THERMOSIPHON SOLAR WATER HEATER USING CO2 AS WORKING FLUID

Abstract

A gravity driven Thermosiphon solar water heating system to harness solar insolation in low sunshine regions. This innovatory system uses CO.sub.2 as the working fluid to collect even mild sunlight to heat the water in sub-zero temperature areas. This solar water heater harnesses solar energy by fitting U-shaped copper heat removal pipes in evacuated glass tubes. This system works automatically by natural thermosiphon circulation force caused by density difference of supercritical CO.sub.2 at different temperatures. This innovatory solar water heater can perform in ice cold temperature areas where water based systems cease to function after freezing.

Claims

1. A thermosiphon solar water heater comprising: an evacuated double-layer borosilicate glass tube further comprising a top opening and a bottom opening, a top plug for the top opening with two holes in the center, a bottom plug for the bottom opening, a U-shaped copper tube further comprising aluminum fins attached in a circular manner around the U-shaped copper tube, and a first end and a second end, wherein both ends are in communication with outside of the glass tube through the two holes in the top plug and wherein the U-shaped copper tube is filled with carbon dioxide and the glass tube is disposed at an angle equal to the latitude of the level of site of use; a hot tubular manifold disposed horizontally and connected along the horizontal axis to the first end of the U-shaped tube; a cold tubular manifold disposed horizontally and connected along the horizontal axis to the second opening of the U-shaped tube; a heat-insulated tank with an inner volume and capable of holding a volume of water in need for heating; a helical-shaped copper tubular coil with a top and a bottom opening and disposed vertically inside the heat-insulated tank; an up-riser stainless steel tube inclined at 45 degrees connecting the hot tubular manifold to the top end of the helical coil; a down-comer stainless steel U-shaped tube down-comer disposed horizontally and connecting the lower end of the helical coil to the cold tubular manifold; and, a source of carbon dioxide connected to the hot tubular manifold with a valve and pressure gauge to fill and maintain a pressure equivalent of 68 bars in the U-shaped tube at −5 C.

2. The thermosiphon solar water heater of claim 1, wherein a plurality of glass tubes are used simultaneously.

3. The thermosiphon solar water heater of claim 1, wherein carbon dioxide circulates through the U-shaped tube in the glass tube under gravity as the carbon dioxide upon cooling becomes heavier and returns to the U-shared tube in the glass tube through the cold manifold.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0010] FIG. 1 shows a schematic diagram of Evacuated Glass Tube Solar Water Heater (EGTSWH) 1 using 9 evacuated glass tubes 2 connected in series. U shaped copper tubes 3 are inserted inside the evacuated glass tubes 2 using wooden corks 6. The U shaped copper tubes 3 are enfolded in aluminum foil 4 to act as a fin between evacuated glass tube 2 and U shaped copper tube 3. The U shaped copper tubes 3 are connected with cold header 7 and hot header 8 using stainless steel connectors 21.The U shaped copper tubes 3 are bent inside the evacuated glass tube 2 such that they do not touch each other maintaining a distance 5 among them. The cold header 7 and hot header 8 are made of stainless steel having high pressure bearing capacity i.e. 220 bar. Temperature gauges 10 and pressure gauges 9 are installed on hot header 7, cold header 8, water tank 13, up-riser 17 and down-comer 18. The hot header 8 is inclined at an angle 5-10° to stop reverse thermosiphon during cloudy weather and absence of sunlight. The CO.sub.2 16 is filled through non return valve 11 with cylinder at a pressure of 68 bar or above. The hot header 8 is connected to up-riser 17 which is made of copper tubes through stainless steel connector 21. Up-riser 17 is connected to helical coil heat exchanger 14 which is placed inside water tank 13 using stainless steel connector 21. Down-comer 18 is connected to helical coil heat exchanger 14 from the bottom of water tank 13. Down-comer 18 is further connected to cold header 7 using stainless steel connector 21. This way the circuit of CO.sub.2 16 is completed from solar collector 1 to water tank 13.

[0011] FIG. 2 shows U-shaped copper tube 3 assembled in a single evacuated glass tube 2 using wood cork 6. The Evacuated glass tube has a dimension of length of 1.8 m made of borosilicate glass with an external diameter of 0.0058 m and internal diameter of 0.0047 m. Wood cork 6 holds the U-shaped copper tube 3 and also insulates the evacuated glass tube 2.

