Abstract
The integrated chemisorption technology driven by ultra-low grade heat for simultaneous electrical power and thermal energy storage has been investigated in this work. A resorption cycle employing low and high temperature salts, with the coupling between manganese chloride (MnCl2), calcium chloride (CaCl2) and sodium bromide (NaBr), was evaluated and compared in the proposed integrated system for a heat source temperature of between 30°C and 100°C. During the energy charging process, working fluid compression was introduced to convert mechanical power or electricity into chemical energy so that it could be stored during the adsorption process at the same time as utilising low temperature heat energy. During the energy discharging process, mechanical power could be generated via the expansion of the desorbed high pressure working vapour from the low temperature salt reactor, there is also a by-product potential of the cooling energy which can be extracted from the cold expansion exhaust. In addition to the power generation and the potential cold energy, upgraded heat could also be provided by the exothermic adsorption process in the high temperature salt reactor. The performance of this integrated cycle in terms of energy and exergy efficiency and energy density have been discussed. With the help of ultra-low grade heat, a 100% round-trip electricity storage efficiency of the proposed system has been found to be achievable using CaCl2–NaBr and MnCl2–CaCl2 pairs when the heat source temperature was higher than 50°C and 60°C, respectively. Also a temperature lift of the heat by 15–33°C and 22–68°C respectively was possible using these two adsorbent pairs.
Highlights
Low-grade heat sources are ubiquitous, with industrial waste heat in the UK estimated to be as high as 40 TW h annually and is enough to heat over 2 million homes a year [1]
Van der Pal et al [15] carried out experimental tests on a hybrid adsorption-compression heat pump based on a roots-type compressor and silica gel-water adsorption to find out the optimal integrating configuration. This tentative work studied physisorption system, but the ultimate goal emphasised is still to develop hybrid ammonia-salt heat pump system because the latter one is more suitable for the targeted high heat-upgrading purpose. This present study proposes a new integrated system that combines chemisorption reactors, compressor and expander and it is aimed to recover ultra-low grade heat for thermo-electric energy storage and exploitation
In the energy discharging process, besides the restored electric power the integrated system would generate a considerable amount of upgraded thermal energy, plus small amount of cold energy depending on heat source temperature and the refrigeration requirement
Summary
Low-grade heat sources are ubiquitous, with industrial waste heat in the UK estimated to be as high as 40 TW h annually and is enough to heat over 2 million homes a year [1]. Investigations have indicated that low grade waste heat accounts for 50% or more of the total heat produced by industry and the total waste heat available above 30 °C from US power plants is estimated to be. Renewable low grade heat, such as from solar thermal and geothermal sources, is extremely valuable and is of increasing interest to support the energy mix. Various thermodynamic cycles, such as the organic Rankine, supercritical Rankine, Kalina, Goswami, trilateral flash, and adsorption cycles have been proposed for the conversion of low-grade heat to electrical power [4,5,6,7,8,9,10].
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