Applications of Heat Pipes in Thermal Management

Abstract

The paper explores the application of heat pipes in thermal management for efficient heat dissipation, particularly in electrical equipment with high heat loads. Heat pipes are devices that transfer heat with high efficiency through the phase transition of the working medium (e.g., water, alcohol, ammonia) between the evaporator and the condenser, while they have no moving parts and are distinguished by their simplicity of construction. Different types of heat pipes—gravity, capillary, and closed loop (thermosiphon loop)—are suitable according to specific applications and requirements for the working position, temperature range, and condensate return transport. An example of an effective application is the removal of heat from the internal winding of a static energy converter transformer, where the use of a gravity heat pipe has enabled effective cooling even through epoxy insulation and kept the winding temperature below 80 °C. Other applications include the cooling of mounting plates, power transistors, and airtight cooling of electrical enclosures with the ability to dissipate lost thermal power in the order of 102 to 103 W. A significant advantage of heat pipes is also the ability to dust-tightly seal equipment and prevent the build-up of dirt, thereby increasing the reliability of the electronics. In the field of environmental technology, systems have been designed to reduce the radiant power of fireplace inserts by up to 40%, or to divert their heat output of up to about 3 kW into hot water storage tanks, thus optimising the use of the heat produced and preventing overheating of the living space. The use of nanoparticles in the working substances (e.g., Al₂O₃ in water) makes it possible to intensify the boiling process and thus increase the heat transfer intensity by up to 30% compared to pure water. The results of the presented research confirm the versatility and high efficiency of the use of heat pipes for modern cooling requirements in electronics and environmental engineering.

1. Introduction

Adequate cooling of static power converters is one of the main requirements for their error-free and safe operation. It was the need to reliably dissipate heat loss from the electronic components of power converters that motivated the experimental research on cooling systems using the specific properties of heat pipes. Static energy converters are predominantly cooled by forced convection (Figure 1a) of heat from power electronic and passive elements through standard finned heat sinks (Figure 1b). For reliable heat dissipation, the cooling air flow is provided by a fan system or liquid circuits with forced movement of the heat transfer medium.

One of the first tasks was to solve the heat dissipation from the inner side of the energy converter (Figure 2), which is normally encased in epoxy insulation.

While fins could be placed on the outer wrapping and substantially increase the heat exchange surface, this was not possible with the inner winding. Therefore, we removed the lost heat from the central compartment with a gravity heat pipe, the evaporating part of which was encased in epoxy insulation and the condensing part of which was cooled by natural convection. Initial experiments and experience with the design of heat pipes have already indicated their considerable application potential in a number of thermal management problems in facilities where heat has to be supplied or removed under specific conditions. The gradual discovery of the remarkable properties of heat pipes has also led to further application areas not only in electrical engineering, but also in environmental engineering and in power engineering.

In this paper, we review the use of different types of heat pipes in the removal of waste heat from electronic components and electrical enclosures, in increasing the efficiency of small heat sources, in energy storage, in the recovery of low-voltage heat from ventilation systems and geothermal ground boreholes, as well as in the use of nanoparticles to intensify the boiling of the working substance in the evaporator of the heat pipe. All the experiments presented in the thesis are carried out at our workplace by the author collective and address the actual issue of heat transport in different thermal devices.

2. Heat Pipe

A heat pipe (Figure 3a,b) is a device used to transfer heat by phase transformation and with a high heat flux density. It contains a working medium (alcohol, water, mercury, ammonia, helium, Freon…) in two phases at a certain pressure dependent on temperature. By heating one end of the tube and cooling the other, the medium is circulated so that at the heated end the medium evaporates and the vapour moves to the cool end where it condenses back to a liquid and, by spontaneous fall or rise through the capillary structure, the liquid again reaches the warm end, where the process repeats itself again. This creates a forced circulation of the working substance associated with heat transfer [1,2].

The heat flux transferred by the working substance from the evaporator to the condenser is many times greater than the heat transferred by a solid body, made of the best heat-conducting material, which has the same dimensions and the same temperature difference between the ends of this heat pipe.

Advantages of using heat pipes:

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Heat transfer even with small temperature differences at the ends of the tubes (around 2 °C);

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They have no moving parts and do not need additional equipment;

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Ease of installation and ease of design.

