Abstract:
A solar cooker in the form of a pot with double-walled body and lid configured to collectively constitute a near-hermetic thermal enclosure including a closed, solar radiation-absorbing and heat-retaining vessel surrounded by a vacuum insulation shield encased in a transparent peripheral jacket. The device intercepts solar energy omnidirectionally, and collects and retains it with sufficient efficiency to pasteurize water and cook food without resorting to outside reflectors, refractors, conductors, or insulators, thereby escaping from the dual requirements of solar concentration and orientation that universally govern the construction and operation of existing solar cookers. Freedom from these restrictions enables designs that are inexpensive, simple, compact, lightweight, sturdy, accessible, and applicable in varied geographic, climatic, economic, social, and culinary contexts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional application Ser. No. 61/001,607 filed Nov. 1, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    This invention relates to cooking devices, and more specifically to solar cookers. 
         [0004]    2. Description of Prior Art 
         [0005]    Solar cookers come in a myriad of designs from different historical eras and various geographic parts of the world, and proponents of solar cooking have devised schemes for classifying the many models and their variants, as discussed on the website, solarcooking.wikia.com/wiki/compendium_of_solar_cooker_designs. At the most basic level, among solar cooking devices in common use, one can distinguish three types: the solar oven, the solar panel cooker, and the solar stove. The term “solar cooker” is used to refer to any type of solar cooking device, including the above and less common indirect cookers. 
         [0006]    In a solar oven, direct and, optionally, reflected solar radiation penetrates the transparent part, or glazed aperture, of an insulated enclosure and is converted to heat at the dark-coated interior walls of the enclosure. The heat is trapped within the cavity and is transmitted to one or more cookpots placed therein, mostly by convection. Alternatively, the interior walls of the oven are reflective and the cookpot is dark-coated, therefore heat is evolved at the wall of the pot. 
         [0007]    A commonly used solar oven is the Sportoven, (www.solarovens.org), shaped to rest in two different positions, presenting its glazed opening at a choice of two angles to accommodate summer and winter solar altitudes. Another is the Sunoven (www.sunoven.com) with built-in external reflectors for increased performance, and adjustable legs for angle setting. 
         [0008]    In a solar panel cooker, direct and reflected solar radiation penetrates a transparent enclosure surrounding a dark-coated cookpot and is converted to heat at the wall of the pot. The essential difference between the solar oven and the solar panel cooker is that, in the former, only the sun-facing part of the enclosure is transparent while the remaining sides are insulated, minimizing conductive and radiative heat loss. The oven has the advantage of retaining more heat, attaining higher cooking temperatures, and functioning in harsher weather conditions, and the disadvantage of being bulkier and more cumbersome to transport. 
         [0009]    A widely distributed solar panel cooker is the Cookit (www.solarcookers.org), which uses a clear plastic oven bag for enclosure. Another is the HotPot (www.she-inc.org), which uses a borosilicate glass container to enclose the black cookpot. The solar panel cooker with a foldable reflector system has the advantage of smaller size and weight, and better portability than a solar oven, and the disadvantage of lower cooking performance and more susceptibility to ambient conditions such as cold and wind. Inexpensive reflectors made of aluminum foil laminated over corrugated cardboard have poor durability, while better units made of anodized aluminum sheets are out of the financial reach of many prospective users. 
         [0010]    In a solar stove, focused reflected or refracted solar radiation is converted to intense heat on a small part of the wall of a dark-coated cookpot. Alternatively, the energy may be focused on a plate on which a cookpot is placed, in which case heat conversion is done at the plate. Direct solar radiation plays a lesser part in the cooking process. The essential difference between the stove and the above two designs is that the stove uses focused light and no enclosure around the pot or pan. The advantages are that the cook has unimpeded access to the food, and the localized heat delivers elevated temperatures suitable for frying. The disadvantages are that a focusing system is more complex in construction, requires frequent repositioning in operation, and presents the health hazards of skin burn and eye damage. 
         [0011]    Naturally, there are solar cooker designs that do not fall neatly under one classification or another, but rather possess a mixture of the characteristics of the basic types in different proportions. Nevertheless, all solar cookers in common use share two disadvantageous traits. The first universal characteristic is the requirement for solar concentration: the total solar intercept area of the cooker, including reflectors and glazed aperture as applicable, is significantly greater than the intercept area of the food container proper. The second universal characteristic is the requirement for solar orientation: there is a preferred arrangement of the cooker with respect to the position of the sun as a function of geographic latitude, season of year, and time of day. 
