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LOOP THE LOOP

GEOTHERMAL ENERGY

Martin Preene examines the long term challenges of using groundwater for heating and cooling buildings.

The potential for groundwater to be abstracted from aquifers for heating and cooling buildings has been enthusiastically embraced by many in the geotechnical and construction industries.

Developers see groundwater systems as a way of meeting obligations to reduce carbon emissions from buildings, while reducing building energy costs and meeting the 'green' aspirations of potential occupiers and purchasers.

Geotechnical specialists see these systems as a new market into which to sell their drilling capability and hydrogeological expertise. On the face of it, the outlook for systems that use groundwater for heating and cooling is rosy, especially as future energy price increases will only enlarge operational savings with groundwater systems.

However, there two constraining factors.

The first is how willing environmental regulators are to grant groundwater abstraction licences to heating and cooling systems. If it cannot be demonstrated that such systems are sustainable, designers and developers may find their applications for licences to abstract groundwater refused.

A sustainable system should use water as efficiently as possible, and have minimal impact on nearby abstractors and on the wider groundwater environment.

The second potential constraint is the need to dispose of signicant volumes of warmer or cooler water produced by the heat exchange system. One initially attractive option is to reinject it back into the aquifer. However, this carries the risk of warmer/ colder water migrating between the abstraction and reinjection boreholes, affecting the long term efifciency of the system.

If these issues are not addressed in a rational and systematic way, clients may be become disillusioned if abstraction licences are not granted or long term system efficiencies are not as great as in theory. If that occurs, the full potential of the environmental and economic benefits of groundwater source heating and cooling may not be realised.

Before looking at how these constraints might be lessened, it is worth revisiting the background to the boom for using groundwater for heating and cooling.

One of the main drivers is to reduce carbon emissions. Buildings are responsible for about 40% of the CO 2 generated in the UK, so one of the design aims should be to reduce carbon emissions through energy efficiency measures and the use of renewable or low and zero carbon (LZC) energy systems.

From this month, specific targets for carbon emissions for buildings, and a benchmark of achieving 10% of the building energy from LZC, will apply in the revised Part L of the Building Regulations.

Strictly speaking, systems that use energy from groundwater (or directly from the ground) 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, systems that obtain energy from groundwater or the ground are classed as LZC because they are very efficient, producing far more heating or cooling than the electrical energy they consume.

A 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 produced to electrical energy consumed can be more than 20.

Use of the ground and groundwater as a source of energy is based on some simple physical principles. In the absence of external influences, soil and groundwater at moderate depth generally mimics the mean surface air temperature, and sub-surface temperatures vary little during the year. The ground, and the groundwater, is a potentially huge and stable thermal mass.

Typical ground and groundwater temperatures in the UK are 10 to 14°C. 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. Effectively, a building can be heated by chilling the ground beneath. Conversely, in summer a building can be cooled either using a direct heat exchanger (or using the heat pump, working in reverse) to shed heat into the ground.

Two different approaches can be taken when using the ground as an energy source or sink. Closed loop systems do not abstract groundwater, but circulate fluid through a loop of pipes buried in the ground. The fluid passes through a heat pump at the surface, and is recirculated through the buried closed loop, to exchange heat with the ground.

These systems generally influence a relatively a limited volume of ground around the buried loop, limiting 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.

Open loop systems abstract groundwater and pump it to the surface where it passes through a heat transfer system. They then dispose of it (at a different temperature than before) either to waste or by reinjection back into the ground.

Open loop systems can draw water from great distances and have the potential to influence a much greater volume of ground, with a corresponding larger capacity to supply energy. Some pumping from chalk in the UK have capacities in excess of 1MW.

Open loop systems can be a realistic option for larger developments, when the economic capacity of closed loop systems may be exceeded.

Although developers and building service engineers might regard them purely as systems to provide energy for a building, open loop groundwater systems are essentially water abstraction systems. They are therefore subject to similar constraints as systems that abstract water for other uses. Two key ones are worth considering here.

The first is the impact on groundwater resources. In England and Wales there is a legal requirement to obtain a licence from the Environment Agency (EA) to permit the abstraction (similar arrangements apply in Scotland from this month under the Controlled Activities Regulations).

Open loop systems can abstract signi. cant volumes of water during a year (up to several hundred thousand cubic metres for a 1MW system).

Such systems have the potential to detrimentally affect neighbouring abstractors or sensitive ecological sites such as wetlands.

As part of the licence application process there is a requirement to demonstrate environmental impacts from the abstraction are acceptable, and that the water use is efficient.

If a licence is refused, an open loop system can not be implemented.

If the EA grants a licence it will set upper limits on the instantaneous flow rates and on the annual volume - these are likely to be significant restrictions for larger systems.

The second major constraint is the disposal of water that has passed through the heat transfer system.

Cooling operations will produce water warmer than when it started, and heating will produce waste water colder than original temperatures.

Effective management of the waste water stream is essential if open loop systems are to be successful.

Where sites are near rivers or lakes it may be possible to obtain a discharge consent to dispose to surface waters. But the different temperature of the waste water may have affect the ecology of the surface waters, so the EA may set a limit on water discharge temperature.

Where there is a sewerage network, it may be possible to discharge the waste water stream there - although disposal charges may significantly affect the economics of the system.

One increasingly considered approach is to dispose of the waste water by reinjection back into the aquifer from which it came via another set of boreholes. This seems attractive, avoiding surface water impact and sewer disposal costs.

It may also make obtaining an abstraction licence easier as it minimises or eliminates net groundwater abstraction. The disadvantage is that if the extraction and reinjection wells are not widely separated, with time, warmer or colder water may circulate between the extraction and injection boreholes.

To avoid significant water recirculation, boreholes may need to be sited several hundred metres apart depending on abstraction/ reinjection rate and the aquifer's nature. On congested urban sites such separation between boreholes may be difficult to achieve.

If the circulation of water between boreholes causes temperature variation of the abstracted water by more than a few degrees C, this can seriously reduce the efficiency of the heating or cooling system. Such a condition may take several years to develop, so there is a need for predictive modelling of the long term interaction between the aquifer and extraction and reinjection systems.

In conclusion, the huge potential to reduce operating costs and carbon emissions using groundwater as an LZC source of energy for heating and cooling, may be constrained by factors affecting the abstraction of groundwater. To overcome these, future projects may need to:

Carry out more detailed assessments of hydrogeological impacts, to support applications for abstraction licences. For larger schemes this may involve development of numerical groundwater models to help predict changes in ground ter levels and quality.

There is also a need to demonstrate efficient water use. This may not be straightforward as the water use depends on the way the heating/cooling system is used by the building operator.

Ensure that predictive modelling of long-term year-on-year changes in groundwater temperature caused by reinjection of warmer/cooler water is integrated into the design process.

Such tools must address the interaction between the aquifer response and heating and cooling loads, modelling relevant aspects of both.

To allow the validity of any predictive modelling to be assessed, there is a need to incorporate long-term performance monitoring (of both the building and the aquifer) into future large-scale groundwater heating/cooling schemes.

Martin Preene is UK ground water manager for Golder Associates (UK).

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