The heat output that is transferred through the heat pipe depends on its operating temperature. This should be identical to the working temperature range of the working substance. At very high working temperatures, when the whole substance is in vapour form, condensation does not occur, and heat transfer stops, and, conversely, at very low temperatures, when the entire substance is in the form of liquid, there is no evaporation, and heat transfer also stops [2].

The selection of the working medium depends on the temperature range of the tube, the required evaporation and condensing temperatures, the pressure inside the tube, and the ambient pressure. An overview of some frequently used working fluids is given in Figure 4 [4,5].

2.1. Distribution of Heat Pipes

Heat pipes can be classified according to the operating temperature (type of working substance), the shape of the construction, the possibility of use, the amount of working substance components, the method of condensate transport, and the method of non-condensing substance control.

According to the condensate transport method, heat pipes can be divided into three basic types [2,6,7]:

2.1.1. Gravity Heat Pipe

This type of heat pipe is the simplest in terms of design, principle, function, and method of condensate return. The gravity heat pipe is a simple type of tube which is made of a smooth-walled tube and filled with a working substance at a defined pressure. The return of the working substance condensate from the condensing to the evaporating part is only possible by flowing down the inner wall of the tube due to gravity. The working position of the gravity heat pipe shall be vertical or inclined, with the condensing part positioned above the evaporating part in such a position that the condensate replenishes the liquid phase in the evaporating part in sufficient quantity due to gravity. The correct operation of the gravity heat pipe is conditional on the amount of working substance being such that, within the range of its operating parameters, there is no insufficient wetting of the surface of the evaporating section, otherwise the heat transfer may be reduced. On the other hand, too much liquid in the evaporator section will lead to the development of large vapour bubbles during boiling and thus shock phenomena. The heat flux transferred is also dependent on the thermal resistance of the liquid film on the wall of the condensing section. The advantage of the gravity heat pipe is that it is capable of transferring large heat fluxes. The disadvantage is that it has a limited working position and operates only in the position with the condenser positioned above the evaporator [1,2].

2.1.2. Capillary Heat Pipe

This type of heat pipes has a capillary system on the inner wall through which the condensate is transported by capillary pressure. They are most often horizontal but can also work against gravity up to a limited height. When the evaporator is heated, the working fluid evaporates from the pores of the capillary-porous wall into the space of the tube. As this happens, the vapour pressure in this zone increases. At the same time, as the condenser cools, the vapour pressure in it decreases. Under the influence of the resulting vapour pressure differences, the vapour in the tube flows from the evaporator to the condenser, where it condenses. The liquid is then absorbed into the pores of the capillary-porous wall structure and transported by capillary forces from the evaporator to the condenser, where it is re-vaporised. In this way, the heat pipe operates on the principle of a closed evaporation–condensation cycle. At temperatures far from the critical temperature, the vapour velocity generally greatly exceeds the fluid velocity in the capillary-porous structure. The high heat-transfer capacity of heat pipes is achieved not only due to the circulation of the working fluid at high vapour velocity, but also due to the high latent heat of vaporisation and condensation.

Despite the high heat transfer efficiency of the heat pipe, this heat transport is influenced by the limits of the heat pipe. Limits (Figure 5) refer to phenomena in the heat pipe that reduce the amount of power transferred through the heat pipe. These limits depend on the choice of working substance, capillary structure, material, dimensions, and operating temperatures. For the operation of the heat pipe, the delivered heat input plays the most important role, on which the mass flow of the working substance in the pipe depends.

The viscous limit occurs when the operating temperature is extremely low, and the applied heat load is also low. The sonic limit usually occurs in heat pipes filled with liquid metal. It can be observed when the heat pipe starts up at low operating temperatures.

The capillary limit in heat pipes occurs when the capillary pressures at the vapour–liquid interface in the evaporator and condenser are not as large as the frictional pressure losses due to fluid motion.

The interaction limit occurs when the mass flow of vapour is large enough to remove droplets of liquid from the surface of the capillary structure, causing drying of the evaporator and primary capillary structure.

The evaporative limit occurs at high heat fluxes. At this limit, bubble boiling occurs in the primary capillary structure and drying of the primary capillary structure occurs [2,6,7,8].

