Introduction: Hot Pot Is a Great Solar Cooker, But Was Designed for the Tropics
But those of you who want to do more research, Google searches are more successful when searched as "hot pot", since the rest of the world misspells the name.
Here's a simple hack (well... maybe not so simple) that enhances the performance of the HotPot:
For those of you who are not familiar with the legendary HotPot™, the original pot assembly comprised a 5-liter black-enameled steel pot, supported by its rim inside a covered, clear, tempered glass bowl. There is a half-inch of air space between the pot and the bowl. The heat retained inside the glass bowl creates a miniature “greenhouse” effect and maintains cooking temperatures.
The pot assembly rests inside the polished aluminum reflector. There is also a less expensive standard reflector, made of aluminum foil bonded to heavy cardboard, but is not featured here. Both reflectors are shaped to concentrate the solar energy onto the pot, they are equally efficient, and both fold easily for storage.
The aluminum reflector model, featured here, also known as the "Morning Star", is made entirely of aluminum: Aluminum reflector plates, aluminum hinges, aluminum hinge pins, and aluminum rivets. It is more durable, more alluring, impervious to termites and unappetizing to goats, but the aluminum reflector is not rugged enough to routinely expose to buffeting winds. But it is easily collapsible: The reflector panels quickly folding together to a flat, 14"x14-1/4" size, allowing for more compact storage overall.
The HotPot™ Solar Cooker, specifically, the original 5-liter model, with the folding polished aluminum reflector was primarily designed for the third world and developing countries, to function most effectively in the Tropics, between the Tropic of Cancer at latitude 23.5°N and the Tropic of Capricorn at latitude 23.5°S (=11.75° average latitude). This amazing device allows for a 6 hour cooking window, on a typical day, in impoverished areas that often have meager resources for fire and cooking.
But most of those who purchased the solar cookers, specifically the original 5-liter model, partially subsidizing HotPot™ ownership in the third world, live in the Temperate Zone (=45° average latitude). And we are often frustrated with the short 2-4 hour cooking window, and that is usually only during the summer months, as well as difficulty in reaching optimum cooking temperatures (at least 180°F / 82°C).
But we're not crybabies on this Instructables.com site. Right? We fix things and get them working!
The issue here is that the optimum solar cooking angle of the HotPot folding aluminum reflector is about 65° (90° is the sun directly overhead, 0° is the sun on the horizon, shining into the front of the HotPot cooker), as measured from the shiny flat aluminum base the HotPot rests on. And tilting the HotPot and reflector together to focus on that optimum 65° solar angle, to really get things cooking, is a messy and dangerous juggling act. For example, where I live, I have to often tilt the reflector, with a full HotPot, about 30°, until the hinge of the angled front reflector is parallel the the ground.
But it's not as simple as latitude. Refer to the handy global map below, which shows the amount of effective solar energy, in hours, received each day on an optimally tilted photovoltaic panel surface during the WORST month of the year (based on accumulated worldwide solar insolation data). But, with the HotPot solar cooker, the hours posted on the global map can, conversely, be taken loosely as the effective daily cooking hours, during the BEST/WARMEST month of the year. This quickly explains why HotPot™ owners in Arizona and New Mexico give glowing reviews of this device, while HotPot™ owners in America's southeast states (same latitude), or Brazil, or Spain or China, quietly repack their HotPot™s and ship them back the their retailer.
But to get to the point, be sure to heed the "world_solar_insolation_data" chart. If you live in a region that has 3.0-3.9 solar cooking hours a day, or less, during the BEST/WARMEST month of the year, taking out and using your HotPot will be a rare, and disappointing, culinary event.
You also have to understand that, in the Tropics and Third World, the typical HotPot user will prepare a dish to cook that is basically a pre-cooking temperature of ambient temperature: Fresh vegetables from the garden, and a freshly killed chicken, or freshly caught fish, and freshly drawn water. Whereas the typical HotPot user, in the Temperate zone, even those who live off-the-grid, will usually prepare a dish to cook, with a combination of fresh, canned, as well as frozen vegetables, plus refrigerated meat. And, as such, it's not uncommon for Temperate zone users to start out with a HotPot cooking temperature of about 38°F (3°C), that has to fairly rapidly be brought to above 140°F, or the food may spoil, putting them at a serious solar cooking disadvantage, besides latitude.
