Energy costs and operating efficiency have gained considerable importance in the minds of many building owners and plant operators in recent years. Current prospects for future prices of energy resources suggest that these issues will become even more urgent as environmental concerns and the high cost of money exert an ever greater impact on building design and operation.
When weather permits, free cooling systems can generate significant savings for the owners of such systems. However, the amount of potential energy savings available depends almost totally on the overall system design and on the selection of equipment for use in the system. In general, the designer must balance higher equipment cost with greater opportunity for energy savings. Fortunately, these savings — and their associated costs —are reasonably quantifiable so that designers can make intelligent choices guided by reliable information.
A quick glance at Figure 1 shows that the chiller uses the most energy in the system, by far. Simple logic leads immediately to the conclusion that the greatest possible energy savings would accrue from turning off the chiller.
This, then, is the goal of free cooling — to avoid the energy costs associated with operating the chiller. Obviously, some other means of producing the necessary chilled water must be available. Under suitable conditions of weather and heat load, the cooling tower can act as the source of chilled water.
Simply stated, free cooling opportunity is the number of hours per year a given system can operate in the free cooling mode. The designer controls the amount of free cooling opportunity, to a very large extent, by the choices made in the design process.
Three primary factors under the designer’s control determine the amount of free cooling: design chilled water temperature, heat exchanger capacity (for an indirect system) and selected cooling tower capacity. Two other significant variables, load profile and local weather patterns are — quite obviously — beyond the designer’s control; but consideration of their impact can help to maximize the availability of the system for free cooling and assist the designer in balancing initial cost with potential operating savings.