Solar-powered lights - recommendations please ??

[Adrian]

HI Folks

Another of those projects that's been on the back-burner for a while is devising 'something' to finish off the stone-built gateposts at the entrance to our place.

Suddenly - bright idea - literally !

Considering making a couple of 'lanterns' with stained glass - powered by solar-recharged high intensity leds (it's a long way to run a mains cable down to the gates from the house).

Mostly for decoration (and for a bit of fun!) - not expecting quartz-halogen-style illumination !

So - any recommendations for solar lighting kit that people have actually used and found to be effective. I can see that prices range from the ridiculously cheap up to 20 quid or so for something that's probably a bit more useful like this from CPC

'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

[zaax]

Solar lights doen't work in the UK, too dark in the winter.

[Tony Bryer]

IIRC also a function of low temperature

[Andy Dingley]

"Decent"? No idea - too subjective.

24 quid gets you a "Tritronic" PIR security light with two nice 3-LED lamps, a 6" square panel and 3 C-cell NiCds. PIR doesn't work in cold weather, but it's quite a good set of parts for a lighting set

Otherwise Maplin's January sale for panels (excellent deals last year) and Ultraleds.co.uk for the LEDs themselves.

[Dave Liquorice]

Niether will the NiCds. Really need Li-Ion but I don't think the charging requirements for those are quite as easy as NiCd.

[meow2222]

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 =A320 kits, they're only rated for summer use, and power collection, storage and light output are all fairly pathetic.

NT

[pete]

OK, here's a strawman spec. - hopefully other people will add their experience to it. The OP says he'll use high-intensity LED. Let's say that's a 1 Watt type and that it'll be asked to provide, worst case, illumination from 4pm to midnight in the winter. (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.

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. On the outrageously optimistic assumption that during the winter, you'll average ONE HOUR of charging at this rate per DAY, 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.

A quick shufti on eBay throws up a 1 square metre (1.49 * 0.67) panel for grabs at £360. Interestingly, this is rated at 100 Watts - gotta love that sense of humour! So it looks like we're talking about £500 just for the solar panels. However, I can't see this being a starter: once the OP realises just how much acreage is involved, and (due to it having to face the sun) the near impossibility of disguising the panels, or siting them discretely so they aren't an eyesore.

[Andy Burns]

My laptop won't allow itself to be powered-on (even when plugged in) when when the li-Ion battery is cold, no doubt the battery has internal protection, not sure whether it doesn't want it to be charged while it's cold, or that it doesn't it want to supply power.

[The Natural Philosopher]

actually, they are easier.

Basically the same as lead acid. Limit the current, to no more than the one hour rate, and limit the voltage to about 4.2v per cell.

They are still crap in sub zero conditions though.

[ARWadsworth]

> I'm guessing that the cheapest devices are going to have very

Solar powered cats eyes?

[meow2222]

ok :) Lets see if we can tidy it up some.

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.

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 =3D 0.42Wh/day

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.

I dont see any basis to assume the rest of the day will deliver zero.

well, by now of course this is miles out.

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.

[Adrian]

Thanks for all the comments so far.....

fwiw, I _do_ have trees nearby - and I'm intrigued to see what you were thinking of doing with them

Adrian

[pete]

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. 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.

What is this "concentration" thing?

True, however the output drops _very_ quickly when the sun goes in. 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.

[Jules]

I think the ambient temperature only changes the field of view, doesn't it

- they don't just stop working altogether. We've had many a cold evening here recently where it's hit -30C (I'm a brit living in the wilds of Minnesota these days) and the PIR sensor on our porch security light has been just fine.

cheers

Jules

[The Medway Handyman]

???

I'm intreegued as well :-)

[Jules]

Hmm, the gadget that ET built to phone home was powered by a tree branch, wasn't it...

Aha: tree branch -> dynamo -> light bulb -> solar panel -> solar lighting? :-)

J. (in an inefficient mood today)

[meow2222]

He's close... tree movement will never drive a dynamo though. You need a mini gen that will produce useful power at under 100rpm, ie a stepper motor from a 5.25" floppy drive or a printer.

Attach plastic string to tree 3/4 way up. String comes down at 45 degrees and is looped once over motor shaft. End of string has a weight on, needs to be sufficient to drive the motor as the tree comes back.

Now, the wind blows the tree, the tree sways, and the stepper produces pulses of relatively high current low voltage output. Pick a stepper that's not stiff, many of the printer ones are very stiff.

Parts cost: a bit of string and a few diodes to rectify the output Advantage: no cost, more output in winter than summer, and with nylon. fishing line its almost invisible.

NT

[Richard Russell]

He also said "mostly for decoration". A 1 watt high-intensity LED would be far too bright for that application, IMHO. My assumption is that what he really meant was 'high efficiency', not 'high intensity'.

I've made basically what he described, except that rather than being for decoration they are to show where the pillars are at night (no street lighting in the vicinity). I bought, very cheaply, a couple of 'solar powered shed lights' similar to the ones linked to in an earlier post. They're designed to give out a reasonable amount of illumination for a short period, so I modified them to give out much less light for a much longer period (five white LEDs at about 1mA each, I think). I also converted them to turn on automatically at night and off in the day (don't ask for the circuit; it relies on germanium transistors I happened to have in the junk box!).

They're working well at the moment. In the depths of winter the solar panels don't provide enough charge to keep them lit all night, but for most of the year they do.

Richard.

reply by email change 'news' to my forename.

[meow2222]

Ah. True for an ohmic source, but a solar panel is a very different animal.

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.

The sun's up for more than 1 hour round here - though it can feel that way some days!

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

[Jules]

The power source seems analogous to a tidal plant - strong, but intermittant, slow and variable over a short range of movement. Can't help thinking that there must be inherent inefficencies in involving the tree though, and there must be far better ways of capturing the wind energy that's moving the tree in the first place...