Abstract
Heat advection caused by groundwater flow can potentially improve the performance of a borehole heat exchanger. However, the required flow velocity is not achieved under most natural conditions. This study focuses on artificial groundwater flow generated by pumping and investigates the associated effect in a lowland area near the Toyohira River alluvial fan, Sapporo, Japan. Thermal response test results are compared under natural and artificial groundwater flow conditions. A pumping well is constructed one meter from the borehole. Temperature profiles are measured in the U-tube during testing, using a pair of optic fiber distributed temperature sensors. The effective thermal conductivity is calculated from the profiles obtained in each 10-m sub-layer; this thermal conductivity is termed the stepwise thermal conductivity. Additionally, the upward flow velocity in the pumping well is measured to estimate the mean groundwater flow velocity at the borehole. The results show that effective thermal conductivity increases at depths less than 50 m, where the pumping creates mean velocities greater than 0.1 m d−1 in each sub-layer (1.5 md−1 on average). Thus, a borehole length of 50 m is more reasonable at the test site for its efficiency in a ground source heat pump system coupled with the pumping well than that used.
Highlights
Ground source heat pump systems (GSHPs) have been increasingly established for heating and air conditioning in residences and commercial buildings owing to their energy efficiency, ubiquitous availability, and environmental friendliness [1,2]
This study focuses on the artificial groundwater flow effect generated by pumping water from a well adjacent to a borehole heat exchangers (BHEs)
This study shows the actual effect of artificial groundwater flow and discusses a reasonable BHE length in terms of efficiency for GSHPs under the artificial conditions
Summary
Ground source heat pump systems (GSHPs) have been increasingly established for heating and air conditioning in residences and commercial buildings owing to their energy efficiency, ubiquitous availability, and environmental friendliness [1,2]. The effective thermal conductivity may increase in aquifers with groundwater flow velocities sufficiently high for heat advection relative to the conduction, i.e., the Peclet number is of order 1 in principle [4]. Several numerical studies have indicated that the threshold of the groundwater flow velocity for the advection effect is commonly on the order of 0.1 md−1 [5]. The threshold velocity was evaluated on the same order by other numerical studies [6,7,8]. Such high velocities of groundwater flow are limited to cases in which both hydraulic gradients and conductivities are relatively high, such as in unconsolidated gravelly aquifers
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