comparing
different storage materials. This makes it possible to consider the energy
storage density and to immediately identify the material that will result in a
higher investment cost for a given storage project. For example in Figure 7,
the costs of material for a long-term absorption based thermal energy storage
for space heating [42, 44] are provided. In short, NH₃-H₂O would be the best
option if the selection criteria were the cost per unit volume (€·m⁻³) or the
cost per stored energy (€·kWh⁻¹) (Figure 7). CaCl₂-H₂O would be the best
option if the criteria were the mass energy storage density (€·t⁻¹).

[Figure 7 about here.]

Actually, the question of cost should be analyzed over the entire life cycle
of the planned storage; however, the necessary data are not always available
to do so easily and quickly. A material may have a low cost per kWh of stored
heat but be very corrosive and operate at very low pressure, leading to the
need for the use of materials adapted to its corrosiveness for the construction
of heat exchangers or reactors and special components to ensure and keep a
high vacuum. Price varies in space and time, and there is currently no defini-
tion of inexpensive material. In European contexts, various authors [84, 85]
have estimated the target of approximately 0.25 €·kWh⁻¹ as desirable for
the energy storage system. More recently, assuming the energy delivered by
an energy storage system should not be more costly than the energy avail-
able from the market, a group of experts from IEA-SHC Task 42 participants
have indicated that the maximum acceptable costs per installed storage ca-
pacity are in the range of 2€·kWh⁻¹ to 4€·kWh⁻¹ for the case of seasonal
storage under the assumption of a very enthusiastic user (payback period =
25 years and low interest rates = 1%) [86–88]. For an ordinary user in the
building sector (payback period = 15-20 years and moderate interest rates =
5%), the acceptable cost is 0.6€·kWh⁻¹ to 1.43€·kWh⁻¹. They conclude
that, among the systems that they examined, only large sensible hot water
storage systems may be economical when considering seasonal thermal en-
ergy storage [88]. Note that for the sake of simplicity, operating costs and
energy production cost evolution over time have been disregarded [86, 88].

4. Performance and real potential of thermochemical storage

In general, at the beginning of the studies, authors most often refer to
the absolute ideal energy storage density of the material, hence sidestepping
some crucial issues that subsequently have major adverse impacts on the
energy storage density of the prototype [15]. The following brief overview of
the use of magnesium sulfate may be considered as an illustration.

Magnesium sulfate heptahydrate MgSO₄ 7H₂O was presented in 2004 as
the most promising material according to a theoretical study conducted at
ECN [89]. This study covered 88 materials with potential for the seasonal
storage of solar thermal energy [90]. The theoretical energy storage den-
sity (absolute) of the reaction is 780 kWh m⁻³ at a reaction temperature of
122°C [89]. However, van Essen et al. [91] performed characterization studies of this material and concluded that its use as a thermochemical storage material is problematic. Indeed, the specific power observed appears to be low, and the material cannot restore the stored energy under practical conditions of use. Vapor generated from a source at 10°C and then 20°C did not allow sufficient reaction kinetics. In 2016, Claire Ferchaud presented, in her thesis at ECN, the results of investigations aiming, among other things, to understand the limitations related to this salt [92]. Her thesis showed that the kinetics is improved for a water vapor pressure of 50 mbar, i.e., saturated humid air at 33°C, during the discharging to reach a temperature of 40°C. In addition, the reactor must be maintained above 36°C to avoid solution formation. The study concludes with these words: "Therefore, the outcome of the present investigation shows strong disadvantages of using MgSO₄·7H₂O as a thermochemical material for seasonal heat storage in the residential sector."

