This page explores the similarities, differences, and best uses of a thermosiphon and a heat pipe.
What is a Thermosiphon and How Does It Work?
A thermosiphon is a passive, two-phase heat exchanger that transfers thermal energy from a heat source to a heat sink using the natural thermodynamic behavior of a working fluid, requiring no mechanical pump or wick structure. Rather than capillary action, a thermosiphon relies on gravity to return condensed liquid to the evaporator. It is used in high-power electronics cooling and other applications where orientation constraints can be accommodated.
The Thermosiphon Loop
A thermosiphon operates through a continuous thermosiphon loop driven by the phase change of a working fluid. Common choices include deionized water, ammonia, methanol, and refrigerants. The device is a sealed, vacuum-evacuated vessel (typically copper or aluminum) that uses the reduced boiling point created by the internal vacuum to initiate phase change at lower temperatures than would occur at atmospheric pressure.
The cycle works as follows:
- Evaporation at the Heat Source: Heat is applied to the lower portion of the device, known as the evaporator. The working fluid absorbs this thermal energy and undergoes a phase change from liquid to vapor. Because the interior is vacuum-sealed, this boiling threshold is significantly reduced — allowing efficient operation even with modest heat inputs.
- Vapor Flow Dynamics: Once vaporized, the working fluid expands rapidly and rises through the central vapor space toward the upper, cooler region of the device. This movement is driven by the pressure differential between the hot evaporator zone and the cooler condenser zone — no external pumping energy is required. The vapor carries substantial latent heat energy as it travels upward.
- Condensation at the Heat Sink: At the condenser (the upper portion of the device, in contact with a fin stack or cold plate), the vapor releases its latent heat. This is where the thermal energy is ultimately transferred to the ambient environment or a secondary cooling medium.
- Gravity-Assisted Liquid Return: Here is the defining characteristic of the thermosiphon: the condensed liquid flows back down to the evaporator purely under the influence of gravity, traveling along the smooth inner walls of the vessel. This is efficient and low-resistance, but it creates an absolute geometric constraint — the evaporator must always be positioned below the condenser. If this orientation is inverted or the device is operated horizontally, the thermosiphon loop cannot function.
Similarities Between a Thermosiphon and a Traditional Heat Pipe
The most basic thermosiphon shares the following elements with heat pipe technology:
- Envelope material made from copper (most common), aluminum, or titanium.
- The use of a working fluid such as water (most common), ammonia, methanol (also used in heat pipes), or some kind of refrigerant.
- An interior that’s been vacuum-sealed to allow the phase change from liquid to vapor at lower temperatures than would occur at atmospheric pressure.
- The ability to have a remote condenser with tubes that can be bent, flattened, and/or plated.
- An evaporator portion of the device where heat is applied, a condenser portion that attaches to a fin stack where the vapor is cooled, and an in-between section called the adiabatic region.
- Thermal conductivity is 10-100 times that of solid copper.
Both devices essentially operate the same way, as seen in the figure below.
When heat is applied to the evaporator portion of the device, some of the liquid turns to vapor and moves to an area of lower pressure, the condenser end. As it passes through the cooling fins, the vapor turns back to liquid. Lastly, the liquid is returned back to the evaporator where the cycle repeats itself.
Differences Between Thermosiphons and Traditional Heat Pipes
Thermosiphon systems rely on gravity to move liquid back to the evaporator.
Because they have no wick structure to produce the capillary pumping action necessary to move the liquid back to the heat source, thermosiphons rely on gravity to do the job. From this perspective, heat pipes should be used anytime the heat sink assembly has the evaporator at the same level or above the condenser. In these situations, heat pipes rely on the wick structure to transport the liquid. This geometric limitation is the central thermosiphon design constraint engineers must account for before specifying one. The heat pipe design guide covers orientation constraints and wick behavior in detail.
For a given diameter, thermosiphons have a higher Qmax than heat pipes.
The amount of vapor space available to carry a given load is a crucial factor in determining the device Qmax (maximum heat transport capacity in watts)
The sintered wick lining the walls of a heat pipe reduces the available vapor space, a key component to determining a device’s Qmax. This isn’t a problem for heat pipes; heat transport capacity is governed by the capillary limit, not available vapor space. The chart below shows Qmax for various heat pipe sizes where the evaporator is directly below the condenser (+90 degree orientation).
Wickless thermosiphons of the same diameter as heat pipes have 100-200% higher Qmax than the heat pipe counterparts, all because of the additional vapor space.
Thermosiphons can transport heat over longer distances than heat pipes.
The simple reason thermosiphon systems operate more effectively over long distances has to do with the ease with which liquid can travel from the condenser to the evaporator. In a heat pipe, liquid returns through a sintered porous wick. In a thermosiphon, liquid flows back along smooth or grooved inner walls with significantly less flow resistance.
A heat pipe’s practical heat transport limit is approximately 1–2 meters. A thermosiphon can operate over distances exceeding 10 meters. For most electronics cooling applications, transport distances are under a couple of meters, so this distinction is generally not a limiting factor in electronics cooling applications.
Thermosiphons are susceptible to freeze damage
The fluid level required for a thermosiphon generally covers between 20-80% of the length of the evaporator, assuming the vessel is in a vertical orientation. This is significantly more than traditional heat pipes and can be problematic if water is used in conditions where they are exposed to freezing temperatures. After repeated freeze/thaw cycles, the expansion of water will form bulges in the evaporator section of the device, eventually fracturing the enclosure wall. For thermosiphon designs using other fluids, freezing is generally not a concern.
Thermosiphon Design Variations
Two variations of the basic thermosiphon design can improve thermal performance and reduce freeze damage risk.
Thermosiphon with a partial sintered wick in the evaporator
Adding a sintered wick to the evaporator section reduces thermal resistance, increases power density capability, and allows the fluid charge to be reduced. The lower fluid volume reduces freeze damage risk. Note that the wick is used only in the evaporator section. The rest of the device remains wickless.
Loop thermosiphon with separated vapor and liquid paths
A loop thermosiphon separates the vapor and liquid flow paths, which increases Qmax. Vapor flows directly to the condenser, where it condenses and returns as liquid to the evaporator. A mechanism at the hollow evaporator prevents vapor from traveling up the liquid return line while allowing liquid to enter. Adding a sintered wick to the evaporator further reduces the required fluid charge and lowers freeze damage risk. For applications requiring planar heat spreading in addition to directional transport, vapor chamber designs should be considered as a complement or alternative to loop thermosiphon configurations.
Using Thermosiphons to Cool Electronics
Thermosiphons should be specified for applications where the evaporator is reliably below the condenser and where passive, pumpless operation is preferred.
Historically, the primary application has been power electronics. For example, stationary motor controllers in industrial settings such as steel mills and mining operations, and transportation systems such as light rail and subway traction inverters. More recently, there has been renewed interest in data center applications as inlet temperatures rise and fan count is reduced. Cooling accounts for approximately 33% of data center operating costs, with fans responsible for nearly half of that.
Thermosiphon systems are also applicable in harsh, fixed-orientation environments (including aerospace applications and defense thermal solutions) where passive operation and high reliability are non-negotiable. For organizations bringing a new thermosiphon-based system to production, early-stage design and prototyping, and thorough quality and testing are essential to validating performance before deployment. Teams unsure whether a thermosiphon cooling system is the right fit for their application can explore the full range of options through custom heatsink development.
In summary, thermosiphons remain a viable technology for cooling high-power applications where the evaporator is below the condenser. Applicable loads include power electronics such as IGBTs, radar systems, RF transmitters, and alternative energy generation equipment.
