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Annual Review of Heat Transfer, Vol. 15, p.131-177 https://doi.org/10.1615/AnnualRevHeatTransfer.2012004651 b cp (2) A high thermal diffusivity of the heat storage material provides a quick response to temperature differences, i.e., quick charging and discharging. A high thermal effusivity yields a large amount of heat being stored. A high thermal diffusivity improves transient heat transfer and hence, shortens the time for charging and discharging processes. The thermal effusivity is important for the transient heat conduction phenomena in a semi-infinite slab assuming constant temperature boundary conditions. Metals and graphite are best suited for quick charging and discharging (high thermal diffusivity a) and for a large amount of heat stored in a given time (high thermal effusivity b). Other solid materials such as stones are much less advantageous. Their respective values are smaller by an order of magnitude. Thermochemical storage system may use powder fills with even lower diffusivity and effusivity values. Small density changes versus temperature can minimize thermo-mechanical stress phenomena. It needs to be considered, that thermophysical properties are not always available and their values may differ among different literature sources. Some thermophysical property values, such as graphite values, are strongly temperature dependent. Also, impurities in the substances can change the properties considerably. For example, impurities in metals cause a drop in the thermal conductivity values. 1.3. Thermal energy storage for concentrated solar power Although other applications are also mentioned, the focus of this chapter is on TES for concentrated solar power (CSP). Hence, the following text gives some background of CSP. Regarding electric grid and quality of bulk power supply, it is the ability to provide dispatch on demand that makes solar thermal power stand out from other renewable energy technologies like PV or wind. TES systems store excess thermal heat collected by the solar field (Figure 3). Storage systems, alone or in combination with some fossil fuel backup, keep the plant running under full-load conditions. Improvements on operational flexibility and energy dispatchability by thermal storage and hybridization are identified as key technology objectives for R&D development. The capability of storing high-temperature thermal energy leads to economically competitive design options, since only the solar part of the plant has to be oversized. This solar thermal power plant feature is tremendously relevant, since penetration of solar energy into the bulk electricity market is possible only when substitution of intermediate-load power plants of about 4,000-5,000 hours/year is achieved. The focus is currently on systems compensating transients caused by clouds and on systems for daily charging/discharging system. Seasonal storage is not considered to be cost-effective within the near future.PDF Image | Annual Review of Heat Transfer
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