Energy Efficiency Measures for Air Conditioning Systems
During summer, air conditioning is the highest energy expense in residential and commercial buildings. This not only represents a high cost for building owners, but also a technical challenge for electric companies. If the power grid has not been upgraded according to demand, the combined air conditioning load from all buildings can overload the system and cause blackouts.
There are three main strategies to reduce air conditioning costs in buildings:
Reducing the need for cooling: This can be achieved by improving the building thermal envelope, minimizing heat gain.
Upgrading air conditioning equipment: For a given workload, equipment with a higher nameplate efficiency has a lower operating cost.
Improving air conditioning controls: Even efficient equipment wastes energy when the thermostat is set too low. Additional savings are possible with a smart thermostat and automatic setback.
How the Building Thermal Envelope Can Reduce Cooling Needs
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Air conditioning systems must deal with internal and external heat sources. Building occupants, appliances and equipment are the main sources of indoor heat, and these are always present. However, an efficient thermal envelope can reduce external heat gain from the warm summer air and sunshine. The most effective building envelope upgrades are the following:
Upgrading to insulation with a higher R-value.
Detecting and sealing air leaks.
Upgrading to energy efficient windows. The highest performance is achieved by triple-pane windows with a low emissivity coating.
Potential window upgrades are evident, but insulation and airtightness issues are more difficult to detect. The best recommendation is getting an energy audit, since consulting engineers can pinpoint building envelope issues with thermal imaging and other methods.
Building envelope improvements are recommended before an equipment upgrade. Since the cooling load is reduced, it may be possible to size the new equipment smaller. This will reduce both its price and its operating cost.
If the lighting systems in a building are old, the cooling load can be reduced further with an LED upgrade. On average, LED products consume 80% less energy than incandescent lamps, over 50% less than HID bulbs, and 20-50% less than fluorescent bulbs. The reduction in heat emissions is not much when only a few lamps are replaced, but a large building may have thousands of them.
Understanding the Nameplate Efficiency of Air Conditioners
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The efficiency of air conditioning equipment is described by the Energy Efficiency Ratio, or EER. This metric is calculated under testing conditions that are provided by the Air-conditioning, Heating and Refrigeration Institute (AHRI). However, the EER assumes a constant outdoor temperature of 95°F. This does not reflect all the operating conditions to which the equipment may be exposed.
Two more useful metrics are the Seasonal Energy Efficiency Ratio (SEER) and Integrated Energy Efficiency Ratio (IEER). These two metrics consider a wide range of operating conditions, and they describe equipment efficiency better. The SEER is used for mini-splits and other unitary systems, while the IEER applies for larger equipment like chillers and packaged rooftop units.
Air conditioning systems are characterized by their variety, and the best option changes depending on the needs of each building. Professional HVAC engineering services are recommended to find the optimal system configuration. Even the most efficient equipment may suffer from low performance when it is poorly matched with the application at hand.
Improving Efficiency with Air Conditioning Controls
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Once the building envelope has been optimized and high-efficiency equipment has been deployed, the final step is adding smart controls. Many buildings have their thermostats set as low as possible, which increases electricity bills even if the equipment is efficient.
The US Department of Energy recommends setting back the thermostat to reduce space cooling costs. According to their analysis, a thermostat setback of 10°F for 8 hours each day achieves air conditioning savings of 10%. The task can be simplified greatly with smart thermostats, which learn this function on their own. Modern chillers, packaged units and variable refrigerant flow systems often have built-in control functions that must only be programmed.
Air conditioning costs can be reduced significantly with an efficient thermal envelope, equipment with a high SEER or IEER, and smart controls. When the three measures are deployed together, a building can reach its lower possible cooling cost.
Michael Tobias is the founder and principal of Chicago Engineers, an Inc 5000 Fastest Growing Company in America. He leads a team of 30+ mechanical, electrical, plumbing, and fire protection engineers from the company headquarters in New York City; and has led over 1,000 projects in Chicago, New York, New Jersey, Pennsylvania, Connecticut, Florida, Maryland and California, as well as Singapore and Malaysia.
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