         [0012]    In practical terms, the property of solar concentration implies that existing solar cookers are heavier, more complex, and more space consuming than possible, while the property of solar orientation implies that they are more demanding in setup and operation, and more time consuming than possible. These dual limitations lead to designs that incorporate manifest compromises between cost and durability, between portability and sturdiness, and between performance and accessibility, resulting in commercial products that are taxing on the user&#39;s physical, intellectual, and motivational resources. 
         [0013]    Solar cookers have been conceived centuries ago and put to continuous worldwide use since. They present a great potential for countering environmental pollution, global warming, deforestation, desertification, and fossil fuel depletion, and for mitigating health hazards associated with wood and coal fires, and with unsanitary drinking water. For all their promise, solar cooking devices have achieved limited success in developing countries, and negligible market penetration in developed economies. 
         [0014]    Even though cultural and other societal factors may be invoked for explaining this failure of clean energy application to food preparation, it is probable that the right concept, with an attractive combination of features, has yet to be presented to the global consumer. Properly designed, a solar cooker can serve not only as a staple appliance in daily use for the environmentally-conscious householder, but also as a convenient camping tool for the outdoorsman and wilderness dweller, as well as an essential component of the emergency and disaster preparedness kit for the survival-minded citizen. 
         [0015]    A step in the right direction was taken by Alex Kee (www.freewebs.com/solarkettle, or solarwyse.cjb.net), who pioneered the cross-application of commercially-available hot water solar collector vacuum glass tubes to solar cooking. The long and narrow form factor is better adapted for water boiling and distillation than food preparation, and the contraption has to be mounted at a slant for adequate solar capture. Although this method is awkward and unsuitable for general kitchen purposes, it demonstrates the feasibility of eliminating the need for solar concentration. 
         [0016]    Ashok Kundapur and J. Samalea (solarcooking.wikia.com/wiki/Box_cookers) have similarly proposed a solar cooker concept based on the Dewar vacuum jar, or thermos bottle principle, in which a dark-coated inner container is vacuum-insulated within a transparent outer container. The lid is shown as being thermally insulated from the body of the pot. Tapani Hakonen (www.netti.fi/˜hakone1/cooker.htm) has also reported on an idea for a Dewar pot with an opaque lid, used in conjunction with a parabolic mirror reflecting sunlight onto the bottom of the pot. Philip Fairey (www.fsec.ucf.edu/en/publications/html/FSEC-CR-1283-01/index.htm) has performed theoretical calculations showing the improved performance of a vacuum jar cooker with low-emissivity greenhouse walls. This analysis however does not examine the design of the lid. 
         [0017]    Solar heated vacuum flasks are shown in U.S. Pat. No. 4,196,721 to Posnansky (1980) and U.S. Pat. No. 4,442,828 to Takeuchi et al. (1984). These both are adapted only to the heating of liquids, similarly to Alex Kee&#39;s solar kettle, and are geometrically configured to operate with the help of reflectors. 
         [0018]    While the concept of a high-performance cooker based on the Dewar vacuum jar principle has been described and analyzed by the above solar pioneers over a period of time, and a specialized tubular application has been demonstrated, no practical implementation has been realized. In addition, these proposals collectively do not teach all the features and benefits of the present invention. 
       OBJECTS AND ADVANTAGES 
       [0019]    An object of the present invention is therefore to provide a solar cooker that is free from the dual constraints of solar concentration and solar orientation. 
         [0020]    Another object of the present invention is to provide a solar cooker of simple and aesthetic design, in the form of a self-contained cooking pot, requiring no additional parts or accessories for its normal operation. 
         [0021]    Another object is to provide a user-friendly solar cooking pot of simple construction, requiring no assembly or setup, and immediately usable out of the box, comprising only two separate components: a body and a lid. 
         [0022]    Another object is to provide a practical solar pot of simple operation, able to function unattended, and requiring no adjustment or planning other than the initial selection of a sunny location for solar exposure. 
         [0023]    Another object is to provide a universally applicable cooking pot that has an extended range of working specifications, delivering satisfactory performance in most geographic locations, climatic environments, and weather conditions. 
         [0024]    Another object is to provide a portable solar pot that is compact, with an overall size not significantly larger than the volume of food being cooked. 
         [0025]    Another object is to provide an easy to handle pot that is lightweight, its individual parts being able to float on water. 
         [0026]    Another object is to provide a sturdy pot that is virtually unbreakable, able to withstand the force of natural disasters. 
         [0027]    Another object is to provide a visible solar cooker that glows in the dark, facilitating its retrieval or recovery in emergency situations. 
