FIGURE 1. The relationship between heat and work in an airconditioning system.

In a chilled-water system, heat that other-wise would be lost to the environment (Figure 1) can be captured and used for other purposes. Although this process, known as heat recovery, is not new, the benefits — decreased energy consumption, reduced greenhouse-gas emissions, more points toward Leadership in Energy and Environmental Design (LEED) certification — have never been greater.

This article examines several methods of recovering heat from chilled-water systems, focusing on the capture of sufficient heat for useful application, the minimization of chiller lift and maximization of chiller efficiency, and the control of hot-water temperature without the loss of chiller-plant operation stability.

USES OF RECOVERED HEAT

Heat recovered from a chilled-water system can be used to generate hot water. This hot water can be used to heat the building, to heat service water, or as part of a manufacturing or industrial process.

Building heating

Understanding hot-water-temperature needs for building-heating purposes is important. Most reheat and building-heat applications do not need 130°F to 140°F water to perform satisfactorily. As discussed later in this article, operating a reclaim chiller at higher leaving condenser-water temperatures (LCWTs) increases lift and reduces chiller-plant efficiency.

In many variable-air-volume reheat applications, hot-water temperatures as low as 105°F can be effective. Lower temperatures maximize chiller-plant efficiency while minimizing system energy consumption. They can be achieved by specifying a two-row reheat coil instead of a one-row coil.

FIGURE 2. Hourly building-heating-plant data.

To better understand building-heating-load requirements and the potential of a heat-recovery system, monthly or even hourly heating- and cooling-load data should be considered. Figure 2 shows a typical hot-water boiler-plant load for an office building in Chicago, while Figure 3 shows typical cooling loads for the same building.

FIGURE 3. Typical hourly cooling-coil loads.

A designer's challenge is to determine when both heating and cooling loads exist and how best to capture heat from a chilled-water system. For building heating, some of the best opportunities exist when a facility operates 24 hr a day, with high internal cooling loads and the possible need for heating for perimeter zones.