175 - Integration of solar collectors in buildings A study of the performance of flat plate and CPC collectors on a building façade

摘要

The integration of solar collectors on building facades is becoming a much more common option, and a true alternative to the usual placement of collectors on terraces or on roofs, seeking an orientation towards azimuth south (on the northern hemisphere) and a tilt equal or higher than latitude.For a stationary collector the choice of azimuth south and a tilt around the value of latitude maximizes yearly energy collection, or Winter energy collection, when a tilt value higher than that of local latitude is chosen.However when collectors have to go on a vertical facade the results obtained are much different, specially on latitudes typical of southern Europe, i.e. from 35° to 55°; from winter to summer, due-south may no longer be the azimuth of choice. In fact, in the summer time, the sun is "very high" in the sky and is basically reflected off the collector glass cover, greatly reducing the collected radiation.In this paper the authors wish to make a collector performance evaluation of three different collectors, operating on vertical surfaces. The three collector types are a flat plate collector and two CPC collectors, one with absorber fins running horizontally (HF) and another with the absorber fins running vertically (VF). The comparison will be made at constant inlet temperature to the collectors. The flat plate collector is a good selective surface one and the CPCs chosen are: (i) a standard CPC manufactured by AO SOL for the VF case and (ii) a new proposed design for the HF case.This HF case is an attempt at having an optics developed to somehow compensate the grazing incidence angles of the sun in the summertime, when the sun is highest in the sky and the azimuth of the vertical wall approaches zero (due-south).For facade azimuth varying between 0° to 90° we calculate the solar energy collected (direct and diffuse radiation) for apparent solar annual motion, and use recent refinements proposed to standard calculation methods [1,2]. A comparison will also be made with the same collectors operating according to the usual "due south-tilt equal latitude" procedure.The analysis is done for two locations Lisbon (latitude 38.75°) and Freiburg (latitude 48°) as two representative cases.The results show that the behaviour of the collectors on vertical walls is more or less the same on an yearly basis for several different azimuths, (a result which was not intuitive to the authors) although they are quite different on a seasonal basis.As for the comparison between the three collectors considered it is seen that the CPC HF is better than the other two, in particular for azimuths closer to zero, both because of its optical behaviour and because of its better thermal performance.