The use of ground water cooling systems, considering the technical and cost implications of this renew-able energy technology
The application of ground water cooling systems is quickly becoming an established technology in the UK with numerous installations having been completed for a wide range of building types, both new build and existing (refurbished).
Buildings in the UK are significant users of energy, accounting for 45% of UK carbon emissions in relation to their construction and occupation. The drivers for considering renewable technologies such as groundwater cooling are well documented and can briefly be summarized as follows:
Government set targets– The Energy White Paper, published in 2003, setting a target of producing 10% of UK electricity from renewable sources by 2010 and the aspiration of doubling this by 2020.
The proposed revision to the Building Regulations Part L 2006, in raising the overall energy efficiency of non-domestic buildings, through the reduction in carbon emissions. This was subsequently updated by 2010 to require a 25% reduction of carbon emissions from the 2006 requirement and in 2013 it is expected that the requirement will be a 44% reduction from 2006.
Local Government policy for sustainable development. In the case of London, major new developments (i.e.
City of London schemes over 30,000 m2) are required to demonstrate how they will generate a proportion of the site’s delivered energy requirements from on-site renewable sources where feasible. The GLA’s expec-tation is that, overall, large developments will contribute 10% of their energy requirement using renewables, although the actual requirement will vary from site to site. Local authorities are also likely to set lower targets for buildings which fall below the GLA’s renewables threshold.
Company policies of building developers and end users to minimize detrimental impact to the environment.
The ground as a heat source/sink
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 around 20 K, the temperature of the ground is far more stable. At the modest depth of 2 m, the swing in temperature reduces to 8 K, while at a depth of 50 m the temperature of the ground is stable at 11–13°C. This stability and ambient temperature therefore makes ground-water a useful source of renewable energy for heating and cooling systems in buildings.
Furthermore, former industrial cities like 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. The use of groundwater for cooling is therefore encouraged by the Environment Agency in areas with rising ground-water as a means of combating this problem.
System types
Ground water cooling systems may be defined as either open or closed loop.
Open loop systems
Open loop systems generally involve the direct abstraction and use of ground water, typically from aquifers (porous water bearing rock). Water is abstracted via one or more boreholes and passed through a heat exchanger and is returned via a separate borehole or boreholes, discharged to foul water drainage or released into a suitable avail-able source such as a river. Typical ground water supply temperatures are in the range 10–12°C and typical re-injection temperatures around 3°C warmer than the extraction temperature (subject to the requirements of the extraction 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.
Open loop systems are thermally efficient but overtime can suffer from blockages caused by silt, and corrosion due to dissolved salts. As a result, additional cost may be incurred in having to providefiltration or water treatment, before the water can be used in the building.
Abstraction licence and discharge consent needs to be obtained for each installation, and this together with the maintenance and durability issues can significantly affect whole life operating costs, making this system less attractive.
Closed loop systems
Closed loop systems 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 therefore suffer fewer of the operational problems of open loop systems, being designed to be virtually maintenance free, but do not contribute to the con-trol of groundwater levels.
There are two types of closed loop system:
Vertical Boreholes– Vertical loops are inserted as U tubes into pre-drilled boreholes, typically less than 150 mm 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 due to associated drilling and excavation requirements.
As an alternative to having a separate borehole housing the pipe loop, it can also be integrated with the piling, where the loop is encased within the structural piles. This obviously saves on the costs of drilling and excavation since these would be carried out as part of the piling installation. The feasibility of this option would depend on marrying up the piling layout with the load requirement, and hence the number of loops, for the building.
Horizontal Loops– These are 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 compared to open systems. Alternatively, coiled pipework can also be used where excavation is more straightforward and a large amount of land is available. Although performance may be reduced with this system as the pipe overlaps itself, it does represent a cost effective way of maximizing the length of pipe installed and hence overall system capacity.
The case for heat pumps
Instead of using the groundwater source directly in the building, referred to as passive cooling, when coupled to a reverse cycle heat pump, substantially increased cooling loads can be achieved.
Heat is extracted from the building and transferred by the heat pump 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 HVAC system.
The use of heat pumps provides greater flexibility for heating and cooling applications within the building than passive systems. Ground source heat pumps are inherently more efficient than air source heat pumps, their energy requirement is therefore lower and their associated CO2emissions are also reduced, so they are well suited for connection to a groundwater source.
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Closed loop systems can typically achieve outputs of 50W/m (of bore length), although this will vary with geology and borehole construction. When coupled to a reverse cycle heat pump, 1 m of vertical borehole will typically deli-ver 140 kWh of useful heating and 110 kWh of cooling per annum, although this will depend on hours run and length of heating and cooling seasons.
Key factors affecting cost
The cost is obviously dependent on the type of system used. Deciding on what system is best suited to a particular project is dependent on the peak cooling and heating loads of the building and its likely load pro-file. 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 in the case of vertical boreholes, the depth and number or bores. The cost of the system is therefore a function of the building load.
