STULZ: Free cooling in data centers - More cost efficiency through dynamic control

Date: 17 November 2019
STULZ: Free cooling in data centers - More cost efficiency through dynamic control

Hardly any data center today operates without free cooling. Depending on size, load profile and external conditions, corresponding air-conditioning systems achieve energy savings of up to 90 percent in power consumption.

But what are the advantages and disadvantages of the various concepts? Benjamin Petschke, Product Manager at STULZ, gives a brief insight into the most common free cooling options and explains the advantages of a dynamic control system.

At what performance level is it useful to think about using a free-cooling solution?

In general, the larger the data center, the more costs can be saved with free-cooling. A few years ago, it was assumed that free cooling only makes sense for larger data centers. The payback period for such a free cooling system was around two to three years. However, development has not stopped here. There is now also a wide range of free cooling solutions for medium-sized and small IT systems. Starting with the free-cool plenum for small server rooms and edge computing from 2 kW up to the 250 kW air-conditioning system with indirect free cooling and external recooler for medium-sized data centres.

When it comes to free cooling, operators today have a wide range of systems to choose from. What are the differences and possible hidden pitfalls?

During the past years, the variety of systems has increased considerably, and free cooling solutions have also become more flexible overall. The main objective of most developments was to maximize free cooling periods over the course of the year. Simple systems can often only switch between mechanical cooling and free cooling, so free cooling times are limited to a few thousand hours per year. In order to avoid such pitfalls, a modern free cooling solution should always support gliding mixing operation. Also technologies such as dynamic control technology can significantly increase the operating time in free-cooling mode. Above all, the basic structure, i.e. the decision for a system with indirect or direct free cooling, needs to be clarified. In addition to efficiency, the question of practical suitability always arises here.

Direct free cooling is regarded as the nonplus ultra when it comes to efficiency. Why is the process not necessarily suitable for every location?

Direct free cooling systems are using cool outside air for heat dissipation. However, the ambient air must first be conditioned at great effort before it can be led into the data center. Here lies one of the greatest weaknesses of direct free cooling: It is strongly dependent on the respective location conditions. If the general conditions are right, direct free cooling promises the greatest savings potential at around 80 percent. This is because there are no heat transfer losses due to additional heat exchangers.

Are there any site conditions that speak against the use of a direct free cooling system?

The temperature and humidity conditions inside a data center alone do not permit direct free cooling without restrictions. In addition, due to air pollution and fine dust pollution, expensive filter systems with a high filter class are often required. In winter, additional humidification is necessary, which can also have an impact on costs. Although these aspects do not fundamentally speak against the use of a direct free cooling solution, operators should nevertheless always consider the total costs of different variants. In addition to the expected energy savings, this also includes detailed costs for filter replacement, maintenance and humidification measures.

What alternatives are available for direct cooling with outside air?

The main alternative is indirect free cooling - either as a combination of free cooling chiller and CW air conditioners or on the basis of DX air conditioners with integrated free cooling and recoolers. In any case, a water-glycol mixture acts as a heat transfer medium between the outside air and the data center during indirect free-cooling, so that there is no direct air exchange with the environment. In order to comply with the occupational safety directive and maintain the necessary overpressure in the server room, only a small proportion of fresh air is added for indirect free cooling. This eliminates the need for expensive filter systems and the corresponding costs for filter replacement and regular maintenance. Even larger openings in the building, which could represent a potential safety risk, are no longer necessary.

Why does the water temperature affect the efficiency of the free cooling system?

Usual indirect free cooling systems work with fixed switching points for mechanical cooling. The period in which the free cooling can be used is therefore very limited. In order to switch to the free cooling mode, a recooler requires a water temperature of 7 °C.  To achieve this value, an outside temperature of 3 °C is required. However, the number of days during which the outdoor temperature is a maximum of 3 °C, for example at the Frankfurt location, is relatively low. With dynamic control, the water temperature is no longer controlled to a fixed value, but adapted to the load requirements in the data center. The cooling capacity of the free cooling system depends strongly on the respective cold water temperature. The higher the water temperature, the lower the cooling capacity. It is precisely this situation, which at first glance appears to be disadvantageous, that is used as an advantage in dynamic control as soon as the data center is no longer fully utilized.

Could you briefly explain this with an example?

A data center in full load operation requires a water temperature of around 10 °C in free cooling mode. This 10 °C cold water can only be produced by the recooler from outside temperatures below 7 °C. However, the same data center requires only 60 percent of the cooling capacity at 60 percent load. The cooling system can supply this cooling capacity with a considerably higher water temperature of 16 °C in free cooling mode, for example. This water temperature can then be produced at an outside temperature of around 13 °C by the recooler.

What is the savings potential behind this approach?

In Frankfurt, outside temperatures of less than 7 °C prevail for around 3,000 hours over the course of the year. By contrast, the temperatures remain below 13 °C for 5,300 hours. This means that the intelligent control of the chilled water temperature can achieve an additional 2,300 free cooling hours per year. Usual free cooling systems still work here in mixed operation or even completely with mechanical cooling.

What do you advise operators of data centres who do not yet use a free cooling system? Are there options for retrofitting?

Of course, conversion and retrofitting are always associated with investments. In view of technical developments, especially in the area of free cooling, a regular analysis of the existing infrastructure as well as an exact consideration of the environmental conditions always makes sense. Current cooling systems have become so efficient that even larger investments quickly amortize. Progress has been made not only in the area of refrigeration components, but also in the area of analysis tools. Today, the combined use of environmental monitoring and modern air conditioning technology can lead to immense energy and CO² savings.

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