Converting Moist Air Into Water and Power

Abstract

A method to process moist air into dry air and water results in a surplus of energy for the process. The sun evaporates water everywhere on earth and expends 2.26 MJ/kg (429.9 Btu/lbm) for each kg (2.204 lbm) of water evaporated. A mass of 1 kg (2.204 lbm) of water with a mixing ratio of 0.3% in dry air represents 2.26 MJ (199 Btu) of latent heat energy distributed in a volume of approximately 333 cubic meters (11,759 cubic feet). A system is described by which ambient water vapor is enriched, condensed with the release of latent heat in a heat-exchange boiler, which vaporizes a working fluid used in a Rankine-cycle turbine generator system. Water vapor enrichment is achieved with a vapor-separation barrier. Fans draw moist air through an air-intake system which brings the air into contact with a large surface-area vapor-separation barrier. The intake of a compressor imposes a vacuum on the extraction side of the barrier at a pressure that is lower than the ambient water-vapor pressure. This pressure difference drives water vapor across the barrier into the compressor. A two-stage compressor is used to maintain the low pressure and convey water vapor at high temperature and near atmospheric pressure to a heat-exchange boiler. Two processes occur in the heat-exchange boiler: 1) water vapor condenses and is pumped out of the boiler, and 2) heat is transferred to a working fluid that vaporizes. The vaporized working fluid drives a turbine in a Rankine cycle with condenser. Exhaust heat from the turbine is dissipated with a water-cooled condenser. An air-cooled rock-bed system is suggested as an alternative, when water cooling is not possible. Current advances in materials, the efficiency of turbomachinery, and the effectiveness of heat exchangers suggest that a system can be conceived that is completely fueled by moist air and produces water and excess shaft horsepower that can be converted into electricity. The analysis treats turbomachinery and heat exchangers as ideal components constrained by the Carnot and isentropic efficiencies. Pumps and fans are treated as components with state-of-the-art efficiency. System computations for an ideal 100% efficient system indicate that approximately 25% of the latent heat can be converted to electricity. For a system made with contemporary state-of-the art components a yield of a few percent is predicted. Principles of operation and engineering details are quantified.