The present invention relates to heat-conducting apparatus, the temperature of which is capable of being regulated, and in particular but not necessarily exclusively to a hotplate forming part of a heat storage cooker, stove or range. The invention also relates to a cooking vessel having such heat-conducting apparatus, and to a method of regulating the temperature of such a heat-conducting apparatus.

Heat storage stoves, cookers and ranges, hereinafter simply referred to as 'cookers', operate on the principle that the metal, typically cast iron, parts of the cooker can readily absorb thermal energy from a low-intensity heat source, such as fossil fuels, for example, oil, wood and coal, which is continuously burning, and as such, the accumulated heat can be used as required, particularly for cooking purposes.

Typically, such devices have at least one oven unit, in addition to one or more hotplates mounted above the or each oven. Said hotplates comprise a platen made from a thermally conductive metal which is physically positioned above and mechanically connected to a cast iron slab. The slab is heated by the heat source of the device, which transfers thermal energy to each hotplate.

As such, each hotplate remains at a consistent temperature whilst the cooker is operational. Whilst many users enjoy this aspect, since there is no delay whilst the hotplate comes up to temperature, it is an enormous waste of thermal energy, as the cooker radiates heat whilst the hotplates are at maximum temperature.

Generally, cookers will attempt to mitigate the heat loss through the hotplate by utilising a plate cover when the hotplate is not in use. This cover may be thermally insulated, but there will still be significant heat losses associated with the cooker.

Furthermore, since the hotplates remain at a consistent temperature, there is no adjustability in the cooking temperature, which may be required depending upon the particular food being cooked. Heat storage stoves, cookers and ranges are usually equipped with a 'boil' hotplate and a 'simmer' hotplate; however, a dish requiring an intermediate temperature cannot be easily cooked on such a device, certainly without careful and attentive monitoring.

The present invention seeks to overcome these problems, by providing a temperature- adjustable hotplate.

According to a first aspect of the invention, there is provided heat-conducting apparatus for enabling heat exchange during a cooking process, comprising: an upper heat- conducting platen, having a flat or substantially flat upper surface; a lower heat- conducting platen disposed below the upper heat-conducting platen; a cavity defined by the upper and lower heat-conducting platens; platen adjustment means for adjusting a separation between the upper and lower heat-conducting platens at the cavity; and a platen separation controller for controlling the platen adjustment means; wherein heat exchange between the upper and lower heat-conducting platens is controllable by adjusting the separation between the upper and lower heat-conducting platens at the cavity.

As mentioned, a major drawback of heat storage cookers is that there is no mechanism for adjusting the temperature of a hotplate thereon in a straightforward manner. For some models, it may be possible to adjust the rate at which the heat source is providing heat in order to somewhat adjust the hotplate temperature, but this is generally too inconvenient to be done whilst cooking, as the hotplate requires time to heat up and cool down. As such, it is highly advantageous to provide a means of adjusting the temperature of such a hotplate in a fashion akin to that of a standard electric or gas cooker. As adjustment at the heat source is undesirable, the present invention relies on adjustment of the rate of thermal exchange between the upper and lower platens of the heat- conducting apparatus, in order to modulate the temperature of the upper heat- conducting platen, and therefore the cooking surface.

By providing a separation between these upper and lower elements, a controllable insulating buffer can be inserted therein, inhibiting the rate of heat exchange under control of the user. Increasing the separation between the platens leads to a retardation of the heat transfer, resulting in a net cooling of the upper heat-conducting platen, this being the cooking surface of the hotplate.

Preferably, the cavity is fluid-pressurisable between the upper and lower heat- conducting platens, a pressure control device is included for controlling the pressure of a fluid within the cavity, the pressure control device being in fluid communication with the cavity, and the platen separation controller is a pressure adjustment means capable of adjusting the pressure of the fluid in the cavity, wherein pressurisation of the cavity results in an increased separation between the upper and lower heat-conducting platens.

One advantageous means of providing a thermal buffer between the upper and lower heat-conducting platens is by inserting a fluid layer between them which is an insulator. The greater the fluid layer, the greater the thermal barrier. To accurately regulate the fluid input into a cavity between the platens, it is advantageous to provide a control device and adjustment means.

