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Title:
BOX-TYPE SOLAR COOKER
Document Type and Number:
WIPO Patent Application WO/2018/185646
Kind Code:
A1
Abstract:
Solar cooker with a core comprising at least one insulating wall and at least one window having a glazing on which is deposited a low-e coating, characterized by the fact that said window(s) is/are oriented along several directions.

Inventors:
MAUREE, Dasaraden (Rue Centrale 22, 1110 Morges, 1110, CH)
SCHUELER, Andreas (2 Ch. du Grandchamp, 1018 Lausanne, 1018 Lausanne, 1018, CH)
DIÉVART, Alexandre (Chemin des Lebrats 642, Montauban, 82000, FR)
BOUVARD, Olivia Valérie Charlotte (Chemin des Cottages 6, 1007 Lausanne, 1007, CH)
Application Number:
IB2018/052278
Publication Date:
October 11, 2018
Filing Date:
April 03, 2018
Export Citation:
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Assignee:
ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE (EPFL) (EPFL-TTO, EPFL Innovation Park J, 1015 Lausanne, 1015, CH)
International Classes:
F24S20/30; C03C17/00; E06B3/67
Domestic Patent References:
WO1994002313A11994-02-03
Foreign References:
FR2640031A31990-06-08
GB2286008A1995-08-02
TW201348667A2013-12-01
US4077391A1978-03-07
US20060090834A12006-05-04
US20090133688A12009-05-28
US20140230869A12014-08-21
US20040248051A12004-12-09
ZA200006680B2001-06-18
US20100139648A12010-06-10
US5195504A1993-03-23
CN204363800U2015-06-03
US20160145151A12016-05-26
Other References:
N.M. NAHAR: "Design, development and testing of a double reflector hot box solar cooker with transparent insulated materials", ENERGY CONVERSION MANAGEMENT, 2001
S.S. GOSH; P.K. BISWAS; S.NEOGI: "Thermal performance of solar cooker with special cover glass of low-e antinomy doped indium oxide (IAO) coating", APPLIED THERMAL ENGINEERING, 2016
S. MAHAVAR; P. RAJAWAT; V.K. MARWAL; R.C. PUNIA; P. DASHORA: "Modeling and on-field testing of a Solar Rice Cooker", ENERGY, 2012
J.-L. SCARTEZZINI: "Building Physics I & II", September 2010, SOLAR ENERGY AND BUILDING PHYSICS LABORATORY, ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE
Attorney, Agent or Firm:
ROLAND, André (P.O. Box 5107, 1002 Lausanne, 1002, CH)
Download PDF:
Claims:
Claims

1. Solar cooker with a core comprising at least one insulating wall and at least one window having a glazing on which is deposited a low-e coating, characterized by the fact that said window(s) is/are oriented along several directions.

2. Solar cooker according to claim 1 wherein the low-e coating gas a thermal emissivity below 0.2

3. Solar cooker according to claim 1 or 2 wherein the window-to-wall surface ratio is above 0.3

4. Solar cooker according to the previous claim comprising one mirror.

5. Solar cooker according to one of the previous claims comprising a corner defined by three walls.

6. Solar cooker according to claim 4 or 5, where at least two of the mirrors form an edge with an angle θ between 60 and 120 degrees.

7. Solar cooker according to one of the previous claims, where at least one of the window pane surface is coated with an anti-reflection layer with Tsol(0°) - Tsol(60°) < 15%.

8. Solar cooker according to one of the previous claims, where each window comprises at least one glass pane and one polymer pane.

9. Solar cooker according to one of the previous claims, where a removable heat storage element with a thermal inertia superior to 0.25 kJ/K can be placed in the core to store heat in order to be used outside the solar cooker to grill meat.

10. Solar cooker according to one of the previous claims, where the windows have a U-value below 1.6 W/( m2.K K)

11. Solar cooker according to one of the previous claims, where the U-value of one or several walls is below 0.8 W/( m2.K K)

12. Solar cooker according to one of the previous claims, wherein at least one insulating wall contains vacuum insulation.

