What are heat pumps?
In simple terms, a heat pump is a device that moves heat from one source to another as efficiently as possible, from warm to cool, or cool to warm. As heat is moved rather than generated this method can be much more efficient and cost effective than conventional heating or cooling solutions. The heat they absorb is considered renewable and as a contribution to the UK government’s net-zero emissions target, heat pumps could be a real contender for the future of home heating. But not many people know what a heat pump is or that the UK government is encouraging homeowners to install them by offering substantial financial incentives. So in order to understand if heat pumps are a viable solution for low-carbon heating, let’s look at the various available options today.
There are three types of heat pumps, geothermal or ground sourced, air sourced, and water sourced.
Ground-source heat pumps (GSHP)
The most common ground-source heat pump solutions are referred to as close-loop systems. They circulate an antifreeze liquid around large plastic tubes that are located under the soil, in some instances 90m-160m deep. The temperature under the earth increases significantly the lower you go which allows the cooler liquid at ground level to heat up on its journey through the system.
Ground-source heat pumps can require a lot of space. A horizontal system of 8kW would require around 700m2 which is considerably larger than your typical garden in the UK. A vertical ground-source heat can be much more costly due to the specialist equipment needed to bore holes in the ground. A drilling rig is needed that will drill 20cm-wide holes, 5m-6m apart, up to 100m deep. Whilst more efficient than horizontal systems, many households may be prevented from installing vertical systems due to ground access restrictions and the geological make-up of the installation site. In addition, a ‘plant room’ or area to house the ground source heat pump inside is needed and this can take another 1m x 3m footprint.
Whilst the technology can operate efficiently and is undoubtably kinder to the environment, some households will not be able to install a ground-source heat pump. Fortunately, air-source heat pumps require much less space for those who have restrictions.
Air-source heat pumps (ASHP)
Air-source heat pumps (ASHP) work by absorbing heat in the outside air and transferring it, via similar technology to what is commonly found in refrigerators, to heat or cool water or air inside the home. If the air-source heat pump is used to heat or cool air, it is referred to as an air-to-air heat pump. Intuitively, if it heats or cools water, that’s an air-to-water heat pump. Air to water can be used to heat traditional water-based heat sources (like radiators) but is more commonly used for either warming the hot water supply or underfloor heating.
ASHPs typically cost less that ground-source heat pumps to install and require much less space, typically the size of an external air-conditioning unit but they also come with their own disadvantages due to the technology being used. Fan(s) are required to move the air around which can become noisy, especially at night and in quiet environments. Another problem is that ASHPs tend to be less efficient in the winter due to how they work. Think of them as similar to refrigerators in reverse. They take the warmth out of the external environment and use that to warm either the internal air or water supply. If the air outside is particularly cold and the internal air or water needs to be warmed particularly high, the difference can cause ASHPs to struggle. Fortunately the technology is advancing rapidly, and innovative solutions are starting to emerge from many of the leading manufacturers.
Water-source heat pumps (WSHP)
Water-source heat pumps (WSHP) are less common in the UK but share a lot of similarities to GSHP. They also require quite a large area to absorb heat from, in this case, water. Taking advantage of a suitable local water source, such as a lake, river, or other body of water, a WSHP can absorb heat from the reasonably stable water temperatures we experience in the UK, somewhere between 8°c and 12°c. They use a similar open or close loop system to pump a refrigerant solution around a length of pipe and use this, in conjunction with a heat exchanger to produce hot water or home heating with radiators, under-floor heating, or air-based heating.
Much of the advantages and disadvantages of WSHPs are shared with GSHP.
Are Heat Pumps the Answer?
Heat pumps are an effective, low-carbon alternative to fossil fuel-using heat/cooling products. They can be subject to a financial incentive from the UK government as part of the Renewable Heat Incentive (RHI) scheme that pays out per unit of heat energy (kWh) for up to seven years. In fact, tariffs for domestic heat pumps were increased on 20th September 2017 after a RHI consultation.
Installations can be quite expensive with a typical 3 bedroom home costing up to £18,000 for horizontal or £30,000+ for vertical ground-source heat pump systems (https://www.energysavingtrust.org.uk/renewable-energy/heat/ground-source-heat-pumps) but can be much cheaper for alternative technologies. It must be stressed that a qualified installer would need to assess the needs of a particular property before any real costings can determined.
Today, heat pumps are technically a very viable solution to low-carbon heating but the barrier to entry, whether that be available installation space, noise pollution, installation costs or something else, are slowing down adoption. If the UK government is able to level the playing field by continuing with financial incentives while promoting more low-carbon alternative heating, heat pumps can be a very important part of the UK’s future heating strategy.
