Comparation And Research Of Borehole Heat Exchangers Heat Transfer Analysis Methods

As an environmentally friendly and energy saving technology,ground source heat pump(GSHP)has been applied widely.Borehole heat exchanger(BHE)is one of the most important parts of GSHP system,and its heat transfer affect the operation of a heat pump system directly.Therefore,the research on borehole heat exchangers heat transfer process has always been an important topic in this field.Nowadays,the methods to analysis the heat transfer process of BHE with their own advantages and disadvantages are various.At present,many studies regard the heat transfer of each borehole as consistent and the influence of boreholes position on heat transfer is usually neglected.However,this kind of method cannot calculate the temperature of circulating fluid in the buried pipes and the effect of circulating water temperature limits on GSHP system operation is ignored.Based on the finite line source model and quasi-three-dimensional model,a modified calculation method is proposed in this paper.The inlet water temperature is determined by trial calculation,which makes the heat drawn or released of each borehole fitting their own heat transfer conditions and as well as promises the total heat flux of the BHE meeting the load demand of GSHP systems.In this method,the circulating water temperature limits of GSHP is considered,i.e.the inlet water temperature of boreholes can only rise or decrease when it does not reach the temperature upper limit or lower limit.When circulating water temperature reaches its limit,the heat transfer of BHE may not meet the building air conditioning demand.This modified calculation method is compared with other two kinds of methods,by which the boreholes inlet water temperature,boreholes wall temperature and the BHE heat flux are calculated and analyzed.The results show that the method proposed in this paper illustrates the BHE heat transfer process more closer to reality compared with the other two methods.Results also indicate that there may be a large deviation in the long-termoperation if the circulating water temperature limits are not considered.The effects of soil thermal properties and GSHP system parameters on the BHE heat transfer are also investigated in this paper.The results show that if the winter and summer load is imbalanced,circulating water temperature limits should be considered,particularly in those area where the soil thermal conductivity or volume specific heat is relatively small When circulating water temperature limits are considered,the GHE design deviation can be avoided which means the GSHP system can meet the demand of building air conditioning even after long-term operation.This paper recommends that engineers can enlarge the boreholes space and chose the heat pump unit with larger circulating water temperature ranges when designing GSHP systems to alleviate ground thermal accumulation caused by imbalanced loads in summer and winter.