Scientists from the University of Žilina in Slovakia have developed a series of systems based on heat pipes, simple yet exceptionally effective heat management tools. Tests have shown that these pipes can improve the reliability of electronic gadgets, home heating and ventilation systems, and even hydrogen storage.
A heat pipe is a sealed metal tube partially filled with liquid. When one end of the pipe is heated, the liquid boils and turns into vapor. The vapor rises to the colder end, where it turns back into liquid, releasing the accumulated heat. The resulting condensate comes back and the process repeats over and over, without the involvement of pumps, fans or an external energy source. This makes it possible to transfer heat virtually without losses, even if the temperature difference between the ends of the pipe is only a few degrees. Moreover, a pipe of this kind can pump heat flows with a power of tens of kilowatts per square meter of surface area.
The Slovak researchers studied three types of heat pipes: gravity, capillary and closed-loop heat pipes (thermosyphons). In gravity heat pipes, liquid comes back due to gravity, requiring the pipe to be vertical. Capillary pipes can operate in any position thanks to a special internal porous structure that draws the liquid back in like a sponge. In closed-loop heat pipes, liquid and vapor move through separate channels, making the system more stable and efficient. The simple yet elegant thermodynamics allows heat pipes to operate autonomously and without wear, lasting for decades even in environments where other cooling systems are simply unfeasible, namely, in sealed devices, satellites or underground.
First, the engineers tested heat pipes on electronic devices, the most sensitive among modern technological components when it comes to overheating. In a power transformer, whose windings are encased in epoxy resin and have virtually no natural cooling, they installed a simple copper pipe filled with water and alcohol. One part of this pipe was located inside the enclosure, where the liquid heated up and evaporated, while the other was located outside, where the vapor condensed and released the heat. This solution proved so effective that the temperature of the windings dropped to a safe 80°C, while the system operated completely autonomously, without fans or pumps.
After this, the researchers created miniature pulsating heat pipes: tiny, powerful heat pumps without any moving parts. The pipes were filled with an insulating heat-dissipating liquid (Fluorinert FC-72), thanks to which they were able to dissipate up to 100 W of heat from power transistors while remaining compact and silent.
During the next stage, the scientists developed sealed cooling systems for electronics cabinets where traditional fans cannot be used due to dust and moisture. In these cabinets, the heat pipe passed through the enclosure wall, absorbing heat from the inside and releasing it into the air outside. Even with a load of up to 1.5 kW, the temperature inside the cabinet did not exceed 70°C.
Equally impressive results were obtained in the area of household energy. In gas-fired fireplaces with a large glass insert, the researchers installed a loop heat pipe that transferred up to 40% of heat to a hot water storage tank. This reduced room overheating, increased comfort and made it possible to use excess energy to warm up the heating system. In small boilers, similar devices brought back some of the heat from the flue gases to the combustion air, reducing the exhaust temperature by 20–50°C and increasing the system’s efficiency by 2–5%.
The team also experimented with geothermal springs. They placed heat pipes filled with ammonia into deep boreholes so as to transfer heat from the ground without pumps. Although their capacity is currently not as high as that of conventional coil pipes, the researchers identified the causes of losses and proposed new solutions for distributing condensate evenly across the inner surface. Once optimized, this system could become a cheaper and more reliable alternative to existing geothermal systems.
Particular attention was paid to improving the working fluids. The addition of nanoparticles of aluminum, copper or titanium oxides to water significantly increased boiling and accelerated heat transfer. Experiments have shown that this nanofluid increases the efficiency of heat pipes by roughly one-third, paving the way for compact and cost-effective heat exchange systems to be used in ventilation and air conditioning.
Finally, heat pipes have even proven useful for hydrogen technologies. The storage of hydrogen in metal hydrides involves significant heat release and absorption, requiring precise temperature control. The Slovak engineers used a closed-loop pipe that automatically cools the system when hydrogen is absorbed and heats it when it is released, by using exhaust heat from a fuel cell, among other things.
In the future, the scientists plan to improve the internal structure of the pipes and the uniform distribution of condensate, develop new nanofluids with more stable properties and integrate heat pipes into solar collectors, batteries and hydrogen storage systems.
