Geology - when is geo feasible? (part 2 of 7)

A GCHP system, by definition, transfers energy to and from the ground. Obviously the temperature of the ground, thermal conductivity and diffusivity of the ground have an impact on how much and how quickly energy can be transferred between the fluid circulating through the GHX and the ground. The geology has an impact on how how much land area is needed to build it and how expensive it will be to build. 

If the site is overlying a an area with limestone bedrock that has been dissolved by water flowing through it for millennium there may be large fractures and caverns that a driller has to drill through. Grouting a borehole riddled with cracks and fractures is more difficult. With large caverns the driller may have to resort to over drilling the hole to insert a casing through the cavern making it more time consuming, more expensive and less feasible. On the other hand, fractured limestone provides good conditions for high-production water wells and it may be a good opportunity to consider an open well system...improving the feasibility of a GCHP system. 

Flowing wells can be difficult and expensive to control in some regions. If an artesian aquifer is shallow the potential of difficult to control flowing wells can limit the depth of vertical boreholes. In other regions pockets of methane gas are can be found in shallow boreholes and drillers have to be prepared to deal with the possibility of releasing methane while they are drilling. These conditions can make drilling more expensive and less feasible. But flowing wells may also be an opportunity for an open well system. 

Excavations and horizontal drilling is easily accomplished in clay and silt, but regions with large rocks and boulders make it difficult to install a horizontal GHX or even the horizontal trenches needed to connect vertical boreholes to the building. 

Geology can provide opportunities to make the installation of some GHX configurations very easy and make it virtually impossible to construct them in other situations. As a designer you need a good understanding of the geology and what contractors in a region can work with. Either way, if you are working closely with the building owner and the design team, and using a detailed energy model as a design tool, the GHX will become smaller and will require less land, making it more feasible to building.  (see Part 1: Integrated Design)