Abstract:
A water heater includes a thermally isolating preheater using a phase change material for increased efficiency. Cold water passes through the preheater and is heated using the phase change material therein. Due to the preheater, heated water is supplied to the water contained in the water heater storage tank.

Description:
[0001]    This application claims priority from provisional patent application No. 62/278,459 filed on Jan. 14, 2016, and which is incorporated in its entirety herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    A water heater includes a thermally isolating preheater using a phase change material for increased efficiency. 
       BACKGROUND 
       [0003]    The latest energy efficiency standard for water heaters is NAECA III, which came into effect on Apr. 16, 2015. This standard mandates that new water heaters with capacities larger than fifty-five gallons have increased efficiency. This effectively limits the traditional and inexpensive water heater technology to capacities of fifty-five gallons or less. Water heater manufacturers are searching for a way to deliver more hot water from smaller capacity water heaters. One way to accomplish this is by storing heat in a material that that has a higher heat capacity than water, given the same volume. Such a material might be a phase change material that has a melting point near the temperature of hot water stored in domestic water heaters. An example of a phase change material is described in U.S. Pat. No. 8,887,762 to Junge et al., which is incorporated in its entirety herein. Simply encapsulating such a phase change material and locating it inside the storage tank of a water heater may improve its heat capacity but only in a limited manner for the added cost. 
         [0004]    Most conventional water heaters draw hot water from the top of the tank and deliver incoming cold water to the bottom of the tank. Some water heaters have a cold water tap on the side near the bottom of the tank. Other water heaters have a cold water tap at the top of the tank. The latter has a cold water dip tube that carries incoming cold water from the top of the tank to the bottom of the tank. When hot water is drawn out of a water heater, the same amount of cold water enters and mixes with the rest of the stored hot water. This mixing of cold and hot water reduces the temperature of the stored water which reduces the amount of hot water that can be used. The term “draw efficiency” describes the percentage of hot water than can be drawn out of a water heater before the remaining water is considered too cold for use. Most conventional water heaters have a draw efficiency of about 70%. For example, a fifty gallon water heater with a draw efficiency of 70% can only deliver thirty-five gallons of hot water from its stored capacity without the help of its heating source. 
         [0005]    There are several drawbacks of current technology utilizing phase change materials in water heaters. One downside is the low temperature difference between the phase change material and water due to cold water mixing with hot water before contacting the phase change material. This low temperature difference between the water and the phase change material slows down the heat transfer process. Another drawback is the added cost due to additional material and complexity in the water heater manufacturing process. 
       SUMMARY OF THE INVENTION 
       [0006]    An improved water heater thermally isolates the incoming cold water from the stored hot water until it absorbs heat from a phase change material. This is accomplished by using a preheater inside the hot water storage tank. In the case of a water heater with a cold water dip tube, the preheater would replace the cold water dip tube. The preheater still provides a channel for incoming water to pass from the top to the bottom of the tank. However, this channel is surrounded by a second enclosure that is filled with a phase change material. This forces the cold incoming water to be heated by the phase change material before reaching the bottom of the tank and mixing with the stored hot water. This ensures a large temperature difference between the phase change material and the cold water, which allows heat stored in the phase change material to pass quickly into the incoming water. 
         [0007]    By replacing the cold water dip tube with a preheater, the improved water heater can be made at a low cost and with minimal changes to existing manufacturing processes. The preheater can be made of similar materials to that of existing cold water dip tubes or other materials with more desirable characteristics. The preheater can be permanently fixed to the water heater storage tank. Alternatively, the preheater can be inserted through a threaded hole in the top of the water heater, similar to how a cold water dip tube is inserted. Another advantage of this method is that the preheater can be maintained or even replaced in the field. 
         [0008]    The invention also includes a method of heating water including the inventive preheater and the preheater alone for situations where the preheater would be used to improve the efficiency of an existing water heater instead of being installed as part of a new water heater. 
         [0009]    The following description and accompanying drawings will further illustrate these and other advantages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a perspective view of a first embodiment of a water heater storage tank of the invention. 
           [0011]      FIG. 2  presents a cross sectional view along line II-II of just the preheater shown in  FIG. 1 . 
