The path to an environmentally-friendly data centre

The Carbon Trust inform us that for every kWh of grid electricity used, our fossil fuel burning power stations will throw around 0.5 kg of carbon dioxide into the atmosphere.

The path to an environmentally friendly data centre therefore involves minimising energy usage, just like we are encouraged to do at home. In this case however, switching off is not an option so it becomes a matter of selecting the most efficient power and cooling equipment, rather than the cheapest.

We shall concentrate here on our review of cooling, or more appropriately air conditioning systems, as this choice offered the greatest scope for energy saving. It also exposed us to a disproportionate amount of verbose and often confusing “Greenwash” from some vendors, whilst checking out supposedly new technologies and associated performance claims.

Air to the servers in our care must be maintained at a temperature between 18OC and 27OC if they are to perform without thermal build up leading to reliability problems. The co-incident relative humidity (RH) is best kept between 40% and 65% to obviate potential issues with static discharge, conductive anodic filament (CAF) growth or even just simple condensation. The foregoing was therefore taken as a baseline specification for air conditioning system performance which, in the interests of our customers, could not be compromised. In our business, if server downtime were to become one of the variables in our equations, payback periods would rapidly tend towards infinity!

As with many legacy data centres, conditions could be achieved most simply using a number of downflow air conditioning units, directly connected by 35 mm refrigeration pipework to matched outdoor condensers. Each “Split” system is autonomous and therefore provides the necessary standards of reliability and flexibility. We take heat directly from the room and reject it outside with only two heat exchange processes, which from a thermodynamic viewpoint is also a very efficient way of getting rid of excess heat.

Option A was therefore to install a number of direct expansion (DX) systems as required to meet our cooling load, with at least one extra system for standby provision. Using GEA Multi-DENCO®, and variable refrigerant flow (VRF) technology with inverter controlled compressors, we could obtain over 100kW of cooling from a single refrigeration circuit, so six systems would meet our 500kW server load, even with one unit offline for maintenance. In practice all systems would be configured to run, not only to provide “Hot” standby, but also to take advantage of the considerable increase in energy efficiency of these systems when running under part load conditions.

Noting that many building services engineers are more familiar with, and therefore gravitate towards, water based cooling we then looked at using chilled water downflow units, matched to external air cooled water chillers.

Using relatively large chillers on a ring main distribution system was discounted simply due to unacceptable issues with autonomy, complexity, flexibility and maintenance that arise from single points of failure.

Option B therefore evolved to incorporate six chilled water downflow units of 100kW cooling capacity, each independently connected to a separate remote air cooled chiller, via 65mm insulated flow and return water pipework. As no qualified refrigeration expertise is required, this system could be installed by any competent mechanical services contractor. Thermodynamically though, our system is now less efficient because we have introduced a third heat exchange process. The refrigeration compressors in our chillers will have to work harder than those in Option A because they have to cool the water to a very low temperature, so that it in turn can cool the air to our servers. To obviate this to some extent, we decided that “Free Cooling” chillers only should be considered for this Option B.

If the West Midlands were somewhat closer to the equator, with associated year round warm climatic conditions, this would probably be the end of our story.

However, for at least 75% of the year, our Northern European climate provides colder air outside than we need for our servers, meaning that simply “Opening the Window” could get rid of some of the unwanted heat. This is known as ventilation, or direct free cooling, and at first glance looked to be an ideal alternative solution.

Large data centres in the US are increasingly cooled using direct free cooling, but these are generally located in areas such as California or Arizona which have ideal climatic conditions. You will also see massive open air storage facilities for aircraft there, as they do not corrode or otherwise deteriorate under these temperate conditions, but we know of no such facilities at our location.

Option C was therefore to use packaged indirect free cooling air handling units. These could provide similar efficiency to the foregoing direct free cooling systems, but without any of the potential issues associated with mixing indoor and outdoor airstreams, due to the use of a very generously sized intermediate heat exchanger.

The six roof mounted air handling units would be rated at 100kW of cooling and connected to the data centre via very low velocity ducting to minimise pressure losses and hence fan power required. This system again could be installed by any competent mechanical services contractor, but further benefits from the fact that it remains an off the shelf product, which can be easily maintained by any competent service organisation.

Thermodynamically our system is
apparently less efficient because we have introduced a heat exchange process. However, by using GEA Adia-DENCO®,
with a carefully designed evaporative spray system, high external ambient temperatures can be reduced by up to 10K, meaning that no refrigeration system is theoretically required at all.

The foregoing options were compared not only on the basis of reliability, energy efficiency, flexibility, scalability, capital cost and plant space required but also on their ability to be easily maintained.

It soon became apparent to us that downflow “CRAC” units incorporating direct expansion refrigeration systems (Option A) had evolved considerably since we last carried out any analysis of this sort. Not only indoor and outdoor fans, but also compressors are now speed controlled so that cooling capacity exactly matches the server heat load.

This gives the obvious benefit of very stable conditions, but also allows the power used to diminish as the cube of any reduction in capacity. A unit running at 80% of rated load therefore uses half the power, and at 50% load, power usage is down to nearer 10%.

Option C also offered reduced energy consumption and a non-refrigerant based solution was thought very attractive. Particularly as for the majority of the time we would be able to utilise true ‘Free Cooling’ from the outside air.

Our evaluation continues but our studies so far have shown that there have been a number of advances in data centre cooling technologies that provide excellent alternatives which have not previously
been available.