الفهرس 
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Abstract
The classical types of energy as oil, natural gas and cool will depleted, as also, the cost of their exploitation is high, and it is the main cause of environmental pollution. For these reasons, the renewable energy becomes the best option for energy types. It is among the modern direct use for solar energy the most popular and it is simplest for heating water. There are different designs models for solar water heating systems. As water heating with forced circulation or heat pipes or thermosyphon system.
The objective of this work is studying experimentally and theoretically the
performance of solar water heaters used heat pipes for heat transfer, from absorber plate of solar collector to water tank. An extensive experimental investigation has been performed under actual weather conditions in the solar energy laboratory, Department of
Mechanical Power Engineering, Faculty of Engineering, Menoufia University, Shebin ElKom, Egypt at Latitude of 30.58 N and Longitude of 31.01
E. The experiments were conducted in two stages. In the first stage, two sets of solar water heaters were designed, fabricated and tested for comparing the performance of solar water heater with gravity assisted heat pipes system with the performance of solar water heater with forced circulation works at two flow rates. The heat pipes system uses methanol and acetone as working fluid, The second stage of the experiments was studying the effect of increasing number of the heat pipes on the performance of the solar water heater with gravity
assisted heat pipes system. The hourly variation of the absorber plate temperature, the water storage tank temperature and solar radiation intensity are measured. Then, the
stored energy and the efficiency were calculated for the three systems.
The results were as the following, the operation point of the conventional system begin at the operating start, while the thermosyphon system starts when the absorber plate temperature reaches to boiling point of the working fluid. In addition, the increase of the
heat pipes number lead to improve the performance of the thermosyphon system. The maximum obtained experimental water temperatures of the conventional system and the hermosyphon systems using ethanol or acetone were 66 [°C], 67.8[°C], and 64.6[°C], respectively. For theoretical calculations were 69.2 [°C], 69 [°C] and [69.3] °C, respectively. The percentage improvement in water temperature for the cases of 9 pipes and 14 pipes ethanol, relative to 4 pipes, were 20% and 30%, respectively. Also, the
corresponding increase in the case of acetone as a working medium was 14% and 20% respectively. Also, the maximum daily average of accumulated stored energy for the forced convection system and thermosyphon system charged with ethanol and acetone was 8.81 [MJ/m2
.day], 9.3 [MJ/m2 .day] and 9.57 [MJ/m2
.day] of net absorber surface,
respectively. The percentage improvement in stored energy for the cases of 9 pipes and 14 pipes ethanol, relative to 4 pipes, were 44% and 74%, respectively. Also, the corresponding increase in the case of acetone as a working medium was 26% and 56%
respectively. In addition, the efficiency of the conventional system is high from the beginning of
the operational time until the peak time, then it starts to decrease, the maximum value of the actual efficiency was ranged between 47~53%, while the theoretical calculations was
between 50~53%. The efficiency of the thermosyphon system is higher than efficiency of conventional system when it reaches to operating temperature. The experimental maximum efficiency of thermosyphon systems was around 55% at the case of installing 14 heat pipes and theoretical efficiency was 55% for same the case. The thermosyphon
system is better performance than forced convection system, because thermosyphon system does not consume electricity. Also, the performance of the thermosyphon system using acetone as working fluid was better than the performance of thermosyphon system
using ethanol, because acetone system has an operating period higher than ethanol system.