Just what is the point of bubblewrap?

If anyone watched Dick Strawbridge on BBC' "Its not Easy Being Green", he used an underground heatsink filled with imploded glass. This was heated during the day with air drawn from the apex of the GH by a solar powered computer fan and battery.The battery kept the fan running at night and thus pumped the warm air up from the heatsink.

I find his choice of material for the thermal mass rather odd since glass has about five times less thermal mass than water. Concrete and brick are more effective than glass, but water seems to have about the best thermal mass of any material.
Our GH (ex pigsty) has a thick concrete ( about half the thermal mass of water) floor and a six foot high, double brick ( about a third the thermal mass of water), south facing wall at the back. The wall at present is white but I think I'll get some masonry paint and paint both wall and floor black as I think it should assist absorption of heat during the day. I'll also try some black painted containers of water, though I think it would take a heck of a lot of them to make a significant difference.

 

Hi Dave,

I don`t think a computer fan would have enough airflow unless it was a very small greenhouse.

Imagine how much air a computer fan moves in an hour, especially with the added static pressure from the storage container.
In order to efficiently move the heat from the greenhouse to the storage, 5 air changes per hour is generally recommended.

On the subject of your south facing wall, now it`s dawned on me that the south facing side is inside the greenhouse :D

Insulate the "outside" face of the wall in winter to prevent the heat escaping.
You could even add a layer of glass on a frame on the inside to magnify the temperature against the wall (you`d need a fan to draw the heat back out at night though)

Remove all the insulation in summer to allow the heat to escape to the cooler night air on the open (back) side of the wall

In summer the wall acts as a cooling mass for the greenhouse.
By dumping the heat from the wall overnight it will be cool by the morning and ready to start over.
In winter it captures what little sun is available to store heat which migrates to the greenhouse interior later in the day.

You must make sure the greenhouse side is cooler than the "open air" side (by insulating the open air side)
..otherwise the heat will travel the wrong way

[ 11. November 2006, 10:05 PM: Message edited by: Hex ]

 

Thanks for the ideas Hex.
Painting internal walls and floors black would undoubtedly be of benefit in summer (as well as winter) as anything that could draw heat would be an advantage as the GH gets too hot even with door and windows open. The tall south facing wall at the back doesn't actually get much sun on its outer surface as half of it forms one wall of a small garden store which I suppose also gives a little insulation against heat loss and the eastern half is more or less in permanent shade.

 

Hi Dave
I`m still not sure if the sun facing side of the wall is inside the greenhouse

Painting them black for summer is probably not a good idea if it has any direct sun.
Absorbing direct radiation will cause it to become hotter than the greenhouse air which prevents it from acting as a cool mass.

You could always cover the (matt) black painted wall in white poly in summer i guess or a swift coat of brilliant white emulsion

In summer white is better to minimise heat gain from direct radiation and also increases light intensity.
Heat from the air will be transferred to the wall mostly via convection and conduction.

In winter ideally you need the direct radiation gains as they heat faster and hotter than the other methods.
You can amplify the air temperature even further with glazing and an airspace in front of the wall.

If you do a google search for "trombe wall" you may get a few more ideas for your wall

 

Hi Hex
The GH and its INTERNAL side of the rear wall faces south so the wall gains little or no heat from the external side.

Just looked at trombe walls and wish we we 30 years younger and in a position to do a new-build home!

 

Most of this technical stuff is very interesting but a bit above my head.
However, how about this: A water filled old radiator painted matt black, mounted onto the south-facing rear wall of the greenhouse. The wall is insulated with polystyrene and behind the radiator a reflective foil surface, effectively heating it from both sides.
At dusk, a lagging jacket is put over the radiator to partially cover it, conserving most of the heat to enable the heat to be released slower but over a longer period. Waddya think?

 

Hi Blackthorn.
I think in essence you've a good idea, but I don't think the radiator will hold enough water to make a heat sink of large enough capacity to provide much heat. I'm sure I read somewhere that it takes about 2 gallons of water for every square foot of surface that absorbs light (I may be well off the mark though).
I'd a similar idea but my thoughts had been along the lines of using a radiator in an insulated box outside to feed a large capacity container or containers inside the GH. The heated water in the radiator would rise due to convection and feed the heat sink. Cooler water from the bottom of the heat sink would flow back down to the radiator.
I've never really pursued the idea and no doubt it's hedged with problems!

 

Hi Blackthorn
Sorry about the technical stuff, it is important to quantify the various heatloss, gain, storage and resistance values, otherwise it`s all guesswork.

Consider the question: How large a heater will i need?
Without knowing the actual heatloss and the lowest likely outside temperature, it`s impossible to answer with accuracy

Once you understand the basics it`s not too difficult honestly.

Your radiator holds a fixed amount of water at a certain temperature. It also has a certain amount of surface area.
The sun heats it by direct radiation,whereas the greenhouse air will be heated mainly by convection from the radiator surface.

If your radiator holds 20lbs of water at 40F which heats to 160F by dusk, it has stored 2400 btu`s of heat (20lbs x 160F-40F = 20 x 120)

To drop the radiator temperature back to 40F it has to lose the 2400 btu gained.

If the greenhouse air is more than 40F it won`t go that low and the smaller the difference between radiator temp and air temp the slower the heatflow will be.

Assuming the radiator only loses heat by convection (which it won`t) the amount lost per hour is dicatated by surface area and temperature difference to the air.

Lets say for example the radiator has 10 sqft of surface and the greenhouse air temperature has "instantly" dropped from 160F to 40F (with a gradient the heatflow is constantly changing value)

So the temperature difference between the radiator surface and the greenhouse air is 120F,
in one hour the air will take 1200 btu`s (10 x 120) from the water in the radiator.

With a small mass (20lbs) it won`t work quite like this.
The reason is that as the btu`s are lost the water temperature of the radiator will cool
..this changes the temperature difference and the amount of heat flowing to the air

In a nutshell, with constant temperature difference and heatflow (not possible in reality) your 160F radiator water would be back to 40F in about 2 hours, which is equivilent to 350w of electrical heat for 2 hours

Probably still quite technical but a good way to judge if / how well something will work without actually building it...cheaper too :D

 

I will think through all this and get back to you
many thanks all.

 

Hex,i never understand anything you say.I`m going for a lie down. :D

 

Hi Elainefiz,
Sorry about that, it`s hard to describe it without getting a bit technical

Have your lie down in the greenhouse..
you`ll generate about 350 btu`s of heat per hour
About the same as a 100w lightbulb :D

Every little helps

 

if you can persuade the whole family to sleep in the greenhouse perhaps you won t need a heater?

 

Hi LG
Well it might be worth a try but you would have to vent periodically which costs some heat..

It is very necessary though, as the alternative is likely to be death by asphyxiation :D

Apart from heat, people also generate co2 at a rate of more than 3 Litres a minute.

so even just one person sleeping in an standard 8x6 greenhouse (well sealed and unvented) would raise the level above 1500 ppm after about 3.5 hours

 

Please don`t apologise Hex,i`m not exactly the sharpest knife in the draw.It`s just that i tend to get a very high pitched ringing in my left ear whenever i read your posts. :confused: :D

 

Hi elaine,its what is known as blinding you with science :D :D :D