Nevertheless, to reduce the impact of these limitations and take advantage of the high energy density offered by the salt, the production of composite materials based on MgSO₄·7H₂O has been proposed in several studies. Work conducted at CETHIL from 2008 to 2011 proposes to impregnate it on zeolite 13X to take advantage of the storage capacity of the zeolite [53]. A composite material consisting of the salt at 15% by weight has thus been identified as optimal. The material, designated ZM15, requires approximately 150°C for regeneration and thus achieves a storage density of 166kWh m⁻³ for heat release at approximately 30°C (discharging temperature). However, studies published by IRCELYON (calorimetry) in 2013 show that a higher or lower sorption capacity is obtained compared to pure zeolite, depending on the
zeolite used to manufacture the composite material [93, 94]. Another the-
sis [95] presented in 2018 at the University of Lyon showed mixed results, at
least in regard to seasonal storage. The study showed that the water sorp-
tion capacities of the zeolite/MgSO4 composites are lower than in the case of
pure zeolites at low vapor pressure (about 10 mbar) and at high temperature
(superior to 35 °C). However, at high vapor pressure (above 15 mbar) and
at low temperature ( 20 °C to 25 °C), there is a significant gain in the wa-
ter sorption capacity of the composite compared to the pure zeolite (15% to
29%). These different studies made it possible to understand more generally
various phenomena involved in thermochemical storage materials.

Figure 8 shows the storage density for the most successful thermochemical
storage prototypes in the literature [1, 45, 53, 59, 60, 77, 96–102].

[Figure 8 about here.]

As previously indicated (Section 3), rigorous comparison between the
processes requires information on several other parameters such as operating
pressures and power densities. Because there is currently no widely accepted
rule for evaluating the energy storage density of a prototype, the comparisons
between the prototypes need to be put into perspective. A few comments on
Figure 8 are nevertheless appropriate:

* Most prototypes target temperature levels that match the range of heating in buildings. The thermochemical storage developed today is initially targeted for high-energy-performance buildings (e.g., Passive houses [44, 54, 97] and low-energy buildings [53]) to meet the heat requirements of a single family house. In these houses, however, the
heating requirements are so low that those of DHW have become pre-
ponderant [103].

* Irrespective of the nature of the process, the energy storage density is less than 200 kWh m⁻<fnref n="3" />, which is lower than the minimum assumed energy storage density for the process to be relevant [61]. The reported value is not directly derived from measurement on the prototype but instead is calculated under the assumption that 100% of the storage capacity of the salt is recovered during charging and discharging [72, 104]. Because the full capacity of the reactor has not been recovered for an acceptable power release condition (limited reaction advancement) [72, 104], the actual energy storage density is then lower than the reported values. Furthermore, this highest energy storage density is exhibited by an open system based on chemisorption under very favorable operating conditions [104] that can hardly be obtained in winter conditions without the installation of additional units such as humidity generation systems and significant energy input<fnref n="3" />. Regarding processes that are likely to provide the temperatures required for DHW, the maximum observed energy storage density is 140 kWh m⁻<fnref n="3" />, again for an open system based on chemisorption.

* For closed systems, the highest storage density observed is 85 kWh m⁻<fnref n="3" /> (commercial machine), whereas other systems provide very low energy storage densities, down to 5 kWh m⁻<fnref n="3" />.

<footnote n="3">³The assumed inlet air in the system during discharging is at 10 mbar and 25 °C. Such characteristics could not be obtained from outdoor air in winter (Figure 5).</footnote>
* Physisorption processes exhibit the lowest energy storage densities. Those with storage densities above 100 kW h m⁻³ require charging temperatures as high as 180 °C, a temperature level that cannot be achieved by ordinary solar collectors with satisfactory yields.

* For chemisorption processes, the storage density of the system may ultimately represent only 5% to 30% of the absolute density of the material.

* Excluding the MONOSORP system, the highest ratio between the prototype energy density and the material energy density is about 50%.

The values shown in Figure 8 assume that heat is taken from the environment free of charge. When this heat cannot be assumed free, as is the case for the storage of heat coming from a micro-cogeneration [105], then the storage densities of two of the most advanced prototypes in the literature, SOLUX [102] and MERITS [99], for instance (Figure 8), fall to 20 kW h m⁻³ and 13 kW h m⁻³, respectively. Note that the bulk of the low-temperature heat source/sink for evaporation/condensation is not considered by authors. It is also important to note that depending on the design, the energy storage density of a prototype may depend, to some extent, on its capacity because the bulk of some accessories may not be directly proportional to the capacity of the prototype.

