Abstract:
The present invention generally relates to solar water heaters, in particular to solar water heaters using glass vacuum tubes as both solar energy collector and thermal energy storage device, without a hot water storage tank. To improve the efficiency of thermal energy storage, a novel medium for thermal energy storage is disclosed, which utilizes the heat of solution of aluminum sulphate, comprising water and 40% to 47% of Al 2 (SO 4 ) 3 . The working temperature range of such energy storage medium is between 50° C. and 100° C. The energy storage medium is contained in plastic capsules, submerged in water and placed in glass vacuum tubes.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to solar water heaters, in particular to solar water heaters using heat of solution of aluminum sulfate inside glass vacuum-tube solar radiation collectors to store energy, together with a water handling apparatus to transfer stored energy to water. The system can operate without a hot water storage tank. 
       BACKGROUND OF THE INVENTION 
       [0002]    To date, for most solar water heaters, both those using flat-plate collectors and those using vacuum-tube collectors, hot-water storage tank is an essential component, because sunlight is intermittent. To keep the water hot for hours and days without sunlight, tank size must be sufficiently large, typically 100 liters to 250 liters. For convection-operated systems, the tank must be placed above the solar radiation collector. The weight of the tank is acting on the roof as a concentrated mechanical pressure. If the heat storage tank is placed no higher than the heat collector, an electric pump is necessary. The electrical pumps, control units, connecting pipes and valves between the solar thermal energy collector and the heat-storage tank are expensive, and require frequent service and maintenance. 
         [0003]    The vacuum tubes as solar radiation collectors are also excellent heat storage devices because of the vacuum sleeve. To one with an ordinary skill in the field of solar water heaters, it is obvious that vacuum tubes can be used to store energy. Therefore, an array of vacuum tubes alone could function as a complete solar water heater without a hot water storage tank. Such a design is advantageous over the solar water heaters with a tank: Comparing with the integrated convection-driven systems, the overall structure is simplified, and the mechanical pressure on the roof is reduced and evenly distributed. Comparing with the separated systems, the electricity-powered pump and control unit can be eliminated; therefore the system can run maintenance free. However, by using sensible heat of water to store thermal energy, the volume must be very large. Vacuum tubes of very large diameter must be used. Therefore, in order to build a tankless solar water heater, two problems must be resolved. First, to find a type of thermal-energy storage medium with the following properties: (1) having a heat capacity much greater than water in the upper range of water temperature; (2) having no hysteresis or incongruent phenomenon during heat cycling; (3) inexpensive; and (4) nontoxic. Second, to design a water handling apparatus to transfer the thermal energy stored inside the vacuum tubes to running water with the following properties: (1) with no need of an electrical power; (2) automatic control based on the nature of materials; (3) ensuring the highest possible efficiency; (4) minimizing heat loss; (5) inexpensive to produce; and (6) easy to install. It is the goal of the current invention to resolve the above problems to construct tankless solar water heaters using vacuum tubes. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    The current invention discloses first a novel medium for thermal energy storage by utilizing the heat of solution of aluminum sulphate. In the temperature range of 70° C. to 90° C., direct experiments show that the heat capacity of aluminum sulfate solution within a well-defined concentration range is more than 2 times the heat capacity of water per unit volume. Therefore, the volume required to store thermal energy is reduced by a factor of more than two. Next, the current invention discloses a design of a water handling apparatus that would satisfy all the requirements stated in the previous paragraph. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  shows the solubility data for aluminum sulfate. 
           [0006]      FIG. 2  shows a chart of solubility of aluminum sulfate from the data of  FIG. 1 . 
           [0007]      FIG. 3  shows the predicted heat capacity of aluminum sulfate solution. 
           [0008]      FIG. 4  shows the experimental cooling curves of aluminum sulfate solution. 
           [0009]      FIGS. 5A-5E  show plastic capsules containing aluminum sulfate solution and the placement of the said capsules in a vacuum tube. 
           [0010]      FIG. 6  shows the production process of thermal energy storage capsules. 
           [0011]      FIG. 7  shows the water handling apparatus. 
           [0012]      FIG. 8  shows the entire solar water heater system. 
           [0013]      FIG. 9  shows a tankless solar water heater mounted on a roof. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Currently, the most popular solar thermal energy collector is the glass vacuum-tube collector. Every year, more than 200 million pieces of glass vacuum tubes are produced and installed. Each said vacuum tube has an outer glass tube and an inner glass tube with a vacuum between the two said glass tubes, and a selective absorption coating on the outer surface of the said inner glass tube to absorb solar radiation and to reduce the heat radiation from the material inside the said inner glass tube. Because solar radiation is intermittent, solar water heaters require a large insulated water tank to store thermal energy, typically 100 liters to 250 liters. Vacuum tubes are superb thermal insulation devices. To one with an ordinary skill in the field of solar water heaters, it is obvious that vacuum tubes can be used to store energy. Therefore, an array of vacuum tubes alone could function as a complete solar water heater without a hot water storage tank. However, by using the sensible heat of water to store thermal energy, the volume needed is very large. Materials with heat capacity much greater than water are advantageous. 
