Patent Description:
Storage boilers, also referred to as dry core storage boilers, heat banks, heat batteries or zero emission boilers, convert electrical energy into heat using electrical heating elements or resistive heating elements and store the heat in a storage medium located in a core. The heat is usually transferred from the storage medium by a fan driving air between the core and a heat exchanger in a closed loop. The heat exchanger transfers the heat to a water and/or central heating system for delivering heated water as required. Typically the storage boilers consume electrical power at times of low demand (or excess generation) across an electricity grid or network, such as during the night, when it has a lower cost. Increasingly this can occur at any time of day due to the increase in generation from renewable sources. In order to store large amounts of heat and have high heat transfer rates in the core, it is generally desirable for the storage medium to have high thermal conductivity and high heat storage density (i.e. a high specific heat capacity and high material density). Further requirements are for the storage medium to have limited thermal expansion and to be low cost. <CIT> and <CIT> disclose typical storage boilers in which the storage medium comprises refractory bricks. Such refractory bricks are commonly used due to their relatively low cost and ease with which they can be formed in different structural shapes. However, refractory bricks generally comprise a binding agent, such as clay, or are manufactured using certain production methods such as clay firing, which decrease their overall heat storage density as compared to the raw heat storage material. <CIT> proposes increasing the heat storage density by packing a first heat storage material in lumpy form in a casting mould and pouring a second heat storage material into the mould to fill the residual cavities in the first heat storage medium. However, the method of <CIT> involves additional processing steps and the rest of the core has to be designed around the shape of the moulded heat storage material.

In order to improve the power output of the storage boiler it is desirable to have high air flowrates and effective transfer of heat from the heat storage medium to the air. The air circulation systems of <CIT> and <CIT> aim to achieve the latter by having the air directly contact the storage medium. However, the systems therefore require chambers before and after the refractory bricks for the distribution of air to and collection of air from the refractory bricks. High pressure drops result due to high turbulence at the sharp changes in fluid direction such that the air circulation fan produces a lower flowrate, thereby limiting the power output of the storage boiler.

In <CIT> and <CIT> throttle valves are used to control the air flowrate and thus control the power output. The temperature of the air contacting the heat exchangers is not directly controllable and the maximum temperature allowable in the core (<NUM> in <CIT> and <CIT>) must be set according to the temperature ratings of the other components of the air circulation loop. In particular, the maximum temperature of the air exiting the core must be set in correspondence with the maximum temperature rating of the heat exchanger, which directly receives hot air from the core. Thus the maximum temperature of the air exiting the core is limited and the power output of the storage boiler is limited. In systems such as those of <CIT> and <CIT>, the refractory bricks can be operated at a temperature higher than the maximum temperature rating of the heat exchanger because the bricks have low thermal conductivity and thus do not raise the temperature of the air to the temperature of the refractory bricks (i.e. the temperature difference between that of the bricks and the transfer fluid is high). Instead, the air is only raised to a lower temperature that is below the maximum temperature rating of the heat exchanger.

<CIT> discloses an electric storage heater with finned heat exchanger for allowing infinitely variable heat control by using flap valve. <CIT> discloses a method of controlling a heating apparatus and a heating apparatus controlled by this method, the apparatus comprising an accumulation block heated by resistances. <CIT>, entitled "Improvements in Storage Heaters," discloses a storage heater comprising a storage core, a main air flow passage for air to be heated by the core, a by-pass passage, a fan casing in circuit with both passages, a valve for apportioning air between the passages, and a fan for recirculating air through the passages.

Objects of the present invention include addressing these problems and providing an improved storage boiler. A particular object is to provide a heat storage medium that has a low cost, a high thermal conductivity and a high heat storage density. A further object is to provide a storage boiler with a more consistent and improved power output across a range of core temperature. The storage boiler of the present invention is preferably a domestic storage boiler for domestic use and may be referred to as a dry core storage boiler.

The present invention provides a storage boiler and method in accordance with the claims.

The storage boiler comprises a base to which the core is mounted. The bypass arrangement is for mixing cooled transfer fluid with heated transfer fluid prior to the mixed transfer fluid entering the heat exchanger. The bypass arrangement is mounted inside the base. The storage boiler comprises a control system configured to operate the fan, by varying the speed of the fan, for controlling the flowrate of transfer fluid through the core and fluid system. The control system is configured to operate the bypass arrangement for controlling the flowrate of transfer fluid through the bypass arrangement.

The bypass arrangement can be operated to control the temperature of the transfer fluid entering the heat exchanger, which is therefore substantially independent of the temperature of the core. As a result, the core can be operated at high temperatures independently of the heat exchanger temperature value and temperature rating of the heat exchanger. The temperature difference between the core and the transfer fluid exiting the core can thus be low. As the temperature of the core is reduced, the temperature of the transfer fluid exiting the core reduces closer to the temperature rating of the heat exchanger. The bypass arrangement can be operated to reduce the bypass flowrate such that the temperature of the transfer fluid entering the heat exchanger is maintained at a heat exchanger temperature value, which may be the temperature at which the heat exchanger operates at substantially maximum power output. Thus maximum power output can be achieved across a range of core temperatures. A heat storage medium with high heat storage density, which stores a substantial amount of heat at a relatively high temperature, can therefore also be selected.

The storage boiler of the present invention may comprise a heat storage medium comprising an aggregate bed of material elements.

The storage boiler of the present invention may comprise a fluid system comprising at least one fluid conduit comprising at least one core fluid passageway extending through the core for receiving transfer fluid.

The storage boiler of the present invention may comprise an electrical system comprising at least one heating element at least partially located in the core for directing heat to the heat storage medium from at least one heating element. The heat storage medium may be sealed from the heating elements and transfer fluid. In particular, the storage boiler may comprise at least one element conduit extending at least partially, preferably entirely, through the core chamber and the at least one heating element may be at least partially located in the at least one element conduit. The at least one element conduit may be sealed to the core housing. The heat storage medium, preferably comprising the aforementioned aggregate bed of material elements, may be distributed at least partially around and in contact with the, preferably exterior of, at least one element conduit.

