In the present study, the effect of wickless heat pipe cross section geometry and its working fluid filling ratio on the
performance of flat plate solar collectors has been investigated experimentally. Three groups of wickless heat pipes having
three different cross section geometries (namely, circular, elliptical and semi-circular cross sections) were designed and
manufactured. Each group of three wickless heat pipes was charged with three different distilled water filling ratios of
10%, 20% and 35%. Each wickless heat pipe was then incorporated into a prototype flat plate solar collector developed
for the purpose of the present study. The prototypes wickless heat pipe flat plate solar collectors have been investigated
experimentally at different inlet cooling water temperatures, two different cooling water mass flow rates and under the
meteorological conditions of Cairo, Egypt. The experimental results indicate that the elliptical cross section wickless heat
pipe flat plate solar collectors have better performance than the circular cross section ones at low water filling ratios. The
optimum water filling ratio of the elliptical cross section wickless heat pipe solar collector is about 10%, while it is very
close to 20% for the circular cross section one. Also, the water filling ratio corresponding to the flooding limit of the elliptical
wickless heat pipe solar collector is lower than that of the circular one. At 20% water filling ratio, the semi-circular
cross section wickless heat pipe solar collector has bad performance compared with that of the other cross sections.