The present invention relates to an electric heated oven with heat-storing capacity.

Background

More than 1 billion people in the world live in areas without access to grid electricity. More than 2 billion people do not have access to clean cooking technology and do their cooking of food on open fire with no or inadequate venting of the smoke gas. The persistent exposure to combustion gases is a serious environmental and health problem. It is thus a need for cooking capabilities which are not too costly, and which do not rely on burning fire wood, coal or other combustion fuels as energy supply.

Nearly all the people with poor access to clean cooking have good solar insolation conditions making sun light an abundant and available source of energy.

Prior art

The energy in sun light may be harnessed thermally or by the photovoltaic effect. The simplest and most efficient harnessing of energy from sunlight is to absorb the sunlight in a material having a low albedo. This approach has the potential of capturing almost all energy of the incident sunlight and converting it to thermal energy. By using a mirror to focus and concentrate incident sunlight from a relatively large area onto a receiving area or object, the sunlight may heat the receiving area/object to several hundred degrees Celsius which is sufficient to cook or bake food.

There are known solar cookers and ovens using mirrors or similar reflective surfaces to harness incident sunlight to cook and/or bake food. One example of such a solar cooking apparatus is disclosed in e.g. document US 3938497. Such solar cookers have the benefit of having a relatively simple and inexpensive construction using free energy, making them very cheap apparatuses for cooking/baking food suited for use by low-income residents. They also have the benefit of obliviating the need for using open-fire to heat food and thus potentially significantly improving the in-house environment and health conditions. However, they have no heat storage capacity and stops working immediately if the sunshine is interrupted. They can only operate in direct sunshine, which significantly constrains indoor use and the very common use in late afternoons. This problem may be alleviated by using a heat storage material to accumulate and store solar energy until later use.

From CN 102 597 649 A, it is known a solar cooking device comprising a solar heat collector to collect and store solar heat, a heat storage and conducting material partially filling said solar heat collector and a set of solar cooking utensils. The utensil has an inner wall which is thermally connected to the heat storage, conducting material and the solar hear collector, so that the solar energy can be collected to cook food. The utensil also has a removable part for opening and closing said utensil during cooking. In one example embodiment, the heat storage material is a phase-change material and there is an electric heating element located in the heat storage material.

From US 4 619 244 it is known a solar oven having a heating chamber mounted on a first axis of rotation, and which revolves through an angle OMEGA at a constant angular velocity. The first axis of rotation is adjustable through an angle theta to the horizontal according to the geographic latitude, and through an angle phi to the axis of rotation according to the season. The heating chamber is an insulated cavity with a small aperture. The sun's rays are focused along a second axis normal to the first axis through a small aperture into the heating chamber. The heating chamber contains a phase change material which melts at about 180 °C, which provides heat at that temperature after sundown. In one embodiment, a structure includes a platen to supply heat to a cooking utensil. The platen has heat-conducting ribs attached to its bottom side, which dip into the phase-change material contained in an enclosure under the platen. The surface of the platen is maintained level by gravity. A major practical challenge with such oven in a single household, however, is the mainte nance challenges related to keeping the solar tracking function sufficiently precise and operational at an acceptable cost.

Panwar et al. [1] provides a good overview of the prior art replicated herein in figure 9 and the various ways of channeling solar irradiative heat directly or through an interim storage into energy for cooking purposes. The simplest solution is the “box type collector” where a glass insulation allows solar irradiation to pass through the glass and into the collector, while the glass is insulating reasonably well the heat that has been captured and stored. The box type collector can be made with or without mirrors to collect more irradiation. “The flat plate collector” has a separate panel collecting solar irradiation and converting it to steam, and then transferring the steam to the cooking utility. The “parabolic reflector direct cooker” has an advanced parabolic mirror that reflects irradiation from a larger area by the use of a parabolic mirror so that a strong concentrated irradiation can be achieved. The “parabolic reflector indirect cooker” uses the same parabolic mirror but directs the sunlight onto a heat storage medium that afterwards or elsewhere (indoors) is used for cooking. Unfortunately, none of the above methods have been able to provide a low cost, reliable cooking method with limited needs for maintenance and which allows the user to decide freely when to cook.

Lameck NKhonera et al. [2] has investigated finned thermal box-type collectors for solar cooking at 150 - 200 °C using the pentaerythritol as phase-change material (PCM) for storing latent heat collected from irradiation. The study investigated the performance of the thermal storage units as function of the volume ratio of PCM : fin in a cookstove designed for use within 1 to 4 hours after being fully charged by the sun. The article discloses a thermal storage unit consisting of an aluminium container being filled with pentaerythritol and a set of aluminium fins standing edgewise in parallel, and which in their upper end is attached to an aluminium top plate having a set of electric heating elements incorporated therein in order to simulate solar radiative heating providing a surface temperature of 220 °C under a glass. The thermal storage unit was insulated with Rockwool on the sides and in the bottom.

