TECHNICAL FIELD
The present disclosure relates to convection driven solar water heaters, and in particular to convection driven two-component solar water heaters.
BACKGROUND OF THE INVENTION
Solar water heating is currently the largest solar energy application by volume, with an accumulated capacity of 126 gigawatts in 2007. However, only about 10% of the buildings in the world have solar water heaters installed. A bottleneck currently exits preventing expanded application of solar water heaters because current designs are either too expensive or difficult to install. Innovations for inexpensive and easy to install solar water heaters are crucial for broader utilization of the sun as a renewable energy source.
Before World-War-Il, the most popular solar water heater was the Day-and-Night type invented by William Baily in 1910 (U.S. Pat. No. 966,070). The heat collector is a copper plate with zigzag copper coil welded on it, and painted black. The water in the copper coil is heated by sunlight. As a result of solar energy driven natural convection, the heated water ascends to an insulated water tank located at a position higher than the heat-collecting plate. No pumps are needed. Continued improvements in the systems have allowed flat-panel solar water heaters to remain the most commonly installed system in the U.S. and Europe, examples of which are disclosed in U.S. Pat. Nos. 4,353,352 and 4,599,994. However, a high manufacturing cost coupled with poor heat collector insulation stifles the potential for broadened growth and acceptance of the system.
Worldwide, a great majority of solar water heaters are based on all-glass evacuated-tube solar collectors, invented in 1911 (U.S. Pat. No. 980,505). The most popular configuration is the integrated systems, in which a number of all-glass evacuated-tube solar collectors are directly connected to an insulated water tank. Similar to U.S. Pat. No. 966,070, the integrated system is also driven by natural convection, completely absent of moving parts. Since the raw materials are inexpensive, and due to the economy of large scale, the manufacturing cost of all-glass evacuated tubes is very low. However, because the integrated solar hot water system has the water tank located directly on top of the evacuated tubes, the weight of the system after it is filled with water is typically 500 to 1000 pounds, and the supporting structure is bulky. Therefore it is impossible to put on top of a great majority of roofs. To date, the integrated system is only installed on a small fraction of residential homes.
The most preferable solar water heater with high efficiency and ease of installation is a system using all-glass evacuated-tube solar energy collectors and a separate water tank. For such a separate system, the water tank can be placed on structures which can support a greater weight, for example, in the attic or the bathroom; while the collector can remain on relatively less supportive structures, for example, on a rooftop. Therefore, the system can be installed in a much broader range of domestic applications.
In order to separate the water tank from evacuated tubes, sophisticated heat transfer devices are commonly used. For example, inserting metal tubes inside the glass evacuated tubes; see U.S. Pat. Nos. 3,952,274,4,002,160, and 4,319,561. In China, many patents have been issued for separated solar water heaters using metal heat-transfer structures; for example, using metal U-tubes (CN 201476230U), concentric metal heat-transfer tubes (CN 101430139A, CN 201043810Y, CN 201237377Y), or thermal siphon heat transfer mechanism (CN 201066206Y, CN 201072245Y). Although those methods allow the solar collector and water tank to be separated, it is at a much higher cost, and requires an electrical pump. Systems such as these are not free of moving parts and consequently, cost is much higher and reliability is compromised.
On the other hand, there are Chinese patents which describe solar water heaters using all-glass evacuated tubes directly connected to a separate water tank (CN 201255511Y, CN 201297783Y). However, the convection force within those designs only comes from the relative positions of the all-glass collectors and the tank, which is very weak, and therefore cannot function properly. Consequently, to date, no mass-produced commercial products appeared based on those patents.
It is well known that for the integrated solar water heaters, the driving force of fluid flow comes from inside the all-glass evacuated tubes. See Reference 1. Therefore, a natural technological progression would entail using the thermal head generated inside the evacuated tubes to drive fluid flow to and from a separated water tank. A design based on this idea was proposed in a 2005 Chinese patent (CN 2704796Y). However, the design has several fatal flaws. The seams between the cylindrical upper collecting pipe and the all-glass evacuated tubes cause serious water leakage, rendering the system unusable. The partition barriers inside the evacuated tubes are not connected to anything, making the system unstable. No working system can be constructed based on that patent. The idea of partition walls in all-glass evacuates tubes was also proposed in a 1975 U.S. Pat. No. (4,016,860) for a completely different utilization: a solar air heating system with forced air circulation using an electric motor.
