Air Energy Recovery

Data center managers are always on the lookout for a means to reduce the heat that their facilities generate to operate more efficiently. During the winter seasons is often brought up to use the warmer air from the data center return stream to heat other spaces, as it seems like
an obvious way to save energy. As all of the IT equipment in the data center rejects a large amount of heat that typically goes to the atmosphere via cooling towers, condensers or similar, on the surface it makes sense to instead reuse when the outdoor temperatures drop and other spaces need the heat.

Often air energy recovery is difficult since the heat rejection and distribution is limited by physical, monetary, and temperature limits.  But with operating temperatures
increasing year by year, the opportunities to reutilize waste heat grows accordingly. 

The choice of energy recovery should not be considered a new concept, as systems, equipment and design techniques have been used in similar applications.  Some of those may seem like an easy way to gain savings but each should be vetted to understand whether the savings is worthwhile.

To keep the data center air separate from other spaces, heat exchangers might be deployed to pass the energy from one air stream to another without risk of mixing.  The main types: air-to-air, which typically uses a crossflow; heat wheel, which can also control latent heat and use a ‘purge’ section to completely prevent mixing; and heat pipes.  In a typical application the air streams need to be adjacent, which may require extra ductwork and fan energy to accomplish.  This can be overcome, especially with existing systems, by instead using a runaround loop that circulates a refrigerant via a pump. 

The location matters, of course, and installing these options might have a much longer payback in hotter regions that cooler.  Using the data center return air to strictly preheat ventilation air for a building will have excellent performance during the winter season due to maximizing the air temperature difference.  Utilizing the warm return air from the data center for heating the air in a mixed air or a central air handler may have less overall efficiency, but the airflow volumes are much more likely to match. 

There are integrated means to utilize return air for a data center, but often this is with an air handling unit that is already dedicated to the data center for ventilation.  While they may be small, their function as a dedicated outdoor air system can more easily be used with the data center by directly premixing cold outdoor air with return air.  This is a typical arrangement
with facilities that are in cooler locations where cold extremes can threaten damage to equipment by freezing but could be readily applied to data centers more universally. 

The installation and maintenance costs must be worthwhile, and a simple heat exchanger, air mixing, or runaround loop system might be the best way to sample the savings.  Each of
these has a potential return on investment of less than 5 years and in some case studies less than 3 for a larger deployment.  The equipment purchases to implement can be further supplemented by energy savings grants through a utility or with energy savings performance contracts, where the risk and cost of energy savings performance falls on the designer and not the owner.

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