If you've been involved in the GCHP industry you've undoubtedly seen projects issued for tender where the "drilling contractor shall provide sealed engineered drawings for the ground heat exchanger (GHX) that provides the 80 tons (280 kW) of cooling capacity", or a drawing that says "GHX boreholes to be located in the area indicated and to provide 480 kBtu/hr (140 kW) of heating capacity". It's very obvious the designer does not have any idea of the design of a GCHP system and is trying to pass on the responsibility for the design to someone else. As a mechanical system designer it's dangerous to leave the design of the GHX to someone without providing the detailed information they need to know.
A GHX is an integral part of the building and mechanical systems, and cannot be optimized without a good understanding of how much energy must be transferred to and from the ground. Ideally, the GHX designer will work with the building owner, architect and the rest of the design team to first reduce the loads, and secondly to balance the energy loads to and from the GHX. The GHX designer should also work closely with the mechanical system designer to ensure the distribution system is optimized to work with a heat pump system (water temperatures, air flows, etc.). It may also be advantageous to consider integrating energy storage into the mechanical system, take advantage of excess heat from large cooling loads to produce domestic hot water, melt snow, or other possible use of the energy.
If the building loads cannot be balanced, it may be advantageous to take advantage of hybrid options (closed circuit cooling tower, auxiliary heat source, etc.) to reduce the size and cost of the GHX and ensure the long term viability of the system.
The point is, the design of the GHX cannot be done in isolation of the building, simply by saying the GHX must provide 80 tons (280 kW) of cooling capacity or that it must provide 480 kBtu/hr (140 kW) of heating capacity, and then rely on a drilling contractor or mechanical contractor to use whatever rules of thumb they would use to design the GHX. It is the responsibility of the system designer to understand and design the GHX.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.