Freecooling and freechilling  

Introduction

In industrial buildings, it is essential to control the temperature in the premises. This is why HVAC (Heating Ventilation Air Conditioning) installations are an important aspect of the design and operation of a building that needs to be properly sized and optimised. 

To limit the consumption related to the conditioning of an installation, it is often interesting to be able to do free cooling, but this can have several meanings, let's try to clarify.  

Freecooling, freechilling? 

In any case, the idea is to use outside air (or a cold source) directly to cool the installation without resorting to a refrigeration machine. 

There is no standard regarding the precise vocabulary in the use of outside air for the energy optimisation of air conditioning, but it is important to distinguish between two cases: 

• Outside air is directly sent into a room because it is colder than the air in the room. This is called direct free cooling  

• Outside air is used via a heat exchanger to cool water (which will ultimately go into a local cooling unit, CRAH). This is indirect free cooling 

And what about free chilling?  

Sometimes the term free chilling is used when pre-cooling the water entering a chiller. This is indeed an indirect free cooling which can be total (the chiller is off) or partial (the chiller supplements the cooling demand). The term chilling is generally used when talking about the production of chilled water while the term cooling concerns "all air" systems.  

In this article we will use the following terminology to avoid ambiguities: 

• Free cooling : direct free cooling  

• Total free chilling : cooling of a chilled water circuit without operating the chiller 

• Partial free chilling : pre-cooling of a chilled water circuit before entering a chiller 

Fluid diagram of the operation of a partial free chilling installation: 

This diagram shows that the process water to be cooled is first cooled through free chilling (since we are cooling water) and then passes into the evaporator of the chiller which completes the cooling demand. The use of a water-cooled chiller as represented above is not necessarily required but is a common practice for high power installations such as data centres. 

Fundamental differences 

In the context of designing ventilation systems for a room, one may have to choose (depending on the installation, dissipations, etc…) between conditioning by supply air only, or by local cooling units. Depending on this, one can do free cooling (by directly blowing in fresh air) or free chilling (via the cooling units). Each solution has its advantages and disadvantages (installation, duct size, water network…). 

If we consider a room for which we seek to maintain 20°C, with a supply set point of 16°C (related to the maximum flow rate of the fans) the theoretical operating limit in total free cooling will be 16°C, beyond which the room would start to warm up and mechanical cooling would need to be used as a supplement. Beyond 20°C, the intake of outside air becomes counterproductive and it is preferable to recirculate the air (often with a minimal amount of fresh air to ensure the function of sanitation-renewal). 

In the case of free chilling, it is rather the sizes of the heat exchangers (pinch*) that define the limits. Indeed, the use of a chilled water loop requires two exchangers to transfer heat from the inside to the outside. One exchanger that extracts heat from the room to be air-conditioned, and one exchanger outside that releases heat to the open air.  

On an air-water exchanger, one can reasonably achieve a pinch of 6K for air-water exchangers. A 5K increase in the chilled water circuit is also assumed. 

• T cold water = 16-6 = 10°C (water entering the local unit) 

• Heating of the water = 10+5=15°C: Water regime 10-15°C

• T total free chilling =10-6 = 4°C (air temperature for total free chilling) 

• T hot water = 10+5 = 15°C (limit temperature for partial free chilling, beyond which the air heats the water) 

In summary, from a hardware perspective, free cooling is limited by fan capacity and duct size, whereas free chilling is limited by the pinch temperature differences of the heat exchangers.

*Pinch temperature difference: the difference between the cold-fluid inlet temperature and the hot-fluid outlet temperature. For a cold fluid (water) entering at 14°C and a hot fluid (air) leaving at 20°C, the pinch temperature difference is 6 K.

Influence of Setpoint Temperatures

As discussed previously, it is clear that the higher the cooling setpoint temperature—whether for air or water cooling—the longer the system can operate in free cooling or free chilling mode.  

Consequently, in the data center industry, the increasing operating temperature limits of modern processors (and therefore the corresponding higher water supply temperatures) make architectures incorporating free cooling systems increasingly attractive. This advantage is even more significant for Direct Liquid Cooling (DLC), which operates with higher water temperatures than conventional air cooling. Higher water temperatures also facilitate heat recovery. 

In the data center context, free chilling is therefore becoming increasingly valuable, significantly reducing the electrical energy consumption associated with cooling.

Conclusion: How to Choose?

In locations with high thermal load densities (such as a data hall in a data centre, an electrical room in a nuclear power plant), it is often impossible and unreasonable to condition directly with outside air. Therefore, we will favour free chilling, which will only be possible if the water temperature is sufficiently high (but in this case, the heat exchangers in the rooms are necessarily bulkier). 

If we implement local conditioning with very low water temperatures (for example, 5-10°C still commonly used in nuclear installations), then we deprive ourselves of the possibility of free chilling and will significantly increase the operating times of the chillers and thus the energy costs of conditioning.