Originally posted by OccamsRazor04
I think my point was missed, I was at work so maybe I was less clear than I should have been. The duck requires energy to move at first, then self
sustains itself as a heat engine.
The bird doesn't require any mechanical energy to move at first, if that's what you meant. It begins
operation as a heat engine:
The initial state of the system is a bird with a wet head oriented vertically with an initial oscillation on its pivot.
The process operates as follows:
The water evaporates from the felt on the head.
Evaporation lowers the temperature of the glass head (heat of vaporization).
The temperature decrease causes some of the dichloromethane vapor in the head to condense.
The lower temperature and condensation together cause the pressure to drop in the head (by the ideal gas law).
The higher vapor pressure in the warmer base pushes the liquid up the neck.
As the liquid rises, the bird becomes top heavy and tips over.
When the bird tips over, the bottom end of the neck tube rises above the surface of the liquid.
A bubble of warm vapor rises up the tube through this gap, displacing liquid as it goes.
Liquid flows back to the bottom bulb (the toy is designed so that when it has tipped over the neck's tilt allows this), and pressure equalizes
between the top and bottom bulbs
The weight of the liquid in the bottom bulb restores the bird to its vertical position
The liquid in the bottom bulb is heated by ambient air, which is at a temperature slightly higher than the temperature of the bird's head.
It will stop eventually.
Why? As long as you keep it supplied with water and as long as the sun shines it will keep going, with maybe a
little maintenance like compensating for wear on the pivots. I couldn't make it work in the desert, but anywhere with access to a sufficient supply of
rainwater or a mountain stream would be enough to keep the necessary water level maintained. I think I could make one that would basically run until
the earth gets so hot the oceans dry up. Since all humans on Earth will be dead by that time what happens after that is somewhat moot to Earthlings.
However it would require large water storage tanks and the economics don't make much sense as discussed below.
It only moves for as long as it does because it does almost no "work", it needs to be light. You don't understand the operation of the
drinking bird. While it does help to make it light, this has nothing to do with how long it moves. So making a duck that did actual "work" would
prevent the duck from moving. I imagine it would have to be a very powerful heat engine to move a heavy magnet, which would require a lot of energy.
You could extract energy from the drinking bird, but of course you are correct that there's a limit to how much energy you can get out, and
the wiki provides this information. The evaporative heat flux from the bird's head is 0.5W, but due to ineffeciencies, the power that can be extracted
is only 1 microwatt. (Pretty inefficient, eh?) Since I need about 1000 watts to run my home, I'd need a billion drinking birds to make that much
energy, and I don't think I can fit that many in my house, but even if I did, there would be nowhere left for me to live.
I'd have a better chance of taking my house off the grid with small antennas that grab free EM energy from the air, because even though I'd need the
same number of those (they might also produce 1 microwatt each), they would take up less space than the birds, since you can print the antennas on
something as thin as a sheet of paper or perhaps even thinner. I ran the numbers, and the payback time on my initial investment would be about 1.5
million years, so all the energy it produces AFTER 1.5 million years from now would be completely free. The birds would probably require at least 10
times the initial investment, so the payback time would be about 15 million years. Since my goal is a payback period of 10 years or less, these
solutions just aren't economical or practical at today's energy costs.
However, these projections are not entirely accurate, because over the next few centuries fossil fuels will become more scarce, and if no alternatives
are in use by then they will be prohibitively expensive. So if fossil fuels become 10 or 100 times more expensive, those payback times might be
reduced by those factors respectively but they are still too long, and they are too impractical. But, what if you could figure out how to get more
than .000001W from that heat engine that's operating at 0.5W? That should be possible.
Some people, like me, actually analyze this stuff. So when roguetechie says it's not economical to make something expensive that produces very little
power, he's probably right, and I've actually run some rough numbers to prove it in these examples.
edit on 10-9-2012 by Arbitrageur