The Problem: Heat pipe systems have been the mainstay of thermal management for a long time. However, this incumbent technology is reaching its limits with devices that have very high power and power density, creating the need for a new thermal management technology. Liquid cooling systems have been developed to address the limitations of heat pipes; however, since liquid cooling is an actively pumped system, there are major reliability concerns, water leakage risks, cost burdens, and additional pump power requirements.
The Solution: Loop Thermosyphons can provide the best of both worlds. Like heat pipes, Thermosyphons are passive as well as reliable, and like pumped liquid loops, Thermosyphons can transport heat over a long distance due to its ultra-low thermal resistance, enabled by two-phase heat transfer. There have been previous attempts at Loop Thermosyphons but they suffer from high evaporator resistance and manufacturing cost. But, at Deeia, we have solved some of these key challenges, enabling true liquid cooling like performance at heat pipe like simplicity and cost.
Working Principle: Our two-phase Loop Thermosyphon is a closed system, partially filled with a working fluid with no non-condensable gases present inside. Just like a heat pipe, it is a hermetically sealed system. The working principle of a two-phase closed Loop Thermosyphon is shown in the figure above. The self-sustained fluid motion is produced by the density difference established when the fluid is simultaneously evaporating and condensing at the evaporator section and condenser section respectively. The loop between the evaporator and the condenser results in separate vapor and condensate path. Heat is supplied at the bottom of the device (in the evaporator), where the refrigerant changes phase and the liquid vapor mixture rises in a two-phase adiabatic section until the mixture reaches the condenser (placed at a relatively higher location than the evaporator). At the condenser, the latent heat is released, producing the condensation of the refrigerant that, through a second adiabatic section, returns to the evaporator, where evaporation occurs again, driving the passive, gravity-driven flow-motion.
Deeia Innovation: With the right selection of working fluid, it is even possible to utilize the height difference within even an 1U server to create sufficient flow to move 500 W of heat in a Loop Thermosyphon. With the right evaporator design, it is possible to achieve 0.01 C-cm2/W and up to 90 W/cm2 of heat flux. Loop Thermosyphons can also be utilized to cool kW of heat passively from an IP rated enclosure.