Chapter 1

1.1 Introduction to thermal energy storage

Developing efficient and inexpensive energy storage devices is as important as developing new sources of energy. The thermal energy storage (TES) can be defined as the temporary storage of thermal energy at high or low temperatures. The TES is not a new concept, and at has been used for centuries. Energy storage can reduce the time or rate mismatch between energy supply and energy demand, and it plays an important role in energy conservation.

One of the important characteristics of a storage system is the length of time during which energy can be kept stored with acceptable losses. If solar energy is converted into a fuel such as hydrogen, there will be no such a time limit. Storage in the form of thermal energy may last for very short times because of losses by radiation, convection and conduction. Another important characteristic of a storage system is its volumetric energy capacity, or the amount of energy stored per unit volume. The smaller the volume, the better is the storage system. Therefore, a good system should have a long storage time and a small volume per unit of stored energy.

There are three basic methods for storing thermal energy:

Sensible Latent Chemical

1.1.1. Sensible TES

In sensible TES systems, energy (or heat) is stored/released by heating/cooling a liquid or solid storage material through a heat transfer interaction. The amount of energy input to a TES in a sensible heat system is related to the mass of storage material and its heat capacity as well as the temperature difference of the storage medium between its initial and final states. This heat transfer Q can be expressed as:

Q = mCpT

where m and Cp are denote the mass and specific heat of the storage material and T is the temperature difference before and after the storage operation. Examples of materials typically used as a storage medium are water, air, oil, rocks, brine, concrete, sand and soil.

1.1.2. Latent TES

Latent heat involves the change of a substance from one phase to another at a fixed temperature. In latent TES systems, energy is stored during the phase change (e.g. melting, evaporating and crystallization). Due to the specific heat of a typical medium and the high enthalpy change during phase change, the latent heat change is usually greater than the sensible heat change for a given system size. Latent heat storage materials are usually useful over a small temperature range . The stored energy during a latent storage process can be evaluated as:

Q = mL

where m denotes the mass and L is the specific latent heat of the phase change material (PCM). Examples of PCMs are water/ice, paraffin and eutectic salts. An example of an industrial PCM is the hand warmer (sodium acetate trihydrate). PCMs are usually packed in tubes, plastic capsules, wall board and ceilings and they are supplied mainly in threeshapes: powder, granulate and board.1.1.3. Chemical Energy Storage

The chemical TES category includes sorption and thermochemical reactions. In thermochemical energy storage, energy is stored after a dissociation reaction and then recovered in a chemically reverse reaction. Thermochemical energy storage has a higher storage density than the other types of TES, allowing large quantities of energy to be stored using Thermochemical storage materials, in this aspect, provide much higher storage capacities per mass or volume compared to sensible or latent heat storage, often by a factor of 10 or more compared to water storage, the most often used storage type . Moreover, thermochemical storage materials can store the heat for infinite time without insulation and are regarded as a key technology for heat transport and long term storage, although short term storage concepts presently seem to be more feasible in terms of economy.

Fig. 1.1 comparison of different types of thermal storage systems1.2 Thermo chemical energy storage

1.2.1 Principles of Thermochemical Energy Storage

The main principle of thermochemical TES is based on a reaction that can be reversed:

C + heat A + B

In this reaction, a thermochemical material (C) absorbs energy and is converted chemically into two components (A and B), which can be stored separately. The reverse reaction occurs when materials A and B are combined together and C is formed. Energy is released during this reaction and constitutes the recovered thermal energy from the TES. The storage capacity of this system is the heat of reaction when material C is formed.

1.2.2. Thermochemical Energy Storage Components and Processes

During the thermochemical storage reaction, expressible as C+ heat A+ BC is the thermochemical material (TCM) for the reaction, while materials A and B are reactants. Substance A can be a hydroxide, hydrate, carbonate, ammoniate, etc. and B can be water, CO, ammonia, hydrogen,etc. There is no restriction on phases, but usually C is a solid or a liquid and A and B can be any phase. In general, a TES cycle includes three main processes:

Charging Storing Discharging

These three processes are illustrated for thermochemical energy storage in Fig. (1.2), and are described individually below: Fig. 1.2 Processes involved in a thermochemical energy storage cycle:charging, storing and discharging.