2.1.3. Heat Pipe with Closed Loop—Thermosiphon Loop

Closed-loop heat pipes (Figure 6, Figure 7 and Figure 8) belong to single-component power pipes, which are divided into capillary and gravity. Loop heat pipes operate on the same principle as conventional heat pipes but have a separate condensate flow from the steam flow. The difference between them is in the way the working substance is circulated. While in a standard heat pipe the working substance flows between the evaporating and condensing parts in the same space, in a closed loop heat pipe the working substance flows between the evaporating and condensing parts in one direction in a closed loop [2,6,8].

The medium in liquid form is fed into the evaporator where it is heated and evaporated. The saturated steam is led from the evaporator through a steam pipe to the condenser, where it condenses and is returned by gravity to the evaporator through a second pipe [1,2,6,7,8].

3. Implemented Heat Pipe Applications

One of the directions of development of today’s electrical components is the miniaturisation of their dimensions and high integration, which often results in an increase in heat loss. Higher temperatures in these devices affect their operating parameters and can also cause their destruction. It is therefore often necessary to increase the heat dissipation from active or passive elements, but also from electrical enclosures.

3.1. Cooling of Assembly Boards and Elements in Electronics

The first application of the use of heat pipes to dissipate waste heat in electrical engineering was the use of a gravity heat pipe to dissipate waste heat from the inside of a static energy converter transformer (Figure 9) for electric locomotives. Figure 10 shows a comparison of the maximum temperatures reached on the transformer.

We used the simplest design of gravity heat pipe made of copper tube DN 18 with water and ethanol filling.

The principle of heat transfer by pulsed heat pipe was also used to cool the mounting plate [10]. The basic mechanism of heat transfer is associated with the pulsed motion of the fluid due to phase changes (evaporation and condensation). For the operation of the heat pipe, capillary forces must be used to form the liquid and vapour phases. An example of the implementation of heat removal from a plate by a pulse tube is shown in Figure 11.

Based on the results obtained, it can be concluded that the constructed Fluorinert FC72 pulsating heat pipe is a suitable device for heat dissipation from electronic components at a maximum heat loss capacity of 100 W.

One of the available and effective solutions for heat dissipation from individual power electronic elements (Figure 12, Figure 13 and Figure 14) by a closed loop heat pipe (LHP) appears to be the very advantageous use of a closed loop heat pipe with Fluorinert FC 72 working substance (Figure 12).

3.2. Airtight Cooling of Electrical Enclosures by Closed Loop Heat Pipe

A static energy converter enclosure was used to test the potential for dissipation of waste heat from the electrical enclosures (Figure 15, Figure 16 and Figure 17) [11]. It is a power electronic device with no moving parts that converts electrical energy parameters of one kind (magnitude and time history of voltage and current, frequency, and number of phases) into other desired parameters. The heat pipe has also proved to be quite a promising device for the dissipation of stray heat in this case. A very important advantage of using a heat pipe cooling device is the possible dust-tight sealing of the electronic equipment and thus the prevention of contamination of the electronic components by the deposition of dust from the cooling ventilation system. The electrical cabinet was hermetically sealed with sheet metal parts without ventilation grilles. The heat loss was dissipated by forced convection heat transfer from the flowing air to the evaporator of the gravity straw tube inside the enclosure. The steam and condensate flowed between the evaporator and condenser through two tubes of corresponding diameters, which had a dust-tight passage in the wall. The finned condenser was cooled outside the interior of the cabinet by natural or forced convection. The HP LOOP charge was water, acetone, and Fluorinert 72 (3M—Czech republic, Praha, www.3M.cz); the maximum temperature inside the enclosure was about 70 °C, the ambient temperature was 25 °C, the heat output dissipated through the heat pipe was about 1500 W, the evaporator was located at the rear wall, and the condenser was above the enclosure. The most efficient was water, then acetone, and Fluorinert 72. Steam and condensate flow in separate pipes; condensate flows by gravity. Analytical models were developed to simulate the heat pipe operation, to simulate the condition in the heat pipe, and the heat transfer at known temperature and flow velocity on the outside of the evaporator and condenser, and their geometries.

3.3. Implemented Applications with Heat Pipes in Environmental Technology

Due to the wide range of issues related to creating thermal comfort, reducing the energy consumption of heating and ventilation, increasing the energy efficiency of equipment and the use of low-temperature heat, environmental technology offers many opportunities to improve existing systems and equipment by exploiting the specific properties of heat pipes.