13. Solar cooker according to one of the previous claims, wherein at least one wall contains aerogel, preferably sealed.

14. Solar cooker according to one of the previous claims, where windows and walls form shape close to a polyhedron.

15. Solar cooker according to one of the previous claims, where windows and walls form a shape close to a parallelepiped, a rectangular parallelepiped or a cube.

16. Solar cooker according to one of the previous claims, where the mirrors are connected to a main body, and can be folded or disassembled for easy stowaway and protection

17. Solar cooker according to one of the previous claims, where the solar cooker comprises a unit that can control one or more external window shading system in front of one or more windows of the solar cooker to control the cooking temperature.

18. Solar cooker according to one of the previous claims, where the solar cooker comprises a thermoelectric generator which uses the heat of the solar cooker to produce electricity. 19. Solar cooker according to one of the previous claims, where the solar cooker comprises an electronic connection which can be used to charge a phone or other electronic device.

Description:
Box-type solar cooker

Field of invention

The present invention relates to cookers that are using the sun as energy source.

Definitions

The following definitions apply in the present document.

Solar cooker. A solar cooker, also called solar oven is a device used to cook food using the thermal energy of the sun. There are three main types of solar cookers: (1) solar cookers equipped with heat storage such as phase change materials or hot water tank, (2) concentration solar cookers such as parabolic mirrors or Fresnel lens and (3) box-type solar cookers.

Box-type solar cooker: A solar cooker made of a box in any shape, where the inner base presents an absorbent face or inner reflectors and where the top and the lateral faces are made of insulated walls and windows.

Window: The glazed surface of a box type solar cooker. It can be made of several panes. Mirror: Reflector of the box type solar cooker, used to redirect solar irradiation to the window and in this way multiplies the input solar irradiation on the exterior surface of the window.

Absorber tray: Interior part of a box type solar cooker with high solar absorbance, which receives solar radiation.

Insulation: Component of the walls of a solar box cooker that reduces the conductive thermal losses through the walls of the oven. In the present text, the term "insulated wall" and "wall" are indifferently used, they have the same meaning.

Low-e (low-emissivity) coating: Microscopically thin metal layer that is deposited on a glazing to help keep heat on the same side of the glazing from which it origlnaied.

Total surface of the oven: Sum of all the surfaces of the core of a box-type solar cooker, comprising walls and windows [m 2 ].

Window-to-wall surface ratio: Sum of the surfaces of the glazed surface divided by the total surface of the oven. Exposed surface of the windows: Equivalent surface of windows that receive solar irradiation at a normal incidence and has the same amount of solar irradiation than the real windows of the solar cooker [m 2 ].

Orientation factor of the windows: Ratio of the exposed surface of the windows divided by the total surface of windows.

Solar irradiance: radiative power per unit area received by the sun, measured on the ground perpendicular to the incoming sunlight [W/m 2 ].

Concentration factor: Ratio of the total solar irradiance received by the windows of the oven divided by the total irradiance received by the windows of the oven without reflectors. G-value: Effective solar heat gain coefficient of the windows: Ratio of the total radiative energy that enters the oven on the interior side of the window divided by the total radiative energy received by the window on its exterior side. It takes into account the energy transmitted through the window and the part of the energy absorbed by the window that is reemitted on its interior side.

Input solar power: total radiative power that enters the solar cooker on the interior side of its windows.

Effective U-value of walls: Effective heat transfer coefficient of the walls of the solar cooker: Total heat flux that passes through the wall surfaces per unit area per degree of temperature difference between hot absorber plate and external air. It comprises the impact of the different layers of the walls especially the insulation and the external surface limit condition of convection exchange. [W/(m 2 . K)]

Effective U-value of the windows: Effective heat transfer coefficient of the windows of the solar cooker: Total heat flux that passes through the windows per unit area per degree of temperature difference between hot absorber plate and external air. It comprises the impact of the different layers of the composition of the windows, the external and internal surface limit conditions of convection exchange of the windows and the internal surface limit condition of convection exchange of the walls. [W/(m 2 . K)]

Effective U-value of the total envelope: Effective heat transfer coefficient of the whole envelope of the solar cooker: Total heat flux that passes through the whole envelope of the solar cooker divided by the total surface of the envelope per degree of temperature difference between hot absorber plate and external air. It can be obtained by computing the average of the effective u-value of the window weighted by the window to wall surface ratio and the effective u-value of the walls weighted by the ratio of surfaces of walls over total surface. [W/(m 2 .K)]

Temperature difference: Temperature difference between the temperature of the hot absorber plate, also called cooking temperature of the solar cooker and the temperature of the external air [K].

