Bret Cahill wrote:
There's no question conventional PV would get a blimp going over 25
mph.

Maybe. Per http://www.goodyearblimp.com/faqs/fa...struction.html :

"The GZ-20A size blimps (Spirit of Goodyear; Spirit of Innovation; Spirit
of America) are 192 feet long, 55 feet in diameter, and 59.5 feet high,
with 202,700 cubic feet of helium and a gross weight of 12,840 lbs."

"The GZ-20's carry two fuel injected Continental I0-360's, producing 210
horsepower each."

"The usual cruising speed is thirty-five miles per hour in a zero wind
condition; all-out top speed is fifty-three miles per hour on the
GZ20."

Jane's "Encyclopedia of Aviation" (ISBN 0-517-10316-8), p. 425, says
that the Goodyear "Mayflower" (built 1978) has a gross volume of
202,700 ft^2, so I am assuming it's the same size as the current blimps.
Jane's also says that the Mayflower has "an envelope surface area of
2,006 m^2 (21,600 sq ft)". Jane's also cites the twin 210 hp IO-360
engines.

So, 420 hp is 313 kw. If your motors are 90% efficient, you need to
deliver 345 kw to the motors. If you covered the entire surface of the
envelope with solar cells, you'd need to _average_ 0.172 kW / m^2 to get
that much power. If you assume that everything is great and you're
getting 1 kW / m^2 coming in, that's 17.2% efficiency. I would guess
that at most half of the solar cells would actually be illuminated, so
that means you need to average 0.344 kW / m^2 or 34.4% to get that much
power. As has been noted, the "40%" cells require a concentrator (lens)
in front of them to get that efficiency. Spectrolab claims their TASC
cells ( http://www.spectrolab.com/prd/terres/tasc-main.htm ), which
don't need a concentrator, are up to 30% efficient.

On the weight side... Googling around says that a Continental IO-360
weighs something like 300 lbs. I have no idea what the fuel burn is
like in a blimp, but for grins let's say there is at least enough fuel
for two hours at 10 gal/hr/engine, or 240 lbs. So dropping all of that
stuff gains you 840 lbs.

Spectrolab says one of their cells is 2.277 cm^2 and weighs 0.234 g.
To cover the entire skin of the blimp, you'd need about 8.8 million
of these cells, or about 2060 kg or 4050 lbs. So you're up about
3210 lbs (25% over gross) and you haven't even strapped on any
electric motors yet.

You might be able to make it work with a smaller blimp - you'd need less
power, but you'd also have less surface area for mounting the cells.

Matt Roberds