地源热泵系统中短期供热性能衰减和长期土壤热不平衡影响着其实际运行效果。为此，本文建立兼顾快速性和准确性的通用型地下换热器分布参数模型，揭示多种因素对钻井换热的影响规律，指出现有地埋管设计存在的问题并提出解决方案；提出了新型复合地源热泵系统及耦合供暖方案，优化了系统设计及运行策略，提高系统可靠性、节能性和经济性。具体工作如下：(1) 建立基于响应系数的通用型地下换热器分布参数模型(RF模型)。定义了用于土壤导热的响应系数，并将其与埋管内流动传热和蓄热体边界导热结合建立了瞬时隐式热源热流求解方法，进而提出了适用于任意时间步、多工况快速准确计算的RF模型。通过合理的边界位置选取、热源数目研究简化了计算工作量，对比发现埋管热源分段数目可为2~4段，各蓄热体边界可各自当作一个整体热源。(2) 对所提地下换热器分布参数模型进行了实验验证及换热特性分析。搭建砂箱实验台对模型进行了验证，并进行了稳态解校核；对比短时间步和变水温工况的埋管出口水温，表明RF模型相对于数值解误差小于0.2 ºC，叠加计算耗时远低于数值解；分析了钻井内部温度分布及不同类型地下换热器的换热特性。(3) 利用RF模型开展地下换热器短期换热特性及设计方法研究。建立基于RF模型的系统模型，分析持续换热产生短期土壤温度分布不均和系统性能下降的问题；指出现有经验估算法存在的不足，总结不同影响因素下的钻井换热特性图，提出图表估算法指导工程设计；分析了钻井换热测试存在的误差并提出修正方法，分析修正后的系统运行效果。(4) 针对大负荷下地源热泵性能衰减、供热不足的问题，提出补热机组耦合供暖运行模式。搭建了系统逐时模型用于系统设计和长期运行模拟，并对耦合供暖启动温度进行了优化。在维持土壤热平衡的同时，耦合供暖使得尖峰负荷的供热能力大幅提高，且钻井数量减少40%、系统COP提高17.4%、初投资降低13%。(5) 为进一步提高补热系统的经济性，构建了空气源补热器型地源热泵系统。建立了系统逐时模型、提出系统设计方法并分析了不同地区的适用性。空气源补热器直接补热平均COP达12.0，初投资仅占系统2~3%，不同地区相对于传统系统节能24%以上。另外，空气源补热器的间歇供暖可促进地温恢复，使供暖期埋管壁面温度最大提高9.1ºC，机组供暖COP提高0.59，有效提高短期供热性能。
The practical application of ground-coupled heat pump (GCHP) is limited by the decline of short-term heating performance caused by continuous heat extraction and the imbalance of long-term soil thermal exchange caused by the unbalanced heating and cooling loads. Therefore, a fast, accurate and general ground heat exchanger model with distributed parameters is proposed in this paper. Based on the model, the law of borehole heat capacity influenced by several factors is discovered, the problems of two common design methods of ground heat exchanger are pointed out and the corresponding solutions are proposed. Besides, a new hybrid GCHP system and a coupled heating strategy are proposed. In addition, the system design and operation strategy are optimized to improve the system reliability, efficiency and economy. The specific work is as follows:(1) A general ground heat exchanger model with distributed parameters (RF model) is proposed based on the response factors. The response factor used for soil thermal conduction is defined and combines with the heat transfers inside the pipes and across the soil boundaries to calculate the transient heat fluxes of all the heat sources in soil. RF model is built for the fast and accurate calculation of ground heat exchanger with ambient time steps and under variable working conditions. The positions of soil boundaries and the suitable numbers of sub heat sources are investigated to simplify the calculation. The analyses show that the numbers of sub heat sources of a U pipe could be 2~4 and each soil boundary can be regarded as a whole heat source respectively.(2) The RF model is validated and its characteristics are analyzed. The sandbox experiment is established to validate the RF model, and the steady solutions of the RF model are verified. The errors of borehole outlet temperatures calculated by the RF model are less than 0.2 ºC for both short term periods and variable inlet temperatures. The inner heat exhange characteristics of a borehole are analyzed based on RF model. The heating characteristics and soil temperatures of different kinds of ground heat exchangers are also explored.(3) The characteristic of short-term heat transfer of ground heat exchanger is investigated and the common design methods of boreholes are analyzed. The GCHP system model based on the RF model is built, through which the unhomogeneous distribution of soil temperature and the decline of heating performance are revealed. The shortcoming of the design method based on the rule of thumb is pointed out, the characteristic diagrams of the borehole heating capacity influenced by several factors (excess inlet temperature, velocity, operation time, thermophysical properties of soil) are discovered, and the design method based on these diagrams is proposed to guide the borehole design. The errors of thermal performance testing of boreholes are analyzed, the correction coefficients are proposed and the performance of system designing with modified boreholes is investigated. (4) The coupled heating mode of heat compensation unit is proposed to improve the heating performance of GCHP system under the large heating loads. The hourly system model is established to conduct a long-term simulation and to optimize the starting temperature of the coupled heating mode. Apart from keeping the soil thermal balance, the coupled heating strategy can improve the system heating performance at peak loads, being able to reduce the borehole number by 40%, to improve the system COP by 17.4% and to reduce the initial cost by 13%.(5) The novel GCHP assisted by an air source heat compensator is proposed to improve the system efficiency and economy. The hourly system model is established, the system design approach is proposed and the applicability of the system is analyzed. The average COP of the air source heat compensator is 12.0 in its direct heat compensation mode. The initial cost of the air source heat compensator is accounted for 2~3% of the whole system. The energy saving rate of the system can exceed 24% in relative to the conventional system. In addition, the intermittent heating of the air source heat compensator is favorable to the recovery of soil temperature, with an increase of 9.1ºC for the pipe wall temperature and an increase of 0.59 for the unit heating COP, improving the short-term heating effectively.