Preferably, the pressure control device may include a vacuum generation means and/or a fluid inlet means in communication with the cavity. It is advantageous to provide some means of inserting pressurised fluid into the cavity in order to provide the insulating buffer between the upper and lower heat-conducting platens, but also to provide some means of reducing the pressure to subsequently increase thermal exchange between the two platens. This pressure reduction may be most readily achieved by utilising a vacuum generation means, such as a pump, preferably in this case being a rotary pump.

Furthermore, the fluid within the cavity may be a substantially non-flammable gas, most preferably air.

The primary benefit of utilising a substance having fluid properties as the buffer between the upper and lower heat-conducting platens is that said fluid can be readily pressurised and/or evacuated. Evidently, given that a hotplate will be exposed to high temperatures, it stands to reason that a non-flammable gas would be ideal, with air being the most suitable and readily available such gas.

Preferably, the pressure adjustment means may be a user-controllable input device. In order to fully replicate the temperature adjustability of a standard electric or gas cooker, it is beneficial to provide a user-controllable input, such as a rotatable dial, in order to permit the user to select the temperature at which they wish to cook. Generally, such dials are demarcated so as to indicate the temperature selected by the user. Advantageously, the platen adjustment means may be a mechanical actuator for physically separating the upper and lower heat-conducting platens.

As an alternative to a pressurised fluid-based platen adjustment means, a simpler mechanical device can be used to forcibly separate the upper and lower heat-conducting platens, thereby providing an insulative buffer. Such a heat-conducting apparatus therefore has the advantage of being simpler to maintain, as no pressurising device will be required.

Preferably, the upper and lower heat-conducting platens are formed from a material having controlled expansion characteristics. In particular the upper and lower heat- conducting platens are formed from a controlled expansion alloy having a low expansion coefficient.

A critical feature required in order for the present invention to operate is that the area or volume between the upper and lower heat-conducting platens is not breached when either the upper or lower heat-conducting platens of the hotplate are heated or cooled, in particular when the two are at different temperatures. To achieve this, it is important to ensure that the material used for either element has well-established thermal expansion properties. Controlled expansion alloys are particularly useful, and in particular those with low expansion coefficients, such as those formed from iron/nickel/cobalt mixtures, for instance Kovar RTM.

Preferably, there may be provided a temperature display for displaying the or approximately the temperature of the hotplate to a user.

A temperature display may advantageously be provided; this may be as simple as the demarcation of the user-controllable input, or could be a more sophisticated temperature gauge which allows the user to accurately see the temperature of the upper heat- conducting platen as it heats.

Preferably, the upper heat-conducting platen may be segmented.

Since the majority of cookware is circular in profile, and since most heat storage cookers are designed with circular hotplates in mind, it is advantageous to provide at least the cooking element of the present hotplate as a circular unit. Whether circular or non-circular, it may in any event be beneficial for the platen to be segmented or partitioned, which thus allows different portions of the cooking surface to be at different cooking temperatures.

According to a second aspect of the invention, there is provided a hotplate for conductively heating items placed thereon, comprising: heat-conducting apparatus in accordance with the first aspect of the invention; and a heating element in thermal communication with the lower heat-conducting platen, the heating element adapted to conductively heat the lower heat conducting platen

The heat-conducting apparatus of the first aspect of the invention has been designed particularly but not necessarily exclusively with hotplates in mind. Typically, the hotplates will be provided with a heating element underneath the lower heat-conducting platen, thereby advantageously providing a heating gradient going from the heating element, to the lower heat-conducting platen, and then to the upper heat-conducting platen. Advantageously, the lower heat-conducting platen is thermally conductive, and therefore by being in communication with a heating element to rapidly heat up, realtime temperature adjustments in the upper heat-conducting platen may be effected. The upper and lower platens are preferably manufactured from the same Controlled Expansion/Low Expansion alloy in order that they can be welded together to create the enclosed cavity. If two dissimilar metals or metal other than Controlled Expansion/Low Expansion alloys are joined in this way, the different expansion/contraction rates when heated/cooled distort the plates with the possibility of breaking or fracturing the join or weld.

