Pot with Integrated Greenhouse for Solar Panel Cooker

David Delaney March 28, 1999 - Version 1.7



 

 The Solar Panel Cooker  was invented by Roger Bernard and described by him in Solar Box Journal #16, February 1994.  The solar panel cooker introduces a transparent greenhouse enclosure around the cooking pot,  replacing  the insulated box of a Kerr-Cole solar box cooker.

The reflective panels of the panel cooker surround  the pot and its greenhouse,  bathing the pot in concentrated sunlight, thereby heating the pot.  The pot then heats both the food in the pot and the air trapped between the greenhouse and the pot.  Because the air is held in place by the greenhouse, it insulates the pot,  reducing heat loss from the pot and food to a rate which is initially lower than the rate at which the sunlight delivers heat to the pot.  The resulting accumulation of heat cooks the food.

In Solar Box Journal #16,  Bernard  describes an inverted glass salad bowl as the greenhouse enclosure. In an associated article in the same issue of SBJ, Barbara Kerr suggests a transparent plastic oven bag,  which is now widely used.  Both these forms of greenhouse enclosure suffer from condensation of cooking vapors,  and require periodic drying.   Since the pot and its lid are sealed in the greenhouse, it is awkward to stir the food, or test for doneness.

Bernard solved these problems (see Solar Box Journal #18, October 1994 )  by suspending the  black cooking pot by its lip inside a glass vessel.  This arrangement  exposes the top of the pot, giving easy access to the food, and venting cooking vapors directly to the atmosphere, rather than into the greenhouse airspace.  Placing the uninsulated top of the pot into direct contact with the atmosphere, however,  increases heat loss from the pot.

This proposal elaborates on Bernard's arrangement, introducing an additional greenhouse enclosure for the lid of the pot, and packaging both greenhouse enclosures as integral parts of an easily handled assembly.  For the cook,  the greenhouse enclosure is no longer a distinct object;   the combination of  two greenhouse enclosures and pot is the pot.  This integration should add substantial convenience to solar panel cooking.
 

Fig. 1 shows a cross section of the pot in use.
 


 

A transparent stand supports the pot, admits sunlight to heat the pot, and contains an insulating airspace.  It forms a greenhouse enclosure for all but the top of the pot.  Handles on the stand make it a convenient accessory for handling the pot, which has no handles.  When the pot is removed from the stand, it is awkward to handle when full of hot food.  The design encourages the cook to keep the pot in the stand while preparing food before cooking, and while serving food after cooking. The lid of the pot has an integrated greenhouse enclosure, including  an inner black heating surface.  Cooking vapors are unlikely to invade either of the greenhouse air spaces, removing the problem of condensation.
 


 

Glass is an obviously suitable material for the transparent parts.  A plastic with suitable thermal and optical properties could be used, and would be lighter, thinner, and might have more easily accessible manufacturing facilities. I have not sufficiently  investigated the technical and economic aspects of the use of plastic to have an opinion as to its relative desirability.  Either kind of material requires fabrication by molding.  This design sketch pays attention to keeping the tooling costs of the molds as low as possible.   In particular, the transparent parts shown here can each be formed with a two part mold.  See Fig. 7.

Aluminum is an ideal material for the pot and the bottom of the lid.  Each of these parts would be formed from a single piece of material by  processes available to any pot manufacturer.  Again, tool development is required. The pot is straight forward.  The lid bottom does have a slight "negative draft" required to form the shape that retains the press fit transparent lid top.  See Fig 3.
 
 


 

The top surface of the lower element of the lid (the pan of the lid)  is, of course, black.   The pan is in contact with cooking vapors but not food.  Since it is much hotter than the food, but is not in contact with the food, the pan will radiate its heat to the food more effectively if its bottom surface is also black.


 

This design tapers the sides of the pot and stand to allow each to be stacked with itself, as shown in Fig. 4.   Stacking the components of multiple pots in this way greatly reduces the space needed for storage or transportation.  My intuition says that the taper should not produce an unacceptable reduction in the amount of energy that reaches the pot, but this needs checking.
 


 
 


 
 
 

Implementation

To my knowledge, such a pot does not now exist, even in prototype.    The principle of the greenhouse lid needs to be tested to make sure it adds enough value to be worth the cost.  I intend to test its principle in the next few months with a crude prototype. I expect it to make a significant improvement relative to exposing a metallic top to the atmosphere.  I would be delighted if others were also to test the idea.   The detailed design presented here is not practical for home construction, but less sophisticated designs may be.   Extremely brief investigation indicates that tooling costs for manufacture would total approximately US $100,000.  The manufactured cost could be very low in volume, as usual.  The cost of the integrated pot would be no greater than the sum of the costs of an inexpensive glass pot and an  inexpensive aluminum pot.
 

Work List

Thermal Effectiveness Testing

I am doing this.  Three configurations will be tested.  The pot and pot stand will be common to  the three configurations, the only differences being in the design of the lids.  The test configurations will be relatively crude prototypes, so will not be suitable for testing some of the ergonomic and  esthetic impacts of the proposed design.   Lid configurations: 1) Uninsulated metallic lid, 2) Greenhouse lid, 3) Insulated lid.

Detailed Design

A possibility for further work subsequent to thermal effectiveness testing.

Ergonomic Testing

A possibility for further work subsequent to completion of design study.  Subsequent to completion of successful testing for thermal effectiveness, a selected design could be fabricated in a small number of parts sufficiently close in form and function to manufactured quality to allow testing of the ergonomic and esthetic aspects of the design.

Manufacturing Cost Study

A possibility for further work involving manufacturers subsequent to completion of detailed design study.

Marketing Study

A possibility for further work.

David Delaney
142 Waverley Street
Apartment 2A
Ottawa, Ontario, K2P 0V4
Canada