WATER STORAGE TANK WITH PASSIVE ENHANCED THERMAL ENERGY MANAGEMENT AND RESISTANCE

Abstract

A water storage tank with passive enhanced thermal energy management is provided. The tank is formed of substantially cylindrical central section comprising a majority of the length along its axis and a dome-shaped section at each end, A first system of managing energy management includes providing insulation covering all or most of the tank in order to prevent heat flow energy leaving the tank. The strength of the insulation, is varied such that one end of the tank has less insulativity than the other end of the tank. Preferably change occurs gradually along the axial length of the tank. The second system for providing passive energy management is using insulation formed of a material that has a glass phase change temperature at or near the temperature of the water when it enters the tank. In order for the temperature of the water in the tank to change from the initial temperature it must first cause the insulation to make that glass phase change in order to either heat up or cool down from its initial temperature point. By varying the mass or thickness of the glass phase insulation along the axial length of the tank the amount of passive energy resistance to change varies thereby causing desired convection currents that serve to maintain a constant temperature within the tank thereby slowing any change of temperature at minimum cost.

Description

BACKGROUND OF THE INVENTION

Tanks used for the storage of elevated temperature water, such as hydronic heating, hot water expansion, buffer tanks, and geothermal heating tanks, are required to maintain a temperature differential with the outside environment, and in some cases, a temperature gradient within the tank itself. The approach to maintaining temperature, or temperature gradient, has typically been to either insulate the tank as a whole, or actively heat or cool the tank, or a combination of both. Previous approaches to the design of these tanks, including the materials of construction for fluid barriers, pressure reinforcement, and insulation, have provided largely uniform properties around the tank, not taking advantage of differences in thermal properties of these materials to optimize the performance of the tank. Meanwhile, active heating and cooling has the disadvantage of consuming additional energy, usually in the form of electricity, natural gas, or other fossil fuels. Therefore, there is a need for novel hot water storage tanks that take advantage of differences in the thermal properties of the materials of construction to provide enhancements in passive thermal management.

GENERAL DESCRIPTION OF THE INVENTION

The present invention provides an efficient, inexpensive passive means to provide a double diaphragm water tank, capable of providing the desired temperature maintenance. In accordance with one preferred aspect of the present invention, there is provided a double diaphragm holding tank with passive thermal management for the storage of water at temperatures up to 250° F. This invention avoids the use of more costly active heating or cooling means. Preferably, the tank comprises a central, elongated, substantially cylindrical housing section 1, joined at two circumferential end locations 2 and 22 to an upper and a lower dome-shaped housing end-sections. Within the tank housing sections, and secured to the inner circumferential surface of the cylindrical housing section is a flexible diaphragm and, preferably, a rigid diaphragm. The flexible diaphragm is preferably secured to the upper circumferential rim of the rigid diaphragm, which in this preferred embodiment is secured to the inner surface of the central housing section and the flexible diaphragm is sealingly secured to the upper rim, and circumferentially internally of, the rigid diaphragm by a removable circumferential clip. This invention provides an insulation layer surrounding the housing sections, which is preferably formed on the elongated central tank section and at least closely adjacent the dome sections, if present. Operatively connected to the bottom of the tank and extending through the bottom of the rigid diaphragm, if present, is a water inlet pipe, and operatively connected to and extending through the top of the tank is a pressure relief valve and nipple.

This invention provides a varying insulative effectiveness of the insulation layer along the length of at least the central elongated section, so that either the upper or lower dome-shaped housing section is covered with a less effective insulation layer, so that an internal temperature gradient is formed in the water, and thus to create convective mixing currents in water within the tank, when the water is at a temperature different from the temperature exterior of the tank. This can be accomplished by varying the thickness of the insulation along its length, or to otherwise change the effectiveness of the insulation, such as by changing its nature.

Another novel aspect of this invention is the use of an insulative material that has a glass phase transition temperature (t_g) of about the incoming temperature of the water in the tank, or preferably slightly lower. This temperature is commonly up to about 250° C. Generally, the insulation material should be tailored to the intended use of the tank, i.e., for holding cool water at room temperature or lower, or to hold hot water, as may be used for a hot water tank in the home or commercial building or factory.

