###### Abstract  
This paper deals with the experimental research and verification of a passive cooling system operating on the principle of a loop gravity heat pipe designed for cooling electrical cabinets. This type of cooling works automatically by changing the state of the working substance and thus saves energy consumption. Since the designed cooling system ensures heat transfer from the interior cabinet to the outdoor space, where the heat can naturally dissipate to the surroundings, it is dustproof. The heat pipe consists of an innovative evaporator concept designed to minimize liquid and vapour phase interference in the refrigeration circuit. The aim of the research was to experimentally determine the limit performance parameters of the refrigeration system for different volumes of working medium in the evaporator and decrease heat loss in the cabinet interior. The designed device was verified experimentally and by mathematical calculations as well. The greatest benefit of the work is that the cooling device was able to ensure temperature conditions inside the electrical enclosure at a heat load of 2000 W under 60 \({}^{\circ}\)C, 1500 W under 55 \({}^{\circ}\)C, 1000 W under 50 \({}^{\circ}\)C, 750 W under 45 \({}^{\circ}\)C and 500 W under 40 \({}^{\circ}\)C.  
loop thermosiphon; passive cooling; electronics; heat dissipation; phase changes +
Footnote †: journal: Applied  
In electrical equipment, heat is usually transmitted by conduction and convection. Convection is used to release and mix air in an open electrical box. If a closed dust-free box is needed, heat is dissipated through the wall by conduction. These two methods of heat removal can be referred to as active and passive cooling. The active cooling system includes external devices in its operation that support the cooling operation. This technology helps to increase the airflow rate and improves heat transfer and heat dissipation. Active cooling systems use forced convection to cool electrical enclosures through various types of fans, heat exchangers and cooling units. Cooling systems with such air intake cannot prevent contamination of the interior of the box. Air coming from external space brings dirt and dust with them. It is this contamination of the interior of the enclosure that has a negative impact on the electronic components, which reduces their service life. When clogging the system with external dirt, the overall maintenance of these devices increases. These negative effects give rise to the innovation of electrical enclosures to prevent dirt and dust from penetrating their interior [8]. Another method of heat removal can be performed by passive cooling. Passive heat dissipation transfers heat mainly from the housing of the electrical equipment. The heat transfer takes place through the flow of heat from a warmer environment to a colder one. Electrical enclosures can be open or closed. With open devices, heat is simply dissipated by natural air circulation. Closed construction is advantageous in terms of airtightness and dust-freeness. A great advantage of passive cooling is the lower cost of manufacturing and operating electrical enclosures. Passive cooling is one of the most commonly used, least expensive and easiest to implement cooling methods. Passive cooling methods include free cooling and cooling with heat pipe technology [9]. The gravity loop heat pipe (loop thermosiphon) belongs to passive cooling methods using heat pipe technology. In this way, it is possible to remove a large amount of heat with low working substance content in the heat pipe. The whole system operates automatically, with low operating noise and minimum electric power consumption. From a constructional view, a loop thermosiphon is one of the simple heat exchangers for heat transfer. Figure 1 shows the parts of the gravity loop heat pipe. It can be oriented in a vertical or slightly inclined position. The name of the gravity heat pipe implies that it uses gravity to circulate the working medium when it operates [10]. Figure 1: Gravity loop heat pipe.