Solar Wind Energy Tower (SWET) has announced that it has released a report explaining "How the Evaporatively Driven Downdraft Power Production System Works." One of the atmosphere's more dynamic systems is the severe downdraft (sometimes called a microburst or downburst) of a thunderstorm, in which rapidly sinking air (kinetic energy) is produced by the evaporation of the rain produced by the storm.
The "Evaporatively Driven Downdraft Power Production System" works in precisely the same way as the thunderstorm downdraft. A continuous spray of small water droplets is introduced at the top of the tower. The cooling due to the evaporation of the water droplets, combined with the weight of whatever liquid water droplets remain unevaporated and fall through the tower, causes the air to become much heavier than the air that was previously in its place in the tower. The heavier, denser air sinks through the depth of the tower, is forced to turn horizontally at the base of the tower, and ultimately passes through the turbines as it exits the tower (though at speeds far less violent than in the most severe downbursts observed in nature!). As air flows out of the base of the tower through the tunnels, air is continuously drawn into the top of the tower owing to the principle of mass conservation, and the spraying of liquid water droplets into the air entering the top of the tower maintains the evaporative cooling.
There is a limit to how much evaporational cooling can occur, and ultimately how much power can be generated, because the air cannot be cooled indefinitely by injecting more and more water droplets. As water evaporates and the air cools, the relative humidity within the tower increases. Evaporational cooling ceases once the relative humidity reaches 100%. The temperature reached at this so-called "saturation point" is known as the wet-bulb temperature.
Very roughly, this temperature lies midway between the ambient air temperature and ambient dewpoint (the dewpoint is temperature at which air would become saturated if cooled while maintaining a constant water vapor concentration, that is, not spraying water droplets into the air).
Assuming that sufficient water is available to be sprayed into the air in order to saturate the air, the amount of cooling, downdraft speeds, outflow speeds at the tower base, and power output increase with decreasing relative humidity. For this reason, a desert region is the most suitable site for a downdraft tower. Moreover, power output would be maximised in the late afternoon (when ambient relative humidity tends to be a minimum) and minimised near dawn (when ambient relative humidity tends to be a maximum).
