[ExI] pentagon wants orbiting solar power stations

Jordan Hazen jnh at vt11.net
Mon Oct 15 15:17:59 UTC 2007

On Mon, Oct 15, 2007 at 09:25:05AM +0200, Eugen Leitl wrote:
> On Sun, Oct 14, 2007 at 07:06:29PM -0400, Jordan Hazen wrote:
> > Have you watched your PV system's activity during sudden changes in
> > cloudcover?  Mine will drop to 10% of full-sun production as a heavy
> Sounds like monocrystalline cells.

Mixed mono & polycrystalline array.  I've read that thin-film
amorphous does better in low-light conditions, but haven't seen any
hard numbers, and that type of panel has its own drawbacks.

> > This extreme variability is fine so long as solar amounts to only a
> > small fraction of total grid generation, as it is now.  Spinning
> Don't think the grid, think in terms of powering your home with it.

Grid-tie is nice for preventing any waste of PV energy, during times
when production exceeds household load... for example, if you're away
from home during peak sun hours, with most equipment turned off.  At
the scale of an individual household, the grid can be treated as an
infinite storage battery-- feed it excess joules during the day
(turning the meter backwards), and draw them back at night.

On the negative side, typical grid-tie systems have no autonomy, their
inverters being required to shut down during a utility outage
("anti-islanding").  Some can revert to standalone mode, with
batteries for buffering, but at greater cost and lower overall
efficiency, as safety requirements in most areas limit DC battery bus
voltage to a nominal 48V.

> > reserves at nearby power plants instantly make up any sudden
> > shortfalls, keeping frequency and voltage within spec.  But, there are
> What's wrong with DC?

At the low DC voltages commonly used off-grid, I^2R losses over any
significant distance (even in house wiring) can be high, unless very
thick conductors are used.  It can work well for low-wattage loads.

For utility-scale transmission and distribution, although DC-DC
voltage converters are gradually improving, they still tends to be far
more expensive, less efficient, and more vulnerable to surges &
spikes, compared to the simple and rugged AC transformer.

High-voltage (700kV - 1M V) DC power transmission is already used in a
few places, though, and may become more popular over time.

For end users, though, there's too much inertia, and legacy equipment
tied to AC for customer power standards to change anytime soon.

(Interestingly, though, most electronics and CFL lighting is perfectly
happy running on DC, at 1.4 times its rated AC line voltage-- i.e.
~160V in 120V countries, or 320V in the EU.  Switchmode power supplies
in this type of equipment tend to immediately rectify the incoming
line, and are sensitive to peak voltage, not RMS.  Active power-factor
correction circuitry may complicate things...)

> > growth), or trying to use PV & Wind for baseload generation will
> > require very large-scale energy storage-- something better than pumped
> > hydro and lead-acid batteries (even today, PbA is still the best
> Yes, I'm planning to run my computers off it. I already have everything
> on UPS, so that won't be much of a difference. Properly recycled
> lead-acid aren't that bad.

Special power supplies are available for running computers directly
from DC.  I do this at home, floating most equipment on a 12V bus, and
avoiding inverter losses.  Just remember the I^2R considerations, and
try to keep all low-voltage cabling as short as possible.


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