Study On Geothermal Closed-loop Heating Systems Integrated With Deep Borehole Heat Exchangers For Heat Extraction And Energy Storage

In order to achieve China’s strategic goals of "Peak Carbon Dioxide Emissions and Carbon Neutrality",the existing energy structure needs to be changed and renewable energy should be vigorously developed.Geothermal energy has become an important part of the development of renewable energy due to its advantages of abundant reserves,cleanliness and stability.Focusing on the utilization of mediumdepth and deep geothermal energy,this study establishes an integrated system of “a geothermal well heat extraction,energy storage and heating",by coupling a deep borehole heat exchanger(DBHE),a heat pump and an energy storage device.The heat extraction performance and economy of the integrated system under different scheduled operation conditions have been analyzed.Two novel DBHE systems,namely “checkvalve type” and “three-layer casing type”,have been designed.The effects of the two systems for heat extraction(using the whole well section)in winter and for heat injection(using the upper well section for energy storage)in summer were studied.Through numerical simulation,it is revealed that this mode of heat extraction in winter and heat injection-and-storage in summer can better maintain the heat balance of the formation,reducing the "waste heat" discharged into the air in summer and alleviating the urban "heat island effect".In addition,considering different-depth formation conditions,performance of the DBHE system applied to deeper formation,integrated with a heat-and–power cogeneration system,has been analized in this study.Based on the research of different daily running hours of the DBHE system,it is found that the heat extraction rates of the DBHE system can be effectively improved by using the scheduled operation modes.The greater the mass flowrate of circulating water,the more obvious the improvement.For the fixed daily operation time,further scheduling the system operation time has little effect on the heat extraction rates.When the mass flowrate of the circulating water is 9 kg/s,the heat extraction rates of the scheduled operation system can be increased by about 115 %.By coupling with the heat pump,the heating rates of the system can be increased by about 16 %.Using the scheduled operation mode,combined with the application of the ground energy storage device as well as using the peak and valley electricity prices,the daily operation cost can then be reduced by about 30 %.The study on the heat extraction in winter and heat storage in summer of two novel DBHE systems shows that the energy-storage rate of the system in summer increases with the increase of the depth of heat injeation,and this rate is more affected by the heat injection temperature in summer.When the depth of heat injection in summer is 900 m and the temperature of heat injection water is 50 °C,the energy storage rate in summer is more than 50 k W during the five-year operation,which is about 27.4 % of the heat extraction rate in winter of that year.The thermal conductivity and the geothermal gradient of the formation have a great influence on the system’s heat extraction rates in winter and energy storage rates in summer.When the thermal conductivity is 3.25W/m·K,the system’s heat extraction rate in winter and energy storage rate in summe can reach 263 k W and 71 k W respectively.When the thermal conductivity is 1.25W/m·K,the system’s heat extraction rate in winter is only 119 k W and its energy storage rate in summer is 31 k W.When the temperature of the heat injection water in summer is 50 °C,the ratio of energy storage rate in summer to heat extraction rate in winter is around 27 %.When the temperature of the heat injection water decreases,this ratio shows a downward trend.Under the condition that the water temperature of heat injection is 50 °C,when the geothermal gradient is 20 °C/km and the depth of heat injection is 1400 m,the energy storage rate of the system in summer is 77 k W,which is about 60% of the heat extraction rate of heating in winter(129 k W);when the geothermal gradient is 40 °C/km and the depth of heat injection is 700 m,the energy storage rate of the system in summer is reduced to 39 k W,which is about 15 % of the the heat extraction rate of heating in winter(259 k W).The study on the integrated system of check-valve type DBHE coupled with heat pump shows that,when the temperature of heat injection water in summer is 40 °C,45 °C and 50 °C respectively,the decrease of the corresponding cooling power of the heat pump is about 19 %,21 % and 24 % respectively.The energy consumption of the working-fluid pump of the heat pump system changes with the change of heating and cooling powers of the system.In winter,the COP(coefficient of performance)of the heat pump is 4.38,the energy consumption of the working-fluid pump is about 22.9 %of the heat pump’s heating power.In summer,the energy consumption of the workingfluid pump increases with the increase of the temperature of the heat injection water.When the temperature of the heat injection water is 40,45 and 50 °C,the COPs of the heat pump system is 2.64,2.27 and 1.98 respectively,and the corresponding heat pump’s energy consumption is about 38 %,44 % and 50.6 % of the heat pump’s heating power.The geothermal gradient of the formation has an obvious influence on the ratio of summer’s cooling power to winter’s heating power.Under the condition that the temperature of the heat injection water is 50 °C and the depth of heat injection is 900 m,when the geothermal gradient is 30 °C/km,the cooling power of the system in summer is 18 % of the heating power in winter;when the geothermal gradient of is20 °C/km,the system’s cooling power in summer is 39 % of its heating power in winter.The study on the integrated system of heat and power cogeneration coupled with a DBHE in deeper geothermal wells shows that when the mass flow of the DBHE circulating water(heat extraction working fluid)is 1 kg/s,2 kg/s,and 3 kg/s,cogeneration should be adopted in accordance with the principle of cascade utilization of energy.It is shown that,when the mass flowrate is 1 kg/s,a maxmum net power output of electricity can be obtained using an organic Rankine cycle(ORC),which can meet the power consumption of the DBHE-cogeneration system.In the heating season,a mass flowrate of 9 kg/s should be used in order to get the maximum heating load.