Heat transfer in vacuum tubes

[emlaurent]

Hello,

I am trying to understand the principles used in evacuated tube solar collectors to transmit solar heat.
And there is a question that I do not solve, maybe you have the solution!

There are (old) systems with heat pipes onto which fins have been grafted which absorb solar radiation.
The whole is inserted into a glass tube under vacuum (for thermal insulation). Those are ok for thermal transfer:
solar radiation> absorption by the fins> transfer by thermal conduction to the heat pipe> transfer to the liquid

The more recent systems use a double-walled vacuum tube whose internal face is covered with an absorber (metal layer deposited under vacuum) and there is a heat pipe in the middle of the tube. Solar heat absorbed by the inner layer is transferred to the heat pipe tube, but how?
- is it by convection / conduction through the air between the heat pipe and the glass tube?
- are there metal fins to transmit this heat from the abs layer. to the heat pipe? (which would seem more efficient to me!)

I take a close-up photo to see the different solutions (or diagrams)! (I don't have such a solar collector at home ...)
merci!