Impeccable source

The ground offers an excellent resource for heating and cooling buildings. There are three distinct regions. The temperature of the first 4 m will vary depending on the season. The next 100 m or so is constant throughout the year due to the large thermal mass of the ground. From about 100 m and below the ground temperature warms due to radioactive decay effects at a rate of between 9 and 48ºC per 1000 m.

The most useful depth for cooling is the first 200 m. Figure 1 (right) shows the measured values of ground temperature as a function of depth for the important top portion of the ground. From this it can be seen that during the summer a depth of 4 m would provide a soil temperature of 10ºC. This remains constant to about ±2ºC and the average temperature is an approximate average of the annual ambient dry bulb temperature. However above this depth there is a swing due to the effects of the seasons, as shown, with the ground temperature out of phase with the ambient temperature due to the capacitance of the ground.

Open loop systems

Ground coupled cooling systems can be either open loop or closed loop.

Open loop systems use groundwater as the source of cooling. Groundwater flows through permeable aquifers and can be extracted through a well drilled into the aquifer. The position of the aquifer varies – a hydrogeological map can be used to establish whether groundwater is available. However, to determine an aquifer’s productivity, more properties are required such as transmissivity and hydraulic conductivity. Hydrogeological maps are available from the British Geological Survey (www.bgs.ac.uk), but it is advisable to drill a test borehole to confirm production rates before committing to a groundwater scheme.

The Environment Agency is responsible for the management of groundwater resources and any groundwater scheme needs to comply with its rules. In England and Wales a scheme will normally need an abstraction licence, and some projects may require an environmental impact assessment which may mean water has to be re-injected into the aquifer. There is a small fee to cover the cost of administering the licensing system and an annual charge is made for abstraction, which is metered. In 2004/2005 the unit charge for abstraction varied from £9.48 to £21.72 per 1000 m3 depending on the region (for details see www.environment-agency.gov.uk).

Figure 2 (below) shows the typical open loop system used at London’s Portcullis House, which includes a secondary heat exchanger to avoid circulation of the primary groundwater within the cooling system. This is because the groundwater is either corrosive and/or has the potential to form scale. Here, groundwater is rejected to the river, but it could equally be returned to the aquifer.

Open loop systems require minimal energy input compared with traditional refrigeration systems because they only need a small amount of energy for the pump (the magnitude depends on several factors including whether the system reinjects to the aquifer).

Open loop direct groundwater schemes have a reported coefficient of system performance (COSP) of between 100 and 10, depending on the application, whereas a traditional system has a COSP of about 3.0. Also, these systems can have a low revenue cost: in addition to the small cost of electricity used in driving the pump there is a small revenue charge for the water they extract. Overall, open loop systems compare favourably with conventional cooling systems – see Table 1.

Closed loop systems

Sometimes the aquifer does not produce sufficient groundwater, water quality may be unsuitable for cooling or extraction costs may be too high. Closed loop systems may be considered in these cases.

Closed loop heat exchangers are buried in the ground. They usually consist of a sealed polyurethane pipe containing heat transfer fluid, which is pumped around the loop. The heat exchanger may be installed vertically or horizontally, but the choice will depend upon the available land, the local soil type and the excavation costs.

Horizontal collectors require a relatively large ground area, free from hard rock, with a minimum soil depth of about 1.5 m. The ground temperature at this depth is not ideal but construction costs are cheaper.

There are three types of horizontal collector: single pipe, multiple pipe and spiral (‘slinky’). To minimise interference between multiple pipes a separation distance of 0.3 m is recommended and trenches should be at least 2 m apart. A slinky coil is used to reduce the ground area and trench length required; the slinky trench is only 20-30% as long as those for a single pipe configuration, but pipe lengths are double for the same performance.

A vertical heat exchanger is used where land area is limited. It can provide 500 W/m2 of ground area compared with a horizontal heat exchanger, which can achieve about 30 W/m2. The bores in the closed loop are generally 100 mm to 150 mm in diameter, with a depth of 15 m to 150 m. Performance degradation can occur if adjacent boreholes are not spaced sufficiently far apart and a separation distance of 5 m is considered a good starting point.

