The hydrogeology of the City of New York is complex and varied presenting an interesting challenge for the design and implementation of geothermal systems. Since the location of the intended system is critical in determining the nature of the geology and, consequently, the type of well that will be used for the system, the designer must consult the mapping that is in this chapter or additional mapping listed at the end of the chapter. It is important that a hydrogeologist is consulted at the initiation of the design process so that the nature and extent of the drilling can be assessed early.
The cost of well drilling is affected by many variables including non-geotechnical factors such as site access and noise restrictions. Various geologic environments present difficulties to the driller that will extend the amount of time necessary to complete a well. A well drilled in an area that has bedrock close to the land surface is considerably easier to drill than a well that has to first penetrate many feet of unconsolidated deposits. The boring through the unconsolidated deposits must be kept open so that the formation does not fall into the boring, locking the drill tools into the earth. This is done by either using high density drilling fluids to drill through the earth or by installing steel casing as the boring is drilled. If the objective is to tap an unconsolidated aquifer, the well must be finished with a well screen to hold back the aquifer material while letting in the ground water. Bedrock wells, in most cases, do not require well screens. However, bedrock wells generally do not produce as much water as wells screening unconsolidated deposits. Therefore, bedrock wells will be significantly deeper than gravel wells.
Certain sections of New York are underlain by highly productive unconsolidated aquifer materials. Wells tapping these aquifers can produces prodigious quantities of water, limiting the amount of drilling necessary for the project. However, if the underlying aquifer is the Lloyd, as it is in parts of north/central Queens, the New York State Department of Environmental Conservation will not issue a permit for use of the Lloyd, since it is a highly protected aquifer. In that case, it is necessary to utilize any overlying aquifer material, if available, or to drill through the Lloyd, isolating the aquifer with steel casing, and continue into the underlying bedrock.
The following reports were referenced in this chapter:
Buxton, H.T., Soren, J., Posner, A., Shernoff, P.K., 1981, Reconnaissance of the Ground-Water Resources of Kings Queens Counties, New York, U.S. Geological Survey Open Report 81-1186, 59 p.
Bedrock and Engineering Geologic Maps of Bronx County and Parts of New York and Queens Counties, New York, Charles Baskerville, Miscellaneous Investigation Series, Published by the U.S. Geological Survey, 1992.
5. b Choosing a geoexchanger
Brooklyn and Queens, and any site with a similar geology, are well suited to the geothermal earth coupling formed by a supply and a diffusion well. A requirement of 3 gpm per dominant ton is required from the supply well and a responsible diffusion well must be capable of receiving this flow rate. It should be noted the typical diffusion well is "double the size" of the supply well. This increase in size is attributed to the relative hydrologic potential between a well with a depressed source cone around the well head as compared to the impressed diffusion cone around the return well.
The diameter of the impressive and depressive cones are a function of the permeability of the surrounding geological strata. The only limiting factors to these type systems is the availability of 3 gallons per minute per connected heat pump ton of flow and a responsible method of returning the water to the environment. Aquifer testing and modeling may be necessary if large, multiple well systems are to be installed. This will allow a proper design which avoids overlapping cones and provides sufficient water flow. Typical design temperatures for the New York City area are 50°F well water the year around, heating and cooling season (ARI standards rate at 50°F all year around; new ISO standards rate cooling at 59°F and heating at 50°F entering water).
Attention must be paid to the source of the water being returned to any given aquifer. Earth recharge via septic or sewers is not permitted. The Upper Glacial Aquifer which lies closest to the surface in Brooklyn and Queens may carry both natural and industrial contaminants. The Lloyd aquifer which is the next aquifer down is considered to be pure and uncontaminated cannot be used as a diffusion well if the source water is not also the Lloyd Aquifer. See also the section on permitting. Existing installations east of the East River favor this two-well system. See the description below of the four Long Island Power Authority buildings.