EP 0 221 575 discloses a heating and/or coating device comprising a heat storage unit (2) surrounded by heat insulation (1), a heating element (3) arranged in said storage unit (2), at least one heat consumer (5) and a heat transfer element (6) emerging from the heat storage unit (2) and routed through the insulation (1) to the heat consumer (5), whereby the heat transfer power of said element (6) is adjustable via a regulator (7). The output of the device is considerably improved and its controllability simplified in a universal manner by the fact that the heat transfer element is a heat exchange tube (6) rising from the heat storage unit (2) to the heat consumer (5) and that the steam/liquid flow in said tube (6) can be regulated by the regulator (7), which may be a valve or a server-valve linked with an electrical or electronic control system.

WO 2017/205864 discloses devices, systems, and methods relating to providing a portable, rechargeable vessel for collecting, storing, and recovering thermal energy are provided. In one aspect, vessel includes a structure defining a well and an open- top portion at the top of the well; a phase- change material, wherein the phase- change material is disposed in the well, the phase-change material being configured to change phase at temperature in the range of 110-700 °C; one or more thermally- conductive fins interleaved in the phase-change material; and a thermally- conductive heat transfer plate disposed at and substantially covering the open-top portion of the structure, in direct thermal contact with the one or more fins, thereby allowing the transfer plate to directly exchange thermal energy with the phase change material.

CN 101 329 137 discloses an electric oven, in particular to a heat-accumulating electric oven which comprises an oven body. An insulating course is arranged outside the oven body, the inner side of the oven body is separated into two parts with the upper half part being a working region and the lower half part being a heat accumulating region, the lower half part comprises a plurality of heat-preserving devices, each of which comprises a refractory corrosion-resistant container, an expansion chamber and an electric heating rod, the expansion chamber and the refractory corrosion-resistant container are communicated, a phase change heat- accumulating material is arranged inside the refractory corrosion-resistant container, and the electric heating rod is embedded in the phase change heat- accumulating material. A resistance wire heating plate is arranged at the upper part of the refractory corro si on-resi stant container, and an air inletting fan and an air exhausting fan are arranged at the two sides of the resistance wire heating plate and used for connecting the upper and lower parts. An automatic temperature controlling system is arranged on the outer surface of the oven body and has a plurality of temperature sampling points that are arranged in the working region at the upper part of the oven body. The heat-accumulating electric oven can accumulate a large amount of heat energy during an electricity off-peak period at night, and does not consume electricity and utilizes the stored heat energy for convenient working during an electricity peak period.

Objective of the invention

The main objective of the invention is to provide a low-cost and reliable electric heated cooking oven for domestic use having heat storage capacity enabling use in periods with power outlet. Description of the invention

The present invention utilizes an electric heating element and a phase-change material to accumulate and store the thermal energy required to bake/cook food. This feature makes the cooking oven suited for use in areas where the electricity is unstable/intermittent by accumulating heat energy when electric energy is available and storing it for later use.

Thus, in a first aspect, the present invention relates to a cooking oven, comprising:

- a first enclosure (1) encompassing an oven compartment (2) on all sides except at a front side, wherein the first enclosure (1) is defined by: i) a lower wall (la) constituting a bottom of the first enclosure (1), ii) a first side wall (lb) constituting a left side of the first enclosure (1), iii) a second side wall (lc) opposite the first side wall constituting a right side of the first enclosure (1), iv) a back wall (Id), constituting a back side of the first enclosure (1), and v) an upper wall (le) constituting a top of the first enclosure (1),

- a heat storage (3) comprising a first phase-change material (4),

- a first electric heating element (6) in thermal contact with the first phase-change material (4),

- a first electric connection (7) electrically connecting the electric heating element (6) to an external source of electric energy,

- a thermal insulation (9) adapted to:

- covering a lower surface of the lower wall (la), a left side of the first side wall (lb), a right side of the second side wall (lc), a back side of the back-side wall (Id), and an upper side wall (le) from above, and

- thermally insulating the heat storage (3) from the ambient environment, and

- a front door (10) pivotably attached to and adapted to open and close the front side of the first enclosure (1), wherein the front door (10) comprises a thermal insulation

(11) covering a front side of the front door.

As used herein, the term “front” refers to the side of the cooking oven which enables accessing the oven compartment, i.e. the side at which the front door is pivotably attached. The term “back” as used herein, is the side of the cooking oven at an opposite end of the front side. The back side of the cooking oven will typically be facing a kitchen wall etc. The terms “left” and “right” as used herein, refer sides of the cooking oven being on the left and on the right, respectively when seen towards the front. The term “lower” as used herein refers to the base/floor side of the cooking oven, while the term “upper” refers to the top side of the cooking oven. The interrelationship between lower, upper, left, right, back and front as used herein is schematically illustrated in figure 1 which is a drawing seen in a perspective view as seen towards the front side of an example embodiment of the cooking oven according to the invention. The drawing illustrates the example embodiment with an open front door (10) such that we see into the oven compartment (2). The first enclosure of this example embodiment is shaped as a rectangular parallelepiped by planar side walls (la, lb, lc, Id, and le) made of a metal or alloy such as e.g. steel or aluminium, or possibly even different such metals or alloys for different walls/surfaces. At this angle of view, the right side-wall (lc) and the upper side wall (lc) are only visible as by their edges at the front side. The left and the right side- wall of the first enclosure may advantageously comprise grooves and/or protrusions

