Thermal Performance Of Ground Heat Exchanger Research Articles

Numerical approach for sizing vertical ground heat exchangers based on constant design load and desired outlet temperature

Vertical ground heat exchangers (GHE), as part of a ground source heat pump system, have been utilized for energy extraction/injection from/to the ground, i.e. a renewable source of energy, in the heating/cooling months to reduce building energy utilization. The current available GHE sizing methods are mostly based on several over-simplified one or two dimensional analyses, which cannot predict its thermal behavior with the accurate and realistic boundary conditions or in series configurations. In this study, thermal performance of vertical GHEs is numerically investigated at a given constant load through solving full transient 3D Navier-Stokes equations inside the U-tubes and the energy equation all over the computational domain, i.e. both fluid and solid parts. There are three interconnected parameters, namely GHE load, the heat transfer fluid outlet temperature, and the GHE depth that influence its thermal performance and consequently, the required investment cost. Therefore, the thermal performance of GHEs is computed for different constant loads at different depths, and it is shown that at each constant load, there is a trade-off between the investment cost and the thermal performance. Furthermore, the GHEs thermal performance in parallel and series configuration is investigated at a fixed depth and different loads. In light of the obtained data, the required size of the GHEs for a test building with an imbalanced energy extraction/injection rate is derived and the thermal performance of the proposed cluster of GHEs is numerically simulated for 20 years.

Laboratory study on the thermal performance of vertical U‐tube ground heat exchanger during short‐term borehole thermal energy storage ( BTES ) and heat extraction process

To explore the thermal performance of vertical U-tube ground heat exchanger (GHE) during short-term thermal energy storage (TES) and heat extraction process, a novel and well-controlled sandbox with a plate heat exchanger at the bottom of GHE was proposed and established. According to the experiments of short-term TES and heat extraction under different inlet temperature with Re between 2677 and 2823, we found that the thermal performance of GHE was mainly determined by the dynamic soil temperature distribution around GHE, which was induced by the inlet temperature and operation process of GHE. Furthermore, the energy was concentrated within a limited radius (0.4 m) around the GHE during the 8 hours TES and heat extraction process, which was of benefit to the efficient operation of GHE and the thermal environment of soil. Moreover, compared to the simple extraction process C10 (−/8°C) without TES, the heat extraction process of C5 (35/8°C) obtained a 47.8% increase of average heat extraction rate. In addition, approximately 35% of the injected heat was left in the experimental soil under thermal balance conditions C3, C5, and C7. These results show the superiority and characteristic of short-term TES and heat extraction for GHE operation.

A numerical study on utilizing horizontal flat-panel ground heat exchangers in ground-coupled heat pumps

Horizontal ground heat exchangers (GHEs) have a lower cost in comparison to vertical ones; however, they require more land area. For the sake of reducing the required land area for horizontal GHEs, flat-panel GHEs have been recently introduced. In this piece of research, a 3-D numerical model has been developed to predict the thermal performance of flat-panel GHEs. In this study, at first, the thermal performance of a flat-panel GHE has been compared with that of linear horizontal ones. The results demonstrate that the flat-panel GHE has a better thermal performance of about 27% in comparison to an arrangement of linear GHEs with a value of 0.5 m for pipe spacing. In the second part of this study, the effects of three different parameters including soil thermal conductivity, buried depth, and the width of the flat-panel GHE on the thermal performance of the GHE have been investigated. The obtained results show that even though increasing buried depth enhances the thermal performance, the rate of enhancement decreases at the same time. Furthermore, it is established that increasing width is not of great significance when the GHE is more than 10 m wide.

Numerical investigation on thermal performance of ground heat exchangers using phase change materials as grout for ground source heat pump system

In order to solve the increasing deterioration of the energy shortage problem, ground source heat pump (GSHP) systems have been widely used. The ground heat exchanger (GHE) is a significant component of GSHP systems and the backfill materials have a dominating effect on thermal performance of GHEs. Therefore, phase change materials (PCM) were proposed to use as backfill material instead of common materials to improve the performance of GSHP systems. This paper first established a finite element model to investigate thermal performance of GHE, which was verified by experimental data of other researchers. Evaluation methodologies, such as effectiveness and thermal effects radius, were then proposed. The thermal performance of four backfill materials, including soil, paraffin RT27, acid and enhanced acid, was discussed, respectively. Finally, the effects and the combined effects of initial ground temperatures and pipe spacing on thermal performance of GHEs were studied. The results show that PCM has certain advantages over ordinary soil, because of its small thermal effects radius and consistent temperature in the phase change process. Meanwhile, the lower initial ground temperature and the larger pipe spacing of U-tube were beneficial to heat transfer, while it increased thermal effects radius and decreased liquid fraction of PCM. Furthermore, the thermal effects radius growth rate with pipe spacing increased with an increase in initial ground temperature.

The importance of surface air temperature fluctuations on long-term performance of vertical ground heat exchangers

Ground source heat pump (GSHP) systems efficiently heat and cool buildings using geothermal energy accessed via ground heat exchangers (GHEs). The thermal performance of GHEs is usually studied with equivalent groups of linear heat sinks/sources or with more advanced numerical models. In most cases, in regions with common geothermal gradients, the ground temperature is assumed to be constant with depth, a simplification that is reasonable for typical borehole GHEs which are at least 50 m deep. However, the increasing use of piles (and other geo-structures) as GHEs, which are typically shallower than borehole GHEs requires a re-assessment of the thermal effects of the surface air temperature changes on GHEs. This work introduces and addresses such issues using the newly developed 3D detailed numerical model of GHEs and highlights its significant influence on thermal performance. Numerical results show that in the cases studied herein, accounting for the surface air temperature fluctuations may result in up to about 11% shorter GHEs than if these effects are ignored.