Incorporating cool and dry outdoor air into commercial HVAC systems can save money and energy, while causing less wear-and-tear on cooling systems. An economizer is a device that works alongside rooftop HVAC systems to augment the system with outdoor air, in order to provide what has become known as ‘free cooling’.
Economizers
Economizers introduce low-temperature or low-enthalpy outdoor air into a building to reduce or eliminate mechanical cooling, depending on outdoor air conditions. [Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system.]
An economizer evaluates the temperature (and sometimes humidity) levels of outdoor air. A combination of logic controllers and sensors both assess the right level of outside air to bring in and operate internal dampers to control how much air is pulled in, re-circulated, and exhausted from a building.
Rather than relying solely on a building’s cooling system, the system leverages cool outside air to accomplish the same task.
Free Cooling
When outside air is cooler than re-circulated air, using more outside air is more energy efficient
When outside air is cooler than re-circulated air, using more outside air is more energy efficient. When outside air is sufficiently cool and dry, no additional air conditioning is needed, hence the term ‘free cooling’.
By minimizing the operation of an A/C unit, economizers can prolong the life of a system, reduce costs, and minimize maintenance with fewer breakdowns.
Enhanced ventilation
Extra ventilation is another benefit of economizers, which is important in the era of COVID-19. An economizer can improve overall air quality by bringing in the fresh air and exhausting the building’s stale air.
Even in older buildings that were not designed with ventilation in mind, economizers can improve air quality. Fresh air can keep employees healthier and minimize an employer’s cost of sick days because of air pollutants.
There are three types of economizers:
- Dry bulb, which evaluates the difference between the temperatures of indoor and outdoor air, but does not consider humidity.
- Web bulb or Single Enthalpy, which evaluate both air temperature and humidity. They are the most popular option.
- Differential Enthalpy, which uses multiple sensors to measure temperature and humidity of return air, as well as the outside air, a popular option for more complex scenarios.
Minimizing total building energy consumption
Energy costs are a big advantage of economizers, helping to minimize total building energy consumption, 45% of which is accounted for by the HVAC system. The use of an economizer provides up to 45% greater efficiency in cooling loads than existing HVAC operations, according to researchers.
Climate conditions are a significant factor affecting the energy-saving impact of air economizers. Researchers evaluated the energy-saving potential of economizer systems in six climate zones, concluding that energy savings in hot-humid summer climate zones were 10% higher compared to cool-dry summer zones. Good controls, valves, and dampers are needed to ensure optimal operation of economizers, and maintenance is also critical.