(12) on their inner surface for releasable attachment of baking trays etc. The outer surface of the walls (la, lb, lc, Id, and le) of the first enclosure and the outer surface of the front door (10) are covered by a heat insulation (9). The heat storage is embedded in the heat insulation (9) and thus not visible on this figure. The cooking oven may be standing on a set of foots (12). A three-dimensional cartesian coordinate system is, for illustrative purposes, overlaid the drawing such that its origin is located approximately at the geometric centre point of the oven compartment. Relative to this cartesian coordinate system, the front side (If) and the back side (Id) of the first enclosure (1) are oriented to be substantially parallel to the xz-plane and located at a negative y-value and at a positive y-value, respectively. Likewise, the left side (lb) and the right side (lc) of the first enclosure (1) will be oriented substantially parallel to the yz-plane and located at a negative x- value and at a positive x-value, respectively. The lower side (la) and the upper side (le) of the first enclosure (1) will be oriented substantially parallel to the xy-plane and located at a negative z-value and at a positive z-value, respectively. The terms “inner” and “outer” as used herein is relative to the oven compartment (2) such that e.g. an inner surface is a surface facing the oven compartment while an outer surface faces the ambient environment of the cooking oven. Likewise, the term “inward or outward direction” as used herein refers to the oven compartment such that an outward direction is towards the ambient environment of the cooking oven and inward direction is towards the oven compartment.

The term “first enclosure” as used herein means any form of container being open at a front end such that it encloses an internal space being accessible through the open front end. The first enclosure may apply any material in the walls (la - If) of the first enclosure (1) known to the skilled person to have the required mechanical strength, thermal conductivity and thermal resistance to be applicable as a load carrying structure of a cooking oven. Examples of suited materials for the bottom wall (la), the first side wall (lb), the second side wall (lc), the back-side wall (Id), the upper wall (le) include, but is not limited to; one or more metals chosen from the group of: Fe, Cu, Al, Zn, Sn, W, or one or more alloys chosen from the group of: bronze, brass, constantan, steel, or pinchbeck.

The term “oven compartment” as used herein, is the internal space enclosed by the first enclosure. The oven compartment, and thus the corresponding first enclosure, may have any shape and dimension able to house food heating tools such as baking trays, ovenproof dishes, ceramic pots, etc. However, in a practical example embodiment, the oven compartment may advantageously be shaped substantially as a rectangular parallelepiped as shown schematically in figures 1 a) and 1 b). In figure 1 b), an example embodiment of the oven according to the invention is drawn schematically as seen in a perspective view towards the front side. In this example embodiment the cooking oven has a rectangular oven furnace defined by the walls (la to le) of the inner enclosure. The front door (10) is pivotably attached to the lover part of the oven and shown in open position. The walls (la to le) of the first enclosure and the front door (10) has a layer of heat insulation on their outer surface. This example embodiment of the cooking oven is standing on foots (12) to obtain a practical working height.

The term “substantially as a rectangular parallelepiped” as used herein means the shape of the first enclosure and its oven compartment may deviate from the strict geometrical definition of the rectangular parallelepiped by having e.g. rounded corners/edges and/or having walls of the first enclosure deviating somewhat from perpendicular relative to each other, etc.

The term “heat storage” as used herein encompasses any way of storing and containing a mass of a phase-change material at a temperature ranging from the ambient temperature and heated up to above its phase-change temperature. The term “electric heating element” as used herein, encompasses any known of conceivable resistive heating elements which convert electric energy to thermal heat. The heating element has the function of accumulating thermal energy in the heat storage (when electric energy is available) and should thus be in thermal contact with the phase-change material. In practice, this may e.g. be obtained by locating the electric heating element(s) inside the thermal storage (in the bulk of the phase-change material). Alternatively, the electric heating element(s) may be located outside of the thermal storage(s) but obtain thermal contact by way of a thermal bridge(s), heat conducting fins etc. The electric heating element(s) is(are) electrically connected to an external source of electric energy enabling the electric heating element to convert electric energy (whenever available) to heat energy being stored as latent heat in the phase-change material. In one example embodiment, the invention may further comprise an electronic switch (8) adapted to cut the electric connection (7) when the phase-change material reaches a cut-off temperature to avoid overheating the phase-change material, and to re-establish the electric connection (7) when the temperature of the phase-change material is below the cut-off temperature to enable accumulating latent heat. This may e.g. be obtained by having a temperature sensor (29), such as e.g. a thermocouple, registering the temperature of the phase-change material (4) feeding the electronic switch with the registered temperature.

