Ground water cooling

The application of ground water cooling systems is quickly becoming an established technology in the UK. This is partly a result of the pressure on the industry to make use of renewable technology from sources including:

• The 2003 Energy White Paper, which sets the UK the target of producing 10% of its electricity from renewable sources by 2010 (and the aspiration of doubling this by 2020)
• The proposed revision to the Building Regulations Part L 2006, which seeks to raise the overall energy efficiency of non-domestic buildings, through the reduction in carbon emissions, by 27%
• Local government policy for sustainable development, such as London’s requirement that new developments of more than 30,000 m

•  The policies of building developers and end-users, which increasingly look to minimise a building’s impact on the environment.

The thermal capacity of the ground can provide an efficient means of tempering the internal climate of buildings. Whereas the annual swing in mean air temperature in the UK is about 20°C, the temperature of the ground is far more stable. At a depth of 2 m, the swing in temperature reduces to 8°C, while at a depth of 50 m the temperature of the ground is stable at 11-13°C. This stability and ambient temperature makes groundwater a useful source of renewable energy for heating and cooling systems.

Furthermore, former industrial cities such as Nottingham, Birmingham, Liverpool and London have a particular problem with rising ground water as they no longer need to abstract water from below ground for use in manufacturing. As a result, the use of groundwater for cooling is encouraged by the Environment Agency in such areas.

Open and closed loop systems

Ground water cooling systems maybe defined as either open or closed loop. Open loop systems generally involve the direct abstraction and use of ground water, typically from aquifers (porous water-bearing rock). Water is abstracted through boreholes, passed through a heat exchanger and then returned through a separate borehole, discharged to foul water drainage or released into a suitable available source such as a river.

Typical ground water supply temperatures are in the range of 6-10°C and typical re-injection temperatures 12-18°C (subject to the requirements of the abstraction licence). Open loop systems fed by groundwater at 8°C can typically cool water to 12°C on the secondary side of the heat exchanger to serve conventional cooling systems.

Such systems are thermally efficient but can suffer from blockages caused by silt and corrosion from dissolved salts. As a result, filtration or water treatment may be needed before the water can be used in the building, adding to the cost of the system. On top of this, an abstraction licence and discharge consent needs to be obtained for each installation, and this, along with the maintenance and durability issues, can significantly affect whole-life operating costs, making this system less attractive.

Closed loop systems on the other hand do not rely on the direct abstraction of water, but instead comprise a continuous pipework loop buried in the ground. Water circulates in the pipework and provides the means of heat transfer with the ground. Since ground water is not being directly used, closed loop systems suffer fewer of the operational problems of open loop systems. However, they do not contribute to the control of groundwater levels.

There are two types of closed loop system: vertical boreholes and horizontal loops. For a vertical borehole system, vertical loops are inserted as U tubes into pre-drilled boreholes, typically less than 150mm in diameter. These are backfilled with a high conductivity grout to seal the bore, prevent any cross-contamination and to ensure good thermal conductivity between the pipe wall and surrounding ground. Vertical boreholes have the highest performance and means of heat rejection, but also have the highest cost as a result of associated drilling and excavation requirements.

Horizontal loops consist of single (or pairs of) pipes laid in 2 m deep trenches, which are backfilled with fine aggregate. These obviously require a greater physical area than vertical loops but are cheaper to install. As they are located closer to the surface where ground temperatures are less stable, efficiency is lower than in open loop systems. Coiled pipework can also be used where excavation is more straightforward and a large amount of land is available. Because the pipe overlaps itself, performance may be reduced but it does represent a cost-effective way of maximising the length of pipe installed and hence overall system capacity.

The case for heat pumps

If groundwater cooling is coupled with a reverse-cycle heat pump, substantially increased cooling loads can be achieved. The heat pump transfers heat from the building into the water circulating through the loop. As it circulates, it gives up heat to the cooler earth, with the cooler water returning to the heat pump to pick up more heat. In heating mode the cycle is reversed, with the heat being extracted from the earth and being delivered to the heating system.