         [0028]    Another object is to provide a secure solar pot that has means for accepting belts and lanyards to tie the body and lid to each other and to external objects. 
         [0029]    Another object is to provide a self-sufficient solar pot that incorporates an integral thermometer for confirming water pasteurization, for monitoring food temperature, and for pushing operation close to design limits. 
         [0030]    Another object is to provide a versatile solar cooker that can be safely used in multiple modes of operation at different heat levels, with the optional assistance of external accessories. 
         [0031]    Another object is to provide a multifunctional solar cooking pot that doubles as a heat-retention cooker, or vacuum cooker. 
         [0032]    Another object is to provide an inexpensive solar cooker that is durable, affording both up-front and long-term economy for the user. 
         [0033]    Another object of the present invention is to provide a packaging for a solar cooker that serves as an accessory solar booster for the cooker, to enhance its performance under adverse conditions. 
       BRIEF SUMMARY OF THE INVENTION 
       [0034]    In accordance with the present invention, an exemplary preferred embodiment solar cooking pot includes a body and a lid of similar double-wall construction. The outer wall is transparent and preferably made of polycarbonate plastic, the inner wall is preferably made of stainless steel covered on its outer surface with a solar selective coating and on its inner surface with a dark-colored non-stick coating, and the space between the two walls is closed and evacuated. When the lid is fitted onto the body, the inner walls of these two parts effectively form a fully enclosed solar radiation-absorbing and heat-retaining capsular vessel surrounded by an insulating vacuum contained within a transparent jacket. 
         [0035]    The assembly, which we could call a “solar thermos”, works as a heat trap that captures solar light energy, converts it to thermal energy, distributes the heat evenly within the central compartment, and keeps it from escaping. The device is capable of attaining moderate cooking temperatures, sufficient for pasteurizing water and preparing food, from direct incident sunlight without employing additional implements such as mirrors, lenses, or thermal conductors or insulators. When a balanced form factor is used in shaping the vessel so that it presents a nearly constant intercept area to light beams impinging from any angle, this energy trap is omnidirectional and obviates the need for orientation to the sun. The present invention is thus embodied as a cooker that is free from the dual constraints of solar concentration and orientation. Solar cooking becomes the simple act of loading a pot with the desired ingredients and leaving it in a sunny spot. 
         [0036]    By virtue of its well-insulated design, this solar cooker doubles as a heat-retention cooker, or vacuum cooker, so that food heated by other means can be transferred to the pot for retained-heat cooking without solar energy input. By the same token, food solarly heated in this device can continue cooking after cessation of sun exposure, or during periods of temporary solar interruption, such as from passing clouds. Additionally, since the pot keeps food hot for a long time, it automatically delivers a hot meal in the evening after being left to cook outside for the day. 
         [0037]    Another feature of the preferred embodiment is the inclusion of a thermometer mounted on the outer surface of the metal capsule within the evacuated space, with special markings for the temperatures of water pasteurization and boiling, and the safe range of operation. The presence of a thermometer enables safe use of the solar pot in a push mode for frying. Additional features of the preferred embodiment provide for safe handling, securing, and locating in the dark. An alternate design of the vacuum space closure extends the range of operation to higher temperatures. An optional packaging is adaptable for use as an accessory external solar reflector for performance boosting. 
         [0038]    An alternate embodiment of the present invention, suitable for occasional or disposable applications, features an inexpensive, all-plastic construction similar to the clear plastic clamshells and bubble packs used extensively in consumer product packaging. 
         [0039]    These novel features present unique combinations of advantages not found in the current art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Drawing Figures 
         [0040]    In the drawings, closely related figures have the same number but different alphabetic suffixes. 
           [0041]      FIG. 1  is a front perspective view of the preferred embodiment solar cooking pot, with the body and lid separated. 
           [0042]      FIG. 2  is a left cross-sectional view of the preferred embodiment solar cooking pot, with the body and lid engaged. 
           [0043]      FIGS. 3A and 3B  are front cross-sectional detail views of closures in the preferred embodiment, respectively with the body and lid separated and engaged. 
           [0044]      FIGS. 4A and 4B  are front cross-sectional detail views of alternate closures in the preferred embodiment, respectively with the body and lid separated and engaged. 
           [0045]      FIG. 5  is a front perspective view of the preferred embodiment solar cooking pot together with an accessory packaging. 
           [0046]      FIG. 6  is a front cross-sectional view of an alternate embodiment solar cooking pot. 
           [0047]      FIG. 7  is a front cross-sectional view of an alternate embodiment solar cooking pot including wall inserts. 
       