The "world_solar_insolation_data" chart indicates, for example, 3.0 - 3.9 solar cooking hours, a day, during the BEST/WARMEST summer month of the year, for my area. Not a good sign. So, with my special four-leg / tilting rig, I should be able to cook, at best, through May, June (warmest month), July, and halfway through August. But for simply heating up ambient temperature tap water, the HotPot season is from April - mid-September.
When using my HotPot, I really want to make the decision, to cook, the day before, so as to be able to plan my meal, and maybe invite dinner guests. And that decision is based on living in a suitable zone on the "world_solar_insolation_data" chart, the time of year, and favorable weather forecasts. Using an out-of-the-box HotPot, in my 3.0 - 3.9 solar cooking hour zone (referenced from the map below), that requires: A 2-quart meal maximum, sunny and clear skies, a minimum high noon temperature of 85°F (29°C), a minimum high noon UV Index of 11 (= extreme), no wind, and a minimum high noon solar elevation of 75°. I know that the 75° minimum high noon solar elevation seems to conflict with the 65° optimum solar angle of the reflector, but I'm trying to make the best of a bad situation. And the out-of-the-box HotPot, simply, will not get hot enough to cook on 65° high noon solar elevation days in my area. This information is also transferable to those who live in other latitudes. And that leaves me with only the month of June for solar cooking. So it's obvious that the standard 5-quart HotPot kit definitely needs to be modified and enhanced, for use in the Temperate zones. Hence the need for this Instructables article.
If the "world_solar_insolation_data" chart indicates that you live in a region that is favorable HotPot cooking, you'll then need to find out what months of the year that you can use it: For your seasonal altitude of the sun, the University of Oregon offers their "Online sun path chart program". It's the simplest, and most intuitive, I've found:
Enter the information for your area:
Step 1: *Latitude (decimal degrees)
*Longitude (decimal degrees)
*(south latitude and west longitude are given in negative degrees)
or... ZIP code, instead of Latitude and Longitude
Step 2: Time zone (for simplicity, American time zones are noted)
Step 3: *Plot dates 30 or 31 days apart, between solstices, December through June
*Plot dates 30 or 31 days apart, between solstices, June through December
*(only select one at a time)
Plot hours in local solar time
Step 4: I simply use default:
Extend azimuth axis from 0° to 360°
Extend elevation axis to 90°
Show hours in AM/PM style
Step 5: I generally title it:
"Noon Solar Altitude for"
"(your City, your State)
Default: Places label in upper left corner
(Step 5 is handy, if you also plan to use your HotPot in different places around the world)
Step 6: PDF (Portable Data Format)
Step 7: Simply enter the posted verification code
Two passes through the above steps (changing only Step 3) will generate two landscape, 11" x 8.5" charts that show the angle of the sun, at high noon (solar time), in your area, for different months and days throughout the year. Don't be distracted by the hours on the chart, as high noon is the only position you should be concerned about.
Both printouts will now serve for years of use, as the information on them will not change. When referencing these charts: 90 degrees, on the chart, is directly overhead. 0 degrees, on the chart, is on the horizon.
With this information, the optimum solar elevation, as well as optimum cooking window, in the Temperate Zone will usually begin at about 9:00am, solar time, and end at about 3:00pm, solar time. But afternoon temperatures are usually warmer than mornings, so on hot days, cooking may continue until 4:00pm. 9:00am may seem too early to begin solar cooking, as the solar altitude is quite low (40°, for example), but the temperature in the HotPot will often rise 40-45°F, by 10:00am, giving the cooking process a head start.
