Economizers Provide Free Cooling

Key Points

  • Economizers save energy by bringing in outside air to supplement or replace mechanical cooling.
  • Dry-bulb controls sense only temperature, while enthalpy controls sense temperature and humidity.
  • A properly operating economizer can reduce cooling energy costs as much as 10 percent.

Summer air conditioning is a significant operating expense for most facilities. For operations with servers and other heat-generating equipment, cooling is a critical, but costly, requirement year-round. Air-side economizers can save energy and money by bringing in outside air to replace or supplement mechanical cooling. Under the right conditions, a properly operating economizer can reduce space cooling energy costs by 10 percent, or more.

How economizers workeconomizersblog

Economizers are located on the supply side of the air-handling system. Components include dampers, sensors, small motors, and controllers. When the air outside is cool enough (generally around 55°F to 60°F) the damper opens, bringing in outdoor air to provide free cooling. When the sensor indicates that the temperature is not right for cooling, the damper is closed.

Two types of economizer controls are available, dry bulb and enthalpy. Dry-bulb controls only sense air temperature, but are generally less costly to install and maintain. Enthalpy units sense both temperature and humidity. Enthalpy controls typically increase comfort by blocking excessively humid outside air, but higher installation and maintenance costs can outweigh their additional benefits.

In some cases, the air-conditioning cooling coil is always off when the economizer is operating. Integrated economizers, however, allow both to operate at the same time, as indicated in the following table.

 

 Mode

Temp (°F) 

Coil 

Economizer 

 Mechanical heating

 <30

On

Closed

 Modulated economizer

 30 to 55

Off

Variable

 Integrated economizer

 55 to 75

On

Wide open

 Mechanical cooling

 >75

On

Closed

One of the most advanced economizer control strategies available is a differential enthalpy system, in which two enthalpy sensors are installed (one for the outside air and one for the return air). Under the differential control strategy, the system uses whichever air source (outside or return) has lower enthalpy when there is a need for cooling.

When do economizers make sense?

While economizers are an effective energy-saving technology, they are not the right fit for every situation. Key considerations include:

  • Equipment life. In the right setting, an outside-air economizer will not only save energy, but also extend the life of air-conditioning system components. When outside air is brought in, the compressor and condenser fans run less, reducing wear on the unit.
  • Climate. Consider the amount of time during the year when the air is cool and dry enough to use for space cooling. Economizers are now required in most climate zones by the ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings for new rooftop units with ≥4.5 tons capacity.
  • Hours of operation. Take into account your operating hours. Cool night air is not very useful unless your facility is up and running at night.
  • Air quality. If your facility is located near operations with potentially harmful emissions, you may need to look at air quality issues.
  • Internal heat load. Office equipment, computers, lights, and production equipment, coupled with a low building heat loss, may even make space cooling with an economizer attractive during the fall and spring.

Economizers require careful installation and maintenance. A poorly installed or malfunctioning economizer can waste more energy than it was intended to save. It is important to evaluate these and other factors when deciding whether an economizer is a cost-effective solution for your facility.

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This article has been prepared solely for the purpose of providing helpful information to users of this service. The information has been compiled by Questline, a contractor to the NHSaves utilities; however, no representation is made by either Questline or NHSaves as to the completeness or accuracy of the information contained therein. In particular, some information may be incomplete, may contain errors or may be out of date. In addition, neither Questline nor NHSaves endorses any product or service mentioned therein.