6 Energy Efficiency
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Energy saving inverters for refrigeration equipment
The use of inverter drives to control the speed of compressors is almost universal in air conditioning and has become increasingly important to the refrigeration sector too. Philip Sloper, managing director of Thermofrost Cryo explains.
INVERTERS ALLOW the capacity of the system to be matched to demand by increasing or decreasing the speed of the compressor(s), and this in turn improves the energy efficiency.
The step-less control of compressor speed, as provided by an inverter, can result in up to 40% reduction in the energy consumption of a single compressor, and up to 25% compared to conventional on/off multi-compressor packs. Inverter technology can also successfully improve the energy performance of chillers, glycol
coolers, trans-critical and sub-critical CO2, dry coolers, condenser fans and pumps. The extent to which the compressor speed can be varied depends on the design characteristics of the compressor, and might typically be between 30Hz and 60Hz – although Bitzer compressors specifically designed for inverters can operate up to 87Hz. This represents a speed (and so capacity) range, compared to that at a fixed 50 Hz, of 60% (at 30 Hz) to 174% (at 87Hz).
While reducing the frequency when loads are low will save energy, the extra capacity available above 50Hz also allows a system to respond to unexpectedly higher refrigeration demands. If the compressor
is sized to match the maximum required load at its highest operating speed, then considerable energy savings will be made when running at part load.
In addition to the energy savings arising directly from the speed control of the compressor, the stable suction pressures achieved gives further energy saving benefits.
The system can run at a higher suction pressure; a 2.5°C rise in suction pressure can yield a 10.2% increase in Energy Efficiency Ratio and each 1.0°C rise in evaporator temperature decreases energy use by four. A steady suction pressure also allows the electronic expansion valve to operate at lower superheats, improving coil and system efficiency and reducing temperature deviation, resulting in a higher relative humidity and less icing of the evaporator.
The resulting decrease in the frequency of defrost cycles reduces the amount of electrical defrost energy used.
An inverter can be applied to an existing installation, after first checking with the compressor manufacturer if their product is suitable, or to new installations where the inverter can be designed in from the outset. In the past inverters have tended
to be applied to large refrigeration compressors, but are now available for smaller compressors too. Sporlan offer PSD inverters right down to 2.5amps. Inverters are also available conveniently packaged into a complete refrigeration condensing unit. For example, the MTH Frigor-Afen range of hermetic inverter units covers applications from 1kW to 9kW, and the Bitzer Ecostar range of semi-hermetic units offers duties from 5kW to 30kW. Both ranges are designed for outdoor installation, making them suitable for many types of application.
In cold stores, inverter units provide a solution to variable loads within the refrigerated space. This is particularly useful in fruit or vegetable stores, where after the initial pull down the load drops dramatically, to which the inverter responds by reducing the speed of the compressor and hence the energy consumption. Dairies also often have wide variations in load, so again the potential for energy saving using inverters is considerable. Matching inverter units with dual circuit evaporators, such as the Küba SG series, allow cold stores to operate over wide variations of load while maintaining good air distribution.
It is also worth noting that with the introduction of new F-Gas regulations, the use of R134a with inverter driven systems provides a bigger energy saving as the power absorbed by an inverter drive compressor for a given duty can be much less than that of a fixed speed compressor using R404A for the same duty. And with a GWP of 1300, R134a meets the new F-Gas requirements. To maximise the benefits of using an inverter, accurate control of refrigerant flow across a wide duty range is desirable. For this reason, most installations use electronic expansion valves like the Sporlan SER stepper motor valve, controlled by a driver such as the Dixell XEV22D. Matching the capacity of the refrigeration plant to the load is a challenge that our industry has faced for many years.
Now, with energy prices higher than ever before and predicted to rise even higher, pay-back times on the higher capital cost of an inverter driven system have fallen making the technology viable for any forward thinking end-user.
A supplement to ACR News May 2014 Visit ACR News online at
www.acr-news.com
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