Preferably, the heating element may be incorporated into the lower heat-conducting platen. Alternatively and more preferably, the heating element may be provided separately to, and disposed below the lower heat-conducting platen.

Such a heating element could feasibly be included as part of the lower heat-conducting platen, or more typically would be a separate metallic slab provided as part of a heat storage cooker. The particular design of the heating element will be dependent upon manufacturing considerations.

Preferably, the platen adjustment means may be sub-divided such that a plurality of separations between different adjustable portions of the upper and lower heat- conducting platens is achievable, and a platen separation controller may be provided for each adjustable portion of the hotplate. Additionally, the upper heat-conducting apparatus may be divided into at least two individual hob elements, the temperature of each hob element being controlled by the pressurisation of the cavity between it and the lower heat-conducting apparatus.

To further enhance the functionality of the present hotplate, it is therefore advantageous to regulate the temperature of different portions of the upper heat-conducting apparatus so as to thereby provide a plurality of individual hob elements, each operating at a different selectable temperature. This allows the user of the hotplate to cook several items at once, increasing the usefulness of the hotplate to a potential chef.

According to a third aspect of the invention, there is provided a temperature-adjustable cooking vessel, comprising: a base portion including a heat-conducting apparatus in accordance with the first aspect of the invention; an upstanding side wall extending substantially from the perimeter of the base portion; and at least one handle. Preferably, the cooking vessel may include a lid.

Whilst it will be appreciated that a hotplate may be the more convenient place in which to position the heat-conducting apparatus, users already owning heat storage cookers will not wish to replace their expensive systems. Since there is no particular requirement for the heat-conducting apparatus to be installed as part of the hotplate, it could in fact be installed onto saucepans, frying pans, or any hob-top cooking vessel or utensil to create the same temperature adjustability. Such heat-conducting apparatus can be integrated into the base of a cooking device, typically being a cooking vessel, therefore, to advantageously provide temperature regulation to those who would otherwise have to replace their expensive existing heat storage cookers. According to a fourth aspect of the invention, there is provided a heat storage cooking unit for providing at least thermal energy for cooking, the cooking unit comprising: a heat-storage body; and at least one hotplate in accordance with the second aspect of the invention; the heating element being disposed within the heat-storage body, and the upper surface of the upper heat-conducting platen being disposed at or adjacent to an upper surface of the heat-storage body.

Heat storage cooking units, preferably being a stove, cooker or range, rely on the principle that it is possible to apply a constant low level source of energy to the cooking unit, which can absorb the heat in a manner suitable for at least cooking food, but often also for providing central heating functionality as well. However, the constant provision of heat to elements of the cooking unit which are not insulated results in the thermal energy dissipating, which massively reduces the energy efficiency of the device.

By providing hotplates wherein the temperature can be regulated, and in particular, cooled down, the radiative heat loss of the cooking unit can be minimised, resulting in increased fuel efficiency. Simultaneously, the cooking unit is made that much more useful; a user can cook their food at the correct temperature, rather than an arbitrary temperature as determined by an internal setting of the cooking unit.

Preferably, the heat storage cooking unit may further comprise a thermally insulative cover for the or each hotplate, actuatable between an open and closed position to uncover or cover the or each hotplate respectively. The provision of insulative covers to protect the hotplates firstly prevents accidental damage and/or soiling of the hotplate surfaces, but also reduces the radiative heat loss emitted due to residual heat in the hotplates. This retardation of heat loss advantageously further increases the fuel efficiency of the cooking unit.

Preferably, the position of the cover may be linked to the platen separation means of the or each hotplate, separation between the upper and lower heat-conducting platens being increased upon closure of the cover to reduce heat exchange from the lower heat- conducting platen to the upper heat-conducting platen of the or each hotplate.

One possible way of further decreasing the radiative heat loss of the cooking unit is by linking the position of the cover to the state of the hotplate below. In particular, when the cover is closed, the user will not be cooking on the hotplate, and therefore it may be advantageous to increase the separation between the upper and lower heat-conducting platens of the hotplate to a maximum, in order to provide the greatest thermal buffer therebetween. As such, the upper heat-conducting surface will be automatically cooled upon cover closure.