The storage boiler of the present invention may comprise (a) an inlet distributor for distributing transfer fluid prior to entry into the core, preferably prior to entry into a plurality of the core fluid passageways and/or (b) at least one distribution baffle located downstream of the core, preferably in a heated fluid passageway extending from the core, preferably from at least one core fluid passageway, to the heat exchanger. The method may further comprise (a) directing transfer fluid through the inlet distributor to distribute transfer fluid between the core fluid passageways and/or (b) directing transfer fluid through the at least one distribution baffle to distribute the transfer fluid from at least one core fluid passageway prior to entry into the heat exchanger.

It will be appreciated from the foregoing that the storage boiler and method of operating a storage boiler in accordance with the present disclosure may comprise the bypass arrangement or a combination of the bypass arrangement and the aggregate bed, fluid conduit(s), electrical system, inlet distributor, distribution baffle(s) and/or any other features disclosed herein. For example, the storage boiler may comprise a bypass arrangement and a core comprising refractory bricks as in the prior art and the fan may circulate the transfer fluid through the refractory bricks rather than any fluid conduits. However, as will be apparent, a combination results in a substantially improved storage boiler. In particular, the bypass arrangement allows for a heat storage medium to operate at a particularly high temperature. The aggregate bed is also an appropriate manner of including a material with a high thermal conductivity and the fluid conduits can be formed from a material with a high thermal conductivity, such that there is a low temperature difference between core and transfer fluid exiting the core. Thereby, in combination with the bypass arrangement, the heat exchanger can operate at maximum power across a wide range of core temperatures.

By way of example only, embodiments of a storage boiler, methods of operating a storage boiler and methods of manufacturing a storage boiler in accordance with the present invention are now described with reference to, and as shown in, the accompanying drawings, in which:.

<FIG> and <FIG> and <FIG> illustrate, respectively, first and second embodiments of a storage boiler <NUM> in accordance with the present invention. Unless otherwise specified, the following description generally applies to both embodiments. The storage boiler <NUM> comprises a core <NUM> mounted to a base <NUM> and an electrical system <NUM> extending into the core <NUM> for providing heat thereto. The storage boiler <NUM> further comprises a fluid system <NUM> extending through the core <NUM> and base <NUM> for heating a transfer fluid in the core <NUM>, for circulating the heated transfer fluid between the core <NUM> and the base <NUM> and for extracting the heat from the transfer fluid for supply to an external heat demand system <NUM>, such as a domestic hot water circuit or domestic central heating system.

The core <NUM> comprises a core housing <NUM> and a heat storage medium <NUM> within the core housing <NUM>. The core housing <NUM> may comprise or define a core chamber <NUM> within it and in which the heat storage medium <NUM> is located or dispersed. The core chamber <NUM> may be a sealed chamber, in particular sealed from the environment external to the core <NUM> and/or storage boiler <NUM> such that gas cannot be communicated into or out of the core chamber <NUM>. The core housing <NUM> preferably comprises a non-oxidising material, such as stainless steel. As illustrated, the core housing <NUM> may be substantially cuboidal such that the core chamber <NUM> is also substantially cuboidal and may comprise an open core main housing <NUM> and a lid <NUM> to seal the opening of the core main housing <NUM>. The lid <NUM> may be removable for access to the core chamber <NUM> for maintenance. However, the core housing <NUM> and core chamber <NUM> may have any other suitable shape and/or construction.

The heat storage medium <NUM> preferably comprises an aggregate bed of material elements as illustrated. For example, the heat storage medium <NUM> may comprise a plurality of steel fragments and/or magnetite dust. The heat storage medium <NUM> is partially illustrated in <FIG>, illustrated in cross section in <FIG> and hidden in <FIG> and <FIG>. In the present disclosure the term "aggregate bed" refers to a plurality of material elements that are loosely and/or randomly distributed in the core chamber <NUM>. The aggregate bed may be an agglomeration of discrete, unfused and disjoined material elements. The material elements may thus be poured into the core chamber <NUM>. The aggregate bed is supported from the lowermost surface of the core chamber <NUM> and may fill at least approximately <NUM>% or at least approximately <NUM>% of the volume of the core chamber <NUM>. The aggregate bed is substantially different from refractory bricks, which are large bricks stacked regularly and non-randomly upon one another. The aggregate bed may be compacted, such as on a vibration platform or by tamping.

The material elements may comprise particles, fragments, small bodies and/or the like. The particles may be in powder and/or fine form. The fragments may be offcuts or recycled materials from other material processes, particularly metal processing industries. Thus the fragments may comprise punchings, shot, shavings, grit, small irregular sheets and the like. The small bodies may comprise balls, cubes, irregular shapes and the like.

The average maximum diameter of the material elements (which may be the average of all of the maximum dimensions of the material elements) is preferably up to approximately <NUM>, approximately <NUM> or approximately <NUM> and/or at least approximately <NUM>, approximately <NUM> or approximately <NUM>.