The cooking oven according to the invention may apply one or more heat storages to accommodate for various needs for heat-storing capacity. The heat storage may be dimensioned and shaped in any manner enabling supplying heat energy from the one or more heat storages to the oven compartment. One example embodiment of the heat storage may be in the form of e.g. a closed container made of metal or other relatively high thermal conductive material being filled with phase-change material and containing the electric heating element(s) and electric connection (s). Examples of suited materials for use as the closed container for the heat-storage include but is not limited to; a metal chosen from the group of: Fe, Cu, Al, Zn, Sn, W, or an alloy chosen from the group of: bronze, brass, constantan, steel, or pinchbeck, or a high temperature resistant polymer like for example high temperature silicones. The closed container may advantageously be shaped congruent with the shape and dimensions of the outer surface of a wall (la to le) of the first enclosure. A phase- change in a material may involve a simultaneous change in the volumetric density leading to an expansion or contraction of the material when passing from one phase to the other. In this example embodiment, the heat-storage may alternatively be made of a flexible material, such as a polymer material, allowing the container to expand and shrink in sync with expansions or contractions of the phase-change material. A further alternative of this example embodiment, is to only partly filling the volume of the heat-storage container with the phase-change material and form a gas/ air-filled pocket inside the heat storage container which absorbs expansions/- contractions of the phase-change material. The air or gas filled pocket may advantageously have a volume constituting of from 70 to 99 %, preferably of from 85 to 98 %, or most preferably of from 95 to 98 % of the total inner volume of the primary heat-storage container. I.e. the gas or air-filled pocket has a volume in the range of from 1 to 30 %, preferably of from 3 to 15 %, or most preferably of from 3 to 5 % of the total volume of the heat-storage container.

In one example embodiment, the heat storage may be located at the outer surface of one or more of the bottom wall (la), the first side wall (lb), the second side wall (lc), the back-side wall (Id), and the upper wall (le) of the first enclosure (1). Figure 2 shows schematically an example embodiment with a heat storage in the form of a container located at the outer surface of the lower (la) and the top (le) wall. The figure is a cut-view taken along the xz-plane (at y =0) of the example embodiment shown in figure 1 as seen towards the front side. In this example embodiment, there is applied two separate heat storages (3) one located at the outer surface of the upper wall (le) and one at the outer surface of the lower wall (la), both being a closed container shaped into a rectangular parallelepiped being in physical contact with and dimensioned to cover the outer surface of the wall of the first enclosure it is located outside of and filled with a phase-change material (4). Each heat storage (3) of this example embodiment comprises an electric heating element (6) being located inside the heat storage, and the electric connection (7) to each heating element (6) has an electronic switch (8) cutting/re-establishing the electric connection in accordance with the phase-change material temperature registered by temperature sensor (29). The operation of the electronic switch (8) may be controlled by a logic command unit (28) which is adapted to receive the registered temperatures form sensor (29) and to cut the electric power to the heating element(s) (3) when the temperature of the phase-change material becomes higher than a predetermined temperature being above the phase-change temperature of the phase-change material (4) and to reengage the electric connection when the temperature of the phase-change material becomes lower than the predetermined temperature. The heat storage(s) (3) are in this example embodiment embedded in the heat insulation (9).

In a further example embodiment, the heat storage (3) may be a double-walled closed structure containing the phase-change material which is shaped/adapted to envelope the first enclosure to cover either: i) the outer surface of the upper wall (le), the left (lb) and the right (lc) side walls, ii) the outer surface of the upper wall (le), the left (lb) and the right (lc) side walls, and the back wall (d), or iii) the outer surface of the upper wall (le) the left (lb) and the right (lc) side walls, the back wall (Id), and the lower wall (la).

In a further example embodiment, the heat storage may be made integral with the first enclosure by the first enclosure comprising double-walled hollow side wall at or more of the walls (la to le) and filling the space in the hollow double-wall with the phase-change material (4). In the case of employing more than one heat storage (3) in the cooking oven, it is also envisioned the possibility of employing different phase-change materials in the heat storages (3) having different phase-change temperatures to enable having e.g. having a higher temperature at the upper part of the oven space (2) as compared to the lower part.

The term “phase-change material” as used herein, means any chemical compound or mixture of chemical compounds going through a reversible phase-change enabling absorbing or releasing a useful amount of latent heat, preferably more than 100 kJ/kg phase-change material, at a temperature in the range from 120 to 500 °C, preferably from 130 to 400 °C, more preferably from 140 to 350 °C, and most preferably from 170 to 300 °C. The phase-change may either be a solid-solid phase change, or a solid-liquid phase change. The invention may apply any phase-change material known to the skilled person having a phase-transition temperature suitable for use in a cooking apparatus. In case of applying a phase-change material having a liquid phase, the heat storage should comprise a closed and leak-proof container.

