What is Indirect Free Cooling and what are the differences with Direct Free Cooling?


Companies that use air-conditioning systems to keep technical rooms cool, because of the prolonged energy consumption, promote pollution and have to deal with rather high costs in the bill.


Two critical aspects that companies are trying to solve by choosing to switch to a new refrigeration system: free cooling.


This is the most attractive system for achieving impressive energy savings, as it uses very little electricity in the air cooling process. As well as a huge step forward from an environmental point of view, this also means considerable cost savings.


When we talk about free cooling, however, we refer to two types of free cooling: direct free cooling and indirect free cooling.


In a previous article we looked at the subject of direct free cooling. Direct free cooling consists of supplying a room containing heat dissipating equipment with filtered air from outside at a lower temperature. At the same time, the internal heat is dissipated and expelled from the room, replaced by the same amount of fresh air.


In a nutshell, outside air with a lower temperature replaces warm indoor air.


In this article we will deal with the other type: indirect free cooling.

What indirect free cooling is and how it works.

In indirect free cooling, to remove heat from technical rooms, such as data centres and TLC rooms, cold water is sent into the AHU (central air handling unit).


To function, it needs at least two heat exchangers: one for the indoor units and one to dissipate the heat outside.


This cooling system becomes a viable option if there is a geothermal plant and a refrigeration unit at its base: the return water from the geothermal probes is cooled and flows inside the heat exchangers, resulting in pleasant summer air conditioning.


In this way, the right climatic conditions are maintained in the environment, in a completely natural way.


In addition to geothermal systems, there is another source that does not require a refrigeration unit and a geothermal system: outside air. This can be used to cool the water in the system, but in this case, favourable climatic conditions must be present, as too high temperatures would make operation impossible.

Advantages and disadvantages compared to direct free cooling

Let’s take a more detailed look at the two types of free cooling, to highlight the differences and critical points.


For example, in the case of indirect free cooling, as mentioned, the system requires at least two heat exchangers: one for the indoor units and one to dissipate the heat outside. The greatest economic benefits are obtained with temperature differences on the exchangers of 3/4 K. To be able to cool the indoor air to 20°C, the outdoor air must therefore not exceed 14°C. This unfortunately reduces the number of days on which indirect free cooling can be used. In that case, the chiller will intervene and the energy consumption will be higher.


Another disadvantage of this system is that, by lowering the room temperature, it raises the relative humidity of the air. Condensation can then form on cold surfaces, which can increase the percentage of humidity in the room.


A scenario that can be avoided by (partially) dehumidifying the air before it is supplied. However, dehumidification requires lower water temperatures than are normally possible. It may therefore be necessary to also switch on the air conditioning system or activate mechanical ventilation.


Direct free cooling, on the other hand, which makes use of the outside air and the cool night-time hours, can make a valuable contribution to the cooling of rooms. Direct free cooling is when (in summer at night) cold air is introduced into the building and the outside temperature is below 18°C (in warm months, this is normally the case between 24:00 and 6:00).


With direct free cooling the savings in energy bills can reach 90%, and you avoid wasting huge amounts of polluting energy, giving your company a green and eco-friendly image.


The excellent scalability of this system and the absence of hydraulic components (such as pipes, pumps and fittings) should also be noted.


Obviously, the investment costs for a direct free cooling system will also be much lower than for an indirect one.