| The utilization of solar energy for clean heating can help reduce the consumption of fossil fuels,alleviate the combustion heating haze,promote energy conservation and environmental protection.Solar-assisted absorption heat pump(AHP)is an effective technology of solar clean heating,but is faced with problems like high driving temperature,low solar fraction and short working time.An absorption-resorption heat pump(ARHP)can be built by replacing the condenser and evaporator in a conventional absorption heat pump(AHP)with a high-pressure absorber and a low-pressure generator,respectively,which can efficiently reduce the working pressure and heat source temperature demand.Compared with thermal-activated(GAX)AHPs,the solar fraction and operation time for solar-assisted ARHPs are greatly lifted,which can effectively realize multiplication of the input solar heat at 70-150~oC for space or water heating output.On this basis,how to expand and transform the ARHP cycle by means of internal stage enhancement and multiple heat recovery,and external coupling is important to lift their adaptability to various heating terminals and colder climate condition,which is also the technical difficulty of promoting ARHPs to efficiently use low-temperature heat source for wider-territory space and water heating applications.Based on solar-assisted ARHP for winter heating,the following main research work is carried out:(1)In order to reduce the driving heat source temperature and lift solar fraction,a novel kind of ARHP cycle were put forward.The mechanism of internal multiple heat recovery for performance improvement and pressure differential expansion for stage enhancement were revealed.A single-stage balanced-type ARHP cycle and a two-stage GAX-based ARHP cycle with multiple internal heat recovery were developed.Theoretical models and evaluation criteria were establised based on thermodynamics and solar-to-heat conversion methods.The coupling selection mechanism of working medium concentration and working pressure was expounded.The effects of working pressures,ambient temperature,and driving heat source temperature on the main performance parameters of the cycle were studied.Compared with the traditional AHPs,the driving heat source temperature of the ARHP is significantly lower,which realizes the utilization of low-temperature solar heat above 70 ~oC for efficient space heating,and greatly improves the solar energy guarantee rate and working time.(2)By integrating with a vapor compression heat pump(VCHP),two kinds of coupled heat pumps were put forward based on the single-stage ARHP by means of subcooling and cascade coupling.The feasibility of using them for space heating in cold climate driven by low-temperature heat source was studied.For the subcooling coupled heat pump,the condensation heat is used for space heating,so the condensation temperature is high.The adaptability to cold climate condition is restricted by the single-stage pressure ratio.For the cascade coupled ones,the condensation heat is used for low-pressure generation process,which requires a lower condensation temperature.The evaporation temperature of the cascade coupled heat pump using R410A as the working fluid can be greatly extended down to-35 ~oC.The lower the evaporation temperature is,the more significant the performance advantage of the cascade coupled heat pump will be.The two kinds of coupled heat pumps both reduce the low-pressure generation temperature,so the driving heat source temperature can be as low as 70 ~oC,which can be effectively driven by low-grade heat sources such as solar heat.(3)A multi-mode ARHP experimental system was established and can be operated by single-stage and two-stage direct heating mode.The two modes were experimentally operated to summarize the operation law and control strategy.The experimental results show that the single-stage mode can be operated when the ambient temperature is above 0 ~oC and the heat source temperature above 90 ~oC,and the water supply temperature can meet the requirements of the floor radiant heating end.The two-stage mode can be operated at ambient temperature above 0 ~oC and heat source temperature above80 ~oC.The outlet temperature can adapt to the end of fan-coil heating.Comparing with the single-stage mode,the two-stage mode can achieve higher water supply temperature and heat output at the cost of COP reduction.The experiment results also verified the feasibility of the cycle and the accuracy of mathematical model.(4)With solar heating as the target,the performance evaluation indexes of solar-assisted single-stage,two-stage and two kinds of coupled heat pump systems are established.Based on the characteristics of driving heat source and ambient air temperature,the optimal selection of collecting temperature,collecting area and solar fraction is carried out when solar energy is matched with the four kinds of heat pumps.The collector type and installation area can be selected according to solar fraction and economical analysis.When the ambient temperature is-25 ~oC to 20 ~oC and the collecting temperature is in the range from 10 ~oC to 200 ~oC,the principle of"higher-efficiency use of high temperature and longer-time utilization of low temperature”solar heat is adopted to increase the proportion of solar heat in winter heating and reduce or totally avoid the use of fossil fuels.As a new type of heat pump driven by low-temperature heat source,the ARHP provides a new idea for increasing the contribution of solar energy in winter heating and an important supplement to the existing solar heating technical routes. |
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