Please help a newbie (I am clueless)

System · November 2008

This is my first experience with solar. I have been thinking about it for a while. I have read many discussions and articles on the subject, but most of it is above my head. I have no technical knowledge, so please bear with me. I am trying to heat my small green house (10' X 12') with solar. What I have purchased so far is a 15 watt- 12 volt panel, the recomended 4a controler for the panel, a 1000w marine inverter (I figured with the humidity it would be best), a deep cycle battery (not sure of the amps, etc.), and an oil filled electric heater with 600, 900, and 1500 watt settings. Last night was the first night I tried it. Needless to day when I went out this morning the battery was too low to run the heater. What do I need more batteries, another panel? If I need another panel do I need another controler?

mike95490 · November 2008

Re: Please help a newbie (I am clueless)

For heating anything, you are much better off (about 60% more efficient), going with solar thermal, using the sun to warm water, and store the warmed water. You can use your electric parts to run a small water pump, but solar PV for electric heat is VERY inefficient.

You might want to look at the solar shed article for more inspiration.
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/collectors.htm
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

and maybe this string of posts:
http://forum.solar-electric.com/showthread.php?t=2809

Cariboocoot · November 2008

Re: Please help a newbie (I am clueless)

You need to spend a ton of money.
Heating with electric is not recommended; it is very inefficient.
That said, I've been working on the design for just such a system for the garage I'm building in the Frozen North. As it is impractical to stoke a wood fire for the garage all night and not a good idea to have an open flame anywhere gasoline is stored, electric is the safe alternative. But when you do the math, it's disheartening.
To run a 600 watt heater off 12 volts (via inverter) requires a significant current draw on the DC side: 50 amps. That means each 50 amp/hrs of battery you have will run the heater for 1 hour. Since you don't want to take a battery past 50% discharge, you actually need 100 amp/hrs of battery for every hour you expect to run that heater @ 600 watts (more, of course, at higher settings). 12 hours over night = 1200 amp/hrs of battery. That is a LOT of battery!
To recharge such a bank you need a charge rate of 5%-10%, or a minimum of 60 amps of (peak) current to recharge in a reasonable amount of time. 60A @ 12 volts is 720 watts; a long way off your 15 watt panel.
Running direct off DC won't save you anything either, as the inverter is a miniscule portion of the usage.
The better insulated the building, the less the heater will run. Greenhouse walls are generally not known for their insulative factor, being basically a bunch of windows.
So you'll need to buy like (rough estimates) $4,000 worth of panels, a $600 MPPT charge controller, $2,400 worth of batteries ... Like I said: a ton of money, just to heat a greenhouse.
Time to consider Plan 'B'!

BB. · November 2008

Re: Please help a newbie (I am clueless)

Short answer is you have way too few solar panels/power, probably not enough battery, and way too much load.

First thing, put the batter on a car battery charger--taking the battery down to 0% state of charge (near dead), will mean that this battery will only last a few weeks or months.

Second, solar electricity is very expensive electricity... Using it to power any sort of electric heater is a waste of money--and unless you have a large system ($10,000 of thousands of dollars or more), virtually impossible to get any useful heat out of it.

When everything is working, off grid solar electricity will cost you around $1.00 or more per kWhr... You are probably paying $0.10-$0.20 per kWhr...

Next, into the details... Your 15 watt solar panel will run your 1,500 watt heater for, roughly, 30-45 seconds per day (based on charging for a sunny day).

To run your electric heater for, say 8 hours per night (just a guess), you would need about $60,000 worth of solar panels and $30-$50,000 worth of batteries in the winter (plus you would still need a back up generator for backup power).

Just for a reference, your 15 watt panel would run a 7 watt nightlight for about 4 hours per night.

So, backing up a whole bunch... What do you need solar electric for, and how much are you willing to pay?

If you need to heat a green house--solar thermal would be a better bet--and lends itself to home building the collector panels and water storage tanks--but for a large green house (depending on temperature and amount of heat needed)--it still would take $1-10,000 to rig something up.

-Bill

Macaw · November 2008

Re: Please help a newbie (I am clueless)

Yo Tomato
I am not an expert but am doing ok with my 10X12 (Vegetable) greenhouse without extra energy. Four things are required for a successful greenhouse of this small, non-optimal size.

Insolation
Insulation
Thermal Mass
Humidty Control / Ventilation / Air circulation

For Insolation I have good sun between 9 and 3
For Insulation I build a house with Solex Top and Sides with 2" thick foam insulation on North wall
For Thermal Mass I have eight 55-gallon drums full of water, painted flat black, against the north wall of the interior of the green house.
For Circulation I have a fan that runs 24/7, otherwise mold problems might arise
The temperature inside gets down to 32F when the outside temperature gets down to 20 F at night. No problem for lettuce leaves, not to mention broccli, swiss chard ect. In the daytime it can get to 80 when outside temps in the 20's. Must have vent doors to open on warmer spring days or else you cook your crops before you know it.
Bottom line from a solar energy viewpoint- Build it right and you don't need any (very little) extra energy. But then again if you are growing orchids in Alaska in the winter...then solar hot water heating is the only way to go.

SolarJohn · November 2008

Re: Please help a newbie (I am clueless)

Tom,

I see that you are a gardener. You might want to cosider growing your own fuel. I use a corn-burning stove to supplement the natural gas furnace in my home. If you have the ability to grow and harvest 1/2 acre of corn, that might be enough to heat your greenhouse all winter long. Try this website for more information: www.iburncorn.com
I've also written about my corn-burning experiences on my blog: http://solarjohn.blogspot.com
You'll have to dig through my archives.

I would also like to learn more from you about growing tomatoes indoors (if that's what you do).

Good luck!

John

Cariboocoot · November 2008

Re: Please help a newbie (I am clueless)

You might want to look here for some interesting "passive" solar plans:

http://www.builditsolar.com/

I'm thinking of adapting some of these ideas for my garage.

Please help a newbie (I am clueless)

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