mike wrote:
Anybody got any real data on this stuff.

There's no shortage of information about this. Useful
keywords are "insolation" and "solar insolation" (the word
"solar" is slightly redundant but it's commonly included).
In summer, you can expect a maximum of 1 kWatt per square
metre to reach the surface of the earth. The units most
commonly used are kW-Hour per square metre per day - I'll
call them Units here.

Insolation tables for the USA can be seen at:
http://www.suntrekenergy.com/sunhours.htm
These figures are somewhat suspect - the difference between
"high" and "low" seems too small (a maximum of 6 Units is
rather low), especially when compared with the following,
which contains some good maps:

http://www.wattsun.com/resources/ins...map_index.html

On this page, click on Flat Plate Collector, Single Axis
Tracker and Double Axis Tracker. The latter can produce up
to 14 Units in summer. The improvement when tracking the
sun's angle is very large. It pays to live in California.

I have seen a similar table somewhere for the UK, showing
that 5 Units is the best that can be expected, and maybe
less than 1 Unit in winter.

Bear in mind that the efficiency of Solar Cells is less than
20% in the very latest state-of-the-art devices, typically
10%, and maybe as low as 5% in reject/hobbyist cells.
Generating hot water directly from flat solar collectors is
probably more efficient, and certainly cheaper, but not much
use if it's electricity you want.

If, on a bad day, the cell voltage is less than the battery
voltage, you can still charge the battery. Look at:

http://www.elecdesign.com/Articles/A...262/6262.html#
This article appeared in Electronic Design, Sept 14 1998.

It describes a circuit for a Maximum-power-point-tracking
solar battery charger. The principle is simple: the
duty-ratio of a switch-mode power supply is continuously
modulated at about 50Hz. The change in output on each cycle
is used to determine whether a higher or lower duty-ratio
would increase the output power. A phase-sensitive detector
and feedback loop determines whether to increase or decrease
the average duty-ratio. It settles at the point of maximum
power.

As the article points out, it works for other energy sources
such as water-wheels and other devices where the shape of
the "energy curve" is not precisely known.

When used as a battery charger the voltage of the battery is
fairly constant, so "maximum power" means "maximum
current". At the solar cell end, we are working at maximum
power, although the voltage may vary. The "maximum power
transfer" condition is when 50% of the power goes to the
load, and 50% is dissipated in the cell. I don't know if
this is precisely true in a solar cell, but it certainly
implies considerable power dissipation in the cell, which
may shorten its life. On the other hand, a cell of 1 square
metre will have 1000 watts of solar power falling on it, and
may generate 100 watts of electrical power, of which we may
get 50 watts into our battery. The 50 watts dissipated in
the cell is much less than the 1000 watts from the sun - so
maybe it doesn't matter.

J.S.Blackburn,
London UK.