In this process the supplied quantity of heat, denoted here by ∆_RH, is used to break the A–B bond into independent species A and B. If the reverse reaction of turning products A and B into the compound A–B is avoided the energy stored in the chemical bonds can be stored without energy loss for any length of time.

An example of this type of reaction is the dehydration of salt hydrates, for example, magnesium hydroxide into magnesium oxide as shown in Eq. 17.1:

The energy required for the endothermic reaction can be provided by heat from any source. If the salt oxide is stored in a hermetically sealed space, then the energetic state can be kept without energy loss for an unlimited period of time. This means that heat losses only occur during charging and discharging. If at any later time the oxide is brought into contact with water or water vapor, hydration occurs and the quantity of heat supplied during dehydration is released.

Typical reversible solid/gas reactions with potential for thermal energy storage are listed in Table 17.1. These reaction types cover a temperature range from below 100 °C up to very high temperatures of over 800 °C. Besides the potential for high-energy storage density this is a reason this technology is attractive for a wide range of energy storage processes.

The basic reaction shown in Eq. 17.2 is very similar to the chemical reaction described in Eq. 17.1. During the energy-discharging process the solid reactant A is in contact with water vapor (H₂O_(g)). The solid adsorbs the water vapor to form the product A⋅x H₂O_(s) and the adsorption enthalpy (∆_adsH) is released in the form of heat. As this is a reversible adsorption reaction the same amount of heat has to be applied to decompose the product A·x H₂O_(s) into water vapor and the adsorbent A. It should be noted that the supply heat during desorption is typically at a higher temperature than the heat released during adsorption.

As in the case of chemical reactions there is no loss of energy during long-term storage, so long as the adsorbent is kept in a dry, water vapor–free environment.

The technology of adsorption is widely used in the chemical industry for a range of processes which include waste water treatment and gas purification. The process of adsorption is very well known and intensively investigated [7,8]. Therefore, only a brief introduction will be given which is necessary to understand the adsorption process for thermal energy storage and to understand the concepts discussed in Section 4.

The process of adsorption describes the attachment of molecules or ions from a gas phase or a liquid to a solid surface. Adsorption is a phase transfer process and can be explained as an enrichment of chemical species from a fluid phase onto the (inner) surface of a solid material. The solid surface provides energy-rich active sites which interact with the gas-phase species. By adsorbing these species the energy level of the surface is reduced to a thermodynamically more stable condition as energy is released. The solid is referred to as the adsorbent, the gas-phase molecule is called the adsorptive, and the adsorbed molecule is the adsorbate. Fig. 17.2 illustrates the adsorption process. As shown in Eq. 17.2, adsorption is an exothermal process and heating up the solid reverses the process, the binding forces are overcome, and the adsorbed species are released from the adsorbent back into the fluid. Thereby the adsorbent is raised to a higher energy level. This process is referred to as desorption.

The process of adsorption and desorption is an equilibrium controlled process. The amount of adsorbed mass per mass of adsorbent is referred to as loading X. Adsorption equilibrium depends on temperature and is defined by the partial pressure p* of the adsorptive in the gas phase and the adsorbent loading X. The equilibrium can be expressed as a function f (p*, T, X). Typically, the solid loading X is given as a function of the partial pressure p* at constant temperature T:

This relation is called the adsorption isotherm at temperature T. In Fig. 17.3, adsorption isotherms of H₂O adsorption on zeolite 13X are depicted. Adsorption equilibrium is described by two dependencies:

This behavior of energy storage defines the boundary condition for charging and discharging of thermochemical energy. For the discharging process it is beneficial to operate at low temperatures and high partial pressure to achieve a high amount of adsorbed mass. In a thermal process, desorption is achieved by increasing the temperature of the storage material. Furthermore, low partial pressure supports desorption.

In addition to adsorption capacity, the heat or enthalpy of adsorption is one of the most important parameters in heat storage. The enthalpy of adsorption is the difference in energy between the adsorption process and the desorption process.