Figure 9 shows the evolution of the reduced energy storage density (i.e. related to the theoretical total volume: volume of aqueous solution + volume of stored water) as a function of the absorption temperature for different absorption couples [42]. It appears that the storage density decreases rapidly with
discharging temperature and vanishes for a temperature below 46 °C in the
case of the LiBr-H₂O working pair, which exhibits the highest energy storage
density. This implies that conventional absorption thermal energy storage
systems based on LiBr-H₂O, or the other aqueous solutions presented here in
Figure 9, cannot provide heat for DHW (application requiring a temperature
of approximately 60 °C [78]). Certainly, the implementation of advanced cy-
cles could allow in some cases one to increase the discharge temperature and
to satisfy the needs of DHW; however, this is done to the detriment of other
performance indicators, especially the energy storage density [54].

[Figure 9 about here.]

For an absorber temperature of 35 °C (i.e., discharging temperature if the
heat exchanger temperature pinch is overlooked), the LiBr-H₂O working-pair,
which offers the best energy storage density, offers a reduced energy storage
density of approximately 100 kWh m⁻³. For an individual house of 120 m²
meeting the Passive House standard (i.e., heating needs = 15 kWhm⁻²year⁻¹),
a 15.1 m³ storage tank is required for the solution and a 2.9 m³ water tank,
i.e., a total useful storage volume of 18 m³. This volume, which concerns only
heating, seems somewhat unacceptable because, for example, it exceeds the
maximum volume of 10 m³ for the complete system sometimes mentioned by
certain authors [99] as their objective to ensure the heating needs and DHW
of a single house. The volume has been calculated without accounting for
heat losses so that the actual necessary volume will be higher. The authors
are not aware of processes, of any nature whatsoever, installed in individual
houses containing so much liquid solution. This also requires 3.2 t of anhy-
drous LiBr, a significant amount of salt. Several tons of salt are required to
cover heating needs, irrespective of the considered salt (Figure 9). Regardless
of the inevitable question of cost that will arise - since the materials today,
even free, will have a cost as soon as a recovery method is identified - there
are the inevitable problems of pollution related to the presence of such a huge
quantity salt in a house, no matter how harmless the salt: salty soil is almost
always unfit for animal or plant life. Given the large quantities of liquids
that are not completely safe to store, there are several safety and therefore
regulatory issues that could arise regarding storage of chemicals in a habitat.

With regard to salt hydrates, despite their predominance as thermochem-
ical storage materials, most salts have led to technical performances below
expectations, e.g., an inadequate or too low discharging temperature, energy
storage density or power density (kinetics), not to mention stability prob-
lems (cyclability). From the material to the prototype, crucial issues lead to
a significant reduction of the energy storage density [15]:

* The need to ensure a minimum porosity or to use a matrix (vermiculite, expanded graphite, pumice, more stable physisorbents, such as silica gel and zeolite, etc.) to improve the heat and mass transfers, as well as the cyclability of the material, can strongly reduce the energy storage density of the reactive material. In different studies [30, 53, 102, 106, 107], the materials ready to go to the reactor already have only 3% to 40% of their absolute density, not to mention the fact that the binding energy could also decrease because of the use of physisorbents.

* The heat exchanger required in the reactor (and possibly the vapor diffusers) usually needs to have a large exchange surface and leads to a significant reduction in the energy storage density [99, 102].
* Possible evaporator or condenser vessels or the corresponding heat exchangers occupy a volume that must be considered.

* The sensible heat losses can be difficult to neglect and are even greater than the storage if the charging load is not done at once.

