SOLAR COOKING APPARATUS WITH HEAT STORAGE CAPACITY

Abstract

This invention relates to a cooking apparatus comprising a container having a bottom wall, side wall, and an upper wall enclosing a first inner chamber of the container, where the bottom wall and side wall of the container are thermally insulating, and the upper wall is thermally insulating except for at least one planar and substantially horizontally oriented cooking zone which is thermally conductive, a first phase-change material located inside and substantially filling the first inner chamber of the container, an electric resistance heating element located in the first phase-change material and electrically connected to a source of electric energy, and a releasable lid made of a thermally insulating material adapted to cover and thermally insulate each of the at least one cooking zone.

Claims

1-27.

28. A cooking apparatus comprising: a first liquid tight container having a bottom wall, side wall, and an upper wall enclosing a first inner chamber of the container, where: the upper wall comprises at least one substantially horizontally oriented cooking zone made of a thermally conductive material, and the bottom wall, the side wall, and the upper wall except for the at least one cooking zone comprises a first thermal insulation, and a first phase-change material located inside and substantially filling the first inner chamber of the first container, one or more electric resistance heating element(s) electrically connected to a source of electric energy and being in thermal contact with the phase-change material(s), wherein: the cooking zone is integrated with and forming a part of the upper wall and is in direct thermal contact with the first phase-change material, or is arranged onto the upper wall, and in that the cooking apparatus further comprises: a releasable lid made of a thermally insulating material adapted to cover and thermally insulate each of the at least one cooking zone.

29. A cooking apparatus comprising: a first liquid tight container having a bottom wall, side wall, and an upper wall enclosing a first inner chamber of the container, where: the upper wall comprises at least one substantially horizontally oriented cooking zone made of a thermally conductive material, and the bottom wall, the side wall, and the upper wall except for the at least one cooking zone comprises a first thermal insulation, and a first phase-change material located inside and substantially filling the first inner chamber of the first container, one or more electric resistance heating element(s) electrically connected to a source of electric energy and being in thermal contact with the phase-change material(s), wherein the cooking apparatus further comprises: a second inner chamber located inside the first container just below and in contact with the cooking zone or the upper wall, and which is filled with a second phase-change material which exhibits a liquid-solid phase-change and has a lower phase-transition temperature than the first phase-change material, and a releasable lid made of a thermally insulating material adapted to cover and thermally insulate each of the at least one cooking zone.

30. The cooking apparatus according to claim 28, wherein the first phase-change material is a chemical compound or mixture of chemical compounds going through a reversible phase-change enabling absorbing or releasing latent heat, preferably more than 100 kJ/kg phase-change material, at a temperature in the range from 80 to 500 C., preferably from 90 to 450 C., more preferably from 100 to 400 C., and most preferably from 110 to 300 C.

31. The cooking apparatus according to claim 28, wherein the first phase-change material is: either a chemical compound or a mixture of chemical compounds chosen from the group consisting of; LiNO.sub.3, NaNO.sub.3, NaNO.sub.2, MgCl.sub.2.6H.sub.2O, NaOH, KOH, KNO.sub.3, or - a mixture of either; 26.8% by weight NaCl and NaOH, 7.2% by weight Na.sub.2CO.sub.3 and NaOH, 5% by weight NaNO.sub.3 and NaOH, 49% by weight LiNO.sub.3 and NaNO.sub.3, or 31.9% by weight ZnCl and KCl.

32. The cooking apparatus according to claim 28, wherein the first phase-change material is: either a chemical compound or a mixture of chemical compounds chosen from the group consisting of; erythritol, acetanilide, pentaerythritol, pentaglycerine, d-mannitol, or dulcitol/galactitol, or a mixture in molar % of pentaerythritol (PE), trimethylol ethane (TME) and neopentyl glycol (NPG) of: 30 PE, 10 TME, and 60 NPG; 45 PE, 45 TME, and 10 NPG; 45 PE, 10 TME, and 45 NPG; or 70 PE 15 TME, and 15 NPG.

33. The cooking apparatus according to claim 28, wherein the cooking zone has an upper planar surface (5) and is made of: a metallic material chosen from the group of: Fe, Cu, Al, Zn, Sn, W, or an alloy chosen from the group of: bronze, brass, constantan, steel, pinchbeck, or a ceramics chosen from the group of: aluminium oxide, crystalline silicon dioxide, porcelain, or pyrex glass.

