Investigation of mechanical heat pump systems for heat upgrading applications
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Three high temperature heat pump systems based on vapor compression cycles are introduced and examined. Four fluids (water, cyclohexane, biphenyl and mercury) are selected and analysed thermodynamically as prospective working fluids for the high temperature heat pumps. These working fluids are used in cascaded cycles to upgrade the heat to a temperature of 600 °C. The equations of state used in performance analysis are Peng–Robinson, NRTL and IAPWS-95. A parametric analysis is carried out to study the effects of isentropic efficiency, sink temperature, source temperature, and ambient temperature on the system performance. Energetic and exergetic COPs of the overall and individual cycles are determined. The COP values obtained are found to range from 2.08 to 4.86, depending upon the cycle and temperature levels. The System 2 (energetic and exergetic COPs are 3.8 and 1.9 respectively) outperforms System 1 (energetic and exergetic COPs are 2.2 and 1.0 respectively) both energetically and exergetically, while operating under same conditions of source temperature 81°C and sink temperature 600 °C. The System 3 achieves maximum cycle temperature of 792 °C while operating under moderate pressure ratios. The high COP values in some instances make these systems promising alternatives to fossil fuel and electrical heating. As a possible sustainable scenario, these pumps can utilise low grade heat from geothermal, nuclear or thermal power plants and derive work from clean energy sources (solar, wind, nuclear) to deliver high grade heat. The high delivery temperatures make these heat pumps suitable for processes with corresponding needs, like high temperature endothermic reactions, metallurgical processes, distillation, and thermochemical water splitting.