BRIEF SUMMARY OF THE INVENTION
The disclosed invention is intended to be a solution to the aforementioned practical and economic problems concerning separated solar water heaters, which up to this point have prevented further growth within the industry. Various exemplary embodiments of the present invention provide a separated solar water heater using all-glass evacuated tubes as the solar energy collector. No metal components inside the tubes are required to facilitate heat transfer and no pump is required to facilitate the flow of fluid within the system. The system can generate a strong hydraulic head utilizing the temperature difference inside the all-glass evacuated tubes to drive heat transfer by natural convection.
According to the present invention, due to the action of the heat separator, the incoming cool water and the outgoing hot water inside an evacuated tube are well separated. Additionally, the temperature difference of incoming cool water and outgoing hot water generates a hydrodynamic head which drives the heated water into a water storage tank through a pipe. Due to the same hydrodynamic head, the cooler water in the water storage tank automatically circulates through another pipe back into the evacuated tubes. The water flow is entirely driven by natural convection and no mechanical pump is required. The design may have four variations: vertically oriented evacuated tubes with a heat separator of parallel configuration; vertically oriented evacuated tubes with a heat separator of series configuration; horizontally oriented evacuated tubes with a heat separator of parallel configuration; and horizontally oriented evacuated tubes with a heat separator of series configuration. The term “heat transfer fluid” is used herein interchangeably with “water” and it is observed that the system disclosed herein functions effectively using many heat transfer fluids including but not limited to water, glycerin and anti-freeze solutions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1C show cross-sectional views of a vertically oriented solar energy collector using a heat separator with parallel configuration and a number of all-glass evacuated tubes according to an exemplary embodiment of the present invention.
FIGS. 2A-2D show cross-sectional views of a vertically oriented solar energy collector using a heat separator with series configuration and a number of all-glass evacuated tubes according to an exemplary embodiment of the present invention.
FIGS. 3A-3E show cross-sectional views of a horizontally oriented solar energy collector using a heat separator with parallel configuration and a number of all-glass evacuated tubes according to an exemplary embodiment of the present invention.
FIGS. 4A and 4B show cross-sectional views of a horizontally oriented solar energy collector using a heat separator with series configuration and a number of all-glass evacuated tubes according to an exemplary embodiment of the present invention.
FIG. 5 shows a perspective view of a direct-use complete solar water heater using a vertically oriented solar energy collector with a heat separator and a number of all-glass evacuated tubes according to an exemplary embodiment of the present invention.
FIG. 6 shows a perspective and partially broken away view of an indirect-use complete solar water heater using a vertically oriented solar energy collector with a heat separator and a number of all-glass evacuated tubes according to an exemplary embodiment of the present invention.
FIGS. 7A and 7B show perspective and partially broken away views of two configurations for an indirect-use complete solar water heater, each using a different type of horizontally oriented solar energy collector with a heat separator and a number of all-glass evacuated tubes according to an exemplary embodiment of the present invention.
FIGS. 8A and 8B show cross-sectional views of both a parabolic reflector and flat reflector; either of which can be placed on the side of the evacuated tubes opposite incident solar radiation to increase overall collector efficiency according to an exemplary embodiment of the present invention.
FIG. 9A-9D show perspective and partially broken away views of both a direct use water storage tank and an indirect use water storage tank, the later shown with a heat exchange coil as the heat transfer mechanism, where each tank has a paraffin wax thermal storage system which increases the overall heat energy storage capacity of the tank according to an exemplary embodiment of the present invention.
FIG. 10A-10C show cross-sectional views of a vertically oriented solar energy collector using a heat separator with parallel configuration and a number of triple-concentric three cavity evacuated tubes according to an exemplary embodiment of the present invention.
