Thermosyphon solar water heater with automatic backflush

Abstract

A thermosyphon solar hot water heating system specifically designed such that it incorporates an integral, automatic backflush configuration. When hot water is drawn from the system, an automatic backflush valve redirects cold "mains" water through the system's solar collector(s) in a flow direction that is opposite to the normal buoyancy-driven flow direction that occurs during solar heating (i.e., "charging"). This backflushing action is completed passively when heated water is consumed for domestic use and does not require other user intervention. The routine introduction of cold water through the solar collector during the backflush, reduces the formation of mineral deposits in the solar collector's flow passages and the reverse flow scours the interior surfaces of the solar collector's flow channels such that the potential of scaling is significantly reduced.

Description

THERMOSYPHON SOLAR WATER HEATER WITH AUTOMATIC BACKFLUSHTECHNICAL FIELD

[0001] The present invention relates to a thermosyphon solar domestic hot water heater designed with and integral, automatic backflush feature that redirects cold makeup water through the unit's solar thermal collectors whenever hot water is drawn from the system, thereby reducing the buildup of mineral scaling in the flow passages that reduces the system's performance and leads to system failure.BACKGROUND

[0002] The inventor has previously developed back-flushing valve assemblies for heat exchangers such as shown in U.S. 7,823,628; U.S. 7,171,972; and US 6,827,091, and has other patents pending. These have proven successful in passively backflushing heat exchangers used in indirect, pumped circulation, solar water heating systems to clear and / or reduce fouling, as explained in Harrison, S. J., 2005, Passive Heat Exchanger Anti-Fouling for Solar DHW Systems, DOI:10.1115 / ISEC2005-76232, ASME 2005 International Solar Energy Conference.

[0003] Throughout the world, it is common practice to use solar energy to heat water for domestic use. A typical thermosiphon solar system (shown in Figures 1A - IE) is a simple and effective solar water heating system that relies on a thermally driven, buoyancy-induced, natural convection flow to circulate water through a solar collector 7 to a storage tank 6 located at a position that is elevated above the solar collector as shown in Figure 1A. These systems are not equipped with circulation pumps and do not require electrical power sources to operate. The most common units circulate cool potable water from the bottom of an elevated storage tank, directly through the solar collector(s), where it is heated and returned to the top of the storage tank. The heated water will normally rise to the upper section of the storage tank and the cooler water will fall to the bottom of the storage tank such that the water in the storage tank achieves a "stratified" temperature profile. The heated water is stored until it is drawn off for domestic use through an outlet port 7 located in the upper section of the storage tank. As shown in FigureIB to IE, as hot water is drawn from the storage it is replaced by new cold water, under pressure, from a municipal water supply 3 or other source (for the purpose of this document referred to as the "mains" water supply). As illustrated in Figures 1A - IE, an optional resistive heating element 6a is shown in the storage tank that is intended to provide auxiliary heat when there is insufficient solar energy to heat the required quantity of hot water. This optional feature, although not required, can be added to all the embodiments shown without impacting the operation of the automatic backflush feature described in the current disclosure.

[0004] The original thermosyphon solar water heater was invented over 100 years ago but has not changed appreciably since then. See US Patent 966,070. Other design improvements and features have been introduced since that time to improve thermal performance and durability (e.g., temperature limits, freeze protection etc.) but none have addressed the fouling of the system due to hard-water conditions. In regions where the water supply contains dissolved minerals such that it is considered "hard water", inverse-soluble salts ( e.g., CaCOs) deposit on the interior of the hot flow passages in the solar collector(s)' absorber plate 77, reducing heat transfer and increasing the pressure drop through the flow channels. See Arunachala , U.C. et al., 2010, Scaling Effect of Direct Solar Hot Water Systems on Energy Efficiency, Solar Energy Division of ASM E for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Vol. 132 / 041012-1. This reduces the flowrate through the solar collector(s) and results in higher temperatures in the solar collectors, significantly reducing system efficiency. See U. C. Arunachala, L. K. Sreepathi & M. Siddhartha Bhatt (2014), Analytical studies on drop of H-W-B constants due to scaling in natural circulation flat plate solar water heater, International Journal of Sustainable Energy, 33:1, 192-202. Continued deposition of hard-water scale will result in blockage of the flow passages and failure of the system. Lugo- Granados H., Canizalez-Davalos L., Picon-Nunez M., 2023, Thermohydraulic Effects of Scaling in Flat Plate Solar Collector Networks, Chemical Engineering Transactions, 103, 421-426. Thermosyphon solar systems are particularly vulnerable to scaling fouling due to their low flow velocities through the solar collector flow-passages, high surface temperatures and low circulation pressure head. To date, disassembly and mechanical or chemical cleaning have been the only options to reduce orremove scale buildup. As this is impractical in most installation, systems are usually disconnected, or components replaced prematurely.

