Five Minute Guide to... Ground Source Heat Pumps
For self-builders with even a modest-sized garden, a ground source heat pump may be a sensible choice, says sustainable homes expert Tim Pullen
Sustainability expert Tim Pullen explains why, even if you have a modest-sized garden, a ground source heat pump may still be a sensible – and cost-effective – choice
Aground source heat pump system comprises a series of pipes buried underground to extract solar energy from the ground in order to convert it into heat for use in the home. It may sound like a 21st-century renewable energy system, but the technology is actually quite old. The first heat pump in the UK was installed in 1952 in the Royal Festival Hall, and systems today are generally reliable and long-lasting.
If you do an internet search on ground source heat pumps you will still see reference to geothermal heating. Geothermal heat is the energy emanating from the earth’s core. In this country you have to go down at least 500m before there is any appreciable input from the earth’s core. Ground source heat pumps generally take heat from 1.2m down (maybe 100m in the case of boreholes) — which is some distance from geothermal energy. They also absorb solar energy — the energy from the sun warming the ground through the summer. This may sound like ‘ expert pedantry’ (and it may well be) but it matters.
The implication of the term geothermal is that the supply of energy is virtually infinite, while solar energy is very finite and very measurable. It means that the pipes buried in the ground have to be properly designed and properly positioned to ensure that the system is not taking more heat from the ground than the sun can replace in summer.
What it Looks Like
A ground source heat pump (GSHP) has two main parts: the ground array, which can be vertical or horizontal, which is buried underground. The noticeable bit in the house is the heat pump itself (see centre). Some of these include a hot water cylinder and can be as big as a three- or even four-drawer filing cabinet. The smallest pumps, such as those in the Kensa heat pump range, can be fitted in an under-sink kitchen cupboard. Noise is often a concern, but heat pumps use the same technology as a fridge or freezer and make the same level of noise.
Choosing a Ground Array
As the name implies, a ground source heat pump extracts heat from the ground and to do that an array of pipes in the ground is needed. As mentioned, there are two forms of ground array: vertical and horizontal.
A horizontal array is a pipe, without joints, laid in a serpentine closed loop in a trench, usually 1.2m deep, connected to the heat pump. Allow 500m2 for a 10kw heat pump in clay soil, and twice that for sandy soil. Pipes can be a straight pipe or in coils, called a ‘slinky’ pipe. In terms of efficiency and cost there is no material difference between the two. Manufacturers will often have a preference.
There are two key issues: capacity (the amount of piping and the length of the trenches needed) and the soil conditions, both of which your installer can advise on. A good installer will dig a hole to check the ground conditions before designing the ground array; a bad one won’t. Says Kensa Heat Pump’s Stephanie Gregory: “Collector arrays are sized to meet the heat needs of the building and the thermal conductivity of the ground conditions are taken into account when sizing the ground collector.”
A vertical array, often called a borehole system, will have a number of boreholes drilled into the ground and connected across their tops, again in a closed loop. The number and depth of boreholes will be dictated by the size of the heat pump and the geology but an 8kw heat pump is likely to need at least three boreholes 70m to 100m deep (or two slightly deeper boreholes). There is a ‘postcode’ lottery element to boreholes, as the cost varies enormously around the country. The price can vary from reasonable to ruinously expensive. Their big advantage is that they do not need a large area of land to be carved up with trenches and if the price is right then a vertical array is a good option.
Heat can also be extracted from lakes or ponds, if the body of water is big enough. But this is a more specialist area and requires careful calculation, usually carried out by the heat pump manufacturer.
Calculating Efficiency
Efficiency is stated as the SCOP (Seasonal Coefficient of Performance). A typical figure for a GSHP might be 4.0. This means, in broad terms, that for each 1kw of electricity
used to drive the heat pump, it will produce 4.0kw of heat. Kensa Heat Pumps helpfully publishes a whole list of SCOP figures, rating the efficiency of its heat pumps at a range of different ground temperatures — from 0°C to 10°C. This shows that at 0°C the MCS (Microgeneration Certification Scheme) SCOP – the figure actually used for calculating Renewable Heat Incentive payments (more on which later) – is 4.23, with a flow temperature of 40°C, which would be typical for use with underfloor heating. One of the main benefits of a GSHP is that the ground temperature does not vary much, winter to summer.
The temperature variation is generally accepted as being from 7°C in winter to 10°C in summer. Given that the heat pump is extracting heat, the winter temperature might fall a little below 7°C, but a well-designed system will avoid over-chilling the ground as the sun would then not be able to get it back to 10°C in summer. Over-chilling would lead to a gradual and progressive reduction in the energy available to the heat pump. This is what we technical people would call a ‘ bad system’.
The figure to focus on is something below 7°C and Kensa provides a SCOP at 6°C (which we might call coincidence or thoughtful) for a 7kw heat pump of 4.95 with a flow temperature of 40°C.
This raises another issue. A heat load calculation will usually take heating parameters of -2°C outside and 21°C inside. And the peak load (the maximum heat the house will need) will be based on those figures and for a modern newbuild might be 8kw. But for most of the year the outside temperature will not be -20C, meaning that the heat pump does not need to produce 8kw. Or it might be colder than that and the heat pump is being asked to produce more than that. Heat pumps with a fixed output tend to struggle to meet that varying demand and as a result become less efficient. Some heat pumps have ‘modulation’ capability, however — they have the ability to vary the output to meet the actual demand. So an 8kw modulating heat pump would be able to put out anything from 4kw to 9kw, minimising the electricity needed to meet the demand.
The Installation and Running Costs
The Energy Saving Trust estimates that the cost of installing a typical GSHP system in a three to four bedroom house will be something over £10,000. The generally accepted ‘ budget’ figure is £1,200 per kw capacity. A 200m2, four bedroom house, built to Building Regulations’ standard is likely to need an 8kw heat pump. The machine itself is likely to cost around £6,000 to £7,000, the balance being the installation cost which can vary significantly with the ground conditions.
That four bedroom house is likely to need around 11,000kwh of heat for space heating and another 4,000kwh for domestic hot water. If we assume a SCOP of 4.5 then the property will need (11,000 + 4,000)/4.5 =
3,334kwh of electricity to run it. Electricity at 15p/kwh (including VAT, standing charges, and so on) gives a running cost of £500 per year. That compares to £890 per year to run a gas boiler.
Ground source heat pumps qualify for the Renewable Heat Incentive (a government scheme under which those with renewable heating technologies are paid back for the heat they generate) and the current rate is 20.46p/kwh, payable for seven years from the date of commissioning. RHI is paid for the renewable element of the heat produced, not for the electricity used to produce it. In the scenario above, we need 11,000 + 4,000kwh of heat, less the 3,334kwh of electricity used, so RHI applies to 11,666kwh, at 20.46p/ kwh = £2,386 per year for seven years.
Are they Worth the Investment?
Ground source heat pumps are expensive to install but they offer the highest RHI return and the lowest running cost of any renewable energy. Systems are generally reliable and long-lasting. The key is to find a good, qualified, experienced installer. That will ensure the GSHP system is designed to properly meet the ground conditions, and the demands of the house, and will run happily for 20 to 30 years.