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Hi,
I'm getting ready to build an experimental solar heater. The goal is to heat the greenhouse after dark in cooler weather. I'm looking for some ideas, advice, lessons you learned, etc.

Our greenhouse is 8'x8'. It has a double layer of 6 MIL plastic. Inside I have two black plastic 30 gallon barrels filled with water. Over the past month I have learned that the mass of water has not heated sufficiently. So, I want to add as much thermal energy to the water in the barrels as possible by night...so they can give off heat at night.

Here's a description of the solar collector. I plan on using existing materials.

* Collector box is 4'x3'x6".
* Inside is insulated with 1/2" insulation sheeting material. Moisture barrier (silver side facing inwards).
* A garden hose is laid out in a serpentine pattern (2 loops...looks like a rounded W).
* Hose is 3/4" inner diameter...~18 feet in length.
* Aluminum flashing is placed over hose so entire inside surface is covered. Painted black.
* Box is sealed with caulking.
* Lexan cover.
* 40°C temperature switch inside...connected to a 120VAC supply plug and feeding a receptacle.
* Submersible pump located inside barrel. Plugged into thermal controlled switch.

Intended operation:
* Collector placed near greenhouse with southern exposure...angle adjusted for time of year.
* Sun warms solar collector.
* When internal temperature is > 40°C pump kicks on and sends water to the collector.
* Water passes through hose absorbing heat.
* Water discharges back to barrel.
* At night, with the pump not running, the barrels radiate heat into greenhouse.

That's what I'm planning. I even have Grace's blessing to get started since I have most of material.

Do you have any suggestions before I get started? Thanks.

Regards,
Chuck

I have no experience in this but it would be nice if you could arrange to do without the pump and rely on gravity. From a plant point of view, plants do not grow much in winter not only because of the temperature but also the lack of light. You do not necessarily want to boost the temperature in your greenhouse by much, more important is to keep an even temperature (probably only a few degrees above freezing). Keeping an even temperature requires a lot of thermal mass, you don't want your heat store being heated to too higher temperature on sunny days and falling to nothing on cloudy days.
I remember the Centre for Alternative Technology used to heat one of their polytunnels from a bore hole drilled into the ground (like a narrow well), this I believe was sufficient to keep it from freezing. Maybe they could help with sizing / approriate calculations.

Hi Graham,
Thanks for the pointers.

In my area the average temp in January is 45°F with an average range of 32° to 57°. The goal, as you stated, is to keep the temp above freezing if it should dip...so we can get an early start on some of our veggies...plus a place for my wife to start growing annuals for spring planting.

We have quite a few greenhouses here in the south that do quite well during the winter months...and they generally supplement their lighting and use some heating source.

I hadn't thought about a bore hole for heating the greenhouse. That's interesting. We're getting quotes for a geothermal HVAC system...and the ground temp holds about 55°F all year. I've already asked about the possibility of grabbing water for are garden usage...didn't think about heat.

Thanks!
Chuck

chuck_n_grace

It might not suit your requirements but do a search on YouTube for a beer can solar heater or collector. Just more food for thought for you.

Hi greenorelse,
I saw that...there's a Canadian business man that builds solar air heaters out of beer cans. Grace has already given me permission to build a test demo. I need to invite a few friends over to empty some beer cans.

I also saw a video on youtube of someone who simply has a black garden hose, coiled up, and placed it on the roof. It may have been 100' long...so it heated up quite nicely. However they live in Florida.

What's bad is we have this nice little greenhouse...that isn't much good with unusually cold temperatures below freezing. Just looking for a cost effective way to warm up the water barrels.

Thanks,
Chuck

Hi,
We've been measuring temperatures in the two 30 gallon barrels in the greenhouse. As the outside temp ranged from 25°F to 60°F, the barrels remained about 54°F. The greenhouse followed the outside temperature. Obviously the barrels (plastic) don't transfer heat either way very quickly.

Took the lids off. This time they ranged between 50°F and 59°F during the same temp ranges. Observed the greenhouse temps about 3°F above the outside temps. This is obviously an improvement.

Sidetracked at the moment with starting seedlings and the fish tank. Hope to improve upon this system soon.

Regards,
Chuck

For what it's worth, if your pump pushes the water through the panel too fast then it will not have time to heat up sufficiently.

Have you thought of painting a house radiator black and using that as your collector?

HTH,

BA

We have a solar heat sink in our polytunnel.

It's basically a solar-powered fan which sucks the warm air from the top of the polytunnel down to a large, gravel-filled, polystyrene box buried under the path. An outlet pipe (these are all small drainpipes) runs along the side of one of the raised beds and has several openings along its length.

Very simple. Seems to work (as long as there's a bit of sunshine of course!)

Green Aura, do you have any temperature readings to judge how effective it is?

No, but you can feel warm air round your ankles. Does that count?

Green Aura, can you give a bit more detail on the set up so I can look at trying it in my PT? What sort/size of fan and size of tubing to get air to ground level? How large a polysyrene box? and is tha sunk under your path? IT sound' almost possible even for someone who is engineeringly challenged like me!

Cassie

Hi,
Interesting idea about the black radiator. I don't have any lying about...maybe I can pick one up at a 're-store'...habitat for humanity store with lots of old house stuff.

The pump rate will certainly be important. I need to do some hose volume calculations and size a pump. Of course I'll most likely start with whatever pump I have to establish a base line for improvement.

That's interesting about blowing air down toward the heat sink. Our greenhouse does heat up to about 40 degrees over ambient here in January...if the sun is shining.

I need to get to that store and to some 'thinking' You know what I mean? You have a conceptual idea so you go into a second hand store and look about for something that might work.

Thanks,
Chuck

OK - I'll have a go, but you might start to feel less technically challenged when I have .

The poly box is one of the ones that they deliver organic meat in, complete with lid. We bought one length of the small bore drainpipe (about 35-50mm -ish diameter) and the thing that connects it to the gutter - don't know what that's called and various connectors - right angles and a couple of T-pieces. The box was sunk into a big hole in the ground, level with the top of the lid and then filled round to give a tight fit. We stood the gutter thing upside down in the box to lift the pipe up off the bottom and connected the pipe to it, then filled the box full to the top with gravel and put the lid on it - you need 2 holes in the lid - for the in and out pipes.

The top of the pipe had a right angle connector fitted and this went into the back of a small wooden box (which we made out of some scrap plywood) that housed the fan. It's not a big fan - came complete with solar panel from fleabay (about £13) can't remember the power (?) of the panel. The box was wedged into the crop bar/polytunnel frame joint.

Then we put a right angle connector into the outflow hole in the box lid and joined several short pieces of the drainpipe to the t-pieces to give various outlet points along its length. This runs along the ground next to one of the raised beds.Then we covered the whole path, including the top of the box, with weed membrane and clinker from the boiler - because we had tons of it and it makes quite good paths.

The only problem we've had is the top of the box is a little bit spongy although it's safe enough, being full of gravel. And the fan box isn't very secure - we've not found a reliable way of attaching it to the frame yet. But that's a minor problem - it doesn't fall down just doesn't stay wedged in IYSWIM.

OH has just reminded me that we cut up a pair of my old tights and covered over the outlet holes to stop soil/ clinker etc getting in. And we siliconed all the joints on the fan box to make it as airtight as possible. And make sure you put the fan the right way round - so it's sucking the air into the pipe not blowing it out!

Green Aura,
Good description...it went over my head just a bit. Thing?

Do you have any pics?

Fleabay? Not sure what that is...sounds funny.

Regards,
Chuck

Hi Chuck

How did you get on with the system. Is it up and running? I am wanting to do something very similar. Thanks