Sites where a concern for surface or ground water quality exists typically utilize closed loop systems. These vertical or horizontal closed loop earth coupling systems are designed to move an antifreeze solution through a series of loops arranged either vertically or horizontally. The material used for this piping is high density plastic pipe with a low friction loss and consequent low pumping effort1. This method is employed in areas with polluted water, e.g. do not meet primary drinking quality standards are encountered. Closed loop systems are somewhat less efficient and more costly. There is a trade off between loop pipe length and minimum design temperature. Existing ARI (ARI-330) and ISO (ISO-13256) standards specify design temperatures at 32°F for the heating season and 77°F for the cooling season.
Horizontal closed loops take one of two forms either a straight pipe of approximately 1,000 linear feet per ton, out and returned to the heat pump in a 4-6 foot deep 500 foot trench or a more recent and popular method call the Slinky®. The Slinky® also employs approximately 1,000 linear feet of high density polyethylene pipe but it is coiled and the extended as a flat map similar to child's slinky toy. In this manner an 80-100 foot trench can be loaded with 1,000 feet of pipe providing a one ton capacity. Generally, if trenching is easily achieved and no sharp rocks exist, a horizontal earth coupling system can be less costly than other systems, with the exception of the two well system described above. Typical Slinky®. Installation is shown in figure 2b-4
Vertical Closed Loop earth coupling utilizes the same design specifications and piping methods and material. Antifreeze solutions in the loops are also required. As the earth is warmer at depth, the heating dominated vertical closed loops typically require only 300-400 linear feet of pipe per ton, see figure 5. The average practical heating dominated borehole is 300-400 feet deep, this implies approximately two tons capacity per bore hole.
A cooling dominated vertical closed loop may require nearly twice that length. (keeping in mind a cooling load not only must remove the sensible and latent loads of the building, but also must remove the inductive heat generated by the heat pump's motors.)
See appendix C for the software modeling available for closed loop systems. Loop length and commensurate cost are reduced as the design temperature limits are further away from the average earth temperature, 51°F in this example.
Performance and efficiency of a typical heat pump in the heating mode at 30°F versus operation at 20°F can be reduced by 15%2. In the cooling mode at 70°F vs. 90°F the reduction in performance is approximately 9%, with a reduction in efficiency of approximately 25%! In this example, these penalties are offset by a reduction on total closed loop length by approximately 80 feet per ton for heating and 90 feet per ton for cooling requirements.
While reducing the cost of the loop field is an important design factor, it also can severely impinge upon a design safety margin. Designing at the heat pump's minimum or maximum entering temperature limits provide no design. An unusually cold winter or hot/moist summer can place a higher demand on the closed loop than published design conditions, leaving no capacity in the ground loop and driving the loop temperatures beyond the heat pump's design capabilities.
Closed loop systems, are designed for heat pump evaporators operating at or below 32°F, e.g. entering water temperatures below approximately 38°-39°F. Because of the probability of creating ice in a heat pump's evaporator heat exchanger, good closed loop design practices require an antifreeze be added to the closed loop. While the antifreeze will somewhat decrease efficiency it permits the heat pump to operate at these lower temperatures. Antifreeze solutions are typically designed for temperatures 10°F lower than the minimum entering water solution. A common solution of 20% food grade propylene glycol, this solutions provides an 18°F freezing point. This implies a minimum of 28°F (i.e. +10°F) minimum entering water temperature from the ground loop.
We recommend the use of propylene glycol as it is not a pathenogenic poison and is environmentally friendly. However, propylene glycol solution tends to become increasingly viscous as temperatures go below 35°F. The designer must consider this increase in viscosity when designing ground loop pumping.
Other antifreezes without the increased viscosity effect, as methyl alcohol (methanol) are equally effective as antifreeze agents, however, its designation as a poison and flammability do not recommend this compound. Several ethyl alcohol (ethanol) based compounds are also available to the designer. Use of these antifreeze solutions should be tempered with a review of the denaturants used in the ethanol solution. Some of ethanol's denaturants are equally poisonous as methanol.
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