In refrigeration systems, the compressor is an energy hog, often consuming about 70 percent of the system’s total electricity. Reducing the amount of time the compressor runs can substantially lower the amount of energy used. However, minimizing compressor run time without affecting the refrigeration temperature is a challenge.
The refrigeration cycle is dynamic and changing. From the time the compressor starts until it stops, the suction pressure, evaporator temperature, rate of heat exchange, refrigerant flow and many other factors are changing continuously. The total efficiency of the system changes through the entire life cycle.
Most modern compressor controls concentrate on the conditioned space temperature, the chilled water temperature or suction pressure. The compressors are switched on in response to a call for cooling and will operate until that demand for cooling is satisfied. To avoid rapidly and repeatedly switching the compressors on and off (short cycling), which will cause damage to the compressors, the control bands usually are set with a minimum differential of approximately 4 psi or 5°C (9°F). In many cases, the compressor control differential is greater than this.
The compressor’s capacity to remove heat is directly proportional to the operating temperature. That is, the higher the controlled temperature, the higher the suction temperature (evaporator temperature) and the faster the rate of heat removal.
For example, figure 1 shows the compressor cooling capacity required at each degree of suction temperature to reduce the temperature from 10 to 5°C (50 to 41°F). When the compressor first starts at the high limit point, it is operating at maximum efficiency with a high suction pressure. As the conditioned space temperature is reduced, the suction pressure drops and compressor capacity is reduced. Each degree of temperature reduction takes a longer period of time and uses substantially more electricity.
As shown in the example in table 1, reducing from 10 to 9°C (50 to 48.2°F) takes only 6.6 minutes and 1.84 kWh, while reducing from 6 to 5°C (42.8 to 41°F) takes 14.4 minutes and 3.57 kWh. The last degree of pulldown uses almost twice the energy and time.
This example uses only a ±2.5°C (4.5°F) suction temperature differential. However, even with this small of a control band, a significant difference in energy consumption exists between the first degree of reduction and the last degree of reduction. Figure 2 illustrates the percentage of energy used per degree. The last degree of pulldown used 28 percent of the total energy for the cycle.
Eliminating the last two degrees by starting at 10°C (50°F) and stopping at 6°C (42.8°F) would provide a differential of 4°C (7.2°F) and reduce energy consumption by 28 percent. Unfortunately this solution also increases the midpoint temperature from 7.5 to 8.0°C (45.5 to 46.4°F), which is unacceptable.
A Temperature-Independent SolutionModern control technologies can reduce the electricity consumption (kWh) and maximum demand (kW/kVA) of refrigeration compressors by improving their performance and maintaining temperature control. Some of the most advanced systems are not controllers themselves but instead supplement the existing control system. They are designed to work with the existing refrigeration equipment, along with the current control methodology, to reduce energy consumption. When a call for cooling comes from the existing controller, the advanced control module takes over to determine when and for how long each compressor or unloader will run.
Because the primary controller is not replaced, the advanced control module can be put into bypass at any time and then returned to operation exactly as it was prior to the installation. This is an important benefit for system repairs or troubleshooting.
These advanced control technologies enable the compressor to maximize the rate of heat removal by optimizing the natural physical properties of the compressor operating cycle. This process, known as “compressor optimization,” can reduce the compressor run time by up to 30 percent without affecting the temperature conditions, as illustrated in figure 3. Through the use of proprietary software, these systems manage the suction pressure of the refrigeration system in order to cool more efficiently (figure 4).
The systems can be used for single compressor applications or multi-compressor parallel racks and packaged units. Additionally, through an intelligent interface module, even the most complex chiller packages can benefit from implementing this technology.
Energy SavingsWhile the technology primarily has been aimed at commercial and retail facilities, a number of industrial process cooling operations increasingly have begun taking an interest in compressor optimization using the advanced control devices.
For example, Carclo PLC, headquartered in Ossett, UK, is a global company with more than 1300 employees worldwide that designs, manufactures and sells injection-molded plastic components. The technical plastics division of Carclo (CTP) supplies multinational customers in the automotive, optical-medical and teletronics industries.
Several years ago, the company tested a System 4000 Energy Saving Module (ESM) supplied by Smartcool Systems Inc. on the refrigeration and air-conditioning compressors at its plant in Mitcham, Surrey, UK. The system provided a total annual reduction of 270,000 kWh - a 30 percent savings - which equates to enough energy to power roughly 90 homes. After installing the devices, the company saw a return on its investment in just over 14 months.
“In the business that we are in, and with the ever-increasing costs of energy, we are always looking at ways to reduce the amount of energy that we use,” said Peter Callistan, group health, safety and environmental manager at Carclo. “Smartcool has provided us with a product that significantly reduces the kWh consumption used by our process chillers with a payback on capital of just over a year. And importantly, all of this is achieved without any detrimental effect to our plant, operations and processes.”
The company has since installed the technology in other operations and at other sites.
At a Lancaster Foods cold storage facility in Jessup, Md., the same technology was installed on three packaged refrigeration compressors. The compressors serve one of many cold storage chambers and must maintain an average temperature of 0.6°C (33°F).
During a two-week test, the electricity consumption of the refrigeration compressors was measured by data loggers. Temperature loggers also were installed to measure and record the temperature of the cold storage chambers and ambient conditions. Results from the evaluation demonstrated that the technology significantly reduces kWh consumption, providing both financial and environmental benefits, without compromising the temperature requirements of the cold storage facility (figure 5).
As energy costs continue to rise, companies with process cooling requirements should evaluate the efficiency of their compressors. Adding intelligent control capabilities can optimize compressor operation, leading to substantial reductions in energy use.
Smartcool Systems Inc., based in Vancouver, BC, Canada, is an advanced energy conservation solutions company that specializes in energy and cost reduction technologies. The company’s wholly owned subsidiary, Smartcool International Inc., is the developer and manufacturer of the Energy Saving Module (ESM). For more information, contact Smartcool Systems USA Inc. at (888) 669-1388, e-mail email@example.com or visit www.smartcool.net.