In the case of vertical boreholes, drilling costs are significant factor, as specific ground conditions can be variable, and there are potential problems in drilling through sand layers, pebble beds, gravels and clay, which may mean additional costs through having to drill additional holes or the provision of sleeving etc. 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 obviously results in the requirement for a greater number of boreholes or greater area of horizontal loop coverage, however in the case of boreholes the associated cost differential between a thermally inef-ficient building and a thermally efinef-ficient one is substantially greater than the equivalent increase in the cost of conventional plant. Reducing the energy consumption of the building is cheaper than producing the energy from renewables and the use of renewable energy only becomes cost effective, and indeed should only be considered, 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 taken out of the ground, this being dependent on the local ground conditions. Reduced performance due to blockage (silting etc.) may lead to the system not delivering the design duties whilst bacteriological contamination may lead to the expensive water treatment or the system being taken temporarily out of operation. In order to mitigate the above risk, it may be decided to provide additional means of heat rejection and heating by mechanical means as a back up to the borehole system, in the event of operational problems. This obviously carries a significant cost. If this additional plant were not provided, then there are space savings to be had over conventional systems due to the absence of heating, heat rejection and possibly refrigeration plant.
Open loop systems may lend themselves particularly well to certain applications increasing their cost effectiveness, i.e. in the case of a leisure centre, the removal of heat from the air-conditioned parts of the centre and the supply of fresh water to the swimming 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 licenses; particularly in areas of high fore-cast energy consumption, such as the South East of England, which needs to be borne in mind when selecting a suitable system.
Whilst open loop systems would suit certain applications or end user clients, for commercial buildings the risks associated with this system tend to mean that closed loop applications are the system of choice. When coupled to a reversible heat pump, the borehole acts simply as a heat sink or heat source so the problems associated with open loop systems do not arise.
Typical costs
Table 1 gives details of the typical borehole cost to an existing site in Central London, using one 140 m deep borehole working on the open loop principle, providing heat rejection for the 600 kW of cooling provided to the building. The borehole passes through rubble, river gravel terraces, clay andfinally 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 any plant or equipment from the well head are not included.
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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 system can operate at around three times the efficiency of conventional dry air coolers, so the payback period is a reasonable one. Additionally, the borehole system does not generate any noise, does not require rooftop space and does not require as much maintenance.
This is representative of a typical cost of providing a borehole for an open loop scheme within the London basin.
There are obviously economies of scale to be had in drilling more than one well at the same time, with two wells saving approximately 10% of the comparative cost of two separate wells and four wells typically saving 15%.
Table 2 provides a summary of the typical range of costs that could expected for the different types of system based on current prices.
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Table 1: Breakdown of the Cost of a Typical Open Loop Borehole System
Description Cost £
General Items
Mobilization, Insurances, demobilization on completion 21,000
Fencing around working area for the duration of drilling and testing 2,000
Modifications to existing LV panel and installation of new power supplies
for borehole installation 15,000
Trial Hole
Allowance for breakout access to nearest walkway
(Existing borehole on site used for trial purposes, hence no drilling costs
included) 3,000
Construct Borehole
Drilling, using temporary casing where required, permanent casing and
grouting 32,000
Borehole Cap and Chamber
Cap borehole with PN16 flange, construct manhole chamber in roadway,
rising main, header pipework, valves,flow meter 12,000
Permanent pump 14,000
Samples
Water samples 1,000
Acidization
Mobilization, set up and removal of equipment for acidization of borehole,
carry out acidization 12,000
Development and Test Pumping
Mobilize pumping equipment and materials and remove on completion of testing
Calibration test, pretest monitoring, step testing
4,000 4,000
Constant rate testing and monitoring
Waste removal and disposal
20,000 3,000
Reinstatement
Reinstatement and making good 2,000
Total 145,000
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Table 2: Summary of the Range of Costs for Different Systems Range
System Small–4 kWth Medium–50 kWth Large–400 kWth Notes
Heat pump (per unit) £3,000–5,000 £30,000–42,000 £145,000–175,000 Slinky pipe (per
installation) including excavation
£3,000–4,000 £42,000–52,000 £360,000–390,000(1) (1)Based on 90 nr 50 m lengths
Vertical, closed (per installation) using structural piles
N/A £42,000–63,000 N/A Based on 50 nr piles. Includes
borehole cap and header pipework but excludes connection to pump room and heat pumps
Vertical, closed (per installation) including excavation
£2,000–3,000 £63,000–85,000 £370,000–400,000 Includes borehole cap and header pipework but excludes connection to pump room and heat pumps
Vertical, open (per installation) including excavation
£2,000–3,000 £45,000–65,000 £335,000–370,000 Excludes connection to pump room and heat exchangers
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