Abstract:
A heat exchanger system, comprising: a storage tank; and heat exchanger apparatus comprising: a heat exchanger; and a plenum structure defining a hollow body comprising a first end that is coupled to the heat exchanger, and a second end remote from the first end; a mounting arrangement to mount the heat exchanger apparatus within the tank in an orientation wherein the heat exchanger is disposed in substantially a horizontal condition, whereas the plenum structure is disposed in substantially a vertical condition.

Description:
FIELD 
       [0001]    Embodiments of the invention relate to heat exchangers. 
       BACKGROUND 
       [0002]    Heat exchangers are devices which allow for heat to be transferred from one fluid to another. In some cases, the flow of one fluid may be forced while the flow of the other is not. In such cases the performance of the heat exchanger can often be improved if some amount of convection in the non-flowing fluid can be induced. 
       SUMMARY 
       [0003]    According to one aspect of the invention, there is provided a heat exchanger wherein the flow of one fluid is forced and the flow of the other is not, wherein the architecture of the heat exchanger is fundamentally planar such that the fluid whose flow is forced flows substantially within the plane of the heat exchanger and the fluid whose flow is not forced flows substantially in a direction perpendicular to the plane of the heat exchanger 
         [0004]    According to another aspect of the invention, there is provided a planar heat exchanger which is coupled fluidically to a hollow plenum which supports the development and isolation of buoyancy driven flows within the plenum from those without. 
         [0005]    According to a further aspect of the invention, there is an induced convection heat exchanger, comprising a planar heat exchanger and a watertight plenum extending vertically above or below the heat exchanger. During operation the temperature difference between the heat exchanger and the fluid within which it is immersed induces buoyancy driven flows which are constrained and isolated by the plenum. These flows improve the performance of the heat exchanger and the planar nature of the heat exchanger maximizes the power output during operation. The heat exchanger and plenum reside within a fluid containing insulated thermal storage tank into which heat is injected or extracted by the heat exchanger. 
         [0006]    According to yet another aspect of the invention, heat which is introduced into or extracted from the insulated thermal storage tank causes the movement upwards or downwards of a thermal stratification layer, a boundary which defines the separation between fluid which is hotter than the fluid which resides below it. 
         [0007]    According to an additional aspect of the invention, heat is injected into or extracted from the insulated thermal storage tank by some combination of heat exchangers, hot/cold fluid inlet and outlet pipes, and electric resistance heaters. 
         [0008]    According to a further aspect of the invention the heat exchanger is fluidically coupled to a municipal water supply outlet and a domestic water supply inlet, and resides in an insulated thermal storage tank which is fluidically coupled to a heat source from a solar thermal array. 
         [0009]    Other aspects of the invention will be apparent from the detailed description below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIGS. 1 a  and 1 b    are prior art illustrating the use of coils in a relevant heat exchanger application. 
           [0011]      FIG. 2  is a diagram illustrating a planar heat exchanger with a plenum. 
           [0012]      FIG. 3  is a diagram illustrating construction detail of one planar heat exchanger design and a means for its fabrication. 
           [0013]      FIG. 4  is a diagram illustrating construction detail of an alternative planar heat exchanger design and a means for its fabrication. 
           [0014]      FIG. 5  is a diagram showing a planar heat exchanger with a straight tapered plenum in operation in an insulated thermal storage tank. 
           [0015]      FIG. 6  is a diagram showing a planar heat exchanger with a curved tapered plenum in operation in an insulated thermal storage tank. 
           [0016]      FIG. 7  is a drawing showing a planar heat exchanger with a plenum and two mechanisms for choking flow. 
           [0017]      FIG. 8  is a drawing showing a planar heat exchanger and plenum positioned to inject heat into an insulated thermal storage tank. 
           [0018]      FIG. 9  is a drawing sequence showing the progression of a thermal stratification line within an insulated thermal storage tank as heat is injected by the heat exchanger. 
           [0019]      FIG. 10  is a drawing sequence showing the progression of a thermal stratification line within an insulated thermal storage tank as heat is injected by a hot/cold fluid inlet outlet tube pair. 