[0005] It would be desirable to address the known issues with hard-water scale in the previously known thermosyphon solar water heaters.SUMMARY

[0006] A THERMOSYPHON SOLAR WATER HEATER WITH AUTOMATIC BACKFLUSH is disclosed herein that incudes a single solar heating collector (Figures 2A - 2E), or multiple solar collector panels (Figure 3), that absorb solar radiation, heating cool water supplied from an elevated storage tank as it flows through flow passages in the collector(s)' "absorber" plate(s), delivering heated water to the elevated water storage tank. The water heater disclosed herein incorporates an integral, automatic backflush configuration (or feature) whereas when heated water is drawn from the system's storage tank, an automatic backflush valve redirects incoming, cold "mains" water, from flowing directly to the storage tank, through the system(s)' solar collector(s) in a flow-direction that is opposite to the normal buoyancy-driven flow-direction that occurs during solar heating (i.e., "charging"). This backflushing action is completed automatically when heated water is consumed for domestic use and does not require other user intervention.

[0007] The automatic backflush is initiated when heated water is withdrawn from the system, causing the pressure in the system to drop below the "mains" supply water pressure. This causes "mains" water to flow into the system, triggering the automatic backflush, whereas, the cooler "mains" water entering the system, is directed through the solar collector(s) in the reverse flow direction, at a relatively high flowrate, effectively cooling the collector flow passages, redissolving mineral deposits and scouring the heat transfer surfaces of any accumulated particulates.

[0008] When the hot water delivery valve is closed and water withdrawal stops, the system repressurizes and the backflush valve opens such that normal thermosyphon flow from the solar collector outlet to the storage tank can be re-established, allowing the storage tank to be recharged as normal by solar heating.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figures 1A - IE show the typical configuration of a common thermosyphon solar water heater without Automatic Backflush. Figure 1A indicates the water flow direction in the system in"charging" mode when water is not withdrawn from the system. Figure IB and 1C - IE illustrate the water flow direction during withdrawal of heated water from the system.

[0010] Figures 2A - 2E shows the configuration of a thermosyphon solar water heater with integral Automatic Backflush. Figure 2A indicates the water flow direction in the system during solar "charging" when water is not withdrawn from the system. Figure 2B and 2C - 2E illustrate the water flow direction during withdrawal of heated water from the system.

[0011] Figure 3 shows a thermosyphon system with an integral Automatic Backflush System incorporating two solar collectors plumbed in a "series-parallel" configuration. Also shown is the location of an optional "dip tube", inserted into the tank through the port to deliver and supply "mains" water from a central position of the storage tank.

[0012] Figures 4A - 4C show a thermosyphon solar water heater with integral Automatic Backflush System with a vertical storage tank.

[0013] Figure 5 shows the disclosed system with the alternative location for the backflush valve.

[0014] Figure 6 shows a thermosyphon solar water heater with integral automatic backflush system in backflush mode with solenoid valve actuated by remote sensor.

[0015] Figure 7 is a side view of a thermosyphon solar water heater with integral automatic backflush system installed on a sloped roof. The installation is shown with optional tank cover.

[0016] Figures 8A - 8C show examples of three flow valves that transition from a normally- open position to a closed position, with respect to the flow of water from point I to point II, in response to a change in flowrate or differential pressure caused by the withdrawal of water from the system. Fig. 8A shows a valve that relies on a neutral density ball that rises in response to a predetermined flow condition, restricting the in-flow of "mains" water (point I) to the system's storage tank through point II, redirecting it to point III through the system's solar collector(s). Fig. 8B shows a valve, shown in the closed position, where it redirects the in-flow of "mains" water from point II to Point III in response to a pressure differential caused by the withdrawal of water from the system. Fig. 8C shows a valve that operates on a "shape memory alloy" that changes shape during the entry of cooler mains water into the system during the withdrawal of water from the system.

