DEVELOPMENT OF A SOLAR COLLEСTOR BASED ON ALUMINUM CONSTRUCTION HEAT PIPES

The object of research is the thermal efficiency of solar collectors based on aluminum structural heat pipes. Solar collectors with heat pipes have a structurally similar design. The heat-absorbing panel secures the heat pipes to the evaporation zone, and the condensation zones are located in the battery tank. The differences are only in the designs of the absorption panels, the areas of the condensation zones and the heat carriers of the heat pipes. One of the most problematic places of research is the justification and development of a new design of a heat-absorbing solar collector panel based on heat pipes.A review of the publications showed that a solar collector based on aluminum structural heat pipes operating in the two-phase thermosyphon mode has a heat loss when transmitting radiation heat exchange in the long-wavelength range between the absorbing flat panel and the heat pipe. The project of a solar collector with a new, absorbing solar rays panel is proposed. To analyze the efficiency of the solar collector based on aluminum structural heat pipes with a new absorption panel, two models of solar collectors were created – with a flat absorber panel and a cylindrical absorber panel.The models of solar collectors based on one aluminum structural heat pipe fixed on aluminum flat and aluminum cylindrical absorption surfaces are investigated by the method of thermophysical experiment.The results of studies of the efficiency of solar collector models are presented. The thermal efficiency of a new solar collector based on an aluminum structural heat pipe with a cylindrical absorbing panel in the initial period of heating water is up to 1 5 % higher than that of a solar collector with a flat absorbing panel, and at the end of heating up to 4 %. The cylindrical surface of the panel plays the role of a concentrator of the reflected part of the radiation and the own radiation of the panel in the region of the heat pipe.Further studies are planned to be conducted in the direction of optimization of the geometric parameters of the heat-absorbing surfaces of solar collectors.


Introduction
When creating heat and mass transfer devices using heat pipes (HP) or two-phase thermosiphons (TPT), it opens up great opportunities for solving problems in new energyefficient systems [1][2][3]. There are various design solutions using HP and TPT in energy saving systems [1,4,5]. And also in systems operating using renewable energy sources [6,7]. HP and TPT have a good prospect for use in solar energy systems. Solar energy is a source of energy, as a step towards reducing dependence on other energy resources. Currently, there is already an industrial production of HP (TPT) and solar collectors (SC) based on them [8]. In the SC HP and TPT, they provide simplification of the collection of structures, high modularity, maintainability and reliability. Apply SC to heat water of active or passive type. An active system uses an electric pump to circulate fluid through a manifold. The passive system does not have a pump and uses only natural circulation. SC with HP or TPT are structurally similar to execution.
HPs (TPTs) are fixed on the heat-receiving panel by the evaporation zone, and the condensation zones are located in the storage tank. Differences exist only in the designs of absorbing panels, areas of condensation zones and heat transfer fluids HP (TPT). The authors of the studies [9,10] developed and created SCs based on aluminum profile HPs with axial grooves of a Ω-shape, operating in the two-phase thermosiphon mode. The collector panel, which absorbs the heat of sunlight, consists of aluminum HPs made of a flat aluminum profile. Heat is perceived by the flat surface of the HP evaporation zone and is transferred to a liquid heat exchanger located on their condensation zones. Such a heat transfer system provides low hydraulic resistance of the solar collector heat exchanger and, accordingly, low energy consumption for pump operation. The disadvantage of this SC is that it has heat losses during the transfer of radiation heat transfer in the long wavelength range between the absorbing flat panel and the heat pipe. Наведені результати досліджень ефективності моделей сонячних колекторів. Теплова ефективність нового сонячного колектора на основі алюмінієвої конструкційної теплової труби з циліндричною поглинальною панеллю у початковий період нагріву води до 6 % більша ніж у сонячного колектора з плоскою поглинальною панеллю, а в кінці нагріву -до 1,5 %. Циліндрична поверхня панелі грає роль концентратора відбитої частини випромінювання і власного випромінювання панелі в районі теплової труби.
The element of the specially profiled absorbing panel is made in the form of a cylindrical surface, in the lower inner part of which there is a rib, at the free end of which there is a zone of evaporation of the HP.
The cylindrical surface of the panel plays the role of a concentrator of the reflected part of the radiation and the panel's own radiation in the region of the heat pipe [11].
To conduct a performance analysis of the new SC based on aluminum structural HP with a cylindrical absorbing panel, two SC layouts are created -with a flat absorbing panel and a cylindrical absorbing panel.
Thus, the object of research is the thermal efficiency of solar collectors based on aluminum structural HPs. And the aim of this research is to develop the design of the SC element, on the basis of which it is possible to increase the efficiency of the SC due to the implementation of additional radiation heat transfer in the long wavelength range between the absorbing panel and the heat pipe with a coolant.