           [0020]      FIG. 11  is a drawing sequence showing the progression of a thermal stratification line within an insulated thermal storage tank as the heat is injected and the extracted via a heat exchanger. 
           [0021]      FIG. 12  is a drawing showing an insulated thermal storage tank with hot/cold fluid inlet/outlet tube pair and electrical resistance heater. 
           [0022]      FIG. 13  is a drawing showing a multistory home with a supplemental hot water heating system installed. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. 
         [0024]    Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not others. 
         [0025]    In one embodiment, a planar heat exchanger is disclosed which in its simplest form comprises an array of hollow parallel tubes which provide a fluidic connection between two larger tubes or conduits known as headers. Flow of a heated or cooled fluid from one header, through the tube array, and out of the other header allows for heat to be input or output from a fluid medium within which the heat exchanger is immersed. 
         [0026]    In another embodiment the tubes comprising the planar heat exchanger may contain within them two or more additional tubes which provide additional interior surface area with which the fluid flowing within can interact with. 
         [0027]    In yet another embodiment the tubes are actually hollow planar conduits which comprise two separate strips that have had features embossed and holes punched through them in such a way as to create a narrow passage through with a fluid can flow from a hole punched at one end to a hole punched at the other. The features may include structures that create turbulence in the forced fluid flow. An array of such conduits can be bonded together using intervening circular tube segments aligned and bonded to the holes at each end. 
         [0028]    In a further embodiment the planar heat exchanger is coupled to a plenum which supports the development of convection currents which induce flow along the exterior surface of the heat exchanger thus improving performance. The plenum may include interior physical features or exhibit a geometry to control the flow within the plenum. 
         [0029]    In general, in a representative application, the planar heat exchanger may be immersed within a fluid inside an insulated thermal storage tank and located at or near the bottom of the tank. For some embodiments, the planar heat exchanger may be located near the top of the tank. If the purpose of the heat exchanger is to inject heat to the fluid, then it will reside near the bottom whereas it will near the top if the purpose is to extract heat from the fluid. In this way the heat exchanger will produce convection currents within the fluid which will flow upwards or downwards depending on the location of the heat exchanger, in a way that is beneficial for inputting or extracting heat, as will be described. 
         [0030]    In general, in another representative application, heat may be injected into the fluid inside the thermal storage tank by some combination of methods including the heat exchanger, a hot/cold fluid inlet outlet tube pair, and an electrical resistance heater. 
         [0031]    In general, in yet another representative application, one or more heat exchangers may be combined with a hot/cold fluid inlet outlet tube pair and an electrical resistance water heater, all of which reside in an insulated thermal storage tank. The heat exchangers and inlet outlet tube pair are used to couple heat from a solar thermal array into the municipal water supply for a residential or commercial consumer for the purpose of providing a supplemental hot water heating source. 
         [0032]      FIG. 1  shows two examples of prior art in the form of helical heat exchanger.  FIG. 1 a    shows a vertically oriented coil  110  immersed in a fluid  102  which fills insulated thermal storage tank  104 . The fluid  102  may comprise water or thermal oil, and serves the purpose of storing heat for applications such as water heating or any applications requiring the storage and distribution of heat. During the day, heat from solar collector panels (not shown) is added to the fluid  102  so that the fluid is at an elevated temperature, for example about 50° C. When heat from the tank is required to heat water, water from a municipal supply is allowed to flow through the interior of coil  110 . For example, the municipal water may flow into the coil via inlet  106  and is released via outlet  108  to flow into a household water supply (not shown). The water inside the coil  110  does not come into direct contact with the tank water  102 . The temperature of the municipal water is raised as it flows through the coil  110  and as it absorbs heat from the fluid  102 . Simultaneously, the temperature of the fluid  102  is reduced as heat is transferred