Examples of suited solid-liquid phase-change materials include, but are not limited to; inorganic salts such as e.g. NaNCb (310 °C, 174 kJ/kg), NaN0 ₂ (282 °C,

212 kJ/kg), MgCl ₂ -6H ₂ 0 (117 °C, 168,6 kJ/kg), NaOH (318 °C, 158 kJ/kg), KOH (360 °C, 167 kJ/kg), KNCE (337 °C, 116 kJ/kg), L1NO3 (261 °C, 370 kJ/kg), and mixtures thereof; mixtures (by weight) of salts such as e.g. 26.8 % NaCl and NaOH (370 °C, 379 kJ/kg), NaOH and 7.2 % Na ₂ C0 ₃ (283 °C, 340 kJ/kg), NaCl and 5 % NaOH, 49 % L1NO3 and NaN0 ₃ (194 °C, 265 kJ/kg); 31,9 % ZnCl and KC1 (235 °C, 198 kJ/kg); organic compounds such as erythritol (IUPAC-name; (2R,3S)- butane-1, 2, 3, 4-tetraol, 121 °C, 339 kJ/kg), or acetanilide (IUPAC-name; N- phenylacetamide, 114.3 °C, 222 kJ/kg), or d-mannitol (IUPAC name; (2i?,3i?,4i?,5i?)-Hexane-l,2,3,4,5,6-hexol; 167°C, 310 kJ/kg), or dulcitol/galactitol (IUPAC name; (2i?,3S ^, ,4i?,5A)-hexane-l,2,3,4,5,6-hexol; 188 °C, 350 kJ/kg). The temperatures and kJ/kg figures set in parenthesis above are the melting point and heat of fusion, respectively, of the phase-change material.

Examples of suited solid-solid phase-change materials include but are not limited to; pentaerythritol (IUPAC-name; 2,2-bis(hydroxymethyl)propane-l,3-diol,

184.2 °C, 222.5 kJ/kg). The temperatures and kJ/kg figures (if given) set in parenthesis above are the solid-solid phase-transition temperature and heat of fusion, respectively, of the phase-change material.

The first enclosure and the heat storage(s) should be heat insulated towards the ambient environment of the cooking oven to enable storing the accumulated the electric energy as latent heat in the heat storage for later use and also to prevent the outer surface of the cooking oven to be excessively hot. The cooking oven according to the invention has therefore a thermal insulation covering the outer surface of all walls of the first enclosure including the heat storage and the outer surface of the front door. In an example embodiment, the heat insulation may advantageously reduce the heat loss to the ambient environment of the cooking oven to retain sufficient latent heat in the phase-change material to cooking/baking food at least two hours, preferably at least 12 hours and most preferably at least 24 hours after a power black-out. This feature enables using the cooking oven more or less independently of an unreliable and intermittent source of electric energy, such as e.g. a solar PV-source of electric energy, a wind-mill, an unreliable grid etc.

The term “thermal insulation” as used herein, encompasses any known way of heat- insulating a surface. The thermal insulation may e.g. be obtained by covering the outer surfaces of the first enclosure including the heat storage(s) and outer surface of the front door by one or more layers of a material having a relatively low thermal conductivity to set up a thermal barrier towards the ambient environment. There is no absolute boundary for thermal resistance across the thermal insulation to obtain the objective of at least 24 hours storing capacity. This depends on the heat of fusion of the phase-change material being applied, the temperature of the phase change material, amount of the phase-change material in the heat storage(s), the surface-to-volume ratio of the container of the heat storage(s), etc. However, in practice, the specific thermal resistivity, R, across the thermal insulation may advantageously be at least 10 mK/W, preferably of at least 12 mK/W, more preferably of at least 15 mK/W, more preferably of at least 20 mK/W, and most preferably of at least 25 mK/W. With this range of thermal resistivity, the heat insulating material (or sandwich of several insulating materials) has an average thermal conductivity in the range from 0.03 W/mK to 0.10 W/mK, and the total thickness of the heat insulation layer(s) required to obtain the above objective of 24 h heat-storage capacity becomes in the in the range of from 5 to 30 cm.

The invention may apply any material as thermal insulation known to the skilled person having the required (low) thermal conductivity and the thermal resilience to withstand the maximum temperatures at which the phase-change material is heated. Examples of suited thermal insulating materials include, but are not limited to; calcium silicate, cellular glass, fiberglass, mineral wool, rock wool, ceramic foam, polyurethane, foamed polyurethane (such as e.g. commercially available under the trademark Puren), and other porous materials having air filled pores, etc. The heat insulation may alternatively be obtained by two or more layers of the above- mentioned materials, where the two or more layers are of the same heat insulating material or being chosen among two or more of the above-mentioned heat insulating materials. Depending on which material being chosen as heat insulation, the cooking oven may apply the outer heat insulation to form the outer surface of the cooking oven. Alternatively, the heat insulation may be enclosed by a second enclosure (13) shaped and dimension to contain the heat insulating material (9), the heat-storage(s) and the first enclosure (1). The second enclosure may advantageously be made of one 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.

The invention may apply any electric heating element known to the skilled person which enables heating the phase-change material to its intended heat storing temperature. The source of electric energy may be any available source, including small-scale wind power, electric power from a photovoltaic panel, grid electricity, battery, windmill, or a combination thereof.