3.3.1. Design of a Device to Reduce the Radiation Output of a Fireplace Insert Using a Loop Heat Pipe

Recently, an interesting trend in the construction of fireplaces and stoves has taken hold. Customers choose fireplace inserts (Figure 18) with large heating elements for the visual effects of burning fuel. The heat output of such a heat source tends to be more than 10 kW and the heat loss of an entire low-energy house with several rooms tends to be well below 10 kW under calculated conditions, so the use of a large fireplace insert leads to significant overheating of the room.

One option to meet the conflicting requirement of significant flames and reduced radiant heat flux to the room is to remove some of the heat output from the heating system and divert it, e.g., to a hot water accumulator (HWA) and use the accumulated heat in the heating system [12]. Also, in this case, it is possible to use the properties of the gravitational loop tube—thermosiphon loop. In order to extract the heat from the fireplace insert, transfer it, and then use it to heat the water in the storage tank, a system of heat exchangers was designed (Figure 19) [13,14].

The results of the experiment in

Table 1. The experimental results showed the use of the Loop HP evaporator built into the firebox of the gas fireplace insert.

Measured Values	Thermal Output of a PKV Using LHP with Accumulator		Heat Output of the PKV Without Accumulation
	(W)	(%)	(W)	(%)
PKV total heat output	7560	100	7680	100
Radiative heat output	1061	14.0	1645	21.4
Heat output to accumulation	2735	36.2	bez	0
Heat output to flue gas extraction	1195	15.8	1528	19.9
Heat output by convection	2569	34.0	4507	58.7

showed that the use of a Loop HP evaporator built into the firebox of a gas fireplace insert (GFI) significantly redistributes the total heat output released by the combustion of natural gas in the burner of the fireplace insert. While a standard PKV without heat storage has a stack loss of approximately 19.9%, and up to 80% of the total heat input of the PKV is diverted to the interior, when the proposed Loop HP heat removal system is applied to the PKV firebox, the stack loss is reduced to 15.8%, up to 36.2% is diverted to heat storage, and the heat output is reduced to 58% of the total heat input to the interior. If in the case of PKV without Loop HP about 6152 W were going to the interior from the tested liner, with the application of Loop HP and heat accumulation only 3630 W, i.e., the heat output to the interior, was reduced by about 41%.

The main criterion for the selection of the heat pipe was, first of all, the price of the additional phase change heat transfer system, then the required heat transfer capacity, and the possibility of incorporating the designed system into the existing equipment.

Initial experiments showed that the cardinal problem of the large-scale evaporator is to optimise the bypass of the evaporator wall by condensate in a closed-loop gravity heat pipe. Therefore, it was necessary to design the condensate distribution system such that the liquid fed to the wall flows around the entire vertical surface of the evaporator uniformly. Figure 20a shows the distribution of the original condensate distribution through the pipe with holes Φ = 1 mm with a uniform spacing of 50 mm, and Figure 20b shows the condensate distribution adapted to the entire evaporator wall.

If the condensate can be distributed over the entire evaporator wall area, then the maximum effect of reducing the radiant flux from the HPC to the heated room can be expected.

3.3.2. Increasing the Efficiency of Small Heat Sources with the Use of Heat Pipe

Currently, small heat sources (SHS) achieve relatively high efficiency (85 to 88%), yet SHS experience heat losses, of which the most significant is the stack loss [14,15,16,17,18,19]. By removing some of the otherwise unused heat from the flue gas, we have the opportunity to reduce the largest heat loss, and by using this heat we can also increase the efficiency of the FGD by removing some of the heat from the flue gas and then using it to preheat the combustion air using a loop thermosyphon (LHP). The proposed device for increasing the thermal efficiency of the MZT consists of a loop heat pipe (LHP), the evaporative part, which is located before the inlet to the chimney, where it removes part of the heat from the flue gas and through the steam pipe of the LHP the heat is transported in the form of steam to the condensing part of the LHP located in the combustion air inlet duct. After the heat has been transferred to the combustion air, the condensate is transported by gravity through the condensing duct back to the evaporator, where it is again converted to steam and the cycle is repeated.

