the kerr-cole large solar panel/propane hybrid stoves

introduction

the field of hand-crafted, low tech solar cooking devices is expanding all the time. it now includes solar box cookers of multiple designs used for family cooking, pasteurization, medical sterilizing by boiling and by baking and for food preservation. it also includes solar panel cookers of  multiple designs with almost the same range of uses. the differences relate mainly to the amount of materials required, the ease of building and use, and type of loads.

both solar boxes and solar panels are very functional and useful solar cookers for individuals and families. they rely totally on solar energy and retained heat components. backup if the weather changes is simply to pick up the pot and move it to whatever alternate fuel is available. they are being used around the globe, however acceptance has been slower and more limited than we would like.

there are at least two expansions that might increase the field of home-crafted, low tech solar cooking: enlarging sizes to deal with institutional and community cooking; and hybridizing to increase reliability. neither of these developments are new; but they have not been widely applied on the hand-craft level. the large kerr-cole solar panel/propane hybrid successfully developed in 1994 utilizes both potentials. integrating these changes increases both the complexity and cost of construction as well as  potentially the cost of use. but, as dr. shyam nandwani testifies, integrating backup in his solar/electrical hybrid adds peace of mind and confidence, while continuing to use maximum free fuel. electricity not being widely available, we have concentrated on integrating other fuels.

design goals

the large kerr-cole solar/propane hybrid meets the following design goals: handcrafted; hybridized to assure no run has to be aborted because of time constraints, excessive load, or changing weather conditions. backup fuel integrated so a large, hot pot never has to be moved to another location because of failing sun. it is capable of firing some of the equipment currently in use -- large community cooking pots and at least one standard pressure pot sold for use in cooking, in food preservation by steam pressure canning and in medical sterilization by steam pressure (autoclaving). it is made with simple, basic tools from materials locally available in u.s.a. neighborhoods. it is of low cost, compactable for storage and light enough to be carried short distances, assembled, and used by a single person or a group. finally, it is transportable in most standard u.s.a. passenger cars. it appears to be adaptable to other cultures and locations.

although the prototype is hybridized with propane, clearly other fuels such as methane, alcohol, charcoal and home- manufactured natural gas could be used as well as electricity where available. some further development is needed to deal with the smoky fumes of kerosene or wood fires.

by incorporating a fuel familiar to relief agencies and government organizations, to institutional cooks and emergency field personnel as well as by placing the backup immediately available, perhaps we can build more confidence and gain support for wider use of solar energy.

additional benefits are that even without solar input, in an exposed location the hood can reduce the use of propane or other fuel. also, the reflective panels, if turned with the back into the wind, can provide stable wind protection both for the cooking pot and the operator during very bad weather. this appears to make it attractive for emergency field kitchens, for groups of homeless people including refugee camps, or for food vendors.

the following are two different solar units that have been used to fire a standard pressure vessel (all-american) this has been preliminary work and both designs need refinement. i will try to be available by email or post to anyone working with these designs. also, research might be done at the sustainable living center in taylor, az usa, where i reside.

construction of a large solar panel/propane hybrid stove

the panels are constructed of two 4' x 8' sheets of 1/4" plywood or equivalent material protected with exterior grade house paint, 1" x 2" framing, four pairs of regular 2" hinges, one pair of 2" slip-pin hinges, heavy duty household grade foil, glue, screws and weights to stabilize in the wind. in addition, a single burner, cast iron propane stove is used as a stand for the vessel and for fuel back-up. a two-section hood is constructed out of uv resistant, flat fiberglass, rivets, silicone seal and four 3" strips of 1/2" velcro.

the dimensions of the fiberglass hood were chosen to be at least 1" larger than any of the pots to be used including the extended control valve and handles on the pressure vessel. the array is roughly twice as high as the pot and 2 1/2 times as wide. dimensions were then adjusted for rational use of material.

regular kitchen pots, darkened, can be used for normal cooking. for pressurization, the all-american medical sterilizer was chosen because it is relatively inexpensive and has been used in rural areas around the world for decades. depending on size, it has either a metal/spring or a neoprene disk for safety release. metal disks that melt as an over-pressure safety release do not work since they melt prematurely from the solar heat that falls on the top of the vessel. this type of pressure vessel was also chosen because it relies on a metal-to-metal seal, eliminating a need to periodically replace a gasket. the vessel was sprayed with barbecue black paint after masking the gauge and the control valve. (various sizes are available from the wisconsin aluminum foundry co., inc., p.o. box 246, 838 s. 16th st., manitowoc, wisconsin, u.s.a., 54220 phone: 414 682-8286) further research is expected to reveal other brands of pressure vessel that would be suitable.

using the solar panel/prpopane hybrid stove

to use, the ground panel is placed on a smooth flat surface. the heavy-duty propane stove (size:12"x12"x6" with metal, heat-resistant tubing, is centered 10" to 12" from the back panel. the empty vessel is placed on the stove; then loaded and covered with the fiberglass hood. preheating time varies with the size of the load and the weather. the unit is rotated to follow the sun.