In a further example embodiment, the second enclosure (13) may be integrated with the first enclosure to a single hollow entity by arranging them concentric to each other such that a space of 5 to 30 cm thickness is formed between the first (1) and second (13) enclosure outside all walls (la to le) of the first enclosure and sealing the space at the front end by a front-wall (14) such as illustrated schematically in e.g. figure 3, which is a cut- view along the xz-plane as seen from left of a similar example embodiment as shown in figures 1 and 2. The space height is indicated schematically by the stapled double arrow marked A - A’ on the figure. In this example embodiment it is employed a heat storage (3) at the outer surface of the lower wall (la), the back wall (lc) and the upper wall (le) of the first enclosure and which are located inside the space between the first (1) and second (13) enclosure. The space between the first (1) and second (13) enclosure not occupied by the heat storages (3) may be filled with a heat insulating material (9). Alternatively, the space between the first (1) and second (13) enclosure may be evacuated to a gas pressure less than 100 kPa, preferably less than 25 kPa, and most preferably less than 1 kPa. In this case, the heat insulation effect is obtained by low pressure gas filling and being confined in the space in-between the first (1) and second (13) enclosure, similar to the heat insulation effect of laminar window glass.

In a further example embodiment, the cooking oven may comprise one or more engageable and adjustable heat-protections (16) where each heat-protection (16) is adapted to substantially cover the inner surface of one of the lower wall (la), the first side wall (lb), the second side wall (lc), the back wall (Id), or the upper wall (le) of the first enclosure (1) by comprising a set of turn-able plates (17) arranged in parallel at a distance from each other in a planar plane being oriented substantially parallel with and at a distance above the inner surface (la, lb, lc, Id, or le), and where the blades (17) are dimensioned to substantially cover the planar plane when the blades are turned to be oriented in the planar plane, and to open up and have a minimum heat-protecting effect when the blades are turned to be standing normal to the planar plane.

Thus, the terms “horizontal” and “vertical” are in this context relative to the orientation of the planar plane formed by the set of the turn-able plates (17), where horizontal orientation means the blades are laying side by side in the planer plane, while vertical orientation means standing normal to the planar plane. The turn-able blades (17) of the heat-shield (16) may advantageously be turned manually by the operator of the oven (or electronically by a thermal controller) in any position from fully closed by being in the horizontal orientation to fully open by being in the vertical orientation so that the heat flow from the inner surface of the wall (la to le) of the first enclosure to the bulk space of the oven compartment may be regulated gradually by varying the ratio of convective and radiative heat transfer from the inner surface to the bulk space of the oven compartment.

An example embodiment of the cooking oven equipped with the heat-shield (16) is schematically drawn in figure 4. The drawing is a cut-view along the yz-plane at x=0 seen from left as in figure 3. This example embodiment applies one heat- storage (3) outside the lower wall (la) and one heat-storage (3) outside the upper wall (le) of the first enclosure. The example embodiment applies one heat-shield (16) located at a distance above the inner surface of the lower wall (la) and one heat-shield (16) at a distance below the inner surface below the upper wall (le) of the first enclosure. The upper heat-shield (16) is in closed position by having its blades (17) horizontally oriented laying in the planar plane indicated by the stapled line marked B-B’, while the lower heat-shield (16) is in open position by having its blades (17) vertically oriented standing normal to the planar plane indicated by the stapled line marked C-C’. The example embodiment is drawn under operation, that is by its front door (10) in closed position and a baking tray (40) holding two bakery products (41), e.g. two breads being baked. The lower heat-shield (16) is open and allows maximum conductive and radiative heat transfer from the heat-storage (3) below the lower wall (la) to the bakery products being baked, while the upper heat- shield (16) is closed to minimise the heat flow from the heat-storage (3) above the upper wall (le) to the bakery products being baked.

In one example embodiment, the heat-storage (3) may further comprise one or more heat-conducting fin(s) (26) extending from the inner surface of the of the heat- storage in outward direction a distance into the bulk of the phase-change material (4) to enhance the heat transfer from the phase-change material (4) to the inner surface of the heat-storage. Figure 5 illustrates an example embodiment including a set of fins (26) in the heat-storage. The drawing is a cut-view along the xz-plane seen from left as in figures 3 and 4. The fins may advantageously be made of metal, such as e.g. aluminium or an aluminium alloy. As used herein, the term “front door” encompasses any conceivable or known to the skilled person openable front closure of an oven compartment.

In one example embodiment, the cooking oven may further comprise, for at least one of the walls (la to le) of the first enclosure:

- a number of at least one indentation (20) extending a distance in outward direction from the at least one wall (la to le) and at least entering a distance into a heat storage (3) and optionally through an heat insulation (9) being intermediate between the at least one wall (la to le) and the heat storage (3), and

- a heating plate (21) being parallel with and located at the inner surface of the at least one wall (la to le), where the heating plate (21) has a number, equal to the number of indentation(s) (20), of protrusion(s) (22) extending in outward direction, and where each protrusion (22) is located and shaped and dimensioned to enter and substantially fill an indentation (20), and

- a mechanical adjustment mechanism (23) adapted to adjust the heating plate (21) to any position from a retracted position where the number of protrusion(s) (22) is fully inserted into and substantially filling the number of indentation(s) (20) to an outstretched position where the number of protrusion(s) (22) is/are at least partly extracted out of the number of indentation(s) (20).