Well aware of the performance issues, a new HotPot™ solar cooking kit is now being manufactured, and introduced to those in the Temperate Zones. It includes the original reflector, but now a much smaller, 3-liter black enameled steel pot. The clear glass lid seems to be the same size as the original 5-liter kit, but the 3-liter steel pot (= 2-quart cooking capacity) and clear glass bowl is much shallower, and with a much lower profile. If you live in the Temperate Zone, just be sure that you're ordering the new 3-liter version. Many HotPot™ enthusiasts, in the Temperate Zone, report success when cooking in a half-full 5-liter HotPot™. But, still, many online vendors have temporarily suspended, or dropped the product because of 5-liter HotPot™'s inability to perform well in the temperate latitudes. In fact, when I received my 5-liter HotPot™ shipment, I noticed that the protective plastic film had been removed from the polished aluminum reflectors, indicating that it had been a returned item. But I didn't complain, since it was a discount purchase.
So, to cook at my house, in the Temperate Zone (latitude: 37.5° north), I have drilled out a 1/2" diameter hole, in the center of the flat reflector base, 3" forward of the very center. I then drilled out a 3/8" hole in the center of a sturdy four legged wooden foundation, from an old chair, and inserted a 6-1/2" long, standard 3/8" threaded rod, vertical (perpendicular to the ground), with the HotPot resting on very top of the pipe, on an inverted round black steel lamp base, bolted to the top, that conforms to the glass bottom of the HotPot. The jet-black makeshift lamp base actually is the hottest spot in the cooking process, as the water begins to boil there, as well as intense heat rises up from the metal base and into the 1/2" double-wall area of the bowl. Capitalizing on this effect, I spray painted the upper 1-1/2" of the threaded lamp pipe, as well as the supporting nut and washer, jet-black. The folding aluminum reflector can now safely be periodically tilted and turned to focus the sun's rays, while the boiling HotPot firmly and safely remains stationary. This provides a potential for a 3-5 hour cooking window, depending on the time of year and latitude.
With this setup, I am able achieve the best results possible, for the latitude. For example, on a sunny day, no clouds, a very light breeze, April 5th, northern hemisphere, 37.5° latitude, I poured a gallon of 60°F water into the HotPot, the ambient temperature was also 60°F. I chose to test one gallon of tap water, just to put the HotPot through a demanding test:
Above times are solar time. And, with these results, it's obvious that, where I live, I'll only be able to solar cook, maybe, 2 months out of the year (June - July). The solar altitudes were included on the chart so anyone can apply these results to wherever they live on the planet. And while one can always tilt and rotate the HotPot to track the sun at the optimum 65° solar angle, the solar cooker becomes less and less effective as one moves away from the equator, since the sun's rays are filtered out and weakened by more and more of the atmosphere as the elevation (angle) of the sun decreases. Ambient temperatures were also noted, as that will prove to have a major effect on cooking temperature, as well as cooking times.
I have now settled into 3 quarts (or 3 liters) of water, for HotPot cooking in the temperate zones. This seems to be a good trade off, an adequate meal, that has a better chance of success. And even though the HotPot™ has a 5 liter capacity, 4 liters (or about one gallon) is just too full to stir, handle, and move the pot around, although hot water can be force-fed through the HotPot, a gallon at a time.
That said, on a sunny / partly cloudy day, a very light breeze, April 21st, northern hemisphere, 37.5° latitude, I poured 3 quarts of 63°F tap water into the HotPot, the ambient temperature was 57°F:
* The above heating test actually ended at 3:00pm solar time. But, on a lark, taking the advice of another HotPot owner, I took two 19" x 23.5" clear plastic oven roasting bags, and slit each open to form flat 38" x 23" sheets, then clear taped them together to form a single 38" x 46" sheet. The sheet was snug-fitted over the reflector, for an additional hour. The oven roasting bag trick worked exceptionally well, pushing the HotPot temperature up to 181°F in the afternoon sun, and shows great promise, even though the trapped air in the reflector area did not get that hot. I will soon fashion a lightweight wooden frame, to stretch and staple the clear oven roasting bags (up to 400°F) over it, as they will provide a long service life, if handled with care. This simple rig will allow one to cook on days with a minimum UV Index of 9 (= very high), expand the months of the year that the HotPot can be used, as well as spread its use through the cooler latitudes of the planet, giving its users ample cooking temperatures and extended cooking times.
