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Development of heat pipes for solar water heaters
Sohr
Energy
Vol. 32, No.
5, pp. 625-631,
0038-092X/84 $3.00+ .I0
1984
Printedin Great Britain.
Pergamon
Press Ltd.
DEVELOPMENT OF HEAT PIPES FOR SOLAR WATER HEATERS MEHMET AKYURT
Mechanical Engineering, King Abdulaziz University, Jeddah, Saudi Arabia (Received 17 August 1982:accepted 10 May 1983) Abstract-Numerous heat pipes were designed, manufactured, and filled on a specially developed filling rig. Each heat pipe was incorporated into a prototype solar water heater developed for this purpose,and was tested under actual insolation conditions. An extensive testing program lasting for more than a year revealed that the heat pipes perform satisfactorily as heat transfer elements in solar water heaters. A special heat pipe featuring a compact and effective condenser configuration was also tested. It was observed to likewise exhibit isothermal behavior and hence promised potential for large scale solar applications. 1. INTRODUCTION
until the late sixties when the aerospace and nuclear industries started making use of it. It soon won widespread application in cooling electronic components and thermoionic converters, and in waste heat utilization.
The conventional flat plate solar water heater is simple, robust, reliable, reasonably efficient, and automatic in operation but is notoriously frost sensitive. Efforts to overcome this serious drawback resulted in closed loop systems, where a suitable fluid is circulated in the heater circuit. The collector in closed loop systems necessarily operates at higher temperatures than in the conventional open system, the difference of mean collector and tank mean temperatures being frequently over 20°C. The higher operating temperatures coupled with the thermal barrier of the heat exchanger result in lower system efficiencies such that typically a closed loop system will have only about 70 per cent of the efficiency of the open system. 1.1 The heat pipe The heat pipe consists essentially of an evacuated container within which a wick may be inserted and a small amount of working fluid is supplied (Fig. 1). When energy is applied on the evaporator, part of the fluid is vaporized, soon saturating the interior of the container with the pure vapor. As the wall of the condenser is cooler due to heat extraction, some of the vapor condenses there, releasing thereby the latent heat of condensation. The condensate is then trickled back to the evaporator in the gravity assisted heat pipe, thus completing the cycle. The heat pipe was selected in this study as the medium of heat transfer in solar water heaters because of the following outstanding features characterizing it: -Suitable working fluids are available for the operational range -50°C to tlOO”C (-5%212°F). -The heat pipe can handle very high axial heat fluxes at essentially constant temperature. -Reverse flow is not possible in the heat pipe, which property minimizes recirculation losses from the storage tank, and -The heat pipe is simple, robust and reliable. It has no moving parts, and works quietly. The first patent on a heat pipe was granted in 1944. The heat pipe remained relatively unknown, however,
I .2 Solar applications : In one of the first solar applications of the heat pipe Bienert and Wolf [l] inserted the evaporator end of a heat pipe in a flat plate collector. The condenser protruded into an elaborate water manifold attached to the upper end of the collector. Their results were neither conclusive nor optimistic. Using single axis tracking parabolic trough concentrators Ramsey et al. [2] obtained collector efficiencies of 50 per cent at 300°C (572%) for a selectively coated heat pipe. Aluminum heat pipes sere used for water and space heating[3] as well as for desalination, refrigeration and power generation. Several other attempts[&91 were made to incorporate the heat pipe in collectors. None of the findings are conclusive or optimistic, and all designs suffer from the weakness of Ref. [l], i.e. the water manifold is so bulky that the energy collected and lost by it easily offsets any advantages the heat pipe may bring. Schreyer [ lo] built a two-phase thermosiphon that ‘was charged with a freon. Force-cooling a heat exchanger, he reported satisfactory performance at near ambient temperatures. In what follows, research work toward the development of a frost resistant heat pipe solar water heater is described. To this end heat pipes were manufactured and incorporated into miniature solar water heaters. Work on full scale solar water heaters will be reported in a separate article. 2. DEVELOPMENTOF HEAT PIPES
A heat pipe can be envisioned like a riser in a conventional flat plate collector such that its condenser protrudes into the water tank. For a riser-to-riser spacing of I5 cm (6in.) and a riser length of 2OOcm (80in.) the maximum power to be handled by the heat pipe becomes 216 W, considering a peak intensity of 1.2kw m-* and a collector efficiency of 0.6. The maximum allowable tem-
625
Energy
Vol. 32, No.
