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Ground (or geothermal) heat exchangers are attracting a great deal of attention as a way of using shallow geothermal energy. This paper provides not only a critical review but also a thorough introduction to the analysis of heat transfer by borehole and foundation pile ground heat exchangers, with an emphasis on different analytical models. The literature is reviewed in a time-scale framework because of the diversity of the time and space scales involved in the thermal processes of ground heat exchangers. We summarize, discuss, and evaluate major advances in this field, including heat-source models, short-time models, models for energy piles, in situ thermal-response tests, indoor sandbox experiments, and parameter estimation as an inverse problem. Of particular note is that the unit-step temperature response (i.e., G -function) of a ground heat exchanger with one U-shaped pipe is calculated; and six analytical models are compared: an infinite cylinder-source model, two infinite line-source models, two finite line-source models, and a composite-medium line-source model. This paper closes by identifying several unsolved problems that require solutions. [ABSTRACT FROM AUTHOR]

The ground and grout thermal parameters are very important for the Ground-Coupled Heat Pump system (GCHPs) design and simulation. An accurate ground and grout thermal parameter will result in a reasonable GCHPs design and operation control strategy. In this paper, a new transient ground and grout parameter estimation method was proposed. In addition to the ground thermal parameters, the transient borehole thermal resistance and the grout thermal parameters were firstly estimated by analytical models with the proposed method. Compared to the measured values by the thermal probes, the parameters estimated by the proposed method was reasonable and much more accurate than the conventional method. Accompany with the transient borehole thermal resistance estimated by the proposed method, the fluid temperature predicted by the ILS model was acceptable for both the early regime and late regime, and the maximum relative error for the fluid prediction result was up to 2.82%. [ABSTRACT FROM AUTHOR]

Horizontal geothermal heat exchangers (HGHEs) with vertical spiral coils have been increasingly used in ground source heat pump (GSHP) systems. Research studies have been conducted using field tests and simulations trying to reveal the heat transfer mechanism of HGHEs with vertical spiral coils and to provide a good heat transfer performance. However, there is no analytical solution has been established for HGHEs with vertical spiral coils. Of particular interest is the temperature variation at the soil surface. The study presented in this paper proposes the development of a mathematical vertical ring-coil model used to describe the heat transfer process of HGHEs with vertical spiral coils. In this new model, the spiral heat exchanger is simplified as a series of ring coils that inject/extract heat in/from a semi-infinite medium. A single ring model in an infinite medium is first introduced. Using images and superposition, a multiple ring-coils analytical solution is then given. The significant influence on the heat transfer performance of HGHEs temperature variation of the ground surface is then considered. A periodically changed boundary condition is being taken into consideration in the modeling process. The validity of the model was examined by a field test and a 3-D simulation model. The analytical solutions may provide a desirable and better tool for the simulation/design HGHEs with vertical spiral coils. [ABSTRACT FROM AUTHOR]