To determine the length of heat exchanger needed to meet a given load, you need to know the thermal properties of the ground. A geotechnical survey should be used to reduce the uncertainty. For large schemes where multiple boreholes are used, a trial borehole and properties test may also be appropriate. Heat from the working fluid is transferred to the soil by conduction and the soil is considered to form a concentric cylinder around the pipe. The external radius of this cylinder is a distance through the soil at which the temperature change is small, commonly referred to as the Farfield Radius (FFR). As the ground is a good insulator the heat transfer rate is quite low at around 5 W/°C per metre of pipe length. Sizing of heat exchangers can be calculated using software.

The simplest application of a closed system is to pass the water directly through a heat exchanger positioned where heating/cooling is required. However, performance is limited by the small temperature difference between the room and the heat source.

In terms of energy input and running cost, a closed loop system connected to a heat exchanger is more efficient than a conventional cooling system; the downsides are capital cost and space for the pipework. For instance, 1 kW of cooling requires a ground source heat exchange (GSHE) 50 m long.

However, thermopiles can reduce the GSHE cost. A thermopile is a foundation pile that incorporates a heat exchanger for heating and cooling buildings. The heat or coolth extracted from the thermopile is similar to that achieved by a vertical or horizontal GSHE, but as the pile is provided anyway there is potentially a very marginal cost associated with it. Typical heat transfer capacities are 80 W/m depth for a 0.6 m diameter pile to 125 W/m for 1.0 m diameter pile.

Heat pump systems

A way of extracting more energy from the ground is to combine a GSHE with a heat pump to give a ground source heat pump (GSHP). This enables a greater temperature differential between ground and water loop, which significantly increases heat transfer and cooling capacity. A typical configuration is shown in Figure 3 (above).

To achieve both heating and cooling a reversible heat pump is used with a four-way reversing valve to change over from heating and cooling. Ground source heat pumps are special versions of conventional water source heat pumps designed to operate over an extended range of entering water temperatures. The operation envelope for the heat pump is controlled by the heating mode. Under this mode typical evaporator temperatures range from - 5ºC to 12ºC, whereas the condenser temperature can be restricted to 50/55ºC.

The operating conditions for the heat pump must be carefully chosen at the design stage in order to meet heating, cooling and hot water requirements but also ensure that the system operates efficiently. The 55ºC limitation means that a GSHP will struggle to achieve the heat output required when used with a conventional wet radiator system. Instead the GSHP can be used to deliver forced warm air heating at around 30°C to 50ºC, lower temperature water for radiator systems at 45°C to 55ºC and hot water for underfloor heating systems at 30°C to 50ºC.

This limitation also means that the heat pump cannot easily provide domestic hot water to the required temperature of 60ºC to minimise the risk of Legionnella. In some systems the heat pump is used to preheat the water to 50ºC with an auxiliary electric heater to provide a boost. Another option is to use a de-superheater coil to transfer excess heat from the heat pump’s compressor to heat the water above 60ºC. The de-superheat capacity is about 20% of the total condenser capacity and normally the superheat temperature is likely to be sufficiently high when the heat pump is operating in the heating mode only. Another option is to run the heat pump for limited periods at higher condensing temperatures as required by the domestic hot water load. The only other limitation concerns the temperature of rejection into the ground as there are concerns that the fluid flowing through the ground loop should be above 0ºC to avoid frost heave in the ground.

The performance of the heat pump can vary widely so it is important to select and operate an efficient system. Control is important and features like variable speed compressors and pumps offer potential energy savings.

Performance of ground source heat pumps

The normal measure of peak system performance for a heat pump is the coefficient of performance (COP). A distinction needs to be made between cooling or heating modes COPc and COPh:

COPc = Evaporator duty / Compressor power

COPh = Condenser duty / Compressor power

Typical COPc values are between 2.8 and 3.6 for cooling to 5ºC, whereas COPh values of between 3 and 4 are common. Recently, variable refrigerant flow (VRF) technology has been integrated with ground-coupled systems. This will present an opportunity to heat and cool separate spaces and recover the heat between them. In a case study an average COP of 3.48 was reported.