to the municipal water through the walls of the coil  110  from the heated tank fluid  102 . Convection of the fluid within the coil  110  is considered to be forced because the flow of the water is driven by the pressure of the municipal water supply. However, there are no such forced flows within the tank. Flows within the tank are in the form of free convection which is driven by buoyancy forces caused by a difference in density between portions of the heated tank fluid  102 . For example, the tank fluid  102  which is in proximity to the coil  110  will generally be cooler (and thus, more dense) than other portions of the tank fluid  102 , by virtue of being cooled by coming into contact with the exterior surface of the coil  110 . This dense portion of the tank water  102 , will tend to flow downward along the segments of the coil in an overall direction indicated by convective flow arrows  114 . As this occurs, water at the bottom of the tank is displaced and forced to flow upwards along the interior and exterior of the coil in an overall direction indicated by convective flow arrows  116 . While the described convection within the tank water  102  helps transfer heat from the fluid  102  to the coil  110 , it is not optimized for several reasons. Firstly, because the coil  110  extends vertically the difference in temperature between the tank fluid  102  and the coil  110  will vary in the vertical direction. Lower differences mean less effective heat transfer. In a situation where the hottest water resides at the top of the tank it is advantageous to have the bulk of the heat exchanger reside near the top of the tank if the goal is to extract heat, because the temperature difference will be high over the entire surface area of the heat exchanger. A second issue with this design derives from the fact that the coil segments are arranged vertically on top of each other. This means that any convection flows near the tube segments are slowed and or obstructed. A tube segment is defined as a single loop of the coil, indicated by box  112 . Given that the tube segments reside immediately above each and are aligned, flow downward along the tubes is non uniform as indicated by the erratic nature of flow arrows  114 . Additionally, the upward and downward convective flows are able to mix freely within the tank and thus have a tendency to reduce any thermal stratification which might exist within the tank fluid  102 . This also results in a rather chaotic flow in either direction and to occasional convection loops  118  which enhance the mixing. 
         [0033]    Referring now to  FIG. 1 b   , a prior design is shown which attempts to resolve the aforementioned problems. In this case, there are two coils  106  and  108  which are oriented in a horizontal fashion (into and out of the plane of the drawing) and therefore individually do not extend vertically to the same extent as the coil of  FIG. 1 a   . In addition, there is a plenum  112  which has been included in order to direct the convective flows into separate paths, as indicated by the arrows, to at least reduce mixing. This helps to minimize the loss of stratification while also encouraging higher flow rates. Higher flow rates on the exterior of the coils mean higher heat transfer rates which also enables the use of less material in the heat exchanger. The plenum takes on the form of a hollow rectangular column. 
         [0034]    Referring now to  FIG. 2 , a top view is shown of planar heat exchanger  220 . Fluid  200  flows into header  202  where its flow is divided up into tube array  204  in a single direction as indicated by arrow  206 , to subsequently flow to header  208  and then out in the form of flow  210 . Planar heat exchanger configuration  240  comprises a planar heat exchanger  242  which is identical to the heat exchanger  220  save the plenum  244  which is mounted beneath the heat exchanger  242 . Referring again to  FIG. 2 , side and bottom and bottom views are shown in  248  and  250  respectively. 
         [0035]    The heat exchanger  220  may be made from a number of materials which can be bonded or brazed together and are compatible with the fluids to be used. Bonding could consist of using a metal compatible thermoplastic sealant or other similar compound to create leak free seals between the tubes and the header. Brazing generally comprises applying a solder or other low melting point material to the joint to be bonded and applying high temperatures in suitable oven to cause the solder to melt and provide a leak free mechanical bond. Copper and stainless steel are two materials are commonly used because of their compatibility with water and other liquids as well as their low thermal conductivity. Other materials such as plastics or ceramics might be used if their dimensions can be tailored to minimize the impact of their lower thermal conductivities. 
