Quantifying the Energy Efficiency, Aging Effects, and Heat Losses of Flat-Plate Solar Collectors in a Temperate Climate

This study evaluates the performance of a flat-plate solar collector thermal system located in Bucharest, Romania, by combining experimental measurements (December 2024–April 2025) with numerical modeling. The system, coupled to a 4000 L tank and using a 30% glycol–water mixture, was monitored for irradiance, wind speed, temperature, flow rate, and humidity. Monthly irradiance averaged 448–584 W/m2, and wind speed 2.3–4.3 m/s. Instantaneous efficiency ranged from 25.09% to 73.25% (mean 52.96%), with CUF (capacity utilization factor) ranging between 6.55% and 9.38%. Total heat losses were 4.80–7.79 kW and dominated by top losses (3.60–5.59 kW). The model reproduced the manufacturer’s efficiency curve within 5% for data obtained in the spring, with winter deviations of up to 37% due to weather variability and aged insulation. The results provide reference values for the CUF and loss coefficients of flat-plate collectors in temperate continental climates and can support retrofit and sizing decisions.