Further, the desired variability of the insulative effectiveness can be achieved by selecting insulation material having a t_g near the desired temperature of the water in the tank.

The insulative effectiveness of material selected for its t_g may also be varied by varying the thickness of the glass phase insulation, or by reducing the percentage of that material in the total insulation, along the axial length of the tank, or even by varying the nature of the insulation material, so as to reduce the t_g of material forming the insulation along the length of the tank, to thereby create different temperatures along the length of the tank, so as to create the desired convection currents.

The glass phase transition temperature can also be useful when it is desired to merely increase the time of maintaining a constant temperature as compared with using the usual selection of material having a higher t_g. glass phase transition temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of an elevated temperature water tank according to an embodiment of the invention, representing the tank charged with air pressure and the space below the flexible diaphragm being not charged of water; and

FIG. 1B is an expanded view of the schematic cross-section of the diaphragm tank in FIG. 1.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

FIG. 1 is a cross section of a double diaphragm tank 11 with passive thermal management.

The upper portion of inside the tank, outside of the rigid diaphragm 31, and flexible diaphragm 32, is charged with air through the nipple 9, and lower portion, between rigid diaphragm 31 and flexible diaphragm 32, may be charged with water. The tank comprises a central, substantially cylindrical housing section 1, joined at two circumferential joint locations 2 and 22 to dome-shaped housing sections 5 and 6, respectively. The overall tank 11, preferably forms a substantially isotensoidal shape. The dome-shaped section 5 further comprises an air valve and nipple 9, which allows one area of the tank to be charged with air or gas. The lower dome-shaped section 6 further comprises a threaded connection 10 through which water can flow, via pipe 110, which extends to the bottom of the rigid diaphragm 31; through a water-tight seal the pipe 110 can discharges water into the volume between the two diaphragms 31,32. The condition of the flexible diaphragm 32, when the volume is filled with water is shown by the dashed line A, in FIG. 1.

The three housing sections 1, 5, and 6 are further reinforced with a pressure barrier, 41, and an insulation layer 42, and an outer shell, 43, substantially surrounding the pressure barrier 41, with openings for an inlet/outlet line 10 and for the pressure relief valve 9, and any other fittings or valve. Although the insulation layer 42 is shown as a rectangular box surrounding the tank, the insulation layer 42 is actually formed adjacent to at least the cylindrical central section 1, as shown in FIG. 1B.

In certain embodiments, the tank sections and housing sections 1, 5, 6 and rigid diaphragm 31, may be independently, or together formed of non-metallic materials, selected from the group including thermoplastic polymers, thermoset polymers, whether plastic or elastomeric, natural rubbers, or multilayer materials comprising the same.

In certain embodiments, the tank segments and housing sections 1, 5, 6 and rigid diaphragm 31 may be formed of materials selected from a group of thermoplastics including polyolefins, polyethylene, polypropylene, polybutylene, nylon, PVC, CPVC, ionomers, fluoropolymers, copolymers, crosslinked polyolefins such as crosslinked polyethylene (PEX, PEX-a. PEX-b, PEX-c or XLPE), or multilayer structures comprising the same. The individual items forming the above-described tank: housing sections 1, 5, 6 and rigid diaphragm 3 may also include a “tie-layer”. A “tie layer” is usually one or a combination of two or more mutually compatible materials that form a bonding layer between two mutually incompatible materials. Tie layers may include, for example, a thermoplastic material that provides adhesion to two adjacent materials, most often through melt processing or chemical reactions; modified acrylic acid, or anhydride grafted polymers or those similar to but not limited to DuPont's Bynel, Nucrel, and Fusabond grades, or those described and referenced, as further examples, in U.S. Pat. Nos. 8,076,000, 7,807,013 and 7,285,333. The melting point or melt index of the tie layer may be selected so that the tie-layer can be post-processed without substantially melting or flowing other non-metallics in the structure.

