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?
Have you not used a low level of concentration - god knows the panel
needs it at this time of year.
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.
you'll need about 700
times the surface area of solar panels that I used, i.e. 17,500 cm2.
That's 1.75 square metres of panel to provide enough charge to keep 1 LED
lit for 8 hours.
well, by now of course this is miles out.
£360.
£500
if you make up some basic concentration and see what current you can
get out of it we can recalc what's needed and add a margin.
NT
PS if the OP has a tree anywhere nearby we could go with a power
source that gives more output at much less cost.