Free cooling is a method of using ambient external air temperatures in the natural environment to reduce the temperature of stored water. This cooled water can, in some instances, entirely replace the chiller in a cooling system, saving on electrical consumption considerably. Various industries employ free cooling systems, from hotels and hospitals, to server farms in computer data centers and businesses that want to employ green technologies to protect the environment.
The idea of free cooling works better in some locations than others. In the UK, for instance, the temperature is below 59° Fahrenheit (15° Celsius) for almost 75% of the year, making free cooling very practical for many needs. As well, the UK offers Energy Efficient Loans for the installation of free cooling systems, which can easily be retrofitted onto standard chilled water units or built into new ones. It is also possible to incorporate free cooling systems into standard air cooled or heating, ventilation, and air-conditioning (HVAC) cooling units, but the efficiency level of this type of upgrade is lower than modernizing a pure water-based chiller.
Computer rooms generate a significant amount of heat and also consume a lot of electrical power, and are, therefore, a primary market for free cooling, often referred to as economizer systems. Economizer systems can reduce energy consumption of the cooling equipment by 30–50%, depending on the outside temperature and humidity conditions. External temperatures are recommended to average within or below a range of 68-77° Fahrenheit (20–25° Celsius) for a free cooling system to function well. Relative humidity should also be below a maximum of 60% to avoid damaging data center equipment by introducing outside moisture.
One of the downsides to this technology, especially where its employed to cool sensitive electronic equipment, is that outside air can be damaging to electronics. An information technology (IT) original equipment manufacturer (OEM) study found that there was an 81% increase in IT equipment failures over a period of 8 months, once a free cooling system had been put in place. High-quality filters in the system are designed to catch outside particulate material, but they are not 100% effective, and add cost to the system the more sophisticated they must become to clean outside air.
Limitations of free cooling systems also require that the external water tower must not be allowed to get below 39° Fahrenheit (4° Celsius), as freezing water will damage the equipment. This makes environments where the temperature routinely drops below 41° Fahrenheit (5° Celsius) unsuitable for the use of free cooling chillers. In locations such as the Northern Hemisphere, this generally means that such heat exchanger systems can be operated for two or three seasons of the year, but must have a more conventional system to back them up during down time.