Preferably, the heating element may include a fossil-fuel burner and thermally conductive slab, the fossil-fuel burner heating the thermally conductive slab and the thermally conductive slab transferring heat to the lower heat-conductive platen of the or each hotplate. Primarily, heat storage cooking units operate utilising such a fossil-fuel burner and slab combination, typically being gas-fired appliances. It is therefore advantageous to provide a cooking unit which utilises such components, in order to be fully compatible with existing household or commercial installations.

Preferably, the platen separation controller may be provided as a user-controllable dial on the heat-storage body.

Because of the prevalence of temperature dials on standard electric or gas cookers, for ease of understanding for the user, it is beneficial to provide a similar means of engagement for the user on the heat storage cooking unit. Certain upper and lower platen separations will correspond to particular hotplate temperatures, and these could be marked on the dial.

According to a fifth aspect of the invention, there is provided a method of controlling the temperature of a cooking surface of heat-conducting apparatus, comprising the steps of: a] providing heat-conducting apparatus having an upper thermally conductive platen overlying a lower heatable platen thereby defining a cavity therebetween, the upper and lower thermally conductive platens being adjustably separable at the cavity; b] altering said separation to change a rate of thermal exchange from the lower thermally conductive platen to the upper thermally conductive platen; wherein: i] decreasing the separation increases the rate of thermal exchange between the upper and lower elements; and ii] increasing the separation decreases the rate of thermal exchange between the upper and lower thermally conductive platens, the fluid acting as a thermal buffer between the upper and lower thermally conductive platens.

According to a sixth aspect of the invention, there is provided a method of controlling the temperature of a cooking surface, the method comprising the steps of: a] providing two opposing heatable surfaces defining an insulative cavity therebetween, at least one of the heatable surfaces being said cooking surface; and b] adjusting a volume of the cavity to thereby control thermal exchange between the two opposing heatable surfaces.

In the fifth and sixth aspects, it is preferable that the heat-conducting apparatus of the first aspect of the invention is utilised. It will be appreciated that the method of adjusting the temperature of a cooking surface of a hotplate by altering the separation between upper and lower platens is not just applicable to a hotplate as described previously, but could be advantageously utilised in a variety of hotplate configurations. Provided that the three core components of the hotplate - the upper and lower elements and the means of adjusting the separation therebetween - are present, then this method can advantageously be used to regulate a temperature of the upper thermally conductive element.

Preferably, a user-controllable platen separation means may be provided.

Whilst the hotplate could be provided as a binary system, with the upper and lower platens being either adjacent or forced apart, it is highly advantageous to allow the user to alter the separation between platens at will, thereby providing the adjustability in temperature of the cooking surface which is most desirable for a given cooking activity.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic representation of a cross-section through a first embodiment of a hotplate, in accordance with the second aspect of the invention and utilising heat-conductive apparatus in accordance with the first aspect of the invention;

Figure 2 shows a plan representation of a second embodiment of a hotplate, again in accordance with the second aspect of the invention and utilising heat- conductive apparatus in accordance with the first aspect of the invention;

Figure 3 shows an exploded perspective view of the hotplate of Figure 2;

Figure 4 shows a diagrammatic representation of a cross-section through a third embodiment of a hotplate, according to the second aspect of the invention and still utilising heat-conductive apparatus in accordance with the first aspect of the invention;

Figure 5 shows a diagrammatic representation of a cross-section through a temperature-adjustable cooking vessel, in accordance with the third aspect of the invention and incorporating heat-conductive apparatus in accordance with the first aspect of the invention; and Figure 6 shows a perspective view of a heat storage cooking unit in accordance with the fourth aspect of the invention but still incorporating heat-conductive apparatus in accordance with the first aspect of the invention.

