How borehole design affects heat pump efficiency

With heat pumps, efficiency is perhaps the number one issue to focus on. The whole point of these heating solutions is to maximise the ratio of heat released to energy put in, to make the most of both their environmental and cost-reduction benefits. 

Below, we outline some of the main design factors that impact how efficient a heat pump borehole will be, exploring details like thermal grout and geological conditions. 

Thermal grout

One of the most important design components when it comes to heat transfer efficiency is grout. 

Grout is essentially a highly conductive material that both seals the U-pipes in place, and ensures a maximally efficient transfer of heat between the surrounding ground and the liquid inside the transfer pipe. 

If this grout is not designed and installed properly, not only can it substantially reduce the longevity of the system, but it can also reduce the amount of heat you’re able to get out of each borehole.

Geological conditions

One of the first things you’ll need to do in a ground source heat pump project is have a series of surveys conducted, primarily to assess the geological conditions of the site. 

Certain lithological conditions, such as those that are constantly saturated with water, contain a lot of sandstone, quartzite or saturated granite, will generally contribute towards a more efficient system. 

On the other hand, more porous conditions, such as those with a lot of sand, gravel, or highly fissured karst limestone, will be less effective when it comes to transferring some of that heat energy from the surrounding area into the transfer liquid. 

If you have a larger property, with a number of different areas available in which to dig your boreholes, it might be possible to improve efficiency by choosing a more lithologically favourable sector. 

Depth

The amount of heat that a borehole provides a heat pump with directly depends on how deep it is. Most boreholes provide around 35 W to 50 W per metre, meaning that a 100 m borehole provides around 3 kW to 5 kW. 

Depending on the geology, it could be that digging deeper has a positive impact on efficiency, especially if, at those deeper parts, the borehole starts to hit more favourable lithological conditions. 

Advanced configurations

In certain situations, it might be possible or necessary to install a more technologically advanced configuration. This could be something like a coaxial design, or a double U-tube, either of which can be used to improve efficiency where space is at a premium. 

While it’s a good idea to personally have some level of understanding of how these systems work, it’s essential that you also hire a competent service provider to do the project for you.

A well-designed borehole can last for between 50 and 100 years, and given the costs involved, you really want to get it right the first time around. Make sure you have a thorough survey conducted initially, and that the rest of your borehole design is based on the findings of that survey to ensure maximal efficiency.