| Ground source heat pump(GSHP)technology has been considered as an important way of exploiting shallow geothermal energy for air conditioning,and it has gained a rapid development in the past several years in China.Due to the regional characteristics of the technology,when it is applied in Chongqing area,it is necessary to study the suitability and application effect of this technology.Firstly,the stratus in Chongqing area is mainly bedrock geological conditions composed by sandstone and mudstone,which is different from the homogeneous soil of plain area.There exist specialties in the temperature distribution characteristics and the underground heat transfer characteristics etc.;Secondly,Chongqing is located in regions which is characterized by hot summer and cold winter,where the accumulated cooling load of air conditioning system is far outweigh than the accumulated heating load.There exists serious heat accumulation problem in the application of conventional GSHP system in this area.In this paper,we have carried out a series of related research on problems that from the heat exchange under the ground to the air conditioning over the ground.Firstly,the paper focused on the analysis of the influences from the test duration on the soil thermal conductivity under bedrock geological conditions during thermal response test(TRT)process.According to the analysis of the measured data,the optimal test duration of 60 h and 50 h,and also the abandoned initial hours of 20 h and 10 h for single-U and double-U were obtained,respectively.Besides,the paper also studied the problem that,when the waiting time between the two successive tests is not long enough for the ground temperature to recover to the original ground temperature,the subsequent TRT may be affected by the residual ground temperature field of the previous test.A new computational method,based on linear superposition theory,was proposed to solve the problem,and the results demonstrate the accuracy and reliability of the proposed method.Secondly,a temperature monitoring system was established in the borehole area of a GSHP system,and the original ground temperature field in time and space distribution was obtained before the system put into use.Based on the monitored data,the periodic variations and amplitude of the underground temperature was cleared,and also the geothermal variation with depth.The variational temperature zone(0~15 m)and the constant temperature zone(under 15m)were divided through the analysis of the ground temperature.After that,the formation of the underground temperature field was analyzed in theory and validated by numerical simulation.After the GSHP system was put into use,the change of the underground temperature field was analyzed.The monitored results show that the ground temperature presents an overall upward trend due to the fact that cooling load of air conditioning system is greater than the heat load.However,the ground temperature variations at different depths are different for the reason that the distribution of the strata is uneven.Based on the actual stratum distribution,the numerical model of non-homogeneous borehole was established,and the TRT was simulated by CFD software,and then,the influence of the stratum distribution to the thermal response of the ground temperature was analyzed.In addition,the GSHP system was monitored with a data acquisition system and the measured data were analyzed,including the inlet/outlet water temperatures,power consumptions,and also EER/COP.The measured system EER of 2013 and 2014 are 3.01 and 2.91 in cooling season,respectively,while the system COP of 2013 in heating season is 2.59.This demonstrated the fact that the system cooling performance declined due to the increase of the ground temperature.A TRNSYS model was established based on the studied GSHP system.The accuracy of the model was validated by the comparison between the measured and the simulated outlet water temperatures of the heat exchanger,and also the ground temperatures.The model was then used to predict the long-term performance of the GSHP system.It has been found that after 20-years running of the system,the heat accumulation under the ground and the deterioration of system efficiency aggravate.At last,for the existing serious thermal accumulation problem of the system,the auxiliary operation of cooling tower is introduced for the system.It has been found that,the cooling tower should be put into use in the third year and the optimum auxiliary cooling ratio is 30%.Basing on this conclusion,the optimum control parameter of ΔT=6 ℃ and To/Tc=30/28 ℃ were confirmed for the temperature difference control strategy and the fixed temperature strategy,respectively.Besides,a scheme that the cooling tower being operated in transitional(mid)seasons was proposed,and also a hybrid GSHP system combined with a chiller cooling system,and their applicability are validated and discussed by TRNSYS software in the paper. |
PDF Full Text Request