State of the art

There is presently a growing interest in the field of solar cookers, particularly in China and India where most of the literature and the patents related to solar cookers are written. Cooking accounts for a major share of total energy consumption in developing countries. Solar cookers appear to be a suitable solution in remote places with abundant solar irradiation. It is a sustainable system that helps to reduce the consumption of fossil fuels, especially firewood, improve domestic air quality and reduce domestic incidents rates.

Improving the design of box-type solar cooker to improve their performances is a major concern in the related literature.

In 2001, a trapezoidal box-type solar cooker with internal reflectors has been described by Joan Beall, Hannelore Bergler, Michael Grupp, Maria Owen Jones and Gerd Schroder [1]. Its single window is placed on top of the body formed by four lateral walls and a lower one, like in most of the previous designs of box-type solar cookers. Several researches consisted in improving the thermal performances of the glazing. Nahar developed and tested in 2001 a design of box type solar cooker with Transparent Insulated Materials (TI M) and a double external reflector [2]. A glazing equipped with TI M has a much lower solar transmittance than a high thermal performance glazing with low-emissivity coating and therefore a much lower solar input power. In 2016, Gosh tested a simple design of solar cooker equipped with a single pane glass with a low-emissivity antinomy doped indium oxide (IAO) coating [3]. It comprises only a single glazing and therefore allows important conductive thermal losses. The low-e coating is self-made and has an emissivity of 0.63. The process used by Gosh to apply its own coating is more expensive than using existing architectural glazing technologies. At last, both of the designs developed by Nahar and Gosh, are based on a standard rectangular shape of box-type solar cooker, with a superior horizontal window and with low window-to-wall surface ratios. Some solar cookers with higher window to wall surface ratioshave been developed and tested. Mahavar published in 2012 an article where he modeled and tested a solar rice cooker that had a rectangular shape, one superior horizontal window and three lateral vertical windows [4]. Its windows are made of transparent acrylic sheets simply slipped into their frame and therefore they are the siege of major conductive heat losses. The effective U-value of those windows can be estimated to be 3.5 W/( m 2 . K K) when the U-value of the windows of the present invention is below 1.6 W/(m 2 . K). Combining the low thermal performances of the windows and the high window to wall surface ratio drastically decreases the U-value of the total envelope of the oven. Consequently, this solar rice cooker cannot be used in very cold. Furthermore, the insulation of the solar rice cooker made of 25 mm of ceramic fiber results in an effective U-value of 4.1 W/( m 2 . K K) for the walls of the solar cooker which also reflect very low thermal performances. The wooden frame of the glazing of the solar rice cooker provokes shadowing directly on the cooking pot which decreases cooking temperature.

In 2010, Noel Bourke conceived a box-type solar cooker with a lateral inclined window for a remote heat collector [5]. The purpose of its second window is to heat the absorber tray of the oven in a second chamber, called remote heat collector and linked to the cooking chamber. Besides, the windows of the solar cooker with remote heat collector proposed by Bourke do not have high thermal performances with low-emissivity coating and its mirrors are not efficiently disposed.

Transportability of the box-type solar cookers is also an important field of research. In 1990, Lane Bert patented a design of portable solar cooker with two mirrors which could be folded on the box [6]. The design of the concentration system formed by the mirrors of this portable solar oven is not the same as the one of the present invention. It is aligned with the single superior window. It also comprises a one-step outer reflector. A light reflective device for solar cooker composed of multiple reflectors which can be folded on a main frame has also been patented in 2015 by Yuan Yinmei and Jia Hu [7]. This system of reflectors comprises one-step outer reflectors and two-step outer reflectors, unlike the invention. A principle of double reflection is used in this system to concentrate sunlight, in a similar way than with a concentration solar cooker. Therefore this device requires important solar tracking.