[0017] Figure 9 illustrates one preferred embodiment of the thermosyphon solar water heater with automatic backflush, operating in charge mode when no water is withdrawn from the system.

[0018] Figure 10 illustrates the preferred embodiment of the thermosyphon solar water heater with automatic backflush as shown in Fig. 9, operating in backflush mode when water is being withdrawn from the systems storage tank and the makeup "mains" water is directed through the solar collectors in a flow direction that is opposite to the charging mode thermosyphon flow.DETAILED DESCRIPTION

[0019] For typical thermosyphon solar systems without automatic backflush, as shown in Figures 1A - IE, when water is drawn from the system, pressurized make-up "mains" water is fed to the bottom section of the elevated storage tank, and is delivered to the domestic load from the top of the storage tank, Figure IB - IE.

[0020] In this invention, as shown in Figures 2A - 2E, an automatic backflush feature is integrated in the system such that, when water is withdrawn from the storge tank through pipe 7 by the opening of a tap or valve (downstream of the system), the system pressure is reduced and makeup "mains" water at higher pressure flows into the system, through supply pipe 3, to replace the exiting fluid, Figures 2B - 2E.

[0021] As the makeup "mains" water enters through pipe 3, a (normally-open) valve 12 closes and the entering "mains" water is diverted through the solar collector(s)' outlet port ? (located at the top of the solar collector 7) and the collector's absorber plate 77, in a flow direction that is in the opposite direction to that naturally occurring during thermosyphon heating when no water is drawn from the system, Figures 2B - 2E. This reverse flow of cold "mains" water backflushes the solar collector and system piping during the consumption of heated water from the storage tank.

[0022] For the purpose of this submission, valve 12, will be referred to as the "backflush" valve, and with associated system configuration, plumbing arrangement and operation, constitutes the integral backflush feature described in this disclosure.

[0023] When water is not being drawn from the system, the pressure in the system will reach a pressure equal to the "mains" water supply, and "mains" water will not flow into the system.In this state, the backflush valve will be in, or return to, the "normally-open" position such that the flow of water from outlet port 2 of the solar collector(s) to the inlet of storage tank 5, is unrestricted.

[0024] During solar heating (aka, "charging") as shown in Figure 2A , when hot water is not being withdrawn (i.e., consumed), such that "mains" water does not enter the system, the system operates as a conventional thermosyphon. In this mode of operation, heating of water in the solar collector(s) induces a buoyancy-driven flow through the solar collector(s), that exits through port ? and flows unrestricted through pipe 4 to the inlet of the storage tank 5. Simultaneously, cooler water from the bottom of the storage tank exits through port and flows through pipe 9 into the lower port 10 of the solar collector(s). This thermosyphon circulation will continue until, the water in the storage tank is fully heated, or there is insufficient solar energy to sustain the buoyancy-driven circulation.OTHER EMBODIMENTS OF THE INVENTION

[0025] Although the majority of the figures illustrating the embodiments of the invention are shown for the case of a system with a single solar collector module, the thermosyphon system with automatic backflush can incorporate multiple solar collectors when they are plumbed in a "series-parallel" configuration as shown in Figure 3. As shown in this figure, the absorber piping of collectors 1 and 1a are connected at points 14a and 14>, such that the water flowrate is equal through both collectors during charging and discharging of the thermal storage.

[0026] Figure 3 also shows the location of an optional "dip-tube", or pipe extension 15 that may be inserted into the tank through port to deliver "mains" water to a central position of the storage tank during backflush. This will reduce the potential of "mains" water flowing into the tank and "short-circuiting" whereby the cool "mains" water is delivered directly to the outlet 7 of the storage tank. When water is not being drawn from the system, the "dip-tube" will supply water from the bottom of the storage during thermosyphon circulation.

[0027] In most cases, the "dip-tube" will not be required as the incoming backflush "mains" water will feed into the lower periphery of the storage tank and will mix with the water in the storage tank, having minimal effect on the temperature of heated water withdrawn from the system. Thermal stratification in the storage tank will also ensure that heated water is delivered to the load during backflush.

