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Gizmag (largely directly reproducing the claims of Solar Wind Energy Inc) explains that the proposed new $US1.5m+ power station project works by:

using a series of pumps to carry water to the top of a tower standing up to 2,250 ft (685 m) tall, where it is cast across the opening as a fine mist. The mist then evaporates and is absorbed by hot, dry air, thereby cooling the air and making it denser and heavier than the warmer air outside the tower.

At the base of the tower, turbines are driven by the downdraft of air, and the water is recovered for re-use.

It strikes me that, to the extent that the wet air is more dense and sinks is merely extracting the potential energy put into the water by pumping it higher in the first place - i.e. it is little more than a claim for a perpetual motion machine, that will actually end up consuming more energy than it generates.

On the other hand, to the extent that the air is cooled down (and hence sinks) by the cool water is effectively extracting energy from the temperature differential between the water and the air. That's a legitimate source of power, but it means the water is unable to be re-used once the water has been warmed up by the air. (It also requires a large supply of cool water in a warm region.)

While they don't claim this is a free energy device (they say it is solar-powered), it is unclear where the sun comes into it.

I hope I am hopelessly missing the point, because it smells to me like a major scam.

Is there evidence (competing towers, working scale models, independent analysis of the plans, etc.) that show that such a tower can generate more electricity than it uses to supply cold water to the top of the tower. Alternatively, are there credible models suggesting it will never be a net-positive power generator?

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    I suspect the air is cooled more by the evaporation of the water, rather than by the water simply being cool (i.e. latent heat), so it wouldn't be just the potential energy. I suspect @EnergyNumbers would be a good person to answer this one. – Dikran Marsupial Jun 25 '14 at 17:04
  • The sun comes into it because the sun has warmed the air at the top of the tower, and this provides the energy to evaporate the water sprayed into it. This makes the air cooler, and hence sink. It isn't perpetual motion because it requires something (in this case the sun) to maintain a temperature differential between the top and bottom of the tower. – Dikran Marsupial Jun 25 '14 at 17:15
  • Hmm... I pondered the evaporative cooling, but presumably the steam would rise and not be recoverable. I will read your paper. – Oddthinking Jun 25 '14 at 17:52
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    I think it would be water vapour rather than steam, and would be at the same temperature as the atmosphere around it (as collisions between molecules would quickly share the available energy and it would rapidly equilibriate). However thermal energy has still been lost in the change of physical state, so the temperature of the air and water vapour combined outght to be lower. However I've put this in the comments rather than my answer as I am not a physicist! – Dikran Marsupial Jun 25 '14 at 18:03
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    Humid air is less dense than dry air at the same temperature and pressure... – DJohnM Jun 26 '14 at 1:38
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Yes, at least according to:

T. Altmann, Y. Carmel, R. Guetta, D. Zaslavsky, Y. Doytsher, Assessment of an ‘‘Energy Tower’’ potential in Australia using a mathematical model and GIS", Solar Energy 78 (2005) 799–808. (www)

Abstract: ‘‘Energy Tower’’ is a technology for producing renewable and clean electricity by means of cooling hot and dry air, which is continuously supplied to arid lands. We assess the potential of an Energy Tower by incorporating topographic and meteorological parameters into a computational model, providing evaluations for the net power production and the electricity production cost. We formulate a highly simplified model for the Energy Towers flow, setup and process a spatial dataset of topographic and Meteorological upper air parameters. The model was applied to the Australian continent. A model simulation of one annual cycle enabled the ranking and selection of promising sites. The highest potential for energy towers is in the Port Hedland region, where favorable meteorological and topographic conditions would result in high average net power (370±160MW), potentially providing the electricity needs of 0.5 million people, for an economically competitive costs (3.5¢kWh).

There are also currently 9 papers that cite this one.

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    I read the paper. While it only briefly mentions the area I am now delighted to see I overlooked - the role of evaporation in cooling - and doesn't explicitly account for any recovery of the water, I am satisfied that this is a legitimate engineering endeavour. – Oddthinking Jun 25 '14 at 18:16
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    Back of the envelope calculation: (1 ton of water = 1000 kg of water = 1000 L of water.) Example value given was 14 tons/s of water distributed = 1.2 Mt/day = 1,200 ML/day, which is a significant fraction of a major river's daily flow. I assume they will need to recover much of the water to make that feasible - I'm guessing the unevaporated water accounts for the majority and would be more easily recovered. – Oddthinking Jun 25 '14 at 18:24
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    Instead of a yes this should be a maybe, or a hopefully. I mean the Ivanpah solar facility were hoping to make a lot more power than they are but currently would make twice as much power if they burned the gas they use conventionally en.wikipedia.org/wiki/… – daniel May 17 '17 at 7:34

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