Research On Optimization And Operation Strategy Of Air Source Heat Pump Water Heater

This paper concentrated on air-source heat pump water heater (ASHPWH) system fordomestic use. In experimental research, researchers pay more attention to the matching ofdifferent components in this kind of system; in theory analysis, most of the scholars havedone lots of work about matching, design parameters and estimation of thermodynamicperformance. However, there was little work reported about long-term operation optimizationof this kind of system.Additionally, ASHPWH are almost in off-design conditions during operation. Correctevaluation of thermodynamic performance of this system under variable working conditionsis the fundamental prerequisite for optimization of design and operation. While, there isinsufficiency when conventional exergy anlysis is applied on this kind of system. In order tobetter utilize the optimization tool—exergy theory in ASHPWH, it is expected that theweakness of design and operation can be correctly found based on the relative model whenthe basic exergy theory and thermodynamic performance of the system are combined. Thus,the emphasis should be taken on matching, components design and performance assessment,so as to find a more reasonable and comprehensive optimization method. As to meet differentdomestic energy consumption, some research work has been obtained by scholars. While forsome sites with different temperature domestic hot water consumption, the conventionalmethod is to meet hot water demand with several types of equipments, such as oil boiler, heatpump and solar concentrator. As a result, higher running cost and complex managementcannot be avoided. In order to solve this kind of problem and take ASHPWH’s advantages onenergy-saving, multi-function of hot water system should be integrated. Therefore, it is veryimportant to save energy and initial cost considering the thermodynamic performance of ASHPWH system. The main works are summarized as follows:(1) An experimental set-up of domestic ASHPWH system was constructed and someexperimental data were obtained. In this research, the thermodynamic performance of thesystem was investigated. Additionally, the mathematical model of this kind of system wasestablished, and the effect of environment and design variants was analysized. The simulatedresults showed that it is uncessary to enlarge the area of heat exchangers when COP increasesto some extant. Based on the case, the optimal thermodynamic performance can be achievedwhen the area ratio of condenser and evaporator is among given range0.1-0.118. It was alsofound that the effect of inlet site of hot water to water tank cannot be ignored, the higher theinlet site of hot water, the higher COP the system will obtain. When the inlet site of hot wateris lift from0.167height of water tank to the top, the COP will increase from2.58to2.70, with4.65%increment. Simultaneously, the average water temperature of water tank decreasedabout1℃, which can be ignored. From the viewpoint of improving thermodynamicperformance of system, the optimal site is on the top of the water tank.(2) In this study, an equivalent exergy analysis model was presented according to unstableworking condition characteristic of ASHPWH system and verified by experiments. In thismodel, the exergy loss of different components and stages could be analysized, and thepriority optimization goal of this system was pointed out. The analyzied result indicated that,based on the assumption, the exergy efficiency of system is very sensitive to the adiabaticcoefficient of compressor. The exergy loss of compressor would reduce2/3when adiabaticcoefficient of compressor increases from0.6to0.85under ambient temperature7℃, whilethe exergy efficiency will increase from35%to45.54%, with10.5%improvement. Thecompressor should be chief optimized when the adiabatic coefficient of compressor is lowerthan0.78. When the temperature difference of heat transfer on condenser decreases from8℃to3℃, and the exergy efficiency of system will increase from41.5%to46.2%, with4.7%improvement. Similarly, when the temperature difference of heat transfer on evaporatordecreases from15℃to10℃, and the exergy efficiency of system will increase from41.4%to46.3%, with4.9%improvement. It could be found that, based on the experiments, thehighest average exergy loss of component is from compressor, and the following is from condenser. From viewpoint of improving system thermodynamic performance and cost,improving the performance of compressor is a long-term process, thus, enhance the heattransfer process of condenser is more urgent, for example, optimize the design of condenserand water tank.(3) Based on the mathematical model of this ASHPWH system and validation, the effect ofdifferent parameters to the system performance was estimated. Additionally, the indicator ofoptimization operation strategy was presented and tested according to domestic hot waterdemand profiles. In this study, the influence of ambient temperature fluctuation, setting watertemperature, starting time and electricity price policy under two typical controlling patternswas also discussed. For the timing controlling pattern, the key factors are ambient temperaturedifference between day and night and electricity price policy. For the thermostatic controlpattern, the delivery coefficient was introduced and as an index which scales the degree ofsatisfaction for customers. The higher the setting temperature, the higher the deliverycoefficient is, while, it also leads to lower COP. In order to balance the meet of degree ofsatisfaction for customers and energy-saving concern,100%was selected as the critical pointof delivery coefficient to optimize the system performance. The optimized results showed thatthe optimal setting water temperature should be adjusted according to different seasons. Takethe climate of Shanghai for instance, it was investigated that the optimal setting watertemperature should be set above46℃,48.2℃and55.4℃, in summer, transitional season andwinter, respectively.(4) A new approach to energy consumption prediction of domestic air source heat pumpwater heater was presented based on grey system theory. Investigations with the modelindicated that grey system theory can be used to predict the domestic hot water heat andelectricity consumption, and satisfactory prediction accuracy can be obtained by the improvedgrey model. It can be found that, during the modeling, the samples could be applied in greysystem theory although they appeared unordered. Furthermore, the experimental resultindicated that correct accuracy can be assured only the data sample interval is no less thanfour weeks. Based on the improved model and weather data of Shanghai, the electricity costsaving, the monthly average heat and electricity consumption and the annual carbon emission reduction related to the use of the ASHPWH for the two typical families were evaluated andcompared with those of the conventional electric resistance water heater. It is noteworthy thatthe developed model requires few data in order to predict energy consumption and theprediction error is reasonable, hence some field test expenditure can be saved.(5) Combined with the experiments and simulation results, the air-sourced heat pumpdouble-stage water heating system was elaborated in this study. And then, the mathematicalrelationship between superheat and latent heat release in the condenser was analysized. Basedon the principle of thermodynamic law and theoretical model, the heat pump refrigerants wereselected and relative test-rig was established. In order to learn the thermodynamicperformance of this system, a new mathematical model of ASHPWH was developed andvalidated, in addition, the effect of different parameters to the system was investigated. Theexperimental results showed that the prototype could supply different temperature ranges hotwater and higher thermodynamic performance can be obtained. For instance, it can be appliedin the sites which consume domestic hot water and floor heating simultaneously. The resultsof modeling revealed that the most important factor which affected the system performancewas inlet water temperature of first condenser. Furthermore, the sensible heat could bereleased in the first condenser only when the inlet water temperature of first condenserincreased to certain critical value. With the increase of ambient temperature, the critical valuewould decrease simultaneously.