Figures 6 a) and 6 b) are drawings illustrating an example embodiment of a cooking oven using a heating plate (21) to heat the oven compartment. The drawing is a cut- view seen from the front taken along the xz-plane at y=0 (see figure 1). This example embodiment applies one heat storage (3) located at a distance above the upper wall (le) to allow having heat insulation (9) lying between the outer surface of the upper wall (le) and the heat storage (3). The heating plate (21) is adapted to cover most of the inner surface of the upper wall (le) (when seen in outward direction) and has a set of protrusions (22) entering a distance into the indentation(s) (20). A mechanical mechanism (23), here a screwing mechanism, enables changing the degree of penetration by the protrusions (22) into the indentations (20) from a retracted position, such as shown in figure 6 a), to an outstretched position such as shown in figure 6 b). In this example embodiment, the electric heating element (6) and its electric connection (7) is not drawn for clarity issues. In the retracted position shown in figure 5 a), the protrusions (22) are extending a maximum length into the indentations (20) and thus having a maximum surface area exposed to the heat of the thermal storage (3), and thus maximising the heat transfer (heat flux through the protrusions) from the heat storage to the heating plate (21) and thus the oven compartment. By extending the heat plate (21) and its protrusions (22) towards the outstretched position as shown in figure 6 b), the surface area of the protrusions (22) exposed to the heat of the thermal storage becomes gradually diminished until reaching a minimum at the outstretched position. In this example embodiment, the protrusions are outstretched sufficiently to be standing inward of the heat storage (3) and thus be more or less thermally insulated from the heat storage (3). By adjusting the mechanical mechanism (23), the heat flux from the phase change material (3) to the baking chamber (2) can hence be varied by the user or according to the user’s desired baking temperature through an electronic regulator.

The term “indentation” as used herein, encompasses any known or conceivable channel, dent, notch, hole, ditch etc. or combinations thereof which extends from a wall (la to le) of the first enclosure in an outward direction to a distance into a heat storage (3). The term “protrusion” as used herein encompasses any known or conceivable rod, fin, projection etc. sticking out of a heating plate and which is shaped to be congruent to the indentation, located and dimensioned enable entering the protrusion into the indentation and substantially fill the space thereof. The protrusion (22) and the heating plate (21) may preferably be made of a metal such as aluminium, steel etc.

In a further example embodiment, the cooking oven may be a cooking stove by comprising at least an upper heat-storage (3) at the outer surface of the upper wall (le) of the first enclosure (1) and one or more circular openings (24) in the heat insulation (9) exposing the outer surface of the upper heat-storage (3). The exposed outer surface of the heat-storage (3) constitutes cooking plates at which cooking pots, saucepan etc. may be placed for cooking food. This example embodiment should also comprise one or more heat-insulating lids (25) adapted to fit into the circular openings (24) and heat insulate the exposed part of the heat-storage (3) when the cooking plates are not in use. Figure 7 is a drawing schematically illustrating an example embodiment of the cooking oven with two cooking plates/circular openings (24) in the heat insulation (9). The figure is a cut-view taken along the yz-plane (see figure 1) and seen from left. In this example embodiment, the cooking over applies three heat-storages (3), one at the outer surface of the lower wall (la), the back wall (lc) and upper wall (le), each heat- storage (3) has a set of fins (26) extending from the inner surface of the heat-storage (3) in outward direction a distance inti the bulk phase-change material. The upper heat-storage (3) at the outside of the upper wall (le), is also comprising a second set of fins (27) extending from the outer surface of the heat-storage in inward direction a distance into the bulk of the phase-change material (4). The second set of fins may advantageously be applied inside of the exposed outer surface of the heat-storage (3) (cooking plates) to enhance the heat transfer towards the cooking plates. Figure 7 illustrates one coking plate (the foremost) being applied to cook food in a cooking pot (30) while the backwards coking plate is not in use and is thus covered by a heat insulating lid (25) to avoid spilling heat energy.

The cooking stove may in another example embodiment advantageously comprise a sealing ring (29) of high temperature rubber, such as e.g. high temperature silicone rubber, running along the periphery of the circular opening in the heat insulation (9) a distance outward of the outer surface of the exposed heat-storage (3). By applying a cooking pot (30) adapted to form a close fit with the sealing ring (29), as shown in figure 7, the sealing ring forms a closed space (34) around the lower parts of the cooking pot (30). This feature enables obtaining an efficient heat transfer rate from the hot outer surface of the heat-storage (3) to the cooking pot (30) by adding an amount of water to the outer surface of the heat-storage just before placing the cooking pot (30) thereon. The water will rapidly vaporize and form super-heated steam which fills the space (34). The sealing ring (29) will also have the function of forming a tight closure towards the heat-insulating lid (25) when the cooking plate is not in use.

