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16 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 • Higher steam release rates will produce wet steam, prevented by ensuring a large enough water surface area, and hence again determining the accumulator size. • The evaporation capacity must be sufficient. This is a function of the pressure at which the water is stored when fully charged (the boiler pressure) and the minimum pressure at which the accumulator will operate at the end of the discharge period (the accumulator design pressure). More flash steam is produced when the differential between these two pressures increases. • The accumulator design pressure must be higher than the down- stream distribution pressure, to create a pressure differential and allow for the required flow. • The steam inlet pipes must feed well below the water surface level to ensure the maximum possible liquid head above them. The steam injectors should be installed at a slight angle to avoid erosion of the vessel, and discharge at a very high velocity to provide turbulence and mixing of the stored water mass. • Both the design of the inlet pipe and the manifold system, to- gether with the location of the steam injectors, must provide even injection of steam throughout the length of the accumulator re- gardless of the actual steam flow-rate. The steam injector capacity will reduce as the pressure in the vessel increases because the differential pressure between the in- jected steam and the vessel pressure is reduced. At very low flow rates, the steam feed will not be uniformly distributed and will tend to issue from the injectors closest to the steam inlet pipe. Supercritical water is water that exists above its thermody- namic critical point (374 °C, 22.1 MPa). Using water as the working fluid at supercritical conditions, the respective efficiencies can be raised from about 39% for subcritical operation to about 45% using current technology. The supercritical fluid is a single non-condensable phase which has physical properties that are intermediary between its liquid phase and gaseous phase, with a density typically being closer to that of water but with transport properties being gas- like. Supercritical water at low density behaves like a non-polar solvent unlike its liquid counterpart due to loss of hydrogen bonding [82]. It has high thermal efficiency, as it combines high tempera- ture, high pressure and its existence as a single phase which benefits from relative simplicity in system design due to not having to handle multi-phase flow. The problems associated with supercritical steam is that the pres- ence of ionic species can induce corrosion. At low ionic density (low pressure), corrosion is typically more severe in subcritical than supercritical water, due to the fact that ionic species are insoluble in supercritical water at low density. This is the reason that com- ponents and/or piping used to preheat or cool fluids in a supercritical water system are more susceptible to corrosion than the reactor itself [83–85]. At high ionic density (high pressure), most salts will be soluble. Therefore, pure distilled water should be used in supercritical water/steam accumulator to eliminate salt-induced corrosion. Pressure vessels are designed to have a thickness proportional to the radius and operating pressure of the tank, and inversely pro- portional to the maximum allowed normal stress of the particular material used in the walls of the container. Theoretically, any ma- terial with good tensile properties that is chemically stable in the chosen application could be employed. No matter the shape, the minimum mass of a pressure vessel scales with the pressure and volume it contains and is inversely proportional to the strength to weight ratio of the construction material (minimum mass de- creases as strength increases). Godall [86] has suggested that the best ratio of diameter to total length of a steam accumulator should be between 1:4 and 1:6 in order to obtain optimal condition in terms of thermal storage capacity and material cost. 3.4.3. Moderate to high temperature PCMs Within the high temperature thermal energy storage option using PCMs, various alternatives have been investigated, as illustrated in Fig. 17. Each of the parallel topics is subsequently dealt with. To il- lustrate the practical use of the results reported in previous sections, design approaches for thermal energy storage installations are further elaborated. This will be demonstrated by describing potential ap- plications of high temperature thermal energy storage in three typical case studies: • The encapsulated PCM is mixed within a high temperature con- crete for passive heat energy storage. • The encapsulated PCM is mixed in a liquid HTF. • The encapsulated PCM is used in a solid–gas conveying system as heat carrier. In these case studies, described in Table 10, the coated PCM in- teracts with a different heat carrier, being solid, liquid or gas. 3.4.4. The growing importance of lithium salts 3.4.4.1. Lithium for sensible heat storage. Lithium has been used to improve the properties of molten salts used in CSP by adding it to their composition. Fig. 18 shows new compositions with lithium and in comparison with the commercial solar salt (34 wt% KNO3 and 66 wt% NaNO3): the salts have nearly similar heat capacities, there- fore they have similar energy density. The benefits of adding lithium to solar salts are noticed in the melting temperature of the com- position, which is lowered. The range of working temperature is extended, whilst improving the thermal stability [91]. Due to the Fig. 17. Potential applications for TES using E-PCMs.PDF Image | Thermal energy storage: Recent developments
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