In essence, it's just a heat exchanger in the apex of the greenhouse roof
What does the heat exchanger do then? Are you exchanging heat between circulating water and the air at the top of the polytunnel for instance?
If so, that might be quite efficient (water is about four times better at storing heat compared with glass), but it does add another layer of complexity. If you just had a bed of crushed glass or whatever, you can suck hot air directly from the apex, down into the glass. The air then heats the glass, and exhausts at ground level, a bit colder than it started. Then at night you continue to run the fan in the same direction. The cool air is warmed by the bed, and exhausts at Ground level, preventing freezing.
What am I missing?
I can't quite think how to convert the heat exchanger numbers into that system, but I'm sure there would be a way...
The simplest calculation on heat stored would be to assume that the bed reaches equilibrium with the hot air at the apex during the day (say 30 degC), and then gives that heat back during the night, until it reaches say 10 degC). Then, Q=m x C x (deltaT). So, for 1000 kg of glass, with a
heat capacity of 0.84 kJ/kgK, you're looking at a storage 'capacity' of 1000 x 0.84 x (30 - 10) = 16,800 kJ.
To put that into context, it's the same heat output as running a 2 kW fan heater for a bit over two hours, so (as long as I haven't messed up the maths
)
actually that looks like quite a useful amount of heat storage?I can probably do the calcs on the heat exchanger once I know what sort of system you're looking at.
OK, I'm a *little* bit more ready for going back to work tomorrow now. Thanks!