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Ground source heat pumps

In many buildings Ground Source Heat Pumps (GSHPs) provide an efficient solution for heating and cooling. Using low temperature latent heat which exists naturally below ground, GSHPs provide solutions by harvesting and transferring this energy for the benefit of the built environment. It is important to understand the feasibility of GSHP prior to designing a system.

 

 

This guidance note has been written to help inform those interested in improving the sustainability and efficiency of heating and cooling systems of a building.

 

WSP Environment & Energy's hydrogeological consultants provide a range of professional advice relating to GSHPs including:

1. The identification of potential energy stored in the earths natural resource
2. Assessment of the suitability of the ground in relation to the potential design and effectiveness of a GSHP
3. Recommendations based on the above information regarding the best system design for given ground conditions

 

 

What is ground source heat?

Ground source heat is a low temperature source of energy which exists below the ground surface. It occurs predominantly via solar energy penetrating the earths surface but also other sources from inside the earth. In the UK this energy is stored in the ground at temperatures of approximately 11-12°C close to ground surface.

 

 

Ground source heat is of little use for input directly to a heating system as the amount of available energy is low. However, by utilising a heat pump this low potential energy can be raised to a higher temperature for use.

 

 

What common types of system are available?  

Closed Loop Systems can be laid horizontally in shallow trenches or installed vertically in deeper boreholes. The system consists of a sealed tube installed below ground connected to a heat pump through which fluid is pumped. No water is ever removed from the ground, nor does the fluid come into contact with the ground at anytime, hence the term closed loop. Heat from the ground is transferred to the fluid in the tube and subsequently the heat pump during heating and vice versa during cooling.

 

Open Loop Systems use water directly abstracted from the ground. This is usually abstracted from a borehole. The abstracted water passes directly through a heat pump which transfers low temperature heat energy for use in the system. This water is then discharged either to a borehole, surface water body or potentially stored for other uses. In the cooling phase, heat is passed into the water for discharge.

 

 

Will it work for you?

There are many factors to take into account when considering which system might be most appropriate. These include:

• Available space, building requirements and sub surface geology and hydrogeology
• For a large scheme the number of boreholes in a closed loop system can be high. However, in equivalent open loop schemes many fewer boreholes might be required and these which may be substantially cheaper to install
• Open loop systems do not use coolants. These schemes generally have a lower potential risk of polluting ground and groundwater
• Both systems have the potential to effect thermal interference and open loop systems can cause irreversible heating of groundwater in the short term
• The permitting requirements for open loop systems are more stringent and therefore a lengthier process

 

 

System Feasibility

At the feasibility stage a number of technical issues have to be addressed. An initial feasibility decision will include consideration of the following:

• Does the ground have favourable thermal properties?
• Is there an aquifer beneath the site or are there only low permeability rocks?
• Is an abstraction licence required and if so what is the likelihood of one being obtained?
• If water is to be abstracted is the water quality appropriate or will it cause later loss of borehole yield or contaminate heating equipment with particulate matter?
• Is the site big enough for the number of closed loop boreholes or separation of open loop spacing to prevent thermal breakthrough?
• Is the cost benefit analysis positive?

 

 

Reducing Ground Costs

A substantial proportion of the cost associated with GSHP schemes relates to the installation of the ground components of the system. Large excavations or deep boreholes can be required which need specialist equipment. Uncertainties on site can rapidly increase drilling and excavation costs. Whilst there is always some uncertainty associated with unproven ground conditions on any site, a professional assessment of the ground conditions can do much to minimise this risk.

 

 

If initial studies show that a system is feasible, then more site specific data such as thermal conductivity and borehole abstraction capacity testing may be needed. Based upon the information collated, detailed feasibility study and design can be undertaken. This will be an iterative process with ongoing consultation as the design evolves.