In some embodiments, housing sections 1, 5, 6 and rigid diaphragm 31 may be filled polymers or comprise solids such as but not limited to particles or flakes of polymers or minerals including glass, talc, carbon and graphite; chopped fibers, discontinuous fibers, short or long fibers, or continuous fibers of polymers or minerals including glass or carbon; nanocomposites; clays; or other fibers, particles, flakes or hollow microspheres. In some embodiments, housing sections 1, 5, 6 and rigid diaphragm 31 are independently or together metals, such as but not limited to steels, stainless steels, aluminum, or the like. In some cases, 5 and 6 may further comprise fittings or valves, including those made of metals or non-metals, including but not limited to threaded fittings, compression fittings, bulkhead fittings, quick-disconnect fittings, clip or crimp fittings, air valves, ball valves, needle valves or the like. In some cases, housing sections 5 and 6 may provide surfaces on which to make additional connections through processes including but not limited to stick welding, butt welding, spin welding, friction stir welding, ultrasonic welding, induction welding, solvent welding, RF/microwave processing, resistance-based fusion, adhesives, tie layers, or the like. These fittings, valves, or other surfaces may be connected by means known to those skilled in the art to additional system components including, but not limited to, heaters, filters, pumps, pipes, tanks, or hoses.

In certain preferred embodiments, housing sections 1, 5, 6 and rigid diaphragm 31 are polypropylene, ethylene-polypropylene copolymers, and glass particle or glass fiber-filled polypropylene and ethylene-polypropylene copolymers. The ethylene-propylene copolymers may be block copolymers. The melting point and melt index of housing sections 1, 5, 6 and rigid diaphragm 31 may be tailored to improve the assembly and processing of the tank. In certain embodiments, the outer surface of housing sections 1, 5, 6 and rigid diaphragm 31 may be independently or together surface modified by high energy treatments including ion implantation, plasma, corona or arc, to improve adhesion to adjacent materials. The inner surface of housing sections 1, 5, 6 and rigid diaphragm 31, and flexible diaphragm 32, can also be modified to change properties, such as, but not limited to, chemical resistance, permeability, and wettability by water. Treatments may include but not limited to fluorination or the technologies employed by NBD Nano, or by metallization through chemical vapor deposition or the like. In certain preferred embodiments, polypropylene, polypropylene copolymers, glass filled polypropylene and glass filled polypropylene copolymers are treated by a flame to improve adhesion to adjacent layers. In some preferred embodiments, housing sections 1, 5, 6 and rigid diaphragm 31, and flexible diaphragm 32 may include antimicrobials, including antifungals, antivirals, or antibiotics, or comprise silver. In other preferred embodiments, housing sections 1, 5, 6 and rigid diaphragm 31, and flexible diaphragm 32 contain antioxidants and stabilizers.

The flexible diaphragm 32 may be comprised of a polymer, elastomer, rubber, RTV, or thermoplastic, or multiple layers comprising the same. In certain preferred embodiments, the diaphragm 32 comprises butyl rubber or EPDM. In other embodiments, the diaphragm may be filled with solids such as but not limited to particles or flakes of polymers or minerals including glass, talc, carbon and graphite; chopped fibers, discontinuous fibers, short or long fibers, or continuous fibers of polymers or minerals including glass or carbon; nanocomposites; clays; or other fibers, particles, flakes or hollow microspheres; or woven or non-woven fabrics; to improve the thermomechanical properties or decrease permeability of gases through the membrane. In some embodiments, these multiple layers of the diaphragm are bonded, but the layers may also be non-bonded. In certain embodiments, the layers include a thin higher modulus layer supported by a thicker, lower modulus layer. The high modulus layer may be selected from chemically resistant polymers, or polymers preferred for contact with potable water, such as polypropylene, polyethylene, polybutylene, or the like. The low modulus layer may be selected for different properties, such as durability, toughness, and low cost, protected from contact with the potable water by the high modulus layer.

The flexible diaphragm 32 may also comprise features to reduce the tendency of the diaphragm to wear or become fatigued in service, or protect it from abrasion or cutting by adjacent structures such as a clinch ring. In some cases, the flexible diaphragm 32 may be of substantially non-uniform thickness or modulus. The non-uniform thickness or modulus may be controlled across the surface to reduce the tendency for the diaphragm to rub against itself, against other structures, abrade or tear. The flexible diaphragm 32 can also be substantially folded, in an accordion, serpentine, or wavy shape. These shapes may allow for more compact or rigid diaphragms to be used, while still allowing extension in service without localized strains exceeding the limits of the materials. The diaphragm may be further molded or installed in the shape or orientation that it is most often in service to reduce the in-situ strains or abrasion.

