solar power in the shade

[zpoff]

I'm working on an outdoor art project that requires solar power under a tree, and therefore shaded at all times. The only panel-mounting location is strapped to the trunk of the tree and we don't have the option of running a trench or overhead lines out into full sun. The current consumption is on the small side. (The load will be a small mp3-player contraption with a 5 watt audio amp, average 600 mAh.) I've done a few small projects in the past with more typical arrangements (15watt panel + controller + SLA in full sun) but I'm wondering if anybody has advice for operating panels in the shade. We have some money to throw around so we can "oversize" the panels to offset the shade handicap, but by how much? I'm thinking of a 40 or 50watt panel (rated 2.5 to 3A in full sun) but I'm wondering if a 12v charge-controller would even cut-in considering the shade. Panel composition is another question. Thin film seems to be the choice, considering its friendliness to shade, but would a thin film still beat a crystaline cell when the whole panel is shaded? (Put another way: Isn't the mono/poly crystaline shading issue more related to the differential between cells across the panel surface?) Anybody have any advice? (besides move the panel). Thanks.

[wdyasq]

Could you possibly direct light to the panel? A slightly oversize and parabolic reflector could direct light to the panel - PRESTO, full output.  Would need to be refocused or be on a tracker.

Ron

[alcul8r]

Is there some artistic reason for not wanting the PV in the sun.  Guess you could run a drop cord out and shine the light on it.....

Seems kind of like pointing a fan at your wind generator to run it indoors for the science project.

Wire is cheap for that size PV.

Rex

[ghurd]

Well, move the PV.  OK, I got that out of my system.

How much of the 5W amp are you using?  5W is fairly loud, I was told standard factory car stereos are only about 7W actual.  I did about the same thing with small 12V computer speakers and an MP3 player. Seems like the entire 12V draw was <300ma max.

"Isn't the mono/poly crystaline shading issue more related to the differential between cells across the panel surface?"  I believe thats correct.

Also, maximun exposure to the sky is important.  It could be better to point the PV toward open southern sky than straight through the leaves to where the sun is at noon.  Likewise, it could be better to point the PV toward the clear shot at a rising sun for 3 hours, or at a setting sun for 3 hours.

What is the purpose of the art or solar?

Will it run 24/7?  How big is the battery?  Is this expected to run 3 days or 3 years.

G-

[dboller]

climb the tree, stick a pole up in the air above the tree canopy with the panel on it, strap the pole to the nearest limb and run the wire down.

[Argie]

What happened to that Canadian new type of solar panels that claimed to produce power in the shade and for at least 16 hours every day by using only the U.V. part of the spectrum?

It came up sometime between July and November last year on the news but I never heard about it again.

[willib]

You could also point the panel twards a parking lot if possible , there is usually a lot of reflecion off of cars,

you havnt discribed what is around the tree in question?

buildings? , i would assume the tree it isnt out in the forest?

also large white  buildings give off a lot of reflected light.

[richhagen]

I noticed in the past that my amourphous type panels started putting out a tiny current before my mono and poly crystaline panels.  I much prefer the mono and poly crystaline panels, but for your application and handicapping of the panel, something like a Uni-solar US-64 might work better.  That is if you are that opposed to moving the panel, or chopping a bunch of branches. :-)  I suspect some experimenting will be in order for your application.  Rich

[zpoff]

Replying to myself - Great suggestions everybody. Thank you. Here's more detail...It's actually somebody else's art (I'm just working on the electronics). It involves recordings of humans making bird songs, based on phonetic representations of the bird songs. (Think interspecies "telephone" game) There are multiple trees being electrified, spread across a few acres with no buildings or cars or grid in sight.  Each needs to run maintenenance-free for 5 months, starting in june in the northeast. We're not shooting for 24/7 but the battery needs to carry us throughout dusk and cover cloudy gaps during the day. Battery capacity hasn't been nailed down. I can't measure current consumption because the player/amp hasn't been nailed-down either. (The 600mAh from the original post is my worst-case estimate based on full-scale clipping on my own 10watt bench amp + the published specs of the industrial mp3 players in question) The "pole lashed to branches" technique might be the perfect plan B if a shaded panel doesn't cut it. That's one of those "why didn't I think of that" ideas that makes me glad I asked! The UniSolars are crystaline panels with a bypass diode on each cell, right? Would it make a difference if I used a 24v panel (at the same wattage)? I know "watts is watts", but is the controller looking for the panel to cross a voltage threshold or a current threshold before it kicks in? (I suspect it doesn't matter as the sides of ohm's wonderful law balance out).

