Some proposed high energy applications, such as high energy lasers and high speed long-range aircraft, have large cooling requirements with limited available electric or mechanical shaft power and limited available heat sinking for conventional vapor and air compression cycle cooling systems. High-energy laser systems have relatively low efficiencies that cause waste heat to be approximately three or more times their beam energy. At the same time, they only operate effectively within stringent temperature ranges. High-speed long-range aircraft produce large engine and airframe heat loads during the major portions of their flights that typically consume the available fuel heat sink capacity. Additionally, the high speed at which such aircraft operate makes ram air heat sinks less suitable due to the high temperatures and drag produced at high speeds.
Some cooling systems have used the latent heat or enthalpy of vaporization for an expendable boiling liquid to assist heat extraction. However, such systems have only been suitable for short-term heat loads, such as during supersonic dash flights.