An experimental study was carried out to investigate the heat transfer characteristics of a new bi-directional thermodiode design. The thermodiode was made up of several rectangular loops filled with a fluid and mounted between two vertical plates. Rotatable joints between the horizontal and inclined segments of the loops enable easy alteration of the direction of heat transfer. The dimensions of the loops were optimized for solar heating application. Both master and slave modules were constructed and went through a series of indoor and outdoor tests. Indoor test results showed an onset time of about 15 min for natural convection to be induced throughout the loops when the thermodiode was forward-biased and subject to radiation from 400 to 800 W/m(2). A large temperature difference was maintained across the thermodiode and the cold-end temperature remained almost unchanged during heating when the diode was reverse-biased. In the outdoor tests the throughflow was induced after 90 min of heating in the morning. The maximum temperature difference between the test cells with and without thermodiodes was about 10degreesC which occurred around noon. (C) 2003 Elsevier Science Ltd. All rights reserved.