  Air cooling is the traditional solution to chill servers in data centers.
However, the continuous increase in global data center energy consumption
combined with the increase of the racks' power dissipation calls for the use of
more efficient alternatives. Immersion cooling is one such alternative. In this
paper, we quantitatively examine and compare air cooling and immersion cooling
solutions. The examined characteristics include power usage efficiency (PUE),
computing and power density, cost, and maintenance overheads. A direct
comparison shows a reduction of about 50% in energy consumption and a reduction
of about two-thirds of the occupied space, by using immersion cooling. In
addition, the higher heat capacity of used liquids in immersion cooling
compared to air allows for much higher rack power densities. Moreover,
immersion cooling requires less capital and operational expenditures. However,
challenging maintenance procedures together with the increased number of IT
failures are the main downsides. By selecting immersion cooling, cloud
providers must trade-off the decrease in energy and cost and the increase in
power density with its higher maintenance and reliability concerns. Finally, we
argue that retrofitting an air-cooled data center with immersion cooling will
result in high costs and is generally not recommended.
