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The temperature field problem described in the paper is analytically solved for the absorber of a new type solar collector. The absorber consists of two plates, one of them being flat and placed on top of the other plate made with a square profile. The temperature field is calculated for four areas using the Laplace equation under stationary conditions. For each area the boundary conditions are written and the Laplace equation solved. The resultant temperature fields are found for each area separately.

Abstract.Outdoor air temperature range in the Baltic region is below average values in Europe. Range of outdoor air temperature in the Baltic region is -3 ºC in winter and +16 ºC in summer. And solar radiation intensity is lower in comparison with the average European values. The average solar radiation intensity reaches 1100 kWh/m2 per year in Baltic region. Therefore, there is a particular need to optimize the solar thermal system in the Baltic region. Solar collector's operation methods investigated on the IPE solar energy polygon according different parameters. The most common types of solar collector's operation were investigated and compared: by the time, by the heat carrier temperature differences of input and output, by the solar radiation intensity and operation control methods in various combinations.Theoretical and practical advantages and disadvantages of using each control methods were analyzed. Solar collector's operation system could be regulated in accordance with the solar radiation intensity, by the boiler lower temperatures and by the outdoor air temperature to determine precisely the solar collector efficiency of the parameter changes. This may help to avoid the previous operation systems testing regime deficiencies.The precision of solar collectors operation depends on the type of operation systems: heat losses and the collector efficiency calculation accuracy, the sensor is accuracy, the time between the regulation regimes, and the range of heat carrier pumps action operation sensors.Currently, companies that offer solar collectors, offer solar collectors complete set with all necessary equipments for the solar collector connection for hot water supply system, or for home heating operation system by the temperature difference of input and output. And no one of them does not offer solar collectors operation systems by the various parameters.

Evaluation of Atmospheric Lucidity and Diffused RadiationBased on measurements of the diffused and direct solar radiation intensity, the authors investigate the atmospheric lucidity and its variations that are characteristic of the Latvian conditions. They also consider the dependence of diffused radiation on the nebulosity. In the work it was established that the atmospheric lucidity in Latvia is usually in the range of 0.77±0.05, with only minor changes, so in the calculations it was assumed constant. The diffused solar radiation intensity was found to increase under medium nebulosity and to decrease at high its level. Since this dependence is also influenced by the type of clouds, for more precise calculations further research is needed.

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