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.
Hybrid chillers that incorporate free cooling into the chilling system reduce electrical consumption, save money and help the environment without compromising production capacity. Hybrid chiller systems use outside ambient temperatures to assist in cooling the water or glycol instead of relying on the chiller compressor for 100 percent of the cooling. In some ways, it is not unlike a hybrid vehicle. In a hybrid electric car, the battery runs the car when the power demand is low; when demand increases, the gasoline engine kicks in to provide the required power. A hybrid chiller works the same way. Free cooling provides the necessary cooling when ambient temperatures allow; when ambient temperatures increase, the compressor kicks in to provide the required cooling.
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.
If a production facility in the Chicago area required a maximum of 70°F (21°C) cooling water, it could not get by with just a cooling tower because the tower could not provide 70°F year-round. Yet a hybrid chiller and free cooling system would be able to provide water at 70°F or colder all year - and for almost 4,700 hours without running the compressors. That means the production facility would realize free cooling for 54 percent of a 24/7 operation. If the production facility required a maximum of 50°F instead of 70°F cooling water, it would still achieve free cooling for 2,200 hours, or 25 percent of a 24/7 operation. Table 1 illustrates the process fluid temperature that can be achieved in the Chicago area at varying ambient temperatures and indicates how many hours per year the necessary ambient conditions exist.
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.
Energy conservation is good for the environment, but it also is good for business. Hybrid chiller cooling systems can help a manufacturing facility save money. Chillers consume a lot of electricity, with most of the energy consumption coming from the compressor. Hybrid chiller systems are designed to minimize the time the compressor must run. Every hour the compressor does not run saves money.
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.
If the production plant can utilize warmer water or if the ambient temperatures are colder, then the energy cost savings increase.
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.