Small project for school.

Old Guy · December 2012

Hi everyone. First time poster here.

I am the maintenance man for a small grade school and the teachers would like to have a simple solar system for educational purposes. I have electrical experience but no solar.

Parents have donated one 100 watt Grape solar panel from Home Depot and one Duracell GC12 golf car battery from Sams Club.

This is what the Teachers have asked for:

They would like to have the panel mounted outside the classroom where the children can see it (about 30' away from the battery).
They would like to use it to run a small 12v radio and optionally a small 12v pump for a aquarium if possible.

I dont even want to try to explain all the technical details to them and am trying to keep this very simple so I have a few questions.

1. Is one 100w panel enough to keep the battery charged? I realize you need to know the draw so let me ask this way... If the battery were discharged to say 50%, how long would a 100w panel take to recharge the battery under ideal conditions? The only rating I see on this battery is 75 amps @ 77 minutes.

2. From reading other posts it seems that a charge controller will be necessary. It would be nice if the children could see the charging activity so I was looking at a unit that has a display like a Steca PR1010 or a Sunforce 60031. Would either of these be OK?

3. Is a 10a controller sufficient for this project? Should I get a 30a controller in case they want or need more panels in the future?

4. Can a charge controller provide power to a device while it charges the battery? The biggest problem I see is they need to use everything during school hours (daylight). I am fairly certain the draw from the radio and pump will be much lower than the 100w panel output. They can also limit the usage time of both if necessary.

I realize there is a lot more to this but I need to keep it as simple as possible and have it work safely (even every other day to allow for charging time would be fine).
I honestly dont have a clue here so please forgive me and let me know if I am way off base here.

Any guidance or advice would be appreciated!

Thanks!

BB. · December 2012

Re: Small project for school.

The Grape 100 watt panel has Vmp~18.5 volts which would mean that Imp is:

100 watt * 1/18.5 volts = 5.4 amps

You do not need large charge controller for that one panel--Steca is a very good brand, so if you can swing that, it would be a good choice.

The Duracell battery is a 12 volt 155 AH battery at 20 hour rate. Normally, we would suggest around a 5% to 13% rate of charge:

155 AH * 0.05 = 7.75 amps minimum
155 AH * 0.10 = 15.5 amps nominal
155 AH * 0.13 = 20.2 amps "maximum cost effective"

So, from the point of view of the battery, you really should be looking at 2-4 100 watt panels for "optimum" performance and battery life. One panel is not really enough for other than float charging and perhaps a little bit of loads (radio will not be much, need more information about pump power requirements).

Running a long cable from the solar array to the battery bank is going to be a bit of a problem too--Using a voltage drop calculator for 3% maximum voltage drop (~0.6 volts) for 5.4 amps would suggest using a minimum of 10 awg wiring (0.4 volt drop)... If you have more panel, the awg will get much heavier and more expensive (3 panels in series would be 16.2 amps and a minimum suggested awg of 6 awg wire for 0.5 volt drop).

Then you have the whole safety issue of a relatively large 12 volt lead acid battery. This is not really "safe" for kids to play with as it can easily supply enough current to almost be an arc welder. Plus hydrogen gas from charging and any spark that can explode the battery/spray diluted sulfuric acid around).

So the whole thing would have to be installed in a locked/vented box.

My suggestion, if this is still a project you wish to tackle--I would go with a much smaller battery--Somewhere around around 25 amp-hour capacity (could go to 50 AH @ 12 volts--but I would keep the energy available and costs down towards the 25 amp range). That would let you use just the one 100 watt panel easily. Be much safer (much less current available), and much less cost to replace every year or two (depending on how well the battery is taken care of and what is done in school). You still should use a small circuit breaker to limit the output current of the battery (assume that kids/teachers will do things "wrong" during the experiments.

-Bill

Old Guy · December 2012

Re: Small project for school.

Thank you BB!

I suspected there was a lot more to it (as usual). :roll:

They will not be doing any experiments. These are very young children and this is a "green" thing the teachers want to promote. I will look into a much smaller system.

niel · December 2012

Re: Small project for school.

there's even more to it than that as you'll need fusing and disconnect switches too. as far as metering goes, it may be better to have a battery monitor to show the power in and out of the battery and not just what the pv will produce.

this sounds like a nice project that i'm sure will fascinate some of those students and will stir up something in their curious and inventive minds. problem is getting a good setup together that will also address the many other aspects solar entails, one of which is safety and all will need to learn a bit about that.

Old Guy · December 2012

Re: Small project for school.

Thank you Niel.

After seeing all this it became apparent they did not understand the complexity of the issue. Maybe they can return the items they have.

I found this small solar panel that runs a little water pump at Harbor Freight. http://www.harborfreight.com/solar-powered-fountain-pump-66093.html.
They can set this in the window, observe the effects (and experiment with it like BB suggested). This should fascinate the children and make their frog happy!.

You guys are great. Thanks again for your time and input.

Small project for school.

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