[ExI] pentagon wants orbiting solar power stations

[Eugen Leitl]

On Mon, Oct 15, 2007 at 11:17:59AM -0400, Jordan Hazen wrote:

> Mixed mono & polycrystalline array.  I've read that thin-film
> amorphous does better in low-light conditions, but haven't seen any

It's less low-light than diffuse vs. direct insolation.

> hard numbers, and that type of panel has its own drawbacks.

Yes, but the aging is not nearly as bad now. I wonder how CdTe and
CuInSe2 does these days.
 
> Grid-tie is nice for preventing any waste of PV energy, during times

PV allows you live off the grid, which is otherwise prohibitively
expensive. If we think of it, then water electrolysis is much better
than letting the electrons idle, even if overall efficiency is much worse
than conventional electrochemical storage.

> when production exceeds household load... for example, if you're away
> from home during peak sun hours, with most equipment turned off.  At

You can wash the dishes, or run the washing machine, or an air conditioner.
Or heat your water tank, for later use.

> 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.

It sure is nifty, especially since subsidizing panels and selling juice
to the grid at artificially marked up prices makes it actually
cost effective (IIRC last time I looked it took about 50 years 
to economicaly ROI at current electricity prices, which is beyond 
of panel life time). 

I just don't believe in cheating physics with economics, that's why
I omitted that obvious loophole.
 
> 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

I'm thinking about running the prototype autonomous, with no connection
to the main grid. The drain would probably be no more than 50 W tops.

> 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.

If I roll my own, I don't have that problem. I would still stay way
below 200 V, though.

> > What's wrong with DC?
> 
> At the low DC voltages commonly used off-grid, I^2R losses over any

I don't see much loss on a neighbourly grid of less than 100 m, if 
several 100 V or up to kV are being used.

> significant distance (even in house wiring) can be high, unless very
> thick conductors are used.  It can work well for low-wattage loads.

I'm quite aware of the requirement. I was thinking of hot-soldering 
the connectors to minimize lossage and heating. Copper is expensive,
but there's not much copper in a short, radial-pattern home setting. 
 
> 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.

DC-DC does great for powering small devices, and it's really efficient
these days.
 
> High-voltage (700kV - 1M V) DC power transmission is already used in a
> few places, though, and may become more popular over time.

Any idea whether 1 kV would be safe in a thinnish, but properly
insulated earth cable? 
 
> For end users, though, there's too much inertia, and legacy equipment
> tied to AC for customer power standards to change anytime soon.

Isn't the new automobile standard 48 V, or hereabouts? That could be
a kickstarting market.
 
> (Interestingly, though, most electronics and CFL lighting is perfectly

Any idea about HQI, with electronic ballast?

> 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...)

I'm a bit afraid to test it, lest it becomes a real smoke-test.

> Special power supplies are available for running computers directly
> from DC.  I do this at home, floating most equipment on a 12V bus, and

I was thinking about going Mac Mini 24/7/365 instead of my current 1/4 kW
box.

> avoiding inverter losses.  Just remember the I^2R considerations, and
> try to keep all low-voltage cabling as short as possible.

One of the plans is to remove the sea of wall warts, which is ugly as sin,
and cumulates the aggregated losses to some 50 W, or more.

Thanks for all the interesting info. Much appreciated.

-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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