In the fourth of our series examining renewable energy technologies, Alistair King talks us through ground-source heat pumps, which provide developers with a Part L-friendly way of keeping buildings warm or cool using the ground beneath our feet
Heated from above and below, the ground is a constant and renewable source of energy. Its thermal mass and unchanging year-round temperature can be exploited for heating and cooling, greatly increasing the efficiency of systems and therefore reducing their carbon dioxide emissions. So, in the wake of tougher Part L regulations and developers’ obligations to use 10% renewables, could the ground provide us with year-round sustainable energy?
What technologies use the thermal qualities of the ground?
Operating through pipework in boreholes or trenches, ground-source heat pumps (GSHP) use heat energy in the earth to help control a building’s temperature.
A less high-tech approach to cooling is achieved with air ducts. Using underground cavities, often in concrete, the air is kept cool, as it would be in a cave, because it is sheltered from the sun. It can then be drawn directly into buildings or air-conditioning units at a cooler temperature than air at surface temperature; this increases the efficiency of the system.
How do GSHPs work?
The basic components are pipe loops in the ground, heat exchangers, a heat pump and pipework within the building to be heated or cooled. The loops can be open or closed. Open loops allow water from a drilled well to be drawn into the system and put back into the ground after use. It’s a risky business – the water supply is not guaranteed and licences are required from the Environment Agency – but it is the most efficient system.
Less efficient, but more reliable and much more commonly used, are closed loops. These use standard polyurethane pipes buried in the ground and filled with a water/antifreeze solution, which is warmed by the ground and circulated around the system. These pipes can be laid vertically in standalone boreholes or within structural piles. Or they can be laid horizontally in trenches, in loops shaped like flattened Slinky springs.
The heat pump acts like a fridge in reverse. The refrigerant, pre-warmed by a heat exchanger in contact with the warm liquid, is moved in cycles, condensing, expanding, evaporating and compressing. This raises its temperature further and, after which another heat exchanger increases the temperature of the water used in the building’s internal pipework.
In the summer, the system operates in reverse to cool the building, and the ground acts as a heat sink. The system’s efficiency is ensured because ground temperature is well below ambient summer temperature.
How green is the technology?
Because a certain amount of power is required for operation, GSHPs are not fully renewable systems. However, their coefficient of performance for heating is about four.
In other words, every unit of power used produces four units of heat. For cooling, their coefficient is much higher – in excess of 11. As a result, GSHPs are greener than traditional heating and cooling systems.
Because they use some power, planners can reject developers’ appeals to install GSHPs as part of their 10% renewables targets. But many will accept them.
What is the temperature of the ground?
Subterranean temperature can be established by local surveys and varies across the country from about 7°C to 12°C. On any particular site, however, the temperature at depths from about 4m to 100m is constant. The temperature at the depth used for horizontal loops – about 2m – is slightly lower.
Surveys should also establish the soil type, a variable that affects excavation costs and heat conductivity. Rock and chalk, for example, are good, while clay and sand are less suitable.
Which types of building can use GSHPs?
New-build properties are best suited for two reasons. First, they will be insulated in line with the new Part L, so heating and cooling requirements will be lower. Second, the underground infrastructure can be incorporated into the construction process, particularly important if pipes within piles are used. Mixed-use developments are ideal because the energy demand from the whole site tends to average out over a day.
Although existing buildings can be retrofitted, there are limitations. Space can be an issue when using horizontal loops in urban areas. Given the relatively low optimum temperatures obtained from GSHPs – between 40°C and 45°C – it is much better to use it in underfloor heating systems rather than conventional radiators, which operate at much higher temperatures.
How much heat can GSHPs produce?
Systems vary but the capacity of closed loop boreholes ranges from 3kW for a short (40m) borehole to 18kW for a long (250m) borehole. Boreholes should be 4m to 6m apart, but site space will limit how many can be drilled.
Horizontal systems are cheaper to install because they use trenches about 2m deep, although this reduces their average output.
A 10m long trench generates about 1kW.
If set 5m apart, about 350m2 of land would be needed for the output demanded by a typical energy-efficient house, which is about 7-8kW.
With all options, it can be uneconomical to size systems for infrequent maximum loads, so back-up systems might be needed. It is important to balance systems because GSHPs are designed to cool as well as heat – the net use over the year might well be to cool.
How much does installation cost?
The cost of a pump for a typical 8kW system ranges from £800 to £1,400. Many sizes and models are available. But the real cost comes from surveys, excavation and connecting it to the internal distribution systems, particularly if retrofitted. Economies of scale keep costs per unit down on larger projects.
Anthony Coumidis, director of engineering services for McBains Cooper, which has been involved in several GSHP projects, says the overall cost of an open loop system using vertical piles is £300-400/kW for systems in excess of 1MW. Similar-scaled closed loop systems cost about £500/kW. At smaller scales, the unit costs are higher.
What are the maintenance and running costs?
Servicing costs are low. Closed loop systems need almost no upkeep but open loop systems require some maintenance. A constant small supply of electricity is needed to power the pumps of both system. A typical 8kW system uses 4,000kWh of electricity a year.
What is the break-even point for cash and carbon?
Given the construction cost variables, it is difficult to generalise about cash break-even points. Based on the cost of the equipment alone, however, some manufacturers quote fewer than eight years.
For similar reasons, it is difficult to make carbon calculations. But on average 0.1kg of CO2 per kWh is saved each year. This is low, but when electricity from a renewable source is used to power the system, the CO2 savings are far greater.
What is the average lifespan of a GSHP?
Companies give guarantees for their pumps of about two years but a well-maintained pump can last for 20 years. The underground network should not need to be replaced.
Is planning permission needed?
Generally planning permission is not needed but, as stated, installing open loops requires a licence.
What government incentives or grants are available for installation?
The Energy Saving Trust makes grants available to householders through the DTI, providing maximum payments of £1,200 for GSHPs.
The Carbon Trust helps businesses and the private sector cut carbon emissions through tax breaks and interest-free loans for installing GSHP systems.
Previous articles in this series are available at www.building.co.uk/renewables2007
Next week: energy from biomass