The flue gas temperature reaches quite high temperatures, so we have chosen water as the working medium, which is one of the most widely used working media in thermal management. It has very good physical properties, it is available, it is without harmful impact on the environment, and it is neither poisonous nor flammable. When selecting a wall material, not only its thermal conductivity, which is ideally as high as possible, is a decisive factor, but also its compatibility with the working medium has a great influence [20]. The right compromise between wetting and chemical stability must be found to avoid corrosion and the formation of non-condensable gases.

The method of increasing the efficiency of fireplace inserts (Figure 21) by removing part of the heat output from the flue gases and using it to heat the combustion air or to use it for heating is proving to be feasible.

By removing part of the heat output using a closed-loop heat pipe (LHP), the flue gas temperature is reduced by 20 °C to 50 °C depending on the flue gas temperature, thus increasing the efficiency of the heat source by between 2% and 5%. The advantage of using a closed loop heat pipe to increase the efficiency of the fireplace component is the synergistic effect of its operation. The heat output is greater the higher the temperature of the flue gas in front of the chimney, which also has a significant effect on reducing the chimney loss of the heat source. However, in order to implement the above method of increasing the thermal efficiency of the MZT, the combustion air inlet needs to be modified to accommodate the Loop HP condensing section and to allow the condensate to flow naturally into the evaporator [21,22].

It has been shown that it is also possible to implement a flat evaporator with a capillary structure that can be placed in the soffit of the fireplace insert without compromising its aesthetic value. The condenser must be oversized rather than undersized to be able to use the condensing heat to the maximum extent possible. The use of finned tubes for the construction of the condenser has proved to be appropriate.

As already mentioned, the system must be considered when designing the fireplace insert. The combustion air supply duct must be located so as to provide a separate combustion air inlet in which the heating of the combustion air will also be carried out directly.

3.3.3. Using Gravity Heat Pipe for Low-Potential Heat Recovery from Rock

Two boreholes were previously built on the University of Žilina’s land for the introduction of ground probes for pumping low-potential ground heat. In one borehole (Figure 22), a traditional earth probe was used with a circulating pump transporting the working fluid. In the second borehole, heat pipes were installed using ammonia as their working fluid [23].

For the primary section, a PE-XA ground probe was chosen (

Table 2. Resulting power measurement values on the heat pipe and ground probe.

	Forced Circulation of the Working Substance (Earth Probe)	Heat Pipe
Max. Power	7922 W	3200 W
Min. power	4950 W	1550 W
Average power	4980 W	1900 W

—bottom right), which is used in common practice for the implementation of primary circuits of heat pumps. These piping systems, which are resistant to the conditions in the rock mass, are designed to work with different heat-bearing substances (ethylene glycol, brine, etc.) [24].

Figure 22. Deep borehole cut and drilling rig (top right), termination of the PE-XA earth probe with weights (bottom right) [25].

Figure 22. Deep borehole cut and drilling rig (top right), termination of the PE-XA earth probe with weights (bottom right) [25].

The difference between the performance obtained by the conventional forced circulation method of the working substance compared to the heat pipe (TT) was considerable, and it is clear that the heat pipe did not operate with the required efficiency. This may be due to a number of factors affecting the correct operation of the TT.

Based on the measurements (Table 2), several hypothetical reasons why the heat pipe did not operate with the same performance as the forced circulation ground probe were determined:

Insufficient surface roughness or wettability—this factor is important in the transfer of heat (α) from the rock to the heat pipe. If the condensed working substance does not flow evenly (spreading the NH3 film over as large an area as possible with as little thickness as possible), the heat transfer performance of the heat pipe may be reduced. If the flowing film occupies only a small percentage of the area of the TT tube, this phenomenon can have a significant effect on the obtained performance.

Unequal dosing of the working substance for individual TT U-tubes is also one of the possibilities that could have occurred in our case. Since TT consists of two independent U-tubes, the dosing of condensate in the exchanger section is not controllable and we cannot ensure equal supply to each of the U-tubes. This fact may cause that condensate from the exchanger section going to the bottom of the TT will flow into only one U-tube. This may result in a drop in TT performance due to the following:

Loss of functionality of one of the U-tubes (since it is empty, there will be no phase transformation in it).