5, pp. 625-631,
0038-092X/84 $3.00+ .I0
1984
Printedin Great Britain.
Pergamon
Press Ltd.
DEVELOPMENT OF HEAT PIPES FOR SOLAR WATER HEATERS MEHMET AKYURT
Mechanical Engineering, King Abdulaziz University, Jeddah, Saudi Arabia (Received 17 August 1982:accepted 10 May 1983) Abstract-Numerous heat pipes were designed, manufactured, and filled on a specially developed filling rig. Each heat pipe was incorporated into a prototype solar water heater developed for this purpose,and was tested under actual insolation conditions. An extensive testing program lasting for more than a year revealed that the heat pipes perform satisfactorily as heat transfer elements in solar water heaters. A special heat pipe featuring a compact and effective condenser configuration was also tested. It was observed to likewise exhibit isothermal behavior and hence promised potential for large scale solar applications. 1. INTRODUCTION
until the late sixties when the aerospace and nuclear industries started making use of it. It soon won widespread application in cooling electronic components and thermoionic converters, and in waste heat utilization.
The conventional flat plate solar water heater is simple, robust, reliable, reasonably efficient, and automatic in operation but is notoriously frost sensitive. Efforts to overcome this serious drawback resulted in closed loop systems, where a suitable fluid is circulated in the heater circuit. The collector in closed loop systems necessarily operates at higher temperatures than in the conventional open system, the difference of mean collector and tank mean temperatures being frequently over 20°C. The higher operating temperatures coupled with the thermal barrier of the heat exchanger result in lower system efficiencies such that typically a closed loop system will have only about 70 per cent of the efficiency of the open system. 1.1 The heat pipe The heat pipe consists essentially of an evacuated container within which a wick may be inserted and a small amount of working fluid is supplied (Fig. 1). When energy is applied on the evaporator, part of the fluid is vaporized, soon saturating the interior of the container with the pure vapor. As the wall of the condenser is cooler due to heat extraction, some of the vapor condenses there, releasing thereby the latent heat of condensation. The condensate is then trickled back to the evaporator in the gravity assisted heat pipe, thus completing the cycle. The heat pipe was selected in this study as the medium of heat transfer in solar water heaters because of the following outstanding features characterizing it: -Suitable working fluids are available for the operational range -50°C to tlOO”C (-5%212°F). -The heat pipe can handle very high axial heat fluxes at essentially constant temperature. -Reverse flow is not possible in the heat pipe, which property minimizes recirculation losses from the storage tank, and -The heat pipe is simple, robust and reliable. It has no moving parts, and works quietly. The first patent on a heat pipe was granted in 1944. The heat pipe remained relatively unknown, however,
I .2 Solar applications : In one of the first solar applications of the heat pipe Bienert and Wolf [l] inserted the evaporator end of a heat pipe in a flat plate collector. The condenser protruded into an elaborate water manifold attached to the upper end of the collector. Their results were neither conclusive nor optimistic. Using single axis tracking parabolic trough concentrators Ramsey et al. [2] obtained collector efficiencies of 50 per cent at 300°C (572%) for a selectively coated heat pipe. Aluminum heat pipes sere used for water and space heating[3] as well as for desalination, refrigeration and power generation. Several other attempts[&91 were made to incorporate the heat pipe in collectors. None of the findings are conclusive or optimistic, and all designs suffer from the weakness of Ref. [l], i.e. the water manifold is so bulky that the energy collected and lost by it easily offsets any advantages the heat pipe may bring. Schreyer [ lo] built a two-phase thermosiphon that ‘was charged with a freon. Force-cooling a heat exchanger, he reported satisfactory performance at near ambient temperatures. In what follows, research work toward the development of a frost resistant heat pipe solar water heater is described. To this end heat pipes were manufactured and incorporated into miniature solar water heaters. Work on full scale solar water heaters will be reported in a separate article. 2. DEVELOPMENTOF HEAT PIPES
A heat pipe can be envisioned like a riser in a conventional flat plate collector such that its condenser protrudes into the water tank. For a riser-to-riser spacing of I5 cm (6in.) and a riser length of 2OOcm (80in.) the maximum power to be handled by the heat pipe becomes 216 W, considering a peak intensity of 1.2kw m-* and a collector efficiency of 0.6. The maximum allowable tem-
625