Standing column wells consist of a borehole that is cased until competent bedrock is reached. The remaining depth of the well is then self-supporting through bedrock for the remainder of its depth. A central pipe smaller than the well diameter is dropped to form a core through which the water is pumped up, and an annulus into which the water is returned. The length of the central pipe at the bottom is coarsely perforated to form a diffuser. Water is drawn into diffuser and up this central riser pipe. The well pump is usually located at some depth below the water table in-line with central riser pipe. The standing column well combines the supply and diffusion wells into one, and is not dependent upon the presence or flow of ground water, although fractures in the bedrock that allow flow across the well can enhance performance, and reduce the length of the water column.
In practice, standing column wells are a trade-off between the open well systems and the below discussed closed loop earth coupling. The standing column well has the an advantage during the design phase in that the performance can be predicted without an extensive hydro-geological study. The savings in design fees and the elimination of the time period required for the hydro-geological analysis are attractive.
Given that standing column wells are unambiguous in projected performance, the use of this type of geothermal heat exchanger in most of Manhattan, the Bronx and northern Queens, where near surface bedrock can be anticipated is recommended. The standing column well can be designed and expected to perform to specification without the need for a test well. A heating or cooling capacity of 480 to 600 MBH (thousands of Btu/hr, equivalent to 40-50 tons of cooling) can be reliably expected from a 1,500 foot deep standing column well.
Should multiple wells be required, the spacing should be at least 50-75 feet. Closer spacing will affect the performance of the well field as the earth has a limited capacity to accept and reject heat. The diminished performance can be projected with available design software (typically GLPRO, see below).
Geothermal re-injection well water is considered a Class V water use and is regarded by the EPA as a 'beneficial' use. Permitting or notice may be required dependent upon average daily water flow rates. SCW wells shall be installed and serviced by qualified and experienced geothermal well contractors.
Long Island Power Authority (LIPA, formerly Long Island Lighting Company) has installed open loop geothermal heat pumps in four of its buildings over the past six years, see table 2a - 1 - LIPA facilities.
Facility__Design Load Completed
Brentwood 180 tons 1994 retrofit of 1958 geo system
Riverhead 80 tons 1997
Garden City 80 tons 1998
Hewett 100 tons 2000
Table 2a - 1 Long Island Power Authority Two Well (Open) Geothermal Heat Pump Installations
Of particular note is the Brentwood facility3, designed with an R-12 ground source heat pump installation in 1958. The facility has two wells, one a supply and one a diffusion well, both operational since 1958. The facility is a 6,500 square feet, two story building, providing office accommodations for 300 operating staff, and hosts a large cafeteria. Some offices, lockers and workshops are located in the basement, which also houses eighteen geothermal heat pump modules. The water to water geo-thermal heat pump modules replaced two 350 kW and one 900 kW gas fired boiler. The original wells and pumps were retained, but with a new variable frequency drive (VFD) which provides additional operational cost savings. A conceptual schematic of the system is shown in figure 2a - 7.
Note the modules are not centralized as in most other commercial applications. Each of the existing air handlers was left unmodified and was provided with matching capacity of heat pump modules. A comprehensive Honeywell system is employed to control each of the modules, air handler pumps and other related controls.
The Brentwood facility has been available to qualified engineering and design professionals for review and is well documented.
Based upon the success of the Brentwood operation, subsequent installations were made at the Riverhead operations facility, the Garden City Office and the Hewett Office. The Hewett Office has tied the geothermal heat pumps into a hybrid cooling tower system.
1 High density polyethylene pipe is specified as 3408 resin with a cell classification of 345434C or 345534C; pipe should be marked along its length with these specifications. Suppliers in the New York area are Driscopipe, Charter Pipe, Vanguard Plastics and others.
2 Typical GSV 048 heat pump for HEATING at 35°F, 36.9 mbtuh (3.56 COP) vs. at 20°F, 31.4 mbtuh (3.12 COP). For COOLING at 70°F, 50.9 mbtuh (17.9 EER) vs. at 90°F 47.1 mbtuh (13.7 EER)
3 Long Island Lighting Co., Brentwood Facility, ClimateMaster 97-BB101-9410-0, July 30, 1994
a. Hydrogeology of NYC
b. Choosing a Geo-exchanger using Hydrogeological Analysis
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