Long-term Heat Storage using ThermoChemical Materials Z. He WET 2007.14 Project Report August 2007 Committee Members prof.dr.ir. A.A. van Steenhoven (TU/e) dr.ir. C.C.M. Rindt (TU/e) dr.ir. R. Schuitema (ECN) dr.ir. V.M. van Essen (ECN) dr.ir. W.G.J. van Helden (ECN) Eindhoven University of Technology Department of Mechanical Engineering Division of Thermo Fluids Engineering Energy Technology Group
Content List of Figures List of Tables Abstract Chapter 1 Introduction Chapter 2 Literature Review 2.1. Heat Storage 2.2. ThermoChemical Materials (TCM) 2.3. Reactor System Chapter 3 Definition of Experiments 3.1. Raw Materials 3.2. Materials Processing 3.3. Materials Characterization 3.4. Experimental Variables 3.5. Result Analysis and Discussion 3.6 Further Approaches Chapter 4 Concluding Remarks and Project Planning References 3 4 5 6 9 9 10 18 21 21 21 21 22 22 23 24 25 2
List of Figures Figure 1 Working principles of thermochemical materials Figure 2 Crystal structure of MgSO 4 H 2 O showing (a) the bulk unit cell, (b) side view of the (100) surface along a direction, and (c) side view of the (100) surface along b direction Figure 3 SEM images of an epsomite crystal before (a and b) and after (c and d) dehydration [6] Figure 4 Schematic diagram of rehydration behavior Figure 5 Basic TCM store model Figure 6 System volume for (a) integrated and (b) separate material stores and reactors 11 13 14 15 18 20 3
List of Tables Table 1 Potential TCM candidates for seasonal heat storage Table 2 Characterization methods for thermochemical materials 12 17 4
Abstract This report was written during the preparation phase of the collaborating project between TU/e (Eindhoven University of Technology) and ECN (Energy research Center of the Netherlands). The collaborating project, long-term heat storage using thermochemical materials, is part of a much larger project, WAELS (Woningen Als Energie Leverend Systeem; houses as energy supplying system), which is coordinated by ECN and financed by SenterNovem. Solar energy could provide durable heat for a domestic environment. However, it is most effective in summer and not in winter when there is a high demand. To accommodate the difference in time between energy production and energy demand, heat storage is necessary. The basic idea behind heat storage is to provide a buffer to balance fluctuations in supply and demand of thermal energy for heating and cooling. Materials are the key issue for heat storage. There are a large number of materials which can be used for heat storage. Thermochemical materials have the highest storage capacity among all storage media. In this report, a literature review related to thermochemical materials for heat storage is given, which covers the concept of heat storage, the review of thermochemical materials, and the description on thermochemical reactor system. Furthermore, the definition of an experiment on thermochemical materials is presented, and also the concluding remarks and the future work are given. 5
Chapter 1 Introduction Today the spotlight in the world is on the increasing demand for alternative and renewable energy sources. Solar energy is one of the most important sources, which could provide durable heat for various applications. The availability of efficient heat storage technologies is one of the key factors for the success of several renewable energy technologies. In particular, a high penetration of solar energy technologies will be hard to realize without the availability of technologically and economically attractive heat storage systems due to the short- and long-term variation of the available solar radiation. The principal gain from heat storage is that heat and cold may be moved in space and time to allow utilization of thermal energy that would otherwise be lost because it was available at the wrong place and the wrong time. Thermal energy storage systems themselves do not save energy. However, energy storage applications for energy conservation enable the introduction of more efficient, integrated energy systems. Thermal energy storage can consequently serve at least five different purposes: 1) Energy conservation utilizing new renewable energy sources; 2) Peak shaving both in electric grids and district heating systems; 3) Power conservation by running energy conversion machines, for instance, cogenerating plants and heat pumps, on full (optimal) load instead of part load. This reduces power demand and increases efficiency; 4) Reduced emissions of greenhouse gases; and 5) Freeing high quality electric energy for industrial value adding purposes. Generally, thermal systems are characterized by a broad range of parameters, such as operation temperature and pressure, capacity, power level, and use of different heat transfer fluids. Consequently, the development of efficient and economic thermal energy 6
