GB2484353A

GB2484353A - Solar oven

Info

Publication number: GB2484353A
Application number: GB1018939.7A
Authority: GB
Inventors: Kevin Wheatley
Original assignee: SOLORNO Ltd
Current assignee: SOLORNO Ltd
Priority date: 2010-09-06
Filing date: 2010-11-09
Publication date: 2012-04-11
Anticipated expiration: 2030-11-09
Also published as: GB2484353B; GB2482205A; GB201018939D0; WO2012032330A3; GB2484353A8; GB201014717D0; WO2012032330A2; GB2482205B

Abstract

A solar oven for cooking purposes comprises an oven body covered in reflective material, a cooking surface, a lid, a heat storage unit and a circulatory fan, where the lid is shaped with a mixture of concave and convex portions. Preferably, the heat storage unit comprises a chamber filled with an ionic liquid, or a chamber filled with a gaseous or liquid hydrocarbon. Alternatively, the heat storage unit may comprise a solid heat absorbent material. The heat storage unit may form the surface upon which cooking takes place. Preferably, the temperature within the oven is homogenised by use of the circulatory fan. The oven may be tiltable upon its base to be positioned in optimum sunlight depending on the position of the sun and the terrain in which the oven is placed. Furthermore, the lid may also be tiltable upon the oven to be positioned in optimum sunlight. Preferably, the cooking surface is either part of the reflective surface or comprises a raised grill situated above the base of the oven.

Description

A solaroven or solar cooker is a device which uses sunlight as its energy source. Becanse they use no fuel andthey cost nothing to run, they may be used worldwide since they reduce deforestation and desertification which is caused by burning wood during cooking.

It is usually necessary to use them only during daylight hours. and in cEimates where there is sufficient solar energy to cook food.

There area vailety. oftypes ofsolar cookers, their basic principles of all solar cookers are as f6flows: S Concentrating simlight: Some' device, usually a: mirror orsorne type of reflective metal, is used to concentrate light and heat from the sun into a small cooking area making the energy more conuenirated and therefore more potent 4 Converting light to heat: Any black on the in sideof a solar cooker, as veli as certain materials for pots, Will improve th effectiveness ofturning light into heat A black t pan ill absorb almost all of the suns light and turn It into heat substantially F improving the effectiveness ofthe cooker. Also, the better a pan conducts heat, the F faster the oven will work.

F & Trapping heat: Isolating the air inside the cooker from the air outside the cooker makes an important difference. Using a clear solid, like a plastic bag or a glass covei; will allow light to entei, but once the light is absoibed and converted to heat, a plastic bag or glass cover will trap the heat inside. This makes it possible to reach similar temperatures on cold and windy days as Ofl: hot days, Plastic Sheet Uses plastic sheets to assure that liquids do not seep through into the oven. Also to prevent staining of the underlying sheet inthe oven.

Alone, each. of these strategies for cooking something using solar energy is fairly: ineffective, but most solar cookers use two or all thice of these strategies in combination to get temperatures sufficient for cooking.

The top can usually be removed to allow dark pots containing food to be placed inside. One or more reflectors of shiny metal or foil-lined material may be positioned to bounce extra light into the intenoi of the oven chamber Cooking containers and the inside bottom of the eookei should be dark coloured or black Inside salls should be iefiective to reduce heat loss S ia radiation and to bounce the light towards the pots and the drk bottom, shich is in contact with the pots.

Until now, the use of optimally shaped hoods for th.e cooker and, optional heat collectors and circulatory fans have not been eniployed Flowevcr these further improve the efficiency of solar ovens such that cooking tempei atures increase compared to pioduets currently on the market or that the solar oven can be used in climates previously thought to be too low to cook food by harnessing solar power.

The solar box cooker typically reaches a temperature of I 0 °C (300 df) This is not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time Food Lontaining a lot of moisture cannot get much hottet than 100 °C (212 °F) in any case so it is not always necessary to cook at the high temperatures indicated ha standard cookbooks.

Because the food does not reich too high a temperaturc it can be safely left in the cooker all day without burning It is best to start cooking before noon, though Depending on the latitude and weather,, food can be cooked either early or later in the day. The cooker can be r 2 used to warm food and drinks and can also be used to pasteurise water or milk If you use an indoor stove fOr your actual cooking, s on can save sigmfieant fuel by using the solar cooker to preheat the water to be used for cooking grains, soups, etc., to nearly boiling.

There are several types of' solar cookers: 1) Panel solar cookers are very inexpensive solar cookers that use shiny panels to direct sunlight to a Looking pot that is enclosed in a clear plastic bag A common modd is the cookit Developed in I 994 by Solar Cookers International, it is often produced locally by pasting a reflective material, such as aluminium foil onto a cut and folded backing. usually cardboard corrugate It is lightweight and folds foi storage When compIetel unfolded, it measures about three feet by four feet (1 m by I 3 m) The CooKit is considered a lo-to-modcrate temperature solar cooker, easily reaching temperatures high enough to pasteurize water or cook grains such as ric& On asunny day, one CooKit can collect enough solarenergy to cook rice5 meat or vegetables.