The material elements comprise a heat storage material, which may be a medium capable of storing thermal energy, principally by sensible heat storage (or antiferromagnetic and eutectoid transition effects discussed herein below), for extraction at a later time. The material elements may comprise different heat storage materials. At least approximately <NUM>% or <NUM>% of the volume of the heat storage medium <NUM> may comprise the same heat storage material. The heat storage material may not be a phase change material, may comprise an oxidising material and is solid. The material elements may comprise a metal and may comprise at least one of an iron oxide and/or a ferrous metal or iron alloy (preferably with at least <NUM> wt% or <NUM> wt% iron content and/or up to <NUM> wt% carbon content). The iron oxide may comprise magnetite (Fe3O4), hematite (Fe2O3), wüstite (FeO) and/or any other suitable iron oxide. The iron alloy may comprise cast iron (e.g. <NUM> to <NUM> wt% carbon), a steel, a carbon steel (e.g. <NUM> to <NUM> wt% carbon content), a stainless steel and/or <NUM> series (according to the SAE designations, or corresponding to EN steel grade <NUM>) ferritic stainless or chrome iron (which may have iron content in the range of <NUM>-<NUM>% and a high chromium content, such as <NUM>-<NUM> wt%). Carbon steel (preferably a carbon steel in accordance with the definitions thereof set by AISI) is particularly preferably due to its high specific heat capacity and high thermal conductivity. The use of carbon steel means that the volume of the heat storage medium <NUM> can be reduced for domestic use. Low carbon steel (e.g. less than <NUM> wt% carbon content) may be preferable due to its relatively low cost. The carbon steel may not be stainless steel by having a low chromium content.

The material elements may comprise an iron alloy, preferably carbon steel or cast iron, that forms austenite above a eutectoid or austenite transition temperature and during operation of the storage boiler <NUM> the temperature of the material elements may be raised (by the electrical system <NUM>) to at least the eutectoid temperature. At the eutectoid temperature, when the iron alloy crystalline structure transitions from body centre cubic to face centre cubic to form austenite, a significant amount of energy is absorbed and thus the iron alloy has a significant increase in specific heat capacity when heated above the eutectoid temperature. In particularly preferred embodiments at least a portion of the material elements comprises an iron alloy having a eutectoid or near-eutectoid composition. The eutectoid composition is the iron alloy composition at which the transition is directly to austenite without intermediate structures in between. The term "near eutectoid" refers to the carbon composition being within <NUM> wt% of that of the eutectoid composition. Preferably the carbon steel comprises a eutectoid composition of approximately <NUM> wt% carbon with a eutectoid temperature of approximately <NUM> or is a low carbon steel (e.g. <NUM> wt% carbon) with an austenite transition temperature in the range of approximately <NUM> to approximately <NUM>.

The material elements may comprise an iron oxide, preferably magnetite or hematite, and during operation of the storage boiler <NUM> the temperature of the material elements may be raised to at least a predetermined material temperature. The predetermined material temperature may be a temperature at which the heat storage material exhibits an increase in specific heat capacity, due to a reversible antiferromagnetic transition and/or a transition to austenite as discussed above. The predetermined material temperature may be between approximately <NUM> and approximately <NUM> and may be approximately <NUM> for magnetite and approximately <NUM> for hematite.

The heat storage medium <NUM> preferably comprises first material elements and second material elements, which may each comprise different heat storage materials. The second material elements may have a lower average maximum diameter than the first material elements. As a result, the second material elements may fill spaces or voids between the first material elements, thereby increasing the overall density and thus heat storage density of the heat storage medium <NUM>. Preferably the first material elements comprises fragments or small bodies and the second material elements comprise a powder. Thus the average maximum diameter of the first material elements may be in the range of approximately <NUM> to approximately <NUM> and the average maximum diameter of the second material elements may in the range of approximately <NUM> to approximately <NUM>. Preferably the first material elements comprises steel (e.g. punchings or shot) and the second material elements comprises an iron oxide such as magnetite or hematite (e.g. in fine powder form). The heat storage medium <NUM> may comprise third and onwards sets of material elements. The first and/or second material elements may each occupy at least approximately <NUM>%, <NUM>%, <NUM>% or <NUM>% of the total volume of the heat storage medium <NUM>.

The base <NUM> comprises a base housing <NUM> defining an internal base chamber <NUM> through which the fluid system <NUM> extends. The base chamber <NUM> may be a sealed chamber. The base housing <NUM> may comprise an open base main housing <NUM> and a mounting plate <NUM> mounted to the opening of the base main housing <NUM>. At least one base passageway <NUM> may extend through the base housing <NUM>, preferably through the mounting plate <NUM>, for allowing fluid communication between the base <NUM> and the core <NUM>.

The storage boiler <NUM> may further comprise an insulation arrangement <NUM> located between and mounted to the base <NUM> and core <NUM> for insulating the components in the base <NUM> from the heat of the core <NUM>. The insulation arrangement <NUM> may be mounted to and between the base housing <NUM>, preferably to the mounting plate <NUM> thereof, and the core housing <NUM>, preferably the core main housing <NUM> thereof. Preferably the insulation arrangement <NUM> comprises at least one insulation block <NUM>, which may be formed from, for example, calcium silicate or microporous board. Insulation passageways <NUM> may extend through the insulation arrangement <NUM>, preferably entirely therethrough, to allow the communication of fluid between the core <NUM> and the base <NUM>. The insulation passageways <NUM> may be aligned with the at least one base passageway <NUM>. In other embodiments the core <NUM> may be mounted directly to the base <NUM>.

Although not illustrated in the Figures, the storage boiler <NUM> may further comprise an external housing and/or external insulation around at least the core <NUM>. In particular, the storage boiler <NUM> may comprise external insulation extending around at least the majority of, and preferably substantially all of, the core housing <NUM>. The external insulation may comprise a plurality of insulation boards located around the sides and top of the core housing <NUM>, preferably with overlapping corners to minimise heat bridging. The external insulation may also comprise a thin layer of mineral wool or the like between the insulation boards and the external housing.

The electrical system <NUM> comprises heating elements (not illustrated) for supplying heat to the heat storage medium <NUM> of the core <NUM>. The heating elements may receive electricity from an external power source (not illustrated), such as an electricity grid or network, and may comprise electrical resistive heating elements for converting electrical power into heat. The heating elements preferably comprise electrical elements or coils for receiving electrical power, each mounted inside a nonconductive sheath, which may, for example, comprise ceramic. The heating elements are sealed from the core chamber <NUM> such that they do not contact the heat storage medium <NUM>.

