... counter-current in a tube-in-tube system. Several single tubes, serially interconnected, or tubes connected to each other via manifold, make up ... Solar Absorption: Solar radiation is absorbed by the evacuated vacuum tubes and converted into heat.Solar Heat Transfer: Heat pipes ... gradually heated. TypeC01-15C01-20C01-25C01-30C01-15BC01-30BNumber of tubes 152025301530Gross area (m²)2.293.053.80 4.671. ...