| In recent years,with the promotion of “coal to renewable energy” project in our country,the application of quasi-two stage compression air source heat pump has been expanded quickly in cold regions such as North China,and good energy-saving effect has been obtained.Meanwhile,the project also promoted the popularization of air source heat pump from constant-frequency compressor to variable-frequency compressor,namely the quasi-two stage variable-frequency air source heat pump(QTVASHP).However,when used in severe cold regions,owing to the lower outdoor temperature and the regulation of heating capacity,QTVASHP encountered the lowtemperature adaptability problem,variation of frosting region and slow defrosting speed problem.To promote its application in severe cold region,in this paper,the above three problems were investigated through the combination method of experiment and numerical simulation,and its heating performance was boosted.To achieve the goals of present study,a quasi-two stage variable-frequency airwater heat pump(QTAWHP)test rig and a quasi-two stage variable-frequency variable refrigerant flow heat pump(QTVRF)system test rig were built in Harbin.For the low-temperature adaptability problem,a segmented control strategy for suction refrigerant state based on outdoor temperature was proposed,and applied in the two test rigs.Based on this control method,the heating performance of the two test rigs was experimental investigated when the outdoor temperature was-28.4–19.1 ?.By adopting the segmented control strategy for suction refrigerant state based on outdoor temperature,the heating performance of the two experimental units was improved evidently.Even if the compression ratio increased to 11.38 when the outdoor temperature and supply water temperature reached-27.5 ? and 50 ?,the discharge temperature of QTAWHP was just 112.0 ?,still lower than the highest allowable temperature of 120 ?.Both the two experimental units could work steadily,and good heating performance was obtained.The seasonal coefficient of performance of QTAWHP in typical meteorological year is 2.38,a little higher than the 2.34 of QTVRF.During the above experiments for heating performance at low temperature,it was found that the variable frequency regulation to adapt to the building heating load could affect the surface temperature of outdoor coil obviously,and further affect the frosting condition.To understand the reasons,on the premise of considering heating load,the operation characteristics of QTAWHP were investigated through summarizing its running performance under frosting condition in the two heating seasons of 2017–2019.Then a new frosting map was developed.The frosting map of variable-frequency air source heat pump in severe cold regions had three obvious characteristics: 1)the critical outdoor relative humidity of frosting decreased with outdoor temperature dropping;2)the upper and lower limit temperatures of frosting region were both lower;and 3)the critical outdoor relative humidity of frosting was far higher.The frosting duration of QTAWHP in severe cold region is long overall(59–462 min),and the fastest frosting speed appeared at the outdoor temperature of approximately-10 ?,but not at 0 ? as previously thought.When analyzing the reasons for the variations of frosting map,it was found that apart from the variable frequency regulation mentioned above,the variations could also be affected by the climatic region and the capacity of experimental heat pump unit.To make clear their effects,a mathematic model of QTAWHP was developed.The frosting region of QTAWHP in cold region,hot-summer and cold-winter region,and different capacities of heat pump unit(90%-150%)was investigated numerically,and the frosting maps in these two climatic regions and different capacities of heat pump unit were obtained.Results showed that when an air source heat pump worked in different climatic regions,its frosting region varied significantly.The main variations were that with the decrease of the outdoor design temperature for heating,the critical outdoor relative humidity became higher,while the upper and lower limit temperatures of frosting region both became lower.The frosting time in the whole heating season would decrease with the increase of the capacity of heat pump unit,and the variation is more obvious in high relative humidity region.To speed up the defrosting speed of QTVASHP,the technology of defrosting with vapor injection was proposed without changing its existing refrigeration cycle structure.Vapor injection during defrosting could increase the displacement of compressor and further improve the quantity of heat brought by discharge refrigerant.Adopting the combination method of theoretical analysis and experiments,the feasibility and effect of this technology were investigated.Results showed that during the defrosting process with vapor injection,there was sufficient pressure difference to inject refrigerant into compressor,and wet compression could be avoided as the injection refrigerant was superheated when the opening of injection electronic expansion valve was appropriate.Vapor injection was beneficial for shortening the defrosting duration,decreasing the power consumption,and increasing the defrosting efficiency.At the optimal opening of injection electronic expansion valve,the defrosting duration was shortened by 20.61%;the heat absorbed from water and power consumption decreased by 8.74% and 17.98%,respectively;while the defrosting efficiency increased by 6.22%.The results in this paper provides practical technical solutions to improve the heating performance of quasi-two stage variable-frequency air source heat pump at low outdoor temperature.These solutions provide solid foundations for its application in severe cold regions.This project is part of the “Innovation and operation rules of the convective phase-change energy storage system for solar assisted air source heat pump”(No.51878209),which is supported financially by the National Natural Science Foundation of China. |
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