Methods of research
For research, the authors develop and create two experimental facilities in accordance with [12]. Fig. 2 shows the design of a solar water heater, made of a highly efficient element heat-absorbing aluminum panel and one aluminum structural HP. HP is fixed on the panel with an evaporation zone, and the condensation zone is placed in a «pipe in pipe» type liquid heat exchanger, which is connected to the heat storage tank. Pentane is used as the heat carrier of aluminum heat pipes.
The study of SC elements based on aluminum structural HPs mounted on a flat (Fig. 3, a) and cylindrical (Fig. 3, b) absorbing panels is carried out using sunlight in the summer. The heat flux of sunlight was determined by the FEP-4 pyrometer (Russia).
A 3D model of the SC structure based on aluminum structural HP, which is mounted on a cylindrical aluminum absorption panel, is shown in Fig. 4.
The experimental installations of solar water heaters (SWH) include the following measuring instruments: -mercury thermometer with a division value of 0.1 °C to determine the temperature of the water in the storage tank of the solar heater; -copper-constantan thermocouples with a wire diameter of 0.15 mm for measuring the temperature field of the absorbing panel, the temperature of the external wall of the HP, the temperature at the inlet and outlet of the heat exchanger; -analog input module; -signal adapter; -personal computer; -FEP-4 pyrometer for measuring the magnitude of the radiation flux of solar radiation incident on the experimental site. The geometric characteristics of a cylindrical absorber with aluminum alloy AD31: -length l = 810 mm; -radius of the cylindrical surface R = 38.2 mm; -thickness S = 1.2 mm. The amount of thermal energy absorbed by water in the SWH is determined by the formula: where c w -heat capacity of water, J/(kg K); M w -amount of heated water, kg; t w1 -average water temperature in the storage tank, °С; t w2 -preliminary average water temperature in the storage tank, °C.
The value of the specific heat flux, perceived heat-absorbing floor of solar collectors, is determined by the formula: where F -the area of heat-absorbing surfaces of a flat or cylindrical collector, m 2 ; τ -SWH operating time between measurements of water temperatures in the storage tank, °C.
The areas of heat-absorbing surfaces of a flat and cylindrical SC are equal.
The SWH efficiency coefficients are calculated as the ratio of the specific heat flux q to the incident radiation flux infrared created by the sun E ≈ 850 W/m 2 : To compare various types of SC structures, a characteristic of their effectiveness is used -a coefficient of performance depending on: where t w -the average temperature of the water in the storage tank, °С; t a -average air temperature, °С, Е -solar radiation flux incident on SC, W/m 2 .
Parameter X is a variable value of thermal resistances of this SC design.

Research results and discussion
The results of studies of flat SC are given in Table 1, and cylindrical -in Table 2.
Symbols in the Tables 1, 2: t 1 -HP temperature in the condensation zone, °С; t 2 -HP temperature in the evaporation zone, °С; t 3 -average temperature of the absorbing panel, °С; t a -ambient temperature, °С; t t -temperature of the heated water in the storage tank, °С; Δt -increase in water temperature in the storage tank between measurements, °С; τ -measurement time, min; Q -amount of thermal energy absorbed by water through a heat exchanger at certain intervals of time, J; Q f -magnitude of the heat flux, is perceived by the heat-absorbing surface of the SC, W; q -specific heat flux is perceived by the heat-absorbing surface of the SC, W/m 2 ; η -coefficient of SC performance; X -variable value of thermal resistances. The performance graphs of SC elements based on an aluminum structural HP mounted on a flat or cylindrical aluminum heat-absorbing panel are shown in Fig. 6, and the time dependence of the increase in water temperature in the heat storage tank is shown in Fig. 7.   Fig. 6 shows that the current SC efficiency based on an aluminum structural heat pipe with a cylindrical absorbing panel in the initial period of heating water is up to 6 % more than in a solar collector with a flat absorbing panel, and at the end of heating up to 1.5 %.

ISSN 2226-3780
During research, the average air temperature was 42.3 °C. The initial temperature of the water in the storage tanks was 43.6 °C. The final temperature of the water in the SWH storage tank with SC based on aluminum HP mounted on a flat aluminum panel was 65.2 °C. The final temperature of the water in the SWH storage tank with SC based on aluminum HP, which is mounted on a cylindrical aluminum panel, was 66.2 °C.

Conclusions
A new design of the solar collector element is developed and investigated, on the basis of which it is possible to increase the thermal efficiency of the solar collector based on aluminum structural heat pipes. It is shown that the thermal efficiency of a new solar collector based on an aluminum structural heat pipe with a cylindrical absorbing panel in the initial period of heating water is up to 6 % more than in a solar collector with a flat absorbing panel, and at the end of heating up to 1.5 %.