Free cooling refers to any technique used to reduce the energy consumed by cooling systems, or the time that the cooling units run, by using the outside temperature of air or water to cool the facility. Generally, it comes from the use of air-side and water-side economizers.
What is Free Cooling & How Does it Save Energy
For facilities with water- or air-cooled chilled water plants, free cooling is an economical method of using low external air temperatures to assist in chilling water, which can then be used for industrial processes, or air conditioning systems. Used in the cooler months of the year, such systems can be made for single buildings or cooling networks. Free cooling can also extend the working life of installed cooling systems, lowering the energy and maintenance costs for facility owners.
The cooling provided, of course, is not completely “free” because the tower, chilled water pumps, and tower fans still must be operated. Nonetheless, it allows cost conscious building or process owners and operators to take advantage of naturally occurring climate conditions to save system operating costs.
Free cooling strategies rely on economizers. Economization is accomplished by taking advantage of the temperature difference between indoor and outdoor ambient conditions rather than running compressors to provide the cooling. The effectiveness of an economizer depends on loads characteristics of the building, type of HVAC system and the local climate.
Economizers draw in outdoor air and mix it with return air from indoors. Airside economization can be accomplished by pulling cool dry air straight into the building, which is the simplest and most efficient option in many cases. Waterside economization, in contrast, uses an indirect method of economization and pulls cool water from a cooling tower or dry cooler that is cooled by outdoor air and runs the water through coils inside the HVAC units in the building.
A water-side economizer eliminates the need for cooling via compressors and is an effective way to maintain temperature and humidity requirements while reducing or eliminating chiller use.
Water-side economizers are best suited in climates where the wet bulb temperature is lower than 55°F for 3,000 hours or more. This describes the majority of the United States, barring areas in the extreme Southwest and portions of the Southeast. They are commonly used in data centers, which produce a near-constant internal cooling load, and provide an extra level of cooling redundancy in the event of chiller failure.
Water-side economizers are preferable to their air-side counterparts for applications in which specific minimum humidity levels are called for, such as laboratories and hospitals. The waterside economizer requires a winterized cooling tower in many climates. Since the tower is expected to operate when it’s cold outside, it cannot be seasonally drained.
Airside economizers are a duct and damper arrangement that allow a cooling system to supply outdoor air to reduce or eliminate the need for mechanical cooling during mild or cold weather. At outside air temperatures below 55 F, the compressors are not required to run, which conserves energy. In turn, cooler outside air is used to cool the space, hence the term “free cooling.”
Common Free Cooling Strategies
Determining the most effective free cooling strategy varies by facility. Below are three of the most common strategies.
Strainer cycle: In this system, the condenser and chilled-water systems are connected. When the outdoor wetbulb temperature is low enough, cold water from the cooling tower is routed directly into the chilled-water loop. Although the strainer cycle is the most efficient water economizer option, it greatly increases the risk of fouling in the chilled-water system and cooling coils with the same type of contamination that is common in open cooling-tower systems. A strainer or filter can be used to minimize this contamination, but the potential for fouling prevents its widespread use.
Refrigerant Migration: In this system, valves are open between the condenser and evaporator of the chiller when the compressor is off. This allows free migration of refrigerant vapor from the evaporator to the compressor and of liquid refrigerant from the condenser to the evaporator.
Plate-and-Frame Heat Exchanger (HX): Another way to reduce the energy consumption of a chilled-water plant is to precool the water in the chilled-water loop before it enters the evaporator. This can be accomplished by piping a plate-and-frame heat exchanger into the chilled-water and condenser-water loops. When the ambient wet-bulb temperature is low enough, the heat exchanger transfers heat from the chilled water returning to the evaporator to the condenser water returning from the cooling tower. Precooling the chilled water before it enters the evaporator lessens the cooling burden, reducing the energy that the chiller uses.
Free Cooling & Proper Controls
Energy savings from free cooling rely on the appropriate design and controls to work optimally. Economizers add another level to the cooling scheme and must be engineered into the air handling system and controls. Interactions between the economizer and mechanical cooling (condenser pump, tower fan, chilled water pumps, fans, etc. ) must also be seamless and well-defined to ensure energy savings are achieved.
Coupling a free cooling system with an optimization solution like Xpress® allows seamless monitoring and management of all cooling equipment. Along with live and historical data capture, Xpress® combines real-time algorithms and extensive HVAC design experience to automatically optimize operating settings for equipment, maximizing free cooling and reducing energy costs.