34. The cooking apparatus according to claim 28, wherein: the bottom wall, the side wall, and the upper wall of the first container are made up of a single layer of a material chosen from the group consisting of; glass, polymer, steel or aluminium, and the bottom wall, the side wall, and the upper wall of the first container has a thermal insulation layer of thickness of from 5 to 30 cm on their outer side facing the surroundings of the apparatus, and where the thermal insulation layer is made of one or more heat insulating materials chosen from the group consisting of: calcium silicate, cellular glass, fiberglass, mineral wool, rock wool, ceramic foam, polyurethane, and foamed polyurethane, or other porous materials having air filled pores.

35. The cooking apparatus according to claim 28, wherein the thermal insulation of the first container is obtained by having the bottom wall, the side wall, and the upper wall of the first container formed by at least two concentric similarly shaped containers where the inner container is the first container and the one or more outer container(s) is/are larger dimensioned such that there is a gap between the inner and the second, and eventually the second and the third concentric containers etc., the least two concentric similarly shaped containers are made of the same or of different materials chosen from the group consisting of; glass, polymer, or a metal such as steel or aluminium, and the gap between two adjacent second concentric similarly shaped containers are filled with a material of either: heat insulating material chosen from the group consisting of: calcium silicate, cellular glass, fiberglass, mineral wool, rock wool, ceramic foam, polyurethane, and foamed polyurethane, or a gas evacuated to a gas pressure of less than 25 kPa, preferably less than 1 kPa.

36. The cooking apparatus according to claim 28, wherein the apparatus comprises two or more thermal insulation layers encapsulating the first container except at the area defining the one or more cooking zone(s).

37. The cooking apparatus according to claim 28, wherein the source of electric energy is one of, or a combination of, a photovoltaic panel, wind mill, and grid electricity.

38. The cooking apparatus according to claim 28, wherein the apparatus further comprises one or more elongated members extending downwardly from the cooking zone a distance into the first inner chamber, and where the elongated members is either made of the same material as the cooking zone, or of a solid-solid phase-change material having a phase transition temperature higher than the first phase-change material.

39. The cooking apparatus according to claim 28, wherein the apparatus further comprises one or more elongated members extending downwardly from the upper wall a distance into the first inner chamber, and where the elongated members are hollow and filled with a second phase-change material.

40. The cooking apparatus according to claim 29, wherein the second inner chamber is designed as a metallic box or cylinder attached and sealed to the lower surface of the cooking zone and/or the lower surface to the upper wall.

41. The cooking apparatus according to claim 28, wherein the apparatus further comprises at least one expansion chamber located inside the inner space of the container, comprising: an inner cylindrically symmetric space being confined by a cylinder-shaped side wall, a first end wall and a second end wall, where the first and second end walls are located at opposite ends of the cylinder-shaped side wall, an opening located in the second end wall, and a slide-able piston located inside and dividing the inner cylindrically symmetric space into a first and a second inner expansion chamber, and the first inner expansion chamber is filled with a gas pressurised such that it seeks to press the slide-able piston towards the opening.

42. The cooking apparatus according to claim 41, wherein the at least one expansion chamber is located inside the first inner chamber of the container and its opening makes the expansion chamber in fluid communication with the first phase change material.

43. The cooking apparatus according to claim 41, wherein the at least one expansion chamber is located inside the second inner chamber and its opening makes the expansion chamber in fluid communication with the second phase change material.

44. The cooking apparatus according to claim 41, wherein the at least one expansion chamber is located in the upper part of the first inner chamber such that the opening of the expansion chamber is in direct contact with the phase change material and extends through the upper wall and at least partly into the heat insulating layer, preferably trough the heat insulating layer and a distance into the second insulation layer.

45. The cooking apparatus according to claim 28, wherein the cooking zone has a planar upper surface and a thickness such that it protrudes downwardly a distance into the into the first inner chamber, and where the first inner chamber has a zone filled with a gas having a pressure varying in the range of from 0.25 to 3 bars, more preferably from 0.4 to 2 bars, and the zone is confined by wall 1 b), wall 1 c), and an outer surface of the cooking zone at the upper part of the first inner chamber.

