Optimum tilt angle for flat plate collectors all over the World – A declination dependence formula and comparisons of three solar radiation models

• Monthly optimum tilt angle is predicted for North latitudes between 0° and 80°.
• From practical considerations, an optimum fixed value all along the year is derived.
• Three solar radiation models are compared from the point of view of total incident solar radiation density prediction.

The paper presents a theoretical study on the optimum tilt angle for flat plate collectors at different geographical locations and different time moments over a year. The solar radiation density is estimated based on three analysis models, namely Hottel and Woertz model (the simplest), Isotropic diffuse model (Liu & Jordan model) and HDKR model (Hay–Davis–Klucker–Reindl), passing from the simplest to the most complex one. Further, the absorbed solar radiation density is computed and the optimum tilt angle is found for different geographical locations (covering the Globe latitudes). The optimum value for the tilt angle is researched for maximum incident solar radiation and also for maximum absorbed one. The results indicate that the same angle is obtained when looking for maximum incident and for maximum absorbed solar radiation. When comparing the three above mentioned models, different tilt angles are supplied by applying the Hottel and Woertz model and respectively the others two (both of them providing the same angle). Although, the predicted absorbed solar radiation is the same in the summertime for all three models, and probably overestimated by the last model in the springtime.The obtained results give an overview over the whole year operation at different geographical latitudes. When using the Hottel & Woertz model for estimating the incident solar radiation, the optimum tilt angle for a flat plate collector should be computed as simplest as βopt = φ − δ function on the latitude and declination. If a fixed value is easier to be used over a specific operation period, an optimum value is provided by this research. The novelty consists in finding this global optimum instead of “local” ones for which monthly adjustment is required.The results are compared to the approximation given in the technical literature according to which the optimum tilt angle should be local latitude plus or minus (for winter or summer seasons, respectively) 10° or 15° (with small variations as it is further presented).