Solar Prius : Toyota to add solar panels to some 2009 Prius hybrid models

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Tuesday July 08, 2008

Solar Prius : Toyota to add solar panels to some 2009 Prius hybrid models - innovation or gimmick?

news.yahoo.com :

Toyota plans to install solar panels on some Prius hybrids in its next remodeling, responding to growing demand for "green" cars amid record-high oil prices, a source said on Monday [7 Jul 2008]. The Kyocera panels would power part of the air-conditioning on high-end versions of the Prius. "It's more of a symbolic gesture," said the source. "It's very difficult to power much more than that with solar energy."

Automakers are racing to come up with alternatives to using fossil fuels, but solar power is not seen as a viable solution to power cars. Solar panels are expensive due to rising silicon prices and storing energy is difficult. Toyota has struggled to keep up with demand for the Prius as soaring gasoline prices put consumers off of gas-guzzling SUV's and trucks. Rival Honda will also step up its hybrid push with the Honda Global Small Hybrid next year and other models subsequently.

- This is the first semi-official acknowledgement of a rumour that has been circulating for many months out in the wild. Imagine! A solar-powered Prius! How innovative is that? How neat would that be? Fortunately (or unfortunately depending on your point of view), the peakoiler community (yours truly included) has been quick to deconstruct any fancy notions of future cars gliding silently by powered only by the sun.

At first thought, a solar-powered hybrid makes perfect sense. If you're driving out there in the daytime, what better way to make use of the solar energy falling onto your car than to put solar panels on top to grab some of that energy? But that's exactly where the problem lies - the available surface on the car roof and elsewhere. It simply isn't big enough, even if we were to use the most generous estimates. Let's say we take the entire (purely theoretical) top surface area of the Prius, which would have dimensions of 4445mm x 1725mm (length x width) - that's 7.67 m².

Next, in order to fit the body contours of the Prius, we would have to be using thin-film solar panels. Suppose we generously estimate 10% efficiency, and again very generously estimate 2KWh per m² of the most extremely hot tropical sunshine falling onto these solar cells. The instantaneous power generated is therefore :

7.67m² x 0.1 efficiency factor x 2 KW per m² = 1.5 KW

Given all the generous estimates we can throw at it, we get 1.5KW, or 1500W of power. It's not enough to run a car. It's just enough to run a hairdryer. Maybe. Hairdryers range from 1200W to 1600W or thereabouts. How about that?

Now, let's say we want to turn this around and ask another related question : how big of a solar panel are we talking about if we want it to power the Prius? The Prius hybrid drivetrain (engine and electric motor) generates about 110hp, or around 82KW, so we have :

82 KW / 0.1 efficiency factor / 2 KW per m² = 410m²

410m² is about 4413 square feet. That's over 3 times the floor area of my apartment, and larger than the entire floor area of an average house. Now try to imagine a Prius running about with a solar panel that is 53 times the size of the car and you will now realize the scope of the problem.

But there's one last thing to think about - what if we were to leave that solar Prius parked under the same hot tropical sun and let the solar panels charge the batteries while we watch a movie for say 2 hours and go shopping for say another hour? How far could we go in pure electric mode on this charge? Okay, let's say we get a generous 8 km per KWh which includes energy recovered from regenerative braking, we get :

1.5 KW x 3 hours x 8 km per KWh = 36 km (22 miles)

Aha! Now we're really getting somewhere. So, if we were to leave a solar-powered Prius parked under the hot tropical sun while we go for a movie and shopping, and given ALL the generous estimates we could possibly throw at it, and the system is able to charge the battery from solar power all this while, we could conceivably drive the car home on solar power. Very cool.

Just one last catch. There always is. At it is now, if you live in the US, the price of an automotive lithium-ion battery of that kind of capacity (say around USD $22,000) would cost you the equivalent of another Toyota Prius. The proportion is slightly less if you live in Singapore, but that's only because our cars cost 3 times more than in America, so for us it's one third. Still, it's a hefty sum. So for this to be even halfway workable, costs will have to come down, and solar cell efficiencies have to go up. Even then, in typical real-world conditions, it would only help somewhat. If this sounds like very familiar territory to peakoilers, this is because it is almost the exact problem we come up against when discussing intermittent wind or solar power versus reliable base-load power. Renewable energy will help, but only somewhat, and it won't be nearly anywhere enough to replace our current energy needs.