Energy Towers – The effect of droplet coalescence on power and the environment

The Energy Tower concept is a renewable energy source suitable for dry and hot climates. It consists in cooling air by spraying water at the top of a tower approximately 1000 m high and thus creating a downdraft with a velocity of approximately 15 m/s. At the bottom of the tower the air goes through turbines and is subsequently exhausted through diffusers to the environment. In this work the influence of heat transfer from the droplets to the air is analyzed, the effect of coalescence being described using the O’Rourque model. It is shown that coalescence is substantial inside the tower and results in the droplets being an order of magnitude larger at the tower bottom and that there is a slight adverse effect on the energy performance, since evaporation from larger droplets is substantially less than from smaller droplets. In the case however that the water spray is a saline solution – which is the only possibility in view of unsalted water being as scarce a resource as energy – it makes it possible to keep the deposited salt inside the tower and the diffuser and in the immediate vicinity of the construction.

► In Energy Towers air is cooled by spraying water at the top thus creating a downdraft.
► Cooling of air by water droplets is analyzed, with coalescence taken into account.
► Coalescence is substantial inside the tower and results in larger droplets.
► Coalescence I shown to have a slight adverse effect on energy performance.
► Coalescence results in reduced salt deposition in the vicinity of the tower.