1.2.2.1 Charging: The charging process is endothermic. Thermal energy is absorbed from an energy resource, which could be a renewable energy resource and/or conventional energy sources like fossil fuels. This energy is used for dissociation of the thermochemical material, and is equivalent to the heat of reaction or enthalpy of formation. After this process, two materials (A and B) with different properties are formed that can be stored. The reaction during charging can be written as:

C + heat A + BDuring the charging phase of a storage system, energy from a heat source is needed to overcome the bonding between the (solid or liquid) sorption material and the working medium. The working medium (gaseous) is extracted by thermal energy at typical temperatures and can be released to the ambient (open systems) or stored (closed systems) in condensed form. Another distinction can be made if all energy, also from condensation, is stored or condensation energy is released (Concepts are sometimes named direct storage or indirect storage). After release, the storage material (sorbent) loses its sensible heat, cools down to ambient temperature and is stored in a moisture-tight reservoir to preserve the charging state.

1.2.2.2 Storing: After the charging process, components A and B are separately stored with little or no energy losses. The materials are usually stored at ambient temperatures, leading to no thermal losses (except during the initial cooling of components A and B after charging). Any other energy losses are due to degradation of the materials.

1.2.2.3 Discharging: During this process, A and B are combined in an exothermic reaction. The energy released from this reaction permits the stored energy to be recovered. After discharging, component C is regenerated and can be used again in the cycle. The discharging reaction can be written as:

A + B C + heat

1.2.3.Temperature range of thermochemical storage applications The range of possible applications for the purpose of heat storage using thermochemical reactions is very wide. Starting from temperatures of around 70C (salt-hydrates and solutions), to typical dissociation processes of hydroxides at around 200-350C, ammonia dissociation at 400-700C, until around 2000C for solar thermal processes in tower plants (i.e. for the production of solar fuels). These specifications refer to equilibrium reaction temperatures. Technical processes in practice need temperature gradients to work: higher temperatures than equilibrium can drive water (or other media) out of the storage material while lower temperatures than equilibrium drive the reverse process. Additional temperature drops may arise in heat exchangers, pipes, etc. 1.2.4 Storage densities The amount of stored energy per unit volume is the key indicator for the quality of heat storage. The energy density of sensible heat storage is limited by the maximum applicable temperature and the specific heat capacity of the storage material. Water, for example, is able to absorb about 210 kJ per litre or 58 kWh/m3 while being heated from 40 to 90C. Water in a tan k can store heat when the tank is thermally insulated, and taking into account the space for insulation realistic energy densities are in the range of 30-40 kWh/m. For example a common buffer-tank with a volume of 1000 litres and 10 cm of thermal insulation can store 58 kWh and has an effective volume of 1.6m3 and an energy density of 36kWh/m.If thermochemical storage (TCS) is compared to water stores, the effective energy density has to take into account the space for the apparatus, storage for charged and discharged material, and all other components. Nevertheless TCS systems have not been realized yet, and practical data is therefore rare. Actual development projects report experimental data of laboratory scale systems with energy densities that are three times the storage density of water. There is a distinctive upper limit of the storage density given by the physical constant of the reaction enthalpy. The reaction enthalpy is the maximum energy that can be transferred during the reaction. In reality the reaction performance is described by the so called free enthalpy which is a constant that is always less than the reaction enthalpy and controlled by temperature and entropy transfer (i.e. the reaction conditions). Further reduction of usable energy amounts happens due to losses of sensible heat to the ambient and heat exchanger losses between source and sink. In general, low efficiency during charging is acceptable since charging happens in (summer) periods with excessive (solar) thermal energy. Energy efficiency and temperature development during discharge is the main criteria for the applicability of storage systems. The lower limit of TCS storage densities in practice is given by heat storage with other (cheaper) technologies. It was proposed that in order to ensure the competitiveness of TCS concepts the TCS storage densities need to be in the range of at 4-8 times higher energy density than water.

