Maximising the benefit of free cooling

Airedale, free cooling, data centres
Free cooling for 95% of the year — George Hannah.

Changing the operating conditions in data centres enables cooling plant to run more efficiently at higher flow temperature and for free-cooling to be used more. George Hannah of Airedale looks at how this can assist data centre managers reduce operating costs.

Data centre power consumption in the UK is estimated at 3.3% of total UK power consumption. Cooling is currently estimated to represent 30 to 50% of power consumption within a data centre. Cooling in data centres is responsible for over 1.75 Mt of CO2 emissions, and the demand for computing is set to increase by nearly 30% in five years.

Developments in server technology are raising the thermal envelopes of data centres. Traditionally, 2 kW 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 30 kW 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.

Many data centres still operate at around 20 to 22°C. However, 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 & Air-Conditioning Engineers) now recommends 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 below 20°C.

Combining higher temperatures in data centres with free cooling can considerably reduce cooling energy consumption.

Where hot spots exist, some operators choose to cope with them by cooling 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.

Higher room temperatures can be offset by increasing the chilled-water temperatures. The control 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.

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 them. 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 to a car radiator cooling the liquid from a car engine. The liquid, now cool, returns to the CRAC units, and the cycle begins all over again.

Fig. 1 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 K — i.e. 2°C. The number of hours at 2°C or below in the UK is about 7% of the year.

Fig 1: 100% free cooling when the ambient temperature is 7°C or less more than quadruples this benefit compared with 2°C. Combining free cooling and mechanical with a higher temperature in a data centre enables some free cooling to provided for 95% of the year.

If we shift the operating temperatures upwards by 5 K, 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. An ambient of 7°C can provide full free-cooling; the number of hours at 7°C or below in the UK is 30% of the year.

The opportunity for some free-cooling is present whenever the ambient temperature is below the room operating temperature, i.e. 98% of the UK 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.

The best systems bring together a mixture of free-cooling and mechanical cooling simultaneously in concurrent free cooling, enabling free cooling to be captured whenever the ambient is below the return-water temperature. When such a free-cooling chiller is linked with an air-handling or rack-mounted unit in a 24/7 data centre with a typical room temperature of 24°C, over 95% of the year can be spent with free-cooling active (cumulative hours, London, UK).

Systems that cannot combine free cooling and mechanical cooling at the same time can only exploit free cooling when it can meets all the cooling requirement. 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, and none is ‘free’.

George Hannah is product development director with Airedale.

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