Measurement has been taken of the power heating a mass of water (0.5 kg) in a dark pot placed
in the focal point of the concentrator. Under 680 W/m² of direct radiation the maximum power measured
was of 1046 W. Assuming linear variation of the power with direct radiation, a 1000 W/m² sun would
provide up to 1535 W. This measurement is consistent with the previous calculation.
Sizing the drum for a maximum of 1 kg coffee is done accordingly to existing roasters. On the
roasters observed, the ratio Vgc/Vd of green coffee volume Vgc to the drum volume Vd is in the range of
24 % to 32 %. The beans close to double volume during roasting reaching a roasted coffee volume Vrc.
This leads to a volume ratio Vrc/Vd of 48 -64 %. The size of the focal point of our concentrator imposes a
minimal diameter of 0.2 m. Keeping this minimal diameter, a length of 0.23 m gives a roaster with low
fill ratio Vgc/Vd of 23.5% and high diameter to total length ratio D/Lt of 0.87. This size is our small drum
reference as summarized in Table 2. Such a design imposes to tap the drum by a glass cover to let the
light enter the cylinder and keep the coffee inside.