Study On Heat Transfer Characteristics Of Heat Source Tower Under Low Temperature Conditions And Its Counter-Seasonal Application

Heat source tower(HST)heat pump has attracted more and more attention from researchers because it combines the advantages of air source heat pump and water source heat pump.It mainly adds a layer of heat carrier medium(antifreeze)between the evaporator of air source heat pump and air,which allows the HST heat pump to effectively prevent frosting problems.In this paper,the heat and mass transfer characteristics of open-type cross-flow heat-source tower under low temperature conditions are studied.The corresponding mathematical calculation model is established and the distribution of the temperature and humidity ratio of the air and solution in the HST is studied.The influence of air and solution parameters on the heat transfer performance and the optimum liquid-gas ratio of the HST is also analyzed.In addition,the heat and mass transfer performance of the HST using volatile organic solution as antifreeze is studied.The different effects of the volatile solution and the nonvolatile solution on the heat absorption of the HST are compared.In view of the problem of the solution concentration drop in the HST heat pump,a method of storing antifreeze in winter and regenerating it in summer without additional energy consumption is proposed,and the feasibility of this method is calculated and analyzed by taking the climate in Nanjing as an example.In order to solve the bottleneck problem of soil heat balance in ground source heat pump application,a HST is used to absorb heat from the air to store heat in the soil to achieve the heat balance in the application of the ground source heat pump throughout the year.The feasibility of this method is calculated and analyzed by taking the climate of Changchun as an example.Through the above research,the following conclusions can be drawn as follows:(1)When the inlet gas-liquid temperature difference is low and the humidity ratio difference is large,the solution temperature will be higher than the air temperature in some parts of the tower.It means that the sensible heat transfer direction is transmitted from the solution to the air,which is unfavourable for the HST to absorb heat from the air.The change of the solution mass concentration has a great influence on the optimal liquid to gas ratio of the HST.During the process of increasing the solution mass concentration from 0.15 to 0.40,the optimum liquid-gas ratio increases from 0.320 to 0.403,an increase of about 25.9%.In contrast,changing the relative humidity of the air has little effect on the optimum liquid to gas ratio of the HST.(2)When a kind of solute volatile solution is used as the antifreeze solution,the heat absorbed by the HST from the air rises slowly as the ambient temperature decreases.Among them,as the ambient temperature decreases,the latent heat absorption resistance caused by the solute volatilization is greatly reduced.By comparing the latent heat of the solute volatilization with the water vapor condensation,it can be found that the latent heat transfer increases with the decrease of the ambient temperature.The increase of latent heat transfer reduces the temperature gradient between the solution and the air,thereby reducing the sensible heat transfer of the HST.Compared with the air-cooled heat pump,the heat transfer of the HST heat pump decreases more slowly with the decrease of the ambient temperature.Compared with other evaporator of air source heat pump,the HST absorbs less sensible heat from air,and has obvious advantages in latent heat absorption under low temperature(below 3 C).(3)When the solution is concentrated and regenerated in summer,the temperature and humidity ratio of the air and solution change little in most areas of the tower.At the border of air outlet and solution inlet(upper right corner),the temperature and moisture content of air and solution vary greatly.In the calculation of Nanjing as an example,the continuous replenishment and the non-continuous replenishment of the solution have little effect on the regeneration time.According to the storage capacity of dilute solution in winter(17.69 tons)and the temporarily variable cooling load in summer,the regeneration of all solutions can be completed in about 8 days under the premise of the system running 10 hours a day in summer.(4)Calculating and comparing the accumulated heat storage during the maximum daily mean temperature and humidity in July in Changchun area when using HST to store heat in soil,we find that the humidity ratio has a very important effect on the total heat storage,especially on the latent heat.When the water flow rate of each buried pipe is 1 kg/s and the liquid to gas ratio of the HST is 1:3,the average heat storage capacity of each buried pipe will decrease with the increase of the water flow rate in the HST,but the reduction is not large.Based on the average meteorological parameters in July,when the water flow in the HST is 5 kg/s,the heat storage demand of 1000 m~2 buildings in the severe cold area can be completed in about 34 days,so as to achieve the annual soil thermal balance of the ground source heat pump.The results show that the energy efficiency ratio(COP)of heat supplement is 27.54.Assuming that the COP of the ground source heat pump system is 4.0,the heating COP and annual COP of the ground source heat pump system are3.605 and 3.677 respectively when the HST is used to solve the problem of soil thermal balance.Therefore,in order to ensure the year-round soil heat balance,it is efficient and low cost to use the energy tower to heat the soil out of season.