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For years, we've been watching as solar-powered race cars travel at relatively high rates of speed for hours on end – as long as the sun happens to be shining, of course. In fact, the World Solar Challenge has been taking place in Australia every few years since 1987, and the actual vehicles competing in the event got so fast that race organizers eventually had to alter the rules in the name of safety back in 2005. Since these solar-powered racers had gotten so fast and capable that in 2007 the World Solar Challenge altered its mission in the hopes that more real-world solar cars could hopefully benefit from the lessons learned during the competition.

But there is a problem with most of these solar racing vehicles. In the name of maximum efficiency, solar racers force their single drivers to lie in all sorts of convoluted positions, lack any sort of active safety systems (like airbags, anti-lock brakes or crumple zones) and are made from extremely expensive, high-tech materials. In other words, none of these solar-powered race cars will ever be construed as something you could possibly drive to work. What's the problem? Why can't we build a workable solar car?

That's the subject of today's Greenlings post. Click past the break to read all about solar-powered cars.

Now that we've discussed what isn't possible with solar cars, let's find out why that's the case. First, a few facts about the sun. Each and every year, the Earth absorbs about 3,850,000 exajoules of solar energy per year. Without getting into a full-blown examination of that amount of energy, let's just say that the Earth receives more power from the sun in just one hour than all of humanity uses in a whole year. In other words, there's plenty of solar power to go around.

So, why not harness that power and use it to power our cars? Well, it's not for a lack of trying. Production of photovoltaic cells doubles every two years or so as more and more people add solar installations to their homes or offices. Unfortunately, even the best photovoltaic panels in the world have an energy conversion ratio of around 23 percent, and the average commercially-available cell is only 12-18 percent efficient. This means that lots of solar cells are required to generate any meaningful amount of power, and they require a good deal of space. Of course, cars only have a limited amount of surface area available for solar cells, and it's not enough.

Although some may argue with this assessment, the fact of the matter is that photovoltaics just aren't able to provide enough juice to power an entire car with the available amount of space to install them. At least not yet.

That doesn't mean solar power isn't useful for automotive use. For example, the new 2010 Toyota Prius uses roof-mounted solar cells to help power the car's interior fan so that a reasonable temperature can be kept inside the cabin. The upcoming Fisker Karma uses a similar solar arrangement. Further, large solar arrays have been used by some automobile designers to at least offer a useful boost in an electric vehicle's overall range on sunny days. Obviously, any power that is harnessed from the sun that's used for a plug-in electric vehicle displaces energy that could have come from a relatively dirty power plant, and that's a good thing.

Automakers like Porsche, Ferrari, General Motors and Nissan have installed large solar installations that are designed to help power the factories where cars and trucks are assembled.

Even if solar cells aren't installed directly on electric cars doesn't mean they can't charge its batteries. Perhaps the most intriguing current developments in the solar space are large solar installations in sparsely populated and usually sunny locations, such as in the deserts of the Southwestern States here in the U.S. These solar power plans add their considerable energy to the already-existing power grid, and it's often possible to purchase power specifically from these plants from your own power provider. There are also a number of new products hitting the market designed to use solar power to charge individual cars when parked.

Finally, some electric car owners – such as AutoblogGreen reader David VM and Tesla Motor founder Martin Eberhard – have taken matters into their own hands by installing photovoltaic solar arrays on their own homes or garages. These systems send power to a bank of batteries and can be used to power either the household's electrical needs, the car's onboard batteries or both. In these instances, an electric car could really be considered a solar car that chooses to leave its power-generating panels at home when it goes for a night out on the town.

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