Glazing with high thermal performances and low-emissivity coating is a well-known feature of the building field and is presented in most of the building physics classes [8]. In 2016, AGC G LASS EU ROPE patented a coated glazing for solar protection and proposed the use of this glazing in oven doors [9]. The main purpose of this glazing is to be installed in building fagade with high exposure to sun irradiation. Its coating reduces solar transmittance to avoid overheating of buildings. This function is completely different than the function of a glazing with low-emissivity coating, which is used to enhance greenhouse effect. The coating used in this solar protection glazing increases the solar reflectance of the glazing. That is why it can be appropriate for a household oven, because it will reflect the heat of the oven within the oven. However, it is not adapted for solar cookers because it reduces solar input power, by reflecting exterior solar radiation to the exterior of the oven.

Description of the invention

The main purpose of a solar box cooker is to heat a cooking pot placed inside it using only solar irradiation as power source. Sunlight is concentrated by several mirrors, passes through the window and heats the absorber tray and the pot inside the oven. Convection exchanges at the surface of the absorber tray and the pot brings the air inside the oven to a higher temperature, which decrease the thermal losses of the pot, and therefore allow the pot to reach a temperature suitable for cooking.

Heat is also radiated by the surfaces of the absorber tray and the pot and reradiated back to the oven by the window. Heat is also conducted through the different walls and the window of the oven. The greenhouse effect provoked by the window and, more generally, the global thermal performance of the window, is crucial for the performances of the oven. Indeed, in most solar box cookers designs, a significant part of the thermal losses are located through the window, which presents low thermal performances. As shown in Table 1, a single glazing generally has a heat transfer coefficient (U-value) between 4.5 and 5.7 W/( m 2 . K K) and a standard double glazing has a heat transfer coefficient around 3.3 W/( m 2 .K K).

Glazing with higher thermal performances has already been developed for buildings. As shown in Table 1, a double glazing where the interior surface of one of the single pane glass has a low- emissivity coating, can reach a U-value of 1.8 W/( m 2 .K K). Even higher thermal performances of the glazing can be obtained with other modern glazing technology features (Argon filling, triple glazing...) A comparative test between a solar box cooker with a standard double glazing and a solar cooker with a low-emissivity coated double glazing has been performed by the inventors. The two ovens had strictly identical designs, except for the glazing and where placed at the same time in the same configuration. The evolution of the temperatures of their absorber trays are displayed in Figure 1 that shows the evolution of the temperature of the absorber trays of a solar box cooker with a standard double glazing (solar cooker A) and a solar cooker equipped with low-emissivity coated double glazing (solar cooker B) . In this test, the inventors surprisingly found that the absorber tray of the solar cooker equipped with a low-e coated double glazing, heated 22% faster and reached a maximal temperature 20% higher.

These previous results reflect the performances gains that can be expected when the glazing of a solar box cooker is improved by adding a low-e coating layer in its composition, without changing its Furthermore, performance gains can be considerably improved if the low-e window(s) of the solar cooker is/are oriented along several directions. This higher window-to-wall surface ratio increases the surface of solar irradiation intercepted by the oven. Thus, a solar box cooker with a greater surface of window or with several windows can be designed. In the prior art box-type solar cookers that kind of consideration has the important drawback of significantly increasing the thermal losses. With a high thermal performance glazing, this optimization becomes relevant and notably improves the performances of the oven.

Therefore, the invention consists in a solar cooker with a core comprising at least one insulating wall and at least one window having a glazing on which is deposited a low-e coating, said window(s) being oriented along several directions.

For a cooker with a single window, the several directions can be obtained if the windows is curved, at least partially, to form e.g. a spherical portion.

Preferably, the low-e coating has a thermal emissivity below 0.2.

Advantageously the window-to-wall surface ratio is higher than 0.3 Other preferred embodiments of the invention are defined below in the description and in the dependent claims. The invention will be better understood with a detailed description illustrated by the following figures:

Figure 1 shows the evolution of the temperature of absorber trays of a solar box cooker with a standard double glazing (solar cooker A) and a solar cooker equipped with low-emissivity coated double glazing (solar cooker B).

