Determination of the energy efficiency of a flat reflector solar air heating collector with a heat accumulator

. The article presents an improved scheme and pilot-industrial copy of a flat reflector solar air heating collector with a heat accumulator. The heat balance equation of the flat reflector solar air heating collector with heat accumulator is constructed, and its energy efficiency is based on it. Considering the local climatic and meteorological characteristics of the city of Karshi, its useful performance coefficient was determined based on the experiment conducted on the flat reflector solar air heating collector with a heat accumulator. This designed device is proposed as an active solar heating system for the anaerobic digestion of municipal solid waste based on energy and resource efficiency. As a result of experimental studies, a flat reflector solar air heating collector with a heat accumulator’s high energy efficiency was determined in the heating heat carriers (air up to + 75℃ and heat accumulators up to +85℃) on February 24, 2023.


Introduction
In the southern regions of our republic, specifically in the city of Karshi, it is of great importance to determine the technical and economic efficiency of passive solar heating systems, taking into account the local climatic and meteorological parameters. To combine the main elements of passive and active solar heating systems, it is important to choose the optimal structural structure, optimize the heat-technical parameters, and verify the research results by conducting experiments. One of the main indicators of passive solar heating systems is their energy efficiency [1].
Research aimed at the development of new energy and resource-saving technologies based on renewable energy sources, in particular, the improvement of the solar air heating system and the optimization of the main parameters of solar devices for the heat supply system, is being conducted in leading scientific centers, including the University of Wisconsin-Madison (USA), Indian Institute of Technology Delhi (India), Tamkang University (Taiwan), University of Tanta (Egypt), Universiti Kebangsaan Malaysia (Malaysia), Xi'an Jiaotong University (China), King Mongkut's University of Technology Thonbure (Thailand), University of Tokyo (Japan), Moscow Energy University (Russia), Academy of Sciences of the Republic of Uzbekistan, scientists of scientific research institutes and higher education institutions. From a number of well-known scientists on the laws of heat-mass exchange processes in low-potential solar devices, including their calculation methods, R.A. Zakhidov, R.R. Avezov, A. Klychev, A.A. Abdurakhmanov, G'.N. Uzakov, N.R. Avezova, E.S. Abbasov, M.A. Umurzakova and others studied in their scientific research [4,6,7]. At the same time, in the above-mentioned researches, efficient heat storage solar panels, taking into account the improvement of the efficiency of solar air heaters and the accumulation of heat, increasing the process of heat-mass exchange, taking into account the meteorological parameters of the local climate and reducing economic costs, optimizing the light receiving surfaces and the volumes of heat accumulators issues of development of air heaters and their use in heat supply systems of thermal processing devices of solid household waste have not been sufficiently studied.

