pete wrote:
On Sat, 3 Jan 2009 07:02:19 -0800 (PST), wrote:
pete wrote:
On Sat, 3 Jan 2009 04:07:57 -0800 (PST), wrote:
zaax wrote:
Adrian wrote:
I'm guessing that the cheapest devices are going to have very
short-lived batteries in them... what's a sensible price-point for a
decent solar lighting kit ??
Thanks
Adrian
Solar lights doen't work in the UK, too dark in the winter.
Correction: inadequately designed solar systems dont work in winter.
Just design the system to give enough output under winter conditions..
NiCd and NiMH have significant self discharge, the cheaper lead acid
will work better.
Some diffuse concentration on the panels for winter only can make a
big difference to how much panel you need.
Forget about cheap �20 kits, they're only rated for summer use, and
power collection, storage and light output are all fairly pathetic.
OK, here's a strawman spec. - hopefully other people will add their
experience to it.
ok 
Lets see if we can tidy it up some.
The OP says he'll use high-intensity LED. Let's say that's a 1 Watt type
ok
and that it'll be asked to provide, worst case, illumination from 4pm to
midnight in the winter.
ok, you could but I'd sooner say no to that. Solar power is a premium
resource, especially in winter, so we dont want to waste 90% of it.
Instead we go for a PIR light, set to run for say 3 minutes. Say there
are 6 in&outs after dark per day on average, so thats a total of 18
minutes run time per day.
That's 0.3Wh/day, down from 8Wh. A factor of about 25 right there.
(After that presume it doesn't matter if it runs
out of puff - there won't be anyone around to appreciate it).
That's 8 Watt-hours per day - so the solar panel should provide (say)
10 WHr daily to charge it.
25% loss is standard for the lead acid, but you've also got to take
into account the charge controller and LED ballast.
Lets say 5% loss on charge controller, 10% on the ballast and 25% in
the battery. Now for 0.3Wh per day we'll need 0.3Wh x 1.4 = 0.42Wh/day
Now, today is a perfectly clear winter's day and the sun is about as high
as it ever gets (at 51 degrees north) in the winter. A quick measurement
with a 6cm x 4cm solar panel gives me 3.7 Volts across a 1K resistor - or
about 14 milliWatts for a 24 cm2 area.
Why have you used a 1k resistor? Can you seriously get no more current
out of it?
I used a 1k resistor, as the load (i.e. 1K) should be the same as the
internal resistance of the panel (in this example, 800 Ohms) to transfer
most power into the load.
Ah. True for an ohmic source, but a solar panel is a very different
animal.
In practice, the load used by the OP will be different,
however the power per square centimetre will be the same - presuming he
uses the same solar panel technology.
Have you not used a low level of concentration - god knows the panel
needs it at this time of year.
What is this "concentration" thing?
1 sun's fine in summer, but in winter concentrating the light hitting
the panel makes a major difference. A _very_ shallow cone that doesnt
block skylight can add some diffuse skylight, or a reflector can add
more direct sunlight when available. Remove the reflectors when the
sun gets hot.
On the outrageously optimistic assumption that during the winter, you'll
average ONE HOUR of charging at this rate per DAY,
I dont see any basis to assume the rest of the day will deliver zero.
True, however the output drops _very_ quickly when the sun goes in.
The sun's up for more than 1 hour round here - though it can feel that
way some days!
Also
the position of the panel will (probably) be fixed, whereas for maximum
output it should rotate to follow the sun, otherwise its output will depend
on cos(angle) between the panel and the sun. When the voltage from the
solar panel drops below the battery voltage, it won't charge the cells
even though it's still capable of supplying power.
Well, that depends on your charge controller. For such tiny powers I
doubt its worth using a charge controller, in which case yes, as you
say. Since this is a real issue in winter, you could maybe pick a
battery with a bit lower voltage than usual for the panel, eg 9v
battery on a 12v panel. That would give a little more dull day
tolerance. Battery capacity will be the main defence against dull days
though.
NT