Pathways to an energy-efficient and environmentally friendly data centre

Data centres are cropping up all over the place. The reason – Entire data dependence of society and the economy. The Swiss Federal Office of Energy expects the power demand of Swiss data centres to double in the next five years. Efficient and environmentally friendly cooling is therefore all-important.  

Mr Brechbühler, the cooling of data centres is devouring more and more power. Given the increasing awareness of the value of power – what do we do about this? 

Christoph Brechbühler: Professional data centre operators, known as colocators, are very aware of the value and the costs of power. And they have been for a while now. They pay meticulous attention to ensuring that the share of total power consumption attributable to infrastructure – especially the generation and distribution of cooling – is as low as possible. They try to reduce power demands by using higher supply air temperatures to cool the server racks, and especially by using free cooling. 

Free cooling in particular significantly reduces the cooling power consumption. 

At least at first glance. But free cooling has a number of drawbacks if you look at it more closely. 

Why is that? 

Data centres and server rooms in companies do not need very "heavy" cooling. A cooling water temperature of 24 °C is enough to ensure a server intake temperature of 27 °C. Indirect free cooling may therefore start at outside temperatures below 20 °C. This is indeed tempting, but only if you ignore what comes out "at the back" of the server rack. This air at around 35 °C is pure energy. If you let free cooling then release this heat into the environment without making use of it, you end up with an absolute waste of energy. Especially in view of the energy transition and achieving the net zero target by 2050, we simply cannot afford to waste potentially useful heat energy. We must benefit from this heat in any way possible. 

Using waste heat is one thing, but the refrigerant also plays an important role in the energy efficiency of data centre cooling. Can you see any emerging trends? 

The choice of refrigerant impacts the investment costs as well as the future operating costs. But aside from the cost issue, there is also an ecological aspect. The first generation of synthetic refrigerants turned out to be damaging the ozone layer. Later we realised that refrigerants have a strong greenhouse effect. Apart from the natural refrigerants ammonia or propane, and isobutane in heat pumps, synthetic refrigerant HFO is therefore almost the only coolant you will find in data centres today. All of these have either a negligible or no impact on the greenhouse effect. 

The most commonly used for data centres is HFO, why is this? 

The synthetic refrigerant HFO has many advantages: Hardly any greenhouse effect, low investment costs and an impressive energy efficiency when it comes to the cold-water temperature of server rooms. As opposed to its "natural" competitors propane, isobutane and ammonia, HFO has the added advantage of being neither toxic nor explosive. 

 There is a simple solution for data centres then: Go for HFO? 

Unfortunately, it’s not quite that simple. True, HFO has become the de facto standard among international data centre operators. But this is precisely what is worrying me. Because despite its advantages, HFO is still a synthetic refrigerant. When it is released into the environment, trifluoroacetic acid, or TFA, forms as a decomposition product. TFA is not biodegradable and accumulates in the water. Even if HFO only contributes a little to the total TFA concentration, it is not completely harmless. In the long term, therefore, we also need a change of thought by data centres in the direction of the natural refrigerants propane and ammonia. 

Now, waste heat recovery is by no means a new concept but already used in countless buildings, server rooms and data centres. 

Our experience is that, within buildings or company boundaries, the heat from data centres is often put to good use. But beyond these confines, it’s a different matter. I know of cases where on one side of the street, the heat from the data centre is discarded via the roof. And on the other side of the street, a heat pump collects warm air to heat the building. 

This may be an extreme example. But practically speaking, isn’t it more likely that data centre operators simply can’t find any consumers for their waste heat? 

You are absolutely right. That’s why an overall view is needed when planning new data centres. This starts with considering where the perfect location is from an energy point of view. In the city of Basel, for example, there are so many heat and waste heat sources that nobody is waiting for a new data centre to open up to source heat from. In Bern and the surrounding area, on the other hand, a data centre would be a sought-after and valuable heat source that could heat entire neighbourhoods. There you would need someone to buy the heat and feed it into their heat network. 

Is there a demand for such solutions? 

As a refrigeration and distribution products and heat pump manufacturer, it is our task to support specialist planners and energy contractors in the development of sensible and sustainable solutions. This cooperation is very productive and offers very good options for action, not least for the end customers. But there are limits. For example, there is currently hardly any demand for hot water flow temperatures above 80 °C, although we could easily supply 100 °C (see table). Such high temperatures are suitable for district heating networks through to industrial processes. 

However, the war in Ukraine has led to rethinking power consumption – away from fossil fuels towards renewable energies and towards a more efficient use of electrical energy. But enterprises take three to six years from deciding to planning to actually commissioning such a solution. I am sure that the demand for high-temperature heat pumps will increase massively in the coming years. 

But doesn’t it take much greater efforts to cool server rooms with natural refrigerants? 

Refrigeration machines running on HFO, ammonia or propane basically share the same machine room equipment. All three refrigerants require high-capacity ventilation, refrigerant detection and a good safety concept. Of course, refrigeration using ammonia requires a little more effort in terms of planning. Due to its toxicity, the dispersion characteristics need to be calculated and the risks assessed to prepare for potential accidents. However, it is often forgotten that ammonia is a proven, reliable and safe refrigerant that has been used in industrial refrigeration systems for more than 100 years. 

If you intend to run your refrigerator on explosive propane, we would generally recommend you to install it outdoors. But technology has progressed rapidly, and indoor installation is also quite feasible today. There is no compromise on safety. Having a well-defined explosion proofing document with a risk assessment is a must. And remember to keep the electrical components consistently separate from the refrigeration circuit – either in another room or by enclosing and hermetically sealing the refrigeration circuit. 

After praising natural refrigerants to the skies – do you still offer HFO solutions at all? 

Of course, we also offer refrigeration machines and heat pumps with the synthetic refrigerant HFO. There are always various factors that need to be considered to find the perfect solution. While HFO is an energy-efficient and economically interesting solution for cooling capacities ranging from 50 kW to around 1 MW, its decomposition products cause the aforementioned pollution. For this reason, we take great care to ensure that as little refrigerant as possible is released into the environment. 

Ammonia systems for chilled water applications in data centres are about as efficient as HFO but, for technical reasons, they won’t be economically feasible at cooling capacities of less than 500 kW. In large plants with a capacity of 1 MW or more, ammonia is very interesting in terms of energy performance if combined with using waste heat for generating high temperatures (70 °C or more). At capacities below 500 kW, I think that HFO is best replaced with propane. The catch: Propane is less energy-efficient and therefore somewhat more expensive to operate. Also, propane facilities currently consume a little more capital than HFO plants, whose large-quantity sales benefit from economies of scale. 

What do you expect from propane? 

If you want to avoid the ecological side effects of HFO in small and medium-sized systems up to 500 kW, there is no alternative to propane. The good news is that in a few years from now, the differences in investment between HFO and propane will disappear. 

Combined with the efficient usage of waste heat, deficits in energy performance can be compensated for. But at the end of the day, clients have to ask themselves: What significance has unburdening the environment for them? Are they prepared to pay more for this? And what do the end consumers think – do they want IT to be as ecological as possible or as cheap as possible? 

This interview was printed in the November 2022 issue of trade magazine HK Gebäudetechnik. Christian Werner conducted the interview. Images: CTA AG and Mitsubishi Electric Hydronics & IT Cooling Systems.