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THIS IS ONE OF THE LEAST EXPENSIVE WAYS TO REDUCE CO2

TECHNICAL & PRACTICE

The third part of our renewables series looks at ground-source heat-pump (GSHP) systems and their rising popularity. This article also appears on www. ajplus. co. uk with more information on implementing these systems.

The rapid increase in the use of ground-source energy for building heating and cooling systems in the UK is a relatively recent phenomenon. We are inclined to view it as a new idea - there are only about 3,000 installations in the UK, most of these domestic.

However, there are more than 900,000 installations in the US and 500,000 on the continent.

In Sweden, more than 30,000 vertical borehole and 10,000 GSHP systems are completed each year, so the industry is well established and extremely robust and reliable. By 2010 Sweden expects to provide 30 per cent of its heating and cooling energy from GSHP systems.

The fuel efficiency of a GSHP system in heating mode can be 50-70 per cent higher than the most efficient gas boiler (even when the inefficiencies of generating the electricity needed to run the heat pump are taken into consideration) and cooling efficiency can be 20-40 per cent greater than alternative air-cooled technologies (see over page). This produces corresponding year-round carbon emission reductions and accounts for the current intense interest in GSHP systems.

Installation costs tend to be higher than with more conventional systems, hence the historic market resistance to the technology. But the value equation has shifted considerably in the past 12 months as legislators have moved to target CO 2 reduction.

This is one of the least expensive ways of achieving a reduction in CO 2 generation and meeting renewable energy targets, taking the view that the earth energy component is a 'renewable'.

WHAT IS A GSHP SYSTEM?

A GSHP system uses the earth, or ground water, or a combination of both as a source of heat in the winter and as a sink for heat removed from the building in summer. Low-grade heat is extracted from the ground by a liquid (normally water with antifreeze), energy is added via a heat pump and the resulting high-grade heat is used for building heating. During summer the process is reversed.

A properly designed system will be able to simultaneously heat and cool different parts of the building as, unlike conventional UK heating/cooling systems, the two systems are interconnected.

This can produce energy savings.

There are two main types of ground-source heating systems, known as 'open-loop' and 'closed-loop'. This basically defines the hydraulic means by which the heat is extracted from/returned to the ground.

A closed-loop system draws heat from the ground using multiple continuous loops of plastic pipe that are either inserted into a borehole contained within a structural pile casing, or looped horizontally 1m or more below the surface of the ground.

The latter is very space intensive and would normally only be considered for domestic installations.

Borehole systems comprise pipe loops contained within boreholes, which can be anything from 20m to 250m deep.

Typically, piles are 150 or 225mm in diameter and are drilled by a single rig on caterpillar tracks. Depending upon the site geology the borehole may be backfilled with a material such as bentonite, to ensure a contact surface with the surrounding ground. Where the borehole is flooded this is not necessary. On a recent visit to a borehole installation in progress in Oslo we observed 250m-deep bores being completed in five to six hours. Pipework installation takes another hour or two, so the whole process of establishing one 'well' can be completed in a working day.

The capacity of a closed-loop system to absorb or reject heat is a function of the ground conditions and the length of the borehole (and hence the contact surface). As a rule of thumb a capacity of 3kW for a short borehole (40m) up to 18kW (250m) would be typical.

However, detailed analysis needs to consider the 'annual energy balance' of the system and not just the peak rate of extraction or discharge. These calculations need to be carried out by a specialist using comprehensive dynamic modelling techniques.

It is normal to space boreholes 4-6m apart on the site and the available site area tends to set the limiting capacity of closed loop systems.

Open-loop systems extract heat from ground water.

This is drawn from a drilled well and pumped to a heat pump unit where the heat is extracted (or rejected in cooling mode). The used water is released back into the ground, usually through a rejection well, so that the system only adds or removes heat and does not pollute. This type of system is highly dependant on geological conditions and the availability of water beneath the ground, which normally needs to be tested and validated by a detailed geotechnical survey and trial borehole.

London has a good resource for extractable water at 100-150m depth and so open loop systems are on the increase and have recently been used on several major projects, including Portcullis House in Westminster and the London Mayor's office.

WHY ARE GSHP SYSTEMS EFFICIENT?

In most parts of the UK the temperature of the ground or of water extracted from the ground is at a reasonably consistent temperature, between 9infinityC and 14infinityC. In heating mode a heat pump can turn this into higher grade energy for heating with the addition of only a relatively small amount of power.

In the summer the benefits accrue by having to supply less energy to the heat pump compressor to dissipate the heat to the cooler ground than would be required to dissipate it to the warm summer air outside.

The efficiency of a heat pump - which is measured as its ratio of energy consumed in running it compared with energy delivered to/extracted from the building - is known as the Coefficient of Performance (CoP) when in heating mode and the Energy Efficiency Rating (EER) when in cooling mode.

Typical CoP efficiencies for closed loop heating range from 2.4-4, and for cooling the EER can be from 11-17. For open-loop systems the CoP is typically 3-4 for heating and 11-20 for cooling.

In heating mode this means that a well-designed system can produce 4kW of heating for 1kW of electrical input. If the electrical energy is supplied from the grid with a typical heat fuel efficiency of 35 per cent this still provides a fuel utilisation of 140 per cent compared to a typical condensing gas boiler efficiency of 85-90 per cent. Hence a GSHP system has a Building Regulations' Season Efficiency of Domestic Boilers in the UK of 1.4 in the SAP and iSBEM calculations. In cooling mode this arrangement is anything from four to five times as efficient as an air-cooled system and in the UK climate effectively recharges the ground with heat during the summer months.

Ideally, the heating and cooling loads through the year should be balanced so that the net effect is not to excessively heat or cool the ground. In Sweden a number of different techniques have been used to catch solar energy in the summer and use it to recharge the ground. This can take the form of solar panels or a water body such as a lake, which will be warmed by the sun during summer, or even waste heat from manufacturing processes.

The impact of failing to balance loads is less clear with large open-loop systems and would seem to depend greatly on geology and water migration across the site. Expert geotechnical guidance should be sought when implementing this type of system and flow modelling is recommended.

SIZING SYSTEMS It is accepted that it is disadvantageous to size ground-source systems to meet 100 per cent of the peak demand. This is because it is expensive to invest in rarely used capacity with this system.

For cost effectiveness, designs should be sized to supply between 50 and 70 per cent of the peak maximum demand load for heating and hot water. A system sized in this way will meet 85-95 per cent of the total energy required for the building.

This implies that auxiliary heating and cooling systems are also required, in the form of a boiler and chiller to meet the absolute peak conditions. Thus the GSHP systems do not completely eliminate the need for boiler flues and heat rejection equipment.

GSHP SYSTEMS - THE FUTURE As the mayor of London steps up the requirements for CO 2 reduction and renewables contributions and other authorities around the country start to adopt the 'Merton rule' for low-carbon planning, it seems inevitable that GSHP systems will increasingly be the cost effective way of meeting these tougher standards.

The big question is whether the expertise for analysis, design and delivery of these specialised systems can expand rapidly enough to meet the demand while also driving down the costs.

An investment in a ground-source-energy company right now looks like a very good bet.

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