Site Evaluation for a Geothermal System
February 1, 2010
Because shallow ground temperatures are relatively constant throughout the United States, ground source heat pump installations can effectively be used almost anywhere. However, the specific geological, hydrological and spatial characteristics of a particular site will help you determine the best type of ground loop for that site.
Geologic IssuesFactors such as the composition and properties of the soil and rock (which can affect heat transfer rates) require consideration when designing a ground loop. For example, soil with good heat-transfer properties requires less piping to gather a certain amount of heat than soil with poor heat-transfer properties. The amount of soil available contributes to system design as well – in areas with extensive hard rock or soil too shallow to trench, you may choose to install vertical ground loops instead of horizontal loops.
Hydrology ConcernsGround or surface water availability also plays a part in deciding what type of ground loop to use. Depending on factors such as depth, volume and water quality, bodies of surface water can be used as a source of water for an open-loop system, or as a repository for coils of piping in a closed-loop system. Ground water also can be used as a source for open-loop systems, provided the water quality is suitable, and all ground water discharge regulations are met.
Before you select an open-loop system, you will want to be sure to fully investigate the site’s hydrology, so you can avoid potential problems such as aquifer depletion and ground water contamination. Antifreeze fluids circulated through closed-loop systems generally pose little to no environmental hazard.
Land AvailabilityThe amount and layout of your site’s land, its landscaping, and the location of under-ground utilities or sprinkler systems also contribute to the system’s design. Horizontal ground loops (generally the most economical) typically are used for newly constructed buildings with sufficient land. Vertical installations or more compact horizontal curly-type installations often are selected for existing buildings because they minimize the disturbance to the surrounding landscape.