Closed Loop Collector Systems
There are 2 main ways to gain access to ground heat via closed loop collector systems – Horizontal or Vertical
- Horizontal System – this involves digging wide trench up to 2 metres deep and burying coiled plastic pipe. With an average requirement of 10 metres per 1kw of power this may lead to a considerable earthworks exercise.
- Vertical System – drilling one or more boreholes of up to 150 metres and installing a circulatory loop primed with a heat transfer liquid and surrounded with a performance enhancing grout.
The following is a summary of the pros and cons of the 2 ways:
Vertical boreholes require far less space than horizontal collectors. The average borehole will be 15cm diameter and 80-250 metres in depth. There may be a number of boreholes in series dependent on the heat output required. The boreholes can be specifically sited near buildings or drives. They are usually recommended on smaller plots or where used as a retrofit.
The pipework is installed in wide trenches evenly spaced out and usually 1.5 metres deep. This requires far more space and is usually considered where open ground is not a premium or perhaps a new development. The usual requirement is 10 metres per 1 kw of power. Once the trenches have been installed the land will no longer be available for future tree planting, pond construction, driveways or any other construction work. The land should be left porous and free from potential root damage.
Boreholes are usually unaffected by soil conditions and can be drilled in any type of ground. The collectors are dependent on transferring temperature from the surrounding ground and the deeper the borehole the less susceptible to seasonal external temperatures and the more consistent is this transfer. The vertical collector is normally shrouded in a bentonite grout which enhances the heat transfer process.
The collectors are usually buried 1.5 mtrs below surface and are subject to much more varied ground temperatures. The pipework is simply laid in the trench and does not have any additional grouting to enhance its performance. Hence, to give the most effective heat recovery the horizontal collectors are best situated in saturated or wetter areas of the site.
Generally a borehole is more expensive due to the specialist drilling equipment required.
Usually up to 30% cheaper than vertical installation due to simpler excavation of the trenches. To maintain equal efficiency it may be necessary to use longer pipework to compensate for higher levels of change in ground temperature and moisture.
A number of other issues may need to be considered:
- poor local geology may require a larger collector field than normal
- protection of the collector may be required for sharp stones or underground features
- landscaping, levelling and reseeding may require subsequent work to be done
Boreholes benefit from their greater depths and the water table. They are not affected by the same temperature fluctuations as horizontal collectors with the result that a heat pump system in cold weather will run at the same efficiency as in warmer weather. When using the cooling element in the summer this is particularly beneficial.
The relative close proximity to the surface means that there is a potential for seasonal changes in ground temperature and a slightly increased heat pump inefficiency during the winter. The same effect can happen in summer if the system is being used to cool a building – when using the warm ground to cool water.