As illustrated, the electrical system <NUM> may comprise element conduits <NUM> extending at least partially through, preferably entirely between opposing sides of, the core chamber <NUM> and at least one heating element in each element conduit <NUM>. The hollow interior of each element conduit <NUM> is preferably sealed from the core chamber <NUM>. The element conduits <NUM> are mounted, preferably at both of ends, to the core housing <NUM>. The element conduits <NUM> are accessible from, such as by at least one end of the element conduits <NUM> being open at, the exterior of the core housing <NUM> such that the heating elements can be accessed easily for maintenance. The storage boiler <NUM> may comprise at least five or at least ten element conduits <NUM> and/or heating elements located therein. The element conduits <NUM> may be arranged in an array such as in at least one row of substantially parallel element conduits <NUM> as illustrated. The element conduits <NUM> may be spaced vertically from one another along the row and may be substantially horizontal. Preferably the each heating element is elongate and extends along at least <NUM>%, at least <NUM>% or at least <NUM>% of the length of the element conduit <NUM> in which it is located.

The element conduits <NUM> comprise a conductive material such that the at least one heating element mounted therein can indirectly heat the heat storage medium <NUM> in the core chamber <NUM>. In addition, by forming the element conduits <NUM> from a highly conductive material, the heat can be effectively and quickly drawn away, thereby prolonging the service life of the heating elements. Preferably the element conduits <NUM> therefore comprise a metal, for example steel or stainless steel, an Incoloy (RTM) alloy or an Inconel (RTM) alloy, or a ceramic, such as silicon carbide.

The fluid system <NUM> comprises a transfer fluid and defines a fluid circulation circuit <NUM> (illustrated only in <FIG>) of the transfer fluid extending through the core <NUM>, base <NUM> and, if present, insulation block <NUM>. The fluid system <NUM> is preferably a closed loop, constant volume system. The transfer fluid is preferably air. The fluid system <NUM> comprises a heat exchanger <NUM> and a fan <NUM>, which may be mounted inside the base <NUM>, particularly the base housing <NUM> thereof. The heat exchanger <NUM> may be located downstream of the core <NUM> and upstream of the fan <NUM>. The heat exchanger <NUM> is configured to extract heat from the heated transfer fluid and transfer the heat to the heat demand system <NUM>. The heat demand system <NUM> may comprise, for example, at least one pipe circulating a fluid, such as water, through the heat exchanger <NUM> as illustrated. The heat exchanger <NUM> may be of any suitable type, such as fin and tube, or material, such as brazed copper. The heat exchanger <NUM> may comprise an exchanger inlet <NUM> for receiving heated transfer fluid from the core <NUM> and an exchanger outlet <NUM> for directing cooled transfer fluid to the fan <NUM>.

The fan <NUM> is for directing the transfer fluid around the fluid system <NUM> and along the fluid circulation circuit <NUM>. The fan <NUM> may be operated, such as by controlling its speed, to control the flowrate around the fluid system <NUM> and along the fluid circulation circuit <NUM>. The fan <NUM> may be located upstream of the core <NUM> and downstream of the heat exchanger <NUM>. The fan <NUM> may comprise a fan inlet <NUM> for receiving cooler transfer fluid from the heat exchanger <NUM> and the fan inlet <NUM> may be mounted substantially directly at the exchanger outlet <NUM> as illustrated. The fan <NUM> may comprise a fan outlet <NUM> out of which it drives transfer fluid and the fan <NUM> may be driven by fan motor <NUM>, such as an electronically commutated (EC) motor. The fan motor <NUM> may be variable to drive the fan <NUM> to provide a variable flowrate and thus control the power output from the core <NUM>.

The fluid system <NUM> further comprises a cooled fluid passageway <NUM> extending from the heat exchanger <NUM> to the core <NUM>, core fluid passageways <NUM> extending through the core <NUM> and a heated fluid passageway <NUM> extending from the core <NUM> to the heat exchanger <NUM>. The cooled fluid passageway <NUM> may diverge into and be connected to the core fluid passageways <NUM> and the heated fluid passageway <NUM> may be connected to and converge from the core fluid passageways <NUM>. The cooled and heated fluid passageways <NUM>, <NUM> may be located in the base <NUM> and, if present, insulation arrangement <NUM>. They may be separated and sealed from one another within the base chamber <NUM> by at least one base wall <NUM>, by the heat exchanger <NUM> and/or by the fan <NUM>. The cooled fluid passageway <NUM> may comprise a main cooled fluid passageway <NUM> extending from the exchanger outlet <NUM> through the base chamber <NUM> and leading to cooled fluid sub-passageways <NUM>, <NUM>, which may comprise the base passageway(s) <NUM> and/or insulation passageway(s) <NUM>. The fan <NUM> is located in the cooled fluid passageway <NUM>. The heated fluid passageway <NUM> may comprise heated fluid sub-passageways <NUM>, <NUM>, which may comprise the base passageway(s) <NUM> and/or and insulation passageway(s) <NUM>, leading to a main heated fluid passageway <NUM> extending through the base chamber <NUM> to the exchanger inlet <NUM>.

The fluid system <NUM> may comprise an inlet distributor <NUM> located in the cooled fluid passageway <NUM>, preferably the main cooled fluid passageway <NUM>, for distributing transfer fluid, preferably substantially evenly, between the core fluid passageways <NUM> (and thus, if present, between the cooled fluid sub-passageways <NUM>, <NUM>). The inlet distributor <NUM> may comprise a distributor plate <NUM> extending across the main cooled fluid passageway <NUM> and across and covering at least one cooled fluid sub-passageway <NUM>, <NUM>. The distributor plate <NUM> comprises at least one distributor aperture <NUM> therethrough and the distributor plate <NUM> may be sealed at its edges to the base housing <NUM> such that transfer fluid can only reach the at least one cooled fluid sub-passageway <NUM>, <NUM> behind the distributor plate <NUM> through the at least one distributor aperture <NUM>. The at least one distributor aperture <NUM> is sized and shaped such that the flowrate of transfer fluid there through ensures that the flowrate of transfer fluid into each cooled fluid sub-passageway <NUM>, <NUM>, including those not covered by the distributor plate <NUM>, is substantially similar.

