Translucent cover design of solar energy equipment for manufacturing of prefabricated concrete components

Translucent cover is one the main design features of solar energy equipment like solar collector. Its material and layers number influence on overall translucency of the cover and, as a result, on efficiency of the equipment. Formwork, equipped by translucent cover, can be considered as solar energy equipment like solar collector due to similar thermal-physical processes, taking place in it. Such equipment can be used during manufacturing of prefabricated concrete components to speed up the curing process by heating the structures using solar energy. The aim of our research work is determination of optimal layers’ number of translucent cover. The research was made in various climatic conditions. The obtained results testify to certain influence of translucent cover design on efficiency of solar energy equipment. However, this influence depends on climatic conditions.


Introduction
The research on development of various types of solar energy equipment is made in different countries [1][2][3]. At present, the most common type is a solar collector. Solar collectors are widely used in different fields, such as household equipment systems, hot water production [4][5][6][7].
Concrete, as the most common construction material in the world, and prefabricated concrete components have considerable energy requirements for their manufacturing, including energy expenses for heat treatment of concrete to speed up the curing process of its curing. Fossil fuels, such as natural gas, oil, coal are required for this purpose [8]. Possibility to obtain temperature of heater about 60-80 °C in the solar energy equipment allows using solar energy for heat treatment of concrete during manufacturing of prefabricated concrete components [9].
Formwork, which is used during manufacturing of prefabricated concrete components, can be equipped by translucent cover and considered as solar energy equipment. Such equipment is comparable with solar collector for thermo-physical processes, taking place in it.
Such method of heat treatment of the concrete components can be made on condition of placement of the formwork with translucent cover on open air. Concrete is direct heated by solar energy in the day time and stores heat in the night time.
Our research results show that this technology can be used for manufacturing of prefabricated concrete elements using solar energy within 5 months for Moscow region (φ=56° N) and 7 months for South of Russia (φ=45° N). The manufacturing period does not exceed 24 hours during this time [10].
However, the efficiency of heat treatment of the concrete components in the solar energy equipment depends on design features of such equipment. Design of translucent cover as one the main design features influences on amount of solar radiation, which can be absorbed by heater during the daytime, and heat loss to environment [11][12][13]. Taking into account the above, the aim of this research work is the determination of optimal layers' number of translucent cover.

Materials and methods of research
The increase of the layers' number reduces the amount of heat, consumed by concrete, due to reduction of coefficient of the solar radiation transmission and the growth of the shaded area of the concrete component (Equation 1).
where Ctrcoefficient of the solar radiation transmission; Cshadecoefficient of shading; Cdustcoefficient of dusting; Ccoverintegral coefficient of the translucent cover transmission (it depends on incidence angle of direct solar radiation, number of layer and material of the cover).
At the same time, the increase of the layers' number reduces heat loss from concrete in the environment through the tranlucent cover (Equation 2).  (2) where Ulosscoefficient of heat loss through the translucent cover, W/(°C·m 2 ); h e conconvective coefficient of heat transfer between the translucent cover and the environment, W/(°C·m 2 ); h e radradiation coefficient of heat transfer between the translucent cover and the environment, W/(°C·m 2 ); radiation coefficients of heat transfer between layers of the translucent cover, W/(°C·m 2 ); h c conconvective coefficient of heat transfer between the concrete component and the translucent cover, W/(°C·m 2 ); h c radradiation coefficient of heat transfer between the concrete component and the translucent cover, W/(°C·m 2 ).
The calculation and experimental research were carried out to determine temperature and strength of concrete, curing in formwork, equipped by translucent cover, in different climate conditions under various design of the cover.
The experimental research in laboratory conditions was made to verify the above calculation model. The temperature conditions of humid subtropical climate were simulated in climatic camera. Walls, floor and roof of the camera were insulated. The electric reflector lamps were used for imitation of solar radiation intensity. They were placed on a special panel on the roof of the camera. The passage of the airflow above the lamps allowed keeping the necessary air temperature. The air temperature in the camera was controlled by digital potentiometer with help of thermocouple. The air humidity was controlled by digital hygrometer.
The intensity of solar radiation changed according to parabolic law with the maximum value of 1300 W/m 2 in the camera. The air temperature changed according to sine law in the range of 24-50 °C, the air humidity -6-39% in the camera.
The research of the concrete strength was made for standard methodic with the employment of series laboratory equipment. Concrete B25 was used in the research. The reference standard to compare the obtained concrete strength was concrete, curing within 28 days in normal conditions.
We made the research of materials of translucent cover. The main materials were the following: glass, polymer foil, polycarbonate, acrylic glass. Polymer foils had a number of advantages as compared with other materials, the main are: the price and usability.
Polyethylene-terephthalate foil with integral coefficient of the transmission of 0.88-0.85 and polyethylene foil with integral coefficient of the transmission of 0.92-0.94 were chosen as the most suitable types after the comparison of the polymer foils characteristics.
Single layer and double layer design of translucent cover was chosen in the research because the increase layers' number more than two layers cause considerable reduction of amount of solar radiation, which can be consumed by heater [10]. Qabs and Qloss in the tables are the amount of heat, absorbed by concrete during the day time and lost through the translucent cover.
The analysis of the research results shows that concrete is heated more intensively under single layer translucent cover during the day time. Its maximum temperature is 1.5-2 °C more than under double layer translucent cover. However, concrete temperature under single layer cover starts decreasing after sundown, while it continues increasing under double layer cover.
The coefficient of heat loss (Uloss) of double layer cover decreases almost in two times as compared with single layer cover. The temperature of concrete decreases slower under double layer cover at Note: size of concrete sample 20х20х20(h) cm; temperature (tconcrete) is in the middle of concrete sample; reference standard is concrete C25 (compressive strength in the age of 28 days is 29.8 MPa); material of transparent cover is polyethylene foil.
Maturity of concrete (3), which is total sum of numerical values of concrete temperatures over the period of concrete curing, is almost the same in both climatic conditions, but under double layer cover it is more than under single layer cover. where Δτitime between the measurements of the concrete temperature.
Nevertheless, it is more for the concrete samples, curing under double layer cover in humid continental climate conditions (table 2). The same situation is for relative age of concrete, which depends on the average concrete temperature and intervals between its measurements. The results of calculation and experimental research of the concrete temperature in conditions of subtropical climate are shown in fig. 1. The similarity of graphs, obtained by the calculation and the experimental research, testifies to adequacy of chosen calculation model and the assessment of the process, taking place during heat treatment of the concrete components using solar energy. 1experimental concrete temperature under single layer cover (climatic camera); 2calculated concrete temperature under single layer cover; 3experimental concrete temperature under double layer cover (climatic camera); 4calculated concrete temperature under double layer cover; 5actual air temperature during the experiment (climatic camera); 6calculated air temperature.
Strength of concrete, curing in formwork, equipped by double layer translucent cover, is better in both climate conditions. However, the difference in the values of the concrete strength decreases with increasing the average air temperature and intensity of solar radiation.

Conclusion
The obtained research results testify to the increase of translucent cover layers in humid subtropical climate (Krasnodar region) does not improve the values of strength, maturity and relative age of concrete. It testifies to inefficiency of double layer cover in such climate conditions.
In spite of better values of strength, maturity and relative age of concrete, curing in formwork, equipped by double layer translucent cover in humid continental climate (Moscow region), we have to admit its inefficiency. This is because we have to take into account the economic component, connected with the equipment costs. The increase of the concrete strength by 1-4% does not justify the increase of the equipment price, connected with addition of one more layer of translucent cover at the time of manufacturing and operation of the equipment.