Information technology is ever hungrier for energy. Servers and data centres in Germany alone account for some nine billion kilowatt-hours per year, equivalent to the output of three or four medium-sized coal-fired power stations. For the user this means considerable costs, but there is also the issue of unwanted carbon dioxide emissions. To increase the energy efficiency of an existing data centre and thus save electricity and money, however, experts say that even simple actions can achieve energy savings of between 20 and 25%.

New technologies are also promising to remedy the situation; examples include server virtualisation and the use of thin clients - which incidentally are now sufficiently developed, thanks to improved software and higher network bandwidths, to be viable for large-scale use. Another possibility, known as cloud computing,provides a variety of applications in a centralised server 'cloud' that can be accessed via the Internet by multiple users such as offices and businesses. Here, however, users are generally faced with high investment costs. So it makes sense first to examine and implement other steps towards optimising energy consumption - steps that are simple and sometimes even cost-neutral to take.

Climate control plays a central role in the improvement of energy efficiency. The proliferation of blades and 1 U servers in recent years has led to a sharply increased concentration of energy and thus of heat into ever smaller spaces. Before 2005 a guidance value of 0.6 to 1.0 kW/m² was used in the planning of data centres, but today it is normal to plan for between 1.0 and 1.5 kW/m². Moreover, short-term peak loads lead to these average values in the space available being exceeded several times over. So what steps can be taken in order to operate an older data centre more efficiently that was not designed for the power densities now required of it?

The first step is an analysis and quantification of the existing situation. Information of many kinds is needed for this, such as:

• How big is the room?
• How high is the ceiling?
• Where is the room situated and what are its features? (External and internal walls; cellar or above ground; windowless or with windows; lighting, sunlight etc.)
• How many servers are running and how much heat do they generate?
• What are the maximum permissible temperatures for the servers?
• Where are the cooling systems?
• How are the cabinets arranged?
• How is the false floor designed?
• Where do air short-circuits occur?
• ...

If all the boundary conditions are known, starting points can be found relatively quickly for improving energy efficiency. Frequently, for example, the temperature in the room is kept too low out of excessive caution. Some data centres are still cooled to 18 °C although today 24 to 26 °C are no longer a problem. Calculations show that a temperature difference of one Kelvin can save some 3 to 4 % of energy consumption by the climate control system. It should also be remembered that heat gain from solar radiation into the room can be kept to a minimum by the use of blinds or reflective film on the windows. Lighting in the room also contributes to higher temperatures. Use of timeswitches or motion detectors can help minimise the time for which lighting is switched on.

Vital to the efficiency of a data centre is an optimised pattern of airflow and the avoiding of air short-circuits, in which cold and warm air become mixed. The desired situation can often be achieved simply using covers and sealed cabinet rows. No holes are permitted either within the cabinet, between the servers or in the cabinet row that could interfere with the intended airflow patterns. For this purpose Schroff offers special filler panels and flexible sealing elements.

Even actions as simple as removing cardboard boxes, chairs, tables etc that have been left in the way, and regularly cleaning circulating air cooler filters, will help to maintain optimal airflow in the room. It may also be necessary to consider a 'cleanup of the false floor. The false floor is often used less as a pressure plenum for the flow of air than as a storage space. Even ensuring that cabling under the floor is correctly fitted can play an important role here. The unimpaired flow of air through the floor is a basic necessity for efficient use of the air conditioning. Equally detrimental are unnecessary or excessively large openings in the floor (e.g. for cable ducting). Enormous quantities of air can escape uncontrolled from such points and so disrupt the correct airflow pattern.

The steps described above involve negligible costs and yet can deliver energy savings of 20 to 25 %. Further actions to these are as a rule more cost-intensive. The decision to implement them depends on how quickly the investment can be repaid by the possible energy savings.

