Reverse natural air convection (hot plate top) was experimentally investigated between two inclined parallel aluminum plates (1 m × 2 m × 3 mm) with a separation distance of 20 mm to 100 mm. The inclination ϑ to the horizontal was varied from 0 deg to 90 deg. The mean temperatures of the plates have been adjusted to 90 °C and 30 °C resulting in Rayleigh numbers Ra between 2.7 × 104 and 3.3 × 106. The experimental conditions correspond to the back side of an absorber in a typical solar flat-plate collector, where the conventional insulation has been removed. The upper hot plate simulates the absorber and was electrically heated by an area heater, while the temperature distribution over the plate was measured. The lower cold plate was held isothermally by integrated water tubes and a thermostat. The side walls of the rectangular cavity were thermally connected to the colder plate and had a distance of 10 mm to the hot plate, comparable to a typical collector casing. The experimentally obtained results for Nu (Ra,ϑ) were mathematically described and compared to rare reverse convection data of other authors, gained at smaller aspect ratios/flow lengths and for adiabatic side walls: The formula of Elsherbiny approximately (within 10%) describes solar flat-plate collectors between 0 deg and 60 deg inclination, while the relations of Arnold, Ozoe, and Inaba show large errors up to 50%. Additionally, we experimentally showed that pure air gap insulation (30–50 mm) has surprisingly acceptable loss coefficients between 1.3 and 2.5 W/m2K depending on collector slope. It can be used as a cheap insulation method for low temperature collector applications. Additionally, inserting an 25–50 μm thick aluminum film symmetrically between the plates, a new and efficient insulation method for the absorber of a solar flat-plate collector was experimentally investigated: At plate distances of 30–50 mm, temperatures below 100 °C and slopes below 45 deg, this compact and cheap film insulation was proven to be equivalent to dry mineral wool and avoids its disadvantage of worsening insulation properties due to humidity.

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