Ground source heat pump systems are renewable heating and cooling solutions with a lifetime of 40+ years, low running costs, a large cost saving in displaced fuel and a financial incentive scheme to support it. The incentive scheme available is the Renewable Heat Incentive (RHI) which was created by the Department of Energy & Climate Change (DECC) and developed through industry and aims to support the growth of the renewable heating sector.
There are two sections regarding the RHI: commercial and domestic. RHI for commercial buildings has been available from 28th November 2011, and this was extended to domestic buildings 9th April 2014; however domestic buildings that installed a renewable heating system during September 2013 through to March 2014 may be eligible for the Renewable Heat Premium Payments. The paybacks consist of quarterly payments for 7 years. The monetary amount depends on the size of the property and the type of renewable heating system. Ones ROI calculation therefore depends on the initial cost of the installation of the renewable heating system, the type of renewable system, if any grants or loans were obtained and the incentives. Therefore it is anticipated by many in the industry, including the DECC, that the RHI will assist in the growth of the renewable heating industry in the UK. As a result of this carbon emissions will be reduced along with reliance on fossil fuels from abroad.
Ground or water sourced heat pumps (GSHPs) are currently underused in UK heating technology, they operate with COPs (coefficient of performance) ranging from 3.5 to 4.5 in the majority of cases. The GSHPs use the ‘low grade’ heat from the ground and turn it into ‘useable’ heat in buildings with various functions. A few metres underground temperatures range from 8 to 12˚C. A ground heat exchanger (pipes) laid in the ground extract the low grade heat.
If the GSHP has been accurately measured and designed for the relevant heat load then the available heat will not be used up and will therefore continue to be useful. Although, heat in the ground doesn’t move very fast and replenishing the heat through solar gain and ground transfers will take time. Therefore if too much heat is taken too quickly then the temperature will decrease and the system won’t perform to its best.
Pipes are laid underground to extract the low grade heat also known as a ground heat exchanger. High grade polyethylene can be used as material for the pipes which are then placed either vertically or horizontally to the required depth/length for the relevant heat to be required.
- Horizontal Systems – Pipe is laid in trenches, generally between 0.7m and 1.5m deep. The pipe can be straight or coiled. The top 10m of the ground is considered ‘shallow’ ground source (GSHPA ‘Shallow ground source standard’ is about to be published)
- Vertical borehole systems – Pipe is laid in one or more boreholes of depths from 10 to 200m depending on heat required/available (GSHPA ‘Vertical borehole standard’ is available).
- Lake loops – Closed pipe laid into a body of water such as a lake or river, which requires specialist consultant involvement.
- Thermal piles – Pipes embedded in the foundations (or piles) of larger, commercial buildings (GSHPA ‘Thermal pile standard’ is available).
- Open loop systems -These use heat stored in bodies of water such as underground aquifers and rivers, as long as the flow of the water is sufficient. Water is drawn into the heat pump, heat extracted and the water returned to source. Permission must be granted by the Environment Agency (GSHPA ‘Open loop standard’ is under development).
Heat is transferred by the use of a thermal transfer fluid (TTF) a solution made up of water, an antifreeze solution, biocide inhibitors along with more is located within the pipes in various concentration levels. The components within the TTF have varying thermal conductivity characteristics and other benefits; the GSHPA is working on a standard focused on this element of geothermal systems.
Once the heat has been extracted a ground heat exchanger returns to the heat pump unit, which raises the temperature and delivers the heat through the central heating circuit into the building.
By aiming for the lowest achievable ‘flow’ temperature to underfloor heating or other heat-emitter circuits, the systems tend to be the most effective. Due to the large surface area of underfloor heating the system can operate a flow temperature 20˚C+ which is less than a system used in radiators powered by gas boilers.
Five years after the proposal, the RHI is here to stay and support the industry along with end-users.
To qualify for the RHI the following steps/tasks must be followed:
- Have an assessment done on the property (example: SAP)
- Decide on what renewable heating system to have installed
- Choose an installer
N.B. Both the renewable heating system and the installer must be certified under Microgeneration Certification Scheme (MCS) or Solar Keymark or an equivalent of these.
Now it is up to the industry to work with the Government and end-users to lead the growth of the renewable-heating sector within the UK and to create the low-carbon future the industry is working towards.
