Latvian summers can hardly be described as the most predictable and lovely in terms of their whether conditions, however, when it comes to the cooling of data centres, our chilly climate can be what is called the name of the game. The use of the outdoor temperature – air-to-air free cooling – is but one of the data centre cooling methods, currently being one of the most innovative solutions having its stellar times both in Europe and America. The key advantage of this cooling method lies in the remarkably low costs of its use leading to cost savings in the long term. This cooling method is also one of the most environmentally friendly.
First, it should be noted that there are two – quite different fundamentally – options of using this method for data centre cooling. One of the options – to let the outdoor air into the data hall, cooling it or slightly heating it. The second option – the outdoor air is passed through a heat exchanger in which the temperature is passed from the outdoor air to the data centre air. The outdoor air is not directed to the data centre premises, it is passed outside. Hence the name of the method – air-to-air free cooling, because the cold outdoor air (available for free as is known) is mostly used in the cooling process.
The first air-to-air cooling method is currently extremely popular both in Europe and America, however, its use is subject to limitations. One of the limitations is moisture. In some places, the average moisture level is very high throughout the year. One should not look far, as a matter of fact. Look out of the window – what is happening out there? It is snowing (wet snow) or raining – moisture. The weather conditions such as we have in the Baltic States or Scandinavia set the rules of their own and seem to prompt – no, we cannot benefit from the air-to-air method in cases where the outdoor air is passed into the data hall. This solution is of no use in such climatic conditions.
It works best and most effectively in places where the air is dry – in desert and steppe areas, where the annual average moisture level is around 30%. Why so? That is because a constant moisture level (of between 40 and 60 per cent) needs to be maintained at any data centre. If the moisture level of the outdoor air is very high, such as, e.g. in the Baltic States after raining (around 80-100%), the air needs to be dried, which is an expensive process in terms of energy consumption. On the contrary, the average moisture level in deserts is around 30% and the air needs to be moisturised, which is not that an expensive or complicated exercise. That is why this solution would not be justified in our latitudes. The risk that along with the outdoor air other matters such as smoke, soot, dust, and dirt could be passed into the premises further renders this solution unpopular in our circumstances. For example, if smoke accidentally gets into the data hall, the fire alarm may go off, and gas will be used to put out the fire, i.e., the activation of a false fire alarm will do more harm than good and will ultimately cost a lot.
The second method – the air-to-air cooling method can also be used in our latitudes – without mixing the outdoor air with the air in the data hall due to a heat exchanger being located among them, as a result of which the cold outdoor air is passed indoors. With time, such a solution can turn out to be quite popular in this country, given that Latvia has a low average air temperature, and the periods of cold weather last for eight to nine months a year rendering this solution very efficient. An ordinary air-to-air equipment has three operational modes depending on weather conditions. The first mode – the system works using the heat exchanger in the six or seven months of the year when the outdoor air temperature is lower than that to be maintained in the cold aisle of the data hall (+22° to +24°C). In practice, throughout all spring and autumn nights, as well as early spring and late autumn days and, of course, days and nights in winters under our climatic conditions, the system operates in the above-mentioned mode – using a heat exchanger. Under the second mode the outdoor air temperature is monitored, and when it fall slightly lower than that necessary in the data hall, the air will be moisturised using the water supply system. When evaporated into the air using the cooling equipment, water cools the air of the particular room, hence the name – the adiabatic cooling method.
Under the third mode, when the whether is warm, the equipment ensures the data centre cooling using an inbuilt compressor capacity. Based on this principle, the air-to-air free cooling equipment is used in late spring, summer and early autumn. The data centre cooling is done on average at 60%, 25% and 15% of the total number of hours under the first, second and third mode, respectively. The energy consumption necessary for cooling accounts for 4%/6%/50%, respectively, of the total energy consumed by a data centre. It means that under the first mode, to cool the IT equipment with a total capacity of, for example, 100kW, only 4kW of energy will need to be used for cooling. The costs of a slight water consumption needs to be added to the costs of the second and third mode, which has an immaterial effect on the total costs of cooling. The leading manufacturers of such type of equipment point out that the minimum energy consumption that can be achieved for cooling amounts to 12-14% of the total energy consumed by a data centre. This parameter may change depending on the average temperature of a particular year and the location of the data centre, however, these factors either improve or aggravate the efficiency of each type of cooling.
There are limitations to both the air-to-air data centre cooling methods. Thus – any data centre needs to have a sufficient capacity. In estimating when the cooling of such type is beneficial, the large manufacturers of equipment have come to a conclusion – a data centre needs to have an IT equipment of at least 100kW of total constant capacity which is an equivalent of 25-40 pillar data centre, provided that the capacity of each individual server does not change. If the data centre is smaller, other technologies and traditional methods would be a better choice.
At this point, the traditional data centre cooling methods are worthy of mentioning. In general, the most widely known is the cooling with dry coolers or the cooling with chillers equipped with a free cooling function. Free cooling, in the direct meaning of the word, stands for the cooling for free. This method of cooling is used in countries where the average air temperature is low. How is it different from the air-to air free cooling technology? The functions are largely similar, the difference being that fluid – water with at least 35% ethylene glycol content – usually circulates between the internal and external coolers. The content of the fluid needs to be in the said proportions so that it does not freeze in winter conditions. This is also considered an effective traditional cooling method in our latitudes. Apart from the above-mentioned methods, freon cooling is also used for which the compressor regulation range rather than free cooling is used. The compressor is operated continuously, only its capacity ranges from 0 to 100 per cent. The required energy consumption for the above mentioned cooling methods amount to 30-35% of the total energy consumed by the data centre.
Returning to the air-to-air free cooling method, it is also important to know what buildings it can be used for. As for the existing buildings, it is complicated, at times even impossible to install the equipment, because it is large and requires large air openings, therefore it is quite difficult to adapt the building to be reconstructed to suit this data hall cooling method. It is easy and relatively simple to install such a cooling equipment in such a new building originally constructed to meet the needs of a data centre, and all the requirements related to the cooling equipment could be identified at the project development stage.
If you have an opportunity to select freely the location of a data centre that has a low average moisture level, the first method should be preferred – letting the outdoor air into the data hall, cooling it and slightly heating it. This is the method selected by one of the industry leaders Facebook for its data centre in Prineville (USA). Conversely, our climatic conditions require that when constructing a data centre in this country, either the traditional cooling method should be selected or the global trends should be followed by selecting the air-to-air free cooling method (the second method), the key advantage of which lies in cost savings in the long term. Compared to the traditional cooling methods the air- to- air free cooling method is twice as much better in terms of costs given almost equal capital expenditure on the data centres with the IT capacity starting from 100kW.
Miks Stūrītis, www.citrus.lv, Chairman of the board of SIA Citrus Solutions