Significant energy savings through chiller plant optimization

In today's market, all owners and managers of commercial properties are focusing on energy usage and savings. The reason is simple; any energy savings immediately benefits the bottom line and increases the profitability of the property. The use of chilled water for air conditioning applications remains a popular choice for energy savings because of increased efficiency, predicable costs for preventive maintenance, as well as precise control. The key to comfort and energy efficiency is having control of individual equipment and the overall larger main systems. Focusing on the central refrigeration plant, the chiller and the evaporative cooling tower have the largest energy consumption. Compliance with the N.Y.C. energy code promotes the selection of equipment that has good individual energy efficiencies. Fine tuning this equipment to work together throughout the full range of weather conditions to achieve the least cost of operation is referred to as "chiller plant optimization". Utilizing modern digital controls, this optimization is achieved through sound judgment on the part of operating engineers and the use of specialized computer software. Chiller plant optimization is a wise strategy to achieve financially beneficial savings and to contribute to the sustainability of buildings. Proven control strategies significantly improve the efficiency of a chiller plant. These can be broadly defined as cooling tower temperature relief, condenser water temperature reset, variable condenser water flow rate and chilled water temperature reset. Until recently, the application of these strategies to chiller plant operation was problematic and expensive. Most past applications caused excessive chiller starts or chiller surging or both. New application methods are now available, which eliminate the problems of chiller surging and excessive starts and result in long term savings. The choices discussed below use some or all of the strategies defined above. Note that all this does not require special knowledge from manufacturers and can be readily retrofitted to existing plants. 1. Condenser Water Temperature Reset Chiller The Condenser water reset option enhances chiller efficiency to reduce energy consumption. Most of the energy consumed by a chiller is used to move refrigerant vapor from the evaporator (low pressure) to the condenser (high pressure). As the pressure differential between the evaporator and condenser increases, the compressor must work harder to move the refrigerant. Lowering condenser water temperature decreases this pressure differential, so the compressor does less work. 2. Cooling Tower Temperature Relief A cooling tower typically consumes 10%-20% of the energy of the chillers it serves. Intelligent condenser temperature relief can achieve system savings (chiller plus tower) of 20% or more, compared to controlling the tower to the design temperature. Condenser temperature relief consists of a control sequence to reduce the cooling tower condenser water temperature during periods of low-loads and/or low wetbulbs (measure of water in air) temperatures. This practice can achieve significant energy savings if properly applied; but can actually use more energy if improperly applied. All can be achieved by use of reliable sensors monitoring of load on the cooling tower and application of preset computerized controlling methods. 3. Variable Condenser Water Flow Rate As stated above, the cooling tower provides condenser water to the chillers in the refrigeration plant, and, ultimately rejects heat from the building. So varying condenser water flow can save energy by reducing the amount of work the condenser water pumps must do. The condenser water flow rate must be controlled by the load on the refrigeration plant and the cooling tower performance. The condenser water flow rate may not be reduced below the chiller manufacturer's minimum for their equipment. While this strategy can save significant pumping energy, the strategies mentioned above are more likely to produce larger energy savings. However, variable condenser water flow and cooling tower temperature relief can be combined for greater savings then the savings achieved by each strategy alone. Performance monitoring of the chiller plant ensures that the measures being taken result in actual operational cost savings. In many cases, this is accomplished with a packaged controller and instrumentation system that is factory preconfigured. The result of chiller plant optimization is not only financially beneficial and but it also contributes to the sustainability of our industry as a whole. Andrew Kozak is a mechanical department director, JFK&M Consulting Group LLC, New York, N.Y.