Solar Engineering of Thermal Processes, 4th Edition

6.11 Effects of Dust and Shading

The effects of dust and shading are difficult to generalize. Data reported by Dietz (1963) show that at the angles of incidence of interest the maximum reduction of transmittance of covers due to dirt was 2.7%. From long-term experiments on collectors in the Boston area, Hottel and Woertz (1942) found that collector performance decreased approximately 1% due to dirty glass. In a rainless 30-day experiment in India, Garg (1974) found that dust reduced the transmittance by an average of 8% for glass tilted at . To account for dust in temperate climates, it is suggested that radiation absorbed by the plate be reduced by 1%; in dry and dusty climates, absorbed radiation can be reduced by 2%.

Shading effects can also be significant. Whenever the angle of incidence is off normal, some of the collector structure will intercept solar radiation. Some of this radiation will be reflected to the absorbing plate if the sidewalls are of a high-reflectance material. Hottel and Woertz (1942), based on experiments with two-cover collectors, recommend that the radiation absorbed by the plate be reduced by 3% to account for shading effects if the net (unobstructed) glass area is used in all calculations. The net glass area accounts for the blockage by the supports for ...