46. The cooking apparatus according to claim 45, wherein the cooking zone has a lower outer surface shaped to be convex and rotational symmetric over an axis located in the centre of the cooking zone and being normal to the planar upper surface.

47. The cooking apparatus according to claim 28, wherein the upper wall forms a closure of the upper end of the container and is given a convex bulge resembling shape protruding downwardly into the first inner chamber, and where the lower part of the cooking zone is given a complementary shape and dimension to form a close fit with the bulge shaped part of the upper wall.

48. The cooking apparatus according to claim 28, wherein the apparatus further comprises an electrical heating element in thermal contact with the cooking zone and in electrical connection with a source for electric energy.

49. The cooking apparatus according to claim 28, wherein the first container and/or second inner chamber is made of a flexible material so that all or parts of the volume change in the one or more phase change material(s) is facilitated by a similar change in the volume of the first container and/or the second inner chamber.

50. The cooking apparatus according to claim 28, further comprising a second phase-change material encapsulated in small micro-containers/capsules dispersed in the first phase-change material, and where the second phase-change material in the micro-containers/capsules has a higher phase-change temperature than the surrounding first phase-change material.

51. The cooking apparatus according to claim 38, wherein: the first container is filled with pentaerythritol as the first PCM-material and made of aluminium, and where the first contained is shaped either as: i) a rectangular parallelepiped with an inner length in the range of from 20 to 50 cm, preferably of 30 cm, an inner width in the range of from 20 to 50 cm, preferably of 30 cm, and an inner height in the range of from 12 to 50 cm, preferably of 15 cm, or ii) a cylinder of inner diameter in the range of from 20 to 50 cm, preferably of 30 cm, and an inner height in the range of from 10 to 40 cm, preferably of 15 cm, and where the cooking apparatus comprises in the range of from 20 to 70, preferably of from 30 to 70 elongated members made of aluminium, and where: the elongated members are adapted to fit inside the inner chamber when protruding downwardly from the cooking zone and arranged in parallel and spaced a distance from each other, preferably evenly spaced from each other, the elongated members has a thickness in the range of from 0.5 to 3 mm, preferably of from 1.0 to 1.5 mm, and the elongated members extend downwardly into the inner chamber to a distance in the range of from 2 to 40, preferably 5 to 20 cm above the bottom wall.

52. The cooking apparatus according to claim 28, wherein the cooking zone further comprises a lifting mechanism adapted to, in a first position, be fully embedded and retracted into the cooking zone and further adapted to, in at least a second position, extend a distance above the upper surface of the cooking zone.

53. The cooking apparatus according to claim 52, wherein the lifting mechanism either: comprises a set of rod members being fully embedded and retracted into grooves in the upper surface of the cooking zone when being in a first position, and where each rod member in one end is pivotably attached to a rod extending from the rod member and a distance out of the cooking apparatus, and thus enabling raise one end of the rod members up from its groove in the cooking zone and protrude a distance above the upper surface of the cooking zone by twisting the rod, or: comprises a set of toothed rod members being located and oriented vertically in the cooking zone and which may be vertically displaced between a first position where the toothed rod members are fully embedded and retracted into the cooking zone and at least one second position where the toothed rod members are protruding a distance above the upper surface of the cooking zone by a pinion wheel attached to a rod extending from the toothed rod member and a distance out of the cooking apparatus.

54. The cooking apparatus according to claim 38, wherein the cooking zone and fins are made simultaneously by extrusion of aluminium.

Description

LIST OF FIGURES

[0044] FIG. 1a is a cut cross-view as seen along the dotted line marked B-B in FIG. 1b, schematically illustrating an example embodiment of the invention.

[0045] FIG. 1b is a cut cross-view as seen along the dotted line marked A-A in FIG. 1a, schematically illustrating the same example embodiment of the invention as shown in FIG. 1a.

[0046] FIG. 2 is a similar cut view as shown in FIG. 1a of a second example embodiment of the invention.

[0047] FIG. 3 is a similar cut view as shown in FIG. 1a of a third example embodiment of the invention.

[0048] FIG. 4 is a similar cut view as shown in FIG. 1a of a fourth example embodiment of the invention.

[0049] FIG. 5 is a similar cut view as shown in FIG. 1a of a fifth example embodiment of the invention.

[0050] FIG. 6 is a similar cut view as shown in FIG. 1a of a sixth example embodiment of the invention.