* Limitations of the kinetics at the reactor level: a complete conversion (full decomposition and full synthesis within the envisaged boundaries), especially during the discharging phase, may require time for closed [102, 108, 109] as well as open processes [53, 72, 110]. Therefore, depending on the operating conditions and objectives, one can expect to charge/recover only a portion of the energy in a "reasonable" time.

* The kinetics may be limited in some cases, especially in open systems, by the deliquescence phenomenon [63, 92], which may limit the operating condition possibilities and ultimately reduce the energy storage density. For instance, magnesium sulfate and magnesium chloride, which have been considered as very promising seasonal thermochemical storage materials in the residential sector, have finally been reported as not relevant for this application, at least in open systems [63, 92].

The mass storage densities offered by salt hydrates (typically between 0.1 kWht⁻¹ to 0.3 kWht⁻¹) also lead to the need for several tons of salts to meet the heating needs of a high-performance building such as a house that meets the Passive House standard.

Admittedly, the implementation of advanced cycles could in some cases make it possible to improve, but always to a limited extent, some performance metrics to the detriment of others [43, 74]. However, this increases
the complexity of a system that is already complicated [44] without globally
addressing the key issues.

**5. Conclusion**

The performances of conventional absorption processes are particularly
limited or even prohibitive for long-term thermal energy storage for house
heating. These performances cannot satisfy DHW requirements unless ad-
vanced cycles, which would result in lower energy storage densities, are used
while simultaneously increasing the complexity of the process. Similarly,
the performance achieved with salt hydrates-based systems is too low to
compete with other simpler technologies, namely, hot water storage [111–
114]. Given the complexity of the process and its potentially high invest-
ment and maintenance costs, an annual cyclical operation (one cycle per
year), as is the case with seasonal storage, is not sufficient to justify it from
an economical perspective. Even for short- and medium-term applications,
the need to justify the suitability of using thermochemical storage remains
unresolved. Ultimately, thermochemical storage needs to be justified on a
case-by-case basis, and its option must depend on the application while ad-
dressing sustainability-related questions. More generally, the relevance for a
single family house to invest in a demanding heat pump with no significantly
high electrical COP may be questioned. In any case, the development of
thermochemical storage processes cannot be achieved by disconnecting them
from the most realistic potential availabilities of the various resources that
are necessary for their implementation, as well as the associated constraints,
including sustainability issues. The authors suggest that before undertaking
any new research activity on this topic, researchers should carefully consider
the challenges pointed out in this paper.

Some conclusions drawn here could be mitigated without being destroyed
if, in the future, the temperatures needed for heating a building decrease
significantly. The current analysis also advocates for a more objective and
transparent methodology for comparing the performances of thermal stor-
age processes, from the definition of indicators to the fixation of operating
conditions.

Acknowledgements

Figure 1 has been reprinted from Applied Energy, 181, NTsoukpoe KE, Osterland T, Opel O, Ruck WKL, Cascade thermochemical storage with internal condensation heat recovery for better energy and exergy efficiencies, 562574, Copyright (2016), with permission from Elsevier.

Table 1 has been reprinted from Energy, 77, NTsoukpoe KE, Restuccia G, Schmidt T, Py X, The size of sorbents in low pressure sorption or thermo-chemical energy storage processes, 983998, Copyright (2014), with permission from Elsevier.

