S. Kalogirou, Higher Technical Institute, P. O. Box 20423, Nicosia 2152, Cyprus, Tel: +357-22-406466, Fax: +357-22-406480, email: skalogir@spidernet. com. cy Y. Tripanagnostopoulos and M. Souliotis, Physics Department, University of Patras, Patra 26500 Greece, Tel/Fax: +30-2610-997472, e-mail: yiantrip@physics. upatras. gr

ABSTRACT: To avoid the monotony of the black colored flat plate solar collectors we can use absorbers of blue, red-brown, green or other color. Because of the lower collector absorptance these collectors have lower thermal efficiency than that of the usual black type collectors, they are however of more interest to architects for applications on traditional or modern buildings. In this paper applications of solar collectors with colored absorbers are presented and analyzed with respect to their performance, aiming to give guidelines for their wider use on buildings. These systems are simulated with TRNSYS on an annual basis at two different locations, Nicosia, Cyprus and Athens, Greece. The results show that the energy output depends on the absorber darkness. For a medium value of the coefficient of absorptance, the colored collectors give satisfactory results with respect to the drop of the amount of collected energy, compared to collectors with black absorbers. This implies the use of slightly larger collector aperture area to have the same energy output as that of typical black colored collectors.

1. INTRODUCTION

Flat plate solar collectors are of black appearance because of the color of the absorber, which is employed to maximize the absorption of solar spectrum. The black view of the solar collectors on building roofs and facades is not usually aesthetically attractive and in some applications, as for example in Mediterranean islands, they are not compatible with the color and the architecture of the buildings. In most of these installations the black color of the collectors is not effectively combined with the white color of buildings, the blue color of sea and sky and the color of the doors and windows. Considering that the aesthetic view is of priority for buildings of traditional or modern architecture we estimate that solar collectors with absorbers of different color than black could be an interesting solution for the building integration of solar energy systems.

Generally, to avoid the monotony of the black color, collectors with absorbers of blue, red — brown, green or other color can be used. These collectors are of lower thermal efficiency than that of the usual black type collectors, because of the lower collector absorptance, but they are of more interest to architects for applications on traditional or modern buildings. The application of solar collectors with colored absorber is a new concept and regarding the total solar system, we notice that the increase in cost (by using larger area of collectors to overcome the lower efficiency of the colored absorber) is balanced by the achieved aesthetic harmony with the building architecture.

An extensive study, including prototype testing and modeling, has been carried out at the University of Patras, Greece [1], and has shown that colored collectors can be of satisfactory efficiency, especially if a dark color absorber is used which have a better performance than the light tone ones. The constructed and tested flat plate collector prototypes were of blue and red-brown absorbers and the tests showed that the obtained efficiency can be considered acceptable compared to that of the usual collector type with black absorber. As part of the same work, lower cost collectors without cover (unglazed
collectors) and colored absorbers are studied, aiming mainly to low temperature applications. The effective use of unglazed colored collectors was recently confirmed for swimming pool application [2], while a method to overcome the disadvantage of the lower optical efficiency was the development of colored selective absorbers [3] that increase the temperature range of collector efficient operation.

In this paper we present the application of solar collectors with colored absorbers in a house heating, multi-flat residential or office buildings, and industrial process heat applications. The collectors are analyzed with respect to their performance and practical applications, aiming to give guidelines for their wider use on buildings. These systems are simulated on an annual basis at two different locations at different latitudes, Nicosia, Cyprus (35°) and Athens, Greece (38°).