Heat your greenhouse and home from broken glass

[Davy stephenson]

Remover by author

[dave45]

I don't understand your greenhouse heating idea... why not just have a large black massive object inside the greenhouse - like a water butt, or painted bricks? I can't see any advantage to using broken glass.

[Davy stephenson]

Remover by author

[dave45]

I'm afraid that reasoning sounds like gobbledegook. There is a given amount of incoming solar radiation - it is either absorbed or reflected. Black stuff absorbs more than other colours. If you are heating a black water butt, convection currents in the water will move the heat about from front to back. Storage of the solar-derived heat in enough quantity and controlling its release is the challenge. And, as ever, insulation comes into it. Have you read "the earth sheltered solar greenhouse book".? .. the author reckons that heat-loss through the glass is the biggest factor - hence his earth-sheltering idea. Makes some sense.

"somewhere the sun is always shining" may be true but it is of no practical use at all. In Lancashire we can go for weeks and months without seeing the sun. Then there IS no usable solar energy. end of. For instance - claims that solar panels create electricity even on cloudy days may be strictly true, but at 1% of the full-sun level - thats near-as-dammit nothing in my book. Proponents of green renewable energy overstate their case. In the UK it is usually scarce, hard to get at, and very expensive. I do speak from experience !

[Colin]

If you heat a smaller mass to a higher temperature the losses increase,its more efficient to heat a large mass by a few degrees
Using glass as a thermal mass is ok if you insulate the outer surface otherwise any heat stored in the glass itself will be lost to the outside faster than the inside..heat moves to cold.

If you check the specific heat capacity of glass you`ll find its exactly the same as brick and sand, pound for pound. The only difference its translucent

[Colin]

I don't understand your greenhouse heating idea... why not just have a large black massive object inside the greenhouse - like a water butt, or painted bricks? I can't see any advantage to using broken glass.

You take up space and are reliant on mostly direct radiation to provide the heating
where do you put the plants so they dont shade out what will be a large amount of mass?

[dave45]

well I'm totally lost now ! a picture is worth a thousand words and all that!

fwiw I have just built a lean-to style greenhouse against a 5-foot garden wall (which I have painted black) along the lines of the earth-sheltered greenhouse book. Finished it a month ago, and we have had but one sunny day since.

I stll can't see how to "Heat your greenhouse and home from broken glass"

[dave45]

>where do you put the plants so they dont shade out what will be a large amount of mass?

I guess when you need the heat the most the plants will be small seedlings !

[Colin]

Hopefully you insulated the rear side of the garden wall

I have 30 tons of thermal mass which takes up no space and cost absolutely nothing. It collects heat from both direct and diffuse radiation and from latent and sensible heat transfer.
It can heat, cool and dehumidify the greenhouse air and also provides a great environment for plant roots.
I can do pictures too

[dave45]

Pictures? - yes please!

no I didn't insulate the garden wall... the greenhouse idea came after I'd built it

[Colin]

If you dont insulate the back of the wall, the heat will be drawn through to the cold side and away into the night

Here`s a few pics of my system during its construction.
The thermal mass uses vertical perimeter insulation outside the concrete foundation ring.


This is my 15-arm subsoil monster that does all the work


This is how the SSM looked after installation. Notice the only thing that encroaches into the greenhouse space are the outlets which are located above the red band shown in this pic.

[dave45]

awesome pics! how does the SSM work? are there active components?

[Colin]

There`s a fan driving the system which is quite a bit larger than it needs to be but i went bigger so i could experiment with higher airflow rates. It has its own fan speed controller and flat out it draws 170w, in normal service its running at less than half of that.

The fan is also controlled by a homemade differential thermostat which senses the temperature difference between the soil and the greenhouse air.
In cooling mode, the fan runs when the air is warmer than the soil by a preselected temperature differential, which is adjustable from 3F - 36F.
In winter or at night when the greenhouse air temperature drops below a preset point (adjustable from 40F to 86F) the air and soil sensors automatically reverse themselves and the thermostat switches into its heating mode.
In heating mode, the soil must be warmer than the air by the selected amount,which is also adjustable from 3-36F.
The sensors automatically revert back into cooling mode when the greenhouse air temperature is above the minimum setpoint. The cooling and heating differential settings are completely independant.

So imagine i set the cooling differential to 30F, the heating differential to 9F and cooling/heating changeover point to 45F.
If the soil mass is at 55F the fan wont run until the greenhouse air reaches 85F.
If the greenhouse air temperature drops to 44F the heating mode becomes active and the thermostat checks that the soil temp is at least 53F before it starts the fan.

There are a few more unique features to the system but i`ve waffled on long enough

[dave45]

wow - that is pretty sophisticated stuff... my ideas are a little more small-scale but similar. I was planning to have a low-down and a hig-up thermocouple to read the air temp difference and circulate the air when the diff was over a preset value. And run an extractor fan if it exceeded some "too-hot" value. I was thinking of using a basic STAMP controller (mainly coz I have one spare) in conjunction with some Lascar temperature dataloggers. But all to be run from a 12V solar-charged system. First step is to find some efficient 12V fans.. any ideas?

Burying a soil monster is a great idea but a bit too late for me !

[Green Aura]

Looks fantastic, Davy, albeit a little over-elaborate.

Our polytunnel has a gravel heat sink. It comprises a large polystyrene box, buried to it's depth, with a hole cut in the lid and a drainpipe running through. The box was filled with gravel, lid replaced and the top of the drainpipe connected to a solar-powered fan. There's a second pipe coming out of the box which runs along the floor with various outlet holes. We covered the box with clinker, which we've laid on the path in the polytunnel.

The lowest temp in the polytunnel over the winter was 4oC, even when it was significantly colder outside - the high wind speeds add a big chill factor.