The flexible diaphragm 32 can be sealably joined at the peripheral edge to the rigid diaphragm 31, by methods known to those skilled in the art. Such sealable joints can be formed using, for example, adhesives, solvent bonding, stick welding, butt welding, spin welding, friction stir welding, induction welding, RF/microwave processing, resistance-based fusion, tie layers, or the like, with or without additional sealants. In certain preferred embodiments, the connection of the peripheral edges of diaphragms 31 and 32 may also be made by the application of a rigid clinch ring 3. Such a clinch ring 3 can be comprised of metal or non-metal and provides a clamping force by means known to those skilled in the art, such as but not limited to crimping, snap coupling, fasteners, adhesives, melt processing, thermoforming or the like. The connection between clinch ring 3, the flexible diaphragm 32 and the rigid diaphragm 31 is accomplished by the use of features or structures that improve the connection and seal such as lips, dimples, ridges, knobs, integral rings, including multiple rings.

In certain preferred embodiments, the flexible diaphragm 32 may extend over both sides of the rigid diaphragm 31 and/or be gripped between the surfaces of a u-shaped clinch ring 3, i.e., a preferably elastic ring with a u-shaped cross-section. In some cases, the clinch ring 3 may have a contour that controls the radius of curvature of the diaphragm and protects the diaphragm from contacting any sharp edges.

In some embodiments, housing sections 1, 5, 6 may be further reinforced by the pressure barrier 41 to increase the pressure carrying capabilities of the expansion tank. This reinforcement may comprise glass (including but not limited to borosilicate, e-, s-, and cr-glass), basalt, quartz, carbon or other inorganic or mineral fibers. The reinforcement may also comprise organic or inorganic polymer fibers such as but not limited to polyester, nylon, polypropylene, aramid, Kevlar, Nomex, PPS or carbon. These fibers or fillers may be continuous or discontinuous fibers, chopped, non-wovens or random oriented mat, or may be in the form of fiber tapes. The reinforcing materials may be in a thermoset or thermoplastic matrix, or present without a matrix. In certain preferred embodiments, the reinforcement is a fiberglass-reinforced epoxy. The reinforcement of pressure vessels by, for example, filament winding is well known to those skilled in the art. In some cases, the reinforcement is a metal such as but not limited to steels, stainless steels, aluminum or the like.

In certain embodiments, the tank does not include a rigid or flexible diaphragm, but is rather largely filled with water. In other preferred embodiments, there may be additional ports into the tank, including through the wall of the cylinder 1 and the pressure barrier 41. In certain embodiments, dome 5 may include additional features that encourage mixing or turbulent flow, or in some cases, laminar flow. In other embodiments additional features or plumbing may extend from fitting 10 into the water chamber to control flow of water within the tank.

In some embodiments, housing sections 1, 5, 6 with or without rigid diaphragm 31 and flexible diaphragm 32, and with or without pressure barrier 41, can be further insulated with insulation layer 42. The insulation layer 42 may be material with a low K value, including continuous or discontinuous fiber insulation, pulverized or aerated materials, flakes, or chopped materials that are poured or blown into an outer shell 43, or may be wet blown onto the surface with or without the use of binders or adhesives. The insulation layer 42 may also be a flexible, rigid, or semi-rigid foam. Although the insulation layer 42 and outer shell 43 are diagrammatically shown as a rectangle, they are preferably formed concentric with the pressure barrier 41. The foam may be comprised of polymers, thermosets, elastomers, ceramics, or the like. The foam may further comprise air, CO2, or blowing agents including hydrofluorocarbons. The foam may be formed by pouring a foaming material, such as a two-part expanding foam urethane, into the 43. Alternatively, preformed foam can be wrapped around the tank or joined as sections such as in a “clam shell”. Some foams, including flexible, semi-rigid, heat formable, or kerfed may be wrapped around the tank. In some preferred embodiments, there is a small gap between the pressure barrier 41 and the insulation layer 42.

