How much work can be extracted from diluted solar radiation?

Solar radiation at ground level may be modeled as diluted thermal radiation. Two upper bounds are obtained for the efficiency of converting the energy of diluted radiation into work. They depend on the dilution factor and the geometric factor of the radiation reservoir and on the nature of the absorber (i.e. absorptance, emittance, geometric factor of emitted radiation). The theory is applied to the case of direct and diffuse solar radiation at ground level. The upper bounds efficiencies derived here are more accurate that the Carnot and Petela-Landsberg-Press efficiencies which are very often used in solar energy applications.