Figure 2 shows an example of a solar cooker according to the invention

Figure 3 shows the solar cooker of figure 2 with different positions of the sun rays

Figure 4 shows a folding procedure of a solar cooker according to the invention

Figure 5 shows the effect of an anti-reflective coating simulated on a single pane glass. Figure 6 shows an assembly details of the edge between two windows which have a different orientation

Figure 7 shows an assembly details of the edge between two walls which have a different orientation Figure 8 represents the external shading systems of the solar cooker Figure 9 illustrates a removable Heat Storage Element (RHSE) Figure 10 shows some examples of solar cookers according to the invention

To illustrate the benefits of the invention, a simplified thermal computation has been carried out to compare several solar cookers with low and high thermal performance glazing and with different window to wall surface ratios (0.25 and 0.45). The results of this computation are displayed in Table 2.

The thermal characteristics of the walls and the windows of solar box cookers 1, 2 and 3 are similar to the thermal characteristics of most of the previous designs of solar box cookers. They have a standard single glazing (cooker 1) or a standard double glazing (cooker 2 and 3). Their insulation is equivalent to 40 mm of rock wool (cooker 1 and 3) or 50 mm of sheep wool (cooker 2).

Solar box cooker 4 and 5 have the same body has solar cooker 1 and 3. Their glazing has been changed to higher thermal performance glazing, with a low-emissivity coated double glazing for solar cooker 4 and a low-emissivity coated double glazing with Argon filling for solar cooker 5. The thermal performance gain between solar cookers 4 and 5 and solar cookers 1 and 3 reflects the gains obtained by upgrading the thermal performance of the glazing without changing the geometry or the insulation of the walls of the solar cookers. For example, by improving solar cooker 3, with a low- emissivity coated double glazing with Argon filling, one gets solar cooker 5, and a 32% gain in the temperature difference can be obtained.

Solar box cookers ranging from 1 to 10 have a window to wall surface ratio of 0.25. Solar box cookers 11 to 20 have respectively the same characteristics with a window to wall surface ratio increased to 0.45. Thus, the benefits of increasing the window to wall surface ratio of a solar box cooker can be quantified. Solar cooker 5 has the same wall composition as solar cooker 3, but a glazing with higher thermal performances. When the window to wall surface ratio of those solar cookers is increased from 0.25 to 0.45, a temperature difference gain of 39 % is obtained for the initial solar cooker 3 and a temperature difference gain of 73 % is obtained for the initial solar cooker 5. This gap in the performance gains between those solar cookers reflects the synergy between a higher thermal performance glazing and a design with a higher window to wall surface ratio. The idea of increasing the window to wall surface ratio is relevant because of the use of a high thermal performance glazing. With a higher window to wall surface ratio, the shape of a solar cooker is more favorable to the installation of multiple mirrors to improve the concentration of the sun irradiation because there are more windows and some of them have different orientations.

To go further, the thermal performances of the glazing of solar cooker 5 can be improved with a triple glazing, as in solar cookers 9 and 10 of Table 2. With high thermal performance glazing, it also becomes relevant to use high thermal performance insulation materials for the insulation of the walls, as in solar cookers 7, 8, 9 and 10 of Table 2. In previous designs of solar cookers, using high thermal performance insulation materials in the walls is not as appropriate because a more significant part of the thermal losses are located through the glazing. Therefore, it would result in lower gains than with a design of solar box cooker with high thermal performance glazing.

The shape of a solar cooker according to the invention is described in Figure 2 that contains the following features :

(1) Glazing; (2) Edge between windows; (3) Insulated walls; (4) Edge between insulated walls; (5) Mirror of the concentration system; θ 1 , θ 2 and θ 3 are the angles of the mirrors of the concentration system.

Concentration system

In a design of solar box cooker with high window to wall surface ratio, the back of the body of the oven can be a corner formed by three different walls. Thus, a wider range of orientations of the sun are covered by the windows in the front part of the solar cooker. Then, it is easier for a concentration system composed of multiple mirrors disposed to form some angles, to concentrate the irradiation of the sun on the different windows of the oven.

