If the external air temperature is lower than the cooling temperature, the cooling potential of the external air can be exploited, so that cooling energy can be generated without operating the compressor of the cooling system (the compressor is responsible for consuming 90+% of the electricity). This is called free-cooling, it is one of the most economical cooling methods.

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The simplest method is direct free-cooling, such as opening a window, but it brings air containing moisture, dust and other contaminants into the refrigerated space, and this method cannot be controlled. A more sophisticated version of this is the construction of a fan and a damper that can be controlled and even a portion of the dust can be filtered out by installing a filter (although filters usually need to be cleaned or replaced frequently), but even this can only exploit the cooling potential of the external air locally.

If we want to make the free-cooling completely controllable, or not only to use it locally, it is possible to install indirect free-cooling, a dry cooler (practically a system component with a fan and a heat exchanger) must be placed in the cooling system. If free-cooling is available, the fans will start.

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In the case of facilities in which the demand for cooling occurs throughout the year (data center, industrial processes), free-cooling can operate during a significant period of the year. With HeatTank, we can increase the utilization of free-cooling, because Heatventors' Thermal Battery can be charged with free-cooling (almost free!) during the period of the year when free-cooling can only be used at night, thus extending the free-cooling period to the daytime.

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In the case of office buildings, hotels and public institutions in most places in Europe, the need for cooling does not occur all year round (although in the case of high-occupancy buildings with large glass surfaces, the cooling period can last all year round). As free-cooling can be used at low external temperatures, the availability of free-cooling and the need for cooling in these facilities often do not occur at the same time, so it is not worth building a free-cooling system. As described above, free-cooling would often be available at night, but there is no need for cooling. By charging the Thermal Battery, free-cooling will be available during the day, so investing in a free-cooling system in combination with Heatventors' system can be more beneficial.