MATERIALS FOR CONCENTRATING SOLAR THERMAL TECHNOLOGIES

Advanced low-temperature absorber coatings

Participants: CIEMAT-PSA (E), WAGNER Solar (E)
Financed by: CIEMAT, WAGNER
Duration: March 2005 – June 2008
Background:
There is currently a wide variety of solar collectors for hot water. Although some of their characteristics are different, all of them have working temperatures clearly below 175ºC. The selective coatings that are usually used on these collectors are economical to manufacture, although their optical and thermal properties are not excellent. Improving the quality of these coatings and their optical properties is of commercial interest only if it can be done without considerably raising the manufacturing cost. CIEMAT-PSA has had long experience in the field of selective coatings for solar absorbers manufactured using the sol-gel technique. This experience can be applied to the development of solar absorbers that have better properties than those currently in use in low-temperature solar collectors, without making their cost prohibitive. Solar collectors that have a better selective coating without significantly increasing their cost will doubtless rank higher in the market.
Objectives:
Develop an advanced absorber for solar collectors working at temperatures below 200ºC. The manufacturing process must be an industrial sol-gel technique competitive with currently available low-temperature coatings.
Relevant achivements:
Activities related to preparation of the low-temperature absorber have concentrated on improving the optical properties and durability of the initial 2-layer absorber, Al/CuMnOx/SiO2, with as=0.940 and e100 =0.041. An additional absorbent-protective CuMnSi0.1Ox layer deposited directly on the aluminum substrate (3-layer absorber) has improved the absorptance up to nearly 0.95, with practically the same emittance of e100 =0.044. Durability of both absorbers (2 and 3-layers) was evaluated in thermal stability (200 h/250ºC) and moisture resistance (600 h/40ºC-condensation) tests recommended by the IEA Solar Heating and Cooling Program’s Task X for low-temperature solar absorbers. All of the samples analyzed (2 and 3-layer absorbers) passed both the thermal stability and moisture tests with the same or even better results as some commercial absorbers. It should be mentioned that in copper oxide and manganese absorbers, moisture resistance is far more aggressive. This is because the aluminum substrate corrodes, causing the appearance of white pores. The 3-layer absorber had a better appearance after this test because of the additional CuMnSi0.1Ox layer.