Free cooling is an economical method of using low external air temperatures to assist in chilling water, which can then be used for industrial process, or air conditioning systems. The chilled water can either be used immediately or be stored for the short- or long-term. With aquifer thermal energy storage (ATES), a form of seasonal thermal energy storage (STES), the cold of winter can be free cooling the next summer. Such systems can be made for single buildings or district cooling networks.
When the ambient air temperature drops to a set temperature, a modulating valve allows all or part of the chilled water to by-pass an existing chiller and run through the Free Cooling system, which uses less power and uses the lower ambient air temperature to cool the water in the system.
This can be achieved by installing an air blast cooler with any existing chiller or on its own. During low ambients a processor can by-pass an existing chiller giving energy savings of up to 75%, without compromising cooling requirements.
Free cooling in HVAC
In heating, ventilation, and air conditioning (HVAC) in winter months, large commercial buildings’ interior spaces may need cooling, even while perimeter spaces may need heating. Free cooling is the production of chilled water without the use of a chiller, and can be used generally in the late fall, winter and early spring, in the Northern Hemisphere. Free cooling is not entirely free since the chiller is still operational.
Methods of free cooling
Assuming that the system can utilize free cooling, there are three ways to use free cooling:
- Strainer cycle - The cooling tower water can be directly linked into the flow through the chilled water circuit. If the cooling tower is open then a strainer is required to eliminate any debris that could accumulate within the tower. The cost savings are associated with the limited use of the water chiller energy. There is an increased risk of corrosion using this method.
- Plate and frame heat exchanger - A heat exchanger will transfer heat directly from the chilled water loop to the cooling tower loop. The exchanger keeps the cooling tower water separate from the coolant flowing through the cooling coils. The chiller water is thus pre-cooled. An energy savings is due reduced chiller loading and thus a reduction in energy consumption. There is an increase in cost due to the pump needed to compensate for the pressure differences.
- Refrigeration migration - A valve arrangement within the water chiller opens a direct path between the condenser and the evaporator. The relatively warm fluid in the chiller loop vaporizes the refrigerant, and the energy is carried directly to the condenser, where it is cooled and condensed by the water from the cooling tower. This method is driven by the idea that the refrigerant tends to move towards the coldest point in a refrigeration circuit. The cost savings associated with this method are due to the compressor’s inactivity, since the blower, fans and pumps are all operational
Free Cooling through the Seasons
When the process return water temperature required is equal to or higher than the ambient air temperature Free Cooling is not suitable. The system’s three way valve will continue to direct the fluid flow through the chillers’ compressors to be cooled to the required set point temperature.
Mid season operation
For mid-season operation the water is partially cooled by the compressor and partially by the ambient temperatures. The percentage of Free Cooling achieved mid-season is dependent on seasonal temperatures although partial Free Cooling commences when the ambient air temperature is 1°C below the process return water temperature. The water is partially cooled through the Free Cooler then flows through the chillers’ compressors to achieve the required set point temperature.
In winter, when outdoor temperatures are low enough, the water is chilled solely by the Free Cooling coil. This allows the chillers’ compressors to stop operating, saving significant amounts of energy. The only electrical power used in winter operation is for fan operation. This can be achieved once the ambient air temperature is 3°C-5°C below the process supply water temperature. More details explained in this brochure.
Freezing can be difficult to avoid once the cooling tower water temperature gets below 3.9 ⁰C. Another limitation is the temperature difference across the heat exchanger. A heat exchanger that has a very low temperature difference across can become economically unrealistic. The economics of the heat exchanger allow for a minimum free cooling water temperature of about 5 ⁰C.
There are four main sources of natural cooling energy
- Deep seawater
- High-altitude coldness
- Night-time coldness
- Subterranean geothermal energy
- Posladek, Gina. "MSc Energy Systems and the Environment". University of Strathclyde. Retrieved 5 October 2010.
- McQuiston, Faye C., Jerald D. Parker and Jeffrey D. Spitler. Heating, Ventilation, and Air Conditioning. Hoboken: John Wiley & Sons, Inc., 2005.
- Kelly, David W. "Free Cooling Considerations". Heating, Piping, Air Conditioning (1996): 51-57.