Closed Loop Geothermal Heat Exchangers

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Close loop earth coupling is used in areas where the installation is simple or there are issues with the permitted use of ground water (aquifers reserved by governing bodies), lack of water or with the water quality. This type of geothermal heat exchanger consists of closed loops of plastic piping.

Geothermal Heat Exchanger

As closed loop piping and the required grouting material add another layer of heat transfer resistance, the closed loop system is designed for lower winter entering water temperatures and higher summer entering temperature. These wider temperature ranges reduce the effective capacity and efficiency of the heat pump system. Closed loop piping costs are typically in the same range as the cost of the heat pumps and thus add to the overall cost of the installation. Generally, these closed loop systems are more first cost than the above systems. A compensating increase in the overall system efficiency can be anticipated by the reduction in loop pumping costs. However, the requirement for slightly larger equipment remains a disadvantage. Closed loops with environmentally friendly propylene glycol may not realize as much pumping advantage in the winter period as the typical propylene glycol concentrations tend to greatly increase solution viscosity. Typical closed loop spacing is 15-20 feet and are 300-400 feet deep. In the areas of NYC (Brooklyn, Queens, parts of Staten Island and limited areas of Manhattan and the Bronx) that allow the installation of vertical closed loops, the typical closed loop bore will provide 2 tons of heat transfer. Closed loop systems are installed and serviced by contractors certified by the International Ground Source Heat Pump Association (IGSPHA).

Ohm-ft Ohms CPM

Resistivity Single-Point Resistance Gamma

0.25-ft spacing Sensitivity = 20 ohms/div Time constant = 5

Ohm-ft Ohms CPM

Resistivity Single-Point Resistance Gamma

0.25-ft spacing Sensitivity = 20 ohms/div Time constant = 5

Three plots are shown here that depict different logging functions made in an open sandstone hole. The single-point resistance log shows a sharp deflection to the left, beginning at 254 ft (77.4 m) and ending at 260 ft (79.3 m). This indicates a low-resistance material (shale). The 0.25 normal resistivity also shows the beginning of low-resistance material at about 253 ft (77.1 m). In addition, the gamma-ray log shows a slightly higher gamma-ray count beginning at 254 ft, indicating an abundance of clay minerals contained in the shale. Notice the excellent correlation of the sandstone layer between 260 ft (79.3 m) and 270 ft (82.3 m) on the single-point and resistivity logs. This layer also shows up slightly as a small deflection to the left in the gamma-ray log. This indicates that the readings were influenced by actual changes in formation, and not simply by changes in borehole diameter (which can affect the single-point resistance log and the 0.25 normal resistivity log). Below 320 ft (97.6 m), single-point and resistivity logs show changes in material that are not shown in the gamma-ray log. In this case, the two resistance logs appear to be influenced by changes in the borehole diameter or the chemical quality of interstitial fluid, and not by actual changes in lithology. This theory could easily be checked by running a resistivity log with greater electrode spacing or a caliper log. (Howe, 1979)

Courtesy:

Groundwater and Wells, Driscoll F.G., 1986 Johnson Screens, St Paul. MI.

Introduction

Hand-carried terrain conductivity devices use electromagnetic waves to measure the canductivity of earth materials. Direct contact with the ground is not required during data gathering. Thus, subsurface information can be obtained quickly in both highly urbanized and rural environments.

Courtesy:

Groundwater and Wells, Driscoll F.G., 1986 Johnson Screens, St Paul. MI.

This chapter is an overview what needs to be known about the hydrogeology of a site before committing to the implementation of a ground sourced heating and cooling system. The various system options, such as closed loop, standing column or extraction/ injection systems are described in more detail in other chapters of this manual. The purpose of this chapter is to assist the engineer in determining what type of system is most suitable for the given geologic conditions for the particular location selected, building configuration and load.

The primary determining factor for the system type that is most suitable for a given location is the geology, or more specifically the hydrogeology. For example, if a system is to be designed using extraction and injection wells, it will be necessary to develop wells that have sufficient capacity to provide the required quantity of water for the system needs (based on the tonnage of the system). Conversely, it will be necessary to develop wells that can accept the quantity of water that is extracted. If, as an example, the geother-mal system requires a flow rate of 100 gallons per minute [gpm], wells capable of providing an excess of 100 gpm and wells capable of receiving an excess of 100 gpm will need to be completed. Such wells are possible in Queens, Brooklyn, eastern Staten Island and limited areas of Manhattan and the Bronx. Specifically areas that do not have highly productive granular aquifers below the surface. Areas that are underlain by glacially scoured bedrock, as is most of Manhattan and the Bronx, are typically only suitable for the development of "bedrock" wells.

