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14 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 Table 8 Comparison of specific heat, latent heat and exergy density of cryogens and some commonly used heat storage materials [69]. Media N2 (liquid cryogens) CH4 (liquid cryogens) H2 (liquid cryogens) Storage methoda S + L S + L S + L Specific heat (kJ/kg K) 1.0–1.1 2.2 11.3–14.3 Phase change temperature (°C) −196 −161 −253 Fusion/latent heat (kJ/kg) 199 511 449 Exergy density (kJ/kg) 762 1081 11987 a In this description ‘S’ indicates that thermal energy is stored in the form of sensible heat while ‘L’ stands for latent heat. decreasing its internal energy while increasing its exergy. The use of liquid air/nitrogen as an energy carrier is not commonly known until fairly recently [59–61]. Cryogenic energy storages can have a relatively high energy density (100–200 W h/kg), low capital cost per unit energy, are benign to the environment and have a rela- tively long storage period. However, it has a relatively low efficiency (40–50%) according to the current energy consumption for air liq- uefaction [61]. Cryogenic energy storage is still under development by Mitsubishi (Japan) [62], the University of Leeds (UK) and the Chinese Academy of Sciences [63]. Specific topics of research involve hydrogen releases [64], the phase change itself [65,66], and inte- grated applications [67,68]. Table 8 and Table 9 illustrate that although the specific heat and phase change heat of the cryogens are of a similar order of magnitude than those of common heat storage materials, the exergy density of cryogens is much higher. Once produced and stored in insulated containers, the cryogen is ready to be delivered. No extra energy is required except for the pumping power consumption which is negligible. The only energy loss of the cryogen is the heat dissipation of the cryogenic tank, which can be less than 1% per day using conventional insulation technologies [70–72]. The exergy efficiency for a cryogenic engine is about 40% as a considerable portion of the exergy is in the form of cold, which is wasted during the heating process. In order to improve the effi- ciency, the recovery of the waste cold energy can be integrated with the direct expansion of the cryogen using cycles such as the Rankine cycle as shown in Fig. 15. Many power cycles, based on Rankine and Brayton cycles, were proposed to recover the cooling capacity of cryo- Table 9 Comparison of physical and chemical exergies of the cryogens [61]. genic liquids [74–77]. Cryogenic liquids vapourizers are commonly used as low temperature thermal sinks for the bottom Rankine cycle (see Fig. 14), with waste heat or low-grade thermal energy (to enhance exergy efficiency) as thermal sources. Its alternative heat source could also be using the atmosphere [60], unlike a conven- tional heat engine that uses the atmosphere as a cold reservoir or heat sink. The combined methods for cryogenic energy extraction have been discussed in detail by Li et al. [61,68]. It has been con- cluded that the combined cycle cryogenic heat engines could have an exergy efficiency of 65–78% under ideal conditions [78]. Chen et al. [79] claimed that the exergy efficiency could be im- proved from ~78 to 117% if the working fluid were superheated to ~100 °C, and the exergy efficiency could be doubled if the waste heat temperature is as high as 300 °C. 3.4.2. Latent heat storage with steam accumulators The accumulator is an insulated steel pressure tank containing hot water and steam under pressure, thus storing energy in the form of latent heat stored in the liquid phase, i.e. water. It releases steam by extracting latent heat when needed and to accept steam when the demand is low. The volume specific thermal energy densities are typically in the range of 20–30 kW h/m3. The released steam powers the steam turbine for electricity production. Storing steam under pressure in saturated or superheated conditions in pressure vessels is generally not economic due to the low volumetric energy density [73]; it is nevertheless applied in specific cases as buffer between e.g. a boiler and the steam-driven process of a paper mills [80]. Cryogen Liquid H2 Liquid N2 Liquid CH4 Thermal exergy (kJ/kg) 11,897 762 1081 Chemical exergy (kJ/kg) 116,528 0 51,759 Gas density (kg/m3) 0.0824 1.1452 0.6569 Liquid density (kg/m3) 70.85 806.08 422.36 Fig. 14. Direct expansion-Rankine hybrid cycle [78].PDF Image | Thermal energy storage: Recent developments
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