         [0015]    As an energy storage medium, phase change materials (PCM) have been studied since 1960s, as shown in U.S. Pat. No. 2677664. There are many books and articles about PCM, a recent review is A. Sharma, V. V. Tyagi, C. R. Chen, and D. Buddhi, “Review on thermal energy storage with phase change materials and applications”, Renewable and Sustainable Energy Reviews, 13 (2009) 318-345. PCMs refer to materials that change its phase at a well-defined temperature, and absorb or release thermal energy during change. An example is paraffin wax, which melts at about 60° C. Sodium sulfate, or the Glauber&#39;s salt, changes its state of hydrate at 32° C. However, in the temperature range of solar water heaters, there are very few useful PCM materials. Many salt hydrates show incongruent melting, super cooling or super heating. Paraffin wax has huge volume change during melting, making encapsulation difficult. 
         [0016]    The advantage of PCM is the ability to keep the system at a well-defined temperature. However, for solar water heaters, a high heat capacity within a suitable temperature range, for example, 70° C. to 90° C. is sufficient. We disclose here that the heat of solution of aluminum sulfate can be used as thermal energy storing medium for solar water heaters. 
         [0017]    Aluminum sulfate is widely used in waste water treatment, paper industry, and food industry. The properties of aluminum sulfate are well known. The solubility data, on page 29 of “Solubilities of Inorganic and Organic Substances”, Atherton Seidel, D. Van Norstrand Co., New York 1919, is shown in  FIG. 1 .  FIG. 2  is drawn based on the data shown in  FIG. 1 . Data points  201  through  205  are taken from items  101  through  105  in  FIG. 1 . The left side scale is weight percentage of Al 2 (SO 4 ) 3  in the solution. The right side scale is weight percentage of Al 2 (SO 4 ) 3 .18H 2 O in the solution. As shown in  FIG. 1 , item  106 , the solid phase is Al 2 (SO 4 ) 3 .18H 2 O, which has a mole weight approximately twice as that of anhydrous Al 2 (SO 4 ) 3 . 
         [0018]    As shown in  FIG. 2 , aqueous system of 40% to 47% Al 2 (SO 4 ) 3  is a mixture of liquid and solid in the useful range of temperature. For example, for a system with 44.7% Al 2 (SO 4 ) 3 , at temperatures above 90° C., the system is liquid only. When temperature drops below 90° C., aluminum sulfate starts to crystallize into Al 2 (SO 4 ) 3 .18H 2 O. As the kinetic energy of the component molecules is frozen, it releases a heat of solution. The heat of solution for Al 2 (SO 4 ) 3 .18H 2 O can be estimated from the transition enthalpy near its melting point, which is 117.7 KJ/mol, or 176.7 KJ/Kg, see “Thermodynamic Properties and Phase Transitions of Salt Hydrates between 270 and 400 K”, by F. Gronvold and K. K. Meisingset, Journal of Chemical Thermodynamics, 1982, pages 1083-1098. The heat of solution at various temperatures can be calculated from its solubility data, as shown in the curves,  301  through  305 . 
         [0019]    Experimental data confirms the above analysis, as shown in  FIG. 4 . By filling various aluminum sulfate solutions in identical test tubes, then let it cool down under identical conditions. The temperature data from an automatic temperature recorder clearly demonstrates the effect of thermal energy storage. For comparison, curve  401  is for pure water. Curves  402 ,  403 , and  404  are for aluminum sulfate solutions of various concentrations. At the temperature ranges of 70° C. to 90° C., the heat capacity of aluminum sulfate solution is more than 2 times of water. Through experiments, it was also found that the aluminum sulfate solution is well behaved: there is no incongruent phenomenon, no overheating and no overcooling. In contrast with paraffin wax, volume change during transition is very small. Therefore, encapsulation is easy. Furthermore, on the international market, the price for Al 2 (SO 4 ) 3 .14H 2 O is $160 per metric ton, whereas the price for paraffin wax is $1400 per metric ton. 