         [0036]    Referring now to  FIG. 3 , heat exchanger tube  300  is shown with interior detail revealed in a cross section showing outer tube  302  surrounding inner tubes  304 . Such tubes are available off the shelf and are manufactured for the electro-discharge machining market in which and they are known as EDM tubes for electro discharge machining. The tubes are used to apply electric sparks within a fluid to carve features into metals. The inner tubes provide a path for a liquid to flow which pushes the debris from the machining process out of the feature being machined. Their use in a heat exchanger is unique and represents a novel application of this technology. A typical tube is comprised of a primary outer tube and two or more inner tubes. The material for both outer and inner tubes is nominally copper but may be of stainless steel or some other material with sufficiently low thermal conductivity and material compatibility with the fluids it will come into contact with. Copper is a good choice because it can be readily brazed, is well known for use in plumbing applications, has low corrosion rates, and has one of the highest thermal conductivities of any metal. Typical dimensions are shown with the outer tube having and outside diameter of 3 mm and a wall thickness of 0.6 mm and the inner tubes having an outside diameter that is appropriate to the number inner tubes present and a wall thickness, in the case of two inner tubes, of 0.25 mm. The method of manufacturing the tubes is well known by those who are skilled in the art and generally comprises threading multiple inner tubes into the outer tube. The function of the inner tubes in this application is to provide increased interior surface area such that the rate of heat transfer on the interior of the tube, a product of both interior surface area and internal flow rate among other factors, can be matched to the heat transfer rate on the tank side of the heat exchanger. The tank side fluid transfer rate refers to the limiting rate at which the fluid contained within the tank, not the fluid in the heat exchanger, can transfer heat based on the temperature of the fluid, the free convection flow rate, and the properties of the fluid These internal features are one way in which the planar heat exchanger design departs from that of a coil as coils generally comprise smooth tubes with no interior surface features. 
         [0037]    Referring again to  FIG. 3 , a method of manufacture is shown wherein heat exchanger tube  300  is brazed to headers  306  and  308  which has an array of holes  310  drilled into its side to receive the remaining tubes. The tubes along with solder rings are mounted into the predrilled holes on the sides of two headers. Then the entire assembly is inserted into a brazing oven where the application of elevated temperature forms a bond between the tube array and the two headers. Essentially, when the assembly rises above a certain temperature within the oven the solder rings melt and create a mechanical and leak free bond between the tubes of the tube array and the holes of the headers. A representative design might comprise an array of 80 copper tubes of 400 mm in length and 4.7 mm in diameter with two inner tubes, with all 80 tubes brazed to two copper headers. Each header is roughly 450 mm in length and 10 mm in diameter. The overall dimensions of the heat exchanger are 400 mm×431 mm×10 mm. Such a heat exchanger is capable of up to 16 kW of energy output under typical residential solar water heating operating conditions. This compares well to a copper coil comprising a length of tubing roughly 36 meters long with an outside diameter of 10 mm. The coil weighs over 5 times more and is roughly 600 mm in diameter and 1 meter in height and is capable of up to 20 kW of energy output under similar operating conditions. 
         [0038]    Referring now to  FIG. 4 , an alternative manufacturing process for a planar heat exchanger is illustrated which uses metal sheets, nominally of copper or stainless steel, to start. Metallic strip  400  has dimensions which correspond roughly to those of the heat exchanger tubes of the design shown in  FIG. 3 , that is to say of length roughly 400 mm, and a width of roughly 5 mm and thickness of 0.6 mm. It is cut or otherwise stamped from a large sheet or roll. Holes  404  have been punched into the strip shown as  402  and recess  408  has been stamped into the strip shown as  406 . Two strips  410  are identical to the strip  406  but have been rotated so as to present a side view and so that their recesses are facing each other. In the next step the two have been brazed together to form heat exchanger planar tube  412  which has also been bonded to two header sections  414  made from the same or compatible metal. The combination of the header sections and planar tubes is called a tube section and an array of tube sections  416  is shown bonded together to form a complete heat exchanger. Fluid flows into heat exchanger  416  via inlet  418 , flows through the planar tubes in the direction of arrow  420 , and then is released via outlet  422 . The bonding steps described here are identical to the ones described for the heat exchanger in  FIG. 3 . This manufacturing process has the potential to utilize cheaper starting materials in the form of metal sheets vs. metal tubing which must then be threaded to create the interior surface features. The surface features residing on the recesses of the strips  406  of  FIG. 4  can be formed when the recess is stamped and may be in the form of miniature posts, ridges, or other structures that provide a means for the fluid to flow but do so in a way that encourages turbulence. 
