The solar thermal performance of a nickel-pigmented aluminium oxide selective absorber was determined by using an experimental model (prototype model). The constructed models were tested outdoors during daylight under a clear sky. Different volumes of prototypes were used. The flat-plate collector used was of the glazed type. The thermodynamic characteristics of each flat-plate collector were determined. The maximum calculated collective flux of the selective absorbers was in the range 590-699 W/m(2). The maximum conversion efficiency for our systems was in the range 0.73-0.78. However, the maximum Carnot efficiency was 0.04-0.16. The absorption of merit of the samples lay in the range 0.91-0.97. The selective absorbers used have absorptivity in the range 92.0-97.0% while they have emissivity in the range 11.0-22.5%. The effects of absorptivity, emissivity, and prototype volume on the collective flux, conversion efficiency, and absorption of merit are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.