The idea of an energy tower has been around for some time. The concept is to construct a tall hollow circular tower and to spray water in a fine mist into the top. In theory evapouration will chill the air and the increased density will cause it to flow down inside the tower. The motion of the air would then be used to drive an air turbine. The claim is that the power extracted from the turbine can be greater than the energy required to fetch, raise and inject the water. I find a number of papers that attempt to analyse the ecconomics of such a system. But does anyone know of an experimental demonstration of net power output? We are building one at Sussex University to try and demonstrate this net power output. The weather in the UK makes this tricky!
This is an answer to my original question. We built a simple small energy tower (just 3.5 m tall, 6" diameter) and on a bright, cool, humid British day we tried it out. We can report that the flow field was established but with minimal energy. We had a fine spray nozzle under significant pressure (circa 5 bar). The dropplets were not measured but can be described as small, maybe in the tens of microns range. However, the rate of evapouration was small and the droplets persisted all the way to the bottom of the tower. These droplets mostly ended up on the ground in a circle about the axis of the tower of diameter about 1.5 m. The velocity was so small that our inclined water manometer could not register any dynamic head. Whilst this is a negative result it was not unexpected and we have a much better appreciation of the potential of these devices. After some theoretical thinking we did notice an issue not discussed in the literature on energy towers as far as we can see. This energy tower relies on the cooling effect of water evapouration this is intended to increase the density of the air and cause by bouyancy a down flow. However there is an effect that opposes the density change in cases with water dissolved in air. Perhaps surprisingly, as one dissolves water in air the density of the resultant mixture actually reduces if the temperature is held constant. This can be seen on a psychrometric chart from the inclination of the specific volume lines. This effect tends to reduce the effectiveness of the energy tower. We do not conclude that energy towers are not possible, however it is clear that tall towers with hot dry atmosphere would be required.
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