The refrigeration industry has spent vast sums of money to switch from CFC and HCFC refrigerants to protect the ozone layer. The next important step in environmental assistance is to reduce the total amount of electricity required for cooling.
Reducing electrical consumption reduces the carbon footprint of the process chiller, emissions from electrical generation power plants and, ultimately, global warming. Reducing electrical consumption also saves money. More energy-efficient motors and compressors are being developed every day. While conserving electrical consumption sometimes means compromising production capacity, it does not have to be the case. The use of hybrid chillers provides a way for process chillers to optimize production capacity and energy conservation.
Process chillers typically provide 50°F (10°C) cooling water. This water is delivered throughout a production facility for machine cooling or process and production temperature control. Yet some points of use in a production facility - for instance, water-saver valves on a hydraulic oil cooling exchanger - restrict the flow of 50°F cooling water to prevent equipment overcooling. Other points of use in a production facility may run the water through secondary temperature control units to maintain a temperature higher than 50°F. This means that many process chillers typically produce colder water than required. This is a waste of energy.
Process chillers run 12 months a year, delivering 50°F chilled water even during cold winter months. How many of us have witnessed these chillers running when ambient temperatures are very low? It does not make sense to run a compressor to develop 50°F water when it is 10°F (-12°C) outside. Ambient temperatures in much of the United States make it possible to deliver cold process water for a significant portion of the year without the use of a refrigeration compressor. Figure 1 shows data collected at Scott Air Force Base outside Chicago, Ill.
For example, 69°F (20°C) process water could be delivered by free cooling in ambient temperatures of up to 59°F (15°C). Ambient temperatures of 59°F or less exist 4,688 hours a year in the greater Chicago area. That means for 4,688 hours per year, the production facility can get all the cooling water or glycol needed with free cooling, without running the chiller compressor.
For example, a 110 hp compressor running at 480 V consumes approximately 100 kW of electricity per hour. At an electricity rate of $0.10 per kilowatt-hour, it costs $10 per hour to run the compressor. If the example production facility outside Chicago requires 70°F water, then 54 percent of the cooling demand can be provided by free cooling, at an annual electricity cost savings of almost $47,000 (table 2).
If the production requires 50°F water, then annual energy cost savings is still almost $22,000. Figure 2 illustrates the relationship between the electricity cost savings and process water temperature at our example plant in Chicago running a 110 hp hybrid chiller and free cooling system.
In addition to these electrical cost savings, many municipalities offer energy rebates for free cooling systems. A typical hybrid chiller cooling system may pay for itself in less than two years. Hybrid chiller cooling systems offer other benefits as well. Reducing run time on the compressor extends the equipment’s life, and the free cooling circuit provides emergency cooling backup in the event of a compressor outage.
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