storage requires the coverage of a broad spectrum of storage techniques and materials as well as thermal engineering issues, effective heat transfer, and system integration aspects. Most of the currently available heat storage technologies still suffer from problems such as excessive investment cost, insufficient energy density, limited efficiency and reliability. These issues are restricting broad application and market penetration of heat storage and, therefore, require more R&D efforts to achieve significant improvements in the above-mentioned areas. There are three main physical ways for thermal energy storage: sensible heat, phase change reactions, and thermochemical reactions. Storage based on thermochemical reactions has much higher thermal capacity than sensible heat. The development and use of new materials offers great innovation potential in storage technology. New materials have already demonstrated to have better properties than the previously used silica gel and zeolite types. Therefore, further research into new materials for effective and economic heat storage systems plays a significant role. The aim of the present project is to gain more insight into the physics of compact heat storage using so-called thermochemical materials. Stored energy densities up to 3 GJ/m3 can be achieved TCM can be used for seasonal heat storage in the built environment to bridge the gap between solar energy supply in the summer and heat demand in the winter. The present project is part of a much large project, WAELS (Woningen Als Energie Leverend Systeem; House As Energy Supplying System), which is coordinated by the Energy research Center of the Netherlands (ECN), and financed by SenterNovem. The goal of WAELS is to make the first steps on the route towards an energy neutral built environment in 2050. The objectives of the post-doc project are to Characterize the thermochemical properties of the material candidates for heat storage; Demonstrate the working principle of thermochemical materials; Investigate the mechanisms of heat and mass transfer on molecular, grain, and component level; Design a concept of a reactor system for heat storage; and 7
Develop a proposal for further research based on the obtained results. The material investigated is magnesium sulfate hepta hydrate (MgSO 4 7H 2 O), which is one of the most potential thermochemical materials for solar energy storage. Other candidates will also be investigated based on the project progress. In chapter 2, a literature review related to thermochemical materials for heat storage will be given, which covers the concept of heat storage, the review of thermochemical materials, and the description on thermochemical reactor system. The definition of the experiment will be presented in Chapter 3. Chapter 4 summarizes the concluding remarks and the future work, respectively. 8
Chapter 2 Literature Review 2.1. Heat Storage The present concern about the increasing demand for energy and the high cost of oil and natural gases has incited researchers to find better ways of using alternative and renewable energy resources, such as fuel cell, solar cell, pneumatics, and animal manure. The energy sources normally used for heating and cooling are oil, gas, coal, and electricity. The energy consumption could be divided for industrial and domestic applications. However, it is not entirely logical, nor efficient, to burn fossil fuels at temperatures up to 1000 0 C in order to create an indoor climate at 20 to 25 0 C. Furthermore, burning of fossil fuels emits greenhouse gases. Neither is it efficient to use electric power, a form of highly processed energy, only for resistance heating [1]. Solar collectors could produce durable heat in a domestic environment. However, it is most effective in summer and not in winter when there is a high demand. To accommodate the difference in time between energy production and energy demand, heat storage is necessary. The basic idea behind heat storage is to provide a buffer to balance fluctuations in supply and demand of thermal energy for heating and cooling. The demand fluctuates in cycles of 24 hour periods (day and night), intermediate periods (e.g. one week), and according to seasons (spring, summer, autumn, and winter). Systems for storing thermal energy should therefore reflect these cycles, with either short term, medium term, or long term (seasonal) storage capacity. When a heat storage need occurs, there are three main physical principles to provide a thermal energy function: sensible heat storage, latent heat storage, and thermochemical storage [2]. - Sensible heat storage this is where thermal energy is stored or released as a result of a change of the temperature of the materials. No change in phase (i.e. remains as solid, liquid, or gas) is involved and the amount of energy stored is 9