FIG. 11 shows a perspective view of a direct-use complete solar water heater using a horizontally oriented solar energy collector with a heat separator and a number of all-glass evacuated tubes according to an exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a vertically oriented solar energy collector assembly according to an exemplary embodiment of the present invention using a heat separator with parallel configuration and a number of all-glass evacuated tubes. The all-glass evacuated tubes 101 are mounted on a heat separator 103 through, for example, silicone gaskets 102. The heat separator 103 is a long water-tight box, having a partition wall 104, which divides the box into two compartments, hot water compartment 105, and cool water compartment 107. The heat separator 103 including the partition wall 104 may be made of plastics, preferably but not limited to polypropylene. Alternatively, the heat separator 103 may be made of metal, such as, for example, stainless steel. The hot water compartment 105 has a fitting 106 for hot water to flow from the heat separator to the insulated water tank. The cool water compartment has a fitting 108 for accepting cool water from the insulated water tank. The heat separator is enclosed by an insulating enclosure 109.
Cross-sectional views of the vertically oriented solar energy collector using a heat separator with parallel configuration according to an exemplary embodiment of the present invention are shown in FIG. 1. In cross-section view F, which is in front of the partition wall, the detailed design of partition wall 104 is shown. It is connected to the top wall and the bottom wall of the heat separator, and has tongues protruding into the evacuated tubes 101. In front of the partition wall 104 is the hot-water compartment 105. In cross-section view R, only the cool-water compartment 107 and the incoming fitting 108 are shown with the partition wall 104 in rear of view.
The function of the solar energy collector assembly is as follows. The entire unit, including the evacuated tubes 101 and the heat separator 103 is filled with water. Sunlight, shown as coming from the upper left side of FIG. 1(A), heats up the water near the absorption surface 110. Due to the thermal expansion effect, the heated water has a lower specific gravity and therefore naturally flows upwards toward the hot-water compartment 105. To maintain flow continuity, cool water 111 from fitting 108 and cool-water compartment 107 flows downwards. The difference in water temperature generates a hydrodynamic head which drives water flow from the cool end, 107 and 108, through the evacuated tubes and finally to the hot end, 105 and 106.
As shown in FIG. 1(B), for vertically oriented collectors, the partition wall does not necessary need to make the hot water compartment and the cool water compartment water-tight from each other. More importantly, the tongues of the partition wall 104 inside the heat separator 103 only need to extend into the uppermost section of the evacuated tubes 101. Convection flow works in the system because hot water has a smaller specific gravity than cool water. As shown in FIG. 1A, the system is oriented with an inclination, where the cool water compartment is positioned below the hot water compartment. Consequently, heated water stays in the hot water compartment, and cool water stays in the cool water compartment. Additionally, inside the evacuated tubes 101, heated water ascends along the sun exposed side, and cool water descends along the shaded side. This effect is proved by detailed calculations and experimentation, such as, for example, in I. Budihardjo and G. L. Morrison, Performance of water-in-glass evacuated tube solar water heaters, Solar Energy, Vol 83, Pages 49-56 (2009). Therefore, even if the compartments are not water tight, and the partition wall only extends into the top of the evacuated tubes, convection flow will take place. Extensive experiments of the system have proved that the natural convection flow cycle works as described.
FIG. 2 shows a vertically oriented solar energy collector assembly according to an exemplary embodiment of the present invention using a series heat separator with series configuration and a number of all-glass evacuated tubes. The all-glass evacuated tubes 201 are mounted on a heat separator 203 through, for example, silicone gaskets 202. The heat separator 203 is a long water-tight box, having partition walls 204, which divide the box such that water flows sequentially through each tube, beginning with the tube nearest fitting 208, which accepts cool water from the insulated water tank and ending with the tube nearest fitting 206, which allows hot water to flow from the heat separator 203 to the insulated water tank. The series configuration of the heat separator 203 creates a system whereby water enters each tube from a relatively cool water compartment 207, and after being heated, exits the same tube into a relatively hot water compartment 205. Accordingly, water compartments created by the partition walls 204 increase in temperature as water moves from fitting 208 to fitting 206.
