(From the project's home page: Everything that is impossible remains to be accomplished -- Jules Verne)
The last time Bertrand Piccard flew around the world in one go, it was in a balloon. He was one of the three self-described "adventurers" who flew the “Breitling Orbiter 3" non-stop around the planet in 1999, a 19th century way of finishing out the 20th. The next time he does so, it will be in an aircraft completely powered by solar energy -- this time, Piccard is looking forward, not back, for his inspiration.
Solar Impulse is his project to construct an entirely solar-powered aircraft and fly it around the world non-stop. To say this goal is ambitious is an understatement; success will require breakthroughs in pv efficiency and materials engineering, not to mention a pilot willing to live for days on minimal amounts of food and water. But for Piccard, Solar Impulse is a way to inspire a new focus on sustainability by accomplishing something on the edge of the impossible:
The development of solar flight will:
* Open up new scientific, ecological, humanistic and economic horizons;
* Stimulate scientific research in entirely new areas of composite materials science and methods of producing and storing energy;
* Attract sources of private-sector funding for scientific research;
* Mobilise public support for a meaningful and inspiring endeavour;
* Actively promote renewable forms of energy;
* Participate in the creation of popular interest in the very idea of sustainable development, a concept that is often misunderstood.
This will not be an easy task. The current record holder for distance traveled in a manned solar-powered aircraft is the Solar Challenger, built by AeroVironment: it managed to fly from France to England in 1981. (AeroVironment has shifted its solar aircraft focus to unmanned vehicles, including the Helios, which holds the non-rocket altitude record, at over 96,000 feet.) The Solar Challenger took over 5 hours to make the 163 mile trip.
Piccard and his team aren't close to making the trip. They have a preliminary design for an aircraft, but it has yet to hit the prototype deployment stage. The European Space Agency is pitching in with technology assistance in the areas of energy management, composite materials, and health monitoring for the pilot.
The physics of solar-powered flight are brutal. Solar energy amounts to about a kilowatt of power per square meter -- enough, in principle, to power a hair dryer -- but photovoltaics are typically around 30% efficient (with in-the-lab stuff getting up to 50 or so). They'll need a lot of area covered in pv to generate enough power to fly. Making matters more difficult is weight: the current higher-efficiency pv panels are heavy with glass and silicon. Add weight, you need more power. The much lighter polymer photovoltaics are also much, much less efficient, requiring more wing surface to soak up power. Add more wing, you add more weight. Solar panels don't generate much power from moonlight, so they'll need batteries to hold power to keep the vehicle up at night. More weight. And to get a human around the world, you'll need to go faster than the ~30 miles per hour of the Solar Challenger. Add speed, you need more power. The slower your speed, the more water and food the pilot will need to have on board to last the trip's duration. Again, we add weight.
The IEEE Spectrum layers on more concerns:
First of all, getting above the clouds is essential, as all available sunlight must be captured. But flying at high altitudes, where the air is thinner, eats more power than flying low, making for nasty tradeoffs involving wingspan, solar-cell carrying capacity, lift, and weight. Requirements are all the tougher because Solar Impulse will fly around the clock and therefore must capture and store enough solar energy during the day to carry it through the night.
To do that, it will need a very broad wingspan, on the order of 60 meters[...]. That adds structural complexity and weight, at some cost to performance. Compounding the difficulty is the need to keep a human comfortable at an altitude of 10 000 to 11 000 meters, where the temperature is around -55 °C. When the plane is ready to test on longer flights, the cockpit will have to be pressurized. And the autopilot, besides taking care of altitude and direction, will have to regulate the entire electrical system and govern allocation of incoming energy.
This doesn't mean that success for Solar Impulse is impossible, just that it's very, very hard. And, success or failure, the research into balancing weight, material and power when using solar photovoltaics will have lessons for other fields. While a solar-powered car is not likely to be found in your garage any time soon, more than one hybrid owner has certainly wondered about the possibility of layering pv on the roof their cars, adding a trickle-charge boost to battery regeneration. A gasoline-electric-solar hybrid would be easy in comparison to Solar Impulse, and such a concept will definitely benefit from Piccard and company's efforts.
In the end, the goal of Solar Impulse isn't to blaze a trail into a future of entirely solar-powered commercial flight. If such a thing ever became possible, it would be built on many more years of research into pv efficiency, the development of extremely strong, extremely lightweight materials, and propulsion systems able to convert small amounts of power into great amounts of speed. Instead, Solar Impulse is supposed to make us imagine such a thing might be possible, and wonder what else we can accomplish.
(Thank you, Lorenzo, for the pointer.)
Very interesting challenge.
The part about how easy it would be to add PV to hybrid-cars made me wonder why they couldn't do the same thing with regular commercial airplanes, using solar energy to partially power electric devices in the plane and thus (I assume) make them more fuel efficient (although it'd have to be very light-weight PVs... It probably wouldn't be cost-efficient for a while, though. But eventually I suppose that all the solar power we can get will be welcome.
*Has* anyone done a solar mod for the Prius? Given that my drive into work takes 8 minutes and then the car spends 8 hours in the sun, I'd be able to be pretty much purely electric for the week, only using petrol on the weekends and maybe the odd evening.
Krisjohn, doesn't the Prius automatically kick on the motor over 20 mph or so? Plus, the specs from Toyota say that the electric motor uses 50kW @ 1200-1540 rpm and the battery has a 21 kW capacity. Offhand, that looks like if you ran on pure battery power, you'd zap the batteries in about 25 minutes.
Anyway, assuming you use energy at the rate of 50 kW/hr, and could run on all-electric, over an 8 minute period it would use 6.7 kWh of power (someone who isn't a social scientist feel to correct my layman's grasp of energy calculations). Anyway, that seems about right.