dependant on the specific heat capacity of the material, its mass, and the rise in temperature. - Latent heat storage this is where thermal energy is stored and released as a result in a change in a materials physical state (e.g. liquid to solid and vice versa). Materials that are used to store latent heat are termed Phase Change Materials (PCM). - Thermochemical heat storage this is when heat is applied to certain materials and produces a reversible chemical reaction and thermal energy is stored and released as the bonds are broken and reformed. Thermal energy is stored during the forward reaction which is endothermic and released during the reverse reaction which is exothermic. Materials that are used to store thermochemical heat are termed ThermoChemical Materials (TCM). 2.2. ThermoChemical Materials (TCM) There are a large number of materials which could be used for thermochemical heat storage. The most common sensible heat medium is water. The classical example for phase change materials is sodium sulfate. Thermochemical materials have the highest storage capacity among all storage media. Solid silica gel has a storage capacity which is 4 times that of water. Some of the materials may even approach the storage density of biomass. The basic reaction process for solar energy storage using TCM is: C (solid) + Q (heat) A (fluid/gas) +B (solid) This reaction is considered in thermodynamic equilibrium, where there is no net heat exchange between the reacting substances. The equilibrium temperature is termed as turnover temperature. During summer, the solid C decomposes into the fluid or gas A and the solid B by adding solar heat at a reaction temperature that is higher than the turnover temperature. Materials A and B are stored separately until winter. In winter, A and B are mixed to start the reverse reaction at a temperature that is lower than the turnover 10
temperature, and the heat is released during the reaction. The schematic diagram of the reaction process is shown in Figure 1. The basic demands on TCM for solar heat storage are: Reversible reactions as required Energy storage density greater than 1-2 GJ/m 3 Reaction temperature 60 ºC-250 ºC Cost of the materials (abundance and easy to mine) Environmental impact and toxicity of the materials Corrosiveness at storage and/or reaction There criteria are chosen to be as far as possible independent of each other. Figure 1 Working principles of thermochemical materials In a survey recently conducted by the Energy research Center of the Netherlands (ECN) and the University of Utecht [3], a list of potential theromchemical materials for seasonal storage of solar heat is shown in Table 1. Among the candidates, magnesium sulfate (MgSO 4 ) possesses the largest realization potential for heat storage. The only common, naturally occurring members of the MgSO 4 nh 2 O series on the earth are epsomite (MgSO 4 7H 2 O, 51 wt% water), 11
hexahydrite (MgSO 4 6H 2 O, 47 wt% water) and kieserite (MgSO 4 H 2 O, 13 wt% water). These three salts are believed to be the only members that occur on the earth as thermodynamically stable minerals [4]. Rare, metastable minerals of the series include pentahydrite (MgSO 4 5H 2 O, 43 wt% water), starkeyite (MgSO 4 4H 2 O, 37 wt% water), and sanderite (MgSO 4 2H 2 O, 23 wt% water). Other hydration states (n= 12, 3, 1.25) are not recognized as minerals but can be synthesized. All of these salts consist of SO 4 tetrahedra and Mg(O,H 2 O) 6 octahedra. Some include extra-polyhedral water (water that is not in octahedral coordination with Mg), see the crystal structure as an illustration shown in Figure 2. Table 1 Potential TCM candidates for seasonal heat storage In 1618 a farmer at Epsom in England attempted to give his cows water, but they refused to drink it due to its sour/bitter taste. However the farmer noticed that the water seemed to heal scratches and rashes. The fame of Epsom salts then began to spread. Epsom salt was originally prepared by boiling down mineral waters at Epsom, England, and later prepared from sea water. It forms as a precipitation from vapors on limestone cave walls and on the walls and timbers of deep-shaft mines. In modern times, these salts are obtained from certain minerals such as epsomite. Magnesium oxide, as mined or extracted from seawater, acts as the starting point for commercial production of magnesium sulfate. The magnesium sulfate is produced via the reaction between MgO and concentrated sulfuric acid on certain prescribed conditions, followed by heat treatment. Epsomite transforms readily to hexahydrite by loss of extra-polyhedral water; this transition is reversible and occurs at 50 55% relative humidity (RH) at 298 K and at lower temperatures as the activity of water diminishes. Kieserite is more stable at lower 12
RH and higher temperature; for example, at moderate heating rates in thermogravimetric analysis the kieserite structure survives to 670 K, compared with 450 K for hexahydrite. However, kieserite converts to hexahydrite or epsomite as humidity increases, yet these phases do not easily revert to kieserite on desiccation. Metastability, kinetic effects and pathway dependence are important factors in the MgSO 4 nh 2 O system. Figure 2 Crystal structure of MgSO 4 H 2 O showing (a) the bulk unit cell, (b) side view of the (100) surface along a direction, and (c) side view of the (100) surface along b direction Reversible reactions of dehydration and rehydration are well-suited processes for heat storage using TCM. These reversible reactions take place under non-equilibrium conditions imposed by a double constraint of temperature and pressure. Such phenomena 13