[0028] Figures 4A - 4C show an embodiment in which the storage tank is positioned in a vertical orientation rather than the more common horizontal positioning of the storage. The operation of the system with a vertical storage tank is effectively the same as the horizontal storage tank case. Figures 4A- 4C depict the system during automatic backflush as water is being with drawn from the system.

[0029] In all embodiments described in this disclosure, the storage tank is elevated "mostly" above the top of the solar collectors such that water in the system may thermosyphon during solar heating when the system is being backflushed.

[0030] Figures 2A - 2E, 3 and 4A - 4C show the placement of the mains water supply 3 and backflush Valve 12 near the outlet port 2 of the solar collector, however an alternative arrangement is shown in Figure 5, where the "mains" water supply and backflush valve are located near the inlet to the storage tank . In all cases, the intent is the same, i.e., to restrict the flow of mains water from entering the storage tank through port 5, and to redirect it through the solar collector outlets? and solar collector absorber 77, such that the system is backflushed during the consumption of heated water from the system.

[0031] Figure 6 shows one embodiment of the system whereby the integral, automatic backflush feature uses an electrically powered solenoid valve 12 that goes from a "normally- open" position to a "closed" position in response to a signal from a remote sensor (e.g., 7? / ) that indicates a change in flow, pressure, or temperature occurring during the withdrawal of water from the system. The closing of the solenoid valve restricts the flow of "mains" water from entering the storage tank through port 5ttriggering the backflush of the system.

[0032] In all embodiments shown, the backflush valve can be actuated (i.e., closed) by one or all of changes in the system pressure, temperature, or flow, or when a pressure differential between the "mains" water supply and the solar system occurs during the consumption of heated water. When water is not being drawn from the system, the pressure in the system will reach a pressure equal to the "mains" water supply, and "mains" water will not flow into the system. In this state, the backflush valve will be in, or return to, the "normally-open" position such that the flow of water from outlet port 2 of the solar collector(s) to the inlet of storage tank 5, is unrestricted.

[0033] In Figures 1A to 6, the configurations shown are usually mounted on a horizontal roof or the ground and are held in position by a support rack such the solar collector can be tilted at a slope 75, and the storage tank can be elevated, such that thermosyphon flow can initiate.

[0034] An alternative arrangement is one in which the solar collector is mounted directly on a tilted roof, and the storage tank is mounted on the roof above the collectors as shown in Figure 7. In certain instances, this arrangement of the system may present a more integrated architectural feature or be required if a flat roof area is not available. Figure 7 also show the addition of an optional cover placed over the storage tank.

[0035] In all the embodiments described, when hot water is not being withdrawn (i.e., consumed) during solar heating (aka, "charging"), such that "mains" water does not enter the system, the system operates as a conventional thermosyphon.PREFERRED EMBODIMENT

[0036] One preferred embodiment of the THERMOSYPHON SOLAR WATER HEATER WITH AUTOMATIC BACKFLUSH is shown in Figures 9 and 10. In Figure 9, the water heater is shown operating in charge mode when no water is withdrawn from the system. The arrangement includes a horizontal water storage tank <5 connected by piping to a "flat-plate" solar thermal collector 7 such that cool water from the bottom periphery of the storage tank is fed through pipe 9, into the lower inlet port 10 of the solar collector. Whereupon being heated by solar energy, a positive buoyancy force is created in the "riser" tubes 77 / of solar collector's absorber11a such that when the top outlet port of the solar collector is connected to the inlet port 5 located in the upper periphery of the storage tank by piping 4, a thermosyphon flow will be created such that water will be heated as it flows through the solar collector. The thermosyphon flow direction of water in the system during "charging" is shown as indicated by the (solid) direction arrows shown on Figure 9.

[0037] Flat-plate solar collectors are common and well understood by practitioners in the solar water heating field. Figure 9 shows a "cut-away" view of a typical solar thermal collector. For this preferred embodiment, the solar collector consists of an insulated shallow "box" case Z, insulated at the sides and back with thermal insulation, 77d. The "top" of the solar collector is fitted with a transparent upper cover ZZ? (e.g., glass sheet) to allow solar radiation to enter the case where it can be absorbed and converted to heat in the solar absorber ZZ. Mounted in the interior of the solar collector below the transparent cover ZZ?, the solar absorber "plate" ZZ is positioned in parallel to the transparent cover such that sunlight transmitted through the cover illuminates it. The absorber plate consists of a thermally conductive sheet attached to a series of "riser" tubes 77 / , connected in parallel to upper and lower "header pipes 10a, 10>, that transport the fluid from the bottom "header" pipe 10a to the top "header" pipe 10> of the solar collector. In the preferred embodiment, the absorber is a thin thermally-conductive sheet (e.g., aluminum or copper sheet < than 1 mm thickness) and coated on the upper face such that it will absorb solar energy, converting it to heat that can be transferred to the water flowing in the aforementioned parallel flow tubes 77 / .