Referring firstly to Figure 1 of the drawings, there is shown a first embodiment of a hotplate, indicated generally at 10, in particular but not necessarily exclusively for use with a heat storage cooking unit 410 such as that shown in Figure 6, typically being a heavy metal framed cooker, stove or range, for example, available from Aga Rangemaster Group pic of Juno Drive, Leamington Spa, Warwickshire, CV31 3RG, United Kingdom. The hotplate 10 comprises primarily an upper heat-conducting platen 12 and a lower heat-conducting platen 14, the separation between which is adjustable using a platen separation adjuster or any similar adjustment means 16.

In this particular embodiment, the separation is adjustable by inserting pressurised fluid into a cavity 18 disposed between the upper and lower heat-conducting platens 12, 14. This fluid will most preferably be a gas, typically air, but could feasible be any sufficiently insulative and non-flammable fluid. By way of example only, Argon may beneficially be utilised.

The upper heat-conducting platen 12 is formed from a material having controlled expansion characteristics, preferably a controlled expansion alloy having a low expansion coefficient such as Kovar RTM, and being substantially disc-like in shape. The element 12 is sized so as to be able to accept at least one saucepan or frying pan, an upper surface 20 acting as the cooking surface 22 of the upper heat-conducting platen 12 being preferably planar such that a pan may stably rest thereon, thereby acting as the heating surface of the hotplate 10. Generally, the upper heat-conducting platen 12 will be a solid block of material, having a depth or thickness in a range of 0.2 mm to 15 mm, and more preferably approximately 0.5 mm, thereby maximising the heat-conducting properties. Preferably, the upper and lower platens are as thin as possible to minimise the time for conduction from the heated metal, such as cast iron slab, through to the pan or utensil in contact with the upper platen. Also, heat dissipation from the upper platen will be faster, allowing a more responsive cooking experience, the expansion/contraction of the cavity will not be affected by the thickness of the plates.

The lower heat-conducting platen 14 is formed from a thermally-conductive metal, preferably being a Controlled Expansion/Low Expansion alloy, such as Kovar RTM for instance, having at least one through-bore 24 through which the fluid may be supplied into the cavity 16. The embodiment shown in Figure 1 shows a cross-section through a hotplate 10 having a single through-bore 24.

The lower heat-conducting platen 14 is sized so as to be substantially the same diameter as the upper heat-conducting platen 12, and the platens 12, 14 are welded or otherwise suitably fluid-tightly engaged to one another about their respective perimeters 26, 28. A lower surface 30 of the upper heat-conducting platen 12 and an upper surface 32 of the lower heat-conducting platen 14 are therefore substantially adjacent one another, with the cavity 18 being defined by the lower surface 30, upper surface 32 and the perimeter fluid-tight join 34 between the platens 12, 14, with the through-bore 24 acting as an inlet to the cavity 18. The combination of the upper and lower heat-conducting platens 12, 14 therefore represents the core elements of heat-conducting apparatus 36 of the hotplate 10.

The heat-conducting apparatus 36 is mounted atop a heating element 38, typically being a solid block of highly thermally-conductive metal such as cast iron, which is heated by a heat source 40 such as a fossil-fuel burner. Heat exchange can therefore indirectly occur between the heating element 38 and the upper heat-conducting apparatus 12 via the lower heat-conducting apparatus 14 and the cavity 18.

Extending from the through-bore 24 is a fluid conduit 42, constructed from a heat- resistant material, which connects the cavity 18 with a pressure control device 44 for controlling the pressure of the fluid within the cavity 18. In this embodiment, the platen adjustment means 16 therefore comprises the pressure control device 44 and the fluid inserted into the cavity 16.

The pressure control device 44 includes a vacuum generator 46, such as a rotary pump, and a fluid inlet means 48, such as a bleed valve 50, for respectively evacuating and introducing fluid into the cavity 18. Whilst a bleed valve 50 is described as being used as the fluid inlet means 48, any appropriate fluid inlet device could be considered. Similarly, although a rotary pump is suggested, other kinds of vacuum generation means can be considered, such as a diaphragm pump.

The fluid inlet means 48 will typically have an exposed end 52 external to the main body of the hotplate 10, being capable of drawing atmospheric pressure air into the fluid conduit 42.