Pressure rises due to the height of the NH3 column in the full U-tube (which would reach up to 60 m in the case of a full build-up in one of the U-tubes). It will increase the hydrostatic pressure and thus the total pressure acting in the well. If this were the case, the boiling of the liquid ammonia would be limited to a depth at which the pressure would not exceed the boiling point at a given temperature.

Uneven distribution of condensate around the perimeter of the heat pipe. This problem is associated with the use of unsuitable installation components designed for the flow of the working substance for individual U-tubes. In Figure 23, these components are highlighted in the red circle [25,26,27,28].

The measurements were carried out on a simulator of low-potential heat transport into heta-pipes [26]. These are actually two simulators. One for the working substance carbon dioxide, and the other for ammonia, shown in Figure 24.

The first circuit is a custom simulator of low-potential heat transfer into a heat pipe. The device consists of a corrugated sewer pipe ø500 mm in diameter and 6220 mm long, around which 600 m of heating Devi cables are wound. These cables are divided into three separately adjustable sections and are used to simulate the action of the ground heat. Using Ni-CrNi thermocouples, the temperature of these cables can be measured, and the electrical power can also be measured.

Based on the large discrepancy between the results measured on the heat pipe and the traditional ground probe, and the listed reasons related to this discrepancy, measures were proposed that would be able to ensure ground heat extraction using the heat pipe at its full potential.

We propose to create four classic gravity heat pipes with plate condensers from two U-tubes above the borehole (Figure 25) and to specially modify the condensate distribution around the entire circumference of the tube, according to Figure 26, so that the condensate wets the entire inner casing of the earth probe tube.

3.3.4. Application of Thermosiphon LHP with Nanofluid for Heat Recovery in HVAC Units

Several types of nanoparticles have been investigated in the research on the effect of nanoparticles on the boiling in a heat pipe evaporator, because current research works do not provide sufficient information on the effect of nanoparticles on the processes in the heat pipe. Based on the research works, nanoparticles compatible with the environment, suitable for heat transfer, with good thermal conductivity and with different grain sizes, were selected.

Table 3. Properties of the nanoparticles used.

Name of the Substance	Nanoparticles CuO	Nanoparticles Al2O3	Nanoparticles TiO2
Particle size	80–100 nm	40 nm	4–8 nm
Density	8.78 g.cm−3	3.97 g.cm−3	3.9 g.cm−3
Boiling point	2580 °C	2980 °C	2972 °C
Melting point	1059 °C	2050 °C	1825 °C

describes the properties of the selected types of nanoparticles. The aim of the experiment was to determine the type and concentration of nanoparticles with a positive influence on the heat transfer properties of the working substance [29,30,31,32].

The properties of LHP in waste heat recovery in air handling systems were tested on the experimental device, according to Figure 27, at the same mass flow rate of air through the evaporator and condenser parts of the duct [33]. The dynamics of heat transfer from the evaporator to the condenser was tested for two designs of the heat pipe (Figure 28a,b and Figure 29) by stepwise heating of the evaporator by immersion in a vessel of warm water of temperature 40 °C.

When comparing (Figure 30) the working substances, the positive effect of the nanofluid on the boiling and on the evaporation of the working substance in the heat pipe was significantly shown. The nanofluids increased the performance of the heat pipe by at least 6% and up to a maximum of 33%.

3.3.5. Use of Thermosiphon LHP for Cooling and Heating of Metal Hydride in Hydrogen Storage

Hydrogen storage (Figure 31 and Figure 32) as a very popular form of green stored energy requires efficient systems that do not require operation at extreme pressures or deep cryogenic temperatures. It is proving to be energetically and safety advantageous to store this hydrogen in metal hydride alloys at relatively low pressures (up to 1 MPa) and near ambient temperatures [34]. During the process of hydrogen accumulation and expulsion from metal hydride, thermal changes occur, which are important to control and optimise.