are limited by mass transfer, by heat transfer, and by the chemical kinetics of the reactive salt. The concept of grain and porous compact is useful as it makes it possible to define two characteristic dimensions in the reactive medium: the grain, which is the basic particle where the reaction takes place, and the porous compact, which is composed of a combination of the reactive particles with or without the presence of an inert binder [5]. In general, dehydration reactions proceed stepwise through a series of intermediate reactions involving the decomposition of one phase and the formation of a new one [6]. If the materials receive the radiated solar energy, the dehydration occurs when the temperature is higher than the turnover temperature. The main steps of dehydration include destruction of the reactant structure, water evaporation, and product nucleation and growth. When the dehydration conditions are maintained, it is observed that crack formation and propagation occurs due to the fact that strain associated with water removal is greater than that which can be sustained by the product structure. Figure 3 shows the observed cracks of the dehydration of an epsomite crystal. Cracks provide channels for water escape. Dehydration results in an overall increase in close packing and density and reduction in volume. 14
Figure 3 SEM images of an epsomite crystal before (a and b) and after (c and d) dehydration [6] The dehydration process is mainly limited by the reaction interface. Once a dehydrated layer is formed on the surface of the grain, the progress of the reaction can be influenced by the behavior of the dehydrated part of the crystal. Gradually, the reaction interface moves toward the interior of the crystal [7]. When the anhydride is exposed to water vapor, rehydration reactions occur. Water molecules are first adsorbed on the accessible surfaces of the grains. When the accessible crystallites are rehydrated, the process of diffusion along channels to inner lattices occurs. Vacancies are produced by the jump of water molecules at the interface to adjacent sites in the lattices. Following the explanation of Mojaradi and Sahimi [8], the reaction is an annihilation process. The progress of the reaction depends on the rate at which diffusing water molecules encounter rehydration sites [9]. As one water molecule diffuses along such a path and encounters the first reaction site, a second molecule continues along the same path until it encounters the second reaction site and so on. The distance covered by diffusing water molecules along such a pathway is proportional to the number of sites encountered. The schematic diagram of rehydration behavior is shown in Figure 4. The initial rehydration of a superficial layer proceeds rather easily, while the subsequent bulk rehydration might be somewhat hindered by the presence of such an outer layer. The rehydration process causes volume expansion of the crystal structures due to the addition of molecules incorporated into lattices, and the heat is released through the porous network to the environment. Figure 4 Schematic diagram of rehydration behavior 15
The progress of dehydration and rehydration is dependent on temperature and pressure. Temperature is a very important factor in controlling physical and chemical reactions. From the kinetic standpoint, increasing temperature increases the reaction rate significantly. Thus, the reaction species could contact each other more completely and effectively [10, 11]. The dehydration process is enhanced at low vapor pressure, while rehydration process is enhanced at high vapor pressure. Compared to dehydration, rehydration proceeds slower because of low mobility of water vacancies in the lattices [12]. The hysteresis behavior between dehydration and rehydration processes suggests that the rehydration rate of the hydrate is proportional to t 1/4, while the dehydration reaction is proportional to t [9]. To enhance the rehydration, it must therefore provide the beneficial channels for water molecules moving to reaction sites through networks, probably associated with grain boundaries and other defects which can produce pathways of similar dimensions to those of a diffusing water molecule. As stated, thermochemical materials are key components for the construction of heat storage reactor systems. Therefore, the performance of the material candidates is critical information and has to be known. The literatures [13-22] related to materials characterization on thermochemical properties were reported. With the state-of-the-art characterization technologies, thermochemical materials could be investigated, and as a result, the important results related to heat storage could be achieved. As a summary, Table 2 gives a comprehensive list of characterization methods for thermochemical materials. 16
