hey thanks for the replys so far the waste heat is hot water at around 80 degrees c
Wow, that's not waste heat, that's er, heat, and enough for a typical domestic house!!
OK, just thinking aloud instead of doing the morning crossword, and this is worth exactly what you paid for it
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The hardest bit of a heat loss calculation is finding a reliable value for the heat transfer coefficient. I couldn't find anything for polytunnel plastic, and even a single glazed window is tricky (lots of references quoting without units, or taking the window frame into account). However, single glazing is typically quoted at 5 to 6 W/m2K, and since most of the resistance to heat loss is actually in the transmission of heat in the air either side of the window, and not the glass itself, that's probably a reasonable figure for plastic too (I told you this would be worth only what you paid for it!
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So, since we're just thinking aloud still, let's go with 6 W/m2K and see what happens:
Your PT is 30ft x 90ft, so that's 10m x 30m close enough. If we assume it's shaped like half a beer can laid on its side, the area of plastic is roughly pi x r-squared (you get a full circle area by combining both ends), plus 0.5 x pi x diameter x length. That gives you plastic area = 3.14 x (5^2) + (0.5 x 3.14 x 10 x 30) = 80 + 470 = 550 m2, near enough.
If you want to maintain 15degC in your PT even when it's say -5 degC outside, that gives you a temperature difference of 20degC.
So, Q=U x A x temperature difference = 6 x 550 x 20 = a heat loss of 66 kW, which is more than you have available
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To be more useful then, what could you perhaps achieve? If you have 22 kW available on a continuous basis, then dT=Q/(UA) = 22,000/(6*550) = a 6.7 degC boost compared with ambient temperatures. So, not exactly tropical during the winter, but enough to keep most things alive during the winter I'd have thought, and surely enough to extend your growing season to almost year round?
If it was me, I'd put up an internal partition in the PT, to make the heated area smaller and easier to heat. I'd maybe even do a tunnel in a tunnel arrangement, which would give a nice hot portion of the PT to grow weird and wonderful plants in, and then a still-a-bit heated main section outside that.
Again, I hope that's helpful. If anyone else fancies telling me where my calcs are all wrong, go right ahead - I have only just got out of bed after all!