           [0012]      FIG. 3  illustrates a cross sectional view along line III-III of the preheater shown in  FIG. 2 . 
           [0013]      FIG. 4  presents a perspective view of a second embodiment of the preheater in a water heater storage tank. 
           [0014]      FIG. 5  shows a cross sectional view of a third embodiment of the preheater of the invention. 
           [0015]      FIG. 6  shows a fourth embodiment of the preheater of the invention. 
           [0016]      FIG. 7  shows a fifth embodiment of the preheater of the invention. 
           [0017]      FIG. 8  shows a sixth embodiment of the preheater of the invention. 
           [0018]      FIG. 9  shows a schematic drawing showing thermostat control of the preheater for a storage tank. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  illustrates a perspective view of a water heater storage tank  10  according to the first embodiment of this invention. Storage tank  10  includes many of the components of a typical water heater which are not shown because many of them remain unchanged. For instance, storage tank  10  typically includes one or more heating source such as electric resistance elements, heat pump refrigerant coils, or a combustion fuel heat exchanger. Storage tank  10  is typically insulated and contains a cold water inlet  11  and hot water outlet  12 . When the cold water inlet  11  is located at the top of storage tank  10 , it is typically attached to a cold water dip tube that delivers incoming cold water to the bottom of the storage tank  10 . 
         [0020]    Referring to  FIGS. 1 and 3 , a thermally isolating preheater  13  is shown oriented vertically, in place of a cold water dip tube. In this embodiment, preheater  13  is primarily a cylinder containing a phase change material  17  and flow channels  15  for the passage of incoming water. The preheater  13  can be fixed to the top of the storage tank  10  or inserted through a threaded hole in the top of the storage tank  10 , similar to the way a cold water dip tube would be inserted.  FIG. 1  illustrates an incoming water flow director  14  which can be curved as shown, or any other shape or size. 
         [0021]      FIG. 2  presents a cross sectional view of the first embodiment of preheater  13 , along the line II-II in  FIG. 1 . The first embodiment of preheater  13  has a cold water inlet  11  at the top, one or more water flow channels  15  extending from top to bottom, and a flow director  14  at the bottom. Preheater  13  has an outer shell  16 . The space between the outer shell  16  and the flow channels  15  is filled with a phase change material  17  and may or may not include an air gap at the top to allow expansion of the phase change material  17  as it changes phase. In the case of multiple flow channels  15 , the incoming water splits into a plurality of flow channels  15  that pass through the phase change material  17  before recombining at the outlet side of the preheater  13  by inlet header  18  and outlet header  20 . 
         [0022]      FIG. 3  shows a cross sectional view of the first embodiment of preheater  13 , along the line III-III in  FIG. 2 . The first embodiment of preheater  13  has several flow channels  15  which are spaced such that they come in thermal contact with the phase change material  17  but not outer shell  16  or other flow channels  15 . The materials for the preheater  13 , i.e., the outer shell  16  and ends thereof can be made of any material suitable for use in the tank of a water heater. 
         [0023]      FIG. 4  presents a perspective view of a water heater storage tank  10   a  according to the second embodiment of this invention. Storage tank  10   a  includes a cold water inlet  112  on the side of the tank near the bottom, and a hot water outlet  122  on the side of the tank near the top. Flow director  14  is opposite the cold water inlet  11 .  FIG. 4  shows the thermally isolating preheater  13   a  oriented generally horizontally. Although a vertical and horizontal orientation of the preheater is shown, the preheater can be in any orientation between the cold water inlet and a bottom of the storage tank  10 . 
         [0024]      FIG. 5  illustrates a cross sectional view of an alternate embodiment of a preheater  13   b,  similar to the sectional view shown in  FIG. 2 . This third embodiment has a cold water inlet  11  at the top and a flow director  14  at the bottom.  FIG. 5  shows one or more water flow channels  152  which pass between the inlet and outlet side of the preheater  13   b  two or more times in a countercurrent flow pattern before exiting the phase change material  17 . In this embodiment, the flow channels  152  begin near the center of preheater  13   b  and turn outwards towards outer shell  16  when reversing directions. Flow channels  152  may reverse directions two or more times before finally exiting the preheater  13   b.  Alternatively, flow channels  152  can take any path that begins at the cold water inlet  11  and ends at the flow director  14 . 
