Solar oven with heat storage usable independently of variations in direct sunlight

Abstract

Solar oven regulated in temperature throughout the year. This regulation is done according to the following process: Solar energy is captured using photovoltaic panels when the sun is present This electrical energy is transformed into heat using electrical resistors and heats the sand We can then use this heat stored in the sand on days when there is no sun The oven is presented as a rectangular parallelepiped in masonry the interior is entirely lined with a thick layer of heat-resistant insulation. Inside the insulation are placed electrical resistors powered by solar panels and the oven itself in which the food will be cooked. The latter is a simple metal box with a well-sealed door. The whole thing is drowned in sand.The device makes it possible to compensate for variations in the availability of solar energy and to ensure that a temperature of around 280°C is maintained, even in the absence of direct sunlight, and therefore, according to theoretical calculations, to allow the device to be viable even in winter and even in the north of France. Figure for the abstract: [Fig. 1].

Description

Title of the invention: Solar oven with heat storage usable independently of variations in direct sunlight

[0001] The invention consists of a solar oven with temperature regulation throughout the year.

[0002] This regulation is carried out according to the following process: - Solar energy is captured using photovoltaic panels (1) when the sun is present - This electrical energy is transformed into heat using electrical resistors (6) and heats the sand - We can therefore use this heat stored in the sand on days when there is no sun.

[0003] The oven is a rectangular parallelepiped (see [Fig.l]) made of masonry (2) and the interior is entirely lined with a thick layer of heat-resistant insulation (3). Inside the insulation are placed electrical resistors powered by solar panels and the oven itself in which the food will be cooked. The latter is a simple metal box with a well-sealed door. The whole thing is submerged in sand.

[0004] The device makes it possible to compensate for variations in the availability of solar energy and to ensure that a temperature of approximately 280°C is maintained, even in the absence of direct sunlight and therefore, according to theoretical calculations, to allow the device to be viable even in winter and even in the north of France.

[0005] Operation: - Capturing solar energy: Photovoltaic panels (1) capture the energy radiated by the sun and transform it into thermal energy using electrical resistors (6). - Storage in the sand tank: The electrical resistors (6) are buried in a semi-buried and insulated tank, filled with sand (7). Note that this sand does not need to have "construction" qualities: desert sand, which is very abundant on the planet but unsuitable for construction, is more than sufficient. Sand, having a high thermal capacity, acts as an effective storage medium to store the heat captured by the solar panels when there is sun and release it when needed. - Tank insulation: The sand tank is surrounded by high-performance thermal insulation (3), minimizing heat loss and maintaining a constant temperature inside the device. Rock wool, which is a very widespread, relatively inexpensive insulator and can withstand high temperatures is perfectly suitable, but nothing prevents the use of an even more efficient insulation. A further study will have to determine whether rock wool will be the most appropriate insulation or whether another insulation should be preferred. In this case, the calculations relating to the thermal balance of the furnace will take into account the insulating capacities of rock wool. - Heat restitution: The oven itself is a simple stainless steel box (4) fitted with fins (5), also fitted with a well-sealed door, the whole thing is submerged in sand, it therefore stabilizes at the temperature of the sand and only consumes heat when the door is opened (negligible) and when food is heated (also negligible).

[0006] Thermal equilibrium calculations:

[0007] Below we will show a simulation of the operation of the device in the north of France (Paris) and then in the south of France (Marseille). For this purpose we will use statistical data of monthly sunshine hours for each of these two regions. As we will see, if the efficiency of the device is particularly interesting in the south of France, it nevertheless remains interesting in the north, despite a lower efficiency which does not, however, prevent acceptable operation of the oven even in winter.

[0008] Calculation of losses through the walls.

[0009] The general formula for calculating the heat flow through a wall using thermal conductivity is given by Fourier's law:

[0010] Q = A * (T1-T2) * (k / d)

[0011] where: • Q is the heat flux in watts (W). • k is the thermal conductivity of the wall material in watts per meter*kelvin (W / mK). • A is the surface area of ​​the wall in square meters (m2). • Tl is the temperature of the hot side of the wall in kelvins (K). • T2 is the temperature of the cold side of the wall in kelvins (K). • d is the wall thickness in meters (m).

[0012] Calculation of the temperature increase of the sand.

[0013] The formula that allows us to calculate the heating of a mass of material (such as sand) as a function of the amount of energy received is given by the temperature change formula:

[0014] Q = m * c * AT

[0015] where: Q is the amount of energy in joules (J) or watt-hours (Wh). • m is the mass of the material in kilograms (kg). • c is the specific heat capacity of the material in joules per kilogram*kelvin (J / kg-K). • AT is the temperature change in kelvins (K).