    
    
     REFERENCE NUMERALS IN DRAWINGS 
       [0048]    In the reference numerals, suffixes A and B on a numeral respectively designate homologous parts of the cooking pot body and lid, which may also be referred to collectively by the same numeral without suffix. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 10 
                 Solar cooking pot 
               
               
                   
                 10A 
                 Pot body 
               
               
                   
                 10B 
                 Pot lid 
               
               
                   
                 12 
                 Liner 
               
               
                   
                 14 
                 Shell 
               
               
                   
                 16 
                 Insulation 
               
               
                   
                 18 
                 Closure 
               
               
                   
                 20 
                 Thermometer 
               
               
                   
                 22 
                 Food compartment 
               
               
                   
                 24 
                 Food 
               
               
                   
                 26 
                 Finger well 
               
               
                   
                 28 
                 Lid loop 
               
               
                   
                 30A, 30C 
                 Body grooves 
               
               
                   
                 30B 
                 Lid groove 
               
               
                   
                 32 
                 Liner core 
               
               
                   
                 34 
                 Liner outer coating 
               
               
                   
                 36 
                 Liner inner coating 
               
               
                   
                 38 
                 Glow strip 
               
               
                   
                 40 
                 Contact 
               
               
                   
                 42 
                 Gap 
               
               
                   
                 44 
                 Alternate closure 
               
               
                   
                 46 
                 Reinforcement 
               
               
                   
                 48 
                 Knob 
               
               
                   
                 50 
                 Liner lip 
               
               
                   
                 52 
                 Shell lip 
               
               
                   
                 54 
                 Seal 
               
               
                   
                 56 
                 Packaging 
               
               
                   
                 58 
                 Body bottom insert 
               
               
                   
                 60 
                 Body side insert 
               
               
                   
                 62 
                 Lid insert 
               
               
                   
                 64 
                 Packaging box 
               
               
                   
                 66 
                 Packaging bag 
               
               
                   