1.2.5 Common Materials The selection of a TCM for a certain application depends on the regeneration temperature levels needed, desired temperature levels of usage, technical requirements like energy demand, power, number of load cycles during lifetime, technical infrastructure and costs of the storage materials. Salt solutions, salt hydrates, ammonia, hydroxides, carbonates and metals are promising candidates for future thermochemical storage applications. The numbers for energy density are theoretical values for pure materials under ideal reaction conditions (thermodynamic equilibrium). Practical values for the energy density have to take into account the limited efficiency of every sub-process and the additional demand of space for storage apparatus and technical infrastructure.

1.2.6 Salt Hydrates Salt hydrates in general are promising candidates for heat storage in the low temperature range because of their relatively low bonding force of the associated water molecules and subsequently low dehydration temperatures. Additionally the physical process mechanisms of salt hydration are very similar to desorption processes in molecular sieves and therefore combined materials like composites zeolite-salt can be developed. The hydration reactions of different types of dehydrated salts (MgSO4, CuSO4, Al(SO4)2) have been investigated. Magnesium-sulphate-monohydrate (MgSO4*H2O) for example, has a high potential as a chemical storage material with the involved condensation heat of water vapour, a theoretical storage density 633 kWh/m can be obtained. This is about 11 times higher than the storage density of a water store with the same volume (at T = 50 K). In practical applications, limitations have to be accepted. A material developed on the basis of the mentioned salt hydrate MgSO4 together with zeolite as a mechanical frame show the capability to store 166 kWh/m and perform a 25 K temperature lift while hydrated. Nevertheless the utilization of salt hydrates for the purpose of heat storage show material-related problems: Structural changes: solid absorption materials undergo structural changes during absorption (changes in volume, rheological behaviour). This can result in degradation and failure of the storage material. Phase change: During the desorption (heating) process the salt can solute in leaking crystal water (incongruent melting). Salt solutions can cause corrosion and mechanical damage to the system. Dissociation: Low stability under hydrothermal conditions can lead to decomposition of the salts and formation of strong acids (like HCl in chloride salts) and related pollution and corrosion problems. Reaction kinetics: The hydration of salts is a sequence of reactions forming successively higher grades of salt hydrates. Kinetic hindrance and thermodynamically stable intermediates can significantly reduce the reaction dynamics and output power of the overall process.

Temperature lift: Reaction temperature during adsorption strongly depends on the vapour pressure (evaporator temperature). Corrosion: Suitable materials for construction and practical tests of reliability over long periods (for lifetimes of 20-30 years).

1.3 Advantages of Thermochemical Energy Storage

Thermochemical TES systems have several advantages over other types of TES:

Components (A and B) can usually be stored separately at ambient temperature, after cooling to ambient conditions subsequent to their formation. Therefore there is little or no heat loss during the storing period and, as a consequence, insulation is not needed.

As a result of the low heat losses, thermochemical TES systems are especially suitable for long-term energy storage (e.g., seasonal storage).

Thermochemical materials have higher energy densities relative to PCMs and sensible storage media. Because of higher energy density, thermochemical TES systems can provide more compact energy storage relative to latent and sensible TES. This attribute isparticularly beneficial where space for the TES is limited or valuable.

1.4 Factors Affecting the Choice of Thermochemical Material

Several parameters should be examined in selecting a thermochemical material, as they affect its use in TES systems. The relevant factors include:

Cost Cycling behavior (reversibility and degradation overlarge numbers of cycles) Availability Toxicity and safety Corrosiveness Energy storage density Reaction temperature Reaction rate Ability to be engineered into a practical system (e.g. heat transfer characteristics and flow properties)

Chapter 2Overview of papers

2.1 A Critical Review of Thermochemical Energy Storage Systems by Ali H. Abedin and Marc A. Rosen, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, Ontario, 2011.

