Data centre power consumption in the UK is estimated at between 2.2% and 3.3% of total UK power consumption. Cooling is currently estimated to represent between 30% and 50% of power consumption within a data centre. Cooling in data centres in the UK has an annual carbon footprint exceeding 1.75 million tonnes of CO2.

Servers generating more heat

Development of server technology is raising the thermal envelopes of data centres.

Traditionally, 2kW of energy per rack would reject air into the room at around 28°C. A 42U rack with six fully populated blade enclosures can reject up to 30kW of heat into the room at a temperature of 47°C. These rising temperatures must be stabilised by the data centre operator to ensure that the IT equipment will operate reliably, yet they present significant opportunities to reduce the cost of running the cooling equipment and increase the potential for free-cooling.

Raising operating temperatures

Many data centres still operate at around 20°C and 22°C but as server technology advances and servers generate more heat, they also become capable of running at higher temperatures. Modern servers are capable of running at 35°C.

Whilst nobody wants to run servers near that temperature, ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) now recommend operating temperature in data centres of up to 27°C.

Running data centres at 27°C would offer substantial benefits to the operator:

• Increase efficiency of cooling equipment, as it runs more efficiently at the higher return air temperature of 27°C
• Increasing the opportunity to use free-cooling, as there are more days cooler than 27°C than those mbelow 20°C

Where hot spots exist, some operators choose to cope with hot spots by reducing the temperature of the entire data centre instead of simply cooling only the hotspots. In doing so, operators will reduce the operating efficiency of the cooling system and destroy any free-cooling potential.

Energy savings from higher temperatures

Higher room temperatures can be offset by increasing the chilled water temperatures. The controls system will monitor the room load via sensors and automatically adjust chilled water setpoints to match this load. By raising these setpoints, plant energy efficiency and the free-cooling threshold can be significantly increased. A 1°C increase in fluid temperature can give an 8.5% increase in chiller energy efficiency.

Taking advantage of ambient temperatures

Free-cooling works by taking advantage of the ambient temperature being cooler than the temperature inside the room being cooled. In a data centre cooled by CRAC units and a chiller system, warm air is returned to the CRAC units and heat within the air is given up to the cooler water being circulated through the CRAC units. Now warmer, the water carries the heat externally to the chillers which cool the water without the need for mechanical refrigeration, in a similar way in which the radiator in a car cools the liquid from a car engine. The liquid, now cool, returns to the CRAC units and the cycle begins all over again.

The graph: ‘Ambient temperatures in the UK’, shows the distribution of hourly UK ambient temperature. A typical air conditioning system operating at 22°C will require chilled water at around 7°C.  To achieve this kind of temperature using free-cooling, the ambient would need to be cooler than the water by about 5°C i.e. 2°C. The number of hours at 2°C or below in the UK is represented by about 7% of the UK ambient year.

If we shift the operating temperatures upwards by 5°C, making the room 27°C, we can lift all the operating temperatures of the cooling fluid by the same amount to achieve the same capacity.  Now, the ambient need only be 7°C to satisfy full free-cooling – the number of hours at 7°C or below in the UK is represented by 30% of the UK ambient year. The opportunity for free-cooling is present whenever the ambient temperature is below the room operating temperature, i.e. 98% of the UK ambient year. There are a huge number of ambient hours around the middle of the bell curve, which are not being captured by most free-cooling systems.

In systems with non-concurrent free-cooling, when free-cooling cannot deliver 100% of the required capacity, free-cooling is sacrificed and replaced by mechanical cooling. There is no ability to mix free-cooling and mechanical cooling. Thus, as soon as the ambient temperature reaches a point where it cannot satisfy the entire cooling demand, all cooling is mechanical, none is ‘free’.

The best systems bring together free-cooling and mechanical cooling simultaneously, enabling free-cooling to be captured whenever the ambient is below the return water temperature.

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Tags: Design & Facilities Management

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