The importance of borehole design for ground source heat pumps

Here is some guidance on the importance of a professionally designed and installed borehole for a ground source heat pump. I have included some technical guidance from the Ground Source Heat Pump Association and MCS GSHP Design Document, regarding the sitting and installation of bore holes. This will hopefully provide you with a better understanding of the correct process for drilling and grouting. Unfortunately a majority of companies do not use a thermal grout, partly due to keeping costs down but also a complete lack of understanding to what is happening in the ground. The water table alone will not suffice.

Borehole spacing and sitting is critical to optimising the extracted energy from the ground and sun. Where boreholes are positioned in close proximity to fixed structures (>7m) the impact of reduced ground surface area on the recovery rate from solar irradiation must be considered.(there is very little geothermal energy extracted, 99% is from the sun). The efficiency and composition of a borehole and probe is calculated using the following criteria;

Abstraction rate for the type of ground\geological make-up.

• Borehole depth
• Borehole spacing (distance between boreholes or structures)
• Solar irradiation
• Borehole probe construction (32mm or 40mm HPDE)
• Type of thermal grout

Yes there is the option of energy piles but would you really use this on a domestic project?

We have been designing and installing ground source heat pump systems for over 15 years and built a reputation nationally as a market leader and pioneer in the industry for both domestic and commercial projects. We provide consultancy, design services and training, not only to our competition but also to heat pump manufactures\suppliers. We are one of only a handful of contracting companies in the UK, who provide CPD training to Architects on ground & air source heat pump design.

We are happy to loose a project to a well designed and installed system, unfortunately there are very few out there which is why we try and provide as much guidance and knowledge as possible even if not our job. Anyone can install a heat pump its no different to a boiler. It comes down to design, best practice and experience. The most important part of a GSHP is the ground array as it provides 75% of your energy demands and it’s the one part you can not see once installed.

If you have any questions regarding the drilling process please speak to our drilling partner ADP to see how one of the largest drilling companies in the UK operates.  www.adpgroupltd.com  Tom Page is the MD and can provide you with impartial advice on how a bore hole should be designed and installed. Dont mention you are speaking with South West Energy Services, and see what he has to say.

2.8 Party Wall Issues 

All systems shall be designed assuming that adjacent systems will be installed and will therefore have a right to the heat under their property.

8.0 GROUND HEAT EXCHANGER GROUT

8.1 General 

Grout thermal conductivity is an important aspect of ground heat exchanger design. The designer shall be responsible for selection of the grout to support the foundation of the ground heat exchanger design. The following standards relate to the materials and methods of testing and verification of properties, as supplied by the manufacturers.

8.2 Grout Thermal Conductivity

Grout material thermal conductivity shall be determined by the following tests or comparable to EU standards:

• Pliable materials – American Society for Testing and Materials (ASTM) D-5334 “Standard test method for determination of thermal conductivity of soils and soft rocks by thermal needle probe procedure”

• Rigid materials – ASTM C-177 “Standard test method for steady heat flux measurements and thermal transmission properties by means of the guarded hot plate apparatus”

Materials that are bentonite-based are classified as pliable materials and cement based products or grout mixtures containing cement that cause the product to become rigid once cured are classified as rigid materials.

9.4 Ground Heat Exchanger Installation

The borehole shall be grouted from bottom to top with a suitable recognised geothermal grouting material and tremmie pipe as outlined in section 8.

MCS GSHP Design

3) MIS 3005 requires that the design of a GSHP system will lead to a minimum entering water temperature (EWT) into the heat pump from the ground of 0°C over the lifetime of the installation. Assuming a possible temperature difference of 3°C, an average minimum ground loop fluid temperature of -1.5°C has been adopted here.

This temperature defines some of the properties of the antifreeze solutions.

A) Heat pump thermal output. This is can be taken as the manufacturer’s MCS approved quoted output at B0/W35.

I know this is a bit War and Peace but I am a big believer in pass on the knowledge, it’s the only way the industry will improve and grow.

Please look at our case studies page for the types of projects we are involved in. http://www.swenergy.co.uk/case-studies/