Bedrock wells which are either standing column wells or boreholes into which vertical loops have been installed, are typically open borings six inches, or larger, in diameter and are drilled deep into the bedrock, as much as 1600 feet or more. Steel pipe is used to isolate the upper portion of the well, that penetrates the soil that overlies the bedrock. Therefore, a bedrock well, from the surface, appears to be a six inch, or larger, steel pipe in the ground. The water that is developed from bedrock wells comes from fractures in the bedrock. The fractures form part of an interconnected system that eventually reaches the top of the bedrock surface where surface water or precipitation recharges the bedrock aquifer. The size and extent of the system of fractures encountered by the well dictates the productivity of the well. Bedrock wells drilled at random locations produce an average yield of water typical for the area, generally less than 10 gpm. Occasionally a well will intercept a large fracture system that will produce sizable quantities of water. Such wells are relatively rare, but the chances of drilling such wells can be improved if the location of high yield fracture systems are known and are reachable from the project location.

In areas that are underlain by unconsolidated aquifer materials composed of sand and or gravel, shallower wells that are more productive can be completed. These areas include portions of Kings, Queens and Richmond counties, and isolated areas in the Bronx and Manhattan. Such wells are generally constructed of six inch, or larger, steel casing or well screen to hold back the soil and slotted steel casing in the most productive portion of the aquifer, to allow the entrance of water into the well. The productivity of such wells depends on various factors including the transmissivity of the aquifer, the thickness of the aquifer and the well design. A poorly designed well will not allow the efficient withdrawal of water from the aquifer, regardless of the productivity of the i

Undisturbed samples are obtained with a split spoon by driving the hollow sampler into the ground. After retrieval, the sampler is split open for visual observation.

Courtesy: Groundwater and Wells, Driscoll F.G., 1986 Johnson Screens, St Paul. MI.

In this sampling procedure, a 4 1/2" (114 mm) OD drill pipe equipped with a diamond or carbide coring bit drills a 4 7/8" (124 mm) diameter hole while cutting a 2" (51 mm) diameter core.

Courtesy: Groundwater and Wells, Driscoll F.G., 1986 Johnson Screens, St Paul. MI.

aquifer. Conversely, a well with a screen slot size with too wide spacing for the given formation will not prevent the aquifer material from entering the well and damaging the pumps and or geothermal systems.

Clearly, the construction of geothermal systems requires some advanced planning to optimize the system's efficiency. Knowledge of the site-specific geology is as critical to the design of the geothermal system as is knowledge of the heating and cooling demands of the project. This chapter will not impart the reader with the site specific geologic information required to design a geothermal system. However, the reader will gain a general understanding of the use of geologic resources available for a particular area of concern so that preliminary design decisions may be made. The designing engineer will be able to assess the likelihood of the type of geo-exchange system that may be employed (most suitable) at a given location. However, since the manual does not deal with site-specific geology, conditions may be encountered at any given location that will give the designer more options to choose from. A part of this discussion will be a suggested procedure for drilling wells that will take advantage of the site-specific geology that may be encountered.

All projects will require a hydrogeologist to recommend test well, and production well design for specific sites. There is coordination required between the hydrogeologist, site engineering, consulting engineer and architect in order to bring the project to fruition.

Carrier

Water Drilling Machine Diagram

Mud pump

Cooler

Hydraulic reservoir

Hydraulic cylinders Wat©r injection Control system station

Traveling blocks

Water swivel

Sand reel Drawworks Gearbox Rotary table

Schematic diagram of a direct rotary rig illustrates the important operational components of this truck-mounted drilling machine. This machine, operating with either an air-based or water-based drilling fluid, can drill more rapidly than a cable tool rig.

Courtesy: Groundwater and Wells, Driscoll F.G., 1986 Johnson Screens, St Paul. MI.

Water Drilling Machine Diagram

Components of a complete drilling fluid circulating system for a direct rotary rig.

Courtesy: Groundwater and Wells, Driscoll F.G., 1986 Johnson Screens, St Paul. MI.

Drawworks Sand Reel

Settling pit

DIRECT ROTARY

REVERSE ROTARY

Pump

Shallow ditch

Booster pump

DIRECT ROTARY

Components of a complete drilling fluid circulating system for a direct rotary rig.

Courtesy: Groundwater and Wells, Driscoll F.G., 1986 Johnson Screens, St Paul. MI.

Scw Geothermal Heating

a. Hydrogeology of NYC

b. Choosing a Geo-exchanger using Hydrogeological Analysis

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