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Table 10 Case studies of high temperature thermal storage applications. H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 17 Case study High temperature concrete as passive storage media Micro-encapsulated PCM in slurry Encapsulated PCM in TES heat transfer fluids Fluidized bed Storage concept Description A three component thermal storage system, combining sensible and latent heat storage was tested by Laing et al. [44] for use in solar power plants. Investigations and experiments supported the hypothesis that high temperature concrete could serve as sensible heat storage up to 500 °C. Within the limitations of the solid concrete structure, the incorporation of phase-change materials is beneficial since capable of absorbing a high quantity of energy (sensible and latent heat), while keeping the temperature of the matrix close to the melting point of the phase change. This use of encapsulated PCM mixed with concrete was further demonstrated for applications at ambient temperatures [16,24,87], achieving high energy savings in cooling power. A micro-encapsulated PCM slurry is a suspension where the PCM is dispersed at 10–20 wt % without significantly altering the physical properties of the liquid (density, viscosity) [88]: PCM is microencapsulated using a polymeric capsule and dispersed in water. A slurry with a 10 wt % concentration of paraffin was conducted by Delgado et al. [89] to investigate the effectiveness of its properties as a thermal storage material and as a heat transfer fluid. Results demonstrated an increase of approximately 25% in the convective heat transfer coefficient when compared to water, as the result of improving the overall heat capacity of the material through applying the latent heat properties of the PCM. The feasibility of using ionic liquids as liquid thermal storage media and heat transfer fluids in a solar thermal power plant has been the subject of additional investigations. According to Bridges et al. [90], nanoparticle enhanced ionic liquids have been shown to increase the heat capacity of the ionic liquid without adverse secondary effects on the ionic liquids’ thermal stability. The ionic liquid physical properties were shown to be only affected in cases of high nanoparticle loading. Those similarities between high and low temperature slurries’ properties tend to indicate that a high temperature equivalent of an encapsulated PCM slurry is possible. Heat storage units of new concentrated solar plants could benefit from a heat transfer fluid which incorporates encapsulated particles of PCM. The heat transfer capacities would be improved by the high conductivity and the high energy density of the PCM. In comparison with a conventional thermal fluid, e.g. Santotherm 350, the specific heat will be increased by a fraction of 3 whilst the energy density will increase by about 5. The main issues for developing salt-PCM modifications would be the loss of fluidity of the high temperature slurry. Further studies have to prove if the addition of particles increases the thermal properties of the HTF without affecting the viscosity and associated pumping power needed. Existing CSPs would then see their global efficiency improved by the higher energy density of the HTF. High heat transfer coefficients can be achieved between a fluidized bed of coated PCM particles and a heat exchanging surface [7]: heat can be captured by the particulate gas–solid flowing suspension. Further research is required, since it was impossible to coat the PCM particles by a non-porous metallic layer (chemical vapour deposition or electrolysis) lower temperatures, it is expected that operational and mainte- nance costs of CSPs are reduced, therefore savings should be obtained [91]. In terms of costs, the U.S. Department of Energy performed a comparison between salts used for CSP, including solar salt (40% KNO3 and 60% NaNO3), and the cost of storing energy with each of them in a two tank system [92]. Results are shown in Table 11: despite the higher price of the salts with lithium nitrate, a lower cost per stored kWth is achieved, which can be attributed to the higher energy densities and the savings in heat tracing to keep the salts molten. 3.4.4.2. Lithium compounds as phase change materials. PCMs in- cluding lithium have been a potential for building applications and for high temperature TES. There are different lithium compounds to be used in building applications which are compared to the well- known octadecane and Glauber salt in Fig. 19. It can be seen that Fig. 18. Heat capacity and melting temperature comparison of lithium compositions used in solar power plants (adapted from Ref. 91).PDF Image | Thermal energy storage: Recent developments
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