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There are three types of steam accumulators, as described and illustrated in Steinmann and Eck [73] and DOE [81]: • Water filled accumulator: 1. Constant pressure accumulator: usually arranged in a verti- cal position to allow for both hot and cold water to be stored in the same vessel via thermal stratification, where the hot water is in the top part and the cold water in the bottom part. 2. Pressure drop accumulator: also called feed water accumu- lators, where pressure and temperature are allowed to vary (see Fig. 15). The accumulator is usually arranged horizon- tally and when fully charged, it is usually at 90% full of water. • Supercritical steam accumulator: this requires a storage condi- tion of a minimum temperature of 647 K at pressure of 220 bar. During the thermal storage process the temperature of the liquid water can be increased by condensation of superheated steam, with little variation of the liquid storage mass; or the mass in the volume is increased by feeding saturated liquid water into the system, where the pressure remains constant, charging indirectly where a heat ex- changer is integrated into the liquid volume. The medium flowing in the heat exchanger needs not be water; hence heat from a heat source at lower pressure can be used. Steam is charged beneath the surface of the water by a distri- bution manifold, which is fitted with a series of steam injectors, until the entire water content is at the required pressure and tempera- ture. If the steam accumulator is charged using saturated (or wet) steam, there may be a small gain in water due to the radiation losses from the vessel. To avoid a pressure drop in a constant pressure water filled ac- cumulator, the application of a separate flash evaporator is required, where the saturated liquid water is taken from the steam accumu- lator, which is later depressurized externally. Cold water is fed into the bottom of the storage vessel to keep the water level constant, where mixing of hot and cold water must be minimized. Thermal stress resulting from filling the pressure vessel with cold water must be considered [73]. An alternative option is to maintain constant pressure by integrating PCM into the storage vessel partly replac- ing the liquid water (Fig. 16). PCMs usually exhibit a low thermal conductivity so layers of this material must be thin to ensure a suf- ficient heat transfer rate or via the encapsulation of PCM in small containers placed inside the liquid volume. PCM is not only attrac- tive regarding the avoidance of thermo-mechanical stress, the characteristic volume-specific storage capacity of PCMs is also de- sirable, which is in the range of about 100 kW h/m3, significantly exceeding water (20–30 kW h/m3) [73]. The design of accumulators should fulfill important criteria, as listed by DOE [81] and summarized below: • Provide sufficient time from the end of one overload period to the beginning of the next, to fully recharge the accumulator. • Sufficient surplus boiler capacity should be available to re- charge the water stored in the accumulator during off-peak times. • The accumulator volume should be large enough to contain enough water to provide the required amount of flash steam during the discharge period. Fig. 16. Constant pressure accumulator with PCM [73]. H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 15 Fig. 15. Pressure drop accumulator [73].PDF Image | Thermal energy storage: Recent developments
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