For cooling small volume compartments, the thermoelectric systems have a number of advantages over the classic compressor refrigerators. Their priority is due to the active components in their design -Peltier modules which are chemically inactive, have no moving parts and do not practically need maintenance [1-2]. The thermoelectric cooling systems have the potential to meet the requirements for storing and transportation of vaccines and other medical solutions at low temperatures posed by the global pandemic of Covid-19. The electrical systems in the vehicles are excellent prerequisite for compatibility with the thermoelectric refrigerators requiring stable power supply.The aim of the present work was to design, realize and study a prototype of a small scale mobile thermoelectric cooling system for negative temperatures based on Peltier modules.

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It has been shown [1, 2] that the energy expended on fragmentation by a bit of constant mass and impact energy depends on the shape of the bit. However, these tests were made with simple impacts on a vertical laboratory impact tester. The fragmented volume was measured at an open surface, not one fragmented by previous blows as occurs at the face of a borehole.To assess the information obtained in the laboratory and to follow the laws which develop in mine conditions, we carried out experimental work. We used a BIS-1 test drill rig based on an SShDS rotary drill on which were mounted pneumatic percussion assemblies.The BIS-1 rig comprises rotary and percussive assemblies. The rotator is driven by a reduction gear and 10 kW motor with interchangeable pulleys. The available speeds of shaft rotation are 60, 90, 100, and 135 rpm. The percussion assemblies, consisting of cylinders and bits, and the tailpieces are interchangeable. Using two cylinders we achieved ax/al thrusts of the instrument on the face from 200 to 2000 kg. The number and energy of the impacts of the bit were determined from the bit travel diagram which was recorded during drilling at the face with the aid of an induction transducer of the transformer type with its core rigidly attached to the bit. The transducer was fed from a GZ-33 audio generator. The signals were recorded by an N-102 oscillograph.We tested 10 impact units with bits of various configuration and characteristics (Table 1). For all the percussion assemblies we used cylinders of 150 mm diameter of three types differing in the constructional travel of the bit: for travel of 33 mm, bits 1, 2, and 10; for travel of 44 mm, bits 6 and 7; and for travel of 55 mm, bits 3, 4, 5, 8, and 9.We used bits of lengths 160, 195, 230, and 300 mm and weights between 4.2 and 9.4 kg. They had various lengthwise mass distributions. We used bits of three constructional types: type a, a short bit with conical main mass in the head end (Fig. la); type b, a bit 1.2-1.4 times longer than the first, and with its mass concentrated in the tail part (Fig. lb); and type c, a bit equal in cross section to the rod, with its mass uniformly distributed lengthwise (Fig. lc).We tested the drilling of boreholes in rocks with hardnesses of 14-16 under the following conditions: axial pressure of instrument on face, 6a0 kg; rate of rotation of rotator, 60 or 100 rpm; mains pressure of compressed air, 5 atm (gauge); water pressure, 4 atm (gauge). The power delivered to the rotator was 10 kW. For comparability, all the holes were spaced 20-30 cm apart.Our work comprised two stages. First we drilled holes to 10 m depth successively with all ten percussion assemblies. Then from the test results we chose the four most promising percussion assemblies, namely numbers 3, 4, 8, and 9, and used them to drill the holes to 20 m. In all we drilled 49 holes of 10 m depth and seven holes of 20 m depth. The total length drilled in this series of experiments was 630 rn.During the tests we measured the average mechanical drilling speed every...