Impact of Groundwater Flow and Energy Load on Multiple Borehole Heat Exchangers

Abstract

The effect of array configuration, that is, number, layout, and spacing, on the performance of multiple borehole heat exchangers (BHEs) is generally known under the assumption of fully conductive transport. The effect of groundwater flow on BHE performance is also well established, but most commonly for single BHEs. In multiple-BHE systems the effect of groundwater advection can be more complicated due to the induced thermal interference between the boreholes. To ascertain the influence of groundwater flow and borehole arrangement, this study investigates single- and multi-BHE systems of various configurations. Moreover, the influence of energy load balance is also examined. The results from corresponding cases with and without groundwater flow as well as balanced and unbalanced energy loads are cross-compared. The groundwater flux value, 10(-7) m/s, is chosen based on the findings of previous studies on groundwater flow interaction with BHEs and thermal response tests. It is observed that multi-BHE systems with balanced loads are less sensitive to array configuration attributes and groundwater flow, in the long-term. Conversely, multi-BHE systems with unbalanced loads are influenced by borehole array configuration as well as groundwater flow; these effects become more pronounced with time, unlike when the load is balanced. Groundwater flow has more influence on stabilizing loop temperatures, compared to array characteristics. Although borehole thermal energy storage (BTES) systems have a balanced energy load function, preliminary investigation on their efficiency shows a negative impact by groundwater which is due to their dependency on high temperature gradients between the boreholes and surroundings.

Finite Element Modeling of Geothermal Source of Heat Pump in Long-Term Operation

“…The software provides Finite Element Modeling of Flow, Mass and Heat Transport in Porous and Fractured Media. The nodes belong to the boreholes were coupled to the rest of the modelled area [11]. Borehole heat exchangers (BHEs) were modeled according to [27].…”

“…Knowing hydrogeological conditions and the performance of the ground source system is very important for its exploitation [9]. Thermal parameters and ground water flow can have a significant effect on the development of borehole heat exchangers systems [10] and the temperature recovery rate [11]. In cases in which the layout is sufficiently constructed, the ground water flow can improve the long-term operation conditions [12].…”

Energy balancing in ground heat exchanger for heat pump systems – a case study with simulations

“…The research shows that longterm of large-scale, multiple-probes installations operation has less sensitivity to heat exchange disturbances in the lower source, e.g. groundwater flow, under the condition of a balanced heating load [5]. In other publications, researchers conduct an analysis of ground temperature changes based on a numerical model for different thermal loads, borehole configurations and indicate the necessity of analyses for changes in the distance between BHE's in further studies [7,8].…”

“…It was emphasized that in the case of a building with unbalanced heating and cooling load, the temperature changes around the ground heat exchanger are more and more visible in subsequent years of operation. The use of these systems in buildings with unstable heating and cooling demand and with frequent peak load operation may even lead to freezing of the ground heat exchanger [4,5]. Therefore, for large systems, the pilot drilling and the thermal response tests are in practice carried out to determine the effective thermal conductivity of the soil.…”

Evaluation of NaCl and MgCl2 heat exchange fluids in a deep binary geothermal system in a sedimentary halite formation

“…A fracture density, of 1 m -1 was assumed, with a range examined from 0.001 to 10 m -1 (Kalinina et al, 2014). The fracture aperture, b is assumed to be "partly open" with a value of 0.3 mm, examined at a range of 0.1 mm to 5 mm (Dehkordi et al, 2014). Assuming water density of 1200 kg m -3 , a viscosity of 2x10 -4 kg m s -1 and acceleration of gravity of 9.81 m s -2 , the density of 1 m -1 and aperture of 0.3 mm results in a K of 2.65x10 -4 and of 9x10 -4 .…”