Angular dependent optical properties from outdoor measurements of solar glazings

A method for careful outdoor characterization of the angular dependent sola r transmittance of solar glazing is developed and used for investigating th e performance of structured and antireflection treated glass. Two identical collectors are series connected in order to get an identical flow. The ref erence collector is always covered with a thin Teflon foil and the test col lector is furnished with the investigated covers. The method is carried out in six steps: (1) The collectors are long term tested in order to got the collector parameters, especially the U-values. (2) The performances of the collector at a temperature close to ambient are monitored during a clear da y. (3) The effect of heat losses is eliminated from the knowledge of the U- values, and the optical efficiencies of the collectors are derived. (4) The ratio between optical efficiency of the test - and the reference collector is calculated. (5) Both collectors are tested with a Teflon film, with wel l-known optical properties, as a cover and the relative optical efficiency is derived. (6) From the relative transmittance between the investigated gl azings and Teflon, the absolute value of the angular dependent transmittanc e is derived. The results show good agreement with conventional optical cha racterization. The commercial antireflection treatment increases the solar transmittance by 4% and the annual output by 9%, The structured glass shows similar angular performance as a flat glass if the structure is faced outw ards, but shows a lower transmittance for incidence angles exceeding 40 deg rees if the structure is faced inwards. This means that the annual performa nce is decreased by 4% by facing the structure inwards instead of outwards. (C) 2001 Elsevier Science Ltd. All rights reserved.