The fluid system <NUM> comprises fluid conduits <NUM>, <NUM>, each comprising at least one core fluid passageway <NUM> and thus a hollow pipe. The fluid conduits <NUM>, <NUM> extend, preferably entirely and continuously, through the core chamber <NUM> for enabling the transfer of heat from the heat storage medium <NUM>, through the fluid conduits <NUM>, <NUM> and to the transfer fluid in the core fluid passageways <NUM> by forced convection. The core fluid passageways <NUM>, and the transfer fluid therein, are sealed from the core chamber <NUM> by, for example, the fluid conduits <NUM>, <NUM> being sealed and mounted to the core housing <NUM>. The fluid conduits <NUM>, <NUM> may therefore comprise fluid conduit inlets and outlets <NUM>, <NUM>, <NUM>, <NUM> located where the fluid conduits <NUM>, <NUM> meet or extend through the core housing <NUM>. As a result, the transfer fluid passes through the core <NUM> without contacting the heat storage medium <NUM>.

In order to further minimise the pressure drop, each of the fluid conduits <NUM>, <NUM> and core fluid passageways <NUM> comprise, between fluid conduit inlets and outlets <NUM>, <NUM>, <NUM>, <NUM>, at least one curved turn and preferably only at least one turn that is curved. The magnitude of the centreline radius (i.e. the distance from the centre of curvature to the centreline of the fluid conduits <NUM>, <NUM> and/or core fluid passageways <NUM>) of the at least one curved turn may be at least one, preferably at least <NUM>, fluid conduit <NUM>, <NUM> and/or core fluid passageways <NUM> diameter. The core fluid passageways <NUM> may therefore not comprise distinct edges at which pressure drops might occur.

As illustrated the fluid conduits <NUM>, <NUM> may extend between, and thus the fluid conduit inlets and outlets <NUM>, <NUM>, <NUM>, <NUM> may be at, the same side or face of the core <NUM>, core housing <NUM> and/or core chamber <NUM>. Thus the at least one core fluid passageway <NUM> may turn through <NUM>° along its length. Such an arrangement provides a compact storage boiler <NUM> and only a single face requires sealing with the fluid conduits <NUM>, <NUM>. However, they may extend between different sides or faces depending upon the arrangement of the fluid system <NUM>. The fluid conduits <NUM>, <NUM> may be arranged in sets of substantially similar and parallel fluid conduits <NUM>, <NUM>, such as a set of inner fluid conduits <NUM> substantially surrounded by a set of outer fluid conduits <NUM>. For example, the inner fluid conduits <NUM> may comprise a <NUM>° curved turn, or U-bend, between straight sections extending to inner fluid conduit inlets and outlets <NUM>, <NUM>. The outer fluid conduits <NUM> may comprise two right angled turns with a straight section therebetween and straight sections between the right angled turns and the outer fluid conduit inlets and outlets <NUM>, <NUM>. The fluid conduits <NUM>, <NUM> may extend at least partially around at least one element conduit <NUM>. The inner fluid conduits <NUM> may extend around a row of element conduits <NUM>. At least one row of element conduits <NUM> may extend between the inner fluid conduits <NUM> and the outer fluid conduits <NUM>. The planes along which the fluid conduits <NUM>, <NUM> extend may be orthogonal to the planes along which the element conduits <NUM> extend. Such an arrangement ensures a compact storage boiler <NUM> as well as providing an efficient arrangement for the provision of heat to and extraction of heat from the heat storage medium <NUM>.

The fluid conduits <NUM>, <NUM> comprise a conductive material such that heat from the heat storage medium <NUM> can be effectively transferred to the transfer fluid. Preferably the fluid conduits <NUM>, <NUM> therefore comprise a metal, for example steel, stainless steel, an Incoloy (RTM) alloy or an Inconel (RTM) alloy, or a ceramic, such as silicon carbide. The diameter of the fluid conduits <NUM>, <NUM> and/or core fluid passageways <NUM> may be substantially constant along their length or may gradually increase along their length and through the core chamber <NUM>. The latter arrangement accounts for pressure drops resulting from friction between the transfer fluid and the walls of the core fluid passageways <NUM>. Although not illustrated, the fluid conduits <NUM>, <NUM> may comprise at least one fin on their exterior walls for improving heat transfer with the heat storage medium <NUM>.

The interior walls of the core fluid passageways <NUM> may be substantially smooth as illustrated. However, if the flow is laminar at the interior walls there may be a boundary layer of relatively high temperature adjacent thereto. As a result, in alternative embodiments, the fluid conduits <NUM>, <NUM> may comprise a turbulence inducing arrangement in the core fluid passageways <NUM> and at the interior walls thereof for inducing turbulence at the interior walls. The turbulence reduces or remove the boundary layer, thereby improving heat transfer to the transfer fluid. The turbulence inducing arrangement may comprise at least one turbulator, such as helical grooves (i.e. rifling), fins, coils, helical strips, twisted tapes and the like.

In order to further reduce the possibility of a pressure drop, the exchanger inlet <NUM> may face or be directly downstream from, and may be arranged to extend across, a plurality of fluid conduit outlets <NUM>, <NUM> to effectively collect heated transfer fluid directly therefrom. The fluid conduit outlets <NUM>, <NUM> may be arranged to direct transfer fluid directly towards the exchanger inlet <NUM> through the heated fluid passageway <NUM>, optionally though at least one distribution baffle <NUM>, <NUM>, <NUM> (discussed below). The heat storage medium <NUM> is preferably distributed at least partially, preferably entirely, around and in contact with the exterior of at least one, preferably every, fluid conduit(s) <NUM>, <NUM> and/or heating element conduit(s) <NUM>. The core chamber <NUM> may be defined as the volume between the interior of the core housing <NUM> and the exterior of the at least one element conduit <NUM> and/or fluid conduit <NUM>, <NUM>.