Materials and methods
The purpose of this work is to develop a mathematical model of the heat balance of a flat reflector solar air heating collector with a heat accumulator, taking into account the local climatic and meteorological characteristics of the city of Karshi, and to evaluate its energy efficiency.
An experimental industrial copy of a flat reflector solar air heating collector with a heat accumulator was created at the "Alternative Energy Sources" training ground of the Karshi engineering-economics institute "Alternative Energy Sources" department, shown in Fig. 1 and 2.  A flat reflector solar air heating collector with a heat accumulator (FRSAHCHA) is equipped with a heat accumulator and additionally, a hinged flat reflector mounted parallel to the solar air heating collector housing, this FRSAHCHA is a heat supply in combination with heat storage allows to increase the effectiveness of covering the heat load of the, etc.
FRSAHCHA works in the following order. Scattered, direct, and total solar radiation, depending on the local climate characteristics, ground albedo, the sun's declination angle and the installation angle of the device, falls on the light exchange surface of the FRSAHCHA, i.e. transparent window 1. Depending on the level of transmittance of transparent glass 1, it transmits solar radiation energy falling on its surface. Solar radiation energy absorbed by the surface of absorber 2 turns into heat. On the basis of heat transfer, it heats the air moving from absorber 2 along air channel 3 and the water and transformer oil in channel 4 of the heat accumulator. Since the heat capacity of heat accumulators (water and transformer oil) is 2.5÷4.2 times higher than air, it is considered an additional source of heat for air. In order to increase the efficiency of the heat exchange process, finned ribs 5 are installed inside the air channel, on the side walls of the heat accumulator channel, and on the basis of heat transfer, it additionally heats the air due to the temperature difference and ensures the energy efficiency of the FRSAHCHA.
In order to increase the energy efficiency of the solar air heating collector equipped with a flat reflector with a heat accumulator, hinged flat reflectors 7 are installed parallel to the sides of its body, and the scattered, straight, and collected rays falling on the reflecting surface of the hinged flat reflector are now reflected (refracts) the solar radiation and returns it to the light exchange surface of the solar air heating collector provided with a flat reflector with a heat accumulator. As a result, in addition to being an additional source of energy, the efficiency of the device increases.
The following heat-technical parameters are considered as primary data for calculating the thermal efficiency of the heat exchanger: ambient temperature, the value of the total solar radiation falling on the sun-receiving surfaces of the heat exchanger, the operating time of the heat exchanger, air flow rate, air and heat initial and final temperatures of accumulators (water and transformer oil), heat-physical parameters and geometric dimensions of materials of FRSAHCHA. The heat balance equation of FRSAHCHA was implemented according to heat-technical calculation methods.
Theoretical studies are based on the computational methods of technical thermodynamics, helio technics, theory of heat and mass exchange, hydrodynamics, and aerodynamics. The main structural element of the proposed system is a solar air heating collector. In order to determine the thermal-technical and heliotechnical parameters of FRSAHCHA, calculations were carried out according to the following procedure.
FRSAHCHA is improved with a heat accumulator and a flat reflector, in which the air is simultaneously heated and heat is accumulated in the heat accumulators. Since the heat capacity of heat accumulators is 2.5÷3 (transformer oil, 2500÷3000 DJ/kg) or 4.2 (water, 4200 Dj/kg) times higher than air, simultaneous heat accumulation together with it additionally heats the air, that is, it is considered an additional source of heat.
In the results of the experiment, the important thermal-physical properties of the heat carrier and heat accumulators are presented in Table 1, which are important for increasing the efficiency and intensity of heat exchange. The useful performance coefficient of the FRSAHCHA η-is determined by the ratio of the amount of useful energy obtained through η-air and heat accumulators to the amount of energy coming from solar radiation on the surface of the IAYRQHQK absorber using the following formula: . coll F -light receiving surface of collector, m 2 .
Useful thermal energy (heat production capacity) is determined according to the following equation: here, -volume consumption of air, 3 ; -air density, 3 ; p c -specific heat capacity of air, ⋅°; tair1 and tair2 -temperatures of air entering and exiting the FRSAHCHA,

℃.
The amount of solar energy falling on the absorber through the reflector is determined based on the following equation:

Results
The thermal efficiency of the proposed FRSAHCHA depends on the area's solar energy potential. An experimental study of the thermal efficiency of a combined solar collector was carried out at an experimental installation in the natural and climatic conditions of the city of Karshi, during operation of the solar active heat supply system for the thermal processing of municipal solid waste.
When studying the efficiency of air heating, air was supplied at ambient temperature. Temperature sensors, thermocouples in the KSP-4 kit and bimetallic thermometers are installed on the pipeline no further than 10 cm from the inlet and outlet of the solar collector. Water flow was measured with an ultrasonic flow meter Portaflow 330 (absolute error 3%). And the air flow using an anemometer AM-70. The intensity of direct and diffuse solar radiation was measured using a Mac Solar actinometer (Germany). The results of experiments and calculations according to the above formulas are shown figure 3 and in table 2.

Conclusions
In order to ensure the temperature regime necessary for the processing of solid household waste, the thermal scheme of FRSAHCHA was developed and a experimental-industrial installation was created. FRSAHCHA energy efficiency was based on the experience conducted in the climatic conditions of the city of Karshi. It was determined that the necessary temperature regime for municipal solid waste processing, taking into account the meteorological characteristics of the local climate, the morphological composition of municipal solid waste, and physical-thermal and physical-chemical properties, is covered by 92% FRSAHCHA. It has been established that in the developed FRSAHCHA, the temperature of the heat carriers at the outlet in the heating mode "air + heat accumulators" reaches, respectively, up to 76℃ (air) and up to 85℃ (water).