Table 2 has been reprinted from Energy Conversion and Management, 85, Zhang X, Li M, Shi W, Wang B, Li X, Experimental investigation on charging and discharging performance of absorption thermal energy storage system, 425434, Copyright (2014), with permission from Elsevier.
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[Table 2 about here.]
<table><caption>Table 1: Thermochemical storage: simplified description of the destination the theoretically required heat for the charging and share of each of these destinations for various thermochemical energy storage processes as reported by the authors. In all the cases, water is the sorbate. The sensible heat only considers the storage material (the inert material is not considered). The sensible heat is not considered for the processes depicted on the right side (data reported as provided by the authors). Sil = Silicagel; Zéo = zeolite; 3 phases = three-phase sorption cycle.</caption><thead><tr><th></th><th>Q<sub>c</sub></th><th>Q<sub>r</sub></th><th>Q<sub>bind</sub></th><th>Q<sub>sen</sub></th><th>Q<sub>srock</sub></th><th>Q<sub>sarch</sub></th><th>Q<sub>latent</sub></th><th>Q<sub>cond</sub></th></tr></thead><tbody><tr><td>charging temperature [°C]</td><td>130</td><td>80</td><td>14</td><td>13</td><td>22</td><td>64</td><td>14</td><td>7</td></tr><tr><td>discharging temperature [°C]</td><td>65</td><td>35</td><td>35</td><td>7</td><td>7</td><td>68</td><td>25</td><td>20</td></tr><tr><td>evaporation temperature [°C]</td><td>25</td><td>6</td><td>5</td><td>27</td><td>27</td><td>55</td><td>18</td><td>25</td></tr><tr><td>condensation temperature [°C]</td><td>0</td><td>44</td><td>15</td><td>11</td><td>90</td><td>90</td><td>11</td><td>23</td></tr><tr><td>storage temperature [°C]</td><td>30</td><td>5</td><td>10</td><td>32</td><td>32</td><td>32</td><td>32</td><td>32</td></tr><tr><td>reference</td><td>[49]</td><td>[44]</td><td>[45]</td><td>[43]</td><td>[43]</td><td>[43]</td><td>[43]</td><td>[43]</td></tr></tbody></table>

<table><thead><tr><th></th><th>Q<sub>r</sub></th><th>Q<sub>vap</sub></th><th>Q<sub>bind</sub></th><th>Q<sub> latent</sub></th><th>Q<sub>bind</sub></th><th>Q<sub>bind</sub></th><th>Q<sub>bind</sub></th><th>Q<sub>bind</sub></th></tr></thead><tbody><tr><td>charging temperature [°C]</td><td>11</td><td>3</td><td>32</td><td>11</td><td>32</td><td>32</td><td>32</td><td>32</td></tr><tr><td>discharging temperature [°C]</td><td>66</td><td>65</td><td>23</td><td>66</td><td>65</td><td>23</td><td>23</td><td>23</td></tr><tr><td>evaporation temperature [°C]</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>condensation temperature [°C]</td><td>–</td><td>–</td><td>23</td><td>–</td><td>–</td><td>23</td><td>23</td><td>23</td></tr><tr><td>storage temperature [°C]</td><td>–</td><td>–</td><td>25</td><td>–</td><td>–</td><td>25</td><td>25</td><td>25</td></tr><tr><td>reference</td><td>[49]</td><td>[44]</td><td>[45]</td><td>[43]</td><td>[43]</td><td>[43]</td><td>[43]</td><td>[43]</td></tr></tbody></table>