The fluid system <NUM> further comprises a bypass arrangement <NUM> for selectively communicating transfer fluid directly from the cooled fluid passageway <NUM> to the heated fluid passageway <NUM> without passing through the core <NUM> (i.e. such that at least part of the transfer fluid exiting the fan <NUM> bypasses the core <NUM>). The bypass arrangement <NUM> thus mixes cooled transfer fluid with heated transfer fluid prior to the mixed transfer fluid entering the heat exchanger <NUM>. Mixing the cooled and heated transfer fluid allows the mixed transfer fluid to distribute itself along the heat exchanger <NUM> to prevent local hotspots. The bypass arrangement <NUM> may comprise a bypass inlet <NUM> located downstream of the heat exchanger <NUM> and upstream of the core <NUM> and may comprise a bypass outlet <NUM> located downstream of the core <NUM> and upstream of the heat exchanger <NUM>. The bypass inlet <NUM> may be located immediately downstream of the fan <NUM>, such that the bypass inlet <NUM> is between the fan <NUM> and the core <NUM>, particularly the fluid conduit inlets <NUM>, <NUM>. The bypass arrangement <NUM> also comprises a bypass gate <NUM> for controlling the flow of transfer fluid between the bypass inlet and outlet <NUM>, <NUM>.

The bypass gate <NUM> may be a pivotable lever as illustrated, valve or other such flow controller and may be located in the cooled fluid passageway <NUM> downstream of the heat exchanger <NUM> and fan <NUM>. The bypass gate <NUM> is moveable or pivotable (illustrated by arrow <NUM> in <FIG>) from a closed configuration (not illustrated), in which substantially no transfer fluid can flow through the bypass arrangement <NUM>, to an open configuration (illustrated in <FIG>, <FIG> and <FIG>), in which at least a portion of transfer fluid in the cooled fluid passageway <NUM> and exiting the fan <NUM> can flow through the bypass arrangement <NUM>. The position of the bypass gate <NUM> may be controlled by a bypass actuator <NUM>, which is preferably a stepper motor, servo or solenoid. The bypass arrangement <NUM> may comprise a bypass passageway <NUM>, in which the bypass gate <NUM> is located or to which the bypass gate <NUM> leads as illustrated, which communicates transfer fluid to the heated fluid passageway <NUM>. As illustrated the bypass passageway <NUM> may be defined between the at least one base wall <NUM>, heat exchanger <NUM>, and base housing <NUM>.

The embodiment of <FIG> illustrate a variant of the heated fluid passageway <NUM> in accordance with the present disclosure. In particular, the fluid system <NUM> may comprise at least one distribution baffle <NUM>, <NUM>, <NUM> located in the heated fluid passageway <NUM> and arranged to distribute and mix transfer fluid from the fluid conduits <NUM>, <NUM> prior to entry into the exchanger inlet <NUM>. Preferably the at least one distribution baffle <NUM>, <NUM>, <NUM> distributes transfer fluid substantially evenly across the exchanger inlet <NUM>. Although such an arrangement can result in turbulence, and thus pressure drop, it can be used to ensure that the temperature and velocity of transfer fluid across the exchanger inlet <NUM> is substantially constant. This avoids "hot spots" that might damage the heat exchanger <NUM> and improves efficiency by distributing transfer fluid across the entire surface area of the heat exchanger <NUM>.

The at least one distribution baffle <NUM>, <NUM>, <NUM> may extend across the heated fluid passageway <NUM> and may be distributed in an array or arrays separated along the heated fluid passageway <NUM>. The or each distribution baffle <NUM>, <NUM>, <NUM> may comprise an elongate rail, which may flare or taper outwardly along the fluid flow direction, such as by having an inverted V-shape. The or each distribution baffle <NUM>, <NUM>, <NUM> may alternatively comprise a tube and tube apertures may extend through the tube and distributed along the length of the tube. At least one first distribution baffle <NUM>, preferably parallel first distribution baffles <NUM> in a first array as illustrated, may be located in and extend across a recess <NUM> in the insulation arrangement <NUM> for distributing transfer fluid exiting the fluid conduits <NUM>, <NUM>. At least one third distribution baffle <NUM>, preferably parallel third distribution baffles <NUM> in a third array as illustrated, may be located in the base chamber <NUM> extending over the exchanger inlet <NUM> for distributing transfer fluid just prior to entering the exchanger inlet <NUM>. At least one second distribution baffle <NUM>, preferably parallel second distribution baffles <NUM> in a second array as illustrated, may be located in the base chamber <NUM> downstream of the at least one first distribution baffle <NUM> and/or upstream of the at least one third distribution baffle <NUM>. The at least one second distribution baffles <NUM> may extend orthogonally to the first and/or third distribution baffles <NUM>, <NUM>.

<FIG> and <FIG> illustrate a particularly preferred arrangement in which the bypass outlet <NUM> comprises apertures through a bypass outlet wall <NUM> and each aperture leads to a second distribution baffle <NUM>. The second distribution baffles <NUM> comprise at least one baffle recess facing downstream, such as by being formed on the inside of the inverted V-shape of the rails. The bypass outlet <NUM> therefore directs cooled transfer fluid into the baffle recess and the baffle recess distributes cooled transfer fluid towards the opposing face of the heated fluid passageway <NUM> to the bypass outlet <NUM>. Alternatively, the second distribution baffles <NUM> may comprise a tube with tube apertures directing the cooled transfer fluid downstream. As a result, transfer fluid is more evenly distributed across the entire width of the heated fluid passageway <NUM>. When combined with the first and/or third distribution baffles <NUM>, <NUM>, the second distribution baffle <NUM> assists in evenly and effectively mixing the heated transfer fluid from the core <NUM> and cooled transfer fluid from the bypass arrangement <NUM> such that the exchanger inlet <NUM> receives a transfer fluid at a substantially even temperature across its surface area. The second distribution baffles <NUM> may be configured to release a substantially similar flowrate of transfer fluid along their length. For example, the inverted V-shaped rails of the second distribution baffles <NUM> may reduce in width (i.e. the maximum distance between the free edges of the "V" when viewed in cross-section) along their length away from the bypass outlet <NUM>. Alternatively, the rails may comprise notches at the free edges that increase in size from the bypass outlet <NUM>. Alternatively, the tube apertures may increase in diameter the further they are from the bypass outlet <NUM>.

