Determination Of Thermal Parameters Of Solar Water Heating Collectors Using The “Dark” Testing Method

In the production of flat solar water heating collectors, thermal testing occupies a special place, the purpose of which is to determine their thermal characteristics experimentally. The corresponding approximation expressions are proposed for determining the coefficient of heat loss, inclined to the horizon at an angle of 300 for the average conditions of their operation in the hot water supply system.


INTRODUCTION
Currently, serial production of flat solar water heating collectors (SWHC) is organized in many countries around the world. The organization of medium production of SWHC and the expansion of the scale of their use in hot water supply systems (HWSS) of objects for various purposes in the Republic of Uzbekistan sets the task of their certification and certification, the solution of which includes the development of a whole system of tests (tests) aimed at determining their thermal efficiency, reliability and manufacturing quality control. A special place in it is occupied by thermal testing, the purpose of which is to determine their thermal characteristics experimentally.

Determination Of Thermal Parameters Of Solar Water Heating Collectors Using The "Dark" Testing Method
The American Journal of Applied Sciences (ISSN -2689-0992)

THE MAIN FINDINGS AND RESULTS
Typical SWHC consist of a blackened radiation absorbing heat exchange panel (BRAP) with built-in heat transfer channels (HRC), laid in a shallow flat box (body), with a translucent cover (TC), a thermally insulated bottom and side walls ( Fig. 1-a). In the blackened BRAP, the energy of solar radiation (SR) is absorbed and converted into heat, which has passed through the TC of the SWHC building. The TC, passing short-wave SR through itself, practically transmits back the long-wave (thermal) radiation of the blackened surface of the BRAP and, thereby, creates a greenhouse effect, excludes through radiant losses from the front surface of BRAP. The thermally insulated bottom and side walls of the housing reduce the conductive-convective heat losses of the BRAP into the environment. Useful energy in the form of hot water with a temperature of 55-65 o is discharged through the built-in HRC in BRAP [1].
The results of thermal testing of SWHC are necessary for designers and manufacturers to work on their further improvement, and for suppliers and consumers to determine their nomenclature, compare them with each other and select a design that is most suitable for specific purposes.
The main indicators of the degree of thermal engineering quality of SWHC directly affecting their useful heat output ( пол ) and thermal efficiency ( ) are reduced to a unit area of the frontal (front) surface of their body. The coefficient of total heat losses to the environment (К пр р−0 ) and the coefficient of thermal efficiency of the BRAP of the considered SWHC ( тп ), characterizing the efficiency of heat transfer from the elements of the BRAP to the water circulating inside the HRC ( тп ) and reduced to a unit area of the front surface of the collector body, the effective absorption capacity of the total SR, which characterizes the optical efficiency of the system "TC-BRAP" of the collector ( 0 ) under consideration, which is determined by the product of the effective integral absorption capacity of the total SR of the blackened surface of BRAP ( рэфф ) by the effective throughput of the total SR TC ( сп эфф ) reduced to a unit area of the front surface of the collector body, 0 = ( р сп ) эфф ..
To determine the values of these indicators of SWHC, a wide range of different methods of thermal testing has been proposed to date: full-scale, laboratory, quasi-stationary, nonstationary [1]. Вт м 2 and the angle of incidence of direct SR on the surface of TC ( пр ) not more than 300 [2].
In the process of thermal testing of the SWHC, the flow rate of water through the collector в ( кг с ),, the temperature of water at the entrance to the collector в вх (℃) and at the outlet of it в вх (℃), the temperature of the outside air 0 (℃), and the surface flux density of the total SR falling on the surface plane of the translucent cover of the collector пад Σ ( Вт м 2 ) are measured.
In (4) and (5) в -, the temperature of the heated water averaged over the length of the heat-removing channel of the radiationabsorbing heat-exchange panel.
Note that for the developers and designers of solar water heating collectors, the values тп and К прро , separately, are very important, and not тп К прро and тп ( р сп ) эфф . Due to the fact that the values of complexes тп К арро and тп ( р сп ) эфф were obtained at around noon, they can only be used for comparison with similar indicators of flat solar water heating collectors of various manufacturers. For this reason, these complexes can be used to determine the daily variation of the heat output of the tested collectors, as suggested in [3].
As follows from the graph in Fig. 1-b., The ordinate of the intersection point of the straight line with the ordinate axis, i.e. at в − 0 пад Σ = 0, is equal to the value of complex тп ( р сп ) эфф , and the value of complex • К пр р−0 is equal to the negative value of the slope of the straight line [3].
By including "dark" experiments in the program of their tests, it is possible to significantly reduce the duration of the thermal testing of SWHC and significantly reduce the labor intensity [4,5,6].
The essence of carrying out "dark" experiments is as follows. At night, hot water is supplied to its BRAP with a temperature of вх at a constant flow rate of ( ). As a result of heat losses of the BRAP through the enclosing elements of the collector body into the environment, the water temperature at the outlet from it drops to в вых ..
One of the main goals of the dark method for determining the thermal characteristics of SWHC is to determine the reduced coefficient of heat transfer from water in the HRC BRAP to the unit of the frontal surface area of its body фр and to the environment (К пр р−0 ) according to the measurement results 0 , в вх , в вых , в and фр .
The value тп according to [3] can be determined from the ratio The value К пр р−0 in this case can be determined from the approximation dependences