Solar Towers

The PS10 Solar Power Tower near Seville, southern Spain

Having been a bit of a sci-fi/fantasy bookworm as a lad, and looking more like a big-budget prop for a Tolkien fantasy-brought-to-screen than an attempt to close the growing energy gap, the tower at right certainly piqued my interest. Accordingly, I thought it was time we had a quick look at these gigantic obeliscoid desert entities (by the way, the image inset is not Photoshopped!).

First, how do they work?

At present there are two main kinds (with sub-variations) of solar tower being proposed or utilised: the ‘Solar Power Tower’, like the 11MW Seville tower above, and the ‘Solar Updraft Tower’ (or ‘Solar Chimney’). Both designs are quite simple in concept, as we shall see:

Solar Power Tower:

This design utilises a great number of huge, flat, movable mirrors (heliostats) that track the sun, converging its rays at a solar collector at the top of the tower. Think of how the neighbour’s kid zaps bugs with his magnifying glass (I just know you and yours wouldn’t do this yourselves), and you’ll get a bit of an idea how it works. The intense heat, up to and over 400’C (750’F) at the top, boils water, with the resulting steam blasted into turbines which generate, in the case of the Seville tower, 11MW of electricity – enough to power 6,000 homes, and saving about 18,000 tonnes of carbon emissions every year. Note that the Seville ‘Planta Solar 10’ (PS10) is just the first part of a larger development, currently underway, that would ultimately (around 2013) generate enough energy for 180,000 homes – equivalent to the needs of the 600,000 strong city of Seville, and saving 540,000 tonnes of annual emissions. The PS10 boasts 624 mirrors, each having 120 square meters of surface area, and the tower’s height is 115 metres (377 feet).


Solar Updraft Tower:

The name of this tower really gives the mechanism away. Instead of mirrors, a large area of land around the base of the tower (about 7 kilometers in diameter) is covered in a translucent material like glass or another alternative. Being raised a few metres off the ground at the edge, and increasing in height as it approaches the centre, light enters the space below with the resulting heat building up under the enormous canopy. Essentially, it’s a mega-greenhouse with a chimney in the centre. Hot air, being lighter than cold air, rises – and the difference in temperature between the air flow at the very top of the tower and that of the canopy at bottom facilitates this even further – creating an updraft that drives an array of turbines set into the tower’s base. The next clip is an artist’s impression (as is the picture inset) of a proposed Solar Updraft Tower:

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Unlike the PS10 at top, there are no commercially operating updraft towers in the world as yet. Australian EnviroMission, and U.S. based SolarMission, merged in March 2007 (PDF) to become the dominant world force in solar updraft technology. Although news on progress is sketchy at best, Enviromission are spying out the land (PDF) in Arizona, after John Howard’s coal- and nuclear-centric government policies discouraged their initial attempts to set up in Australia. The proposed 1,000 metre high, 130 metre wide tower, it is claimed, would produce 200MW, or enough to power 500,000 typical Australian homes.


How Do They… er… Stack Up?

Significantly, the emissions generated from initial construction are said to be neutralised within 2-3 years of operation – as, with the exception of minor ongoing maintenance (you can just picture all the guys with squeegees cleaning the mirrors, can’t ya!), the towers are emission-free once operational (although EnviroMission did stoop to encourage increased emissions by way of driving tourists to view the massive structure).

Some have said these projects use too much land, but this is a dismissable argument, at least for the first of the two designs.

Solar thermal power plants are big and seem to use a lot of land, but when looking at electricity output versus total size, they use less land than hydroelectric dams (including the size of the lake behind the dam) or coal plants (including the amount of land required for mining and excavation of the coal). While all power plants require land and have an environmental impact, the best locations for solar power plants are on land such as deserts, for which there are few other uses. –

With any solar technology, there is always the problem of only being able to ‘make hay while the sun shines’, as it were. The PS10 tower at top is currently able to store energy in the form of steam to generate power after the sun has set – but only for an hour or so (the completed larger project is expected to perform better). With the Updraft Tower, heat absorbed into the ground beneath the canopy is given off after the sun sets. Additionally, the Updraft Tower is less dependent on strong direct sunlight, and works almost as well on cloudy days as sunny – as it is the temperature differential between the air at the top of the tower and that passing through the turbines at the base that keeps the air moving. There is also the possibility of having water-filled tubes inside the canopy that can store, and subsequently give off, additional heat. In this respect the Updraft Tower gets bonus points, as it has a more reliable projected base-load capacity. Unreliable input can wreak havoc on electricity grids.

The proposed Australian updraft tower,
as compared to other architectural icons

Perhaps the most obvious concern with the Updraft Tower, however, is the size required for the tower itself. According to Enviromission, one very large tower allows for a greater combined generating capacity than a number of smaller towers, due to the physics of the updraft dynamics themselves – the taller the tower, the stronger the updraft effect. In other words, bigger is better. Their proposed tower, at 1,000 metres (3,280 feet), would be the tallest manmade construction on the planet, a full 350 metres higher than the Burj Dubai (not counting the latter’s antenna).

The sheer size must equate to a gargantuan problem of physics, particularly as deserts can be windy, unstable places…. The world’s first solar updraft tower, a modest 195 metre prototype built in Spain in 1982, was eventually decommissioned after seven years of optimisation testing due to experiencing “severe structural instability close to the tower due to induced vortices”.

The costs and logistics of building a reinforced concrete structure at such a phenomenal height, also makes the Floating Solar Chimney variation an interesting proposition.

In 1931, in a conversation with Henry Ford and Harvey Firestone, Thomas Edison is quoted as saying:

I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that. – Thomas Edison

We won’t be running out of coal anytime soon, but our apparent peaking in oil supplies, and the fact that the consequences of climate change are becoming increasingly obvious to the global public, certainly makes Mr. Edison’s sentiment seem inspired today. The original prototype Updraft Tower was built in an age of cheap oil, so never received further investment.

Map of world solar potential

Although conservation is always going to be the best form of energy ‘generation’ (as the old saying goes, a penny saved is a penny earned), harnessing energy from the sun is arguably the least controversial and cleanest approach to creating power. Unfortunately the majority of the world’s populace may never draw power from a solar tower, as, like other renewable technologies like wind and waves, it’s a very region-specific solution, and many of the countries with the most sunshine are among the poorest in the world – where they cannot afford the large initial investment costs.

Technical and financing hurdles aside, these towers are certainly potential additions to the renewable, distributed generation mix. For myself, though, home or village scale systems would be far more attractive. The large scale centralised systems decribed above take about two years to build. If one were destroyed for any reason, that’s a long time for a great many people to wait before they’re plugged in again….