The outer shell 43 can be comprised of metal or non-metals, including thermoplastics or thermosets. It may be formed as a continuous shell by means such as but not limited to extrusion or rotomolding or it may be applied as a wrap. The wrap may be joined by overlap joints, but welds, seam welds, or mechanical fasteners; the mechanical fasteners may further engage the insulation layer 42. Outer shell 43 may also have the tendency to shrink by either induced stress from the fabrication process, application of additional heat (such as heat shrink), or by the elasticity of outer shell 43. In a preferred embodiment, there is a small gap between the outer shell 43 and the insulation 42. Insulation layer 42 may be non-uniform around the tank. In one preferred embodiment, there is extra insulation on the top of the tank, and less or no insulation at the bottom. The pressure barrier 41, insulation layer 42 and outer shell 43 may also include barrier layers, such as plastic or elastomeric moisture barriers, thermally reflective foils, metallized layers, or the like. In one preferred embodiment, insulation layer 42 and outer shell 43 may each independently have a reflective lining 50, such as shown in FIG. 1B.

All of these materials and design elements are well known to those skilled in the art and are common in the industry. The novel invention, disclosed herein, is the use of novel passive designs to enhance the thermal management without the use of active heating or cooling, specifically for applications where the temperatures exceed about 65° C.

Smart Mixing

In one embodiment of Smart Mixing, a tank shown in FIG. 1 is capable of operating from −40° F. to a maximum temperature 150-250° F. The tank comprises a dome 5, fiber reinforcement 41, insulation layer 42, and outer shell 43, that independently or together produce a K value that is at least 10% higher through the dome 5, than the rest of the shell, or more preferably at least 25% higher. This decreased insulation at the top of the tank changes the near-field temperature of the fluid inside. The temperature differential between the top surface of the tank and the rest of the fluid in the tank has been found to induce convective currents, which stir the fluids, improving the temperature distribution as well as cleaning the surfaces from fouling, all without any additional added forces, stirrers, or heaters. Similarly, the Smart Mixing tank can have a lower dome 6, fiber reinforcement, bottom insulation, or other features which produce a K value that is 10%-25% lower than the rest of the shell, causing near-field temperature inversion, increasing convection within the tank and stirring the sediment from the bottom of the tank, without the need for additional moving parts or energy inputs.

In another embodiment of Smart Mixing, a multi-layer composite tank is capable of operating from −40° F. to a maximum temperature 150-250° F. The inner surfaces of housing sections 1, 5, 6 are manufactured with internal upsets. These upsets, or small baffles, may be as large as 0.1″ or as small as 0.05″, protruding into the inside of the tank, such that when fluids are moving within the tank, either from bulk or from convective currents, the flow is disrupted and the increase in turbulence improves the mixing within the tank and cleaning of surfaces without the need for additional moving parts or energy inputs.

Smart Strain Relief

In another embodiment of the improved passive thermal management, a tank shown in FIG. 1 is capable of operating from −40° F. to a maximum temperature 150-250° F., and is fitted with additional ports through the side wall. Ports or openings that penetrate the outer wall of the tank, especially through housing section 1 and pressure barrier 41, are well understood to be weak points in the structure of the tank, and the location of stress risers. These areas of weakness and enhanced stress are further exacerbated by temperature differentials with the outside environment. In this embodiment, metal is incorporated in and around ports. This serves to draw temperature from the tank, into the ports, and moderate the temperature transition between the tank and the outside environment and reduce thermal stress on the sensitive joint structures, without the need for additional fiber reinforcement.

Phase Transition Materials

In another embodiment of the novel passive, integrated thermal management of the present invention, a multi-layer composite tank shown in FIG. 1 is capable of operating from as low as −40° F. to a maximum temperature 150-250° F. and the thermal management is imparted to said tank through the use of selected phase transition materials as insulators. Phase transition materials may include thermoplastics or thermosets that have a glass transition temperature within the operating window of said tank, preferably at or below the initial inlet temperature of the water entering the tank. These phase transition materials are used to maintain the stored water temperature near the phase transition for longer than if the phase transition was outside, especially far above, the water temperature.