2) The HotPot cooking vessel consists ofa dark pot suspended inside a clear pot with a lid To use a panel cooker, it is folded into a howl shape. Food is placed in a dark coloured pot, covered with a tightly fated lid. The pot is placed in a clear plastic bag and tied, clipped, or folded shut The panel cooker is placed in direct sunlight until the food is cooked, which usually requires several hours for a full family-sized.meal. For faster cooking, the pot can be raised on sticks or wires to allow the heated air to circulate underneath it High-temperature plastic bags (oven roasting bags) can be re-used for more than a month but any plastic bag will x ork, if measuies (such as sticks or wires) are taken to keep the bag from touching the hot cooking pot and melting to it The purpose of the plastic bag is to flap heated air next to the pot, it may not be needed on sery bright, windless: days.

A rccent dcx elopment is the HotPot developed by US NGO Solar Household Energy Inc I he cookmg vessel in this cooker is a large clear pot with a clear lid into which a dark pot is suspended This design has the advantage of very even heating since the sun is able to shine onto the sides and the hottom of the pot during cooking. An added advantage is that the clear lid allows the food to be observed while it is cooking without remox ing the lid 1 he HotPot provides an alternative to usrng plastic bags in a panel cooker.

3) Solar kettles are solar thermal devices that can heat water to boiling point through the ieliance on solar eneigy alone Some of them use exacuated solai gass tube technolog) to capture. accumulate and store solar energy needed to power the kettle Besides heating liquids, since the stagnating temperature of solar vacuum glass tubes is a high 220 °C @25 °F), solar kettles can also deliver dry heat and function as ovens and autoclaves Moreover since solar vacuum glass tubes work on accumulated rather than concentrated solar thermal energy, solar kettles only need diffused sunlight to work and needs no sun tracking at all If sola' kettles use solar vacuum tubes technologies,, the vacuum insulating properties. will keep previously heated water hot throughout the night 4) Scheffler cooker The reflector has an area of 16 m2, and produces 3 kW of heat Although these types of solar cookers can: cook. as well as a conventional oven, they are difficult to construct. Parabolic cookers reach high temperatures and cook quickly, hut require frequent.adjustrnent and superVision for safe operation. Several hundred L 3 F thousand exist, mainly in China. They are especially useftil for targescale F institutional cooking.

5) Parabolic reflectors that have their centres olmass coincident with their thea! points are useful. They can be easily turned, to follow the sun's motions in the sky, rotating about an axis that passes through the focus. The cooking pot therefore stays stationary Tfthe paiaboloid is axially symmctncai and is made ofmaterial of uniform thickness, this condition occurs ifthe depth ot the paraboloid is I 8478 times its focal length..

It is possible to use two parabolic troughs, curved in perpendiuular directions to bring suiffight to a point fous as does a paraholoidal iefleetor The incoming light stiikes one oftbe troughs, which sends it toward a line focus The second trough intercepts the converging light and focuses it to a point A Compaied with a smgle paraboloid. using tsco partial tioughs has important advantages The troughs are "single curvest, which can be made by bending a sheet of metal without an need for cutting crumpling or stretching Also, the light that reaches the target -the cooking pot -is duected approximately do\sn aid, which reduces the danger ofdamageto the eyes ofanyone nearby. On the other.. hand, there are disadvantages. More mirror material is needed, increasing the cost and the light is reflected by o sirfaees instead ofone chich inevitably increases the amount that is lost.

Experimental arrangements of this kind have been. made, and have worked well. The two troughs have been held m a fixed orientation relatne to each other by being both fixed to a wooden flame The whole assembly of frame and troughs has to be moved to track the sun as it moves in the sky 6) The Solar Bowl is a unique concentrating technology used by the Solai Kitchen in India Unlike neatly all concentrating technologies that use tracking reflector systems, the solar bowl uses a stationary spherical reflector This reflector focuses light along a line perpendicular to the sphere's swface and a computer control stcm moves the receiver to intersect this line Steam is produced in the solai bowl's receiver at temperatures reaching 150 1 and then used for process heat in the kitchen where 2,000 meals are prepared daily.

7' A hybrid solar oven is a. solar box cooker equipped with a conventional. electrical heating element for cloudy days or night time cooking Hybrid solar ovens are therefore more independent However, they lack the cost advantages ot some other types of solar cookers, and so they have not caught on as much in third world countries where electricity or fuel sources simply do not exist.

A hybrid solar grill consists of an adjustable parabolic ieflectoi suspended in a tripod with a movable grill sui face I hese outpertonn so'ar box cookers in temperature range and co6king times. When solar energy is. not available,, the design uses any conventional fuel, as a heat source, including gas.,, electricity, or wood.