[0051] FIGS. 7 a) to c) are a schematically presentation of an example embodiment of a lifting mechanism for adjusting the heat transfer rate from the cooking zone to a cooking tool.

[0052] FIGS. 8 a) to c) are schematically presentations of an eighth example embodiment of the invention.

[0053] FIG. 9 is a schematically presentation of a further example embodiment of the invention.

[0054] FIG. 10 is a facsimile of FIG. 3 from Panwar et al. [4] showing state of the art of solar cooking.

EXAMPLE EMBODIMENTS OF THE INVENTION

[0055] The invention will be explained in more detail by way of example embodiments.

First Example Embodiment of the Invention

[0056] The first example embodiment of the invention is schematically illustrated in FIGS. 1a and 1b. FIG. 1a is a cut cross-section view seen from the side taken along the dotted line marked B-B in FIG. 1b, while FIG. 1b is a cut cross-section view seen from above taken along the dotted line marked A-A in FIG. 1a.

[0057] The first container of this example embodiment is shaped as a vertically standing cylinder of inner diameter D and inner height H. The cylindric container (1) is made up of a cylinder section (1b) being closed at the bottom by a disc shaped bottom (1a). A ring-shaped disc (1c) constitutes the top of the cylindric container; and has a concentric circular opening into which a circular disk (4) constituting the cooking zone is located and made fluid tight integral with the ring-shaped disc (1c). The circular disc (4) is made of a thermally conductive material, such as e.g. an aluminium alloy, steel etc. The bottom (1a), cylinder section (1b) and the ring-shaped disk (1c) may be made of e.g. a thin steel or polymer casing of e.g. 1 mm thickness. A first layer of a thermal insulating material (3) is applied to the outer side of the bottom (1a), cylinder section (1b) and the ring-shaped disk (1c). The material of the first thermal insulation (3) may be e.g. rock wool. When not in use to cook food, the cooking zone (4) is covered/closed by a lid (7) made of a thermal insulating material.

[0058] The inside of the container is filled with a phase-change material (8), which for example may be a mixture of NaOH and 7.2% by weight Na.sub.2CO.sub.3, based on total weight of the mixture. The phase-change material may be mixed with a liquid to enable efficient heat transport inside the container also when the major part of the phase-change material is in the solid state.

[0059] An electric resistance heating element (9) is placed in the phase-change material (8) and in the proximity of the bottom (1a) to heat the phase-change material to above its phase-change temperature, which in this example embodiment is 283 C. The electric resistance heating element (9) may in this example embodiment advantageously be shaped as planar plate or as a cross of plates fixed vertically in the centre of the cylinder (1) and at a distance above the bottom (1a), and it may advantageously be equipped with electric conductors (not shown) electrically connecting the element to an appliance plug socket (not shown) for enabling feeding electric energy to the heating element.

[0060] The cooking apparatus may advantageously further comprise an optional temperature sensor (10) located in the phase-change material (8) which terminates the electric energy supply to the heating element (9) when the temperature of the phase-change material reaches a predetermined temperature, which in this example embodiment may be set to e.g. 300 C., and naturally at another temperature if another phase-change material is being applied. The phase-change material (8) may either be exhibiting a solid-solid phase-change or a liquid-solid phase-change.

[0061] This example embodiment further comprises a second thermal insulation (6) covering the first thermal insulation (3). The second thermal insulation (6) is optional but may advantageously be made of a rigid solid material, such as e.g. cellular concrete or other foamed ceramic material, to provide both an increased heat insulation of the container and providing load carrying capacity to the cooking apparatus enabling it being used without any additional load carrying or mechanically stabilising structures. It may simply be placed onto a floor and being used as it is to cook food (after heating up the phase-change material). This example embodiment has an especially simple construction enabling an especially low-cost manufacturing of the cooking apparatus. Remark that the heat insulating walls may consist of one or more layers of insulating materials since different materials may be cost effective at different temperatures.

Second Example Embodiment of the Invention

[0062] The second example embodiment of the invention, shown in FIG. 2, is similar to the first example embodiment, except that in order to alleviate a potential problem of gradually poorer thermal contact between the cooking zone (4) as phase-change material just beneath the cooking zone (4) is cooled due to heat being transferred to the food being cooked, there is one or more elongated members (11) extending from the cooking zone (4) a downwardly distance into the bulk of the first inner chamber (2) to function as heat conducting bridge(s) transferring heat from lower lying and thus hotter phase-change material (8) to the cooking zone (4). This example embodiment has the advantage of enabling a prolonged period of relatively high heat transfer to the cooking zone than the example embodiment shown in the first example, both when applying a phase-change material exhibiting a solid-solid phase-change or a liquid-solid phase-change.