The storage boiler <NUM> comprises a control system (not illustrated). The control system may comprise a controller in communication with at least one sensor and at least one actuator, including the fan motor <NUM> and bypass actuator <NUM>. The controller may comprise at least one processor, at least one memory and at least one network adapter for allowing communication between the at least one processor and an external network, such as the Internet. The at least one sensor may comprise at least one temperature sensor. In particular, the control system may comprise at least one core temperature sensor located in or at the core <NUM> for determining the temperature of the core <NUM> and heat storage medium <NUM>. The control system may comprise at least one exchanger temperature sensor located in the heated fluid passageway <NUM>, preferably adjacent to or at the exchanger inlet <NUM>, for determining the temperature of transfer fluid entering the heat exchanger <NUM>. The control system may comprise at least one fan temperature sensor located in the cooled fluid passageway <NUM>, preferably adjacent to or at the fan outlet <NUM>, for determining the temperature of cooled transfer fluid exiting the fan <NUM>. The control system may therefore control the fluid system <NUM> and electrical system <NUM> based upon the output from the at least one temperature sensor. The control system may also monitor the electrical power consumption of the electrical system <NUM> and/or the frequency of the electricity grid to which it is connected and use the resulting data to control the time of heating of the core <NUM>.

During operation of the storage boiler <NUM>, the control system controls the electrical system <NUM> to draw electrical power from the electricity network. The heating elements convert the electrical energy to heat, which is conducted through the element conduits <NUM> to the heat storage medium <NUM> in the core <NUM>. The heat storage medium <NUM> stores the heat therein for later extraction by the fluid system <NUM>. By controlling the electrical power received by the heating elements, the control system controls the heating of the heat storage medium <NUM> to a maximum core temperature <NUM>. Thus if the control system determines that the temperature of the heat storage medium <NUM> is below the maximum core temperature <NUM>, or below a desired core temperature, the control system operates the electrical system <NUM> to heat the core <NUM>. If the temperature of the heat storage medium <NUM> reaches, exceeds or is expected to reach the maximum core temperature <NUM> or desired core temperature, the control system operates the electrical system <NUM> to stop or reduce heating of the core <NUM> by drawing less electrical power.

In order to extract heat from the storage boiler <NUM>, the control system operates the fan <NUM> to circulate transfer fluid around the fluid circulation circuit <NUM>. The fan <NUM> directs transfer fluid from the heat exchanger <NUM> (in which it has been cooled) through the cooled fluid passageway <NUM> and into the core fluid passageways <NUM>. The control system may control the fan <NUM> to control the overall flowrate of transfer fluid around the fluid system <NUM>, particularly the flowrate through the heat exchanger <NUM>. The control system may control the bypass arrangement <NUM>, particularly the bypass gate <NUM>, to control the proportion of the overall flowrate that goes through each of the bypass arrangement <NUM> and the core <NUM>. Thus, at a fixed speed of the fan <NUM> and thus a fixed overall flowrate through the fluid system <NUM> and heat exchanger <NUM>, the flowrate through the core <NUM> can be increased or decreased by adjusting the bypass arrangement <NUM> and particularly the position of the bypass gate <NUM>.

In particular, the transfer fluid may pass through the main cooled fluid passageway <NUM> into the cooled fluid sub-passageways <NUM>, <NUM>, with some of it passing through the distributor plate <NUM> if present. The transfer fluid is driven through the core fluid passageways <NUM> in which it receives heat conducted through the fluid conduits <NUM>, <NUM> from the heat storage medium <NUM>. The heated transfer fluid is then driven from the core fluid passageways <NUM> through the heated fluid passageway <NUM> into the heat exchanger <NUM>. In particular, the heat transfer fluid is directed through the heated fluid sub-passageways <NUM>, <NUM>, converges into the main heated fluid passageway <NUM> and passes into the exchanger inlet <NUM>. If present, the heated transfer fluid is mixed by the at least one distribution baffle <NUM>, <NUM>, <NUM>, optionally with cooled transfer fluid from the bypass arrangement <NUM> provided that the bypass gate <NUM> is open.

<FIG> illustrates the power output of the storage boiler <NUM> (i.e. the heat extracted by the heat demand system <NUM>, on the Y-axis) against the core temperature, which is the temperature of the heat storage medium <NUM> (on the X-axis). The maximum power output of the storage boiler <NUM> at a given core temperature is illustrated by maximum power curve <NUM>, which extends between the maximum core temperature <NUM> and minimum core temperature <NUM> with an intermediate core temperature <NUM> therebetween.

The control system controls the power output of the storage boiler <NUM> by varying the speed of the fan <NUM> to control the transfer fluid flowrate through the core <NUM> and by controlling the flowrate of cooled transfer fluid through the bypass arrangement <NUM>. The control system controls the power output of the storage boiler <NUM> at a given core temperature, determined by the at least one core temperature sensor, by controlling the speed of the fan <NUM>, thereby controlling the transfer fluid flowrate through the core <NUM>. In particular, the control system may determine or receive a desired heat power output and/or desired water temperature at the heat demand system <NUM>, for example based upon at least one temperature sensor located in the heat demand system <NUM> such as at a hot water outlet, and/or a desired transfer fluid flowrate through the core <NUM> and operate the fan <NUM> to at least one of a plurality of fan speeds (e.g. selecting one speed from a continuous range of speeds) based upon the desired heat power output, desired water temperature and/or desired transfer fluid flowrate. Hence, when the core <NUM> is at a certain temperature, if less than the maximum power output is required, the speed of the fan <NUM> is reduced from its maximum.

