Critical Review on Efficiency of Ground Heat Exchangers in Heat Pump Systems

Abstract

Use of ground source heat pumps has increased significantly in recent years for space heating and cooling of residential houses and commercial buildings, in both heating (i.e., cold region) and cooling (i.e., warm region) dominated climates, due to its low carbon footprint. Ground source heat pumps exploit the passive energy storage capacity of the ground for heating and cooling of buildings. The main focus of this paper is to critically review how different construction and operation parameters (e.g., pipe configuration, pipe diameter, grout, heat injection rate, and volumetric flow rate) have an impact on the thermal efficiency of the vertical ground heat exchanger (VGHE) in a ground source heat pump (GSHP) system. The published literatures indicate that thermal performance of VGHEs increases with an increase of borehole diameter and/or pipe diameter. These literatures show that the borehole thermal resistance of VGHEs decreases within a range of 9% to 52% due to pipe configurations and grout materials. Furthermore, this paper also identifies the scope to increase the thermal efficiency of VGHE. The authors conclude that in order to enhance the heat transfer rate in VGHE, any attempt to increase the surface area of the pipe configuration would likely be an effective solution.

Estimation of In-Situ Heat Capacity and Thermal Diffusivity from Undisturbed Ground Temperature Profile Measured in Ground Heat Exchangers.

“…Furthermore, these results were in the range of thermal properties for geological materials of the Quebec-city area [48]. Hence, the obtained results validate the model assumptions and the parameter estimation procedure and, therefore, the stepwise implementation of this new method (Figure 3) can be conducted in the scope of ground-coupled heat pump system design [1,3,4,10,11,19]. Moreover, this method proposes a new damping depth equation which does not rely on the temperature amplitude as the previous methods adopted to evaluate the in-situ TD [20][21][22][23][24][27][28][29][30].…”

“…That depth at which TUGT is found or at which thermal disturbances from the surface are not perceived is called the depth of zero temperature amplitude (Figure 1) and it defines the boundary between thermocline and the thermostatic zones, where the geothermal gradient can be observed [34]. Usually, estimation of the depth of zero temperature amplitude requires monitoring ground temperature profile at different depths on a yearly basis [9,34], which we want to avoid here to fulfill the TRT practice [3,7,10,16]. For this new method, the acquisition and analysis of ground temperature observations need to be short enough for the test to be reasonably implemented during prefeasibility studies of ground-coupled heat pumps.…”

“…Thermal response tests (TRTs) made in borehole heat exchangers (BHEs) are commonly used to infer in-situ thermal conductivity (TC) [1][2][3]. However, an estimation of the in-situ thermal diffusivity (TD) and heat capacity (HC) is also needed for the analysis of a TRT when applying the infinite line-source equation to reproduce observed temperatures [4].…”

“…The main reason is the time required to continuously measure the ground temperature profiles during several days to a year, in order to record a periodic cycle of heat diffusion in the subsurface [10]. As a matter of fact, field measurements of prefeasibility studies for the design of ground-coupled heat pumps must be conducted within a few days, for instance 2-3 days when considering the heating period of conventional TRT or up to 5 days if the recovery phase of the TRT is included in the analysis [1][2][3][4]10,11,16,33]. Recently, Márquez et al [10] proposed a methodology for the indirect evaluation of the in-situ TD.…”

“…Thus, bearing in mind the importance of a quick and accurate assessment of in-situ HC, this research study had the objective of developing an alternative heat tracing approach to evaluate this thermal property considering the main guidelines of the TRT [1][2][3][4][5][6][7][8][9][10][11]. This study was carried out within the scope of a TRT performed in a pilot BHE [10,16].…”