         [0025]      FIG. 6  shows a cross sectional view of an alternate embodiment of a preheater  13   c,  similar to the sectional view shown in  FIG. 3 . This fourth embodiment of preheater  13   c  has several flow channels  15  which are spaced such that they come in thermal contact with the phase change material  17  but not outer shell  16  or other flow channels  15 . In this embodiment, heat sink fins  22  run lengthwise along an outer surface of the flow channels  15  to improve heat transfer between the incoming cold water and surrounding phase change material  17 . Alternatively, heat sink fins  22  can be in any orientation and any shape that extend from a surface of the flow channels  15 . 
         [0026]      FIG. 7  presents a cross sectional view of an alternate embodiment of a preheater  13   d,  similar to the sectional view shown in  FIG. 3 . The fifth embodiment of preheater  13   d  has star-shaped channel that has a main flow channel  24  with narrow channels  26  extending out from the main flow channel  24  such that they come in thermal contact with the phase change material  17  but not outer shell  16 . 
         [0027]      FIG. 8  shows a cross sectional view of an alternate embodiment of a preheater  13   e,  similar to the sectional view shown in  FIG. 3 . This sixth embodiment of preheater  13   e  has several flow channels  15  which are spaced such that they come in thermal contact with the phase change material  17  but not outer shell  16  or other flow channels  15 , similar to those in  FIG. 3 . In this embodiment, the outer shell  162  has an angular shape to allow it to maintain its structural integrity as it expands and contracts due to changes in volume of the phase change material  17 . The shape of  FIG. 8  is an example of a textured outer shell shape and the outer shell shape can have other undulations, curves, or textures, e.g., be non-circumferential, to allow for expansion and contraction. 
         [0028]    In operation and referring to the  FIG. 1  embodiment, a plumbing system would draw hot water in a normal manner from the storage tank  10 . As hot water is drawn from the hot water outlet  12  at top of the storage tank, the same volume of cold water enters through the cold water inlet  11 . The entering cold water passes directly into the water flow channels  15  located inside the thermally isolating preheater  13 . As the cold water passes through the flow channels  15 , the water rapidly absorbs heat stored in the phase change material  17 . One advantage of this design over the prior art is that the incoming cold water is thermally isolated from the stored hot water until it comes in thermal contact with the phase change material  17 . This thermal isolation results in a large temperature difference between the incoming water and phase change material which promotes a more rapid heat transfer than is achieved by the prior art. 
         [0029]    Another advantage of this embodiment is that the preheater  13  prevents incoming water from mixing with stored hot water until it has been substantially preheated by the phase change material  17 . Thus, rather than cold water emerging from the inlet  11  or dip tube as is the case in conventional water heaters, hot water emerges from the exit end of the preheater  13 . Since a 70% draw efficiency can be achieved with direct mixing of cold and hot water, higher draw efficiencies can be achieved using the inventive preheater  13 . 
         [0030]    Over the course of a long hot water draw, the phase change material  17  will become solid and temporarily depleted of its stored heat. As this occurs, water leaving the preheater  13  will become cooler. One or more thermostats can be provided to recognize the change in temperature and activate the heat source for the water heater. Long hot water draws can be achieved by setting the thermostat to activate the heat source before the preheater  13  is fully exhausted. 
         [0031]      FIG. 9  shows a schematic drawing with a thermostat  30  sensing temperature at the outlet header  20 . The heater  31  is turned on when the temperature of the water exiting the preheater is below a set temperature and turned off when the temperature of the water reaches a desired temperature. Once the long draw has finished, the heat source will continue to reheat the water until one or more thermostats is satisfied. The one or more thermostats can be set such that it is not satisfied until hot water has replenished the phase change material  17 , returning it to its high energy liquid state. 
         [0032]    As such, an invention has been disclosed in terms of preferred embodiments thereof which fulfills each and every one of the objects of the present invention as set forth above and provides a new and improved way to increase the efficiency of a water heater. 
         [0033]    Of course, various changes, modifications, and alterations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claim.