The cooling and heating of a metal hydride storage tank (Figure 33 and Figure 34) are important processes that affect its performance, lifetime, and safety. In the hydrogenation and dehydrogenation of metal hydride, heat must be removed from the metal hydride alloy and supplied to the process and the required temperature must be maintained. And also, in this case we use the specific characteristics of a closed-loop heat pipe. In the case of metal hydride hydrogenation, the metal hydride must be cooled by boiling at an absolute pressure of approx. 14 kPa and a boiling point of 50 °C. In this case, the reservoir body acts as an evaporator and the condenser is cooled by ambient air. In dehydrogenation, at a temperature of about 160 °C and a vapour pressure of 0.618 MPa, the container body is the condenser of the LHP and the vapour is generated, for example, by the waste heat from the fuel cell in the evaporator. The thermal management [37] is adjusted by a system of valves which change the functionality of the metal hydride storage body.

In order to investigate the above functionality of the metal hydride stack and to analyse the heat transfer processes, we have designed and implemented a metal hydride stack simulator with field tubes that simulate the filling of a costly metal hydride charge. The heating and cooling of the metal hydride stack simulator is performed using the closed-loop heat pipe operating principle due to the use of more efficient heat transfer during the phase transformation (boiling and condensation).

4. Conclusions

Heat pipes are an exceptionally efficient and flexible tool in thermal management, allowing reliable heat loss while maintaining simplicity of design and high heat transfer efficiency. In this paper, their basic principles, types (gravity, capillary, and loop), and practical applications in electrical, environmental, and energy engineering are presented in detail.

Based on the experiments carried out by the author’s team at their workplace and their analyses, it can be concluded that heat pipes represent an effective thermal management tool with a high variability of applications and measurable benefits in the investigated areas:

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Cooling of power electronic components: Both gravity and loop heat pipes were able to stabilise the temperature of power transistors at a lossy thermal power up to 100 W. The use of Fluorinert FC-72 working fluid ensured long-term reliability without external forced airflow.

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Fireplace inserts and small heat sources: The implementation of loop heat pipes in gas fireplace inserts has demonstrated a reduction in radiative transfer to the interior by up to 40%, significantly improving the controllability of the output. At the same time, it was possible to divert part of the energy to the storage tank, where up to 36% of the total heat output was stored (2735 W out of 7560 W). Convection transfer to the room was thus reduced from 58% to 34%, demonstrating the effectiveness of the condensate distribution treatment.

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Heat recovery in combustion plants: Measurements have shown that the inclusion of loop heat pipes in the flue gas extraction system resulted in a reduction of 20–50 °C in flue gas temperature and an improvement of 2–5% in the efficiency of small heat sources. This effect also reduces stack loss and increases operational safety.

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Geothermal applications: When compared to a traditional ground probe, the average heat transfer value of the heat pipe was 1900 W, whereas it was 4980 W for forced circulation probes. Although the efficiency was lower, the results point to the need to optimise the condensate distribution and the wettability of the evaporator surface.

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Nanofluids: The use of nanoparticles (e.g., Al₂O₃, TiO₂, CuO) in appropriate concentrations can increase heat transfer efficiency by up to 30%. The tested particles with a particle size of 4–100 nm showed an improvement in the transport properties of the working medium.

Experimental applications in the field of cooling of power transistors, mounting plates, static converters, or electrical enclosures confirmed that the heat pipe not only allows us to increase the reliability of devices, but also to seal them without the need for active ventilation. The significant reduction in heat output to the interior with loop tubes when applied to fireplace inserts (up to 40%) demonstrates their potential in environmental engineering as well. Another interesting direction is the use of nanofluids, which can increase the efficiency of the tubes by up to a third, as well as the implementation in heat recovery systems in air conditioning or in metal hydride hydrogen storage tanks.

Despite the lower efficiency in some specific applications (e.g., geothermal heat recovery), heat pipes remain a promising solution, with the possibility to optimise the design and distribution of the working medium. It has been shown that a properly designed heat pipe can provide stable heat transfer without the need for external actuators and with the ability to operate in different positions and temperature ranges.

Overall, heat pipes allow heat flux densities of up to the order of tens of kW/m² to be transferred at small temperature differences (as low as 2 °C). This makes them a promising solution not only in electrical engineering, but also in the optimisation of small heat sources, in air conditioning, and in modern hydrogen storage systems.

In conclusion, the development and application of heat pipes have great potential, not only for increasing the efficiency of cooling systems, but also for reducing energy losses and the overall sustainability of technical solutions. Further research could focus on the optimisation of condensate distribution, the development of advanced working fluids, and the integration into systems with high demands on reliability and energy efficiency.