Performance Optimization Of Vapor Compression Moderate/High Temperature Heat Pumps

Compared with the traditional heating method by burning fossil fuels, the heatpump technology can save energy when the system COP(coefficient of performance)is guaranteed. The researches on the high-temperature and high-efficiency of themoderate/high temperature heat pump technology aims to broaden the application ofheat pump into a higher temperature area, supplying a method that recyclesmoderate/low temperature industry waste heat and satisfies the heating needs athigher delivery temperature. This dissertation firstly compared and analyzed theperformance of two major kinds of heat pump namely vapor compression heat pumpand absorption heat pump/transformer. Then on the basis, the optimal performance ofsingle-stage cycle and the optimal comparative study on the two-stage cycle methodsfor vapor compression moderate/high temperature heat pump are conducted.The results of comparison of the performance between vapor compression heatpump and absorption heat pump/transformer show that each kind of them has its ownadvantage in primary energy ratio, high-temperature potential, system cost, powersupply for the cycle. Both kinds of the heat pumps can be utilized in moderate/hightemperature area through the researches on high-temperature and high-efficiencytechnologies, respectively for different cycle power supply namely electricity (ormechanical work) and low-grade thermal. The main factors that limit thehigh-temperature level are lubricant thermalstability that restrict the upper limit ofcompressor discharge temperature and the safety of hermetic compressor’s motor coil.The measures which are helpful to achieve the high-temperature level include:adopting dry working fluid, wet suction process or wet compression process withusing rotary compressors for wet working fluids, employing open-type compressorsand oil-free compressors, multi-stage cycle and so on.Performance of the compressors for single-stage vapor compressionmoderate/high temperature heat pump system was compared in the improvedexperimental system. The comparisons between the open type reciprocatingcompressor and hermetic scroll compressor were carried out at condensingtemperature of75,85,95oC with the circulating temperature rise of45,35oC, usingHFC245fa as the working fluid. The results show that under the same conditions, the discharge temperature of open type reciprocating compressor is lower than thehermitic scroll compressor; the former’s constant entropy efficiency is higher thanthat of the latter, but comprehensive efficiency of open-style piston compressor islower than that of the hermetic scroll compressor. This implies that the open typereciprocating compressor is more suitable for high-temperature conditions. And itsmain optimization measures are improving transmission efficiency and optimizingelectromechanical matching relationship considering compressor load.The performances of four different two-stage cycle modes are investigatedadopting improved theoretical cycle performance calculation method. The theoreticalanalysis with HFC245fa as the working fluid was conducted at the condensingtemperature of70-130oC with the temperature lift of40-65oC. The optimal resultsshow that the COP of four two-stage cycle modes are all obviously higher than that ofsingle-stage cycle and two-stage throttling with superheated gas at the inlet of secondstage compressor has the greatest improvement. In the case of wet fluid as workingfluid, the two-stage cycle modes can significantly reduce discharge temperature, butfor a given dry or isentropic fluid as working fluid situation, the exhaust temperaturereduction of two-stage cycle modes is limited.