Table 2 Characterization methods for thermochemical materials Facility-Technology Parameter-Behavior Differential scanning calorimetry (DSC); Enthalpy of formation; Gibbs free energy; Solution calorimetry Entropy; Energy storage density Thermogravimetry (TG) Thermochemical stability Thermogravimetry (TG) - Differential Dehydration and Rehydration scanning calorimetry (DSC); Sorption isotherms Differential scanning calorimetry (DSC) Heat flow rate; Heat capacity Dilatometry Thermal expansion Laser flash Thermal diffusivity X-ray diffraction (XRD) Composition and Phase Inductively coupled plasma - mass Element spectrometer (ICP-MS), Energy dispersive X-ray analysis (EDX) X-ray photoelectron spectroscopy (XPS) Valence Fourier transform infrared spectroscopy Energy bonding (FTIR) Raman spectroscopy Densimeter Density Particle sizer Particle size Scanning electron microscopy (SEM), Microstructure Transmission electron microscopy (TEM) Pressure sensor Vapor pressure 17
2.3. Reactor System The TCM storage system consists of two chemical reactors with heat exchangers for the reaction C (solid) + Q (heat) A (fluid/gas) +B (solid) and a separate material buffer for each of the three reactants, as depicted in Figure 5. Figure 5 Basic TCM store model The materials A, B and C are modeled by their enthalpy function at constant atmospheric pressure, so that sensible heat as well as latent heat is taken into account. Also heat losses are taken into account. The two reactors are thermally well insulated, and the three material store are poorly insulated. When solar radiation is added to the dissociation reactor, Material C is transported from material store C to the dissociation reactor. In the reactor it is heated up to the reactor temperature and partly dissociated into materials A and B. Next the hot materials A and B and the remaining part of material C are transported back to their respective material stores, where they are allowed to cool down. When heat is extracted from the association reactor, Materials A and B are transported from their material stores to the association reactor. In the reactor, the materials are heated up to the reactor temperature and partly associate into material C. Next the hot 18
material C and the remaining part of the materials A and B are transported back to their respective material stores, where they are allowed to cool down. Each material store is considered as a large material filled container with a fixed heat loss coefficient of 100 W/K between the store and its surroundings. Its heat content is calculated from its enthalpy function with respect to some reference temperature. Its temperature is the result of inflowing and outgoing enthalpy flows. The volume needed for a material store is calculated from the minimum amount of material needed in the store, and the storage density of that material. The volume of a fluid or gas (material A) is equal to its mass divided by its mass density. The volume of a solid (materials C and B) however is not, because it is stored as granulate or fine powder. From commercial abrasives (these are fine powders) it was found that on average the store volume is about 1.5 times the volume calculated from its pure material mass density. The transportation of materials between the material stores and the reactor vessels costs energy. For each material flow a characteristic value of 10 kj/kg is used that was derived from the energy consumption of a commercial feeder. Reacted matter has to be stored as compact as possible. As in general it is not readily available as a fine powder, it has to be grinded before feeding it back to the material stores. For the energy consumption of grinding a characteristic value of 50 kj/kg was derived from the breaking and grinding of natural gypsum on an industrial scale. The influence of the losses mentioned above on the effective energy storage density cannot be avoided, but it can be decreased in some ways. One way is to decrease void volume in the energy storage system. Choosing separate material stores and reactors instead of integrated stores and reactors in the storage system can do this. This is illustrated in Figure 6. The system with separate material stores and reactors has a much smaller system volume because it does not need the reaction volume of the total material mass present, but only a relatively small reaction volume associated with the amount of material that is actually being converted in the chemical reaction. 19
(a) 1 (b) 2 Figure 6 System volume for (a) integrated and (b) separate material stores and reactors 20
Chapter 3 Definition of Experiments Since the novel concept of solar energy storage using thermochemical materials is just initiated, research outcome related to this field has been rarely reported so far. The performance of thermochemical materials should be first assessed due to the key role of TCM on heat storage system. The aim of the present project is to gain more significant insight into the physical and chemical aspects on potential candidates of TCM. To achieve the target, the design of experiments, especially for thermochemical characterization, is necessary. It will cover the evaluation on basic parameters, the investigation on thermodynamic and kinetic mechanisms for the two reversible reactions, and the analysis of the effects of internal (grain size, porosity, mass) and external (heating rate, cooling rate, holding time, humidity) factors on materials thermochemical behavior. 