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Design issues in commercial open-loop heat pump systems.

Article Excerpt
INTRODUCTION

Open-loop or groundwater heat pump (GWHP) systems are the oldest ground-source heat pump system, with the first large building systems installed in the late 1940s. Despite this, little formal design information has been available for them until recently.

Several variations on the open-loop system are in use. The most common of these are illustrated in Figure 1. Direct use of the groundwater in the heat pump units is largely an extension of residential design and is sometimes practiced in very small commercial applications.

[FIGURE 1 OMITTED]

The building loop in such systems is susceptible to water-quality-induced problems, the most common of which are scaling or corrosion of the refrigerant-to-water heat exchangers. This design is recommended in only the smallest applications in which practicality or economics precludes the use of an isolation heat exchanger and/or groundwater quality is excellent (the determination of which requires extensive testing).

The standing-column system is distinguished by the return of groundwater to the same well from which it was produced. Like the direct groundwater system, it too is subject to water-quality-induced problems in the absence of ground-water isolation. In general, water quality in the area where most of the installations have been (New England) is extremely good with moderately low pH and hardness (little scaling potential).

Standing-column systems are used in locations underlain by hard rock geology, where wells do not produce sufficient water for conventional open-loop systems and where water quality is excellent. Well depths are often in the 1000 to 1500 ft (320 to 460 m) range and the systems operate at temperatures between those of conventional open-loop and closed-loop systems. In colder climates, this sometimes precludes the use of a heat exchanger to isolate the groundwater.

Indirect open-loop systems employ a heat exchanger between the building loop and the groundwater. This eliminates exposure of any building components to the groundwater and allows the building and groundwater loops to be operated at different flows for optimum system performance. Water is typically disposed of in an injection well for aquifer conservation or to a surface body (lake, river, creek, etc.) if one is available and aquifer depletion is not expected. Properly designed indirect open-loop systems can offer energy efficiency comparable to closed-loop systems at substantially reduced capital cost. Due to the elimination of water quality and geology limitations, this system type is widely applicable geographically and constitutes the focus of the remainder of this paper.

DESIGN STRATEGY

Although seemingly simple in nature, these systems require careful consideration of a host of factors including groundwater flow, well pump control, water chemistry, heat exchanger selection, water well design, particulate removal, production/injection well separation, and other issues so that an efficient and reliable system results. The key task in the design is the identification of the optimum groundwater flow. Groundwater flow in a conventional open-loop system is analogous the loop length in a closed-loop system. The larger it is, the higher the performance of the heat pumps.

In general, the approach is to evaluate the performance of the system over a range of groundwater flows to determine the flow that results in the maximum system performance in the dominant mode (cooling in most applications). This flow defines the design of the wells, well pump, heat exchanger, and other major components of the system. Well pump control is then configured to maintain this optimum performance at off-peak conditions.

Key to this approach is the concept of system performance. In this context, system EER is defined as the cooling capacity divided by the sum of heat pump power, loop pump power, and well pump power. System COP is similarly defined. In any conventionally configured open-loop system, the greater the groundwater flow for which the system is designed, the more favorable the operating temperatures for the heat pumps. This results in a decreasing power requirement for the heat pumps as groundwater flow is increased. Focusing only on the performance of the heat pumps would tend to lead to the selection of relatively high (2.5 to 3 gpm/ton [0.045 to 0.054 L/s kW]) groundwater flow rates. When the power requirements of the well pump are incorporated, however, it is apparent that any system would exhibit a performance similar to that shown in Figure 2. As groundwater flow is increased, system performance increases to a point where it reaches a maximum after which additional increases in flow result in a decrease in system performance. On the left side of the curve, incremental increases in groundwater flow result in a larger decrease in heat pump power requirement than the well pump power necessary to produce the flow. On the right side of the curve, well pump power increases associated with flow increases outweigh improvements in heat pump performance. Any indirect open-loop system will be characterized by a similar curve. The shape of the curve is a function of the site characteristics (static water level, specific capacity, etc.), and in the absence of a design method that simultaneously considers well pump power and heat pump performance, it is not possible to identify the optimum groundwater flow for the system. Arbitrary groundwater flow selection by designers or determinations based on heat pump performance have often resulted in systems operated at excessive groundwater flow. Operation at high groundwater flow compromises energy efficiency and increases capital cost for the owner.

[FIGURE 2 OMITTED]

WATER WELL TERMINOLOGY

Wells are the foundation of open-loop systems, and as such it is useful to review certain key terms prior to a detailed discussion of system design. Figure 3 provides a generalized diagram of a water well. In any well there will be a water level at which the water stands in the well under nonpumping conditions. This level is indicative of the water table level in unconfined (or water table) aquifers or the piezometric level in a confined (or artesian) aquifer and is known as the static water level (SWL). When the pump is started, the water level will normally drop to a new, lower level referred to as the pumping level. The pumping level is a function of the rate at which the well is being pumped--the greater the rate, the lower the pumping level. Both SWL and pumping level measurements are referenced to ground surface. The difference between the SWL and the pumping level is referred to as the drawdown. Drawdown at a given pumping rate divided into the rate results in a value known as specific capacity with units of gpm/ft (L/s*m). Specific capacity is a useful value for indicating the ease with which the aquifer produces water. A high value (10 gpm/ ft [2.1 L/m*s]) indicates favorable production, whereas a value of 0.5 gpm/ft (0.1 L/m*s) indicates a poorer producer. For artesian aquifers, specific capacity will be a constant value over a broad range of flows. In water table aquifers, specific capacity will diminish as pumping rate increases but may be treated as a constant over a small range of flows in many cases.

[FIGURE 3 OMITTED]

The drawdown at a given rate is...

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