The enthalpy of adsorption can be described as the sum of heat of condensation (for gas adsorption) and additional bonding forces which are weak Van der Waals forces or electrostatic forces. The value of these bonding forces is characteristic of the adsorbent material. For example activated carbon shows rather small bonding forces while zeolites have higher bonding forces. Due to the fact that the surface energy of adsorbent is inhomogeneous, preferential sites of adsorption exist. The first molecules adsorb on the active sites with the highest bonding forces. Adsorption enthalpy is, in most cases, a function of the loading X. In Fig. 17.4 the heat of adsorption of water vapor on zeolite 13X is depicted as a function of loading X. Starting at a high energy state, enthalpy decreases continuously and ends up slightly above condensation enthalpy.

To take advantage of high energetic adsorption sites, almost complete desorption of the storage material is necessary. Compared with Fig. 17.3, very high temperatures or nearly zero water vapor pressure is mandatory to reach this energetic state. However, the adsorption processes of typical storage applications start at values of (0.05–0.1) g g^–1 and, therefore, high energetic sites are not included in the process.

The thermal storage capacity provided by an adsorption process is the product of adsorption capacity and adsorption enthalpy. The specific mass amount of thermal energy stored can be expressed as:

and taking the mass of adsorbents into account the absolute value can be calculated as:

where Q is stored energy; m_ads is the mass of adsorbents; X_ads is the amount of adsorbate after adsorption; X_des is the adsorbed amount after desorption; and ∆_adsh is mean adsorption enthalpy in the interval (X_ads – X_des).

Storage density can be determined by dividing the value of Q by the volume of the storage material. Adsorption capacity and adsorption enthalpy are both dependent on the operating conditions during adsorption and desorption. Therefore, storage density must be specified in terms of the adsorption and desorption conditions.

Classical adsorption materials are silica gel, zeolite, and active carbon. Their suitability depends upon the individual application. Active carbon is a cheap and robust adsorption material with rather small adsorption capacity and low adsorption enthalpy. It is often used for the adsorption process in purification, heat pump, or cooling applications. Silica gel and zeolite show better performance for energy storage because the product of adsorption capacity and adsorption enthalpy is much higher (compare Table 17.3). Most adsorption storage systems described in the literature use silica gel/water or zeolite/water as working pairs [15]. Compared with zeolite, silica gel shows a lower desorption temperature (t < 120 °C) which results from the lower adsorption enthalpy. From the technical point of view this may have advantages. On the other hand, the adsorption kinetics of silica gel are slower than for zeolites. Adsorption measurements carried out by Jähnig et al. [12] showed that the achievable temperature lift decreases with increasing loading of the silica gel.

Temperature lift denotes the temperature increase occurring in the bed during adsorption. If temperature lift becomes too small for effective heat transfer the remaining adsorption capacity is lost.

An important advantage of zeolite is its high adsorption kinetics. The fast adsorption of gas-phase molecules in combination with high adsorption enthalpy yields high heat release. Steep adsorption fronts occur which is beneficial for process control. Even very low adsorptive concentrations can be completely adsorbed which makes zeolites perfect for working with moist air.

Of the wide range of different zeolite types, types A, NaX, and NaY have been tested successfully for energy storage. Type A zeolite (technically known as 4A and 5A) is a robust and inexpensive material with good hydrothermal stability. NaX (13X) and NaY-type zeolites show higher adsorption enthalpies than 4A and are very good candidates for thermal energy storage. Further improvements have been achieved by synthesizing novel binderless zeolites of type 4A and 13X [16]. This new class of zeolite is characterized by improved adsorption capacity, sufficient hydrothermal stability, and higher adsorption enthalpies.

