Two types of cooling technologies are commonly used to cool processors: passive cooling and heat pumping.
Passive cooling channels heat to ambient surroundings by means of convection, liquid flow and/or gas carrying heat away from the heat source.
This process is limited by the ambient temperature. Coolant temperature will not fall below the ambient temperature. This is in accordance with Fourier's law, which mandates that the rate of heat transfer is proportional to the negative temperature gradient. As a result, heat absorption is significantly limited by the ambient temperature.
In chip cooling, where heat density is ever-increasing, passive cooling technology alone is insufficient. Indeed, in any computer room, communication rack or data center, ambient temperature must be intensively cooled to keep chips within their temperature specifications.
It may be claimed that coolant temperature can be cooled below ambient using various types of chillers. However, this two-phase solution is highly problematic in terms of efficiency, size and capital costs.
Passive cooling encompasses all current chip cooling technologies, heat pipes, DCLC (water cooling), air-cooled heat sinks and their derivatives.
Active cooling, also known as heat pumping, uses energy to reduce coolant temperature to the desired temperature. This can be achieved by material phase change from liquid to gas or by highly inefficient electro-cooling.
The most common material phase change active cooling technology is pressurized gas, which is used in air-conditioners and refrigerators. Gas undergoes forced condensation under high pressure (usually > 10 atm) and then evaporates by means of pressure decrease (usually >3 atm).
This technology is mature and efficient. However, it has one major flaw: the use of high pressure. This results in a low working volume to sealing surface ratio (and therefore a larger size). It also requires the use of high-pressure proof, strong and rigid materials, as well as extra sealing. These requirements prevent active cooling technology from being a feasible solution for chip cooling.