All-Variable Speed Chiller Plants

Hartman (2001) presents the application of multiple, variable-speed hydronic pumps with energy-efficient variable speed chillers.

New, all-variable speed chiller plant operating strategies have been developed that are based on the specific performance characteristics of variable speed chillers and the variable flow characteristics of condenser pumps and cooling towers. Substantial annual energy use reductions can be expected from optimized all-variable speed centrifugal chiller plants compared to constant speed chiller plants with the same first cost.

According to Hartman (2001), when sequencing and controls are optimized, these plant configurations will use 28 percent less energy, on average, than conventional constant speed chiller plants. Several concepts pertaining to all-variable speed chiller plant's "wire-to-water" performance are reviewed including "natural curve" sequencing and power-based speed control. The following advise is provided:

A strong tie exists between the operation of the chillers and the heat rejection systems. Several rules concerning the application of variable speed chillers have been derived from such analyses of variable speed plants. These rules are:

1. When variable speed chillers are used, optimum performance and simplicity of operation is achieved when all chillers in the plant are variable speed and have identical performance characteristics.
2. To concurrently achieve the benefits of reduced condensing temperatures and low condenser pump/tower fan power consumption, condenser pumps and tower fans should also use variable speed, making the plant an all-variable speed chiller plant.
3. The focus must be on operating chillers at equal loading and as near as possible to their natural curves, and also on coordinating chiller, condenser pump, and tower fan power to minimize the overall plant power consumption at each load condition. Such coordination is most simply achieved by controlling pump and fan speed directly from percent of maximum input chiller power rather than to meet specific tower or condenser water temperature setpoints.