A note on estimation of diffuse radiation collected by all-glass evacuated solar tube collectors

To simplify the estimation of diffuse radiation from the sky and ground collected by all-glass evacuated solar tube collectors (ETCs), four methods were proposed and used in the past. Among them, the one by assuming the directional intensity of diffuse radiation projected on the plane coplanar with cross-section of solar tubes (2-D model) to be isotropic is simpler as mathematical expressions for the estimate of diffuse collectible radiation can be directly derived in this case. However, such assumption is not true for tilted evacuated solar tubes (ET), and the three-dimensional isotropic diffuse radiation (3-D model) should be used. In this note, a trail is made to estimate diffuse radiation collected by tilted-arranged ET collectors (T-type ETC) based on 3-D model and compare with those obtained based on the 2-D model. Results show that the diffuse radiation collected by T-type ETCs estimated based on 2-D and 3-D models are highly in agreement, and the difference between two estimates is dependent on the tilt-angle (beta) of ETCs. For the case of beta = 0 and 90 degrees, the both estimates are completely identical as expected. For T-type ETCs with ETs sizing 47/58 mm in diameter of inner/outer tubes and spacing (B) between centers of two adjacent ETs less than 80 mm, the relative deviation between two estimates for diffuse radiation from the sky is less than 0.65% and that for diffuse radiation reflecting from the ground is less than 1.7% when beta > 20 degrees. This means that the 2-D model is reasonable and advisable for a quick estimation of collectible diffuse radiation of ETCs.