By combining the opposing effects of the vacuum generation means 46 and fluid inlet means 48, the pressure control device 44 is therefore able to control the pressure within the fluid conduit 42. The separation between the upper and lower heat-conducting platens can be adjusted using a platen separation controller 54, in this instance, a pressure adjuster or any similar pressure adjustment means 56. The pressure is set by the pressure adjustment means 56, typically a user-controllable interface 58 external to the hotplate 10.

The pressure control device 44 therefore may include a pressure gauge to determine the pressure within the fluid conduit 42; however, the user-controllable interface 58 will be a rotatable dial or other input means which will be marked or have a display in terms of temperature to allow a user to select the temperature attainable by the upper heat- conducting platen 12. The temperature selected by such a user-controllable interface 58 will typically be displayed on the surface of the dial as delineated portions corresponding with particular temperatures, for example. Alternatively, it is possible that a physical temperature gauge may be provided as a part of the hotplate 10 to give the user a greater degree of feedback; it may be that particular pressures created by positioning the dial do not necessarily correspond with exact temperatures. Therefore, it may be preferable to allow the user to themselves determine the temperature of the hotplate 10 via a temperature gauge.

The central feature of the present invention is that the temperature of the heat- conducting apparatus 36 is adjustable. This is achieved by utilising the cavity 18 to act as an insulative barrier to thermal exchange between the upper and lower heat- conducting platens 12, 14.

It is well known that metals expand marginally when heated, and contract when cooled. To achieve maximum cooking temperature, the pressure control device 44 is set so as to evacuate the cavity 18 of fluid. Thermal exchange from the lower heat-conducting platen 14 to the upper heat-conducting platen 12 will be uninhibited by intermediate fluid, with the upper heat-conducting platen 12 expanding slightly so as to minimise the cavity 18 size. As such, the thermal exchange between the upper and lower platens 12, 14 will be maximised. In this case, the upper and lower heat-conducting platens may thus contact each other at cavity 18. As a user attempts to reduce the temperature of the hotplate 10, they are able to manipulate the pressure adjustment means 56 to permit a flow of fluid into the cavity 18. The pressure of the incoming fluid will be dependent upon the temperature selected by the user. When the fluid flows into the cavity 18, there will be two effects: firstly, the upper heat-conducting platen 12 will be cooled, causing it to contract marginally, effectively enlarging the cavity 18; and secondly, the cavity 18 will be filled with the incoming fluid, which acts as a thermal buffer, inhibiting the thermal exchange between the upper and lower heat-conducting platens 12, 14.

By providing the inhibiting buffer between the upper and lower platens 12, 14, the upper heat-conducting platen 12 is able to cool to the required cooking temperature, thereby allowing the user to regulate the hotplate 10.

Whilst it is advantageous to provide a temperature-adjustable hotplate, one shortcoming is that the hotplate 10 can only ever be set to a single temperature at any one time. To compete with standard electric or gas hob cooking units, it would be beneficial to provide a plurality of individual hobs which are individually controllable. A second embodiment of the invention is therefore shown in Figures 2 and 3, and is indicated generally at 110. Similar or identical references refer to similar or identical parts to those described above, and therefore further detailed description is omitted for brevity. The upper heat-conducting platen 112 is sub-divided into four individual hob elements 160, each of which is preferably but not necessarily exclusively substantially circular in profile. Each hob element 160 is formed from a controlled expansion alloy, set into and extending through a main body 162 of the upper heat-conducting platen 112. The main body 162 may preferably be formed from a less thermally conductive material, such as ceramic, in order to thermally isolate the individual hob elements 160 from one another. In this instance, therefore, the cooking surface 122 is formed by the upper surfaces 164 of the individual hob elements 160 rather than the whole upper surface 120 of the upper heat-conducting platen 112.

Again, the upper heat-conducting platen 112 is welded or otherwise fluid-tightly connected to the lower heat-conducting platen 114, with a fluid-tight cavity being defined therebetween to form the heat-conducting apparatus 136. The cavity will be sub-divided, however, into smaller further or secondary fluidly-isolated cavities 118, such that fluid cannot pass between the secondary cavities 118. This will be achieved by integrating at least one divisor or fluid-tight partitioning element 166 separating the secondary cavities 118.