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
       [0049]      FIG. 1  is a front perspective view of the preferred embodiment of the solar cooking pot  10  comprising two mating parts shown separated, a body  10 A and a lid  10 B. Body  10 A is a double walled container composed of two material layers, a metal liner  12 A and a transparent plastic shell  14 A, separated by a vacuum insulation layer  16 A. Lid  10 B is similarly composed of two layers, liner  12 B and shell  14 B, separated by insulation  16 B. Lip  18 A is an inward projection or fold of shell  14 A that engages liner  12 A and forms a closure for the double wall of body  10 A, sealing its vacuum space  16 A. Lip  18 B similarly forms a closure for lid  10 B, enclosing its vacuum gap  16 B. A thermometer  20  is mounted on liner  12 A, within vacuum gap  16 A. The display screen and indicator on thermometer  20  includes glow-in-the-dark indicia for ease of temperature reading in the absence of ambient light. 
         [0050]      FIG. 2  is a left cross-sectional view of pot  10 , with lid  10 B fitted on body  10 A by apposing closures  18 A and  18 B. Body liner  12 A, shell  14 A, and vacuum insulation  16 A are mated to the corresponding lid components  12 B,  14 B, and  16 B. Body and lid liners  12 A and  12 B form a compartment  22  for containing food  24 . Thermometer  20  is seen in cross-section mounted on liner  12 A. Additional features kept out of in  FIG. 1  for simplicity are shown in  FIG. 2 . Lid shell  14 B has three evenly spaced finger wells  26 , of which one is visible in the drawing, akin to a bowling ball&#39;s grab holes, for safe handling of the lid in the presence of steam escaping from the pot opening. Embedded loops  28 A and  28 B in the body and lid shells are adapted for accepting lanyards, and circumferential grooves  30 A and  30 C in the body, and  30 B in the lid, are configured to accept belts adapted for securing the body and lid together, for carrying the pot, and for tying it to external objects. 
         [0051]      FIG. 3A  is an enlarged cross-sectional view of the areas surrounding lips  18 A and  18 B with the lid and body separated, detailing the structure of the liners and the arrangement of the lips. Liners  12 A and  12 B respectively have a central core  32 A and  32 B coated on its entire exterior surface with a solar selective coating  34 A and  34 B, which features high absorbance and low emissivity, and on its entire interior surface with a dark-colored non-stick coating  36 A and  36 B, as is commonly done with cookware. 
         [0052]    Circumferential strips of glow-in-the-dark paint  38 A and  38 B respectively on the inside surfaces of shells  14 A and  14 B in the vicinity of lips  18 A and  18 B respectively render the pot visible at night. Alternatively, body and lid shells  14 A and  14 B can be molded from clear resin doped with glow dye or pigment, in which case the entire pot is lit in the dark, at the cost of reduced solar performance resulting from lowered shell transmittance. 
         [0053]      FIG. 3B  is a similar view with the lid and body mated. Liner  12 A protrudes past the axial extent of the corresponding contiguous part of lip  18 A over a short distance, and liner  12 B recesses from the axial extent of the corresponding contiguous part of lip  18 B over a slightly smaller distance, so that when the lid is fitted over the body, the liners form an intimate contact  40  while the lips are held slightly apart, separated by a gap  42 . Lips  18 A and  18 B are shaped complementarily with a nonlinear profile such that no part of liner  12 A or  12 B has line-of-sight visual access to the external environment, and with a centripetally downward slope such that any material or water condensation that collects in gap  42  would preferentially drain to the outside. This arrangement effects positive mechanical engagement of body  10 A and lid  10 B when the pot is closed, assures shielded, intimate contact of liners  12 A and  12 B for thermal continuity and sealing, and prevents foreign matter from entering inner compartment  22  and contaminating food  24 . 
         [0054]      FIGS. 4A and 4B  are similar to  FIGS. 3A and 3B  respectively, showing an alternate design of closures  18 A and  18 B in which integral body and lid lips  18 A and  18 B are respectively replaced by annular parts, or rings  44 A and  44 B composed of a material selected for high heat tolerance and low heat conductivity, such material likely being an opaque plastic, as commonly used in the construction of handles for cooking pots and pans. Ring  44 A is bonded at its inner edge to liner  12 A and at its outer edge to shell  14 A. Ring  44 B is similarly bonded at its inner edge to liner  12 B and at its outer edge to shell  14 B. This arrangement gives pot  10  a greater temperature range of operation. 
         [0055]      FIG. 5  shows an optional packaging system  56  for pot  10  in the form of a box  64  composed of reflective panels that, when opened up and properly arranged, act as an external solar reflector, and a clear plastic bag  66  with drawstring closure that, when fitted around reflector  64  and pot  10 , acts as an external insulator. Packaging  56  thus serves as an accessory solar performance booster for cooking pot  10  to enable its function under adverse sun and weather conditions. Reflector  64  may also be provided as a pleated aluminum foil cup that contains pot  10  in the fashion of a cupcake paper cup, and that can be then shaped into an advantageous geometry for capturing solar rays at the time of use. 