[0063] The elongated members (11) may advantageously be made of the same metal/-material as the cooking zone (4), and may be shaped as elongated fins, circular rods etc.

Third Example Embodiment of the Invention

[0064] The third example embodiment of the invention, shown in FIG. 3, is similar to the first example embodiment except that it further comprises a second inner chamber (12) located inside the container just below and in contact with the cooking zone (4). The second inner chamber (12) is filled with a second phase-change material (13) exhibiting a liquid-solid phase-change at a lower phase-transition temperature than the first phase-change material (8) filling the rest of the first inner chamber (2).

[0065] The second inner chamber (12) may advantageously be formed by a metallic box/cylinder etc. attached and sealed to the lower surface of the cooking zone (4) such that there is no exchange/leakage of phase-change materials between the first inner chamber (2) and the second inner chamber (12).

[0066] This example embodiment has the advantage of ensuring that the phase-change material (13) in contact with the cooking zone (4) is in the liquid state until a large fraction of the material (8) has changed phase and ensures thus an efficient convective heat transfer from the phase-change materials (8, 13) to the cooking zone (4) as long as the first phase-change material (8) is still at least partly in the high-temperature phase. I.e., the user has not emptied the thermal reservoir of the first phase-change material (8) to an extent which has transformed a large fraction of the first phase-change material to its low-temperature phase. This embodiment thus gives the user a longer time available for high power usage.

Fourth Example Embodiment of the Invention

[0067] The fourth example embodiment of the invention, shown in FIG. 4, is similar to the third example embodiment except that it further comprises a similar plurality of one or more elongated members (11) as in example 2.

[0068] The elongated members (11) may advantageously extend through the second inner chamber (12) and further a distance into the first inner chamber (2) to enable an improved thermal contact between the phase-change material inside the first inner chamber and the phase-change material inside the second inner chamber. The members (11) may also, in another embodiment, only be installed below the second inner chamber (12).

Fifth Example Embodiment of the Invention

[0069] The fifth example embodiment of the invention, shown in FIG. 5, is similar to the first example embodiment except that the upper wall (1c) is disc shaped and forms an upper closure of the cylindric container (1), the cooking zone (4) is laid onto and in thermal contact with the upper wall (1c), and it applies a heating element designed as a as a cross of two plates, and further that it comprises an expansion chamber (20) located in the inner space of the container (1) of the cooking apparatus.

[0070] The expansion chamber (20) comprises an inner cylindrically symmetric space being confined by a cylinder-shaped side wall (21), a first end wall (22) and a second end wall (23) having an opening (24). The first (22) and second (23) end walls are located at opposite ends of the cylinder-shaped side wall (21). A slide-able piston (25) is located inside and divides the inner cylindrically symmetric space into a first (26) and a second (27) inner expansion chamber. The first inner expansion chamber (26) is filled with a gas pressurised such that it constantly seeks to press the slide-able piston (25) towards the opening (24) and thus squeeze out the phase-change material being present in the second inner expansion chamber (27).

Sixth Example Embodiment of the Invention

[0071] The sixth example embodiment of the invention is similar to the fifth example embodiment except for having the expansion chamber (20) located in the upper part of the first inner chamber (2) extending through the ring-shaped disc (1c) and at least partly into the first (3), and if present the second (6), heat insulation with the opening (24) of the expansion chamber (20) in direct contact with the phase change material (8). During expansion and contraction, the liquid phase-change material may then move up and down inside the expansion chamber (20), and a piston (25) is not required.

[0072] In this design it is required to use a phase-change material (8) which has its lowest density in liquid state, and it is advantageous then to use a small resistive heater placed inside the expansion chamber (20) and extending down to the main heating element (9) so that when heat is added to the chamber (2) and an expansion starts, the liquid phase-change material in the first container (2) is in direct contact with the liquid in the expansion chamber (20).

