Geothermal and Geology

Partridge Well Drilling was established in 1892. Their first drilling rigs used wooden masts, were powered by steam, and transported from site to site by horse drawn wagons. Today, they are a cialis 5mg full service well drilling company and also an IGSHPA (International Ground Source Heat Pump Association) certified geothermal loop installer.

Merritt Partridge works for Partridge Well Drilling and spent some time talking with me last week about how underground geology affects a geothermal installation. Because underground geology changes from location to location, he informed me that it is important to find a local well driller who is familiar with your area. Local well drillers should either have experience in your area and know the geology personally, or be able to pull records and if drill logs to get an idea of make-up of the soil underneath your yard. Another benefit of using a local well driller is that they will be more familiar with local laws and permits regarding rules like drilling through aquifers and other regulations regarding geology.

Closed Loop or Open Loop?

One geothermal design factor that is driven by geology is the decision to install a closed loop or open loop system. If a large aquifer is easily accessible and has the proper quality and quantity of water, an open loop system may have an advantage over a closed loop system. Merritt explained that they install a large amount of open loop systems because they have a large aquifer with clean water. As the size of a geothermal loop field increases, the advantage of installing an open loop system often increases as well. This is because the cost associated with open loop systems is more of a fixed cost and does not increase as steadily as closed loop systems when the size of the loop field grows. While there are some methods such as placing two loops into a single borehole or using modified loops to increase the capacity of closed looped systems, additional loops often need to be drilled in order or to increase capacity. Open loop systems may require a stronger pump as the size of the system grows, but drilling two boreholes is often sufficient even for larger residential homes. The cost of drilling is a large part of a geothermal installation, so less drilling often equals a less expensive system.

Depth of the Loops

Even if the decision is made to install a closed loop system, underground geology can still affect the design. To explain this, Merritt told me about a geothermal project they did for the Florida Air National Guard. Before going into the story, there are a few drilling terms you should know. Circulation is the term that well drillers use to explain the drill’s ability to keep the hole that is being drilled open and prevent it from collapsing. A mixture of drilling mud is mixed at the surface and this is pumped through the drilling rods and down through the bit as the drill is rotating and cutting. This mud serves many purposes such as lubricating the drill bit, pushing cuttings up to the surface, and preventing the borehole from collapsing. The mud actually coats the walls of the borehole, known as the filter cake, and prevents the borehole from collapsing, which can trap the drill rods and prevent the loop from going into the ground.

The original plans on the Florida Air National Guard project called for loops that were each 200 feet deep. Merritt and his team knew that problems with circulation happened after 100 feet, so they changed this project by installing twice the number of loops originally specified, but at 100 feet instead of 200. This is an example where local knowledge and records helped save time on a project and prevented unnecessary headache.

The geology of the soil underneath your yard can definitely affect the decision for a closed loop or open loop system. It can also affect the design of a closed loop system and depth of the loops. Hiring a local driller who is familiar with your area can help with these decisions. For a glimpse of the geology in your yard, visit the online Web Soil Survey.

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