With such concentration system, a bigger surface of solar irradiation is intercepted and redirected to the oven than with a standard concentration system. The impact of this improved concentration system on the global performances of the oven can be quantified in Table 2. Solar cooker 6 is identical to solar cooker 5, with the exception of its concentration system which is improved. A 14% gain in the performances of the oven is obtained with a better concentration system. The same improvement, performed on the same solar cookers with a higher window to wall surface ratio (solar cookers 15 and 16), results in a 24% gain in the global performances. This comparison shows the relevance of the installation of an improved concentration system on a solar cooker with a high window to wall surface ratio.

Beyond the improvement of global cooking performances, this geometry with several window orientations and mirrors disposed in angle to redirect sunlight to those windows also help the user to track the sun.

Indeed, a baking period can last several hours, therefore the altitude angle and the azimuth angle of the change during the use of the oven. With this concentration system, a wider range of solar ray orientations are successfully redirected to the oven. As shown in Figure3, when the azimuth and altitude angles of the sun vary during the cooking period (positions a to d), the solar irradiation is redirected by different mirrors with a simple or a double reflection, to the oven.

The angles formed by the different mirrors are between 60 and 120 degrees and can be different around the oven. Those angles can be adapted in the different embodiments of the invention, depending on the targeted cooking period, latitude of use, climate of use and season of use. For example, with an angle close to 120 degrees between two mirrors redirecting solar irradiation to the superior glazing of the oven, a longer efficient baking time is provided because there will be a longer time (greater solar azimuth angle variation) between the situations (a) and (d) displayed on Figure 3. With an angle close to 60 degrees, between two of the lateral mirrors, the use of the oven is more efficient with a low altitude angle of the sun, like in winter or in places with high latitudes. Indeed, if a mirror is close to a vertical position with low sun rays, those sun rays can hit the mirrors approximately at normal incidence, and therefore be redirected towards their origin instead of the oven. Mirrors have to be oriented towards the floor, and other mirrors, in order to guide solar irradiation to the oven. Therefore, angles close to 60 degrees can be used between two mirrors.

That disposition of the concentration system brings the possibility to fold the mirrors on the core of the oven to form a shape easier to carry and store than the shape of the oven in use. The folding of the mirrors also protects the glazing and the reflecting surfaces of the mirrors against dust and exterior aggressions. It can also give, the oven another function, such as a chair or a storage box for example, and a more elegant design when it is not active. This operation is also crucial to decrease the exposure of the oven to the wind if it is stored outside. The folding process is detailed in Figure 4.

Windows characteristics The windows are made with high thermal performance glazing. Each window comprises two or more panes and at least one of their surfaces is coated and has an emissivity below 0.2. Several examples of high thermal performance window compositions are described in Table 1, along with their thermal characteristics. In the invention the windows have a U-value below 1.6 W/( m 2 . K K).

The windows are disposed in a way that aims at increasing the exposure to solar irradiation during the cooking period. Therefore, the windows can be curved or have several orientations. In a preferred embodiment of the invention (see e.g. figure 2), one lateral window is curved to cover two sides of the solar cooker and another one is flat and covers the top of the oven.

The windows can be made of polymer to decrease the weight of the oven, to improve the resistance of the windows against the impacts and to simplify the fabrication of curved or folded windows. The interior pane can be made of glass for better resistance at high temperature and the other panes made of polymer.

It is crucial for the performances of the oven to maximize the solar transmittance of the windows. With a higher solar transmittance, more solar irradiation power passes through the windows to heat the absorber tray and the cooking pot. This results in a higher cooking power for the solar cooker. To improve the transmittance of the windows, clear glass with low iron content or polymer with high transparency can be chosen and some of the surfaces of the window panes are coated with an antireflective layer. An anti-reflective (AR) coating on a single pane window can improves the global optical transmittance of a window and therefore its solar transmittance. In Figure 5, a standard anti reflective coating (with n=1.28 and thickness = 100 nm) applied on a single side and both sides of a single pane window results in respectively 4% and 8% gains for the optical transmittance at 555nm, which is the most intense wavelength of solar irradiation, for angles of incidence ranging from 0 to 60 degrees. This result is obtained with a simulation performed with TF calc.

