Low-e confined air chambers in solar flat-plate collectors as an economic new type of rear side insulation avoiding moisture problems

• For flat plate-collectors alternatives to mineral wool insulation were developed.
• A low-e bounded air gap is flat, very cheap & insulates less by only 0.5 W/m2 K.
• Aluminum foil insulation is equally economical and even slightly superior.
• Total collector height can be reduced by 1–3 cm, moisture problems are avoided.
• Reversed rear side convection between parallel plates is studied by CFD-simulation.

Flat-plate collectors usually are insulated by a 40–60 mm thick mineral wool layer. Under dry conditions, typical thermal conductivities are between 0.035 and 0.060 W/m K depending on absorber temperature. Therefore, in brand-new flat-plate collectors, absorber rear side losses are in the region of 1 W/(m2 K). However, during operation, the rock wool can absorb humidity from ambient air and the thermal insulation deteriorates substantially. As an alternative, we show theoretically and experimentally, that if the mineral wool is removed, the emerging air chamber, if adequately confined, has acceptable insulation properties (rear side absorber losses 1.5–2.0 W/(m2 K)) simultaneously reducing the collector height substantially down to 20 mm. This is due to wall-adhesion and inner friction preventing air from convection, while the radiation heat transfer is suppressed by low-e walls confining the air chamber. Additionally, an efficient insulation system consisting of two air chambers has been established: A cheap (below 1 $/m2), thin (below 50 μm) and low emissive (thermal emissivity below 0.05) film mounted parallel between absorber and rear casing shows similar insulating properties as mineral wool and is not sensitive to humidity. Especially Al-foils are well applicable. Simultaneously, the total height of flat-plate collectors can be reduced by 10–20 mm. Theoretical calculations of convective and radiative rear side losses of the absorber have been done. On the base of series collectors, prototypes with rear side film insulation have been constructed and successfully tested at an outdoor test facility. The nightly stationary loss measurements and daytime efficiency measurements according to ISO 9806:2013 corroborate the theoretical modeling and the good efficiency of the insulation: For insulation thicknesses between 30 and 50 mm, the film insulation shows comparable and even slightly better insulation values as conventional dry mineral wool. The long term stability of the Al-film, including mounting, has been investigated theoretically and experimentally over 2 years and shows the practical applicability of the new insulating technique in flat-plate collectors.