I know that a panel heater works best when built into a greenhouse type frame, and insulated at the back, but what about the evacuated tubes?
I am retro-heating our house with a mixture of passive heating (making a greenhouse lean-to on our sun-facing wall with thermal masses added) and radiators that are heated with evacuated tubes solar panels to store heat for rainy days.
My question is whether it is better for the evacuated tubes to be within the warmer greenhouse, or if they will work better sitting outside getting direct radiation from the sun. We have average winter temperatures here of 2C to 13C and never on record under -4C as we are 7km from the beach, so freezing is not an issue. (And my design will be drain-back anyway).
Outside for sure if freezing is not a concern. The evacuated tube provides all the insulating you need much like a thermos bottle. They are used here into the -40 range with glycol. Inside your greenhouse you will loose an additional 10 percent of your heating potential due to the extra layer the light has to pass through. On top of the loss of grow space. I would love to see pics!
Evacuated tubes aren't effected by cold weather as much as most solar thermal collectors and in your case it doesn't even get very cold.
Depending on the type of glazing used in your greenhouse, the amount of visible light blocked can be quite high, around 10% for glass and 20-30% or more for plastic.
And that is visible light, IR blocking is even higher.
My opinions are barely worth the paper they are written on here, but hopefully they can spark some new ideas, or at least a different train of thought
I think you should place evacuated tubes outside. Actually solar evacuated tubes work performance is decided by sunshine, not temperature. So if you put it inside, its received sunshine would be reduced.