Open Loop Ground Source Heat Pump System
Heat pump efficiencies vary from type to type and between make and model. As a general rule, the most efficient type of heat pump is an Open Loop Ground Source Heat Pump (OLGSHP).
The usual way of measuring a heat pump’s efficiency is by reference to its Seasonal Performance Factor (SPF 1). As a guide:
· Air Source Heat Pumps (ASHP) have an SPF of 2.8- 3.4
· Ground Source Heat Pumps (GSHP) have an SPF of 3.0- 4.0.
· Open Loop Ground Source Heat Pumps (OLGSHP) have an SPF of over 4.0.
To qualify for the RHI, heat pumps have to be MCS approved and have a SPF in excess of 2.5
In an Open Loop Ground Source Heat Pump scheme, commonly known as a water to water heat pump, ground water is abstracted, usually from an aquifer or possibly from a river, and passed through the ground water heat exchanger before being returned to the ground.
Groundwater temperatures are relatively stable at depths of 10–15 m below the surface (approximating the annual air temperature at that location) and with further depths increase according to the geothermal gradient (UK average 3ºC per 100 m depth). As a result, there is a temperature difference between above-ground (air) temperatures and below-ground (including groundwater) temperatures for most of the year, with the ground/groundwater being colder than air during summer and warmer than air during winter.
Ground source heat pump (GSHP) systems exploit this natural temperature difference for heating or cooling demands. As described above, in Open Loop Ground Source Heat Pump systems groundwater is abstracted at ambient temperature from the ground, passed through the heat exchanger before being re-injected back into the ground or discharged at the surface. This type of OLGSHP system is highly efficient and can offer greater returns than a normal GSHP due to the relatively high temperature of ground water. However, the installation can be more complex and OLGSHP systems are usually confined to commercial buildings or larger domestic properties.
To be feasible, OLGSHP systems rely on certain hydro-geological, economic and regulatory conditions being met. Including:
- The presence of an aquifer of sufficient productivity and water quality (a constant supply of water is needed).
- Reasonable installation and pumping costs.
- Schemes extracting in excess of 20m3 a day require a license (although this is usually straightforward to acquire).
We are fortunate in the south east of England as in many scenario’s ground conditions are suitable to enable a OLGSHP system to be viable. (See link to BGS – British Geological Survey map below 2)
Drilling a borehole for this purpose is not a simple procedure and requires specialist knowledge of the ground conditions to ensure a trouble free and effective installation. Here at the Reina Group we have worked with several regional drilling companies to assist in a seamless process where boreholes are required. In particular, we have found the likes of OT Drilling Ltd, a company with specialist knowledge and years of experience of analysing the geology for water abstraction and who focus on Open Loop systems. They can provide an analysis and survey of the site to ensure all geological factors are considered when installing the GSHP borehole pipe, and have a wealth of experience in drilling through the various types of geological strata found in the South East, chalk, sandstone, clay and limestone.
If you require further information about OLGSHP systems then please call the office and speak with members of the heat pump team
1 A heat pumps SPF is dependent on the type of heat pump, energy source and how the heat is distributed throughout the home. Anyone of these factors can contribute to a low SPF
2 BGS Link – http://mapapps2.bgs.ac.uk/gshpnational/home.html