pressure canning data

the test run was held on october 12, 1994. the maximum sun angle at this location is 47.1 degrees on that day. it was a clear sunny day with calm winds and moderate temperatures. the u.s. dept. of agriculture information bulletin, no. 539. may 1989. "complete guide to home canning." advises dried beans be run at 15 pounds pressure for 1 hour and 15 minutes. three additional pounds are needed to compensate for the altitude of taylor (5800 ft.). the decision was made to run at 20 pounds to allow time to cope with changes needed in the event of falling pressure.

one cup of dried pinto beans had been placed in each of 17 glass quart jars the previous night. water had been added to fill. the jars had stood overnight and the beans were well hydrated. water was changed, dome lids were placed and sealing rings were tightened.

the metal-to-metal sealing surfaces on the pressure vessel were wiped with a soft cloth and petroleum jelly (vasaline) and the vessel sealed. the fiberglass hood with vent closed was placed over the vessel and the fiberglass skirt drawn tight and secured. the reflective panels were placed and the unit aimed directly at the sun. the sun angles were low, so the perpendicular setting was used. for high sun angles, the hinged portions at the bottom of the side panels would have been folded out so the panel array leaned at approximately a 15 degree angle.

test run:

• 9:20 am started on solar alone.
• 12:20 pm 3 hours and no steam. appears to require backup. propane started.
• 1:00 pm a solid plume of steam had been established. the steam ejector valve was closed.
• 1:28 pm 20 pounds pressure. propane turned off. (68 minute burn)
• 2:45 pm 20 pounds pressure held steadily on solar alone for the required 1 hour and 15 minutes.

evaluation: this is a successful heavy duty solar reflective panel/propane hybrid array which meets the design goals. the required pressure or above was maintained for the duration of the run. all of the jar lids sealed. the beans were soft and tasty and none spoiled in coming months. with this solar/mass ratio, solar energy could not completely handle the job. backup in place was successfully accomplished. the fuel used was 4.5 hrs solar plus 17,000 btu of propane supplied by a burn of 1 hour and 8 minutes on a single burner propane stove rated for 15,000/hr. resulting in use of solar plus 1000 btus per quart canned. backup could have been methane, gasoline, charcoal, alcohol, electricity or other clean fuel.

medical sterilization by steam pressure (autoclave)

the solar panel/propane hybrid unit that was used to can 17 quarts of bean on october 12, 1994, was temporarily changed from the canning to the medical sterilization mode on october 26, 1994. the wisconsin aluminum foundry supplies a conversion kit to hold the surgical packs completely away from the water when the vessel is used as an autoclave. this is necessary as packs often contain towels, sheets, cotton balls, etc. the conversion involved 1) lifting the canning rack out and inserting a solid aluminum bucket inset, 2) screwing an exhaust tube below the control valve and 3) leading the tube to the inside bottom of the bucket to assure complete removal of air so steam penetrates throughout the bucket. it took only a few minutes.

it was a foggy morning which finally cleared shortly before 11 a.m. after that there was full sun with calm winds for this location. (maximum sun angle for taylor, arizona, u.s.a. on october 26 is 42.1 degrees.) the goal was 19 pounds pressure for 35 minutes. the load was 5.7 pounds of small surgical/suture packs individually wrapped in cotton sheeting which filled the basket about 1/2 full. one quart of water was used to produce steam. total mass also included the pressure vessel which has a shipping weight of 40 pounds and the weight of the propane stove, approximately 8 pounds.

data on the kerr-cole solar/propane hybrid:

• 11:00 a.m. load started. preheating.
• 1:20 p.m. hood was full of steam and running condensate all over the inside. exhaust/control valve closed.
• 2:00 p.m. 19 pounds pressure. front ground reflector removed to slow it down.
• 2:07 p.m. 22 pounds. array rotated to reduce sun.
• 2:10 p.m. 23 pounds. rotated so west wing was pointing at sun. back bottom panel removed.
• 2:18 p.m. 20 pounds. rotated slightly more toward sun.
• 2:20 p.m. 20 pounds.
• 2:25 p.m. 20 pounds.
• 2:35 p.m. 19.5 pounds. hood removed. control valve opened and steam blown off.

all indicators showed a successful run (pressure tape, diack and thermalog). additional runs would have been faster since the vessel, water, and propane set-up (which was used only as a stand during this run) were already heated. subsequent medical packs could have been preheated in a standard solar box oven to decrease time needed to run any following loads.

evaluation:

this sterilization load was far less than the capacity of the unit. small loads in good weather can be totally solar. the solar panel/propane hybrid design could be useful to medical units relying on limited fuel supplies, as well as useful to reduce the costs of mass feeding. in both cases, it significantly reduced fuel usage. the design could be adapted for different size equipment by changing dimensions.

barbara kerr