The GSHP can also be used advantageously to provide energy efficient cooling for other refrigeration applications. For example in cooling a cold store to -20ºC, a GSHP would have a COP of 2.35 compared with a COP of 1.7 for an air source system and without the acoustic problems associated with operating fans at night. Many applications, such as supermarkets, have large areas of car park under which to bury the ground source heat exchanger. This has the additional benefit of providing an ice-free car park in winter.

The performance for heat pumps is also measured on a seasonal basis using the seasonal performance factor or SPF. It is defined as below:

SPF = Heat delivered (kWh/season) / Energy supplied (kWh/season)

Tables 2 and 3 show how the GSHP system compares with alternative methods of heating and cooling. It can be seen that when used in cooling mode, the GSHP produces 16% less CO2 emissions compared with an air source heat pump. In heating mode, the GSHP saves 40% in revenue and 50% in CO2 compared with a condensing boiler installation.

Achieving a minimum COP is essential in order to save carbon and revenue, compared with conventional technology. An efficient heat pump system is critical. Below ground the first constraint is the impact on groundwater resources. The second is the disposal of water that has passed through the heat transfer system.

If cooling is carried out the water will be warmer than when it started; while water heating will produce a waste stream colder than the original temperatures. Most open loop systems dispose of “used” groundwater either by discharging to a surface water body or injecting back into the aquifer.

These types of open loop systems, known as pump-and-release or pump-and-dump systems, are relatively simple to implement, and for commercial or institutional applications, offer energy efficiencies comparable to closed loop systems at substantially reduced capital costs.

However, they have potential to cause long-term warming or cooling of the ground as well, and to suffer from a reduction in system efficiency, or even failure, caused by excessive warming (or cooling) of the aquifer. This is particularly true in cases where injection of waste heat is not countered by removal of that heat at a later time.

If the Environment Agency considers abstraction for heating and cooling is having an unacceptable impact, a licence may be refused. This would mean an open loop system could not be implemented. Instead, systems that thermally balance the aquifer over time, such as aquifer thermal energy storage (ATES) or borehole thermal energy storage (BTES), could be considered.

Thermally balanced systems

ATES and BTES both allow water reinjection to the aquifer to balance the load. The concept of ATES storage operation is shown below in its first UK application, at London’s Westway Beacons scheme (BSj 05/06).

A system will usually consist of a warm well and a cold one. In the summer, groundwater from the cold well is pumped to the building. The groundwater absorbs heat from the building and is discharged into the warm well where the thermal energy is stored. In winter ground water from the warm well is pumped to the building and releases heat energy. The water temperature cools down and returns to the cold well. This inter-season storage system works efficiently and has a high performance.

The ideal ATES is one in which the net seasonal demand is in balance, but in the case of imbalance heat can be added or subtracted through either summer heat capture or winter heat rejection to atmosphere.

Because of the lower groundwater temperature in summer and higher temperature in winter, ATES open loop systems can offer increased energy efficiency and long-term cost savings over pump-and-dump systems. The lower summer temperature may enable cooling without additional mechanical cooling or an improvement in system COP. The higher winter temperature may enable a higher COP when operating in heat pump mode. The typical improvement in COP when used for comfort heating is 6% compared with a conventional ground source system. And rejecting coolth to the aquifer during winter enables the cold well to deliver cooling at 6ºC during the summer, potentially avoiding the need for mechanical cooling and thus achieving a very large COP and consequential carbon savings.

The BTES system uses banks of closed underground heat batteries through a borehole field with closed loop tubes grouted in. One bank of boreholes is used for storing coolth and the other for storing heat. While ATES can only be used where a suitable aquifer exists, BTES can be used anywhere.

No rules of thumb can be used to determine whether a particular site would be suitable for ATES without a feasibility study. For this you need a hydrogeologist. If the area is well developed and much is known already about the hydrogeology, then investigative costs can be minimised. In uncharted territory, more effort may be needed.

As the ATES system can operate on a seasonal basis, its seasonal performance must be assessed in any viability assessment. To achieve this a transient model of integrated ATES, heat pump and building application, known as the TRNSYS system simulation software has been developed to calculate life cycle, energy, revenue and carbon performance over time. It also provides environmental characteristics of the aquifer to enable assessment by the Environment Agency.