Cooling Calculations and Outcomes


Possibility of "Free cooling" in Scotland

The objective in cooling is to keep the temperature and relative humidity in the data centre space within the required ASHRAE specifications as already presented under Data Centre Cooling page.

To determine the extent to which "free cooling" is possible in Scotland we obtained climate data for Lerwick and Glasgow from MERIT. Merit is software tool developed by University of Strathclyde to support development of new and renewable energy schemes.

Glasgow was chosen to represent high end temperature scenario and Lerwick was chosen to represent low end temperature scenario. Data bins or classes were created to aid in the counting of the number of hours temperature or relative humidity fell within a given temperature or relative humidity interval. This spreadsheet shows the results obtained from this operation.

Presentation and interpretation

For Lerwick the temperature was below ASHARE range 99.5% of the time and within the range for 0.5% of the time for the entire year. There were no hours when the ambient temperature went above the ASHRAE recommended range. Relative humidity was 95.3% of the time above the ASHRAE recommended range, 4.7% within and 0% below.

Also for Glasgow, there were no hours where temperature went above the ASHRAE recommended temperature range. However it was 15.4% within the range, and 84.6% of the time below the recommended range.

Deciding on "Free Cooling"

The above results suggested the following ideas to us:

Discussion

Although it was sensible to conclude that "free cooling" was available all year round, it wouldn’t be technically easy to implement a "free cooling" solution to tap into it.

The layout proposed in the Cooling Approached page, however suggests an air-side economiser arrangement that takes some of the hot aisle air, mixes it with the cold in coming ambient air to that extent that resulting cold mixed air going into the cold aisle is with in the required ASHRAE temperature range.

Moreover, the relative humidity is above the ASHRAE range both in Lerwick and Glasgow. However, by placing a de-humidifier in the mixed air supply duct, its relative humidity can be brought into the recommended range.

Also the question of achieving the right proportion of mixing hot-aisle and fresh cold air from outside can be answered through the use of variable speed extract and supply fans. But also, a system of automatic dampers can be implemented.

This implies that a robust control system to intelligently monitor and operate all these components as and when they are needed depending on the conditions in the data centre would be needed.

Outcomes

We further approximated the energy savings and carbon dioxide emission savings resulting from implementing “free cooling” decision with reference to an average data centre having a PUE of 2 and and IT demand of 312kW. Energy savings of about 716MWh annually were possible. This would result into carbon dioxide emissions amounting 317 tonnes a year based on a CO2 rating of rating of 0.4422 kg/kWh.