The application of an anti-reflective coating improves solar transmittance for a large set of angles of incidence. As shown in Figure 3, in a design of solar cooker with a high window to wall surface ratio, several window orientations and no manual tracking required during the cooking period, solar irradiation hits the windows with multiple angle of incidence. Therefore, the use of an anti-reflective coating can have a very positive effect. The anti-reflective coating of the invention is chosen in a way that aim at having: Tsol(0°) - Tsol(60°) < 15% for at least one of the single pane. In most of the previous designs of solar cookers, the joint around the window acts as a thermal bridge that decreases the thermal performances of the oven. To avoid the formation of major thermal bridges at the edges between the different windows of the invention, several assembly designs are proposed and described in Figure 6. The different windows can be juxtaposed on one another (b) without any frame or the different panes of the windows can be folded or curved to form the next window (c, d and e). The solution (c), (d) and (e) result in a lower thermal bridge than the solutions (a) and (b). They also cause less shadowing, which improve the global solar input power into the solar cooker.

Insulated walls

With high thermal performance glazing, it is appropriate to use high thermal performance insulation materials for the insulation of the walls of the solar cooker. Indeed, in previous designs of solar cookers with standard glazing, a more significant part of the thermal losses is located through the glazing. Improving the thermal performances of the walls is therefore less relevant than in a design of solar cooker with high thermal performance glazing. The thermal characteristics of several insulation materials are displayed in the Table 3.

In that table, one can see that in order to obtain approximately the same U-value as a 25 mm vacuum insulation panel, 165 mm of rock wool is needed. Vacuum insulation and aerogel are high performance insulation technics that are not yet developed enough to be adopted in the buildings. However, they can be appropriate for the smaller scale of the solar cookers.

The walls of the invention are insulated with high thermal performance materials, such as aerogel or vacuum insulation. At least one of the walls has a U-value below 0.8 W/( m 2 .K K). In Table 2, solar cookers 7, 8, 9 and 10 have walls insulated with vacuum insulation. Solar cooker 7 and 8 have the same characteristics as solar cooker 6, with the exception of the insulation which is a standard insulation of 40 mm of rock wool for solar cooker 6 and 15 mm or 25 mm of vacuum insulation for solar cookers 7 and 8, respectively. The gains in the thermal performances for those solar cookers are respectively 56% and 100%.

Solar cooker 9 and 10 are the same solar cookers with a triple glazing, with argon filling and low- emissivity coating on two surfaces. One can see that with solar cookers 19 and 20, equivalent to solar cookers 9 and 10 with a high window to wall surface ratio, temperature difference between cooking temperature and ambient temperature of 245 and 296 degrees can be reached. With those solar cookers it is even possible to cook during a cloudy day, when solar irradiation is two times lower than during a sunny day as a temperature difference of 122 and 148 degrees can be reached.

To avoid the formation of major thermal bridges at the edges between the different insulated walls of the invention, several assembly designs are proposed and described in the Figure 7. The insulation can be realized with panels (a). It can also be a multilayered assembly of several panels where the joints of the different layers are not aligned to avoid the formation of a straight thermal bridge (b). The insulation can also have a more complex shape than panels. It can be continuous with a curved or angular design (c). The insulation can be sealed with an external protective layer that covers the whole oven or only the edges (d). This external protective layer can also be used with a multilayered insulation.

In the different embodiments of the invention, the interior surfaces of the walls can comprise one or more interior reflectors that redirect solar irradiation to the pot. They can also comprise an absorber tray covered with a high thermal conductive layer that diffuses heat in the several inner surfaces of the solar cooker, and a second layer with a high solar absorbance that store heat coming from solar irradiation. The absorber can comprise fins to enhance the thermal exchanges between the absorber and the air inside the oven.

A door is located in the walls of the solar cooker to access the cooking pot.

In the different embodiments of the invention, the solar cooker can comprise temperature sensor and a data acquisition unit. This unit can comprise a micro camera filming the cooking pot. This unit can have a cable or wireless connection to a data analysis unit or the smartphone of the user. This unit can be linked to an application on the smartphone of the user that may help the user to set up the orientation of the solar cooker and of its mirrors at the initiation of the cooking period, display information such as cooking temperature, cooking pot visualization or estimated remaining cooking time and warn the user at the end of the cooking time. This unit can comprise an electronic connector, for example an USB port, which can be used to connect or charge a mobile phone or other electronic device. This whole electronic system is powered by a solar photovoltaic panel, a battery or a thermoelectric generator which uses the heat of the cooking chamber of the oven to produce electricity.

