If you are considering an open loop borehole for your ground source heat pump system, there are a number of points which need to be taken into consideration:
Seek relevant permissions from the Environmental Agency:
- In order to drill or test pump a water supply borehole, you will require a consent to investigate a groundwater source under section 32 of the Water Resources Act;
- To operate an open loop scheme you will need an abstraction licence (if the abstraction is greater than 20m3/d) and an environmental permit to discharge water.
In addition to permission requirements you should consider the design of any scheme to ensure its longevity and efficient performance:
- Employ the services of a professional hydrogeologist and/or groundwater engineer to ensure you have a well-designed open loop ground source heat pump system;
- Ensure that there is adequate separation of the abstraction and discharge boreholes to ensure that large quantities of discharged water do not feed back into the abstraction borehole affecting the temperature of the abstracted water and hence performance of the system;
- Ensure the discharge borehole is well designed so that it does not clog within a short period of operation. Careful control of the gas content, water chemistry and the particulate content of the water is required as small bubbles of gas and particulates can result in rapid clogging of the borehole or aquifer. Biofouling with bacterial growth can also become problematic;
- The hydraulic efficiency of the scheme will reduce over time, even with a good design, due to the clogging of the face of discharge as detailed above and especially if thermal breakthrough occurs between the abstraction and discharge boreholes. Background temperature of the groundwater may also change over time if more schemes are constructed in the area.
- To avoid possible freezing and corrosion of the heat pump it is advised that the heat pump and water source are separated by an intermediate plate heat exchanger as shown below. It is also important that the water is filtered and bacterial growth minimised to avoid fouling. This does mean that a regular maintenance schedule is required.
- The signal for them extraction pump can be taken (via a relay) from the 24V supply between the controller and compressor contactor, contact Kensa Engineering for further details.
- A check valve is also required on the water source side to ensure that the system does not drain down when the extraction pump is turned off.