The performance of a new design of non-metallic unglazed solar water heater integrated with a storage system has been studied. In this system, the collector and storage were installed in one unit. All parts of the system have been fabricated from fiberglass reinforced polyester (GFRP) using a special resin composition that provides good thermal conductivity and absorptivity. The storage tank has a capacity of 329 1. The design of the storage system was sandwich construction, with the core material made out of polyurethane foam, which combines stiffness and lightness of structure with very good thermal insulation. The width and length of the absorber plat were 1.4 and 1.8 m, respectively. The performance of the system has been investigated by two methods. In the first method, the storage tank was filled up with water the night before the test. The tank was then drained during the night, refilled and made ready for the next day's test. The tests were repeated under varied environmental conditions for several days. The maximum water temperature in the storage tank of 63 degreesC has been achieved for a clear day operation at art average solar radiation level of 700 W m(-2) and ambient temperature of 30 degreesC. The decrease of water temperature with and without the thermal diode is 10 and 20 degreesC, respectively. In the second method, the testing was of the same way, but in this case without draw-off or draining of the hot water from the storage tank. All data readings were recorded from sunrise to sunset over the same period. The temperature was recorded for several days and ranges of 60-63 degreesC were obtained in the storage tank. A system efficiency of 45% was achieved at an average solar radiation level of 635 W m(-2) and ambient temperature of 31 degreesC. (C) 2004 Elsevier Ltd. All rights reserved.