Examples of polymers that could be used to prepare insulation are shown in the several listed in Table 1, below, together with their glass phase transition temperatures (t_g). There are many well-known texts providing the transition temperatures of available synthetic polymers as well as natural materials.

TABLE I
                                 Tg
Polymer                          (Glass Transition Temperature)
Nylon 6/6                        50° to 60° C.
Polycarbonate                    140° to 150° C.
Polyethelene Terephthalate (PET) 70° to 80° C.
Polymethyl Methacrylate (PMMA)   85° to 105° C.
Polyphenylene Sulfide            85° to 95° C.
Polystyrene                      90° to 110° C.
Polytetrafluoroethylene (PTFE)   120° to 130° C.
Polyeurethane (Thermoplastic)    120° to 160° C.
Polyvinyl Alcohol                80° to 90° C.
Polyvinyl Chloride (PVC)         65° to 85° C.

Ranges of temperatures are often shown due to the fact that such values are often dependent upon the particular molecular weight of the polymer, its method of manufacture and many factors well-known to the polymer chemists who prepare such materials. The novelty of the present invention does not reside in the method of making the polymers, but rather in the novel way in which they are being used to maintain temperatures of stored water, especially hot water. Other polymers may be used for purposes of this invention without departing from its scope.
The listed Tg is usually some middle point within the range over which the polymer transitions from a softer state to a rigid glass. Therefore, it should be understood that the phase change effect on temperature may linger over a range of dropping temperatures.

Air Gap Inducers

In one embodiment of integrated passive thermal management, a multi-layer composite tank in FIG. 1 is capable of operating from −40° F. to a maximum temperature 150-250° F. Novel features imparted into the wall of housing section 1, pressure barrier 41, insulation layer 42, and outer shell 43 intentionally provide spacing between the two layers. In one preferred embodiment, the tank includes air gap inducers 51. These stand-offs are designed to create small, air filled spaces between pressure barrier 41 and insulation layer 42, and between insulation layer 42 and outer shell 43. In one exemplary embodiment, these stand-off inducers decreased the heat loss from the tank by 20%. In another embodiment multiple air gaps are used. These air gaps can be individually selected between the rigid diaphragm 31 and dome 6; housing sections 1, 5, 6 and the pressure barrier 41; the pressure barrier 41 and the insulation layer 42; and the insulation layer 42 and the outer shell 43.

These air gap inducers have been found to maintain the temperature of the tank for 2× longer without the need for additional heat input. In one preferred embodiment, the air gap is greater at the top than at the bottom to optimize temperature striation, ideally suited for applications such as solar hot water storage tanks.

In another embodiment, the air gap between surfaces is controlled by changing the internal pressure or modulus of the materials of construction during the curing of the matrix of pressure barrier 41. In a preferred embodiment, the modulus of the housing sections 1, 5, 6, is decreased by 5% or more through the use of temperature and the internal air pressure is increased by at least 10%. By increasing the air pressure inside the tank, and decreasing the modulus of the inner layers, the gap between layers can be reduced to less than 0.01 which has been found to be optimal for passive thermal management.

In another embodiment, the thermal management of the tank controlled by tailoring the amount of thermoset matrix, or fiber volume fraction, in the tank. In one preferred embodiment, the matrix is an epoxy, the fibers are glass, and the relative concentration of the epoxy is lower on the top of the tank than on the bottom. In another preferred embodiment, the amount of epoxy is below the amount theoretically needed to fill the spaces between the fibers resulting in extremely small voids which help serve to insulate the tanks and retain heat.

In another embodiment, a small air gap is provided to said tank by reducing the bonding between the housing sections 1, 5, 6 to the pressure reinforcement at joints 2 and 22. This reduces the heat loss at this location, increasing the temperature and reducing the modulus at joints 2 and 22 to improve the ductility and toughness in this high stress location. This air gap is most effective if maintained at roughly 1″ wide and less than 0.05″ h.

