Heating my greenhouse

[takingtime1]

Have a greenhouse 16'x 8'x7' (896 qf) I wish to use 3 solar panels @ 15w ea. (45W) through a charging regulator to a set of batteries @12v to power my 1000w continous MSW inverter to power a 1000 watt coil heater. I plan to use 4 deep cycle marine batteries giving me about 400 ah. the heater will have a thermostat set a 54 deg. and should be on about 7 to 9 hours (nights), will this setup work ?

[solar_dave]

You are better off adding thermal mass wall to the back of the greenhouse to absorb day time radiation. I knew a guy in Colorado that used milk jugs with water in them.

45 watts of solar panels will do nothing to run a 1000 watt inverter or charge 400ah of batteries. In a short time the batteries will be DOA.

[Cariboocoot]

Welcome to the forum.

If you want to see the math it looks a bit like this:

45 Watts of panel * 4 hours of good sun / 2 = 90 Watt hours of AC power.
That would run a 1000 Watt heater for about 5 minutes.

The batteries could supply 200 Amp hours @ 12 Volts or 2400 Watt hours.
That would run a 1000 Watt heater for about 2.4 hours.

Neither includes the system efficiency losses.
Electric heat is terribly inefficient. Solar Dave's suggestion of mass storage (and a bit of insulation) will work far better.

And stay tuned, for next year I'm supposed to redesign a conventional "kit" greenhouse into something that will work in the Cariboo without suffering from the 50C daily temperature swings. Don't ask me how I'm going to do it, but she who must be obeyed commands it.

[mike95490]

.....
And stay tuned, for next year I'm supposed to redesign a conventional "kit" greenhouse into something that will work in the Cariboo without suffering from the 50C daily temperature swings. Don't ask me how I'm going to do it, but she who must be obeyed commands it.

I'm tracking this thread, as I also co-habit with a SWMBO.

[solar_dave]

I'm tracking this thread, as I also co-habit with a SWMBO.

Ditto from me too! In fact this coming weekend is one of those!

[ggunn]

Every year I say I'm going to build a lean-to greenhouse. Every year I put it off until too late and the plants take over the kitchen for the winter. Every year my wife complains when I move the plants inside. Every year she complains when I move them back outside in the spring because she has gotten used to them being in the kitchen.

Some fights you just can't win.

[takingtime1]

Thank you for your input, it is something to think about. I wonder if having an abient temp. of about 65 - 70 deg. during a sunny day and a 1000w heater to heat the jugs.... how fast the heat from the jugs would take to drop from 65 or 70 deg. Do you by chance have a link to the Colorado man ?

Thanks again

[Cariboocoot]

Haven't worked out the details, but the basic principal hinges on being able to control both the thermal gain and the thermal loss.

So for gain it is best to have double-pane glass with a southerly exposure and good ventilation for when the temp starts to get too high.
For controlling the loss you need good insulation on the non-solar surfaces and enough mass to heat up during the day and radiate back at night keeping the temp above "too cold" (which will vary according to the plants).

Unfortunately the kit greenhouses fail on all of these counts, being basically aluminium (heat conductive) frames with single-sheet acrylic or polycarbonate sheets in-between for all surfaces.

I see potential in adding R5 foam insulation (at least) between the frame pieces, Reflectix in some areas, additional solar-powered venting, and some ceramic tile floor mass at least.

[TnAndy]

Electric heat is terribly inefficient. Solar Dave's suggestion of mass storage (and a bit of insulation) will work far better.

Minor point of correction: electric heat is 100% efficient, you put one kilowatt of energy in, and get one kilowatt of heat out.....zero waste.

The problem is the generation of electricity can be extremely inefficient....which I'm sure is what you meant.

In this case, 45w probably won't even keep up the self discharge on that size battery bank, much less leave any to work with.

Takingtime1 would do far better to do as ya'll have suggested, and build a well insulated greenhouse with lots of thermal mass to minimize the amount of heat needed to start with.....then use solar for direct heating of air or water, and store it for overnight/cloudy days.


Hey...that's what I'm doing !


When I first set up my PV system, I need a "power room" out as near to the arrays as possible to house the gear and batteries. So I benched out a place in the south facing side of the mountain, and built a combo solar room/greenhouse ( SWMBO got the greenhouse part ).

Initial construction:

The small 4x8 section on the front left side is my power room, the 12x20ish on the far end is her greenhouse.



The back, far end and front wall are all earth bermed. The front is a permanent bed of strawberries outside now.



Homemade rafter/trusses made of 2x6 treated lumber/plywood gussets, on 24 centers. You can see the trackers I built for my two initial arrays before the drive actuators or panels were installed.



Same shot, taken from the house garage door.

[TnAndy]

Completed greenhouse, two arrays behind with actuators installed now.....my two helpers in the pic. The glazing is 16mm triple wall polycarbonate panels (4' wide, custom ordered for length). REALLY nice stuff to work with.



Inside shot. Knee walls are 2x6 with fiberglass insulation, and 1" pink foam inside.



Outside. Raised bed, gutter into water barrel.




First year we did raise tomatoes, but even this design requires a lot of supplemental heat to keep the overnight temps up to where tomatoes will set fruit, so last couple years, she overwinters her deck flowers, and raises bedding plants for the garden to get a head start there.