Open loop water collectors can be the most efficient and cost effective form of collector for a ground source heat pump. Coefficients of performance of over 7 are possible using this method. It makes sense to consider for larger installations over 200kW, in areas where a known water source is available and when space for alternative solutions is not available. Open loop systems are popular in the London area where the qualities of the underlying chalk aquifer are well understood.

Open loop boreholes or ‘wells’ are like water wells and drilled in a similar way to closed loop boreholes. There are some differences in the flushing mediums used and the borehole flushing and cleaning regime is more rigorous, for example “acidisation” within chalk may be used. Usually, the casing around the unstable, upper ground remains in the ground as a permanent part of the installation. In closed loop boreholes the casing would be removed prior to backfilling the hole. Where the ground conditions allow the borehole to stay open in the zone at which the water is to be abstracted, the borehole remained un-lined. Where this zone is unstable, it is lined using a permeable liner made of stainless steel or plastic. This enables the borehole to remain open and stable over the lifetime of the well.

Once drilled, flushed and lined, a suitably sized submersible pump is lowered into the borehole to the designed depth within the abstraction zone. This is connected at the borehole header, which runs through to the plate heat exchanger and heat pump.

Once extracted and used the now slightly cooler water is discharged to a separate ‘injection’ borehole drilled and constructed in the same way as the abstraction borehole. The two holes are normally separated by a distance of at least 20m in order that the water from the discharge borehole does not flow back into the abstraction borehole affecting the performance of the overall system.

Standing Water Column Well

Standing Water Column Wells are essentially the same as standard open loop borehole systems except that the abstraction and discharge of water occurs within the same borehole but at significantly different depths so that the discharge water does not directly influence the hydrogeology at the depth of abstraction. This type of collector is highly efficient and can yield between 3.5 and 7kW per metre of collector!
Standing water column boreholes may be drilled to depths of up to 300m below ground level, depending upon ground conditions, hydrogeological conditions and the required heating/cooling requirements.

A good understanding of groundwater flow and geology is key to the success of an open loop scheme. Econic’s experts can assess the viability of such a scheme for your project as well as help with the necessary permissions that have to be sought.

For further information on receiving Environment Agency permissions for Open Loop systems - click here