The thermodynamics of a solar thermochemical cycle for the capture Of CO2 from air are analyzed. The cycle encompasses 3 reactors: an aerosol-type carbonator for capturing CO2 from air using a spray of Ca(OH)(2) aqueous solution, a solar calciner for thermally decomposing CaCO3 into CaO using concentrated solar energy, and a conventional slaker for regenerating Ca(OH)(2). Two approaches are examined: (1) a closed-material cycle that delivers pure CO2; and (2) an open-material cycle that, additionally, co-produces hydrogen. The 2nd approach features the same components as those of the closed-material cycle, except that the calciner co-produces CaO and syngas by the combined CaCO3-decomposion and CH4-reforming processes, and syngas is further processed to separate streams of H-2 and CO2. Its thermodynamic efficiency, defined as the ratio of Delta G degrees(298K)vertical bar(H2+0,.05 -> H2O) for the H-2 produced to the thermal energy input (solar energy+ heating value of CH4) is 22.7%. The Solar chemical reactor technology for the calcination and for the combined calcination-reforming is presented. (c) 2005 Elsevier Ltd. All rights reserved.