Thermodynamics of Twisters
The energy of hurricanes is mainly a result of sea-to-air heat transfer from water spray. Spray droplets are cooled to their wet bulb temperature by evaporation before falling back into the sea, thereby transferring a huge amount of heat from sea to air. The heat transfer mechanism is the same as in wet cooling towers; in both cases the heat transfer can be in the order of 50,000 W/m 2. The temperature and relative humidity of the air rising in the eyewall of a hurricane are typically 24.5C and 97% respectively. Saturated air at 24.5C can be produced by spraying 26C water in air. Eyewall sea surface temperature (just outside the eye of a hurricane, where the strongest winds are) is typically 26C. Tropical sea surface temperatures can be as high as 32C.
In a vortex, turbulence is inhibited because when a particle of air moves inward, its tangential velocity increases to conserve angular momentum. This produces an increase in centrifugal force, which in turn pushes the air particle outward. As a result, the flow in the vortex is laminar instead of turbulent, as shown by the smooth thread shape often observed in waterspouts. Centrifugal force stabilizes the flow, thereby reducing turbulence and friction losses. The rising air is like a spinning top being raised; there is little decrease in the angular momentum of the large mass of rising air in the time required for the air to rise from the bottom to the top of the troposphere. The kinetic energy of the spinning air is recovered as the vortex diverges at its upper end.
Given our understanding of these natural processes, the next step is to estimate the energy production potential of the AVE. Ideal work in a gas or steam turbine, where there is no change in potential energy, is the reduction in enthalpy of the working fluid; ideal work in a hydraulic turbine, where there is no change in enthalpy, is the reduction in potential energy. In contrast, the ideal AVE work is the reduction in enthalpy minus the increase in potential energy.
In the AVE, the gas expands as it rises and is compressed as it descends. Compressing subsiding air is extremely efficient; it only requires piling more air on top of the subsiding layer. The AVE’s heat-to-work conversion efficiency corresponds to the efficiency of a Carnot engine, where the hot source temperature is the surface temperature and where the cold source temperature is the tropopause temperature. Approximately 35% of the heat received at the surface is converted to work during the convection process regardless of whether the heat is received as sensible or latent heat.
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