In addition to classical adsorption materials, new materials are under development. The material class of aluminophosphate (AlPOs) and silicoaluminophosphate (SAPOs) represents a group of microporous materials with a zeolite-like crystalline structure. These materials are less hydrophilic than zeolite and may have the potential to fill the gap between zeolite and silica gel regarding optimization of adsorption strength and the ability to be desorbed at low temperatures. The structural types of ALPO-17 and ALPO-18 as well as SAPO-34 look very promising for heat storage with respect to the adsorption equilibrium. Experimental investigations [17] have shown that under moderate operating conditions the water uptake is much higher compared with zeolites. Even at desorption temperatures below 100 °C, significant adsorbed quantities of 25% in mass have been obtained. However, the application of AlPOs and SAPOs as heat storage material is at present not suitable due to cost reasons. The high production costs of the material are caused by the organic template which disappears during synthesis.

Very promising adsorption results has been obtained with another class of microporous materials called metal–organic framework (MOFs) materials. This material class is characterized by a huge internal surface and outstanding adsorption capacity. Water uptake values up to 1.4 g of water per gram of MOF have been measured [18]. MOFs exist in a wide range of different compositions. The large variety and adjustability of the pore structures enables tailored modification to improve the material with respect to heat storage. Due to these attractive properties the technology of MOFs is undergoing fast development. The component MIL-101 has been identified as material with excellent water adsorption properties. Important improvements have already been achieved concerning hydrothermal stability. Some MOFs are already commercially available but are still very expensive.

In the following, typical difficulties encountered during hydration and dehydration are discussed using the magnesium sulfate as an example. Magnesium sulfate monohydrate (MgSO₄ ^. H₂O) has high potential as a chemical storage material. Taking into account the enthalpy of condensation of water vapor a theoretical storage density of 2.3 GJ m^–3_heptahydrate (633 kW h m^–3_heptahydrate) can be obtained. This is about 11 times higher than the storage density of water storage with the same volume (∆T = 50 K).

Experimental investigations have been carried out by Bertsch et al. to analyze the reaction behavior of MgSO₄ in a fixed bed reactor [20]. The reactor had a length of 20 cm and a diameter of 3.5 cm and moist air was passed through it. The inlet conditions of a fixed bed reactor can be adjusted with regard to mass flow rate, temperature, and water vapor partial pressure. In experimental investigations it was found that hydration does not reach the heptahydrate state. Starting with magnesium monohydrate, water uptake was much smaller than expected. The uptake of only three to four moles of water per mole of MgSO_4. ^. H₂O has been measured. There are different hydrates of magnesium sulfate, that is, mono-, bi-, tetra-, penta-, hexa-, and heptahydrate, even though not all of these hydrates are stable or crystalline. The equilibrium curves of mono-, hexa-, and heptahydrate are depicted in Fig. 17.5. To calculate equilibrium curves at atmospheric pressure, thermodynamic data have been taken from [21], [22], and [23]. Fig. 17.4 also depicts the reactor inlet conditions of the air flow (temperature, partial pressure of water vapor) of each experiment (black dots) and the maximum temperature lift achieved (bars to the right). In experiments 1, 2, and 3 the reactor inlet temperature was comparatively low. Furthermore, the inlet conditions were very close to the equilibrium of magnesium sulfate hexahydrate to heptahydrate. Both effects result in a slow reaction rate and a moderate temperature lift. In experiment 4 and 5 the higher inlet temperature favors hydration of the anhydrate and a higher reaction rate, a shorter hydration time, and a higher temperature lift was observed. The highest temperature lift of 30 K was obtained in experiment 6. However, a fully hydrated state of the magnesium sulfate cannot be achieved under these conditions as the reactor inlet condition is to the right of the equilibrium curve of hexahydrate to heptahydrate.

With increasing hydration of the salt a decreasing reactor temperature was observed in all experiments. This is due to the approach of reaction equilibrium as hydration of the salts proceeds. A decreasing reaction rate is the result and the lower enthalpy of reaction yields a small temperature lift. Furthermore, the maximum achievable reactor temperature is limited by the equilibrium of the intermediates. Magnesium sulfate monohydrate can absorb water at a higher temperature than a more hydrated salt such as magnesium sulfate hexahydrate. The storage densities achieved were below the theoretical value for all experiments.