         [0056]      FIG. 6  shows a front cross-sectional view of an alternate embodiment solar cooking pot  10  of an all-plastic construction which is amenable to manufacturing by die cutting, vacuum forming, and heat sealing. Liner cores  32 A and  32 B, and shells  14 A, and  14 B, can all be made from the same clear plastic sheet material. In body  10 A, lips  50 A and  52 A respectively of liner  12 A and shell  14 A are heat-bonded to form seal  54 A. In lid  10 B, seal  54 B is similarly formed from lips  50 B and  52 B. Liners  12 A and  12 B respectively include a solar selective coating,  34 A and  34 B. Dimples  46 A in body shell  14 A and  46 B in lid shell  14 B represent a multitude of such reinforcing shell projections that press against the corresponding liners and help assure structural integrity in the presence of a vacuum in spaces  16 A and  16 B and atmospheric pressure outside. Protrusion  48  of the lid shell serves as handling knob. 
         [0057]      FIG. 7  shows a similar sheet plastic design of pot  10  in cross-section as well, in which structural reinforcement of the walls is derived from the inclusion of corrugated plastic inserts in the vacuum spaces. In this cylindrical container with discoid lid, food volume is maximized with respect to overall cooker size. Body  10 A is strengthened with a bottom corrugated insert  58 , seen here across the flutes, and a side corrugated insert  60 , seen here along the flutes, and lid  10 B is propped with a similar insert  62 . The inserts, illustrated as corrugated sheets, could be provided in other forms as well, such as a three-wall lamination with central corrugation, or a dimpled sheet, or a reticulated sheet, etc. This construction allows use of thin plastic material, such as found in disposable food containers and consumer product bubble packs and clamshell packaging. This inexpensive design is suitable for occasional or disposable applications, such as packaging instant noodles, ready for in-container cooking in the sun, with a cupful of water being the only additional requirement, and with the added advantage that the soup stays hot until the end of the meal. Liners  12 A and  12 B may be made of black plastic sheet material, obviating the need for painting or coating in the manufacturing process. In this case, solar booster packaging  56  plays an important role. 
       Principle of Operation 
       [0058]    When body  10 A and lid  10 B of solar pot  10  are mated, liners  12 A and  12 B form a substantially continuous and closed solar selective capsule  12 , shells  14 A and  14 B form a substantially continuous and closed transparent jacket  14 , spaces  16 A and  16 B form a substantially continuous and closed vacuum shield  16 , and the whole assembly acts as a vacuum-insulated capsule. Selective coatings  34 A and  34 B form a substantially continuous and closed skin  34  that allows radiative heat to flow only in the inward direction through liner core  32  formed by  32 A and  32 B, non-stick coating  36  formed by  36 A and  36 B, to compartment  22 . To achieve an even distribution of energy within cooking chamber  22 , coating  36  is preferably a radiant material with high emissivity. From a thermal perspective, cooking pot  10  is thus a near-hermetic solar heat valve, or heat trap. It can be conceptualized as a Dewar vacuum jar, or Thermos bottle, that will absorb heat from the sun but not release it. When exposed to the sun, this device acts as a solar energy collector to cook food and boil water. In the absence of insolation, it can function as a retained-heat cooker after an initial load of thermal energy. 
         [0059]    Short-wave solar light penetrates clear shell  14  and gets converted to heat at the dark exterior surface of capsule  12 , which then gets transmitted through the capsule wall to food  24  contained within. Dark non-stick coating  36  on the interior surface of the capsule facilitates internal radiation and even distribution of the captured energy. Selective coating  34  on the exterior surface of the capsule keeps the energy from being radiated back outward as long-wave infrared, thus preventing heat loss through radiation. Vacuum  16  between the two solid walls prevents heat loss through convection. Heat loss from the capsule through conduction is limited to the lines of contact with the shell in the area of the opening, at body lip  18 A and lid lip  18 B. Body liner  12 A protrudes past the axial extent of lip  18 A over a short distance, and lid liner  12 B recesses from the axial extent of lip  18 B over the same distance, to assure positive engagement of lid and body, and to prevent foreign matter from entering the food compartment. 
         [0060]    The performance of pot  10  depends chiefly on the quality of the selective coating, the level of vacuum, and the extent of liner-shell contact. Perfect insulation is not desirable: total heat loss must be sufficient to keep the stagnant temperature at a safe level, so that the pot does not self-destruct when left to sit empty in the sun. 
       Fabrication 
       [0061]    This solar cooking pot is a specialized vacuum jar, and the manufacturing techniques used in the production of thermos bottles, well-known to the art, can be applied to this device as well. The double wall structure is inherently sturdy, therefore the metal liner can be thin, drawing on the strength of the plastic shell, and yielding a low total weight. Stainless steel thermos bottles use liner wall thicknesses down to 0.5 mm. Conventional cookware use thicker walls to better distribute the applied energy, typically the intense heat of a flame localized to the bottom. This is less of an issue for a solar cooker, where the energy influx is milder and more diffuse. The liner wall thickness can therefore be closer to that of a thermos bottle than that of a regular cooking pot. 
         [0062]    The transparent shells can be made from molded or vacuum formed clear polycarbonate, a commonly used plastic that is very strong, has good optical properties and chemical resistance, can be UV-protected, and can withstand relatively high temperatures. Multi-walled polycarbonate sheets are used extensively in greenhouse glazing applications. A 6 mm shell wall imparts exceptional toughness to a 3 liter solar cooking pot of the proposed design. 