[0038] The system will have a solar collector array (i.e., the section of the system consisting of single or interconnected multiple solar collectors) with total absorber area of between 1.5 and 8 m2, and a cylindrical water storage tank of between 100 and 400 L volume. The storage tank is insulated with 2.5 to 10 cm of glass-wool or foam insulation. Interconnection piping in the system is between 1.2 and 2.5 cm ID and preferably insulated with 1.2 to 6 cm of fiber orfoam insulation to reduce heat loss. The solar collector(s) are fixed at a tilt ranging from 20 to 90 degrees to the horizontal (indicated as 75 in the attached Figures) such that they receive solar irradiance during all or part of the day. In the preferred embodiment shown, the system is supported by a separate "support rack" 13a that can be supplied with the system or fabricated by the installer or user.

[0039] The riser tubes 77 / in the solar collector are made of copper (or other heat conducting material) with inner diameter of 6 to 12 mm and are attached to the underside of the absorber sheet at 7 to 15 cm spacing, such that the generated solar heat is efficiently transferred to the water circulating in the tubes. The "riser tubes" 77 / are thermally and mechanically attached to the absorber plate 77 by one of welding, soldering, bonding or clamping.

[0040] The flow tubes are hydraulically connected to upper and lower "headers" 10a, 10>, (typically of 1.2 to 2.5 cm inner diameter) such that a "harp" flow configuration is created with a nearly equal flowrate of water in each riser tube.

[0041] During thermosyphon flow, cool (dense) water entering the collector through port 10 is delivered through the lower header pipe 10a to the riser tubes where it is heated, lowering its density, thereby creating a buoyancy force. This drives the flow of water in each riser tube to the upper header pipe 75 / , exiting through the outlet port of the solar collector and the backflush valve 12. This flow of water exits the backflush valve and flows through a connection pipe 4 to the upper port 5 of the water storage tank, entering and accumulating in the upper periphery of the storage tank. This process continues until the majority of the water in the system is heated or sufficient solar irradiance is unavailable to "drive" the thermosyphon flow. Typical thermosyphon circulation flow rates depend on the temperature distribution of the water in the storage tank, the geometry and orientation of the system, and the availability of solar generated heat. Typical thermosyphon flowrates range from 0.1 to 3 L / min. In systems intended forresidential installations, water consumption rates range from 1 to 20 L / min depending on user needs.

[0042] Figure 10 shows the system in automatic backflush mode that is initiated by the withdrawal of heated water from the upper periphery of the storage tank through exit ported. It is noted that the location of exit ported, to withdraw water from the storage tank, can be placed in various locations on the storage tank, e.g., either end of the storage, the top, etc. It is a similar case for exit port that feeds cool water from the storage to the bottom of the solar collector. It can be placed at any position in the lower periphery of the storage tank, preferably close to the bottom of the storage tank.

[0043] In this preferred embodiment, the "backflush" valve 12 is located between the outlet of the solar collector and the upper inlet to the storage tank 5. Mains water is supplied to the valve through "mains" supply pipe 3, such that during the consumption of heated water from the system through exit port &, the valve goes from a "normally-open" condition to a closed condition, restricting the flow of "mains" water through pipe 4 to the storage tank. This action simultaneously redirects the incoming "mains" water through the solar collector(s) in a flow direction that is opposite to that occurring during the thermosyphon flow.

[0044] The withdrawal of heated water from the storage tank lowers the pressure in the storage tank, such that the mains water entering the system "backflushes" the solar collector as indicated by the (solid) direction arrows shown in Figure 10.

[0045] Various backflash valve configurations can be used to control the flow of incoming "mains" water into the thermosyphon system with integral backflush. Three suitable valve configurations are shown in Figure 8. All three shown transition from a normally-open position to a closed position, in response to a change in flowrate or differential pressure or temperature caused by the withdrawal of water from the system. A mechanical / electrical valve could also be used acting on a signal generated when water is drawn from the storage tank.