         [0039]    Referring now to  FIG. 5  an insulated thermal storage tank  500  is shown filled with hot fluid  502 . Insulated thermal storage tank  500  is nominally un-pressurized that is to say that there is an airspace at the top of the tank which is open atmosphere and the pressure within the tank is at ambient atmospheric pressure. In some configurations the tank may be pressurized though such tanks tend to be more expensive to both manufacture and install. Planar heat exchanger  504  is shown mounted to the top of plenum  506  near the surface of the fluid and is oriented so that it is parallel to the surface. Municipal water flows into and out of the headers  516  of the heat exchanger in the direction indicated by arrows  508 . Since the municipal water is cold and the tank water is hot, the tank water near the heat exchanger is cooled and begins to sink to the bottom of the tank in a direction indicated by arrow  512  within plenum  506 . This represents free convection flow which drives flows upwards in the direction of arrows  514  due to the displacement of the water at the bottom of the tank. The flows are constrained and separated by plenum  506  which is a watertight form that can be made from a number of materials that exhibit long lifetimes in water at elevated temperatures such as acrylic, butyl rubber, or EPDM. It is advantageous that the plenum material also have low thermal conductivity. The geometry of the plenum can be rectangular or cylindrical and the top of the plenum is secured to the sides of the heat exchanger so that all flows through the top of the plenum must pass through the tube area of the heat exchanger. The geometry may be vertical or exhibit a taper as shown in  FIG. 5 . The geometry of the taper is selected to achieve an optimal free convection flow rate based which is also influenced by the geometry, dimensions and thermodynamic properties of the heat exchanger, thus the two are effectively designed in tandem to achieve the requisite performance. In one design goal, for example, the goal is to maximize the heat transfer rate for a given set of forced (municipal water) convection flow rates while minimizing the amount of metal used in the heat exchanger design. The combination of the heat exchanger and the plenum is what induces the convective flow. The heat exchanger because it has created the temperature differences and the resulting changes in the water density. The plenum, because it channels the flow and prevents mixing between the downwards and upwards flow, since mixing would lower the average density difference and inhibit the flow. 
         [0040]    In one embodiment, the plenum may be tapered in order to manipulate the nature of the flow within. In particular, it may be shaped to minimize the potential for convective loops or non-vertical flow patterns from developing within the plenum. Referring now to  FIG. 6 . a plenum  600  is shown with a curved taper as opposed to the straight taper illustrated in  FIG. 5 . These and other symmetric or asymmetric plenum architectures are possible and potentially useful. Referring now to  FIG. 7 . Plenums  700  and  702  may also contain flow inhibiting structures such as vanes  704  and or porous mesh  706  whose function is to slow the rate of flow within the plenum and therefore the overall convective flow rate within the tank. Vanes may be in the form of lateral planar structures which extend across the area of the plenum and slow the overall convective flow because they act as obstructions. Meshes act in a similar fashion with the density of the mesh and the number and dimensions of the vanes determining their impact on the flow. Both vanes and meshes obstruct and/or direct the flow in such a way as to slow it down. During a typical course of operation, the fluid in the tank begins at a uniform high temperature of 50° C. for example. As municipal water flows through the heat exchanger it emerges at a higher temperature. As cold water sinks to the bottom of the tank the temperature at the bottom of the tank gets lower and a thermal boundary is formed between the hot water at the top of the tank and the cold water at the bottom of the tank. As more heat is extracted this boundary rises towards the top of the tank. The rate of heat extracted i.e. power output from the tank is driven by the flow rate of the municipal water as well as the temperature difference between the heat exchanger and the water surrounding. When this temperature difference decreases the power output decreases. This serves to illustrate another inherent advantage to a planar heat exchanger for as the thermal boundary rises, the temperature difference between the heat exchanger remains high until almost all the heat is extracted and the thermal boundary reaches the top. For a heat exchanger design such as a coil which extends vertically, some portion of the heat exchanger will extend across the thermal boundary well before the all the heat is extracted. As a consequence the average temperature difference between the water and the heat exchanger will be lower during operation and therefor the average power output over time will be lower. 
