While the majority of geothermal installations use a closed loop system, another option is an open loop system.
Instead of using an antifreeze solution sealed inside the buried piping, an open loop system uses water from a surface or underground source - such as a pond, lake or well.
The water is pumped into the heat pump unit where the heat is extracted; the water is then discharged back into the original source or into a return well.
Well water designs are the most common and most cost-effective. The well supplies both household water and water for the heat pump. Approximately three gallons per minute of well water are needed per ton of cooling capacity. A 3,000-square-foot, well-insulated home would typically require 10 to 15 gallons per minute.
Water quality is an important issue with open-loop systems. Mineral deposits can build up inside the heat exchanger, iron and other impurities can clog a return well, and organic matter from ponds and lakes can quickly damage a geothermal system. Water should be tested for acidity, mineral content and corrosiveness.
Open loop systems are generally avoided and even prohibited in some areas because of environmental concerns. Aquifers can be depleted if the water is not reinjected.
Also, the risk of contamination is an increasing problem; improperly installed wells can be a path for surface water run-off that carries pesticides, fertilizers, organic materials and other contaminants into underlying aquifers.
Learn more about: Designing an Open Loop System