Effect of undisturbed ground temperature on the design of closed-loop geothermal systems: A case study in a semi-urban environment

Abstract

This paper presents temperature measurements in four Borehole Heat Exchangers (BHEs), equipped with fiber optics and located in a semi-urban environment (campus of the University of Liege, Belgium). A 3D numerical model is also presented to simulate the heat loss from the surrounding structures into the subsurface. The mean undisturbed ground temperature was estimated from data during the preliminary phase of a thermal response test (water circulation in the pipe loops), as well as from borehole logging measurements. The measurements during water circulation can significantly overestimate the ground temperature (up to 1.7 °C in this case study) for high ambient air temperature during the test, resulting in an overestimation of the maximum extracted power and of the heat pump coefficient of performance (COP). To limit the error in the COP and the extracted power to less than 5%, the error in the undisturbed temperature estimation should not exceed ±1.5 °C and ±0.6 °C respectively. In urbanised areas, configurations of short BHEs (length < 40 m) could be economically advantageous (decreased installation and operation costs) compared to long BHEs, especially for temperature gradient lower than -0.05 °C/m.

Experimental and numerical investigation of a long-duration Thermal Response Test: Borehole Heat Exchanger behaviour and thermal plume in the heterogeneous rock mass

“…In between them, shale/siltstone layers were detected of lower thermal diffusivity . The negative gradient of the undisturbed ground temperature is attributed to the heat loss through structures into the subsurface, located close to the boreholes (Radioti et al, 2017).…”

Performance analysis of horizontal ground source heat pump for building cooling in arid Saharan climate: thermal-economic modeling and optimization on TRNSYS

“…The design of geothermal heat exchangers depends strongly on the undisturbed soil temperature (i.e. the soil temperature before any heat injection or extraction) as they affects soil resistance [13,14], which limits the heat exchange between the soil and the GHX wall [7], and thus affects the GSHP performance coefficient as well as the design parameters of the ground heat exchanger.…”

“…The properties of the soil, actual loads of buildings and meteorological factors play a significant role in the design of the geothermal heat exchangers. In recent years, many studies dealing with the influence of soil thermo-physical characteristics on ground source heat pump performance have been published [13,14]. Most researchers have found that the undisturbed soil temperature and ground resistance, which vary with time and space strongly affects the amount of geothermal energy that can be transferred to or extracted from the ground.…”

Validating Models for Calculating the Efficiency of Earth-to-Air Heat Exchangers Based on Laboratory Data for Fall and Winter 2016 in Northeastern Poland

“…Ground temperature distribution is also an important consideration in other engineering applications such as borehole heat exchangers [33] and heat pumps [34].…”