[0016] The calculations of the thermal equilibrium of the furnace will be made successively for Paris (north of France) and for Marseille (south of France). They will use the following known constants:

[0017] Specific heat of sand: 830 J.kg-lK-1

[0018] or 0.23055581 Wh.kg-lK-1

[0019] or 0.23055581 kWh.Tl.K-1

[0020] Number of hours per month 720.00 h / month

[0021] Thermal conductivity of rock wool 0.042 Wm- 1k-1

[0022] Density of sand 1600 Kgm-3

[0023] Oven dimensions

[0024] Width 0.5 m

[0025] Depth 0.5 m

[0026] Height 1 m

[0027] Rock wool thickness 0.3 m

[0028] Masonry thickness 0.15 m

[0029] Total width 1.40 m

[0030] Total depth 1.40 m

[0031] Total height 1.90 m

[0032] Volume of sand 0.25 m3

[0033] Mass of sand 400.00 kg

[0034] Rock wool surface 2.50 m2

[0035] SOLAR PANEL DIMENSION FOR PARIS

[0036] Panel surface 5 m2

[0037] Panel power 250 wc per m2

[0038] Total power of the panels 1250.00 wc

[0039] Efficiency at the level of electrical resistance 75%

[0040] Oven temperature setting 280°C

[0041] (A) (B) (C) (D) (E) (F) (G) (H) (I)

[0042] January: approximately 85 hours 85 79688 wh -1 98 70812 wh 8876

[0043] February: approximately 92 hours 92 86250 wh -2 99 71064 wh 15186

[0044] Mars: approximately 145 hours 145 135938 wh 6 96 69048 wh 66890

[0045] April: approximately 183 hours 183 171563 wh 8 95 68544 wh 103019

[0046] May: approximately 206 hours 206 193125 wh 16 92 66528 wh 126597

[0047] June: approximately 215 hours 215 201563 wh 22 90 65016 wh 136547

[0048] July: approximately 240 hours 240 225000 wh 26 89 64008 wh 160992

[0049] August: approximately 226 hours 226 211875 wh 28 88 63504 wh 148371

[0050] September: approximately 176 hours 176 165000 wh 25 89 64260 wh 100740

[0051] October: approximately 120 hours 120 112500 wh 17 92 66276 wh 46224

[0052] November: about 72 hours 72 67500 wh 10 95 68040 wh -540

[0053] December: about 70 hours 70 65625 wh 4 97 69552 wh -3927

[0054] SOLAR PANEL DIMENSION FOR MARSEILLE

[0055] panel surface 3 m2

[0056] power of the panels 250 wc per m2

[0057] total power of the panels 750.00 wc

[0058] efficiency at the level of electrical resistance 75%

[0059] oven temperature setpoint 280°C

[0060] (A) (B) (C) (D) (E) (F) (G) (H) (I)

[0061] January: approximately 138 hours 138 77625 wh 2 97 70056 wh 7569

[0062] February: approximately 138 hours 140 78750 wh 2 97 70056 wh 8694

[0063] Mars: approximately 186 hours 186 104625 wh 8 95 68544 wh 36081

[0064] April: approximately 228 hours 228 128250 wh 10 95 68040 wh 60210

[0065] May: approximately 276 hours 276 155250 wh 18 92 66024 wh 89226

[0066] June: approximately 313 hours 313 176063 wh 24 90 64512 wh 111551

[0067] July: approximately 359 hours 359 201938 wh 28 88 63504 wh 138434

[0068] August: approximately 340 hours 340 191250 wh 30 88 63000 wh 128250

[0069] September: approximately 267 hours 267 150188 wh 28 88 63504 wh 86684

[0070] October: approximately 210 hours 210 118125 wh 19 91 65772 wh 52353

[0071] November: approximately 149 hours 149 83813 wh 12 94 67536 wh 16277

[0072] December: approximately 124 hours 124 69750 wh 6 96 69048 wh 702

[0073] (A) (B) Monthly hours of sunshine (weather statistics data)

[0074] (C) Total monthly energy supplied by solar panels in wh

[0075] (E) Average ambient temperature in °C (statistical data)

[0076] (F) Loss flow in w (calculation formula above)

[0077] (G) Total monthly energy lost through the furnace walls in wh

[0078] (I) balance

[0079] The two tables above show that once the temperature is reached (280°C) the heat input from the solar panels is more than sufficient to balance the heat losses

[0080] The oven can be heated either by a temporary supply of external heat or by leaving the oven to heat for several months if it is started in the middle of winter or in just one month if it is started in summer.

[0081] Apparently, there is a problem in the months of November and December in the Paris region (in the north of France) due to the lack of sunshine. This can be remedied to obtain an oven that is operational all year round either by increasing the surface area of ​​the solar panels or by further increasing the insulation to obtain greater thermal inertia.

[0082] Note however that even with the current configuration the furnace takes almost a month to cool completely, which means that we can overcome the lack of sunshine in the north of France by relying on the thermal inertia of the furnace.

Claims

[Claim 1] Claims Solar oven with heat storage characterized in that it comprises a stainless steel box of rectangular parallelepiped shape (4) provided with fins (5) and a sealed door, the whole enclosed in a masonry (2) whose interior is entirely lined with a thick layer of heat-resistant insulation (3). The oven comprises photovoltaic panels (1) which capture the energy radiated by the sun and transform it into thermal energy using electrical resistors (6). Said electrical resistors (6) are buried in a semi-buried tank surrounded by thermal insulation, filled with sand acting as a heat storage medium (7).