Emerging Technologies: Variable-Speed Drives. The IRC's Todd Jekel's economic analysis of a typical cooler and freezer application showed that variable-speed drives, also called variable-frequency drives (VFDs), installed on evaporator fans provide payback in fewer than three years in a system with the fans running continuously. For a system cycling the fans, savings are diminished somewhat, but payback still is fewer than four years. Jekel reported that varying compressor suction pressure in response to load variations provided energy savings. However, the savings may be not large enough to justify the additional controls that would be required for implementation. For a complete copy of this presentation, go to www.irc.wisc.edu/activities/rtforum.php.
Field Evaluation of a VFD Screw Compressor. Two speakers made presentations on this topic. First, Dan Webb from CF Industries, Long Grove, Ill., presented energy-saving results from CFI and IRC's joint research on two 150-hp low-stage screw compressors installed in early 2003 at an ammonia storage facility. The two compressors are identical except only one unit is equipped with a variable-frequency drive. Monitoring equipment included power transducers to measure true electric power demand by the compressor and a pitot-tube type flowmeter to measure discharge gas flow from the compressor. Other data were taken to allow calculation of refrigeration capacity and efficiency.
Data collected for the variable-speed compressor operating under fixed-speed and variable-speed modes show that when the compressor operates near full load, the variable-speed compressor's performance is slightly worse than at fixed speed, which is attributable to drive losses. However, as the compressor's part-load ratio decreases, the variable-speed operation begins to show a considerable efficiency advantage.
In this application, the variable-speed compressor would save approximately $4,000 per year in energy costs for an approximate five-year payback. Other benefits include better process control and possible maintenance savings.
Second, in the same session, IRC's Doug Reindl looked at how floating-head pressure control increases condenser-fan operation to reduce compressor energy use. In a system with significant reserve condenser capacity, such as one designed to maintain peak condensing temperature in the range of 85 to 90oF (29oC to 32oC) and 152 to 165 psig, there is a point at which running additional condenser fans to reduce head pressure actually uses more energy than is saved.
University of Wisconsin graduate student Kyle Manske analyzed energy use of such systems with five different control options, including various approaches with fan cycling, two-speed motors and variable-frequency drives. Results showed that the variable-frequency drive control for condenser fans had the lowest total energy use for the system when sequenced properly.
The optimum head pressure control setpoint generally is as low as possible on a system with condensers selected for a design with condensing temperature of 95oF (35oC) and 181 psig. On systems with additional condenser capacity, the optimum head pressure setpoint is a linear function of the outdoor air wet-bulb temperature.
For a complete copy of the presentation, go to www.irc.wisc.edu/activities/rtforum.php.