Similar results have been reported by other researchers [24]. Their experiments show that the practical use of pure magnesium sulfate is difficult because of its low power density. They concluded that the application of magnesium sulfate as a thermochemical storage material is problematic. In addition, many authors report that MgSO₄^.7H₂O melts or boils in its crystal water before degrading to lower hydrates [25,26]. This leads to crystal growth and bonding which hinders the hydration process by increasing diffusion resistance.

These findings are transferable to most other salt hydrates investigated for thermochemical energy storage. However, good experiences with pure strontium bromide as storage material were reported by Wyttenbach et al. [27].

In most cases zeolite is used as an active matrix for this purpose. A passive matrix can be any porous medium. Materials with a well-defined pore structure and pore size distribution have shown interesting results. Some of these storage materials already show good characteristics in terms of reaction kinetics, energy storage density, and mechanical stability. Composite salt and zeolite materials are prepared by impregnating commercially available zeolites (e.g., zeolite 13X or zeolite 4A particles) with a salt solution. Early experiments showed an increase in energy storage density compared with pure zeolite of approximately 20% [28,30–32]. The composite material showed similar behavior to that of pure zeolite—a high reaction rate associated with high temperature lift even at low water vapor pressures. To date these new composite materials have not been characterized in detail and the mechanism of interaction between salt and zeolite is not fully understood. Deeper insight into the reaction mechanism is necessary to develop composite materials which fulfill the expected targets which include tailored properties for energy storage.

Fig. 17.6 compares the experimentally achieved storage density of some thermal energy storage materials. Commercially available materials are silica gel, zeolite 4A, zeolite 13X, and a new type of binder-free zeolite (13XBF). In addition, the results of two noncommercially available composite materials are depicted. Composite A is made of a passive carrier matrix impregnated with MgCl₂ and reported by Zondag [33]. Composite B is zeolite 13X matrix impregnated with MgCl₂. The volume of the probes was 200 mL. All materials were desorbed at 140 °C in a dry air stream (p* < 10² Pa or 1 mbar). Adsorptions were carried out at 30 °C and water vapor pressure of 20 × 10² Pa (20 mbar) For comparison reasons the storage density of water (sensible heat storage) for a change in temperature of ∆T = 50 K is also depicted.

In recent years a wide range of thermochemical storage materials have been analyzed and tested and new materials are under development [34]. Good progress has been made regarding increasing performance, hydrothermal stability, and cost reduction.

The process of TCES can be subdivided into three steps. These three steps are illustrated in Fig. 17.8 and are described as follows:

In Fig. 17.9 the operation of an open adsorption store is depicted schematically. Again the process can be divided into three steps:

The open adsorption process is illustrated in Fig. 17.9. The adsorber, or more generally, the reactor is the key element of the system. The reactor is the apparatus where the chemical reaction/adsorption takes place. Two general approaches of reactor design can be found in the literature: the integrated reactor concept if the material reservoir is also the reactor (Fig. 17.10 left) and the external reactor concept if the material is stored separately from the reactor (Fig. 17.10 right).

Charging the storage system (material desorption) is done by heating the storage material via the internal heat exchanger. Desorbed water vapor is condensed in the condenser and pumped into the water reservoir. During the discharging process (adsorption) water from the water reservoir is pumped into the evaporator to produce vapor. The vapor is adsorbed by the storage material and the heat of adsorption is released. The heat is transferred to the space-heating loop via the internal heat exchanger.

