Techniques such as variable-speed control, duty cycling, demand refrigeration, intelligent engine room control, load shedding and demand limiting can greatly reduce energy costs. All of these techniques are easy to implement with a computerized control system.
1. Variable Speed ControlEquipment is sized for peak capacity requirements, but often it is oversized for the existing load, causing fans, pumps, compressors and other rotating equipment to waste electrical energy. Notable savings can result from reducing the motor speed to match the capacity of the equipment to the load. This goal can be accomplished by installing a variable-frequency drive, which is an electrical device that is designed to operate standard AC induction motors over a speed range of 0 to 100 percent.
For example, in refrigeration applications, evaporator fans often are operated continuously at full speed even when the heat load is minimal. In this situation, the fans produce approximately 70 percent of the refrigeration (heat) load. In addition to the negative impact that full-capacity fan operation has on energy efficiency, the product quality also can be adversely affected. Research has shown that excess evaporator capacity tends to result in less stable temperature control, which is detrimental to highly perishable products. Also, the high air velocity that results from full-speed fan operation can affect the rate of moisture loss, or transpiration, from the product. This situation equates to less product left to sell (as much as 2 to 5 percent weight loss) and lower quality due to shrivel.
Variable-speed evaporator fan control is a technique that typically operates the fans over a range of 40 to 100 percent of the full speed. The energy consumption of the fan motor drops off dramatically as the speed is reduced (figure 1). In addition, the reduction in energy required to drive the fan motors results in less heat being added to the refrigerated space, thus reducing the load on compressors and condensers as well.
Variable-speed technology also is well suited for refrigeration compressor and condenser fan applications. The system uses intelligent control algorithms to operate these critical components at the speed required to match the refrigeration load, eliminating the need to start and stop the motors continually to control capacity. Also, the compressor slide valve can remain 100 percent loaded while the motor speed varies for maximum efficiency.
2. Duty CyclingDuty cycling is a low-cost method of reducing equipment capacity. Some applications permit on/off cycling of equipment to achieve energy savings and match the capacity to the load. In the case of refrigerated storage facilities, the control system provides the capability of cycling zones off at regular intervals. An override feature will restart refrigeration if the zone temperature rises significantly. Adjustable refrigeration on/off schedules are entered easily using a graphic screen. A typical schedule would operate the evaporators only a small fraction of the time.
3. Demand RefrigerationDemand refrigeration is a method of saving large amounts of energy in facilities that permit turning off the refrigeration system for several hours at a time. This technique is different from duty cycling, where refrigeration is shut off in individual zones on scheduled intervals to reduce energy consumption. With demand refrigeration, the entire facility’s refrigeration system is shut down when all of the zone temperatures are reasonably close to setpoint. The system remains off until any single zone requires refrigeration again, indicated by a rise in temperature. At that point, refrigeration is resumed in the entire system until the zones are all back to the desired temperatures. This cycle is repeated continuously.
4. Intelligent Engine Room ControlIntelligent engine room control makes it easy to select capacity control and compressor sequence parameters to fit a plant’s changing control needs. The operator decides which compressors will run under different load requirements and can even define parameters for auto-suction pressure reset control. The plant thereby can ensure that it has the capacity it needs, when it is needed, without wasting energy or accumulating unnecessary run hours on compressors.
Plants that do not use a wet-bulb approach to determine the optimum discharge pressure are working their compressors harder than necessary. Efficient condenser control is as simple as tracking ambient wet-bulb temperature (the ideal limit of evaporative cooling) and continuously resetting the discharge pressure setpoint to use the currently available condensing capacity. Add variable-speed control of condenser fans to achieve optimum energy efficiency.
5. Load SheddingIn load shedding, selected loads can be cycled off at a preset time of day and back on at a later time to reduce energy consumption and time-of-day demand charges. One set of times might be used for weekdays and another for weekends. This same scheme can be used to lower fan speeds or control other aspects of a plant’s operation.
6. Demand LimitingTo reduce demand charges, the rate of kilowatt usage can be monitored by the system, and control algorithms can be implemented to keep the demand from exceeding a maximum limit. As the electrical demand approaches the limit, certain equipment can be inhibited from starting or further loading. If the demand rises above the limit, equipment will begin to be unloaded or cycled off. When the facility load returns to a normal level, the equipment will return to normal operation.
7. Intelligent Defrost Control: Several BenefitsRefrigeration systems typically defrost evaporator coils at preset intervals. This approach often results in unnecessary defrosting, which actually adds heat to the refrigerated space and reduces the time available for product cooling. In some cases, defrost cycles have the effect of reducing the moisture content of highly perishable products. Automated on-demand defrost control can save energy and help improve product quality and weight.
On-demand defrost control is designed to hold off defrost cycles until they are necessary as indicated by the performance of the evaporator coil. A demand defrost condition is detected when the zone temperature is above setpoint and the temperature drop of the air passing through the coil is less than normal. If these conditions exists for a preset delay period, a defrost cycle is initiated.