In one example embodiment, the heat-storage (3) may comprise a first heat-storage (31) and a second heat-storage (32), each heat-storage being encapsulated by a metal container and wherein the second heat-storage (32) is located outwards of the first heat-storage (31) and filled with a second phase-change material (33) having a higher phase-change temperature than the first phase-change material (4) of the first heat-storage (31). This example embodiment may advantageously further comprise a set of fins (26) extending in outward direction from the inner surface of and through the first heat-storage (31) and further a distance into the bulk of the second phase-change material (33) of the second heat-storage (32). In this example embodiment, the electronic switch (8) should be adapted to cut the electric connection(s) (7) at a temperature above the phase-change temperature of the second phase-change material (33).

List of figures

Figure l is a drawing seen in a perspective view towards the front side of an example embodiment of the cooking oven according to the invention.

Figure 2 is a drawing illustrating schematically an example embodiment of the invention with a heat storage at the bottom and at the top of the oven compartment. The figure is a cut-view taken along the xz-plane at y =0 (see overlaid cartesian coordinate system in figure 1).

Figure 3 is a drawing illustrating schematically an example embodiment of the invention with a heat storage at the bottom, the back and at the top of the oven compartment. The figure is a cut-view taken along the yz-plane at x =0 (see overlaid cartesian coordinate system in figure 1).

Figure 4 is a drawing illustrating schematically an example embodiment of the invention with a heat storage at the bottom and the top of the oven compartment, and which has an adjustable heat-shield at the lower and upper part of the oven compartment. The figure is a cut view taken along the yz-plane at x=0 (see overlaid cartesian coordinate system in figure 1).

Figure 5 illustrates the same example embodiment as figure 4 including a set of fins (26) in the heat-storages to enhance the heat transfer towards the oven compartment. The figure is a cut view taken along the yz-plane at x=0 (see overlaid cartesian coordinate system in figure 1).

Figure 6 a) and b) illustrate an example embodiment of the invention using a heating plate in adjustable thermal contact with the heat-storage to heat the oven compartment. The figures are cut-views taken along the xz-plane at y =0 (see overlaid cartesian coordinate system in figure 1).

Figure 7 is a drawing illustrating schematically an example embodiment of the invention with a heat storage, with fins, at the bottom, the back and at the top of the oven compartment and cooking plates defined by circular openings in the upper heat insulation exposing the upper heat storage. The figure is a cut-view taken along the yz-plane at x =0 (see overlaid cartesian coordinate system in figure 1).

Figure 8 is a drawing illustrating schematically an example embodiment of the invention having a two-compartment heat-storages having different phase-change materials. The figure is a cut-view taken along the yz-plane at x =0 (see overlaid cartesian coordinate system in figure 1).

Figure 9 is a facsimile of figure 3 of [1] showing different types of prior art solar cooking heaters.

Verification of the invention

An example stove may have lithium nitrate as phase change material and have a normal stand-by storage temperature around 260 °C - just around the phase change temperature. The top and bottom heat storage containers may together have a total outer surface of 0.12 sqm on the assumption that they are 30x20 cm ² large. The other interior surfaces of the stove (that are not facing the heat storage containers) may have a total surface area of 0.52 sqm (0,64-0,12 sqm) assuming a stove chamber of 40x30x30 cm ³ .

For ease of calculation it is here then assumed that during stand-by situation the interior surfaces are holding 240 °C while the heat storage container holds 260 °C and that the ambient has a 25 °C temperature. We further assume that the stove is equipped with 15 cm insulation on all surfaces, that heat is only flowing perpendicular to the surfaces (minor simplification), and that the average thermal conductivity of the insulation is 0.04 W/m K. (The latter can be achieved with a combination of polyurethane and Rockwool.) In this situation the thermal losses can be estimated at 37 Watt. For comparison a human body is estimated to have thermal losses of about 60 Watt so the stove will not get hot in any way. Over 8 and 24 hours the stove losses are equivalent to about 300 and 900 Wh.

A solar module of 350 Watt will typically deliver 1000-2000 Wh per day. This will be sufficient to cover the heat losses as well as recharge the stove in one or two days. Since the stove is continuously hot, the amount of power required to maintain it hot during cooking is already included in the loss calculation above. The amount of energy required to bring a stew (3 liters of water plus a typical glass container) to boiling point is about 0.3 kWh. For baking purposes, Baker Pacific estimates a net need of 0.42 kWh to cook 2 kg of biscuits. One would hence be well equipped with 1 kWh of energy stored in the phase change material, and this would be equivalent to about 10 kg or 6 liters of lithium nitrate. This amount will again easily fit into the top and bottom containers for phase change material as described above.

If one would want to be certain that the stove could be used every day, one would simply need one more solar module, and/or one may increase the size of the heat storage container(s) in order to increase capacity further.

References

1 Pam war et ah, “State of the art of solar cooking: An overview”, Renewable and Sustainable Energy Reviews 16 (2012) 3776- 3785 2 Lameck NKhonera et ah, “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