Seventh Example Embodiment of the Invention

[0073] The seventh example embodiment of the invention, shown in FIG. 6, is similar to the fourth example embodiment except that it further comprises a similar expansion chamber (20) as described in the fifth example embodiment made in fluid communication with the second phase-change material (13) inside the second inner chamber (12).

Eighth Example Embodiment of the Invention

[0074] The eighth example embodiment of the invention has a cooking zone (4) having a planar upper surface (5) and a thickness such that it protrudes downwardly a distance into the into the first inner chamber (2) as shown schematically in FIG. 8 a).

[0075] Alternatively, the cooking zone (4) may at its lower part be given a convex shape and be rotational symmetric over an axis located in the centre of the cooking zone (4) and being normal to the lines marked A-A and B-B in FIGS. 1 a) and 1 b) respectively (i.e. the axis is normal to the upper planar surface (5), as shown schematically in FIG. 8 b).

[0076] Due to the part of the (4) protruding a distance into the first inner chamber (2), there will be formed a zone (14) at the upper part of the first inner chamber (2) being confined by parts of the walls 1 b) and 1 c) and the cooking zone (4). This zone (14) may be utilised as expansion chamber by being filled with a moderately pressurised gas, such as e.g. from 0.25 to 3 bars, preferably from 0.4 to 2 bars. Due to its compressibility, the gas filled zone (14) will absorb volume changes in the liquid phase change material (8). In a further alternative, as shown schematically in FIG. 8 c), the upper part of the container (1) is shaped similar as the bottom of champagne bottle, i.e. the upper wall (1c) forms a closure of the upper end of the container (1) but has a convex shaped bulge protruding downwardly into the first inner chamber (2). The bulge may advantageously be rotational symmetric over an axis located in the centre of the cooking zone (4) and normal to the upper planar surface (5). The lower part of the cooking zone (4) should be given a complementary shape and dimension to form a close fit with the bulge shaped part of the upper wall (1c).

[0077] A cooking zone (4) having a lower part protruding somewhat into the first inner chamber (2) and a gas filled zone (14) may be applied in all aspects of the invention and inn every example embodiment described herein.

Ninth Example Embodiment of the Invention

[0078] The ninth example embodiment of the invention applies elongated members similar as in the second example embodiment, except that the elongated members are made of a solid-solid phase-change material having a phase transition temperature higher than the first phase-change material. The elongated members may for example be plates hanging or mounted vertically below the cooking plate so that they have large vertical surfaces in direct contact with the first phase-change material, and so that they can stimulate a convective flow of liquid in the first phase change material when they are releasing heat due to the solid-solid phase change.

Tenth Example Embodiment of the Invention

[0079] The tenth example embodiment of the invention applies flexible materials in the walls of the first container (1) and/or second inner chamber (12), such that all or parts of the volume change in the one or more phase change material(s) is facilitated by a similar change in the volume of the first container (1) and/or the second inner chamber (12).

Eleventh Example Embodiment of the Invention

[0080] The eleventh example embodiment of the invention further comprises a second phase-change material being encapsulated in small micro-containers/capsules which are dispersed in the first phase-change material, and where the second phase-change material in the micro-containers/capsules has a higher phase-change temperature than the surrounding first phase-change material.

[0081] A second phase-change material being encapsulated in small micro-containers/-capsules dispersed in the first phase-change material may be applied in all aspects of the invention and inn every example embodiment described herein.

Twelfth Example Embodiment

[0082] A twelfth example embodiment of the invention has a rectangular first container (1) made of aluminium with inner dimensions of 303012 cm.sup.3 being filled with pentaerythritol as the first PCM-material and a set of 50 evenly spaced aluminium fins/elongated members (11) of dimensions 0.141030 cm.sup.3 extending downward from the cooking zone (4) (which may also be made of aluminium) such as illustrated in e.g. FIG. 2. The aluminium fins will be spaced with a gap of about 4.5 mm between them.

[0083] Calculations show that such a configuration will obtain a storage capacity of around 1 kWh of heat and enable supplying the cooking zone with heat flux controllable up to around 2 kW (with a temperature difference between the cooking zone and the phase-transition temperature of the PCM-material of around 80 C.).

[0084] In the case of using 30 aluminium fins, instead of 50 and otherwise equal, the solar cooker will be able to store about 1.5 kWh of thermal energy and provide a heat flux to the cooking zone of about 1 kW even when around half of the phase-change material has changed phase. In this case the gap between the aluminium fins becomes approximately 8 mm.