3.1. Raw Materials Commercially available magnesium sulfate hepta hydrate (MgSO 4 7H 2 O) powders with an average particle size of 38 µm are used as the initial materials. 3.2. Materials Processing The mass of the powders is measured using a highly precise balance. For obtaining the powders with different particle sizes, the grinding, milling, and sieving process is made using mortar, pestle, and siever. Die pressing is applied for forming the powders into compacts with different porosities. 3.3. Materials Characterization The density of the compact is calculated by weight and geometry measurement. The composition and phase of the material is characterized using X-ray diffraction (XRD). 21
The microstructural observation and the elemental analysis are made using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). To investigate the thermochemical processes of dehydration and rehydration, combined thermogravimetry and differential scanning calorimetry (TG-DSC) technique is employed. The materials are heated from the room temperature to 300 0 C, held isothermally, and then cooled down to room temperature. In case of the investigation on cycling behavior, the process is repeated up to 3 times. The processing and characterization facilities are available at Mechanical and Chemical departments of TU/e and Energy Storage Laboratory of ECN. 3.4. Experimental Variables It is expected that the thermochmical properties of the materials will be influenced by the internal and external factors. Therefore, the experiment is designed using the combination of different variables: Mass: 10, 20, 50 mg Grain size: 5, 15, 25, 35 µm Porosity: 10, 20, 30% Heating rate: 0.5, 1, 5, 10 0 C/min Holding time: 15 minutes to 20 hours Cooling rate: 0.5, 1, 5, 10 0 C/min Humidity: 30, 50, 70% 3.5 Result Analysis and Discussion With the obtained results from TG-DSC, the enthalpy of formation, the Gibbs free energy, the entropy, and the energy storage density are evaluated. The influences of heating rate, cooling rate, holding time, and humidity on dehydration and rehydration processes are analyzed and discussed. The interpretation of the effects of mass, particle size, and porosity on energy storage density and cycling behavior is made to understand the nature of mass transfer and heat transfer during hydration processes on molecular, grain, and compact level. Finally, the optimal conditions of MgSO 4 7H 2 O as potential thermochemical materials for solar energy storage are determined. 22
3.6. Further Approaches Based on the research progress, further approaches will also be considered to potentially enhance the performance of thermochemical materials. The first approach is synthesis, processing, and characterization of nano MgSO 4 7H 2 O. It is originated from the idea that nano particles have much higher surface energy and hence higher reaction activity to increase the reaction efficiency. On the other hand, shorter diffusion path due to nanostructures could improve the reaction rate of the hydration processes. The second approach is formation of hetero-valence doping MgSO 4 7H 2 O. The replacement of Al 3+ or Na + for Mg 2+ in the lattices could introduce more defects and vacancies inside the grains, which could provide more pathways for diffusing water molecules during rehydration. The third approach is formation of MgSO 4 7H 2 O, (Al) 2 (SO 4 ) 3 18H 2 O, and/or CuSO 4 5H 2 O composites. For single phase TCM, there are only several characteristic temperatures at which the hydration reactions occurs. The incorporation of different compositions into the composites could increase the overall characteristic temperatures, extend the reaction temperature range, and if designed well, obtain more released heat. 23
Chapter 4 Concluding Remarks and Project Planning Solar energy could provide durable heat for a domestic environment. However, it is most effective in summer and not in winter when there is a high demand. To accommodate the difference in time between energy production and energy demand, heat storage is necessary. The basic idea behind heat storage is to provide a buffer to balance fluctuations in supply and demand of thermal energy for heating and cooling. Materials are the key issue for heat storage. There are a large number of materials which can be used for heat storage. Thermochemical materials have the highest storage capacity among all storage media. The design of the experiments provides more significant insight into the physical and chemical aspects on potential candidates of TCM. The project is planned to be accomplished in one year. In addition to the first-3-month preparation phase, the remaining 9 months are for experimental implementation and proposal development. Months 4-9: Performing the detailed research work according to the design of experiments, as presented in Chapter 3. Months 10-12: Proposing a proposal for future research based on the obtained results. 24
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