Initial geological considerations before installing ground source heat pump systems

Abstract

The performance of an open or closed loop ground source heat pump system depends on local geological conditions. It is important these are determined as accurately as possible when designing a system in order to maximise efficiency and minimise installation costs.Factors that need to be considered are surface temperature, sub-surface temperatures down to 100 -200 m, thermal conductivities and diffusivities of the soil and rock layers, groundwater levels and flows and aquifer properties. In addition rock strength is a critical factor in determining the excavation or drilling method required at a site and the associated costs. The key to determining all of these factors is an accurate conceptual site scale model of the ground conditions (soils, geology, thermogeology, engineering geology and hydrogeology).The British Geological Survey has used the modern digital geological mapping of the UK as a base onto which appropriate attributes can be assigned. As a result it is possible to generate regional maps of surface and sub-surface temperatures, rock strength and depth to water. This information can be used by designers, planners and installers of ground source heat pump systems. The use of appropriate geological factors will assist in creating a system that meets the heating or cooling load of the building without unnecessary over engineering.2

Transparent Soil to Model Thermal Processes: An Energy Pile Example

“…A series of tests on the transparent soil using a KD2Pro thermal needle probe indicated it had a thermal conductivity of 0.33 W Á m À1 Á k À1 for the density and saturation conditions tested. This is lower than typical values reported for soils in literature, in the region of 0.7-1.5 W Á m À1 Á k À1 for sand and clay, respectively (Busby et al 2009); nevertheless, it was still suitable to model thermo-dynamic problems.…”

Factors Influencing the Thermal Efficiency of Horizontal Ground Heat Exchangers

“…During the observation period, a minimum of 7. Moving down from the surface, the ground temperature decreases exponentially with depth, due to the high thermal inertia of the ground: T oscillations are quite completely dumped bgl and a phase shift can be observed between T air_surf and T ground [12,15,23,55]. The amplitude of the T waves is already reduced at 0.60 m depth (T ground ), reaching an average value of 7.9 • C, even if seasonal variations are still visible and shifted in time compared to T air_2m (Figure 3a).…”

“…In the T depth profiles two main behaviors are identified, typical of non-operating (i.e., 15 (Figure 5c). Once shutting down the heat pump on 28 November 2016, the non-operating behavior is completely recovered after one week ( 5 December 2016), while already after three days (1 December 2016 the temperature difference between 0.40 and 0.80 m depth is annihilated.…”

“…The operating principle in both cases is the same: in winter time (heating mode) the heat exchanger extracts heat from the ground, taking advantage of the higher ground temperature than the air temperature; in summer (cooling mode) it reinjects heat into the ground by withdrawing it from the warmer external atmosphere [14,15]. In order to assess the performance of different GSHPs, properly size the shallow geothermal applications and estimate cost and payback time of the system, several heat transfer models are available, according to the geometry of the GSHP selected, the environmental conditions and the building features [16][17][18].…”

Geophysical Methods for Monitoring Temperature Changes in Shallow Low Enthalpy Geothermal Systems

“…In situ tests, such as thermal response tests [12,13] or laboratory measurements [14] are sometimes possible, but the values obtained may deliver only well-centered information or may not always (if not at all) be representative of in situ conditions at a larger scale. Such data are often scarce if not missing and authors often have to rely on standard calculation charts, values found in the literature, or simply default values implemented in standard software (e.g., [15][16][17][18]). In addition, the heterogeneity of the material properties and their potential anisotropy, which are difficult to detect with standard integration methods, make the problem more complex.…”