What lies beneath
Martin Preene discusses the carbon-reducing benefits of heat-pump technology. Unlike geothermal methods, it can extract even moderate heat from the earth - while cooling it
The designers and developers of new-build and refurbishment projects for commercial and residential use are increasingly aware of the need to adopt the principles of sustainability in their work. Buildings are responsible for around 50% of the world’s generation of carbon dioxide. It follows that one of the primary objectives in the drive for more sustainable buildings should be to reduce the carbon emissions of a building through energy efficiency measures and the use of renewable or low- and zero-carbon (LZC) energy systems. Specific targets are now in place for carbon emissions for buildings.
In the public perception, renewable energy sources most commonly considered include those from above such as solar or wind power. Technology in these fields is improving, but applications are constrained by climate and location. An alternative approach is to look for energy from below – in the ground beneath the building.
Traditionally, the way to obtain heat from the ground was via true geothermal energy. This involves drilling boreholes down into specific types of rock, which are kept very warm by the decay of radioactive minerals, and extracting the warm water directly for heating use. The drawback with this approach is that it is dependent on the presence of hot bedrock. This is comparatively rare, and suitable geologic conditions exist in only a very few UK locations. This effectively prevents wide application of true geothermal sources.
In the absence of geothermally hot rocks, the temperature of soil and groundwater at moderate depth generally mimics the mean surface temperature. In the UK, between a few metres and 100m depth, temperatures are fairly constant all year around. Typical ground and groundwater temperatures are 10-14oC. These temperatures are too low to be used for direct heating. However, heat-pump technology, which operates on a similar principle to domestic refrigerators, allows heat to be extracted from low-temperature energy sources such as the ground and groundwater. Effectively, the building is heated by chilling the ground beneath. Conversely, in summer the building can be cooled either using a direct heat exchanger (or using the heat pump, working in reverse) to shed heat into the ground.
Systems that extract or exchange energy with the soils, rocks or groundwater are generically known as ground-source systems. The two principal approaches used in ground source systems are open-loop and closed-loop. Open-loop systems abstract groundwater from the ground, pump it to the surface where it passes through a heat-transfer system, before being disposed of (at a different temperature than before) either to waste or by re-injection back into the ground. In contrast, closed-loop systems do not abstract groundwater, but instead circulate a fluid through a loop of pipes buried in the ground. The circulating fluid passes through a heat pump at the surface, and is then recirculated back through the buried closed loop, to exchange heat with the ground.
Closed-loop systems generally influence a relatively limited volume of ground around the buried loop, and this tends to limit the peak heating or cooling loads that can be supported by such systems, even if multiple heat pumps are used. To date, most of the applications of closed-loop systems have had peak
capacities of less than a few hundred kW.
In contrast, because open-loop systems actively pump groundwater, they can draw water from great distances and can influence a much greater volume of ground, with a corresponding larger capacity to exchange heat with the ground. Some open-loop systems currently pumping from the chalk in the UK have capacities in excess of 1MW. Because of the potential for greater capacity, open-loop systems can be a realistic option for larger developments, when the economic capacity of closed-loop systems may be exceeded.
Strictly speaking, open-loop ground-source systems are not renewable energy systems. This is because they require electrical energy for water pumps (and, if used, heat pumps) to operate. This is in contrast to renewable sources such as wind, wave and solar, which do not require an external energy source. Nevertheless, open-loop ground-source systems are classed as LZC systems because they are very efficient, producing far more heating or cooling than the electrical energy they consume.
An open ground-source system that uses a heat pump can produce three to four times more energy as heating/cooling than it consumes in electrical energy. If a system can be run to provide cooling via a simple heat exchanger instead of using a heat pump, efficiency is greatly increased, and the ratio of cooling energy to electrical energy consumed can be in excess of 20.
If a ground-source system is used to replace conventional heating (gas-fired boilers) and/or air conditioning cooling systems, in addition to the significant reduction in carbon emissions, reductions in utility costs (through reduced gas and electricity use) will result. At 2005 gas prices, the cost savings are already worthwhile. But in the current energy-supply climate, gas prices are likely to continue climbing. This makes the substitution of gas-fired heating boilers with a ground-source system look attractive on a variety of scales of projects.
The lower operating costs of open-loop ground-source systems compared with traditional heating and cooling systems must be offset against the capital cost of installing the necessary boreholes, pipework, heat pumps and other equipment.
Nevertheless, there is potential to reclaim the capital costs through operational savings within a few years. Payback periods for commercial applications can be further reduced by taking advantage of the government’s enhanced capital allowances available on LZC technologies.
While open-loop ground-source systems can offer significant advantages, it must be recognised that there are two key constraints that apply to such systems.
The first constraint is the impact on groundwater resources. Open-loop systems can abstract significant volumes of water during a year. Dependent on the nature of the strata from which the water is abstracted, such systems have the potential to detrimentally affect neighbouring abstractors or sensitive ecological sites such as wetlands, by reducing the availability of groundwater.
In England and Wales, there is a legal requirement to obtain a licence from the Environment Agency to permit the abstraction.
The second major constraint is the disposal of the water that has passed through the heat-transfer system. If cooling is carried out, the water will be warmer than when it started, or conversely, heating will produce a waste stream of water colder than original temperatures. Successful management of this stream of wastewater is essential for the successful implementation of groundwater-source systems.
Where sites are located near rivers or lakes, it may be possible, subject to obtaining a discharge consent, to dispose of the water to surface waters. However, the different temperature of the wastewater in relation to the surface water may have an impact on the ecology on the surface waters.
One approach is to dispose of the water by reinjecting the wastewater back into the soil, or rock strata from which it came, via another set of boreholes. On face value, this seems very attractive as it avoids surface-water impacts, and avoids the cost of sewer disposal.
The use of groundwater as an LZC source of energy for heating and cooling buildings has real potential to contribute to reductions in carbon emissions. There are also operational cost savings to be obtained by adopting such systems. However, the future of these systems may be constrained by two factors: the willingness of the environmental regulators to grant groundwater abstraction licences to cooling systems, and the need to dispose of vast amounts of warmer or cooler water.
Martin Preene is the groundwater manager for Golder Associates (UK) Limited, whose website is www.golder.com
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