[ExI] pentagon wants orbiting solar power stations

[Jordan Hazen]

On Mon, Oct 15, 2007 at 05:52:36PM +0200, Eugen Leitl wrote:
> 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.

For a 50W constant load, you'll probably want at least 300W of PV.

50W*24 -> 1200 Wh/day, dividing by, say 5 effective sun-hours per day
(look up worst-case winter values for your location) would give a 240W
minimum, but you want a little extra to allow batteries to gradually
recharge after a cloudy day.

Battery requirements depend on how many consecutive low-sun days you
want to allow for.  Four of "golf-cart" size (6V, 220 Ah) would allow
two sunless day's operation without dropping below the recommended 50%
depth-of-discharge limit.

Requirements could be reduced if you're willing to shut down under
adverse conditions, or occasionally recharge from the grid.
 
> DC-DC does great for powering small devices, and it's really efficient
> these days.

Yup.  Non-isolated "buck" converters (common ground, voltage reducing)
can exceed 96%.
  
> Any idea whether 1 kV would be safe in a thinnish, but properly
> insulated earth cable? 

The insulation on most power wiring in the US is rated for only 600V.
I don't know how EU standards compare... your 400V 3-phase would reach
566V peak-to-peak, so perhaps insulation intended for that type of
service is thicker.  Best double-check, though.
 
> > 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.

They're calling that standard "42V", but it's 36V nominal (18 2V cells
in series, equivalent to three 12V batteries), rising to 42V when the
alternator's running.

48V (24 cells; 52-55V float) is a commonly used for for telecom
equipment.  Some countries use a 60V (65-68V float) telco voltage
instead.

  
> > (Interestingly, though, most electronics and CFL lighting is perfectly
> 
> Any idea about HQI, with electronic ballast?

Don't know... I've never tried that type of lighting.
 
> > 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.

It's best to have a look inside first.  Check where the incoming AC
line goes-- if it connects to anything but a diode rectifier bridge,
don't try it.

Measuring resistance across the AC power contacts (while any 'hard'
power switch is turned on) can be helpful.  Any reading below a few
hundred Kohm would be worrying.  Above that, it may not work, but
shouldn't blow up if you try.  Half-wave rectifiers will want a
particular polarity; full waves don't care.

> > 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.

Consider also a mini-ITX board, or laptop w/ automotive adapter.

> > 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.

Good idea. I removed the last of those a few months back.  A few
devices depend on the wall-wart's transformer isolation, and need 
extra work to avoid conducted interference, ground loops etc.

-- 
Jordan.