Prior art power curve <NUM> illustrates the maximum power output for a prior art storage boiler, such as that disclosed in <CIT> and <CIT>. As illustrated, the prior art maximum core temperature <NUM> is less than the maximum core temperature <NUM> of the storage boiler <NUM> of the present disclosure. This is because prior art maximum core temperature <NUM> is set in accordance with the maximum temperature rating of the heat exchanger. However, by incorporating the bypass arrangement <NUM> of the present disclosure the maximum core temperature <NUM> can be much higher, whilst the temperature of the transfer fluid passing through the exchanger inlet <NUM> controlled in accordance with the maximum temperature rating of the heat exchanger <NUM>.

Therefore, when the control system determines from the core temperature, which may be determined based upon the output of the at least one core temperature sensor, a determination of the fan speed and/or the at least one exchanger temperature sensor, is between the intermediate core temperature <NUM> and maximum core temperature <NUM>, the control system operates the bypass gate <NUM> in its open configuration. The control system operates the bypass gate <NUM> to control the flowrate of transfer fluid through the bypass arrangement <NUM> to maintain the temperature of the transfer fluid at the exchanger inlet <NUM>, which may be determined from the output of the at least one exchanger temperature sensor, at a heat exchanger temperature value. The heat exchanger temperature value is substantially constant, and thus maximum power output of the storage boiler <NUM>, when the core temperature is between the intermediate core temperature <NUM> and maximum core temperature <NUM>. As the core temperature decreases from the maximum core temperature <NUM> towards the intermediate core temperature <NUM> the bypass gate <NUM> is closed by the control system further to reduce the flowrate of transfer fluid through the bypass arrangement <NUM>. Thus the bypass arrangement <NUM> allows the storage boiler <NUM> to provide maximum power output over a wide range of core temperatures, as opposed to providing maximum power output at only the maximum core temperature as in prior art storage boilers. Furthermore, the heat exchanger <NUM> can be designed to have a lower temperature and/or power rating to that of the prior art and can thus comprise lower cost materials.

The intermediate core temperature <NUM> is the core temperature that results in the temperature of the transfer fluid at the fluid conduit outlets <NUM>, <NUM> being substantially at the heat exchanger temperature value and/or the temperature of the transfer fluid at the exchanger inlet <NUM> being at the heat exchanger temperature value when the bypass gate <NUM> is closed. Effectively, the core temperature has reached a sufficiently low temperature that there is no need to mix the heated transfer fluid with cooled transfer fluid to ensure the transfer fluid is at or below the heat exchanger temperature value at the exchanger inlet <NUM>. When the control system determines that the core temperature, which may be determined from the output of the at least one core temperature sensor, is less than the intermediate core temperature <NUM> and above the minimum core temperature <NUM> the bypass gate <NUM> is closed. At this point the maximum power output is dependent upon the core temperature. When the core temperature is below the minimum core temperature <NUM> the storage boiler <NUM> cannot effectively provide the required heat to the heat demand system <NUM> and thus the control system may operate the fan <NUM> to stop circulating transfer fluid around the fluid system <NUM>.

The maximum core temperature <NUM> may be up to approximately <NUM> or approximately <NUM> and may be at least approximately <NUM> or at least approximately <NUM>. The intermediate core temperature <NUM> may be up to approximately <NUM> or approximately <NUM> and may be at least approximately <NUM> or at least approximately <NUM>. The minimum core temperature <NUM> may be up to approximately <NUM>, approximately <NUM> or approximately <NUM> and may be at least approximately <NUM> or at least approximately <NUM>. The heat exchanger temperature value may be up to approximately <NUM> or approximately <NUM> and may be at least approximately <NUM> or at least approximately <NUM>. The illustrated maximum power curve <NUM> is for a storage boiler <NUM> according to the present disclosure with a minimum core temperature <NUM> of approximately <NUM>, an intermediate core temperature <NUM> of approximately <NUM>, a maximum core temperature <NUM> of approximately <NUM> and maximum power output of around <NUM> kWt.

Claim 1:
A storage boiler (<NUM>) comprising:
a core (<NUM>) comprising a heat storage medium (<NUM>);
a base (<NUM>) to which the core (<NUM>) is mounted, and
a fluid system (<NUM>) extending through the core (<NUM>) and comprising:
a heat exchanger (<NUM>);
a cooled fluid passageway (<NUM>) extending from the heat exchanger (<NUM>) to the core (<NUM>);
a heated fluid passageway (<NUM>) extending from the core (<NUM>) to the heat exchanger (<NUM>);
a fan (<NUM>) for circulating a transfer fluid through the cooled fluid passageway (<NUM>), the core (<NUM>), the heated fluid passageway (<NUM>) and the heat exchanger (<NUM>); and
a bypass arrangement (<NUM>) for selectively communicating at least part of the transfer fluid from the cooled fluid passageway (<NUM>) to the heated fluid passageway (<NUM>) without passing through the core (<NUM>) and mixing cooled transfer fluid with heated transfer fluid prior to the mixed transfer fluid entering the heat exchanger (<NUM>), wherein the bypass arrangement (<NUM>) is mounted inside the base (<NUM>); and a control system configured to operate the fan (<NUM>), by varying the speed of the fan (<NUM>), for controlling the flowrate of transfer fluid through the core (<NUM>) and fluid system (<NUM>) and to operate the bypass arrangement (<NUM>) for controlling the flowrate of transfer fluid through the bypass arrangement (<NUM>).