NEW IDEAL PROCESS DESIGN BASED ON THE EXERGETIC ANALYSIS. COLD PRODUCTION USING RENEWABLE ENERGY HEAT
Résumé
Cold production processes generally use harmful refrigerants that increase ozone depletion and global warming effect and it is thus important to design new systems that are more environmentally-friendly. A major issue is to use refrigerant fluids which are neutral with respect to ozone depletion and global warming problems. Another goal is to correlate the quality of the energy used to the quality of energy produced by these processes. The exergy analysis applied to ideal thermochemical dipoles is an efficient way to deal with these questions. Thermochemical dipoles connect two thermal processes exchanging gas with each other. The working gas can be environmentally inert, ammonia or water for example. Original couplings of the dipoles enable to design new processes, for heat or cold production at a desired temperature level. Our work is aimed to create an original process that could use low-grade energy (produced for instance from a thermal solar collector or a geothermal drilling at around 70°C) to provide lowtemperature heat (below -23°C, in order to store deep-frozen food). Indeed, existing solar cooling systems can provide cold at a temperature of around 0°C and produce ice but up to now none is able to ensure long term food conservation. Analysing several possible dipole couplings, one cycle was found to be particularly adapted to a solar heat source. The whole of the exergy entering this ideal system is used and no cold in excess or unused energy are produced. The ideal coefficient of performance of this system is 50%. A numerical simulation of the process evolution proved the feasibility of the concept and showed that its expected coefficient of performance is about 11%, which is similar to the up-to-date solar cooling systems, but at higher cold temperatures. The system exergetic yield is expected to attain 40% in average over the year.
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