         [0041]    Referring now to  FIG. 8 , It should be noted that the same heat exchanger and plenum design can be used to inject heat into the thermal storage tank. In this case the heat exchanger  802  is located near the bottom of the tank  800  and the plenum  806  extends upwards towards the surface. During operation a hot fluid  804  flows inside the heat exchanger which heats the cold fluid  812  within the storage tank. This induces flows in a direction towards the surface indicated by convective flow arrow  810  within the plenum and corresponding displacement flows downward exterior to the plenum. The fluid  804  emerges from the heat exchanger in a cooled state  808 . 
         [0042]    Referring now to  FIG. 9 , thermal storage tank  900  is shown with a uniform temperature of the fluid contained within. Hot fluid  904 , which is hotter than the fluid  902  contained within the tank, has just begun to flow into the heat exchanger where it will emerge as cooled fluid  908 . As heat is added to the tank over time the fluid at the top of the tank becomes heated and begins to force colder fluid to the bottom of the tank. This results in the creation of a thermal stratification boundary at a high position  910  between hotter fluid  912  and colder fluid  914 . The thermal stratification boundary moves from the top of the tank towards the bottom over the course of charging the tank with heat. Its position is indicated by  916  as the tank approaches the end of the charging process. 
         [0043]    Referring now to  FIG. 10 , an insulated thermal storage tank  1000  is shown with a heat exchanger  1002  which is positioned to extract heat from the tank. Also fluidically coupled to the tank are hot/cold inlet outlet tube pair  10084 / 1006 . These tubes are insulated pipes which allow for fluid to be injected and extracted from the tank. In this case the thermal storage tank is charged by the direct input of heated fluid  1008  into the top of the tank and the simultaneous flow of colder fluid from the bottom of the tank. In most embodiments the storage tank is part of a fluid loop wherein the cold fluid  1010  flows to a heat exchanger or other heat source (not shown) and returns heated in the form of fluid  1008 . The level of the colder fluid within the tank declines as indicated by the cold fluid regions  1012  and  1014  as the tank is charged with hot fluid. At some time after the tank is partially or fully charged or even during the charging process, fluid flows  1014  may be initiated to extract heat from the tank in a manner discussed earlier. 
         [0044]    Referring now to  FIG. 11 , thermal storage tank  1100  is shown with two heat exchangers, heat exchanger  1102  near the surface and heat exchanger  1104  near the bottom with plenums oriented accordingly. In this fashion heat can be alternately or simultaneously injected or extracted from the tank without any fluidic exchange with the fluid contained within the tank. Tank  1100  is being charged with heat as indicated by the movement of the thermal stratification boundary  1106  in the downward direction and the fluid flow 1108  into heat exchanger  1104 . The storage tank is shown in a charged state  1110  has the boundary layer  1112  lies near the bottom of the tank. The thermal storage tank is shown in the process of having heat extracted  1110 , via heat exchanger  1116  as indicated by the fluid flow  1118  and the movement of stratification boundary layer  1120  in the upwards direction. 
         [0045]    Referring now to  FIG. 12 , thermal storage tank  1200  is shown with immersed heat exchanger  1202  configured to extract heat from the tank. In addition hot/cold inlet outlet tube pair  1210 ,  1212 , are with associated flows indicating that heat is being added to the tank by virtue of these flows. An additional means for injecting heat is shown in the form of electrical resistance heating element  1206 . This component is identical to the kind of heating element found in most conventional domestic hot water heaters and is well known in the art. In this case, voltage from driving unit  1208  is used to drive a current through a waterproof high resistance conductor which gets hot as a result. The heat is transferred to the fluid within the tank via conduction through the walls of the heating elements housing and as more heat is added the thermal stratification boundary descends in a manner consistent with the other means described for injecting heat into the tank. Also contained within control unit  1208  are electronic circuits and systems which allow the heating element to be turned on and off quickly at a rate that could be as high as 1 Hz. The electronics further enable the action of the drive unit and therefore the action of the heating element to be dictated by a remotely located control system (not shown). This control system may be located at the same facility or premises that the thermal storage tank is located. It may also be located many miles away. Communication from the remotely located control system may be facilitated by any number of different kinds of communications networks including but not limited to the internet, wifi and wide area wireless networks, as well as cellular telephone networks. One advantage of having a heating element which can be controlled in this fashion is that supplemental heat may be added from the electrical grid at times and in a fashion that makes it economically advantageous. For example, late at night when electricity costs tend to be lower than average, heat may be added at a discount to the cost of using electricity to add heat during the day. 
