Experimental validation of an analytical model for performance estimation of natural convection solar air heating collectors

In this paper, the experimental validation of an analytical model that describes the thermal behaviour of double parallel flow air heating collectors working by natural convection is presented. This model was validated with data measured on a collector of these characteristics at the National University of Salta, in the northwest of Argentina. Comparisons between measured and simulated values show an excellent agreement for both the air output temperature and the useful energy produced by the collector, with RMSE values smaller than 6%. The instantaneous thermal efficiency curve of the collector was determined experimentally in winter of 2015. Daily values of efficiency ranged between 0.48 and 0.5 during sunny days, and between 0.41 and 0.46 during semi-cloudy days. A novel correlation between the velocity of the air movement by natural convection and the temperature difference between the building and the collector's interior was obtained by means of linear regression with a correlation coefficient R2 = 0.94. A new correlation Nu = f(Ra) to estimate the average convective coefficients by natural convection was obtained, based on an effective convective heat transfer coefficient calculated with the temperature values measured inside the collector and the absorbed solar irradiance.