h a l f b a k e r yNot so much a thought experiment as a single neuron misfire.
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Clear Pyrex cylinders house pistons which fill with water on their intake stroke. Skipping compression and combustion, the water is turned to steam as rotating cowls uncover magnifying glasses which focus sunlight onto the black bottom of each cylinder.
Slowly, by internal combustion engine standards
but with incredible torque, the pistons turn a crankshaft which imparts energy to a heavy flywheel.
The driver uses the flywheels' energy to accelerate the vehicle at a rate less than that which the pistons impart to it.
Exhaust is then cooled, condensed, and re-enters the fuel tank.
The pack would leave such a car in their dust for a large part of the race but by the middle or last leg of the race this vehicle should be able to reach speeds unattainable by any battery powered racer.
With careful design it just may win.
Solar concept race
http://www.cnn.com/...lar.race/index.html
[2 fries shy of a happy meal, Mar 22 2008]
Arco solar steam engine without load.
http://www.acrosola...crosteamengine.html
[2 fries shy of a happy meal, Mar 24 2008]
[link]
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// this vehicle should be able to reach
speeds unattainable by any battery
powered racer.// |
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This is a nice idea, but how do you
support the above claim? The available
power is the solar energy deposited in
the water, which will be at best about
600W, or roughly one horsepower, per
square metre of lens. So, with a
manageable lens area, you're looking at
a few horsepower at most. |
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I'm sorry [MB] but it seems to me you're reading to much into this. 1 HP per square metre /at the Earth's surface/, maybe, but what about on the sunny side of Mercury, huh? |
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On the sunny side of Mercury, your Pyrex
cylinders will go all runny. |
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//On the sunny side of Mercury, your Pyrex cylinders will go all runny//
Pyrex softens at >800 degrees C, mean surface temperature of Mercury < 200 degrees C. |
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Pyrex softens sufficiently to be a problem
at about 500C. The only significant
difference from normal glass is it's ability
to withstand thermal shock, not thermal
extremes. 500C is itchingly close to the
temperature of Mercury's sunny side. |
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// this vehicle should be able to reach speeds unattainable by any battery powered racer.//
//This is a nice idea, but how do you support the above claim?// |
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I can't really support my claim with numbers but even 1 hp of force spinning a flywheel can store one heck of a lot of energy.
It is this energy that the driver would be tapping into. Since the temperature only needs to reach slightly more than 100 degrees celcius and the first Mercury race isn't scheduled to commence for another twenty three years, squidgy pyrex shouldn't be a problem. |
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May I ask for a link to your 1 hp per 1 square metre of lens [MaxwellBuchanan]? |
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Why use Pyrex for the cylinders? They don't need to be transparent. You already noted you're painting the bottoms black. |
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//fill with water on their intake stroke// er... then it's stuck. Where's the piston going to go after that? You need to have the water fill be only a small portion of the stroke length - the rest needs to be expansion stroke. |
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You've basically moved the boiler portion of the engine inside the cylinder. One of the major problems you face in this situation is that it takes a lot of energy transfer to get over that phase change to make water into steam. The energy transfer takes time. A typical steam engine does that part in the boiler, then transfers the working fluid under high pressure to the cylinder to expand and do the work. You're putting liquid phase water into the cylinder itself, and will have to sit and wait while the magic happens. |
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On the plus side, you've eliminated the need for feedwater pumps to force new water into the boiler against pressure. You'll get to deal with a reduced pressure version between the condenser and the sump, but, hey. |
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I think mirrors would be much more effective than lenses in power per mass. |
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First, you can't recover more energy
from your flywheel than you put in. So
you could spend 10 minutes spinning
your flywheel up to speed, and you'd
then be able to drive with 10hp of
power for 1 minute. With a massive
flywheel, you might be able to 'charge
it' for a longer time, and get more
advantage. But you're not going to win
any speed or endurance records. |
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Re the 1hp per square metre, just
Google for solar flux. It's about
500-600 watts per square metre in
good sunlight, or roughly 1hp per
square metre. |
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At the top of the atmosphere, the total solar flux is 1380 watts per square meter. Admittedly, we don't drive around at the top of the atmosphere; and you may have a difficult time making a system that utilizes every single wavelength that's counted in the flux total. I don't know if [MB] is quoting figures for what might be reasonably convertible to heat, or an absolute maximum that gets to the ground in the UK. <wink> |
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500-600 Watts is the total flux reaching
the earth, averaged across the whole sunlit
face (so, more near the equator, less near
the poles); it's probably about right for
the UK. |
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I was assuming that a perfect lens and
absorber could capture all of this as heat
(unlikely). |
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Looks like mirrors would be the way to go. 2nd [link] |
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//Why use Pyrex for the cylinders? They don't need to be transparent. You already noted you're painting the bottoms black.// |
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I thought that by projecting the light onto the inner bottom surface of each cylinder all of the heat would be transfered to the water.
By heating the bottom from the outside some of the heat would be lost to the air. |
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True. It would be interesting to see how the size of that loss would compare with reflection off the glass. |
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Agreed.
Hmmm, refraction would have to be taken into account as well. |
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I thought sunlight approximated to 1kW/m2. |
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If the flywheel is allowed to spin up for hours before the race then it will have plenty of power for the driver to tap into, otherwise he will only be able to get out, the 1hp that is going in. |
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You could have interesting regenerative braking where the hot brake pads heat the water. |
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You'd be a lot better off using a working fluid that boils at a lower temperature. |
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Fresnel lenses are much cheaper and lighter than mirrors. For the car to be as efficient as possible, the lenses would need to be gyro-controlled to react to changes in direction. |
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Has anyone ever entered anything other than a PV car into a solar race? |
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//I thought sunlight approximated to 1kW/m2.//
AC Clarke's "A Slight case of Sunstroke" gives a value of about 1 HP (PS, whatever) per square yard. |
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Metric, I say.
1 horsepower = 745.7 watts
1 square yard = 0.836 square metres |
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Wouldn't condensing the steam be an issue on Mercury? Or were you just doing a once-through scenario? |
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That'd be wasteful, though - I bet water is scarce on Mercury. |
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1 kW/m^2 is pretty much the standard assumption for full sunlight on a flat plate collector. A tracking parabolic mirror is capable of concentrating this up to aproximately 200x. However, the only transparent portion of this device should be the glass shell that contains the vacuum around the cylinder. The cylinder itself can/will be made out of high temperature materials.
The big problem with a concentrating collector, however, is that it only works with direct, not diffuse, sunlight. This means, unlike photovoltaic electric power schemes, you will get essentially zero power on cloudy days (flat plate colletors can pull in ~60% of nominal even under fairly overcast conditions). |
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