[ghurd]

The UniSolar Rich mentioned is amourphous silicone.  The US-64 has a large area, guessing 2 x 3'.

My Sony MP-3 player uses very little power, runs ~40 hours from a single 'AAA' battery.

The self driven 12V speakers used less 12V battery amps than a lower wattage 9V pair run from a 7809.

The speaker amps vary depending on the sound (or no sound) being produced, and naturally the volume. The ammeter looked like an equalizer bar going up and down with the music.  No sound took very little from the battery, 20ma. if I remember right, but the devices surely vary.

Don't series two 12V PVs for 12V use. That would be a waste of panel.

Don't use a blockibg diode with a controller.

I personaly like the Morningstar SG-4 controller for low power use, like 64W or less.  Nice & simple, PWM, temp compensation, cheap ($30).

I would expect a 12 to 15AH SLA battery, SG-4, and US-32 PV (if you can find one- they have been discountinued) would give excellent results. Naturally, it would be better if the panel was mounted for a hour or 2 of sun a day, and if the audio devices were shut off at night.

It would be cool to see a couple pictures when it's done.

G-

[zap]

I know it's not yet April 1st but why not use the trees for power.

[Clifford]

I presume it depends on the setting...  

Deep Forrest vs relatively open with shade.  I presume you would point the panel in the direction that you have the most ambient light.  Don't try to shove the entire panel too deep into the canopy.

I have panels mounted flat on sloped roof.  It isn't as efficient as I would like.  However, I've compared sunny days vs cloudy days.  

I get about 20-30 Ah in sunny days, and about 5 Ah on the darkest cloudy days.  I presume that in the shade you get a high amount of reflected ambient light.

Volts are normally ok.

So, I would guess that if you oversized your panels by about a factor of 10, then you would probably do ok.

If you are looking at approx 5 watt need, then you might consider a 50 - 200 watt panel.

What is your budget?  The sky's the limit?

I have seen ads for double sided solar panels (bifacial space).  I assume that the silicone wafer is coated on both sides.  It makes sense if you want to minimize surface area.  

Search on E-Bay for:    solar panel* (bifacial,space)

Try either the 50/30 panel or the 90/60 panel.

As always, nothing beats emperical testing.

[Gary D]

The biggest concern I'd have would be theft of the panels. As an art exibit it kind of exposes you to this possibility.

 On a lighter note, it would serve the birds right to irritate them as much as a mockingbird did by repeating our portable phone ring a few years ago, chasing us to answer um no one. Took us a few days to figure that one out... Gary D.

[The Crazy Noob]

Isn't what they are using the same as a earth battery? (powered by "ether"?) I don't think that you realy need trees for that purpose...

[ghurd]

What happened?  Same thing that always happens to the new 1 cent per KW of solar panel.

It didn't exist.

[Clifford]

I might have to try a "tree battery".

I wonder if the tree has to be living, or if a wooden fence post would do the same.

I believe that all living cells create a potential energy difference across their cell walls using sodium and potassium pumps.  However, this sounds like something different.

Another discussion I saw about trees earlier was to use the natural swaying of the tree in the wind to generate electricity.  Presumably connect a hydrolic cylinder to the tree and the ground (or the equivalent mechanical linkage) and use it as a "natural windmill".  There is probably a bit of wasted energy, but you might be able to make a small, unique generator.

[Argie]

Don't you remember those experiments using potatoes?

The same difference so I fail to see how it could be patented.

It has to be a joke.