In some preferred embodiments, the rigid and flexible diaphragms 31 and 32, domes 5, 6, or the cylinder 1, may include layers that serve to reduce heat transfer, including reflective layers, such as metalized layers, or may be light in color to reduce the amount of radiative heat loss.

Smart Susceptors

In one embodiment of integrated thermal management, a multi-layer composite tank is capable of operating from −40° F. to a maximum temperature 150-250° F. A susceptor such as carbon, graphite, or metal is added to housing sections 1, 5, 6 so that it may be heated from an external energy source such as induction, RF, or microwave, without significantly heating the insulation or fiber reinforcement. In another embodiment, ports in and out of the tank comprise metals which may be heated by said external power supplies.

Claims

1. A double diaphragm tank with passive thermal management for the storage of water at temperatures up to 150° C., while avoiding the use of active heating or cooling means, the tank comprising a central, substantially cylindrical housing section 1, joined at two circumferential locations 2 and 22 to an upper and a lower dome-shaped housing sections, and within the tank housing sections, and secured to the inner circumferential surface of the cylindrical housing section is a rigid diaphragm, and a flexible diaphragm, the upper circumferential rim of the rigid diaphragm being secured to the inner surface of the central housing section and the flexible diaphragm being sealingly secured to the upper rim, and circumferentially, internally of the rigid diaphragm, by a removable circumferential clip; and

an insulation layer surrounding the housing sections;

there being operatively connected to the bottom of the tank, and extending through the bottom of the rigid diaphragm, a water inlet pipe; and operatively connected to and extending through the top of the tank is a pressure relief valve;

the improvement comprising, varying the insulative effectiveness of the insulation layer, axially along the central housing section 1, so that the either the upper or lower dome-shaped housing sections are covered with a less effective insulation layer, so as to create an internal temperature gradient in any water held in the tank, so as to create convective mixing currents in such water within the tank, when the water is at a temperature different from the ambient temperature exterior of the tank.

2. The double diaphragm tank of claim 1, wherein the insulative effectiveness of the insulation layer is varied by providing a sealed insulation and varying the width of the air gaps within portions of the insulation layer.

3. The double diaphragm tank of claim 1, wherein the insulative effectiveness of the insulation layer is varied by changing the thickness of the insulation layer.

4. The double diaphragm tank of claim 1, extending vertically, wherein the insulative effectiveness of the insulation layer is varied such that the Kvalue of the insulation layer in one of the upper or lower dome sections is at least 10% greater than the insulation in the rest of the tank, intended to create convective currents in water in the tank to improve temperature distribution in the water and to prevent fouling of the internal surfaces of the tank.

5. A double diaphragm tank with passive thermal management for the storage of water at temperatures up to 250° F., while avoiding the use of active heating or cooling means, the tank comprising a central, substantially cylindrical housing section 1, joined at two circumferential locations 2 and 22 to an upper and a lower dome-shaped housing sections, and within the tank housing sections, and secured to the inner circumferential surface of the cylindrical housing section is a rigid diaphragm, and a flexible diaphragm, the upper circumferential rim of the rigid diaphragm being secured to the inner surface of the central housing section and the flexible diaphragm being sealingly secured to the upper rim, and circumferentially internally of, the rigid diaphragm by a removable circumferential clip; and an insulation layer surrounding the housing sections; there being operatively connected to the bottom of the tank and extending through the bottom of the rigid diaphragm is a water inlet pipe, and operatively connected to and extending through the top of the tank is a pressure relief valve; the improvement comprising securing to the housing sections a layer of insulating material having a glass transition temperature not greater than the desired temperature of the water in the tank, in order to maintain the temperature of the tank near the glass transition temperature.

6. The double diaphragm tank of claim 4, wherein the insulative effectiveness of the insulation layer is varied by varying the quantity of the glass phase transition material within the insulation layer.

7. The double diaphragm tank of claim 4, wherein the insulative effectiveness of the insulation layer is varied by varying the thickness of the glass phase transition material within the insulation layer.

8. The double diaphragm tank of claim 4, wherein the insulative effectiveness of the insulation layer is varied by varying the spacing of the glass phase transition material insulation layer from the surface of the tank shell.