Theoretical And Experimental Study On Capacity Modulation Of Air Source Heat Pump System

The demand of protecting the ozone layer, saving energy and improving room comfort is a developmental trend to air source heat pump system in recent years. Composition modulation is one form of capacity controls, which changes system capacity by varying composition of nonazeotropic mixtures. This method not only modulates system capacity by using refrigerants of environmental protection, but also avoids additional energy consumption. It is likely to gain better modulation effect than other methods. This thesis theoretically and experimentally studies the winter performances of an air source heat pump system, which is newly designed and has a variable composition equipment. The main research contents are as follows:1 .Build a steady-state simulation model of a general heat pump system, calculate system performances of using R22 and (30/70wt%)R32/R134a as refrigerants respectively, and get their performance curves as the working conditions change. Comparing with the system of R22 at the same working condition, it is indicated that heating capacity and work consumption of the system of (30/70wt%)R32/R134a are inferior to that of the former, but its heating COP is higher than the former at the lower temperature condition. Therefore, it shows that mixture replacing pure substance has advantage of energy savings in low temperature environment.2.Build a mathematic model of variable capacity system, simulate the steady-state performances of three different structures, and obtain their trends of performances and compositions with inlet temperature of evaporator descending. It is concluded that it may be disadvantageous to capacity modulation at low temperature environment if the condenser area is smaller and throttle set is capillary.3. At the same equipment and the same working conditions, compare performances of three kinds of systems, which use R22, mixture R32/R134a with a composition of 30/70 (mass) and variable composition mixture R32/R134a as refrigerants respectively. It is proved that variable capacity of mixture has obvious advantages comparing tolimitative composition systems of pure substance and mixture, and it is a feasible method by controlling low pressure, which can be used to vary mixture composition.4. Experimentally study the performance of limitative composition mixture R32/R134a. Compared theoretical results with experimental data, it shows that heating/cooling capacity reduces when inlet temperature of evaporator is decreasing, which is not satisfied room comfortable demand at low temperature environment in winter condition.5. Through experiments on variable capacity system, some problems are found. And some viewpoints about improving equipment and some defects in experiment are pointed out.