Optimal Control And Experimental Study On Variable Capacity Direct Expansion Solar Assisted Heat Pump Water Heater

Solar energy is a kind of clean energy which can reduce the dependence on traditional energy sources to some extent.The technology of solar heat pump integrates effectively solar energy heat utilization and heat pump technology,which is used for heating and supplying domestic hot water.Through the integration of collector and evaporator,higher evaporating temperature and collector efficiency are achieved in the direct expansion solar assisted heat pump(DX-SAHP),which results in the higher coefficient of performance(COP).It is one of the effective technologies to achieve building energy conservation,renewable energy utilization and pollutant emission reduction.Based on a large number of experiments,the thermal performance characteristics and variation trend of various parameters of variable capacity DX-SAHP water heater is studied and analyzed in this paper.The main contents are as follows:A DX-SAHP water heater prototype,using R134a as refrigerant,is designed and built.It mainly consists of a bare flat-plate solar collector/evaporator,a variable-frequency rotary-type hermetic compressor,a micro-channel condenser,a hot water tank,and an electronic expansion valve(EEV).The working principle of the experimental platform and the construction process of the whole machine are introduced in detail,including the introduction of the structural parameters of the four components of the DX-SAHP system,the compilation and construction of the data acquisition and monitoring system,the connection of the solar radiation meter,pressure sensor,temperature sensor,anemometer,humidity sensor,power meter,data acquisition controller and other hardware devices.The system experiment data acquisition and control software based on Kingview is compiled to complete the monitoring,acquisition and storage of the operating parameters of the experimental platform.And the system could control the EEV opening and compressor speed automatically.The operation control strategy of DX-SAHP water heater is proposed,after analyzing a large number of experimental data.The overall operation control strategy mainly includes:the fuzzy control strategy before the system starts;the calculation of the initial opening of the EEV and the initial speed of the compressor in the system start-up stage;the regulation of the speed adjustment of the compressor during the normal operation period and the opening control strategy of the EEV to ensure that the superheat degree is in the range of target superheat degree.And according to the environmental conditions and initial water temperature,a reasonable expected heating time for the system is also established.The real-time adjustment and automatic control of the speed of the compressor,opening of the EEV and superheat degree of the collector/evaporator outlet are realized.Therefore the proposed control strategy is conducive to the safe,stable and efficient operation of the system.Through the experimental test of the DX-SAHP system under various working conditions throughout the year,the proposed frequency conversion control strategy is verified and analyzed.Under the sunny day or cloudy day conditions,the system COP is higher than 4.0 and 3.0,respectively,and even in extremely bad weather conditions,the COP is still higher than 2.5.In addition,the expected heating time is in good agreement with the experimental data of the actual heating time,the error is within 20 minutes.Thus,the DX-SAHP system using the proposed frequency conversion control strategy can obtain a higher COP and control the heating time effectively under different operating conditions.In addition,the variation characteristics of various operating parameters of the system at the constant speed and variable speed mode are analyzed,respectively.That is the compressor speed is negatively correlation affected by the solar radiation intensity,and the EEV opening is negatively correlation influenced by the compressor speed.And the variation trends of the compressor power,compressor speed and discharge temperature of compressor under different heating conditions are studied.With the increase of water temperature,the compressor speed increases gradually,and the corresponding electric power of the compressor increases sharply.The condensing pressure also increases,as does the discharge temperature of the compressor,while the evaporating pressure remains a relatively low level with smaller variations.