A pilot system of the sorption store was installed and monitored in a building located in Gleisdorf. The system consists of a 32 m² flat plate collector area, a 900 L water buffer store, two 500 kg sorption stores, a separate water store for hot-water preparation, and an auxiliary heater (wood pellets). Such a system has been successfully tested. However, experimental results remained below expected values for storage density. One main reason for the insufficient storage density originates from the working pair of silica gel and water vapor. The achieved temperature lift decreases with time during the adsorption process. A necessary temperature lift of at least 10 K was only reached during the first 35% of total storage capacity.

This research has been continued within the framework of the European project COMTES (Combined Development of Compact Thermal Energy Storage Technologies) [39]. The aim was to overcome these drawbacks using storage materials with higher performance and further improved system technology. A hydraulic scheme of the system is depicted in Fig. 17.12. A storage volume double the 1000 L adsorption store was realized in stainless steel vessels. Binderless 13X zeolite was used as the storage material which is characterized by high adsorption capacity and fast adsorption kinetics [9,40]. High-performance vacuum flat plate solar thermal collectors serve as the heat source for desorbing the storage material during summer. The heat for evaporation is taken from the environment, using an air heat exchanger unit. In addition, the solar thermal collectors deliver low-temperature heat for evaporation in case of insufficient ambient conditions.

The focus has been heat and mass transfer inside the sorption unit. A heat exchanger design was developed which provides a high heat transfer rate. First experiments have shown that utilization of the storage material and the achieved temperature lift inside the store were successfully increased. The steam generated by the developed evaporator/condenser unit was continuously controlled and allowed constant power extraction of several kilowatts from the storage system. The demonstration plant was able to analyze thermal performance under realistic conditions. Fig. 17.13 shows the experimental setup and the solar thermal collectors installed at AEE Intec.

A similar closed adsorption storage system has been developed in another European project [41]. ZeoSys together with the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB) has developed a compact adsorption storage system with the aim of storing industrial waste heat and heat from combined heat and power systems [41]. Basic investigations have been carried out in a small 1.5 L storage system where the performance of different sorption materials has been tested. Important process parameters, such as temperatures and pressure ranges, have been analyzed with respect to storage density and power output. Power is a crucial issue when using a solid storage medium with very low thermal conductivity such as zeolite (λ ≈ 0.1 W m^–1 K^–1). Several designs of heat exchangers have been tested in a medium-scale reactor of 15 L volume. A good solution has been found with a heat exchanger configuration that showed a heat power rate more than 60% higher than measured with a parallel copper plate heat exchanger in the bulk. In the final step the improved system concept has been tested successfully in a fully functional storage system with 750 L of storage volume.

Both projects are promising examples for a technology that appears to have a number of advantages for heating applications as well as for industrial process heat applications.

The solar heating concept implemented is based on a solar thermal system in combination with a sorption heat store and is designed for very high solar fraction (up to 100%). Solar fraction is the amount of energy provided by the solar thermal system divided by the total energy required. Fig. 17.15 illustrates the operating mode of the solar heating system during summer and winter schematically. The sorption store is integrated between the controlled ventilation of the building and the indoor air leaving the space-heating zone. In this case the required moisture to achieve space heating by adsorption is supplied by indoor air. In winter, indoor air at room temperature (∼20 °C) is allowed to pass through the sorption storage. The storage material adsorbs moisture from the air. The heat of adsorption is released and air flow heats up significantly above 20 °C. This warm air is then allowed to pass through the heat exchanger, where it heats up incoming fresh ambient air. The incoming air flow affects room heating. The power of the heating system is limited by the humidity of indoor air and the air change ratio. Under typical operating conditions the system delivers power of (1–1.2) kW. This is sufficient approximately 85% of the time in the heating season.

In the summer months, when solar radiation is present in excess, the storage material is regenerated to charge the sorption store (desorption). Vacuum tube solar collectors are used to heat the ambient air. The hot air is then allowed to flow through the sorption store system to regenerate the storage material. The warm air leaving the sorption system is then passed through the heat exchanger to preheat incoming ambient air, before it enters the solar collectors.