         [0063]    The solar selective material coating the liner may be a plating, such as black chrome, or a coating, such as Solkote (www.solec.org). The plastic and metal parts can be made separately and then bonded together with a suitable adhesive. The intervening space can then be evacuated through a small passage in the shell, which is then sealed. The vacuum gap serves primarily as insulation, but has the positive side effect of lowering the overall density of the part. With proper sizing of the gaps, both body and lid can be made light enough to float in water, an appealing quality for boaters and residents of flood-prone areas. 
       User Operation 
       [0064]    In normal or regular operation mode, the user loads the pot body with food to cook, closes the lid, and sets the cooker out in an area that will remain sunny for the duration of the cooking operation. The selective coating and vacuum insulation features maximize energy capture and minimize loss, thereby obviating the need for solar concentration under typical weather conditions. The capsular shape of the food container ensures even exposure to the sun under different angles of insolation, thereby obviating the need for solar orientation. 
         [0065]    The food will cook and stay hot for a long time after the sun has disappeared. It is possible to exploit the heat retention property of the device to extend cooking time beyond the period of solar exposure. This is useful when one has limited access to the sun, in which case the minimum requirement is the time it takes to bring the food up to cooking temperature. For example, a cook concerned about the security of her food left unattended can bring the cooker inside as soon as the desired temperature has been reached. 
         [0066]    In push or overdrive mode, the user places the solar pot in a field of concentrated sunlight, such as produced by a conventional solar oven, solar panel cooker, or solar stove. He then monitors the temperature to ensure that the safe limit is not exceeded. Thanks to its construction, this pot will attain higher temperatures than a regular cookpot, or will achieve faster cooking times, depending on the type of food. In weak solar conditions, this device will still cook when a regular pot will not. In unfavorable geographic or climatic situations, the solar pot can be used regularly in the place of an ordinary utensil. 
         [0067]    For service as a conventional heat-retention cooker or vacuum cooker, the user heats food in a regular oven or stove, then transfers the hot food into the solar pot, letting it continue cooking from the retained heat without need for solar exposure. This unit can also function as a thermos bottle in storing hot contents. 
         [0068]    In the regular or normal cooking mode, the unit operates unattended and independent from any accessories, with moderate heat and self-limited temperature, suitable for pasteurizing water and cooking most foods. In the high heat mode, with the assistance of concentrated solar radiation such as provided by a panel reflector, the unit can achieve active boiling of water, or can perform cooking under unfavorable solar situations such as low angle, or under adverse atmospheric manifestations such as smoke, haze and clouds. In the operator-monitored extra high heat mode, or push mode, the boost of focused solar energy such as provided by a parabolic reflector enables the unit to attain heat levels sufficient for frying food. 
       Applications 
       [0069]    Aside from its obvious role in household cooking, the solar pot of this invention is also well suited for transitory and mobile applications, being small, light, securable to a variety of objects, and operable in the presence of motion. For the student or office worker with access to a sunny spot, it can deliver a green hot lunch. For the camper, it can be hung from a tree or pole to gain a better insolation or to shield the food from undesirable ground circumstances. For the hiker, it can be mounted on the outside of a backpack for direct heating, or tucked inside for retained heat cooking on the trail. For the boater, it can swing over an unsteady deck, and floats if dropped overboard. For the city dweller who prefers the convenience of a microwave oven, it can be left in the closet and ignored until a power disruption happens, or until utility rates reach the financial pain threshold. The glow-in-the-dark paint or resin inclusion is a useful feature for campers and disaster victims. 
       CONCLUSION, RAMIFICATIONS AND SCOPE 
       [0070]    Thus, the reader will see that the present invention provides an unique solar cooking pot that achieves full freedom from the dual constraints of solar concentration and solar orientation that govern solar cookers of the current art. It does so while presenting simplicity in design, construction, and operation, coupled with universal utility, versatile functionality, and clean aesthetics. This device fits various applications in daily, recreational, and emergency situations, and will appeal to disparate categories of consumers. Being comparable to a thermos bottle in affordability, durability, and merchantability, this product holds the promise of breaking through in many markets so far unpenetrated by solar cooking devices, aided by the urgency of global environmental, climate, and energy changes. 
         [0071]    While the above description contains many specificities, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of preferred embodiments thereof. Many other variations are possible. As an example, the solar cooker may be sized and shaped as a coffee mug, with the addition of a handle to the body. As another example, to minimize the weight for a backpacker, aluminum may be used for the liner. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.