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[Discuss] Eclipses Re: Great talks last night, however...

On 7/22/2017 1:14 AM, Gregory Galperin wrote:
> You seem to be claiming here that the lithium ion batteries Musk is selling
> have less than a 10% charge/discharge efficiency.  But when I look for a
> number on charge/discharge efficiency of lithium ion batteries I find
> numbers in the range 80%-90% efficient depending on battery age (with
> measured numbers like 86%).  This says you'd need ~15% more solar power to
> compensate for the losses, not 1000% more.  Can you point me at something
> which explains this discrepancy?

Certainly. Musk's figures are marketing. Actual Tesla, Leaf and Volt
owners see much lower figures, as low as 50% for newish battery packs
(75-80% is typical). As a Li-ion battery wears and its charge density
decreases the thermal waste increases.

> OK, so the 10,000 km^2 would have to be 11,500 km^2 if you put all of it up
> at the Canadian border above NY & VT.  Not sure why anyone would do that
> instead of using some area in TX, but even if someone did, this doesn't
> seem like a big deal -- 11,500 km^2 is 0.15% of the land area of the
> continental US, rather than 0.13% of it for 10,000 km^2.  Hard to see how
> that 0.02% would be a dealbreaker even if Texas did secede.

This assumes that the land up north is flat like Texas. Hilly terrain
creates shade which reduces the time per day that PV panels are
effective. While there is flat land up north, much of it is either farm
or forest. Good luck stripping that for large solar farms.

> Actually, the Great Plains (which is known for being flat) tends to widen
> as you go North.  It's at its narrowest in Texas (including the NW Texas
> site described) and is noticeably wider in Kansas, the Dakotas, Montana...:

Point: you. I did mean northeast and northwest. I should have been more

> ranges (California to Canada)...  Do you know of an analysis showing that
> northern states have "much less flat land"?

Look at a map. Like this one:

The eastern half of the country is off the table because it's forests
and I see clear-cutting to make room for solar farms to be a hard sell.

The Great Plains are partially off the table as well because that's
where the bulk of our livestock production comes from and that's our
number two in agriculture after corn (maize).

The Nevada and southern California deserts might be good choices.

The Pacific Northwest is out due to forests and mountainous terrain much
like the Atlantic Northeast.

> Oh, maybe you mean that a panel on the equator is in the dark half the day,
> and you're arranging to put your solar panels in a sun-synchronous orbit so
> they're never behind the Earth?  OK, that would make for a factor of pi
> difference, but that's only halfway to the "at least an order of magnitude".
> What accounts for the rest?

Partially. An equatorial GBSP station is actually only good for about
7-8 hours a day, not 12. Efficiency drops off as the angle increase and
more atmosphere needs to be penetrated (problem shared by northern
ground stations).  Combine this with PV efficiencies that are much
greater than equatorial PV (the figures I've seen say at least 140%
greater). You can do the math but I think that's close enough to a order
of magnitude for a first order approximation.

Rich P.

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