Cross-sectional views of the vertically oriented solar energy collector using a heat separator with series configuration according to an exemplary embodiment of the present invention are shown in FIG. 2. The partition walls 204 are connected to the top wall and the bottom wall of the heat separator 203, and have tongues protruding into the evacuated tubes 201. Water flow directed into a tube 201 by a partition wall 204 exits the heat separator 203 from a relatively cool water compartment 207, and after being heated in the tube, returns to the heat separator creating a relatively hot water compartment 205.
FIG. 2(A) and FIG. 2 (B) combine to show a vertically oriented solar energy collector assembly according to an exemplary embodiment of the present invention using a series heat separator with series configuration with rigidly bent partition walls. FIG. 2(C) and FIG. 2 (D) combine to show another vertically oriented solar energy collector assembly according to an exemplary embodiment of the present invention using a series heat separator with twisted partition walls.
FIG. 3 shows two different horizontally oriented solar energy collector assemblies according to exemplary embodiments of the present invention, each using a heat separator with parallel configuration and a number of all-glass evacuated tubes. FIG. 3(A), FIG. 3(B) and FIG. 3(C) show a collector designed to allow for evacuated tubes to be mounted on two sides of a parallel heat separator and FIG. 3(D) and FIG. 3 (E) show a collector designed to allow for evacuated tubes to be mounted on one side of a parallel heat separator. The all-glass evacuated tubes 301 are mounted on a heat separator 303 through, for example, silicone gaskets 302. The heat separator 303 is a long water-tight box, having a partition wall 304, which divides the box into two compartments, hot water compartment 305, and cool water compartment 307. The hot water compartment 305 has a fitting 306 for hot water to flow from the heat separator to the insulated water tank. The cool water compartment has a fitting 308 for accepting cool water from the insulated water tank. The heat separator is enclosed by an insulating enclosure 309.
Cross-sectional views of the horizontally oriented solar energy collector according to an exemplary embodiment of the present invention using a heat separator with parallel configuration are shown in FIG. 3. In cross-section view F, which is in front of the partition wall, the detailed design of partition wall 304 is shown. It is connected to the right side wall and the left side wall of the heat separator, and has tongues protruding into the evacuated tubes 301. In front of the partition wall 304 is the hot-water compartment 305. In cross-section view R, only the cool-water compartment 307 and the incoming fitting 308 are shown with the partition wall 304 in the rear of view. Unlike in vertically oriented collectors, the distance that the tongues protrude into the evacuated tubes has a greater effect on the flow within the system, whereby in general, longer tongues lead to better flow, up to a point whereby the tongue length can diminish flow. This precise length is a function of the system design.
FIG. 4 shows a horizontally oriented solar energy collector assembly according to an exemplary embodiment of the present invention using a heat separator with series configuration and a number of all-glass evacuated tubes. The all-glass evacuated tubes 401 are mounted on a heat separator 403 through, for example, silicone gaskets 402. The heat separator 403 is a long water-tight box, having partition walls 404, which divide the box such that water flows sequentially through each tube, beginning with the tube nearest fitting 408, which accepts cool water from the insulated water tank and ending with the tube nearest fitting 406, which allows hot water to flow from the heat separator to the insulated water tank. The series configuration of the heat separator creates a system whereby water enters each tube from a relatively cool water compartment 407, and after being heated, exits the same tube into a relatively hot water compartment 405. Accordingly, water compartments created by the partition walls 404 increase in temperature as water moves from fitting 408 to fitting 406.
Cross-sectional views of the horizontally oriented solar energy collector according to an exemplary embodiment of the present invention using a heat separator with series configuration are shown in FIG. 4. The partition walls 404 are connected to the right side wall and the left side wall of the heat separator, and have tongues protruding into the evacuated tubes 401. Water flow directed into a tube 401 by a partition wall 404 exits the heat separator 403 from a relatively cool water compartment 407, and after being heated in the tube, returns to the heat separator 403 creating a relatively hot water compartment 405. Unlike in vertically oriented collectors, the distance that the tongues protrude into the evacuated tubes has a greater effect on the flow within the system, whereby in general, longer tongues lead to better flow, up to a point whereby the tongue length can diminish flow. This precise length is a function of the system design.