[0085] The twelfth example embodiment is a particularly low-cost version of the present invention and is envisioned having either a cylindrical first container (1) having an inner diameter in the range of from 20 to 50 cm, preferably 30 cm, and a height of from 10 to 40 cm, preferably 15 cm, or alternatively a rectangular first container (1) having a length and width of from 20 to 50 cm, preferably 30 cm and a height of from 12 to 50 cm, preferably 15 cm. The inner chamber (2) is preferably filled with pentaerythritol as a first PCM-materials and contains a set of from 20 to 70, preferably of from 30 to 70, aluminium fins/elongated members (11) arranged in parallel at a distance from each other, preferably evenly spaced, and protruding downwardly from the cooking zone (4) a distance in the range of from 2 to 40 cm, preferably of 5 to 20 cm, above the bottom wall 1(a) of the first container. The thickness of the aluminium fins/elongated members (11) is in the range of from 0.5 to 3 mm, preferably of from 1.0 to 1.5 mm. The aluminium fins/elongated members (11) may in one alternative be adapted to extend over either the length or the width of inner chamber (2), or alternatively if the inner chamber is a cylinder, to extend form one side to the other over the horizontal cross-section of the cylinder. I.e. the fins will be gradually wider and wider towards the centre axis of the cylindrical inner chamber or be equally wide in the case of a rectangular inner chamber.

[0086] Verification of the Invention

[0087] A solar cooker as described in the first example embodiment having a cylindrical inner container of inner radius of 10 cm and inner height of 30 cm may contain about 20 kg of solid phase NaOH mixed with 7.2% Na.sub.2CO.sub.3 as phase-change material (density approx. 2.2 kg/litre), enabling storing around 2 kWh as latent heat in the phase-change material. This latent heat will be released from the phase-change material at a temperature of 283 C.

[0088] In the case of using a layer of rock wool of thickness of 10 cm (thermal conductivity of 0.06 W/mK) as the first insulation layer and a layer of foamed polyurethane of thickness of 10 cm as the second insulation layer (thermal conductivity of 0.025 W/mK), the container of the solar cooker obtains and outer height of 70 cm and outer diameter of 60 cm.

[0089] The heat flux across the bottom and bottom 2 and upper 3,4 surfaces may be considered being almost equal to the heat flux across a planar composite wall in contact with free-flowing air (natural convection). Such heat flux may be calculated by the relation [ref 1], page 37-38:

q=U(T.sub.contT.sub.air)

where q is the heat flux in unit [W/m.sup.2], U is the overall heat transfer coefficient in unit [W/m.sup.2K], T.sub.cont is the temperature in unit [ C.] inside the container (which is the phase-change temperature of the phase-change material), and T.sub.air is the temperature in unit [ C.] of the surrounding air. The overall heat transfer coefficient for a composite wall of a number of i layers is given by the relation [ref 1], page 37, 38:

[00001]$\frac{1}{U}=\frac{L_1}{k_1}+\frac{L_2}{k_2}+\mathrm{.Math.}+\frac{L_i}{k_i}+\frac{1}{h_{a.Math.i.Math.r}}$

[0090] Here L.sub.1 is the thickness in unit [m] of the first insulation layer, L.sub.2 is the thickness of the second insulation layer etc, and k.sub.1 is the thermal conductivity in unit [W/mK] of the first insulation layer, k.sub.2 is the thermal conductivity of the second insulation layer etc, and h.sub.air is the heat transfer coefficient towards the ambient air for natural convection.

[0091] The h.sub.air for a vertical standing plate is about 5 W/m.sup.2K (ref [2]). By assuming the same value for the top and bottom surface (it will be somewhat less due to the surfaces being horizontally oriented) and that the inner steel casing is 1 mm thick and has a thermal conductivity of 16 W/mK (ref [3]), the heat flux across the bottom and upper surface of the container of the solar cooker becomes 44.8 W/m.sup.2, or a heat loss across each of the bottom and top surface of 1.4 W (if the lid 7 is being equally insulating as the rest of the wall).

