Waste Heat Reuse

Servers have a wide range of energy use, depending on traditional use or specialized for applications, with a typical server using around 350 watts of power.  This average server also produces about 1,200 BTUs of heat every hour and for years many of us have aimed at capturing and reusing this heat.  With potentially 50 servers or more in a rack, this adds up to a 60,000 BTU per hour when operating at full capacity.  This is, for comparison, about the same BTU as a larger-than-average barbecue grill.

Well, why don’t we get started?  Let’s talk about a few roadblocks first.  The hurdles with most locations is that the racks are cooled with air and at a lower temperature.  For air, it is a bit harder to keep the hottest air, the discharge from the servers, from blending with cooler bypass air, which reduces the overall temperature difference.  For the temperature, the lower it is the amount of heat that can be transferred is less, making it hard to move the heat energy to other places it could be used.  Another issue has been that data centers produce this heat rather constantly, while the need for the heat isn’t as consistent.  This requires the same cooling system that would normally be required and thus reducing the cooling equipment sizes isn’t an option.  Therefore the costs of heat capturing systems remains to be offset by reducing the heating systems and/or the energy used by them. However, as the data center temperatures go up, air-to-air and air-to-water heat capture becomes more viable.

So then how can we capture those BTUs?  Generally the most viable cases for recycling heat is ‘in-house’, using the heat to support other systems in the same building.  But there are cases where heat, whether from data centers or other industrial facilities, is moved to other locations.  Using water as the heat moving mechanism, a loop between the heat source (data center) and heat sink (such as an office building) is possible.  In Seattle, Amazon is reusing the waste heat from the Westin Exchange building using a similar hydraulic setup.  Their system has a 400,000 gallon tank that provides heat storage that can also act as an emergency water supply as needed.  The water is cycled through a number of chillers that bring the temperature up from 65 to 130 degrees F, sufficient to provide heating water for the 37 story building.  In total, there is about 5 MW of heat moved to the Amazon campus building, at a much more efficient rate that the main boilers that provided the same heat.  The cost savings for this project is equivalent to about $200,000 per year, offsetting the costs of adding the equipment to capture, exchange, and move the heat between the sites.  Given that the system is expected to operate for the next 25 years, it will easily pay for itself many times over, with the initial payback period on track of about 2.3 years.


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