27 Nov 2019

Open-loop GSHPs meet the spec

At a small luxury development in south Buckinghamshire, open-loop ground source heat pumps (GSHPs) use water from the chalk aquifer to provide renewable heat for the hot water systems. The water is also used for domestic consumption and irrigation purposes.

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HD Services was approached by Acanthus Developments and asked to tender for the installation of water supply boreholes to serve three prestige properties. Closed-loop ground source heating was initially being considered; however, HD Services informed Acanthus Developments of an alternative. Due to the availability of a reliable groundwater supply from the chalk aquifer – which could provide renewable heating and hot water to the properties – the developer chose to install open-loop GSHPs at each property.

Using the postcode of the site to assess the hydro-geology, a quotation was submitted for the construction of both the water supply borehole and soakaway at each location. Both the water supply boreholes and soakaways were constructed and water pumped from one to the other to prove their worthiness before payment was requested. It was several months later, once the construction of the properties had been completed, that the heat pumps could be installed and commissioned. In line with the MCS, each renewable heat installation is covered by a workmanship warranty. Separate annual maintenance contracts for each of the renewable heating systems were also provided.

The renewable heating systems

Working from building plans and SAP reports, HD Services was able to calculate the heat demand of each property and; therefore, was able to identify the size of heat pump required at each. The quote allowed for the supply and installation of Soleco three-phase heat pumps; two 18kW and one 26kW. Three-phase power supplies are an advantage as it means the borehole pumps and heat pump are cheaper to run.

The water supply and soakaway boreholes were drilled using a cable-tool percussion drilling rig – the method preferred by the Environment Agency when drilling into the chalk aquifer. Cable-tool percussion drilling is a cleaner way of drilling than the rotary option. While rotary drilling can be much quicker and; therefore, cheaper, often the backwash of the drilling process means that fissures can become blocked and; therefore, the flow of water affected. Rotary drilled water supply boreholes often require ‘treatment’ to re-open the fissures to allow the flow-rate to recover. This treatment is generally an additional expense.

As cable-tool percussion ‘cuts’ through the strata cleanly and no backwashing of cuttings is required, the fissures are unaffected by the drilling process and remain open. Very rarely is treatment required immediately after drilling when using cable-tool percussion.

Borehole logs were submitted to both the British Geological Survey and Environment Agency to protect the abstractions from derogation by a third party. An advantage of open-loop ground source heat pump installations is that the discharge water can feed a harvesting tank, from where it can be pumped for garden irrigation or other ‘greywater’ uses. Harvesting tanks that are only connected to a rainwater system will quickly run dry during a prolonged dry spell as they are not replenished. In contrast, a harvesting tank fed from a heat pump will refill whenever the heat pump is in operation.

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