Thermal insulation with paper honeycombs with solar gain

In this contribution we describe the concept and the model for the heat flux and the effective U-value of paper honeycombs (PHC) used as efficient and cheap transparent insulation material. With this thermal-insulation-material static U-values of U=0.25 W/(m²K) are obtained due to the very low thermal conduction value λ=0.04 W/(mK), which is comparable to thermal insulators as PU-foam or mineral wool. Contrary to conventional insulation materials PHC also gathers solar radiation due to its geometry, thereby providing heat flux into the interior of the building. Because the angle of incidence of the sun in wintertime is low, the direct solar radiation is absorbed approximately within the outermost 3 centimeters of the PHC. Even at ambient temperatures below 0°C, this region is warmed up to 60°C. By conduction the heat is brought to the brick wall underneath, which acts as reservoir and gets to temperatures between 15 and 30°C. Calculated across the full heating period, it is shown, that effective U values of 0.14 W/(m²K) are reached by using PHC, reducing the brick wall U value by a factor of 3/4. Contrary to other transparent thermal insulation systems, e.g. developed by the Fraunhofer Institute for Solar Energy Systems, this system does not overheat during summertime, because the capillary structure is shielding the solar rays. A Windows based program solves the heat conduction equation with finite element methods