<table><thead><tr><th></th><th>Q<sub>r</sub></th><th>Q<sub>vap</sub></th><th>Q<sub>bind</sub></th><th>Q<sub> latent</sub></th><th>Q<sub>bind</sub></th><th>Q<sub>bind</sub></th><th>Q<sub>bind</sub></th><th>Q<sub>bind</sub></th></tr></thead><tbody><tr><td>charging temperature [°C]</td><td>11</td><td>3</td><td>32</td><td>11</td><td>32</td><td>32</td><td>32</td><td>32</td></tr><tr><td>discharging temperature [°C]</td><td>66</td><td>65</td><td>23</td><td>66</td><td>65</td><td>23</td><td>23</td><td>23</td></tr><tr><td>evaporation temperature [°C]</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>condensation temperature [°C]</td><td>–</td><td>–</td><td>23</td><td>–</td><td>–</td><td>23</td><td>23</td><td>23</td></tr><tr><td>storage temperature [°C]</td><td>–</td><td>–</td><td>25</td><td>–</td><td>–</td><td>25</td><td>25</td><td>25</td></tr><tr><td>reference</td><td>[49]</td><td>[44]</td><td>[45]</td><td>[43]</td><td>[43]</td><td>[43]</td><td>[43]</td><td>[43]</td></tr></tbody></table>
<table><caption>Table 2: Performance comparison of various sorption thermal energy storage prototypes, as provided by Zhang et al. [56]. ESD = energy storage density</caption><thead><tr><th>Type of sorption</th><th>Working pair</th><th>Charging rate</th><th>Discharging rate</th><th>Storage capacity</th><th>ESD based on prototype</th></tr></thead><tbody><tr><td rowspan="2">Closed liquid</td><td rowspan="2">LiBr-H<sub>2</sub>O</td><td>kW</td><td>kW</td><td>kWh</td><td>kW h m<sup>-3</sup></td></tr><tr><td>7 kW at 80 °C</td><td>7 kW at 7 °C</td><td>9.8</td><td>Cooling: 0.51</td></tr><tr><td>absorption</td><td></td><td></td><td>7 kW at 43 °C</td><td>25.8</td><td>42</td></tr><tr><td rowspan="2">Closed liquid absorption</td><td rowspan="2">NaOH-H<sub>2</sub>O</td><td rowspan="2">1 kW at 95 °C</td><td rowspan="2">1 kW at 70 °C</td><td rowspan="2">7 kW at 65 °C</td><td rowspan="2">20.8</td><td rowspan="2">Heating: 1.03</td></tr><tr><td>88</td></tr><tr><td>Closed 3 phase</td><td>LiCl-H<sub>2</sub>O</td><td>15 kW at 87 °C</td><td>8 kW at 30 °C</td><td>35</td><td>110</td></tr><tr><td>absorption</td><td></td><td></td><td></td><td></td><td>Heating: 0.97</td></tr><tr><td>Closed liquid absorption</td><td>LiBr-H<sub>2</sub>O</td><td>2 kW to 5 kW</td><td>at 30 °C to 40 °</td><td></td><td>85</td></tr><tr><td>Closed solid absorption</td><td>silica gel</td><td>1.0 kW to 1.5 kW</td><td>at 75 °C to 90 °</td><td>0.5 kW to 1.</td><td>Cooling: 54</td></tr><tr><td></td><td></td><td>at 88 °C</td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>Table 3: Comparison and main features of open and closed thermochemical energy storage systems - extracted from [37].</caption><thead><tr><th>Process</th><th>Pros</th><th>Cons</th></tr></thead><tbody><tr><td rowspan="2">Closed</td><td><ul><li>Higher discharging temperature</li><li>No mass exchange with the environment</li><li>Better control of the pressure and the mass transfer</li><li>Higher power density (no inert gas)</li><li>Can be used as an adsorption heat pump for cooling and heating or as heat transformer within a large range of adsorbate pressure (even above atmospheric pressure)</li></ul></td><td><ul><li>A heat exchanger is normally required in the adsorber resulting in a decrease of the overall energy storage for at least two reasons: the volume of the heat exchanger and the temperature difference between both sides of the heat exchanger; decrease in the thermal efficiency due to the thermal capacity of the heat exchanger</li><li>Difficulty in managing vacuum in large reactors. In addition, the presence of non-condensable gases can reduce strongly the system performance</li><li>The adsorbate or reaction component has to be stored resulting in a decrease of the overall storage density</li><li>Requires condenser/evaporator for heat dissipation/supply resulting in a further decrease in the overall storage density</li><li>Generally, heat transfer is limiting compared to mass transfer</li><li>Typically, this process is complicated by the low thermal bed conductivity of the adsorbent powder, which is due to the low conductivity of the gas in the pores between the particles</li><li>The vaporisation energy of the adsorbate has to be provided, usually by the environment (e.g. borehole)</li></ul></td></tr><tr><td><ul><li>Favorable for the system storage density because no adsorbate has to be stored and no heat exchanger is required in the bed</li><li>The heat transfer fluid is the gas carrier (hummid air): no heat exchanger is needed in the adsorber and there is no temperature approach</li><li>The flow rate can easily be managed for the desired temperature increase</li><li>Heat transfer favored by forced circulation</li><li>Operation under atmospheric pressure</li></ul></td><td><ul><li>Restriction of the size of the storage system in a fixed bed configuration: high flow rate, which yields in high pressure drop, is usually needed because the thermal capacity of the gas is low</li><li>Generally, mass transfer is limiting compared to other heat transfers</li><li>Mechanical energy (usually a fan) for the gas circulation</li><li>Only water is used as reaction partner (gas) because of cost and safety issues</li><li>At the same water vapour pressure, the sorbent may not take up the water vapour under atmospheric conditions</li><li>The humidity in the air is not usually available in particular for seasonal storage (basically, up to 80 % of relative humidity is needed): a humidifier is required in most cases</li><li>Small particles may flow out of the reactor if fluidized bed is used</li></ul></td></tr></tbody></table>
# LIST OF SYMBOLS