FIG. 5 shows schematically a complete solar water heater according to an exemplary embodiment of the present invention using a heat separator 502 which can be configured either as a parallel or series heat separator, with all-glass evacuated tubes 501. As shown, the inlet 507 of the heat separator 502 is connected to the bottom of an insulated water tank 205 through an insulated pipe 506. The outlet 503 of the heat separator 502 is connected to the upper portion of the insulated water tank 505 through an insulated pipe 504. The water tank 505 may be made of plastics, with an inner shell made of preferably but not limited to polypropylene, an outer shell made of preferably but not limited to polyethylene, and insulation made of preferably but not limited to foam polyurethane. Alternatively, the inner and outer shells of the tank 505 may be made of metal, preferably but not limited to stainless steel; and insulation made of preferably but not limited to foam polyurethane. When sunlight is incident upon the top side of the evacuated tubes 501, hot water is generated and flows into the water tank 505. To maintain flow continuity, cool water from the bottom of the water tank 505 circulates back through pipe 504 to the heat separator 502, and subsequently into the evacuated tubes 501. Theoretical analysis and direct experiments have shown that under the conditions where the tank is placed higher than the heat separator and sunlight is incident on the collector, water flow starts and sustains automatically due to natural convection without requiring a pump. Hot water for residential and domestic purposes can be obtained from the water tank through outlet 508, and cool water from the water main can be supplied to the water tank through inlet 509.
FIG. 5 only shows schematically the basic components of a complete solar water heater. A manufactured, commercially available water heater can have parts of secondary importance, intended to supplement the overall efficiency and performance. FIG. 6 shows some but not all of these potential secondary components. For example, the potable water can be heated through a heat-exchange coil 610 positioned inside the insulated water tank 605 which is commonly but not exclusively made of copper. A heat dissipation unit 611 can be placed to prevent overheating. A backup electrical heater 612 can be employed in case of prolonged cloudy days. A thermostat 613 can be used to control the backup electrical heater and the heat dissipation unit. Heat storage components including but not limited to paraffin wax can be placed inside the insulated water tank. An example of a possible design for a paraffin wax thermal storage system is shown in FIG. 9. As stated, there are many additional supplementary components which can be added to the solar water heating systems disclosed herein, not all of which are shown.
FIG. 7 shows schematically a complete solar water heater using a vertically oriented heat separator 702 according to an exemplary embodiment of the present invention which can be configured either as a parallel or series heat separator, with all-glass evacuated tubes 701. As shown, the inlet 707 of the heat separator 702 is connected to the bottom of an insulated water tank 705 through an insulated pipe 706. The outlet 703 of the heat separator 702 is connected to the upper portion of the insulated water tank 705 through an insulated pipe 704. When sunlight is incident upon the evacuated tubes 701, hot water is generated and flows into the water tank 705. To maintain flow continuity, cool water from the bottom of the water tank 705 circulates back through pipe 704 to the heat separator 702, and subsequently into the evacuated tubes 701. The horizontal orientation of the collector corresponds to a vertically oriented heat separator. This results in an increased range over which the collector can be placed relative to the tank. When sunlight is incident on the collector, flow will start and be maintained automatically by natural convection without a pump if the base of the tank 705 is positioned either at the same height or above the base of the heat separator 702. Hot water for residential and domestic purposes can be obtained from the water tank through outlet 708, and cool water from the water main can be supplied to the water tank through inlet 709. The water tank 705 may also include a heat-exchange coil 710.
FIG. 8(A) shows parabolic reflector 802 according to an exemplary embodiment of the present invention and FIG. 8(B) shows flat reflector 803 according to an exemplary embodiment of the present invention, either of which can be placed on the bottom side of the evacuated tubes 801 in order to increase overall system efficiency. The purpose of the reflectors 802, 803 is to redirect a light ray 804 which falls between two evacuated tubes 801 back up toward the tubes, thereby increasing the likelihood that a given light ray falling within the collector area will be absorbed. Both reflectors shown, as well as any other reflector configuration not shown which increases the amount of solar energy captured by the evacuated tubes, will increase the efficiency of the collector for all of the collector configurations shown herein, including those with varying heat separator configurations.