In this work a comprehensive review of various types of TES systems, highlighting thermochemical TES, has been presented. Principles of thermochemical TES and recent advances have been reported. The possibility of achieving more compact systems, little energy losses during the storing operation and higher energy densities compared to other types of TES are the most prominent advantages of thermochemical TES systems. Further research is needed to improve understanding of the scientific and engineering characteristics of thermochemical TES systems and to help improve various aspects relating to the performance and implementation of these systems. The thermochemical material is a critical component of such systems. The cyclic behavior and degradation of thermochemical materials, as well as their cost, availability, durability and energy density, are important parameters affecting the selection of a thermochemical material. Further research is needed on these topics, as well as on design factors, safety, size and efficiency, installation, maintenance and economics for thermochemical TES systems. Also, comprehensive analyses of these systems based on energy and exergy are needed, as such assessments can assist in design optimization and improvement, and such work is the subject of ongoing research by the authors.

2.2 Low temperature chemical heat storage an investigation of hydration reactions by F. Bertsch, B. Mette, S. Asenbeck, H. Kerskes, H. Mller-Steinhagen , Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, Germany.

In this paper two different salt hydration reactions have been studied with respect to possible low temperature heat storage applications. First results of experiments with magnesium sulfate monohydrate and copper sulfate monohydrate, hydrated by a moist air flow, are presented. Emphasis has been placed on the point of reaction equilibrium regarding temperature and humidity, which can be approximated theoretically from literature data, and on the achievable temperature lift. The equilibrium of the sulfates with humid air have been approximated with literature data. The hydration experiments have shown that for both, magnesium- and copper-sulfate-monohydrate, a high water vapor pressure is needed for a sufficiently high reaction rate. The reaction of copper sulfate monohydrate with humid air is limited by the equilibrium to a maximum operating temperature of 60 C. Although higher temperature lifts and operating temperatures were observed using MgSO4, this is not directly correlated with a faster reaction rate. To determine the reaction rate, further research has to be done in micro- and macro- scale to understand the reaction kinetics and mass transfer processes, especially with respect to the different hydrates of magnesium sulfate. However, a water vapor pressure of more than 13 mbar cannot be provided easily from ambient air during the heating period. Hence the application of magnesium sulfate or copper sulfate as the only thermo-chemical storage material seems to be critical. Further material research has to be done.

2.3 Calcium sulphate hemihydrate hydration leading to gypsum crystallization by a) N.B. Singh, b) B. Middendorf . a) DDU Gorakhpur University, Chemistry Department, Gorakhpur, India ,b) University of Dortmund, Department of Building Materials, Dortmund, Germany.

In this paper hydration of calcium sulphate hemihydrate (CaSO4 0.5H2O) leading to the crystallization of gypsum (calcium sulphate dihydrate CaSO4 2H2O) has been investigated. In this review article an overall picture of the subject is presented. The properties of the two hemihydrates (a- and b-), their hydration characteristics, the mechanism of their hydration and the crystal growth of gypsum are discussed. Additives modify the microstructures of the hardened gypsum and reduce its strength. Depending on the process of industrial production, hemihydrate occurs in two different forms (a- and b-forms); a-form is prepared by wet method and b-form is prepared by dry methods (e.g., calcining) from calcium sulphate dihydrate. In the laboratory b-hemihydrate is prepared from the dihydrate by heating under a low water vapour partial pressure, i.e., in dry air or vaccum, between 45 C and 200 C whereas a-hemihydrate is prepared from the dihydrate under a high partial pressure of water vapour, e.g., above 45 C in acid or salt solutions, or above 97.2 C in water under pressure.

Hydration reactions of both a- and b-hemihydrates are highly exothermic in nature and proceed by closely related routes, but with some differences. The differences are due to the methods of preparation, specific surface area, crystal size and habit, imperfections and surface topography.

