Basic Questions About Small-Scale Solar System For Under $300

solarStudent

Hello Everyone,

Solar novice here, just wanted some pointers and feasibility judgements about my small scale solar design.

The idea here is to use a solar panel to charge a 12-volt battery. Once every 2 hours or so, the battery would need to drive a water heating element to generate about 200 watts to boil water for 20 minutes.

I plan on using a pre-built 12 volt battery charger. How do I determine the specs for such a device, to ensure that it will remain charged?

Can I hook up the charger to the battery without a charge controller, and the battery directly to the heating element?
Here are the types of elements I'm looking at:

Solar panel charger: http://www.batteryjunction.com/6wpanel.html?gclid=CNqb9a28jbwCFa9lOgodaRcAlw

12 volt battery: http://www.walmart.com/ip/25955654?wmlspartner=wlpa&selectedSellerId=3&adid=22222222227018915744&wl0=&wl1=g&wl2=c&wl3=35550451356&wl4=&wl5=pla&wl6=53508072156&veh=sem

Heating element: http://www.ebay.com/itm/DC-Water-Heater-Element-for-wind-turbines-and-solar-setups-12-volt-300-watt-/400520753250?_trksid=p2054897.l4276

Anything that will prevent this from working?

Cariboocoot

Re: Basic Questions About Small-Scale Solar System For Under $300

Welcome to the forum.

Math. It's not fun, but it's necessary.

200 Watts for 20 minutes is 67 Watt hours. On 12 Volts that's 5.6 Amp hours for one 'cycle'. Now how many cycles do you propose in a day? If it's every 2 hours for a whole day that's 12 cycles, or 67 Amp hours used.

The battery you point to is not deep cycle (hence the cold cranking Amps rating) and would not be suitable for this application. You'd need something along the lines of this: http://www.solar-electric.com/cr185am12vod.html

You'd also need some significant panel to recharge it. The panel you point to wouldn't do anything for the amount of battery required to supply that much Watt hours as it is only 400 mA. You would be looking at about 475 Watts of panel and a 30 Amp controller.

Really, when you look at the Watt hours you expect to use and work out the battery and recharging capacity from there (the only way to do it) you find this small equipment doesn't really do the job.

bill von novak

Re: Basic Questions About Small-Scale Solar System For Under $300

solarStudent wrote: »

The idea here is to use a solar panel to charge a 12-volt battery. Once every 2 hours or so, the battery would need to drive a water heating element to generate about 200 watts to boil water for 20 minutes.

Assuming this runs 12 hours a day, then you are looking at 2 x 16 amp = 32 amp-hours. For a 5 hour charging day (not a bad average) you would need 6 amps from your panel. Accounting for losses you're looking at 8-10 amps, which means a 180 watt panel. Below you have a link to a 6 watt panel, so you'd need 30 of them. You have also listed a car battery which won't work. You will need a ~60 amp hour battery rated for deep cycling.

I plan on using a pre-built 12 volt battery charger. How do I determine the specs for such a device, to ensure that it will remain charged?

A Morningstar 10 amp controller will run you around $50. If you can live with 120 watts of element power you can use the LVD in the controller, which will help prevent you from killing your battery.

Anything that will prevent this from working?

You don't have nearly a large enough system (battery or solar) to make this work.

solarStudent

Re: Basic Questions About Small-Scale Solar System For Under $300

Thanks for the replies!

I've been reading a lot of thread and I've got a few follow up questions to clarify my understanding.

So I see that the car battery I pointed to would not work, as it is not deep cycle.

However, let's say I had the proper battery.
Every day, I would need to put 67 amp-hours of charge back into the battery. If I had 12 hours to do it, wouldn't that mean I need a panel that can provide 5.58 amps per hour. Given that the battery is 12 volts, wouldn't a 70 watt solar panel be good enough?

Cariboocoot

Re: Basic Questions About Small-Scale Solar System For Under $300

Unfortunately it is not that simple because charging isn't linear. It is not a case of a panel supplying 'X' Amps for 'Y' hours amounting to 'XY' Amp hours. And there is almost no chance that you will have 12 hours to do it in because the peak output of a panel is dependent on direct, full sunlight not sun at an angle shining through a lot of atmosphere. This is why we speak of "equivalent good sun hours".

You need to start by working out how many Watt hours the battery has to supply, and keep that to number that's less than 50% of the battery's total capacity as discharging deeper than that soon results in a battery that doesn't recharge.

Recharging is not merely Amps over hours either; there has to be sufficient current to actually bring about charging, and most battery manufacturers recommend that to be at least 5% of the battery's '20 hour' Amp hour rating (not including any simultaneous load draw). Beside that it must charge quickly enough (high rate) to recharge the battery within the actual time period of sun available, knowing that as the battery comes up in charge the rate will decrease. It takes longer to 'put back' that last 20% than the first 20%.

westbranch

Re: Basic Questions About Small-Scale Solar System For Under $300

since you appear to be in Canada, it is better to use at max 4 hours of GOOD charging time per day, over the whole year... so in winter depending on your location you may get as little as 1/2 hr charging per day or less...
so if you use 4 hours you will be in the ball park.
Nest you have to account for line losses Charge controller losses , inverter losses, and general panel production not the perfect conditions found in the lab that tested the panel for wattage rating. Use 73% for all these together...

hth

waynefromnscanada

Re: Basic Questions About Small-Scale Solar System For Under $300

Unfortunately, using solar electric power to heat water is a very uneconomical use for solar. You're looking at a system that, not including ANY of the normal losses in such a system, must supply an average of 33 watts, 24/7, which adds up to almost 800 watt hours per 24 hour day. And remember, that's NOT taking into consideration any of the losses such systems have. The actual wattage required will be substantially more.
By comparison, that 800 watt hours per day would run 4 of my fridges. (Converted upright freezers)
Not a "small" system and would definitely cost far more than just $300.
We are sorry to be bringing you such bad news, but if we are going to be honest and fair with you, we must present a truthful view of the situation.

Cariboocoot

Re: Basic Questions About Small-Scale Solar System For Under $300

Another way of looking at it:

The panel linked to is 6 Watts. If there were no losses at all, in 4 hours of daylight it would harvest a maximum of 24 Watts hours. How well would that do for running a 200 Watt element for 20 minutes?

The power you use has to be harvested from the sun by the panel(s), plus extra to account for all the system losses.