Funny the It's Not Easy Being Green heater idea should be mentioned as that's almost exactly what part 2 of my solar heating ideas consists of:
While I was rebuilding the greenhouse on the plot I took the opportunity to dig a large hole under where the greenhouse would be, put a recycling box in there (insulated), and filled it with broken glass, bits of rubble, mortar, and anything that would act to store the heat; then put the pipework in so it'll draw warm air in from the top of the greenhouse into the box underground, then vent it out at night. the theory being that if the solar heater warms up the greenhouse in the spring / autumn mornings, the heatsink will pull down some of the heat in spring & autumn and vent it at night to help stabilise the temperature a bit. In the summer the heatsink will pull down the heat in the greenhouse to cool it a bit, and then release it at night.
What makes it all complicated is I'm going to need 3 timers to run it all
1 for the fan on the solar heater (the fan is a 140mm pc fan which is 12 volt) - to run in the mornings
1 for the "in" on the heatink to pull warm air into the underground box(a 80mm PC fan in an enclosure on the pipe) - to run mid morning till late afternoon
1 for the "out" fan pipe (a 40mm PC which I've bodged into the 40mm waste out pipe to pull warm air out of the box at night (and maybe help the in fan pull warm air in by creating a bit of negative air pressure)
As I mentioned I got the idea from INEBG, but I also found this on youtube that gave me some ideas (and I built the box pipework as he did) 1hYxKa_CW0E (if the link doesn't work because I'm new and anti-spam and all that, add "www." to youtube.com/watch?v=1hYxKa_CW0E ) - the bloke is American and hence mangles the word cement
But more about the heat sink another time
I'm going to have to experiment with the whole heater setup to work out the temperature difference between sunny and overcast days - probably time to invest in a probe thermometer. Apparently the boxes can get up to 60 degrees on a sunny day, so if the air can get to 30 I'd be happy. if the temperature difference between sunny & overcast is significant I may have to make the rig a bit cruder and wire a 5 volt panel directly to the fan, that way it'll only turn with speed and draw warm air in when direct sunlight hits the panel (and hence the box would be hot).... In fact thinking about it I might give that a whirl first - or are there any advantages from a ventilation perspective to drawing cool air in on cool days?
I'll probably also paint a few 2 litre pop bottles black and put them in the greenhouse on the really sunny side to soak up heat on hot days then release it back out - I've only got a 8x6 greenhouse so large tanks are a no-go (though have seen videos of people using blue barrels full of water as heat stores
Either way - it's my first growing year on the plot, so it's all experimentation