Evaluation of measures to improve the performance of an open loop ground source heat pump system in the chalk aquifer: a case study

Abstract

A case study documenting the development of a groundwater-fed district heat network in Colchester, UK, is presented. The performance of an open loop groundwater heating and cooling system (also known as a ground source heat pump (GSHP)) is a function of the performance of individual boreholes and interactions between the boreholes. When performance does not meet its design capacity or decreases with time, various measures can be undertaken to improve either the performance of individual wells or the performance of the system as a whole.Output from the first exploration borehole was less than expected, placing the business case for the development in jeopardy. Consequently, refinements to the remainder of the drilling programme were implemented including three to improve the performance of individual wells and two to improve performance of the system in its entirety. Results of these refinements are presented and may be used to inform the design of new open loop groundwater heat pump systems (GSHPs) and/or the rehabilitation of existing systems that have experienced diminished performance.Yields from three wells drilled using the reverse circulation method were more than double those drilled with the direct water flush method. A significant improvement in the performance of abstraction wells due to reinjection was observed. Specific capacity in abstraction wells increased by c. 40% due to reinjection, where the distance between abstraction and reinjection locations was 535–717 m. Allowing an excess pressure of up to 0.2 MPa in the reinjection boreholes meant that reinjection could be achieved with fewer wells.Outputs from abstraction wells were not increased by extending the depth of boreholes from 135 to 200 m or implementing additional acid treatments.