[0092] The heat loss across a cylindrical side wall of length h having a laminar wall structure of a first layer having outer radius of r.sub.1 (this layer is facing the inner space of the cylinder), a second layer of outer radius of r.sub.2, etc. up to an i'th layer with outer radius of rain, may be calculated by the relation, ref [1], page 40:

[00002]$q=\frac{\left(T_{\mathrm{cont}}-T_{a.Math.i.Math.r}\right)}{R_{\mathrm{tot}}}$

where R.sub.tot is the overall thermal resistance in unit [K/W] over the cylindrical wall, and which is defined by:

[00003]$R_{\mathrm{tot}}=\frac{\ln \left(r_1/r_{\mathrm{cont}}\right)}{2.Math..Math.k_1.Math.h}+\frac{\ln \left(r_2/r_1\right)}{2.Math..Math.k_2.Math.h}+\mathrm{.Math.}+\frac{\ln \left(r_i/r_{i-1}\right)}{2.Math..Math.k_i.Math.h}+\frac{1}{h_{a.Math.i.Math.r}.Math.A_{a.Math.i.Math.r}}$

[0093] Here r.sub.cont is the inner diameter of the container and A.sub.air is the outer surface of the cylinder facing the ambient air.

[0094] Applying the same wall structure for the cylindrical part as given for the bottom and top surfaces above, the heat loss across the cylindrical side wall becomes 18.4 W, such that the overall heat loss from this example embodiment becomes approx. 21.2 W as long as the phase-change material releases sufficient latent heat to maintain the phase-change material at 283 C. Over a time-span of e.g. 24 hours, this example embodiment will lose at most about 0.5 kWh of heat when being stored indoors (in contact with air at free convection flow conditions). In practice, the heat loss will be somewhat smaller than calculated because due to the heat loss, phase-change material in proximity of the walls of the container will gradually solidify and thus increase the thermal resistance across the container wall.

[0095] This example embodiment, of about 20 kg of NaOH mixed with 7.2% Na.sub.2CO.sub.3 as phase-change material, may store around 2 kWh as latent heat, such that after 24 hours of storage, about of the cooker's available latent heat content remains for cooking food.

[0096] Similarly, a cylindrical container of inner diameter 15 cm and inner height of 30 cm being filled with pentaerythritol (IUPAC-name; 2,2-bis(hydroxymethyl)propane-1,3-diol) as the phase-change material may store about 1.8 kWh as latent heat (density 1.4 kg/litre, phase-change temperature 184.2 C. and heat of fusion 222.5 kJ/kg). The heat loss over 24 hours storage becomes just above 0.4 kWh, i.e. about of its total the latent heat content.

[0097] These calculations verify that the solar cooker according to the invention may store and hold sufficient amounts of heat at a temperature enabling cooking food for at least 24 hours. A solar panel of 300-500 W peak power is sufficient for accumulating around 2 kWh of heat energy over a single day of sunshine in sub-tropical and tropical areas.

[0098] It should be remarked that the main usage mode in combination with a PV module will be that the energy is added in day time and a large fraction of it is used in the same afternoon 3 hours later. The residual liquid in the phase change material will during the night start solidifying along the coldest walls and thereby gradually add insulation so that in many cases it is likely that 80-90% of the energy added in day time will be available for cooking use for dinner and breakfast/lunch.

[0099] The example embodiments above display the cooking apparatus according to the invention as a vertical cylinder having a single cooking zone. This should not be interpreted as a limitation of the invention. The container may alternatively be shaped as a box, i.e. a rectangular parallelepiped of length A, width B, and height C, or any other conceivable design, and the cooking apparatus may be provided with two or more cooking zones.

REFERENCES

[0100] 1 Adrian Bejan, Heat Transfer, John Wiley & Sons, 1993. [0101] 2 Retrieved from the Internet: [0102] https://www.engineersedge.com/heat_transfer/convective_heat_transfer_coefficients_13378.htm [0103] 3 Retrieved from the Internet: [0104] https://www.engineeringtoolbox.com/thermal-conductivity-d_429. html [0105] 4 Pamwar et al., State of the art of solar cooking: An overview, Renewable and Sustainable Energy Reviews 16 (2012) 3776-3785, doi:10.1016/j.rser.2012.03.026 [0106] 5 Lameck NKhonera et al., Experimental investigation of a finned pentaerythritol-based heat storage unit for solar cooking at 150-200 C., Energy Procedia, 93 (2016) 160-167, doi:10.1016/j.egypro.2017.07.165