June 2008
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Stop throwing it away Energy Recycling

Once you start thinking about waste heat as a resource, it will lead a long way from where you started.

Toby Considine
Toby Considine
Systems Specialist,
Facility Services, University of North Carolina Chapel Hill
The New Daedalus

Contributing Editor

Everyday, data centers are throwing out a valuable resource, one they pay top dollar for. Everyday, buildings here in the American southeast are throwing it away twice.

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I first started to think about this when UNC was initially planning its Renaissance Computing Center, to be home to the Renaissance Computer Initiative (RENCI). RENCI was being planned as the first building on a new campus, one filled with purposes from graduate school to office park. As the new campus is on one of the largest undeveloped sites in town, the entire project had drawn considerable community interest and attention.

Sustainability was in the air, and part of every public discussion. Most of it was what I call pretend sustainability, the sort of ill-considered mish-mash of slogans that might impress at the freshman mixer thereafter flourishes only near college campuses. But one part was breathtaking once said aloud. We did not need a steam plant for this campus.

RENCI was planned at that time with a 15 megawatt (MW) data center. This is a substantial, but not exceptionally large, data center. Data centers are the most efficient way ever created to convert electricity to heat. Typically data centers use as much energy again in cooling as they do in generating this heat. If we but collected this heat and concentrated it rather than throwing it away, we had our district heating plant.

Once you start thinking about waste heat as a resource, it will lead a long way from where you started.

UNC is in the humid southeast, in which every building is heating and cooling year round. In the summer, the air is regularly what I call double 90s, with 90 degree at 90% humidity. For my non-HVAC readers, this means we must over-cool intake air to drive out moisture, and then heat it back up because no one wants icy air blowing on them, even in the summer. We use district steam, a byproduct of electricity generation, and district chilled water as the primary supply for hot and cold.

[an error occurred while processing this directive] One of our in-house engineers got to thinking about energy recycling in general, and heat recycling in particular. He looked at the parallel heating loop and chilling loop running to each air distribution unit. The heating loop sent out hot water and returned warm water to the basement. The cooling loop sent out chilled water and returned cool water to the basement. A simple heat pump to bridge the two, removing heat from the chilling loop and returning it to the heating loop would, he calculated, save 40% of our summer air conditioning costs. We are now waiting for approval as others, worried about the district utilities are calculating the effects on their operations.

As much as half of your data energy may be used up by the conversion from AC to DC, particularly if you have a UPS and batteries. Convert that data center to AC, and take a third of the heat generated out of the building to the high-grade transformer. Design it right, and you may be able to run an absorption chiller off of the water cooled high temperature transformer.

All the most successful recycling is based on energy. Aluminum leads the way because it is cheap dirt converted into a usable metal by binding energy; aluminum recycling is recycling frozen energy. Once you start thinking of every energy store, of every heat gradient as an energy source, building problems look like opportunities. Convert that data center to water cooling and use it for winter heat, and summer re-heat. Use that solar thermal system, so nice for the winter, as part of your summer cooling. Go wild, and put a Stirling engine generator on your flue gas.

Zero net energy buildings can be summed up in the phrase local generation, storage, and conversion of energy. You can start on local conversion today.


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