On 7/10/2014 4:42 PM, Russell Standish wrote:
On Thu, Jul 10, 2014 at 01:23:35PM -0400, John Clark wrote:
On Thu, Jul 10, 2014 at 3:28 AM, Russell Standish <[email protected]>
wrote:
IIRC, the average insolation is something like 1kW/m^2, so that would
make John's solar cells around 3% efficient
At noon under ideal conditions a square meter of solar cells can produce
about 150 watts, but midnight is not ideal conditions and I was averaging
over 24 hours. 30 watts sounds about right to me. If you rigged the solar
panels to follow the sun you could do a bit better but it would be
dramatically more expensive.
I assume that the quoted 333MW engines for a 767 was actually the maximum
power, which is only required for take off,
No. The number I started with was 140 megawatts, and that's the AVERAGE
amount of power a 747 uses during a flight, during take off it would be
considerably more than that. But after I sent my post I did realized I made
a mistake, I was assuming that the jet engines on the 747 were 100%
efficient which is nonsense, 50% would be more like it. So the factory
needed to provide the fuel to keep just one 747 in the air would cover
closer to 6 square miles of the Earth’s surface than 3 as I originally
said. And that my friends just isn't practical.
Of course, you're free to clarify where your figures came from, but usually
consumption is quoted in litres per hour, and power generated is the
maximum power output of the engine. Think about how you car is rated,
which is usually done using the antiquated units of miles per gallon
for consumption and brake horsepower for peak power.
So you would need to have multiplied the aircraft's consumption by the
calorific value of avgas to arrive at your starting figures. Since you
didn't say that, I assumed you were using the maximum power figures,
which seemed more likely to be at hand.
Actually, doing a quick Google, I see you figure of 140MW is splashed
around by Wikipedia. But it still sounds suspicious.
Looking at http://en.wikipedia.org/wiki/Boeing_747#Model_summary gives
a maximum thrust of 223kN for each of the 4 Rolls Royce engines. Given
that maximum thrust and power would be developed at takeoff, we can
multiply this value by the takeoff speed to determine the maximum
power of the aircraft, which I estimate to be somewhere around 30 m/s
(100km/hour approx).
So 223kN x 4 engines x 30 m/s = 26MW
Although that's maximum thrust (46,000lbf) it's not maximum power. The thrust doesn't fall
off with speed. They probably produce virtually the same thrust at 250m/s.
Brent
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