By Loz Blain
An ambitious solar energy project on a massive scale is about to get underway in the Arizona desert. EnviroMission is undergoing land acquisition and site-specific engineering to build its first full-scale solar tower – and when we say full-scale, we mean it! The mammoth 800-plus meter (2625 ft) tall tower will instantly become one of the world's tallest buildings. It s 200-megawatt power generation capacity will reliably feed the grid with enough power for 150,000 US homes, and once it's built, it can be expected to more or less sit there producing clean, renewable power with virtually no maintenance until it's more than 80 years old. In the video after the jump, EnviroMission CEO Roger Davey explains the solar tower technology, the Arizona project and why he couldn't get it built at home in Australia.

How Solar Towers Work

Enviromission's solar tower is a simple idea taken to gigantic proportions. The sun beats down on a large covered greenhouse area at the bottom, warming the air underneath it. Hot air wants to rise, so there's a central point for it to rush towards and escape; the tower in the middle. And there's a bunch of turbines at the base of the tower that generate electricity from that natural updraft.

It's hard to envisage that sort of system working effectively until you tweak the temperature variables and scale the whole thing up. Put this tower in a hot desert area, where the daytime surface temperature sits at around 40 degrees Celsius (104 F), and add in the greenhouse effect and you've got a temperature under your collector somewhere around 80-90 degrees (176-194 F). Scale your collector greenhouse out to a several hundred-meter radius around the tower, and you're generating a substantial volume of hot air.

Then, raise that tower up so that it's hundreds of meters in the air – because for every hundred metres you go up from the surface, the ambient temperature drops by about 1 degree. The greater the temperature differential, the harder the tower sucks up that hot air at the bottom – and the more energy you can generate through the turbines.

The advantages of this kind of power source are clear:

Because it works on temperature differential, not absolute temperature, it works in any weather;

Because the heat of the day warms the ground up so much, it continues working at night;

Because you want large tracts of hot, dry land for best results, you can build it on more or less useless land in the desert;

It requires virtually no maintenance – apart from a bit of turbine servicing now and then, the tower "just works" once it's going, and lasts as long as its structure stays standing;

It uses no 'feed stock' – no coal, no uranium, nothing but air and sunlight;

It emits absolutely no pollution – the only emission is warm air at the top of the tower. In fact, because you're creating a greenhouse underneath, it actually turns out to be remarkably good for growing vegetation under there.

The Arizona Project

While this is not the first solar tower that has been built (a small-scale test rig in Spain proved the technology more than a decade ago) EnviroMission has chosen to build its first full-scale power plant in the deserts of Arizona, USA.

The Arizona tower will be a staggering 800 metres or so tall – just 30 meters shorter than the colossal Burj Khalifa in Dubai, the world's tallest man-made structure. To put that in context – it will stand more than double the height of the Empire State building in New York City, and it'll be as much as 130 meters in diameter at the top. Truly a gigantic structure.

Currently undergoing site-specific engineering and land acquisition, EnviroMission estimates the tower will cost around US$750 million to build. It will generate a peak of 200 megawatts, and run at an efficiency of around 60% – vastly more efficient and reliable than other renewable energy sources.

The output has already been pre-sold – the Southern California Public Power Authority recently signed a 30-year power purchase agreement with EnviroMission that will effectively allow the tower to provide enough energy for an estimated 150,000 US homes. Financial modelling projects that the tower will pay off its purchase price in just 11 years – and the engineering team are shooting for a structure that will stand for 80 years or more.

Considering that a large city like Los Angeles requires total power in the region of 7,200 megawatts, you'd have to build a few dozen solar towers up to the same size as the Arizona project if you wanted to completely replace the existing, primarily coal-based energy supply for that city's 3.7 million-odd residents. So it's not an instant solution – but then, its short projected payback period and virtually zero operating costs make it a very sound economic proposition that competes favorably against other renewable sources.

Under the terms of the pre-purchase agreement, the Arizona tower is due to begin delivering power at the start of 2015. Watch this space!

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