[0012] FIG. 3 shows side view of density driven (thermosiphon) EGTSWH. The hot header 8 is connected to up-riser 17 which is elevated at an angle of 45° with respect to hot header 8 and cold header 7. The mediating fluid CO.sub.2 16 is evaporated in presence of solar insolation and travels to the elevated water tank 13 by flowing through copper tubes circuit. In water tank 13 it gives off the collected heat and returns to the solar collector through down-comer 18.

[0013] FIG. 4 shows isometric view of the EGTSWH 1 with elevated heat exchanger 14 wrapped in water tank 13. The water tank 13 is filled with fluid water 15 that will be heated by the circulating fluid CO.sub.2 16 in heat exchanger. The elevation of water tank is on upper side of the EGTSWH 1 to support buoyancy forces produced in the mediating fluid i.e. CO.sub.2 16.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Generally, water is widely used as the working fluid in water heating systems but it can only be used above 0° C. When temperature drops to −15 to 25 in cold blizzards and sustained snowfall water inside the solar water heater freezes itself. The CO.sub.2 refrigerant has the inherent capability to derive geothermal heat from subsurface (if connected) in the absence of sunshine. Other common working fluids are ammonia and silicon oil in use today for temperate regions. Ammonia is toxic and silicon oil is difficult to handle because of its higher viscosity. They do not exhibit supercritical behavior at low temperature as CO.sub.2. Among natural refrigerants, CO.sub.2 has a favorable property in terms of heat transfer and thermodynamic characteristics, having a freezing point at −76.5° C. renders it viable for being chosen as natural refrigerant.

[0015] Carbon dioxide (CO.sub.2) refrigerant easily attains 75° C. during 30 to 35° C. ambient temperatures. When the hot refrigerant is passed through shell-and-coil type counter flow heat exchanger the inlet water temperature increases from 26 to 55° C. giving off temperature gradient of 29° C. The maximum temperature difference in the heat exchanger is 52° C. Solar insolation acts as driving force starting Thermosiphon effect on CO.sub.2. This system provides 23° C. greatest temperature difference (GTD), 14° C. lowest temperature difference (LTD) and 18.13° C. log mean temperature difference (LMTD). Special arrangement in manifolds and inside the evacuated tubes makes it possible to stop reverse thermosiphon.

[0016] An Evacuated Glass Tube Solar Collector (EGTSC) consisting of 9 glass evacuated tubes with U-shape copper tubes fixed inside the evacuated glass tubes for removal of collected heat. The U shape copper tubes can withstand with supercritical pressure of mediating fluid. The EGTSC is inclined at angle of site latitude (33°) is shown in FIG. 1.

[0017] The solar water heating system shown in FIG. 1 designed and installed at COMSATS Islamabad, Energy park mainly consist of Heat collection unit (EGTSC) 1, Hot manifold 8 and cold manifolds 7 (hot & cold header), up-riser 17, water tank 13 and down-comer 18. The U shape copper tubes 3, Hot 7 & Cold 8 headers, up-riser 17, helical coil heat exchanger 14 and down-comer 18 are filled with CO.sub.2 initially at a pressure of 68 bar or above. The system is also equipped with high pressure filling and discharging system that can withstand up to 220 bar pressure as shown in FIG. 1.

[0018] The heat collection unit is composed of borosilicate glass evacuated tubes 2 of 58×47×1800 mm fixed on aluminum stand at an angle of 33°. The solar collector was built by inserting aluminum foil 4 enveloped U-shaped copper tubes 3 of size 6.36 mm (OD), 4.6 mm (ID) in evacuated glass tubes 2 using wooden corks 6. U shaped copper tubes 3 were connected to the upper 8 and lower 7 Stainless Steel headers (SS) of size 800.89×21.5×15.5 mm which was connected to upper 17 and lower baffles 18 of helical coil heat exchanger 14 as shown in FIGS. 2 and 3.

[0019] Heat collected inside the EGTSC 1 is transferred to water tank 13 elevated above the solar collector through copper tube called up-riser 17 of size 9.25 mm OD. Copper tubes were fitted to stainless steel using argon welding through SS connectors 21. The up-riser 17 has tilted an angle 45° and length 482.6 mm and further 366.6 mm to the water tank 13 as shown in FIG. 4.

[0020] A double layer insulated water tank 13 was fabricated using steel materials. The urethane thermal material was wrapped around the internal 23 liter tank 13 holding helical coil 14 for heat transfer. Temperature gauges 10 and pressure gauges 9 were fitted on top side for measuring temperature changes during steady state operation. Pressure and temperature were monitored by analog check gauges fitted to heating fluid and heated water circuits.