         [0020]    Aluminum sulfate is non-flammable, non-toxic, but corrosive to metals. Appropriate materials for capsules are high melting-point plastics, including but not limited to polypropylene homopolymer and high-density polyethylene. Because solid-phase aluminum sulfate has a low thermal conductivity, to speed up heat transfer, it is advantageous to encapsulate in tubes, with small circumferences, see  FIG. 5A , where  501  is a tube with a cap  502 . After the medium is filled,  503 , the tube is sealed with the cap,  504 . To better transfer heat to and from the medium inside the tubes, the cross section of the tubes should be well designed, see  FIG. 5B . The cross sections include but not limited to circular,  505 ; rounded rectangular,  506 ; rounded rectangular with two grooves,  507 ; and rounded rectangular with multiple grooves,  508 .  FIG. 5C  shows how those capsules are packed together. The corners and grooves provide paths for water to pass through. For fast heat transfer, every point of the medium in the capsule should have a small distance to nearest water, preferably less than 10 mm. The tubes are packed in the vacuum tube; where  FIG. 5D  is a horizontal cross section view, and  FIG. 5E  is a vertical cross section view. Here  509  is the outer glass tube,  510  is the vacuum space,  511  is the inner glass tube,  512  is plastic capsules with heat-storage medium,  513  is the central tube for water flow, and  514  is water. Item  515  in  FIG. 5E  is a mechanical support plate for the capsules. 
         [0021]      FIG. 6  shows a continuous process of filling heat storage medium into plastic tubes.  601  is a bath filled with oil  602 , including but not limited to Canola oil. An electrical heater  603  with thermostat keeps the temperature of the oil between 110° C. and 120° C. A container filled with aluminum sulfate and water,  604 , is placed in the oil bath, and the temperature is kept at around 100° C. to 110° C. Therefore, the medium is in liquid phase. An electromagnetic valve  605  controls the flow of the liquid into the plastic capsules  606 , which are mounted on an automatic assembly line, moves to the left hand side as shown here. When a capsule is under the spigot  605 , a fixed amount of liquid medium  607  is filled into the capsule,  608 . Then a cap  609  is place on the capsule, and sealed by heating,  610 . 
         [0022]    The energy storage medium, aluminum sulfate and water, works best in the upper temperature range of water, 70° C. to 90° C. To collect maximum solar radiation, to minimize heat loss, to prevent overheating, and to transfer stored thermal energy to water without using an electrically driven pump, a water handling apparatus is designed, as shown in  FIG. 7 . 
         [0023]    The central component of the water handling apparatus is a horizontal chamber  701  of rectangular cross section with side dimensions of 50 to 75 mm (2 to 3 inches), referred to as the hot water chamber. Its length is determined by the number and the external diameter of vacuum tubes. It is enclosed in a thermal insulating cage  702 , made of foam polyurethane or a material with similar properties. The hot water chamber  701  has a number of vertical tubes  703 , each has at least one gasket  704  fitted on the outside of each said vertical tube. The gaskets are made of silicone elastomer, nitrile butadiene rubber, or a similar elastic material. The dimension of the gasket is designed to seal against the inside of a glass vacuum tube. The distance between the axes of the neighboring vertical tubes is 2 to 10 mm (⅛ to ⅜ inch) larger than the outer diameter of the vacuum tube. When the vertical tubes are inserted into the glass vacuum tubes, the distance between adjacent vacuum tubes is only 2 to 10 mm (⅛ to ⅜ inch). 
         [0024]    Inside the hot water chamber  701  is a cold water pipe  705 , with diameter 10 to 25 mm (⅜ to 1 inch). The cold water pipe  705  has a number of branch pipes  706 , each connected to a pipe  707 , extending to the bottom of a glass vacuum tube (same as pipe  513  in  FIG. 5E ). 
         [0025]    On the upper side of the hot water chamber there are one or more outlets  708  open to air. Under operational conditions, the hot water chamber  701  is half filled. The outlets  708  keep the pressure of the water surface in the hot-water chamber  701  equals to the atmosphere pressure. If the water in the hot water chamber  701  reaches the boiling point of water, 100° C., steam escapes through the outlets  708  to dissipate excess heat and then avoid overheating. 
         [0026]    At the lower end of the hot water chamber  701  is a hot-water outgoing pipe  709 , which is the same as pipe  816  in  FIG. 8 . 
         [0027]      FIG. 8  shows a complete solar water heater. Cold water from cold water line  801  flows to a cold-water supply chamber  802 , where the fill valve  803  and the outlet  804  keep the water level  805  constant. Fill valve  803  is the same as used in toilets, for example, Fluidmaster universal fill valve, and Kohler K-1068030 fill valve. Cold water flows through pipe  806  (same as  705  in  FIG. 7 ), and the branch pipes  807  and  808  (same as  706  and  707  in  FIG. 7 ), to the bottom of each glass vacuum tube  809 . 
