Abstract
In the 21st conference of the parties (COP21) of United Nations, both renewable energy and energy efficiency were recognized as essential means for low-carbon future. It is estimated that around 20-50 % of energy input to industries is lost as waste heat. Low-grade heat is abundantly available from renewable sources like solar, biomass and geothermal. There is a need to develop technologies which can utilize very low-grade heat (<100 °C). This paper reports the design and optimization of heat driven systems for utilizing very low-grade waste heat in tropical conditions. The Ejector Refrigeration System (ERS) has been selected, designed and simulated to producing cooling from low-grade heat. Organic Rankine Cycle (ORC) has been designed and simulated for producing power and a novel hybrid configuration is proposed to produce power and cooling simultaneously. The ERS and ORC optimization involves the selection of most suitable working fluids, working conditions, sizing of components, and optimizing configurations. For system level modeling with real properties of working fluids, EES (engineering equation solver) and EBSILON (a commercial thermal system design tool) are being used in this work. For design optimization of ejector, EES and ANSYS-FLUENT (a commercial CFD solver) are being used. With EES model, the ORC system has been found to operate at 5.2 % overall efficiency. The results from EES model have been verified by the EBSILON model. The EES model for ERS gives the COP of 0.48. The EES model developed for ejector nozzles can be used to design any supersonic nozzle for the desired inlet and outlet pressers and exit velocity. A novel hybrid cycle configuration has been proposed. This novel configuration uses ejector simultaneously for two purposes; to reduce the work-producing-expander outlet pressure and to compress vapor for cooling cycle. The preliminary modeling results show that the output of this configuration is 27 % higher than simple ORC.
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