a simple solar (sunflower) autoclave design
adaptable for any sunny area around the globe particularly at low latitudes
at least since the middle 1800s concentrating dish-shaped solar collectors have been utilized to raise temperatures to boiling and above. this well known basic configuration has been utilized to forge metals, make steam, produce electricity and for many other processes. all models to date have been constructed of relatively high tech materials and with elaborate supporting structures and focusing arrangements. our goal is to design a version of the standard concentrating dish shape utilizing inexpensive and readily available materials which can be worked employing tools and skills commonly available in household or rural settings.
efforts to devise the very simplest functional solar furnace suitable for use at any latitude and capable of firing a pressure medical sterilizer or pressure canner has brought us to the following design which for convenience is referred to as the sun flower configuration.
in these tests we eventually settled on a cast aluminum pressure medical sterilizer suitable for use with any heat source. (the wisconsin aluminum foundry co., inc.1-414-682-8286, p.o. box 246, 838 s. 16th st., manitowoc. wi 54220 usa. all-american sterilizers). these pressure vessels have a gasket-free seal and a spring release safety valve. 1) we found that some gasket material on other pressure devices will not hold up to heat flowing primarily through the top of the unit, although some will withstand these temperatures which are higher in this location than are experienced by the customary bottom heat. using a metal-to-metal seal eliminated this uncertainty. 2) safety pressure release devices made of lead or other low temperature metals do not work as they may melt prematurely in top heat. 3) since solar heat is gentle, it may not provide the burst of steam needed to seat a neoprene emergency pressure release disk. in some cases the valves were held manually until the pressure built up to hold them. in other cases neoprene disks have been secured by a very light application of silicon sealant fragile enough to be easily broken by the pressure if emergency release were needed. spring release safety valves have worked well and avoid these problems.
data on the kerr-cole sunflower autoclave design
during early tests the solar device has fired 17 pounds (7.65 kg) of mass consisting of a pressure sterilizer and a quart of water. the unit raised temperatures in the pressure vessel to 270f and held 20 pounds pressure for hours at a time. theoretically larger loads would simply require larger reflectors, but i have not yet run that actual test. instructions follow for constructing the unit that fired an all-american pressure vessel, #1915 1/2x.
the 22" tapered cardboard reflectors are covered with lightweight aluminum foil pasted on one side with diluted glue or with wheat paste. they are taped to cardboard sections 22" x 18" to provide a broad prop for focussing. assemble each section by laying two pieces to form a flat strip and taping the backs together at the 18" edges. turn over and fold so foil is outside and cardboard pieces are back to back. tape together with cellophane tape (to provide a clear, flexible hinge.) a tab of duct tape goes off the middle of the 9" edge. (to anchor the reflector at the base of the vessel in a "break-away fashion)
hood diameter was determined by the widest part of the autoclave including handles plus 1". height was determined by the depth of the insulation plus the height of the bottom metal solar adsorber, plus elevators, plus the height of the autoclave with controls extended, plus 1". the side of the hood was prepared first. using a 48" width roll of glazing required joining two strips get the 54" circumference. this was done by laying the pieces flat with the seam allowances overlapping and then drilling 1/8" holes for pop rivets. the two pieces were then riveted together and the strip rivetted into a circle. the edges of both seams were sealed with silicon sealant on both sides.
for the size of vessel being used, the top of the hood was an 18 1/2" circle. with the circle on a flat surface, silicon sealant was laid about 1/4" from edge and the circle for the sides laid in it adjusting it to be approximately round. when the hood assembly was dry, the excess glazing at the top was trimmed off with shears and rasped smooth to prevent shadows. in the top of the hood over the position of the exhaust control valve, a 2" access hole was cut with an expansion bit. a 3" piece of glazing was riveted in position where it could rotate to cover the access hole.
bottom insulation was a 17 1/2" circle cut from rigid wall insulation although any substance that would prevent heat flow from the bottom of the unit into the ground would have been adequate. there was a layer of aluminum foil on top of the insulation. there should be small insulating supports to elevate the tray about 1/2 inch above the foil utilizing a minimum of contact points to reduce heat transfer. these may be three or four blocks of wood 1" by 1/2," or small stacks of 1 inch squares of cardboard glued together. these supports for elevating the tray should not be metal. the metal tray was a 17" circle of rigid sheet metal painted black on the top surface.
instructions for use:
select a relatively flat area of ground twice the length of reflectors plus the diameter of the hood. dig a shallow pit to sink the insulation to ground level. lay reflectors in a circle, sightly overlapping around the pit. if tabs are being used, stretch them toward the center so they will be held by the weight of the central assembly. lay the insulation in the pit with the foil side up, lay the small supports on the foil. place the metal tray with the black side up. load the autoclave including 1 quart of water for steam and place on the tray with the exhaust control valve extended. cover with the glazed hood which slips down over the insulation and rests on ground cutting off air infiltration.
raise each reflector separately until the brightest reflection produced is hitting lower half of the hood. prop with the outer section of the reflector units and secure them in position with a rock or sandbag. depending on the location of the sun, each reflector will have a different position which will need to be changed in about an hour. if there is question about the position of reflectors, look at a multi-petaled flower which is naturally following the sun such as a sun flower, and mimic the angles.
allow approximately two hours for initial heating. when the autoclave is boiling, steam will condense on the hood. by the time condensed steam begins running down the inside of the hood, tests have indicated the air has been exhausted from the pressure vessel. the unit will never send out a powerful jet of steam such as is produced on a wood fire.
lay down the two reflectors directly facing the sun so the operator can approach the hood. reach through the access hole and close the exhaust control valve. rotate the hood until the pressure gauge is visible through the access hole. for medical sterilization, pressure can be allowed to reach the safety release level. the release valve will occasionally release to prevent over pressurization. this simple design for solar heat is gentle and will not produce the rapid rises in pressure that makes explosive pressures with other fuels. the unit also will not boil dry during the normal length of processing. to reduce steam in the hood, after the exhaust valve is closed set the access cover slightly off the hole. close the hole as soon as visibility is restored.
note: when using the unit for pressure canning, a customized pressure release valve can be set for the pressure required by returning it to the factory or adjusting the spring. then the unit can be fired without concern for over-pressurizing food as it will automatically release at 10 pounds or whatever is appropriate for the altitude.
for sequential firings, time can be saved by preheating subsequent loads in a solar box cooker or other heat trap while the initial run for the day is being processed.
this unit is light weight, portable and relatively inexpensive. it functions well enough to process three runs on a sunny day. adequacy of the process was confirmed both by heat-sensitive tape and by a minimum/maximum thermometer imbedded in the simulated minor surgery pack. the size of the reflectors might be increased for quicker heating or for processing in larger vessels.
in gusting wind, occasionally a single reflector blew away. however, the tabs pulled out from under the center assembly and the autoclave, which must not be moved while under pressure, was not disturbed. the safety factor of having break-away reflectors around a heavy center is good. reflectors need a little more holding power, but not by tying them to the autoclave base. perhaps having reflectors of a heavier material or pegging them individually into the ground would work. i think they need to be kept as separate units so they can be focussed and respond to wind individually. any insulation available would serve and can be tested for adequacy by hand, feeling no warmth on the ground under the insulation at the end of the run.
thank you very much for any work you can do on these vitally needed designs.
barbara p. kerr research consultant solar cookers international