The Prius is 175 inches long by 67.9 inches wide (roughly 7.7 square meters). Imagine a solar panel that size (assuming you could cover that much surface area) and also assume you could get 15 percent efficiency, which is typical for a photovoltaic panel. Since sunlight would produce 1 kilowatt per quare meter at 100% efficiency, a normal panel will produce 150 watts per square meter.
So, a big rectangular panel the length and width of a Prius operating at 15% efficiency (a pipe dream in most places, not to mention the issue of getting the sun to hit the panels at the best angle over an 8 hour period, but let's go with it anyway) should be able to generate 9.24 kWh of electricity, or more than the car would need for an eight minute trip.
Seven of these would provide roughly amount of power you would need - though they're getting 12.3% efficiency, not 15%. So, it'd be $3,000 for the solar panels alone and they'd weigh over 200 lbs just by themselves.
Of course you wouldn't rig a giant rack of panels to your roof like that, and I can't imagine that there are more flexible options that could adhere to the contours of the vehicle and provide enought output, so it's hard to see how something like a solar-powered Prius commuter would be practical at this point.
Perhaps someone can correct me if I'm way off the mark.
Correction: the panels would cost $4,200.
I think a better idea would be, if the Prius spends those 8 hours always parked at the same place, to either plug it in at that place (with a plug-in mod, or with a future version of a plug-in hybrid), or to install solar panels on the building and plug-in the Prius.
As I've often wondered: why lug all that equipment around? Cars should be electrical, research should be made in battery technology and the fuel cells shouldn't be in people's garages/community parkings.
Same with solar panels. Why lug them around? You probably have much more surface area and a better angle to the sun at the place where you are parked.
I wasn't necessarily proposing "lugging them around", just wanted to see how much surface area would be involved, even if the car were able to run that way -- which apparently it's not even engineered to do.
Unless he's parking in the same place every time, then you're talking about setting up a solar recharging infrastructure. If my estimates are in the right ballpark, the panels alone are going to cost $4,200 to "fuel" 16 minutes of driving per day. Assume he drives 20 mph for 8 minutes -- that means he'd cover 5.3 miles round trip per day, which in a normal Prius would cost about 20 cents in gasoline at $2.50 per gallon. That means a 10.5 year payback period just for the panels -- and as you know, solar installations include all sorts of hardware other than panels, plus they need to be installed, maintained, etc.
Seems like one hell of an effort just to make the cleanest, most efficient normal production car more "perfect". It seems society would be better off spending those dollars to make less clean, less efficient vehicles more clean and more efficient.
You can see my comment here on other ways we might start thinking of making things better.
Just my opinion on this.
Nice responses. My turn.
* There is no building near the car at work. Just one big parking lot, some of which is in full sun all day, some of which has shade.
* When I say "mod" I'm not referring to a hack with flat panels that screws up the aerodynamics, I mean something more like a replacement roof or bonnet with panels embedded. I would also wish to include a way to manually disable the petrol engine for the duration (though I believe Toyota has the computing locked down, making this "impossible").
* It's not about any sort of payoff. The Prius itself would not pay for itself compared to my existing '85 Gemini. It's about being to roll to work silently, and not generating any fumes all week, most weeks.
Basically, a Prius that had solar panels in the roof with a system that allowed the batteries to drain a little before starting the engine and didn't feel the need to fill the batteries from the engine (always left room for some solar charging) would be both an impressive display of technology and strong environmental statement. And for those of us who live close to work and park in the sun, we'd only need to fill up once every couple of months. Imagine only having to go to the gas station six times a year -- it really would be like those Prius ads where the kid asks her mum what the place is they're stopped at because she's never seen one before.
As I said before, I'm not saying you should screw panels onto the roof. I just used the basic length and width of the car to get a sense of how much surface area the car has. Obviously it has far less than it's basic dimensions indicate. The point was to get a sense of whether such a modification was in the realm of possibility, which it seems that it probably isn't, given current technology.
If you could barely generate enough power with panels the same width and length of the car, given optimal conditions (full sunlight, sunlight perpendicular to the panel surface), then a real-world situation where the usable surface area is far less, the efficiencies of conversion of flexible panels and similar technologies being lower than rigid panels, plus the lack of optimal sun conditions on any given day, would lead to a conclusion that you are probably not going to do a successful modification, regardless of the economics. It doesn't even address the fact that the car would still need to recharge when you got home, so I'm not sure how the solar would help at that point.
And, as I also said earlier, Toyota seems to be making the case that you don't want to mess around too much with the power system on the vehicle, as it could damage it, which would not only probably negate any warranty they offer, but could end up wasting the batteries etc, which kind of defeats the point of attempting to make the car "more pure".
The Prius is already strong environmental statement. No one's going to mistake it for a gas guzzler - most everyone knows what the car is about.
There's just some basic physics at work with moving around 3,000+ pounds of mass, and it's really not something that solar is very good at doing.
You didn't really define the speeds and conditions of your commute, so you'd still need to address the fact that the Prius can only go "all electric" up to about 20 mph. If those speeds are the ones you'd be going, then you'd move a lot more efficiently in something like a Gemcar (which weighs 1,078 pounds and costs $6,995). I did some quick calculations, and it's probably 10 times more efficient at utilizing electricity directly. A Segway would be even more efficient than that, and an electric-assist bicycle even more efficient than that.
But I can only assume you're going at speeds greater than a Gemcar or Segway could handle, in which case the Prius simply isn't engineered to go all-electric at those speeds.
I'm no rocket scientist, but that seems to be the basic numbers for the situation. Let me know if someone can puzzle out a solution for you.