A segmented storage system, subdivided into individual sorption units, has been developed. The sorption store has a cubic shape and a volume of 4.2 m³. For efficient operation, it is necessary to divide the sorption store into individual segments. Due to segmentation a significant reduction in pressure drop is achieved. At the same time, the mass of the storage material involved in the adsorption/desorption processes is reduced. This leads to reduced heat losses due to smaller thermal capacity. Furthermore, regular and homogeneous flow distribution within each segment is an important requirement for the efficiency of the store. These considerations result in the storage concept, which is schematically depicted in Fig. 17.16. Four areas arise as a result of two vertical partitions on the diagonals, each of which is subdivided by horizontal planes.

At INSA de Lyon, the concept of an open adsorption system using exhaust air from the building as the water vapor source has been built. Hot air with a temperature between (80 and 150) °C will be provided in summer by air collectors for desorption. A new composite storage material, consisting of zeolite 13X as carrier matrix and 15% mass percent of magnesium sulfate, has been developed. Composite materials are characterized by simultaneous adsorption and hydration reaction. In a laboratory test reactor with 200 g of composite material a temperature lift of over 25 °C has been measured during the combined adsorption/hydration process (air flow rate of 8 L min^–1 at 25 °C and relative humidity of 50%). A volumetric energy storage density of 166 kWh m^–3 has been achieved. This is 65% of the theoretical storage density of the material (257 kW h m^–3) and a 27% increase compared with pure zeolite 13X (131 kW h m^–3). Microcalorimetry experiments revealed that energy density can be maintained over at least three charge and discharge cycles. Further research will focus on improving the carrier matrix and enhancing the energy storage density.

An open sorption system for long-term solar heat storage with magnesium chloride on a carrier matrix as storage material is under investigation at the Energy Research Centre of the Netherlands (ECN) [33,42]. A prototype reactor, containing 15 L of storage material has been successfully tested. The temperature for dehydrating the material was set to 130 °C. During hydration experiments a temperature lift of the airflow from (50 to 64) °C has been measured in the material bed [12 × 10² Pa (12 mbar) partial pressure of water vapour]. With further improvement of the reactor design and the heat exchangers it is expected that high energy density of approximately 280 kW h m^–3 of the material can be technically reached.

The external reactor concept has been developed at the University of Stuttgart’s Institute of Thermodynamics and Thermal Engineering (ITW). A new process design for solar thermal long-term heat storage has been developed [43] and a solar thermal combination system has been extended by incorporating a thermochemical energy store. In Fig. 17.17 a schematic drawing of the system concept is depicted. Similar to the concepts of the Solar Spaces project, it is an open adsorption/hydration system using ambient or exhaust air to provide humidity for the reaction.

The thermochemical energy store works as a low-power heating system and is connected to the combistore of the solar system via the collector loop heat exchanger. The thermochemical energy store consists of a material reservoir for the storage material and a reactor where heat and mass transfer take place during the reaction. The external reactor concept separates the storage material (a composite material of zeolite and salt) from the reactor. This has the advantage that the reaction is reduced to only a small part of the total storage material amount at a time. Thermal heat capacities and heat losses especially during the regeneration process are reduced. Furthermore, only the reactor has to withstand high temperatures whereas for the material reservoir low-cost materials can be used. Storage material transport between the material reservoir and reactor is done by a vacuum conveying system allowing very gentle material transport with low energetic expense.

The reactor is designed as a cross-flow reactor. The material enters the reactor from the top and runs gravity driven through the reactor. The air enters the reactor from the side and transports the humidity and heat into or out of the reactor. In heating mode, the heat released is transferred from the air flow to the water loop by an air-to-water heat exchanger. For material regeneration the air flow direction is reversed and the heat exchanger is used for transferring regeneration heat from the solar loop into the reactor via the air flow. A sketch of the reactor design and a picture of the laboratory prototype are depicted in Fig. 17.18. A detailed description of the reactor design and reactor operation mode is given in [44].