Effects of measurement conditions on operation limits of solar horizontal heat pipes

In this work operating limit measurements are presented for one self-fabricated and one commercial solar heat pipe at horizontal and small inclination angles. While for the self-fabricated test sample heat transfer is limited by entrainment, dry-out limit is observed for the commercial solar heat pipe. Operating limit measurements of the commercial solar heat pipe show good reproducibility, whereas the detected operating limit of the self-fabricated heat pipe varies drastically. When tested at slight inclination, high temperature oscillations indicating geyser boiling effects are observed for the commercial test sample. Finally, operating limit definitions given by the literature are discussed regarding their applicability for solar thermal applications.