One possibility is to arrange the cabinet rows consistently into hot and cold aisles. Situations in which the warm air from one cabinet is blown straight into the inlet of the next are still found more commonly than one might think. However, before making the considerable effort of repositioning of the cabinets - which in any case is not always possible - it is worthwhile to set up divider plates (Fig. 1) to prevent warm air from one cabinet being sucked into another as 'cold' air. The air inlet elements of the room air conditioner should then be concentrated directly above the resultant hot aisle. If this is too difficult or costly, it may be easier instead to close over the cold aisles that have also resulted (Fig. 2) from above and provide these with an optimal supply of cold air via slit plates in the false floor. This will cause an increase in the back pressure of the cold aisle and thus also of the volume of cold air available. Such a retrofitted hot aisle/cold aisle arrangement will prevent the mixing of hot and cold air and allow the air conditioner system to function more efficiently.

An important condition for a retrofitted 'hot aisle/cold aisle' configuration of this kind is sufficient space between the cabinet rows. If the space is not sufficient, however, there are other ways of minimising the mixing of hot and cold air such as the use of rear doors fitted with radial fans that direct warm air upwards towards the ceiling and not into the next cabinet. While this does not create a hot aisle as such, a significant improvement in separation of hot and cold air is still obtained.

If a cold water supply is available, doors that are supplied with cold running water and act as a cooling register can be used on the server cabinets. Hot air from the servers is blown through the cooling register and cooled passively, without fans or a control system. This option is particularly suited to peak-time use in cabinets that dissipate particularly large volumes of waste heat. In one customer's design such register doors, fed with cold water at 14 °C, stabilised the room temperature at 23 °C and even allowed the existing air conditioner to be switched off. Register doors can also be fitted with fans. This solution is of course conditional on the availability of cold water to the data centre.

Another option might be to define one or several cabinets in the data centre as hot spots, sites that create the greatest heat, and to use cabinets fitted with air/water heat exchangers (AWHE) for these. These cabinets are then cooled independently of the room and do not affect either the room temperature or the air conditioning. It may then be possible to lower the air conditioning setting, so further saving energy.

If the cabinets are already arranged into cold/hot aisles and the optimisation procedures given above have been carried out, efficiency can still further be increased by means of aisle containment. Either the cold aisle or the hot aisle is contained. For rooms with room air conditioning, cold-aisle containment is advantageous. Hot aisle containment isolates hot spots and is thus also suitable for rooms without air conditioning. With this method there is also no real need for a false floor.

What generally makes containment difficult to install in an existing data centre is the fact that a cabinet row frequently consists of a wide variety of cabinets from various manufacturers, with differing heights and widths, plus differing top profiles and fixings. In such cases the standard containment solutions available on the market can rarely be used, if at all - unless the user is prepared to replace all the cabinets in order to use the standard containment housing of a given manufacturer. Very few operators will do this. There are however also cabinet manufacturers such as Schroff, of Straubenhardt, Germany, that offer individually tailored containment for existing data centres, irrespective of the cabinets being used. The company takes all relevant measurements on site, then designs, fabricates, delivers and installs the containment housing.

Schroff bases such individual containment on modified elements from the Varistar cabinet programme. The 45°-angled surface of the VARISTAR frame with its typical meander-style grid holes in the profile offers many universal fixing options. The powder-lacquered frame in steel plate with safety glass panels links between cabinet rows positioned opposite one another and closes off the aisle overhead (Fig. 3). If there are larger holes in the cabinet rows, which must likewise be completely sealed, so-called dummy elements are fitted that can be closed with a continuous front door. These dummy elements can be later replaced, should the data centre be expanded, by normal server cabinets. If the cold or hot aisle containment is sealed overhead, doors can be added to each end of the aisle to seal it completely. These can simply be double doors (e.g. from the Varistar cabinet programme), sliding or revolving doors, electric or mechanical, with or without lock and with motion detector, etc.

The containment of the cold or hot aisles is the best optimisation step as regards infrastructure. In planning a new data centre with Varistar cabinets, however, there always remains the option of retrofitting the cabinets with an AWHE at some later date. The containment is additionally cooled at one or more points, or warm air is drawn off, according to the type of containment.

Further efficiency measures than this are a matter of the IT equipment itself. One possibility might be server consolidation to give a more uniform distribution of heat dissipation in the cabinets, etc.

IT Infrastructure: Custom solutions for your data centre