| With increment of energy conservation and emission reduction efforts and promotion of renewable energies,the application of renewable energy and renewable energies in building energy conservation have attracted more and more attention.Direct-expansion solar-assisted heat pump(DX-SAHP)system which combined the functions of solar collector and evaporator of the heat pump into one collector/evaporator unit,in which the refrigerant is directly evaporated by absorbing the heat from the surroundings.Due to it,system’s evaporation temperature and coefficient of performance are improved.In this paper,the influence of regulation modes of compressor speed and degree of subcooling on the operation parameters and system performance are studied and analyzed both in theory and experiment.The main work is as follows:A R134a direct expansion solar heat pump hot water system experiment prototype was built,which includes the heat storage tank of the built-in external wound microchannel condenser,the electronic expansion valve,the bare plate solar collector/evaporator and the inverter compressor.Besides,the data acquisition system is briefly introduced,the evaluation standard of system performance is proposed,and the uncertainty analysis of experimental error is carried out.The results show that errors of the average and instantaneous COP are within the allowable range.In addition,typical operating conditions in various seasons and the operating conditions of the system are analyzed,which show that components of the DX-SAHP experimental platform match well and run stably.The annual average COP of the system maintains above 3.00,and the water temperature rise can be close to 40℃ under harsh environment.A simulation model is established to study the thermal performance of DX-SAHP hot water system from the theoretical point of view.Among them,A lumped parameter method is used to establish the models of a compressor and an electronic expansion valve,and a uniform flow distributed parameter method is used to establish the models of an unglazed flat-plate solar collector/evaporator and a winding micro-channel condenser;and the physical properties of R134a are from REFPREF.Taking superheat degree and subcooling degree as iterative criterion,the simulation program is compiled in MATLAB.The mathematical model predicts well the experimental data within a maximum relative error of 10%,which verifies the reliability of the simulation model.Under constant environmental parameters,the influence of three regulation modes of compressor speed on the operation parameters is studied.Among them,condensation temperature and mass flow rate are positively correlated with compressor speed;evaporation temperature is negatively correlated with compressor speed.In addition,the acceleration mode of the compressor speed enables to stabilize the evaporating temperature,and the deceleration mode of the compressor speed enables to reduce the pressure ratio.Under variable environmental parameters,the influence of four regulation modes of compressor speed on system performance is researched.The simulation results show that there is little effect of the regulation mode of the compressor speed on the average system coefficient of performance under the same operating and environmental parameters.Furthermore,when the average compressor speed decreases from 2910 to 2650 r·min-1,the average COP increases by 7.5%.That’s to say,the system performance could be improved by reducing the average compressor speed.In order to control subcooling and superheat at the same time,a throttling valve(TV)and a liquid receiver are added to the DX-SAHP system,forming a new system—double controllable DX-SAHP(DCDX-SAHP)system.The optimal subcooling degree of the system is determined by theoretical analysis and simulation calculation,and the influence of the subcooling on the system is analyzed.The simulation results show that under various operating conditions,when the subcooling degree is 4 K,the DCDX-SAHP obtains the maximum COP.In addition,the outlet temperature of the condenser will approach to the water temperature with the increase of subcooling degree,and the condensing temperature is increasing.Compared with the DX-SAHP system which proposed in this paper,the DCDX-SAHP hot water system working under subcooling of 4 K can obtain higher COP. |
