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I am trying to simulate a receiver tube for a concentrating solar power system. The tube consists of a fluid inside a copper pipe undergoing turbulent flow, and a glass cover outside of the copper pipe with a vacuum or air annulus in between the glass and copper.

My problem right now is with the radiation in particular. I need to be able to send in a certain amount of solar radiation from outside the glass. Most of the light in the visible spectrum should pass right through the glass and be absorbed by the copper tube, while the infrared radiation should be reflected off the glass tube.

I have set up a very simplified 2D system to try and model radiation through a transparent media in which there is a copper block separated by empty space with a glass block. "Radiation from an external Source" is chosen and sent in the direction of the glass block. It is my hope to have the radiation pass through the glass and become incident on the copper block. If a diffuse surface is only defined on the top surface of the glass then all other surfaces are considered thermally insulated. However when the simulation is run the copper block cools down by 70 deg. C. I am unsure how this could result when there are no temperatures below ambient and the copper block is considered thermally insulated.

Attached is the simply system.