I welcome a debate as the way we seem to advance is through the ‘head in the clouds’ people coming up with ideas, the ‘feet on the ground’ people shooting them down, and then the ones left over get developed. I feel however that it would be more worthwhile if you are going to engage in this kind of dialog to read the reference material that the current ideas are based on and try and find flaws in that (and when you have, do a quick internet search and see if others have come to the same conclusion and see the counter-arguments made for them).

I know thats not quite fair in that I’m interested/passionate about the subject and willing to spend time and money doing research and you are a skeptic and not inclined to spend precious resources on what you consider a useless pursuit but the total extent of my space elevator reference library is the internet plus 2 books:

The Space Elevator: A Revolutionary Earth-to-Space Transportation System (2002) by Brad Edwards and Eric Westling
Liftport: Opening Space to Everyone (2006) (of which half the book is sci-fi short stories with a space elevator in them)

Available from Amazon:
http://www.amazon.com/Space-Elevator-Earth-Space-Transportation/dp/0972604502
http://www.amazon.com/Liftport-Space-Elevator-Opening-Everyone/dp/1592221092

The reason for that semi-rant is simply that the numbers you state, as pointed out by jtkare, are nowhere near the correct ones and that similar ones to jtkare’s are used in a book published 6 years ago which is considered (by me at least) to be the best place to start for a technical description of lots of different parts of the space elevator. I like the way you think and I think you could make valuable contributions ‘to the cause’ if you were just primed with the right information. Buy the book and find flaws in that rather than risk being accused to constructing ’straw-man’ arguments. Heck, I’ll buy you a copy if you think you’ll actually read it.

Anyway, I thought I should do a short summary of the two sets of figures so far proposed and then look at what the figures in The Space Elevator book are:

From the original post:

4 MW total power for 10 ton climber
42% solar cell efficiency
9,500 square meters est.
1.62×0.992m 20kg = 12.293kg/m^2

 = 256,934 lbs

From jtkare’s comments:

4 MW total power for 10 ton climber
50% efficient
6x sunlight = 8000W/m^2
1000m^2
1-5kg/m^2

= 1-5000kg = 2200-110000 lbs

From The Space Elevator (2002) book pg 63

“For our scenerio we will need 2.4MW of power delivered to our 20 ton climbers
…
 For the longer term a 1×10^6kg climber would reuire 120MW of power delivered
…
If the Compower FEL system is selected, specifically designed GaAs phtovoltatic cells can be used with 59% conversion efficiency, 82% filling factor and a sizzling power density of 540kW/m^2 (D’Amato,1992, and Charlie Chu at Tecstar, private communication)
…
The smaller climbers [900kg mass - mainly used to build up the ribbon to take bigger climbers] with a 4m diameter base has 12m^2 of area and needs to produce 100kW of power. That only requires 8.3KW per square meter of PV cells, an easily achieved capacity given the generous margin from the 540kW/m^2 potential. These cells would also work well with a large laser diode array as proposed by Kwon, 1997.”

So we have:
2.4MW total power for 20 ton climber so we’ll say 1.2MW for a 10 ton climber
59% effiicient
lower bound of 8300W/m^2
1.75kg/m^2 (from the chapter before in the book the 12m^2 cells weigh 21kg of the 900kg climber)

= 253kg = 557 lbs

The key difference to the numbers from jtkare is simply the needed power which he just took as read. I’m not sure where the 2.4MW power needed for 20 ton climber (and the 120MW for the 1000 ton one) comes from in the book as I just opened it at the power beaming chapter but I’m sure if you’re interested you can find out :-)

Photovoltaics illuminated with lasers can be more efficient; the ones we use are roughly 50%.  

>Here, I’m assuming laser energy to be equivalent to the sun on solar cells.  

Generally the laser flux is much higher; the limit is heating of the cells.  2x sunlight is easy, 10x is feasible but takes some effort in space — our cells run at 10X sunlight but are air-cooled.  Assuming 8000 W/m^2 (~6x sunlight) incident and 50% efficiency, a 4 MW output array is 1000 m^2, not 9500.

Finally, the mass of PV arrays for high performance applications is much lower than your estimate.  Current satellite solar arrays are around 5 kg/m^2.  Advanced arrays such as the Entech stretched lens array  are around 1 kg/m^2.  That would make a 4 MW laser receiver mass around 1000 kg, or 11% of a 9000-kg climber.  A full PV power system would mass a bit more, but on the other hand both PV cells and arrays are still improving.

There are certainly reasons to be skeptical about space elevators, but the laser power transmission system really isn’t one.

  Your alternate arrays may be lighter, to be sure.  However, I could not spend the time to research all photovoltaic cells for minimum weight/power ratios.  Don’t forget, also, that they still need a frame, and your lower efficiency cells would have made a monstrously unwieldable size of a field of photocells to handle on the elevator.

  In any case, the severity of overweight indicates that no photovoltaic cells would have fit anywhere near within the original ten tons, and the array is still a predominant factor of weight.