What is free cooling?

Free cooling means that cooler outside air—or the temperature difference between the system and the environment—is used for cooling. The English term for this is “free cooling.” In chillers with free cooling, an additional air-cooled heat exchanger takes over part or all of the cooling of the medium in the load circuit as soon as ambient temperatures are low enough. As a result, the mechanical refrigeration cycle needs to operate less frequently or not at all.

No, free cooling is not completely free. Although the cooling effect does not have to be actively generated continuously by a compressor, electricity is still required to power fans, pumps, control systems, and system operations. Compared to purely mechanical cooling, however, the mechanical refrigeration cycle can be used less frequently.

A chiller with free cooling features an additional air-cooled heat exchanger located on the air side, upstream of the condenser. If the outside air is sufficiently cooler than the returning fluid, the fluid is pre-cooled or fully cooled by the outside air. The greater the temperature difference between the outside air and the fluid, the higher the proportion of free cooling in the total cooling capacity. The existing page cites approximately 2 K as a possible starting point and approximately 10 K for full free cooling capacity.

Free cooling operation can begin even with a small temperature difference between the water inlet temperature and the outside air temperature. With the appropriate chillers, operation can begin with a temperature difference as low as approximately 2 Kelvin. Full free cooling capacity is generally achieved with a larger temperature difference, starting at approximately 10 Kelvin.

Hybrid cooling refers to the combined operation of mechanical cooling and free cooling. Especially during the transitional seasons, the outside air is often sufficient for pre-cooling, but not yet sufficient to handle the entire cooling load. The refrigerant is then pre-cooled via the free-cooling heat exchanger, while the compressor handles only the remaining required cooling capacity.

In 100% free cooling mode, the medium is cooled entirely via the free-cooling heat exchanger. The mechanical refrigeration cycle remains deactivated. In this operating mode, the refrigeration cycle is deactivated and the unit mainly requires energy to power the fans within the unit. However, pumps and control systems may still consume energy.

Free cooling is particularly well-suited for applications with year-round or long-term cooling needs. Typical applications include data centers, manufacturing processes, telephone exchanges, and buildings with a high proportion of glass or high internal heat loads.

Yes, data centers are among the traditional applications for free cooling. Servers, storage systems, and networking equipment continuously generate heat, so cooling is often required year-round. By utilizing cool outside air, the reliance on mechanical cooling can be reduced—especially when system temperatures are designed accordingly.

Yes, free cooling is suitable for commercial and industrial applications with high or continuous cooling demands. These include, for example, production processes, equipment rooms, telephone exchanges, or buildings with high internal heat loads. Key factors include the location, the required system temperatures, and the number of operating hours during which outdoor temperatures are sufficiently low.

For conventional residential buildings, free cooling using chillers is generally not a standard application. The technology is more relevant for larger buildings, technical applications, or systems with continuous cooling requirements. However, free cooling can also play a role in larger residential complexes or buildings with a central cooling system.

Free cooling can reduce compressor runtime and influence operating costs. At the same time, maintenance costs can be reduced because the mechanical refrigeration cycle is used less frequently. With proper design, the investment can pay for itself more quickly. The reduced operating time of the refrigeration cycle can also contribute to a longer service life and help preserve the value of the chiller.

System temperatures are a key factor in efficiency and cost-effectiveness. The higher the permissible fluid temperature can be set, the earlier and for a longer period of the year free cooling can be utilized. A larger temperature difference between the supply and return lines can also optimize operation and allow free cooling to begin earlier.

Free cooling depends on the outdoor temperatures at the site, the required system temperatures, and the actual cooling demand. If very low room or process temperatures are required on a continuous basis, free cooling alone is often insufficient and must be supplemented by mechanical cooling. Dehumidification requirements must also be taken into account during system design.

When outdoor temperatures are very low, it is important to prevent the medium from cooling down too much and the outlet temperature from falling below the setpoint. To do this, the fan speed is first reduced. When temperature differences are very large, a modulating 3-way valve can be used. It mixes in warmer medium from the unit inlet, thereby keeping the target temperature stable.

Free cooling operation can begin even with a small temperature difference between the water inlet temperature and the outside air temperature. With the appropriate chillers, operation can begin with a temperature difference as low as approximately 2 Kelvin. Full free cooling capacity is generally achieved with a larger temperature difference, starting at approximately 10 Kelvin.