## Abbreviations

DHW domestic hot water

## Latin symbols

<table><tr><td>p</td><td>pressure</td><td>Pa</td></tr><tr><td>Q</td><td>energy</td><td>J or kWh</td></tr><tr><td>T</td><td>temperature</td><td>K or °C</td></tr><tr><td>X</td><td>adsorbed water mass ratio</td><td></td></tr></table>

## subscripts

<table><tr><td>bind</td><td>binding</td></tr><tr><td>c</td><td>charging</td></tr><tr><td>cond</td><td>condensation</td></tr><tr><td>d</td><td>discharging</td></tr><tr><td>evap</td><td>evaporation</td></tr><tr><td>eq</td><td>equilibrium</td></tr><tr><td>latent</td><td>latent</td></tr><tr><td>s</td><td>sensible</td></tr><tr><td>surch</td><td>superheated</td></tr><tr><td>vap</td><td>vaporization</td></tr><tr><td>w,stock</td><td>condensate in the storage</td></tr></table>
<figure><img src="image_1.png" /><figcaption>Figure 1: Long-term solar thermal energy storage methods for buildings. Figure adapted from Figure 1 in [43].</figcaption></figure>
<figure><img src="image_2.png" /><figcaption>Figure 2: Diagram of the structure of the paper.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 3: Closed thermochemical heat storage process.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 4: Open thermochemical heat storage process.</figcaption></figure>
Figure 5: Monthly averages outdoor psychometric in some locations for which thermochemical heat storage has been envisioned in the literature. Data extracted from Retscreen [115].

<figure><img src="image_5.png" /><figcaption>(b) Air temperature</figcaption></figure>

<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6: Definition of the bulk volume (particles + closed and open pores + space between particles), the apparent volume (particles + closed pores) and closed pores volume.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 7: Costs comparison for various absorbents considered for long-term thermal energy storage in the same operation conditions. For the operation conditions, see references [42, 44].</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 8: Energy storage density of materials and some prototypes presented in the literature [1, 45, 53, 59, 60, 77, 96–102]. The reported figures correspond to the charging and discharging temperatures. For example, MERITS 80/65 means that the storage density is reported for a charging temperature of 80 °C and a discharging temperature of 65 °C. The energy storage densities of the materials are absolute for chemisorption and bulk for others.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 9: Evolution of the reduced energy storage density as a function of the absorption temperature for different absorption couples considered for seasonal storage. Evaporator temperature = storage temperature = 10 °C, concentration of the solution at the end of the charge at the saturation limit [42].</figcaption></figure>
> Salt hydrates offer very low potential for thermochemical heat storage

> The performances of absorption processes are prohibitive for long-term heat storage

> Thermochemical storage seems not adapted to long-term solar heat storage for habitat

> Crucial issues to consider before any new work on this technology are identified
**Declaration of interests**

[x] The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[ ] The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

<figure><img src="image_11.png" /></figure>