The open type of geothermal heat exchanger consists of one or more supply wells which supply water to the heat pump loop, and return or diffusion wells into which the same water is re-injected into the aquifer from which it was drawn. These wells are relatively shallow (several hundred feet in depth), and are generally cased to the depth where the water is available for pumping, or in the case of a diffusion well, can be accepted by the geologic formation. Aquifers that can furnish water at high flow rates are generally of coarse material such as gravel, but not clay, sand or bedrock.
The water must be drawn into the supply well and re-injected into the diffusion well through a screen. Screens consist of perforated pipe wrapped at a calculated interval with wire having a V-shaped cross section. This permits the well to remain clear of sediment and maintain the required flow rates. Screens for diffusion wells must be twice the size of screens for supply wells which compensate for screens not being optimized for diffusion (return) and the need to overcome the earth's natural resistance to return water to an aquifer (hydrologic tension).
An open well earth coupling can be the lowest cost and the highest efficiency method. A hydrogeological study is required to determine if the required water flow rate of three gallons per minute per ton of air conditioning will be available for the life of the well. A sustainable method of returning the water must also be determined. Disposing of the water into the sewer system is never an option. This type system is most economical in large portions of Brooklyn, Queens and parts of Staten Island. As the availability of water from a given well unknown, test wells and monitor-
ing wells must be drilled in these areas. Monitoring wells are called 'piezometers', and are generally one, two or four inches in diameter which allows a monitoring device called a 'troll' to be installed. The piezometers are placed at specific intervals from the source well as determined by the hydrogeologist, and allow the draw-down of the water table to be measured. Diffused water at the re-injection point may also need to be measured.
There are limited areas of Manhattan, Bronx and Staten Island with intersecting bedrock fractures that allow open well systems. Staten Island has geologic formations similar to Brooklyn and Queens in the eastern part.The ideal design spacing between a ^^
supply and a diffusion (return) well can be in the 100 - 500 foot range and is generally defined as:
Environmental permitting for open wells in NYC is defined by the diffusion flow rate. Any system 50 tons or over will probably need a permit. The EPA considers geothermal re-injection well water as a beneficial use (formally called a Class V use). Permitting or notice maybe required dependent upon average daily water flow rates. Open wells may be serviced by any qualified well contractor.
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