[0046] In the preferred embodiment, the "backflush" valve will be of the type shown in Figure 8B, that isolates (e.g., closes) the flow path between the incoming mains water pipe 3 and the system when the system is not in backflush mode, but when water is withdrawn from the storage tank, hydraulic forces (e.g., pressure and flow) opens the "mains" water connection and simultaneously closes the flow path from the "mains" inlet to the storage tank through pipe 4. The action of the valve redirects the incoming "mains" water back through the solar collector actuating the backflush. When the water withdrawal ceases, the system repressurizes to the "mains" water supply pressure, isolating the "mains" water supply, and opening the flow connection from the solar collector to the storage tank allowing thermosyphon flow to initiate.

Claims

CLAIMSWe claim:

1. A Thermosyphon Solar Water Heater with Automatic Backflush that heats hot water by a buoyancy-induced thermosyphon flow, the water heater comprising: a solar collector through which water is adapted to circulate; a thermal storage reservoir located at an elevation primarily above the solar collector; a make-up water connection via which makeup water is adapted to enter the solar collector at an elevated end thereof as heated water is withdrawn from the storage reservoir to provide an automatic backflush action; and a heated water outlet.

2. The water heater of claim 1, further comprising an integral, automatic backflush valve that, when the heated water is withdrawn, is adapted to direct the makeup water through the solar collector.

3. The water heater of claim 2, wherein when water is withdrawn, the automatic backflush valve directs the makeup water through the solar collector and associated piping, in a flow direction that is opposite to a normal buoyancy-driven flow direction that occurs during solar heating.

4. The water heater of claim 1, wherein the makeup water adapted to be redirected, from flowing directly to the storage reservoir, through the solar collector in a direction that is opposite to a normal buoyancy-driven flow direction.

5. The water heater of claim 2, wherein when the heated water is not withdrawn from the system, the integral backflush valve does not restrict a buoyancy-driven thermosyphon circulation of water from the thermal storage reservoir, through the solar collector, returning to the thermal storage reservoir.

6. The water heater of claim 1, wherein the automatic backflush action is completed automatically, without direct user interaction(s), when the heated water is withdrawn.

7. The water heater of claim 1, wherein the automatic backflush action occurs when a change in at least one of a flow, pressure, or temperature occurs during the withdrawal of the heated water.

8. The water heater of claim 1, wherein, as heated water is withdrawn, in response to a pressure differential between the makeup water and a pressure in the water heater, an automatic flow valve directs incoming makeup water through the solar collector in a direction that is opposite to a normal buoyancy-driven flow direction in the water heater.

9. The water heater of claim 2, wherein the automatic backflush valve, during the withdrawal of the heated water, restricts a flow of water from an upper outlet of the solar collector directly to the thermal storage reservoir.

10. The water heater of claim 9, wherein, during the hot water withdrawal, the valve goes from a normally-open position to a closed position in response to a change in at least one of a system pressure, temperature or flow condition.

11. The water heater of claim 10, wherein, the integral, automatic backflush valve comprises an electrically powered solenoid valve that goes from a normally-open position to closed in response to a sensor signal that indicates a change in at least one of: flow, pressure, or temperature occurring during the withdrawal of heated water.

12. The water heater of claim 1, wherein the solar collector comprises multiple individual thermal solar collector units, plumbed such that a normal buoyancy-driven, thermosyphon flow is adapted to occur from a lower periphery to the thermal storage reservoir, which is elevated relative to the lower periphery, and as heated water is withdrawn, in response to a pressuredifferential between the makeup water and a pressure in the water heater, an automatic flow valve directs incoming makeup water through the multiple individual thermal solar collector units in a direction that is opposite to the normal buoyancy-driven flow direction in the water heater.

13. The water heater of claim 1, wherein the automatic backflush action reduces a formation of fouling and "hard water" scaling in solar thermal solar collector.

14. The water heater of claim 2, wherein at a cessation of the heated water withdrawal, thermosyphon flow is re-established, uninhibited by the integral automatic backflush valve.

15. The water heater of claim 1, wherein the solar collector comprises a combined photovoltaic and solar thermal collector.

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