ASHRAE Journal - March 2009 - 60

cooling is the dominant mode, this tends to occur at the building loop return/groundwater leaving end of the exchanger. The lower the approach temperature at this point, the more favorable the operating temperatures in the building loop. As in any heat exchanger decreasing temperature difference between the two sides is accompanied by increasing surface area and cost. Evaluation of this issue4 indicates that reducing the approach from 9°F to 3°F (3°C to 1.7°C) (while holding all other parameters constant) in a variety of applications will increase system EER by approximately one full point and the economic optimum (considering increasing exchanger cost and decreasing system operating cost) lies in the range of 2°F to 4°F (1°C to 2°C). When using an open loop design program or spreadsheet of the type illustrated in Table 2, the approach value can be varied to determine its impact on system design. Exposure of groundwater to air can result in undesirable changes to the water chemistry arising from oxidation of dissolved constituents (most often ferrous iron) or evolution of dissolved gasses such as carbon dioxide. As a result an effective strategy for minimizing chemistry-related maintenance is to maintain the groundwater side of the piping system full of water under all conditions. One way to accomplish this is to place a pressure-sustaining device on the disposal end of the groundwater piping as close to the disposal point as possible to maintain a slight (a few psi) positive pressure at the high point of the piping. A variety of devices (diaphragm-operated valve, actuated valve responding to pressure transducer, spring loaded check valve, etc.) can be used to provide the necessary action. A key consideration is that the device be characterized by sufficient controllability to be effective at the minimum system flow rate. The type of well pump control, either variable speed or intermittent constant speed, tends to influence the selection of the pressure-sustaining strategy in a given application. The groundwater disposal method recommended for large open loop systems is injection. By eliminating any net water withdrawal, it ensures the operation of the system will not have an adverse impact upon the aquifer water level over time. In many states with water rights legislation, injection is promoted by the awarding of “inferior” water rights if surface disposal is proposed. Most importantly, injection by “recycling” the water back to the source aquifer helps to preserve the positive environmental expectations the public associates with GSHP systems. Though the design of water wells is outside the scope of this article a few issues relative to the injection well are useful to mention here. Water must always be returned to the well though an injection tube (or drop pipe) that extends below the static water level. This helps to reduce turbulence and entrainment of air in the injected water. Injection wells equipped with screens are typically designed with a screen exit velocity (velocity of the water passing though the openings of the screen) of 0.5 ft/sec (0.015 m/s) maximum or one half that of recommended production well screen entrance velocity (0.1 ft/sec [0.03 m/s]).8 Additional information on water well design is available in the references.8,9,10,11 60 ASHRAE Journal 600 Spacing (ft) 400 200 0 0 100 200 Flow Rate (gpm) 20 40 80 300 400 100 Figure 4: Minimum production/injection well spacing requirements. Well Site Selection Locating the production and injection wells on the project site involves both practical considerations such as rig access, avoidance of existing subsurface utilities/structures and coordination with other construction activities. In addition, there are the aquifer considerations of gradient and separation distance. Water in underground aquifers is moving—from areas of recharge to areas of discharge. Velocities are low—in the ft/ day (m/day) to ft/yr (m/yr) range—but moving nonetheless, and this issue impacts well siting. It is desirable to place the production wells upstream on the site and the injection wells downstream in terms of the aquifer flow. Identifying aquifer flow direction is a matter of measuring water level in two or more wells and correcting the water levels to sea level elevations using the ground elevations at the well heads. Aquifer flow is in the direction of the water level gradient defined by these elevations. Placing the injection well down gradient of the production well helps to ensure that the natural aquifer flow will move injected water (at less desirable temperature) away from the production well. Spacing is as important in open loop systems as it is in closed loop bore holes—and for the same reason. The greater the distance between two wells, the lower the potential for interference between them. Figure 4 presents a summary of spacing requirements based on a variation on a method12 developed some time ago. Using a value of 50% of the peak groundwater flow for the system in question, and reading vertically to the intersection with the curve representing the aquifer thickness at the site and then reading horizontally, the minimum necessary spacing between the production and injection wells appears on the vertical axis. Separation distance correction factors for the porosity of the aquifer materials are applied in the event this value departs from the 20% on which the curves are based. This approach to well spacing is appropriate for unconsolidated aquifers (sand, gravel clay, and similar materials). Fractured rock aquifers are more complex and projects in such settings should involve a groundwater ashrae.org March 2009

ASHRAE Journal - March 2009

Table of Contents for the Digital Edition of ASHRAE Journal - March 2009