A review of known prior art is as follows:

FR2787867 discloses the use of"a supple mirror made especially from a plastic with a ieflective or metallised surface in the form of an umbrella vsith ribs that allow it to be deployed for use or"folded for transport".

CN2366794 discloses "a light condensing oven which' . can automatically track the sun. The utility model is composed of a bracket, a monitor, a main shaft, a water supply pipe, a gas

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outlet pipe, a light condensing cover, a self-ignition water tank, a valve, etc., wherein the monitor is arranged on the bracket and a power shaft of the monitor is connected with the main shaft, The power shaft of the monitor is connected with a spring of a clock, a gear wheel of the monitor is engaged with a gear wheel of a.display and a dial plate of the clock is arranged on the. display. While in use, as long as. the light, condensing oven is dialed every morning to face to the sun, the spring can be automatically and tightly wound and the light condensing oven can automatically track the sun: after the spring is released. Therefore, water in the self-ignition water tank can be heated to preduce water vapor which is used daily for cooking, showering, heating, dc" cNi200467 discloses a multi-purpose heat-storing solar furnace consists of two portions. the fonner is day lighting and I ght transmission means and the latter is the furnace equipment It is characterized by that the sunlight focussed by collector lens is fed into optical cable through sunlight funnel, and the orientation of the sunlight is changed by optical cable, the output plug of terminal end of optical cable is inserted into the input socket of the furnace equipment so as to attain the goal of releasing heat or storing heat and storing energy. The output plug of terminal, end of optical cable ean be used'as cutting and. welding machine, and also can be matched with furnace equipment, for using as roasting plant, hot-water stove, steam oven or smelting furnace. When said invented product is used as steam boiler or smelting furnace having need of large powçr, several dayl.ighting and light transmission devices can be used simultaneously to input high-density large-flux daylight beams into one furnace equipment.

None of these documents disclose the improvements in sun oven, technology that we have found.

Whilst it is widely accepted that sun ovens should:have reflective sheeting on its inner surface and have insulated walls and base, we have found that solar oven efficiency can be significantly improved by use of specific lid designs, by using heat storage chambers and by minimisirig heat loss through the place where most loss occurs --the lid. Furthermore, a circulatory fan helps to achieve consistent cooking performance.

Lids of known designs are either a flat cpver (in line with the top of the walls of the oven) or at best a domed shape. However neither of these optimise the effieicny of the oven, The lid preferably is configured into a predetermined shape incorporating a concave and / or convex portion or multiple concave and/or convex portions formed. from.a substantially transparent material. This enables the collector to be of any shape, i.e. domed, hexagonal (honeycombed),. rectangular, "golf ball", parabolic. Wavy, patterned, flat, oval, with different heights to fit different permutations, i.e. if space is limited where the device is fitted or to maximise the absorption Or retention ofthe solar radiation:.. A mixture of concave and convex shapes has been found to offer niaxinium heat retention. The cover may itself be coated in a paint spray or film which improves solar energy adsorption and/or helps prevent reat from radiating outwards and/or with self cleaning properties.

Mininiising heat loss through the lid can be achieved in a number of ways. The material' used, its thickness are the main routes, but applying reflective films or sprays can enhance heat retention within the oven or reduce loss by radiation from the oven. Furthermore. the lid can be double' or treble walled or a cavity within the lid may be filled with an essentially

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transparent solid, liquid or may be a vacuum. All of these routes typically reduce lqss of heat through the lid.

eferahly, the sun, oven fUher comprising a tilting mechanism for tilting said device about an axis. This exposes as much of'the ovens inner surface to direct simlight, without the use of reflectors, to create heat within the oven. The unit itself could be angled or the lid could be angled/preferably shaped, or the tubing in relation to the dome could he angled for niaximwn performance.

No sun ovens presently use heat retention units to store heat. If there are clouds present, or if day is turning to dusk then traditional sun ovens, will coot down or cease to be used.. By employing a heat storage unit, the sun oven can use used. despite passing clouds or for a period of time. after dusk. The energy collector is preferably retained in close proximity to the cooking area. Its purpose is to heat up and store heat when exposed to thermal energy'. In one embodiment the energy collector may be the surface upon which cooking occurs..

Alternatively, for optimal design within some sun ovens, there may be several discrete solar connectors within.one oven which may either be connected or be separated. to one another.

Energy collectors may also be chambers filled with gaseous material. Energy collectors may alternatively be solid materials which absorb and store heat, Such soUd material may be metals, volcanic rocks, stone, ceramic or the like.

Optionally a fan i.s attached to the body of the sun oven. This may itself be powered by wind energy, solar PY energy, batteries, electrical mains or other means. Use of a fan will help to evenly distribute thermal energy throughout the oven and help achieved consistent cooking performance from one part. of the oven to another.