Each secondary cavity 118 has a through-bore 124 extending therethrough, each extending into fluid conduits 142 connected to the pressure control device 144, forming the platen adjustment means 116. Each fluid conduit 142 is capable of being set to a different pressure by the pressure control device 144, again including a vacuum generation means 146 and fluid inlet means 148, and as such, each secondary cavity 118 is able to be individually pressurised.

The pressure within each further cavity 118 is controllable by the platen separation controller 154, here a pressure adjustment means 156, there therefore being a user controllable input 158 for each individual hob element 160. This enables the individual hob elements 160 to be separately brought up to temperature, for instance, the user could only uses one hob element 160 when cooking, therefore avoiding the associated energy wastage with utilising all elements 160 at once.

Whilst the pressurisation of the cavity within the hotplate may be a preferred mechanism of providing an adjustable separation between upper and lower heat- conducting platens, for simplicities sake, it may be desirable to utilise a mechanically simple heat-conducting apparatus instead.

A third embodiment of the hotplate 210 is shown in Figure 4, showing in particular the heat-conducting apparatus 236. Upper heat-conducting platen 212 and lower heat- conducting platen 214 are welded together about their respective perimeters 226, 228, as in the first embodiment, the upper heat-conducting platen 220 having an upper surface 220 through which heat is conducted. Again, parts which are similar to those described previously utilise the same or similar references, and further detailed description is omitted.

Connected to the lower surface 230 of the upper heat-conducting platen 212 is a mechanical actuator 268, in the form of an actuatable rod 270, controlled by a platen separation controller 254. The rod 270 can be actuated along its longitudinal axis, in this case being the vertical axis, via a throughbore 224 in the lower heat-conducting platen to apply an urging force to the upper heat-conducting platen 212 to increase the separation between the upper and lower heat-conducting platens 212, 214.

As the separation between the upper and lower platens 212, 214 is increased, air can seep into the cavity 218 via the throughbore 224, thereby creating the thermal buffer between the upper and lower platens 212, 214.

Such an actuatable rod 270 is clearly not the sole way a mechanical actuator 268 could be installed within the hotplate 210, however. For example, a wedge could be introduced laterally through the thickness of the apparatus in order to urge the upper and lower platens apart. The upper and lower platens could thus be biased towards each other such that they close towards each other as the wedge is retracted. Equally, the upper and lower platens could be screw-threadingly engaged with each other, whereby relative rotation enables the axial separation to be adjusted. The insulative buffer between the upper and lower platens is the important factor in the present invention. It will therefore be appreciated that the heat-conducting apparatus does not necessarily need to be integral with the hotplate, in order to achieve temperature regulation. The heat-conducting apparatus could also be provided on the base of a cooking vessel or other utensil, and an example of such a device is illustrated in Figure 5, indicated generally at 310. As before, references which are similar to those used previously refer to like parts, and further detail is omitted.

A heat-conducting apparatus 336 comprises the base 372 of the cooking vessel, in this embodiment, a saucepan 310, with the lower heat-conducting platen 314 forming the bottom face 374 of the saucepan 310. The saucepan 310 further comprises an upstanding perimeter side wall 376, projecting from the upper perimeter 326 of the upper heat-conducting platen 312, and terminating in an uppermost rim 378. At the rim 378, a handle 380 is positioned, by which a user can safely hold onto the saucepan 310.

In this particular embodiment, the saucepan has a mechanical actuator 368 acting as the platen adjustment means 316, having a jack arrangement 382 to forcibly separate the upper and lower heat-conducting platens 312, 314. The actuation of these jacks 382 may be controlled by a platen separation controller 354 affixed to the saucepan 310. The platen separation controller 354 will typically include a means by which air can be inlet into the cavity 318 of the saucepan 310.

