Six Feet Under: A Geothermal Systems Overview
Due to the current global energy pinch, alternative and more environmentally friendly methods of heating and cooling our buildings have been developed. One such system, the geothermal heat pump system, uses an alternate and renewable energy source – the Earth – as a means of heating and cooling a building. Our planet stays at roughly 55 degrees year round and has an almost unlimited ability to absorb, store, and give up thermal energy. Therefore, geothermal heat pump systems use the Earth as a huge heat sink to pull heat from and reject heat to.
“…the geothermal heat pump system uses an alternate and renewable energy source – the Earth – as a means of heating and cooling a building.”
How exactly does this system work? Within a building, a geothermal heat pump system uses a distributed network of small heat pumps to provide cooling and heating to all of the spaces. The heat pumps are linked to a centralized piping system, or heat pump loop, through which water is pumped. During the heating season, the heat pumps pull heat from this loop. In the summer, they reject heat to it. This water is then pumped out to a geothermal well-field. The well-field is a series of deep wells that act as a massive heat exchanger, transferring heat from the building to the ground in the summer and in the opposite direction during the winter. The wells are typically 6 inches in diameter and are bored from 200 feet to up to 400 feet deep. Flexible plastic piping is looped from top to bottom in each well, and all the wells are daisy chained together to form a continuous loop. The piping system is a complete closed loop so that water in the piping does not interact with any ground water systems.
When considering a geothermal heat pump system, a building owner must first determine if there is enough space on site to accommodate the well-field. A single geothermal well can be expected to provide 2 to 3 tons of cooling. For instance, a 20,000 square foot office building that requires 70 tons of cooling will require 23 to 35 wells. Wells are usually spaced 20 feet on center to maximize heat dispersion through the ground. Therefore, the small office building in our example requires an 80’ by 120’ (quarter acre) open space to accommodate the wells. On a small lot, this extra space may not be available. However, geothermal wells can be located beneath parking lots and athletic fields, allowing these areas to serve dual purpose.
Another matter to consider is whether or not the project can support the cost of the well-field. Recent project cost data shows that a well-field costs between $5,500 and $6,500 per well installation. For our office example, the well-field would add between $6 and $11 per square foot to the construction cost. While this is significant, remember that energy costs are spiraling. Paybacks have consistently been calculated to be between 7 and 10 years, which is typical and wholly acceptable for any large HVAC system change.
Though a substantial initial investment, the geothermal heat pump system is proving a sensible method for building heating and cooling with minimal impact on the environment. By using our planet in order to regulate indoor building temperatures, these systems are ushering in a new world of ‘Earth conscious’ heating and cooling.
– Julie K. Good, PE, LEED AP