RHI stimulus for ground- source heat pumps
With the Renewable Heat Incentive for ground-source heat pumps now near the top of the range for all forms of renewable energy and air-source heat pumps near the bottom, Jake Salisbury of the Ground Source Heat Pump Association reviews the technology.
A renewable heating (and cooling) solution with low running costs, a significant cost saving in displaced fuel,* lifetime of at least 40 years and a financial incentive scheme to support it? Yes, that’s right, a ground-source heat-pump system may be just the solution you’re looking for.
Ground- (or water-) sourced heat pumps (GSHPs) are known to operate with COPs (coefficient of performance) ranging from 3.5 to 4.5 in a large number of cases.
Launched by the Department of Energy & Climate Change (DECC) and developed through industry consultation, an incentive scheme is now available to encourage and support the growth of the renewable heating sector — the Renewable Heat Incentive (RHI).
The RHI is split into two streams — commercial (non-domestic) and domestic. DECC, along with the industry, anticipates that the RHI will facilitate the growth of the UK’s renewable-heating industry, in turn reducing our carbon emissions and dependence on imported fossil fuels.
For those that don’t know much about GSHPs as currently under-used (in the UK) renewable-heating technology, here is a brief overview. GSHPs make use of ‘low-grade’ heat stored in the ground, which is 99%+ accumulated solar energy, and upgrade it to ‘useable’ heat in buildings of all types.
|Heat can be extracted from the ground using shallow coils. (Photo: Kensa Engineering)
The average temperature in the ground, from a few metres below the surface, is 8 to 12°C. It is warmer in south-west Cornwall and cooler in north-east Scotland. A ground heat exchanger (pipes) laid in the ground extracts low-grade heat.
One may be concerned that this freely available heat will be used up and the ground will no longer be useful. However, as long as the GSHP system has been appropriately sized and designed for the heat load of the building in question, this will not be an issue.
Note, however, that the heat in the ground does move slowly. The replenishment of this heat will take time (through solar gain and transfer through the ground). If too much heat is taken from an area of ground too quickly, the ground temperature will fall, and the system will perform less well.
Laying pipe in the ground isn’t quite as simple as it sounds. The ground heat exchanger (in this instance termed pipe) is high-grade polyethylene pipe (minimum PE100/PE-RC/PE-Xa) which is laid horizontally or vertically to the required length/depth for the heat required.
• Horizontal systems. Pipe is laid in trenches, generally between 0.75 to 1.5 m deep. The pipe can be straight or coiled. The top 10 m of 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 200 m 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 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)
|Boreholes exploit heat deep in the ground. (Photo: Geothermal International)
With pipes correctly installed in the ground, how does the heat transfer? Within the pipes (both external and internal) is a liquid called a thermal transfer fluid (TTF). This is a solution of water, an antifreeze solution, biocide inhibitors (and more) in varying concentrations. The TTF is made using a number of components with differing thermal conductivity characteristics and other benefits; the GSHPA is working with industry on a standard focused on this important element of a geothermal system.
Having discussed what goes on in the ground and how the low-grade heat is extracted, a little about where that heat goes and how we use it.
The ground heat exchanger returns to the heat pump unit, usually located in a plant room (or the utility room, for example, in a domestic property) The heat pump upgrades the heat (raises its temperature) and delivers that heat through the emitter circuit (central heating circuit) into the building.
For the most effective GSHP systems, one needs to aim for the lowest possible ‘flow’ temperature to underfloor heating or other heat-emitter circuit). The benefit with underfloor heating is that its large surface area enables it to operate with a flow temperature of around 40°C compared to the 60°C+ seen , for example, in radiators served by a gas-fired boiler.
The RHI was first proposed over five years ago and is at last here to support the industry and end-users. Non-domestic tariffs were launched in November 2011 and revised on 28 May 2014, and the domestic scheme was launched on 9 April 2014.
|Latest RHI payments for heat-pump technologies.
There has been much work from the Ground Source Heat Pump Association and industry in general, with DECC and others in the development of this scheme, and we are confident that it will have the desired effect.
The tables above indicate the tariff available for heat-pump technologies. Tariffs are paid per unit of renewable heat generated.
It is now the responsibility of industry, working with Government and end-users, to drive the growth of the renewable-heating sector in the UK. Strong relationships have been forged over recent years, which will continue and grow as we work increasingly together to forge the low-carbon future we are working towards.
Jake Salisbury is head of operations with the Ground Source Heat Pump Association.
* Cost savings will vary depending on the fuel displaced, with greatest savings made on LPG.