Abstract
The object of the study is a water-based flat-plate solar collector operating under the sharply continental climatic conditions. The problem addressed is the insufficient justification of the installation angle and seasonal heat balance, which reduces the reliability of efficiency assessment in long cold periods. A three-dimensional computational fluid dynamics (CFD) model of a collector with an aperture area of 1.88 m2, glass cover, aluminum absorber, and 12 copper riser tubes were developed and validated. Calculations were performed for average monthly meteorological conditions representing summer, transition, and winter regimes under long cold-season operation, south-facing orientation, and tilt angles of 10–90°.The model accounted for heat conduction in solid elements, laminar heat-carrier flow, natural convection in the air gap, radiative heat transfer, and losses through glass cover and casing. It was established that the rational fixed angle for year-round operation is 50–55°. With seasonal adjustment, 35–40° is recommended in summer and 60–70° in winter, increasing annual useful heat output by 8–12% without changing the collector design. The average thermal efficiency for 12 monthly regimes was 49.3%, while the solar-radiation-weighted efficiency was 57.6%. In winter, the glass cover is the main heat-loss pathway, through which 37–41% of incident solar radiation is lost. Validation using 78 measurement series confirmed agreement between calculated and experimental data: MAPE for efficiency was 7.52%, and R2 was 0.94. The results can be used to select the installation angle, evaluate seasonal heat output, and assess solar hot water and low-temperature heating systems operating under long cold-season conditions