This unit can be linked to an automatic window shade to control the cooking temperature of the solar cooker by adapting the aperture of the external shadings in order to stay at the desired cooking temperature. The shading system is composed of external shadings placed in front of the windows and which can be rolled down or folded into the walls of the solar cooker. This system is described in Figure 8. It can be composed of lateral shadings only (a), roller blinds on the lateral and superior surfaces (b), a single shading on the superior surface that is folded and unfolded the same way as a fan (c) or a combination of a shading fan on the superior glazed surface and roller blinds on the side windows (d)

A removable heat storage element adapted to the shape of the oven can be placed inside the solar cooker in direct contact with the absorber tray. Thus, the removable heat storage element is heated inside the solar cooker. This element comprises a specific material with a thermal inertia superior to 0.25 kJ/K. It can be a cooking stone, a concrete plate or a close recipient which contains a phase change material. It has sufficient thermal inertia to store enough energy to grill meat outside the oven. This cooking stone can also be used to store heat and be able to cook at night with the solar cooker. Depending on the desired thermal charging time and the quantity of food to cook using the thermal inertia of the removable heat storage element, elements with different thermal inertia can be used in the oven, or several elements can be assembled to form an element with a higher thermal inertia.

The removable heat storage element can be placed and taken out of the oven with a removable handle. The element can be placed on an appropriate tray which protect the table from the heat of the cooking stone and can comprise one or several emplacements to dispose meat, side food, condiments or cooking utensils. The emplacement of the removable heat storage element on the tray can comprise an upper layer with low thermal conductivity or several minimalist supports and a specific layer with high thermal reflectivity in order to reduce heat losses from the heat storage element to the tray and to improve the protection of the table.

An embodiment of the removable heat storage element (RHSE), the removable handle and the protective tray is shown in Figure 9.

More precisely figure 9 (a) shows RHSE placed in the solar cooker for heating; figure 9 (b) shows a RHSE with more thermal inertia and longer cooking time place in the solar cooker for heating; figure 9(c) illustrates the placement of the RHSE in the solar cooker with a removable handle; figure 9 (d) shows the displacement of the RHSE with the removable handle; and figure 9 (e) shows a RHSE placed on an adapted tray to protect the table during cooking.

Examples

The shapes of some examples of embodiments of the invention are described in Figure 10.

Embodiment (a) is the preferred embodiment, composed of one lateral curved glazing and one superior plane glazing. Embodiment (b) has a shape close to a cube or a rectangular parallepiped. It has 3 different plane windows. Embodiment (c) has one single curved glazing, with two axis of curvature. Embodiment (d) has 3 curved windows. The windows of embodiments (e), (f) and (g) have shapes close to polyhedrons. Embodiment (h) has a shape close to a distorted parallepiped with tilted windows.

As well as for the concentration system, the general geometry and composition of the invention are adapted according to the targeted cooking period, latitude of use, climate of use and season of use. Indeed, the course of the sun and the external temperature and wind condition are not the same when those different conditions vary. Thus, the positioning of the windows and the geometry can be changed to match the course of the sun. The general shape, number of edges and composition of the walls can be adapted to optimize the performances of the solar cooker according to the external conditions.

References

[1] Patent application ZA 2000/6680

[2] N. M. Nahar, Design, development and testing of a double reflector hot box solar cooker with transparent insulated materials, Energy Conversion Management (2001) [3] S.S. Gosh, P.K. Biswas, S.Neogi, Thermal performance of solar cooker with special cover glass of low-e antinomy doped indium oxide (IAO) coating, Applied Thermal Engineering (2016)

[4] S. Mahavar, P. Rajawat, V.K. Marwal, R.C. Punia, P. Dashora, Modeling and on-field testing of a Solar Rice Cooker, Energy (2012)

[5] Patent application US 2010/0139648 [6] Patent US 5 195 504

[7] Utility model CN 204363800

[8] Building Physics I & II, J.-L. Scartezzini, Solar Energy and Building Physics Laboratory, Ecole Polytechnique Federale de Lausanne (September 2010)

[9] Patent application US 2016/0145151