摘要
北京某住宅小区设有412个钻孔埋管换热器,自2014年冬季开始采用地源热泵系统为小区内的高层住宅和联排别墅供暖制冷.至2020年11月,埋管区域的地温已从最初的14.78℃降至13.00℃,而且系统在冬季的稳定运行也存在一定风险.该文整理此系统2014-2020年的运行数据,对地源端供回水温度、冷热负荷及系统运行效率进行分析.在利用单孔3-D有限元模型获得土层综合热物性参数后,基于所建立的群孔2-D有限元模型,结合系统冷热负荷特征及地源端换热能力,综合考虑地温变化、运行中的停机状况、地源端供水温度及系统取/排热量等几个方面,对系统的长期稳定性进行评估和预测.结合小区复合供暖系统的特点,对3种运行策略(冬季停机1个季度、增大夏季用量和冬季采用燃气锅炉调峰)进行比较,结果表明采用燃气锅炉调峰是目前最合适的应对地温降低和冬季运行不稳定的措施.
Abstract
[Objective]In the context of China's promotion of the development and utilization of renewable energy,ground source heat pump(GSHP)systems based on shallow geothermal energy has been extensively used.With increasing service life of GSHP systems,the risk increases if the operation and maintenance are not properly performed.The operation analysis and operation strategy optimization of existing large-scale GSHP systems are of great value in standardizing the design and operation management of GSHP systems.[Methods]In this work,the GSHP system of a residential community in Beijing is used as a case study.The heating,ventilation and air conditioning(HVAC)system in the community is a compound system,which is composed of a GSHP system,a chiller,a cooling tower and a gas-fired hot water boiler.The GSHP system has been used for heating and cooling buildings since 2014 with no other equipment.According to its operational records from 2014 to 2020,the current operating characteristics and rules of the system are summarized.A 3-D finite element model(FEM)was developed to perform back analysis of the soil thermophysical parameters.A 2-D FEM was designed to analyze the heat transfer of the buried heat exchanger group.The characteristic parameters of the heating and cooling loads of the GSHP system were determined by comparing the 2-D numerical results with the operating data.Furthermore,using 2-D FEM,the changes of ground temperature in the borehole area,shutdown situation,and water supply/return temperature of the ground source end were predicted for the next 5 years for optimal operation strategies.Finally,the long-term stability of the system was assessed using these strategies.[Results]As an introduction,there are two GSHP subsystems in the residential community,one(#53)with 108 boreholes and the other(#54)with 304 boreholes.Each borehole is 120 m deep,15-18 cm in diameter,and 3.6 m in spacing.The operational data involved the supply and return water temperature and flow velocity in the ground source.From 2014 to 2020,every year,heat extraction was higher than heat rejection.For #54,the average heat imbalance rate reached 16.2％from 2017 to 2019.The ground temperature in the borehole area decreased from 14.78℃to 13.00℃.For the #54 system,the results of the 2-D FEM analysis revealed the following.(1)Using current operational measures to continue for 5 years,it was observed that the temperature of the water supply would still be at a low level in winter,there was a risk of shutdown,and the heat extraction and output level would be further reduced.(2)Three possible operation strategies were predicted:(a)stop the operation of the GSHP system in one winter season;(b)increase the use of the GSHP system in summer(considering the two unbalance rates of 5％and 10％);and(c)guarantee a certain source of heat and use gas-fired boiler peak regulation.All three measures alleviate further decreases in ground temperature and enhance downtime.The heat imbalance rate is maintained at 5.0％-8.3％for(b)and(c).[Conclusions]In this case,because of imbalanced heating and cooling loads,after the GSHP system had been running for several years,the ground temperature had decreased and energy efficiency of the system had decreased in winter.The operational strategy requires adjustments.In view of the change in ground temperature,shutdown situation,water supply temperature at the ground source,and heat extraction/rejection in the next 5 years,using the gas-fired boiler set in the composite system is the most appropriate operation strategy.
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