The use of heat pumps provides greater flexibility for heating and cooling applications than passive systems, which simply use the groundwater source directly in the building. Ground source heat pumps are inherently more efficient than air source heat pumps, which means their energy requirement is lower and their associated CO₂ emissions are also reduced, so they are well suited for connection to a groundwater source.

Closed loop systems can typically achieve outputs of 50 W/m (of bore length), although this will vary with geology and borehole construction. When coupled with a reverse cycle heat pump, 1 m of vertical borehole typically delivers 140 kWh of useful heating and 110 kWh of cooling a year, though this depends on hours run and length of heating and cooling seasons.

Key factors affecting cost

Costs can vary greatly between systems and which system is best suited to a project depends on the peak cooling and heating loads of the building and its likely load profile. This in turn determines the performance required from the ground loop, in terms of area of coverage in the case of the horizontal looped system, and the depth and number or bores in the case of vertical boreholes. The cost of the system is therefore a function of the building load.

In the case of vertical boreholes, drilling costs are significant factor and there are potential problems in drilling through sand layers, pebble beds, gravels and clay. The costs of excavation obviously make the vertical borehole solution significantly more expensive than the equivalent horizontal loop.

The thermal efficiency of the building is also a factor. The higher load associated with a thermally inefficient building results in a need for more boreholes or greater horizontal loop coverage. In the case of boreholes, the associated cost differential between a thermally inefficient building and a thermally efficient one is substantially greater than the equivalent increase in the cost of conventional plant – as a result, the use of renewable energy only becomes cost effective when a building is energy efficient.

With open loop systems, the principal risk in terms of operation is that the user is not in control of the quantity or quality of the water being abstracted, which is dependent on local ground conditions. Reduced performance as a result of a blockage may lead to the system not delivering the design duties and bacteriological contamination may require expensive water treatment or that the system is temporarily taken out of operation. In order to mitigate this risk, additional means of heating and cooling by mechanical means can be installed as a back up. However, this obviously carries a significant cost.

Open loop systems are particularly suited to certain applications, increasing their cost effectiveness. For example, in a leisure centre they can remove heat from air-conditioned areas and supply fresh water to the pool.

In terms of the requirements for abstraction and disposal of the water for open loop systems, there are risks associated with the future availability and cost of the necessary licences, particularly in areas in which high energy consumption is forecast, such the South-east. This is a factor that should be considered when selecting a suitable system.

While open loop systems would suit certain applications or clients, for commercial buildings the risks associated with this system tend to mean that closed loop applications are the system of choice. When coupled with a reversible heat pump, the borehole acts simply as a heat sink or source so the problems associated with open loop systems do not arise.

Typical costs

Table 1 gives details of the typical cost of the installation of an open loop borehole system on an existing site in central London, using one 140 m deep borehole and providing heat rejection for the 600 kW of cooling provided to the building. The borehole passes through rubble, river gravel terraces, clay and chalk, and is lined above the chalk level to prevent the hole collapsing. The breakdown includes all costs associated with the provision of a working borehole up to the well head, including the manhole chamber and manhole. The costs of plant or equipment from the well head are not included.

Heat is drawn out of the cooling circuit and the water is discharged into the Thames at an elevated temperature. In this instance, although the boreholes are more expensive than the dry air cooler alternative, the operating cost is significantly reduced as the the system can operate at about three times the efficiency of conventional dry air coolers, so the payback period is reasonable. Additionally, the borehole system does not generate any noise, does not require rooftop space and does not require as much maintenance.

Table 2 provides a comparison of the costs of a closed loop borehole system with a conventional heating and cooling system for a new gallery in the south of England with a gross floor area of 2400 m^2. In this analysis, the pipework forming the energy loop was attached to the structural piles, so the costs of drilling and excavation are not included. Also, the ground loop does not deal with the building’s total cooling and heating loads, so extra refrigeration and heating plant is provided.

Davis Langdon Mott Green Wall would like to thank EarthEnergy for its assistance in preparing this article