Comparison of the organic rankine cycle and variable phase cycle for low-grade waste heat recovery
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AbstractThere is increasing global interest in the recovery of energy from low temperature streams for which low-grade waste heat (<130°C) is captured to increase the energy efficiency of various facilities. While many sources of this waste heat exist, the challenge to utilize the heat economically still remains. Currently, the most common technology for the capture of low-grade heat is the Organic Rankine Cycle (ORC). ORC is an economically viable technology at temperatures >140°C and the expander units in this temperature range can generate 1 to 2 MW or more. However, for lower temperatures, commercial expanders generally produce a maximum of 500 kW of power. Power generation of 1 MW or greater would normally require several units, but the utilization of many small units is not economical for low temperature energy recovery from large waste heat sources. An alternate technology incorporates an expander based on the variable phase cycle (VPC) that seeks to economically increase the power generation beyond 1 MW from low-grade waste heat sources. To date, comparisons of the two cycles were based on the use of water as the condensing fluid, which keeps parasitic loads to a minimum, however this thesis considers air as the cooling medium. Furthermore, evaluations of ORC performance are usually based on the First and Second Law efficiencies whereas, for an overall comparison of the ORC and VPC, numerous performance parameters need to be considered. This thesis compares the ORC and VPC in the temperature range of 110°C to 140°C for waste heat. The evaluation showed that, in this lower temperature range with air as the cooling medium, the ORC had a smaller parasitic load and higher specific work than the VPC.
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