2.4 Application of MgCl26H2O for thermochemical seasonal solar heat storage by H.A. Zondag,V.M. van Essen,L.P.J. Bleijendaal, B.W.J. Kikkert, M. Bakker 5th International Renewable Energy Storage Conference IRES November 2010, Berlin, Germany.

This paper investigates the thermochemical properties of magnesium chloride., and its feasibility of thermo chemical storage. When heating the material at a rate of 1C/min, these measurements showed that the dehydration of the hexahydrate started at about 50C and the formation of monohydrate started at about 120C. These results indicate that the dehydration of the material occurs in a temperature range that is within reach of solar thermal collectors.The dehydration of MgCl26H2O and subsequent hydration at 12 mbar vapor pressure (corresponding to evaporation at a typical air temperature of 10C) gives a good temperature rise of 20C to the flowing air . This is a promising result for the future use of this material for seasonal domestic heat storage for space heating and tap water heating.

Chapter 3Conclusion

Thermochemical heat storage materials (TCM) have been subject of research for quite a long time. Since water as a reactant, as it is non-toxic, less corrosive and ubiquitous, and can be stored in its liquid form at ambient temperatures, provides certain advantages over carbon dioxide, ammoniac and hydrogen, for example, research is more or less focused on adsorption and desorption of water and hydration/dehydration processes. First qualitative descriptions of the underlying reactions can be found published in the 1820s. Reversible chemical reactions, which could be useful for thermochemical heat storage, have been studied since the 1930s .First theoretical considerations on a possible use as a thermochemical heat store can be found published in the 1980s, whereas system designs first appeared in the early 1990s, for example as an application to store solar heat in the concept of steam-process based electricity production for a lunar base, using calcium hydroxide as TCM . Later, different systems have been described using adsorption/desorption of water on zeolites and the concept of a thermochemical heat pump using magnesium hydroxide. Whereas zeolite systems can be regarded to be more or less functional, TCM storage using salts and salt hydrates is still in development. Key questions are still the appropriate basic process design, which has to be fitted to the targeted applications, and materials research to optimize key parameters as there are cycling stability, power density and storage efficiency, whilst storage capacity is of course still one of the most important factors. These key parameters, too, are to be weighted differently, depending on the system design and application. In any case, recently published material indicates that for certain temperature levels and applications only a small number of materials have been successfully tested, MgCl2 6 H2O and CaCl2 6 H2O being most promising for temperature levels useful for space and water heating and Mg(OH) 2 and Ca(OH) 2 for higher temperature levels > 500 K . Whilst Ca(OH) 2 shows good reactivity and overall behaviour, whilst being only relevant for high-temperature heat storage, Mg(OH) 2 and the hydrated sulphates more or less showed less sufficient reactivity in the hydration process. The calcium sulphate can be used only for low temperature applications.The chloride salt hydrates on the other hand, whilst showing good reactivity under all circumstances, are known to be quite corrosive and show a certain tendency to over-hydrate, developing a gel-like consistency which significantly hinders the ability to store and release heat . Moreover, when using MgCl2 6 H2O, care is to be taken, as the material shows thermal decomposition releasing HCl above certain temperatures. Whereas reviewed papers state that thermal decomposition of MgCl2 6 H2O starts at temperatures above 130-140C, in their experiments at low pressures.Hence comparing all factors observed from above papers , it is concluded that magnesium chloride is the best material to be used for thermochemical application in the charging temperature range of 100C to 140C.

Chapter 4Scope for future work

A household water heater can be designed , which will be capable of working in the absence of solar or other external heating source.

A thermo chemical heat storage system with magnesium chloride as the thermo chemical material.

System could be charged by either solar energy or process heat. And could be stored for very long time without any loss.

Heat could be discharged by supplying moist air to the reactor containing the magnesium chloride.

The discharge of heat could be controlled by controlling the supply of moist air.

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