         [0028]    Each vertical tube  810  on the hot-water chamber (same as  703  in  FIG. 7 ) is inserted into a vacuum tube  809 , sealed with a gasket  811  (same as  704  in  FIG. 7 ). Most of the previous solar water heaters with glass vacuum tubes have gaskets placed on the outside of the vacuum tube to seal with an opening in the water handling apparatus, for example US Patent 5163821 Keller et al., and a journal article “Water-in-glass evacuated tube solar water heaters”, by G. L. Morrison, I. Budihardjo, and M. Behnia, Solar Energy, 76 (2004) 135-140. According to those previous designs, there is a large space between adjacent vacuum tubes, typically comparable with the diameter of the vacuum tubes, which would lose efficiency by about a factor of two. 
         [0029]    The current invention places the gasket inside the vacuum tube to seal with the outside of a said vertical tube; the distance between adjacent vacuum tubes is reduced to a few millimeters. The current design also has the following advantages: First, the total length of the hot-water chamber is reduced; therefore, the heat loss is reduced because the surface of the hot-water chamber is reduced. Second, it facilitates assembly and prevents leakage, because the system is assembled at room temperature, and operating at a higher temperature. The thermal expansion coefficient of Pyrex glass is almost zero. The thermal expansion coefficient of plastics or metals is 0.01% to 0.02% per ° C. If plastic or metal structure is placed to the outside of glass tube, at operating temperature, due to thermal expansion of plastics or metal, the seal of the gasket becomes looser. The current invention places the plastic tube inside the glass tube; at operating temperature, due to high thermal expansion of plastics, the seal of the gasket becomes tighter. 
         [0030]    Because the cold-water supply chamber  802  and the hot-water pipe  813  is connected, the water level  815  in the hot-water chamber  813  equals the water level  805  of the cold-water supply chamber  802 , which is controlled by the fill valve  803 . The fill valve is adjusted to keep the hot-water chamber  813  half filled. The outlet  814  thus serves as an overheating protection device. If the water temperature reaches the boiling point of water, 100° C., steam escapes through the outlets to dissipate excess heat. 
         [0031]    During operation, cold water flows from the bottom of pipe  808  through the thermal energy storage capsules  812 , and then being heated up. Because hot water is lighter than cold water, hot water always stays at the top, especially in the hot-water chamber  813 . 
         [0032]    At a lower point of the hot-water chamber  813 , there is a hot-water outgoing pipe  816  (same as  709  in  FIG. 7 ). Because the output from the hot-water chamber  813  could be very hot, to prevent scalding, hot water from pipe  816  first goes into a thermostatic mixing valve  817 . By mixing with cold water from pipe  801 , warm water  818  with a preset temperature is generated. Thermostatic mixing valve is a commonly available commercial product, covered by American Society of Sanitary Engineering standards ASSE 1016. Examples are Honeywell AM101R-US-1, Watts LF 1170, Zurn Wilkins ZW1070, etc. 
         [0033]      FIG. 9  shows a tankless solar water heater mounted on a roof. The roof  901  should incline to the correct orientation: South in the Northern hemisphere and North in the Southern hemisphere. A basically flat mounting frame  902  can be placed directly on the roof if the roof is adequately angled. An end support structure  903  is attached to the mounting frame  902 . The vacuum tubes  904  with heat storage capsules are supported by the water handling apparatus  905  on the top end, and the supporting structure  903  on the bottom end. The steam outlets  906  are at the highest points of the water handling apparatus. The cold-water supply chamber  907 , with an air outlet  908 , is connected to the water handling apparatus  905 . Tap water runs through pipe  909  into the cold-water supply chamber  907 , where the water level is controlled by the fill valve in the chamber  907 . Sunlight heats up the energy storage medium in the vacuum tubes  904 . If the temperature reaches 100° C., excess heat is dissipated as steam through outlets  906 . Hot water flows through the hot water outgoing pipe  910  into the thermostatic mixing valve  911 . By mixing with cold water, warm water  912  with a preset temperature is generated. Therefore, the mechanical pressure on the roof is light and evenly distributed. The entire system runs on natural law with no electric pump and no control system, thus can run maintenance free. 
         [0034]    Here is an estimate of the size of the tankless solar water heater. An available glass vacuum tube has an inner diameter 102 mm and length 1500 mm. The volume of each tube is about 12 liters. Because the heat capacity of aluminum sulfate solution is two times of that of water, it equals to 24 liters of water. A system of 10 vacuum tubes has a total equivalent water volume of about 240 liters, sufficient to support a 4-person family. The price for 150 Kg aluminum sulfate is about $30 on the international market, an insignificant cost. 
         [0035]    While this invention has been described in conjunction with the exemplary embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.