Much in the same way as the hotplates 10, 110, 210 described above are able to control the temperature of the cooking surface, so too is the saucepan 310 able to control the temperature of the upper surface 320 of the upper heat-conducting platen 312. The lower heat-conducting plate 314 is heated from below by a hotplate, for instance, and heat exchange to the upper heat-conducting platen is inhibited to a certain extent, depending upon the separation between the upper and lower heat-conducting platens 312, 314.

Whilst the each of the hotplates 10, 1 10, 210 of the present invention have thus far been described in relatively abstract terms, it will be appreciated that they are specifically designed for use with a heat storage cooking unit 400, since such devices utilise conduction between substantial metallic elements to provide heat to their hotplates.

As shown in Figure 6, it is possible to provide such a heat storage cooking unit 410, in this embodiment, shown having a heat-storage body 484, being a substantially cuboidal ground-level item having at least a generally flat upper surface 486, with one or more ovens 488 forwardly mounted on the heat-storage body 484. Onto the upper surface 486 of the heat-storage body 484 are mounted two hotplates 110, in this instance, in accordance with the second embodiment described above. Pivotably mounted to a rear edge 490 of the upper surface 486 are two covers 492 being sized so as to be able to cover one hotplate 110 each.

The covers 492 each have a handle 494 mounted thereon projecting forwardly when the covers 492 are positioned in a closed configuration wherein the hotplates 110 are covered. The covers 492 are formed from a largely thermally insulating material to permit a user to open and close the cover 492 without burning themselves, by pivoting the covers 492 about the rear edge 490 of the upper surface 486.

As an energy-saving measure, the covers 492 may be linked to the platen adjustment means 116. If a cover 492 is in a closed position, it will necessarily be covering the hotplate 110 thereunder, and there will be no need to cook using the hotplate at that point in time. It is therefore possible to link the position of the covers 492 to the pressure of the cavities 118 of the hotplate; when the cover 492 is closed, the pressure of the fluid in the cavities 118 is maximised, thereby minimising heat exchange from the lower platen 114 to the upper platen 112. Therefore, the upper platen 112 will cool, reducing heat wastage. The fluid will preferably remain in the cavities when the lid is opened, until the user determines the required temperature of the hotplates.

The heat storage cooking unit 410 contains a heat source, which will typically be a fossil fuel burner, which is able to heat a cast iron slab, being in physical communication with the lower platen 114 of each hotplate 110. This is the standard mode of operation of a heat storage cooking unit, the difference in the present invention is in the form of the hotplate 110, utilising the cavities 116 between the upper and lower platens 112, 114 to regulate the temperature of the hob elements 160.

It will be appreciated that although the present invention is described as being for use in conjunction with a heat storage cooking unit, that the principle of providing cavity- based regulation of the temperature of the hotplate is not necessarily dependent upon any intrinsic property of a heat storage cooking unit. It is therefore possible to utilise the present invention with a different heat source, for instance, an electric heating element, in order to provide thermal energy to the lower platen of the hotplate. Along the same lines, were a different heating element utilised, a different form of cooking unit could be utilised. The present hotplate designs could therefore be integrated into a standard electric or gas hob, or could be formed as part of a freestanding hotplate unit, perhaps to be used in a more mobile setting, for instance, as part of a camping stove. The above embodiments of the hotplate show the cases for a single uniform heating surface in the first embodiment of the first aspect of the invention, and a multi-faceted heating surface having four hob elements in the second embodiment. It is acknowledged that these are merely examples of arrangements of hotplates, and any number of hob elements could be integrated; this is entirely dependent upon the number of cavities provided.

It is therefore possible to provide a means of regulating the temperature of a heating surface of a hotplate which is heated from below by a thermally conductive element by providing the hotplate with upper and lower heat-conducting apparatus, separated by a cavity. The cavity can be pressurised with a non-flammable fluid to alter the effectiveness of heat exchange between the two elements, thereby providing a means of temperature adjustment.

Typically such a hotplate would be used in conjunction with a heat storage cooking unit, having a cast iron slab therein, heated by a fossil fuel burner, with the slab directly heating the lower heat-conducting apparatus, which in turn indirectly heats the upper heat-conducting apparatus through the cavity.

The words 'comprises/comprising' and the words 'having/including' when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined by the appended claims.