         [0046]    In addition, the ability to control the heating element at a relatively high frequency makes the combination of the storage tank and heating element capable of supplying a service to the electrical grid that facilitates the regulation of the grid&#39;s frequency. All electrical grids must maintain the operating frequency within some acceptable range of say 60 Hz in the U.S. Appliances like electric water heaters, heat pumps, and air conditioners which are attached to the grid can have an impact on the grid frequency by adding load, i.e. turning on, or subtracting load, i.e. turning off. The changes in load alter the frequency of the grid by some amount based on the size of the load being added or subtracted. Normally the grid operator provides a signal which indicates the need to add or subtract load in order to maintain the requisite frequency in response to the addition or subtraction of other loads. The control of the grid frequency is a form of ancillary service known as regulation. 
         [0047]    The remotely located control system that is used to dictate the action of the heating element can be configured to respond to the utility&#39;s regulation signal in order to help maintain grid frequency stability. This configuration could include setting a particular time period during which the heating element should respond to the grid control signal. There is a value attached to the regulation service and as a consequence this capability enables further economic benefits to supplying supplemental heat to the thermal storage tank via the electrical resistance heater. In this regard the heat can be injected into the tank at an additional discount to normal electricity rates by subtracting the revenue generated by providing the regulation service from the cost of the electricity during the time which the regulation service is being provided. 
         [0048]    Referring now to  FIG. 13 , multi-story home  1300  is shown with solar thermal array  1302  installed on its roof, hot/cold inlet outlet tube pairs  1304  mounted on an exterior wall, and components of a supplemental water heating system installed in the basement. The components in the basement include municipal water line  1308  which directs the incoming flow of municipal water  1306 . Insulate thermal storage tank  1310  which is of the type previously described. Heat exchanger  1312  is configured to extract heat from the storage tank and transfer that heat to incoming municipal water  1306  which emerges and is directed via transfer pipe  1314  to a primary water heating system  1316 . Heated water flow  1318  is subsequently available for use by the occupants of the home. Detail of the basement  1322 , also reveals electrical resistance heating element and heating element control unit  1320  the role and functionality of which has been previously discussed. Multi-story home  1300  could be any building or facility with a hot water need ranging including but not limited to a single or multi family home or complex, a hospital, a hotel, or a commercial enterprise such as a restaurant, beverage bottler or a commercial laundry. In all of these examples there exists a primary water heating source  1316 , which can take on a variety of forms but is configured to satisfy all of the hot water needs of the facility in which it resides. 
         [0049]    Solar thermal array  1302  comprises an array of flat panels, evacuated tube arrays many of which are well known in the art of converting sunlight into heat. Heat from the sun is advantageous in that, taking into account the cost of the solar thermal array and related components, the cost of heat may be less expensive than that of a primary water heater over time. In this case the heat which is produced by the array is ultimately transferred to a water loop comprising inlet outlet tube pair  1304 , which transfers the collected heat to the thermal storage tank  1310  in a manner which has been previously described. In general heat can be injected into thermal storage tank  1310  by some combination of sunlight derived heat, available during days of sufficient sunlight, and electrical resistance element  1320  heat, derived from the utility electrical grid, and made available according to the dictates of the control system. Given the variable circumstances under which heat may be made available from the combination of the solar thermal array and the electrical resistance heater, it is necessary that hot water heater  1316  serve as the primary source of heat. In this fashion hot water is always available to the consumer. In general, however the cost of heat to the consumer can be lowered if lower cost heat is provided to the municipal water supply from the thermal storage tank to supplement the heat provided by the primary water heater  1316 . The primary water heater may experience less overall usage as a consequence, thus lowering the average price paid by the consumer for hot water over a period of time.