Investigation On Performance Evaluation And Operation Strategy Of Air Source Heat Pump Heating System Under Partial Load

The air source heat pump(ASHP)has been widely used in the clean heating project in northern China due to the easy installation,flexible usage,high efficiency and energysaving features.However,the ASHP heating capacity decreases with decreasing outdoor air temperature,while the building heating demand presents an opposite increasing tendency.The coupling between the dynamic variations of the supply and demand in the heating system results in the long-term partial-load operation of the ASHP.In this context,there exist noticeable problems in practical applications,such as the insufficiency of evaluation methods for the partial-load performance of ASHPs,the lack of quantitative analysis methods for weighting the influencing factors of the ASHP heating system,and the need for further improvement of the partial-load operating strategies of the ASHP heating system.Aiming at the above problems,this paper conducts a comprehensive and in-depth investigation on the performance evaluation and operation strategy of the ASHP heating system under partial load,based on the mechanism research of the partial-load energy efficiency losses.Firstly,a partial-load-factor(PLF)characterization method coupling the impacts of the start-up loss and the stand-by loss is proposed.Through mechanism analysis,the influence mechanism of start-up and stand-by process on energy efficiency loss under partial load is revealed.Based on the simultaneous consideration of these two losses,the correction of the PLF calculation methods in the ASHP evaluation standards both at home and abroad is conducted,which solves the divergence in application caused by considering one single loss factor.Secondly,a quick experimental method to test the partial-load performance of ASHPs is developed.The transient experiment of ASHPs under on-off cycle is designed and used to study the partial-load performance of 3 ASHP units with different structures and configurations.The results show that,the on-off cycle losses are influenced by various factors including the operating conditions and the unit configurations,and the start-up performance of different units varies greatly.Through the developed quick experiment,the accuracy of the PLF characterization method coupling the impacts of the start-up loss and the stand-by loss is verified.Then,an energy efficiency analysis model of the ASHP heating system is established.The operational performance of 103 ASHP heating systems in Beijing have been monitored.According to the field test results,the energy efficiencies of different systems are obviously discrepant.By introducing the Thermodynamic Perfectibility to characterize the steady-state performance,the analytical relationship between running time,stand-by time and energy efficiency losses is clarified,and an energy efficiency analysis model of the ASHP heating system is established,which can quantitatively calculate the steady-state performance degradation,start-up loss,stand-by loss and defrost loss.Based on this model,the weights of the energy efficiency influencing factors of the 6 systems are analyzed quantitatively,presenting different on-off cycle losses between 9% and 17%.Finally,a dynamic hysteresis control algorithm based on the matching between supply and demand is put forward.Based on the study of the influence mechanism of operation hysteresis on the energy efficiency improvement under partial-load conditions,and considering that the thermal inertia of buildings and the system functions to restrain the fluctuation of room temperature,the optimization of operation hysteresis is adopted to reduce the start-up loss,and the interaction start-stop control of the circulating water pump is adopted to reduce the stand-by loss.A dynamic simulation model of the ASHP heating system is established and the analysis results have demonstrated the feasibility of the proposed control strategies.The field test results of a practical project show that,on a typical day with a daily average outdoor temperature of-1.7?,the proposed control strategies could decrease on-off cycles by nearly 80%,improve the system energy efficiency by more than 25%,and reduce the system power consumption by more than 33%.This paper comes up with performance evaluation methods and system operation strategies of the ASHP heating system under partial load,which are of great significance to the energy efficiency improvement,the promotion and application of the ASHP heating system.