FIG. 9 shows a paraffin wax thermal storage system placed inside water storage tank 901 according to an exemplary embodiment of the present invention for the purposes of increasing the heat energy storage capacity of the tank. The paraffin wax thermal storage system consists of a number of sealed containers 902 of paraffin wax 902. The system can function using many different types of containers 902 for holding the paraffin wax 903 including but not limited to long heat sealed plastic tubes with elliptical or rectangular cross-section, or long plastic bottles with elliptical or rectangular cross-section sealed with a cap. The paraffin wax containers 902 are submersed in heat transfer fluid 904 which may or may not be water. Ideally, the paraffin wax 903 has a melting point near the optimum operating temperature of the storage tank. The tank is surrounded by insulating material 905.
The paraffin wax thermal storage system according to an exemplary embodiment of the present invention functions as follows. For a storage tank with heat transfer fluid 904 and paraffin wax 903 both at a typical ambient temperature well below the operating temperature of the tank, as heat is added to the storage tank either by a solar collector or any other mechanical means, the temperature of the heat transfer fluid present in the tank increases. Heat transfer occurs between the heat transfer fluid 904 and the paraffin wax 903 stored in the paraffin wax containers 902. If this process is allowed to continue until the tank reaches its operating temperature, the paraffin wax 903 will have melted into a liquid and eventually will reach a temperature in equilibrium with the heat transfer fluid 904. The liquefied paraffin wax 904 stores heat energy proportional to the heat capacity of liquid paraffin as well as the latent heat energy obtained by the paraffin wax 903 during the melting process. This results in an overall net increase in the amount of energy storage capacity of the tank.
Heat energy can then be drawn from the system, either by removing the heat transfer fluid 904 directly as in systems like the one shown in FIG. 9(A) and FIG. 9(B), or by using a heat exchange mechanism including but not limited to heat exchange coil 906 as shown in FIG. 9(C) and 9(D). In the case where the heat energy is drawn from the tank by a heat exchange mechanism such as heat exchange coil 906, the paraffin wax 903 will replenish heat energy lost by the heat transfer fluid 904. Furthermore, if the paraffin wax 903 is chosen to have a melting point near the operating temperature of the system, then the reservoir of latent heat stored in the paraffin wax 903 allows the system to operate longer at a higher temperature. Finally, as heat energy is added to the heat transfer fluid 904 in the tank, in order to replace heat which has been removed, the process is facilitated by the relatively hotter paraffin wax 903.
FIG. 10 shows a vertically oriented solar energy collector assembly according to an exemplary embodiment of the present invention using a heat separator with parallel configuration and a number of triple-concentric three cavity evacuated tubes. Each triple-concentric three cavity evacuated tube 1001 contains an empty glass tube 1002 which reduces the volume of water in the tube, thus accelerating the heating process and reducing the weight of the collector. The triple-concentric three cavity evacuated tubes 1001 are mounted on a heat separator 1005 through, for example, silicone gaskets 1004. The central cavity 1002 is supported by sheet metal springs 1003 from the evacuated tube 1001. Fluid flows in the space between the outside of the central cavity 1002 and the inside of the evacuated tube 1001 and is not blocked or restricted by the metal support springs 1003. FIG. 10(B) and FIG. 10(C) show views in front and in rear of the central cavity 1002 of the evacuated tube 1001 respectively.
FIG. 11 shows schematically a complete solar water heater according to an exemplary embodiment of the present invention using a heat separator 1102 which can be configured either as a parallel or series heat separator, with all-glass evacuated tubes 1101. As shown, the inlet 1107 of the heat separator 1102 is connected to the bottom of an insulated water tank 1105 through an insulated pipe 1106. The outlet 1103 of the heat separator 1102 is connected to the upper portion of the insulated water tank 1105 through an insulated pipe 1104. The heat separator 1102 can be oriented horizontally, vertically or, with an angle inclined to the sun as shown. Additionally, the evacuated tubes 1101 can be angled away from the sun. The heat separator 1102 can have a trapezoidal cross-section, or any other non-rectangular cross-section which assists a water-tight connection between the heat separator 1102 and the evacuated tubes 1101.
While particular embodiments of the invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications may be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.
Patent Application
20120132195