3 quarts of tap water weighs about 6.42 pounds. This is a convenient reference, as a small chicken, vegetables, and water can be planned so as to not exceed the 6.42 pound solar cooking limit, in the temperate latitudes, and have a better chance of success.
I suspect that the maximum heating capacity of the HotPot, filled with 3 quarts of water (or food) is, mavbe, 110°F above ambient temperature, since the HotPot is not an insulated type of solar cooker. But with the oven roasting bag rig, mentioned above, that limit may well be pushed up to, maybe, 125°F above ambient temperature. Many Temperate Zone HotPot™ users report that their solar cookers can maintain temperatures of 250-300°F, and even a maximum of 350-400°F, but I've found that those temperatures are only possible in an empty HotPot, and pointless information to post and circulate. Wind conditions also reduce the heating capacity of the HotPot.
The sun's rays warm the ground at approximately 1,000 watts, per square meter, at high noon, for a surface perpendicular to the sun's rays, at sea level, on a clear day. And, at the 65° optimum solar angle for the HotPot, that amount is reduced to approximately 906 watts, per square meter. The HotPot relector, at 65°, still presents about 3.5 square feet directly into the sun's rays = 326 watts shining into the reflector. The HotPot+relector combination probably has a solar energy efficiency of maybe about 15-20%. Compare this to a typical 5 foot diameter parabolic reflector solar cooker, commonly used in the temperate latitudes, and priced at about one-third more than the HotPot™ (but the parabolic reflector does not include a pot), with a 19.6 square foot area aimed directly into the sun's rays = about 1,826 watts of solar energy shining into the reflector. The point I want to make is: With the HotPot solar cooker, in the temperate latitudes, careful planning is everything.
Compare this to my 5-quart electric Crock-Pot™ slow cooker. Like the HotPot™, it cooks continuously. After starting off, for about an hour, with a 250 watt "high" setting, the contents warm up to about 142°F, the electric cooker is then switched to the 165 watt "low" setting, slowly heating up to a maximum of 185°F at the end of the day (about 12 hours of cooking). Likewise, with the HotPot, the trick is to get above 70°C (158°F), but stay below 100°C (212°F), as steam is just a waste of energy. But, unlike my electric Crock-Pot slow cooker, the HotPot doesn't have the luxury of extended cooking times. My electric Crock-Pot slow cooker, on the 165 watt "low" setting, compares to the performance of the HotPot+relector combination.
But the HotPot+relector+oven-roasting-bag combination, plus careful pan and tilt tracking, raises its solar energy efficiency and compares to the performance of the Crock-Pot slow cooker on the 250 watt "high" setting (170-205°F). We now have a solar cooker that will reliably function in the temperate latitudes.
The hot water, from the solar cooker test, was poured into a one gallon glass-lined vacuum airpot. A stainless steel-lined vacuum airpot, while far more tough and rugged, simply does not hold their heat for more than a couple of hours. But a glass-lined vacuum airpot will keep my hard-earned hot water, hot: Scalding hot 24 hours later, hot 48 hours later, and warm 72 hours later. A 3/4 gallon airpot of warm water may not sound like much of a resource, but someone in a developing country would be able to completely bathe, shave, and rinse off, and also hand wash and rinse their from the day before. 3-day old warm water can also be much more quickly brought back up to boiling temperature by pouring it back into the HotPot and reheating it. A real convenient feature of airpots is that they have a convenient push-button, or lever-pump, top that allows it to function as a portable hot water faucet, to be placed throughout a summer cabin in the kitchen, dining room, bathroom, or generously loaned to a neighbor.
As mentioned, some HotPot™ owners, in temperate latitudes, report success by fashioning a clear plastic oven roasting bag and stretching it over a lightweight frame, forming a reusable, snug-fitting cover over the reflector. I have tried that method, and it actually does raise the HotPot temperature to where it needs to be, as well as providing strength and rigidity against wind gusts. Other HotPot™ owners suggest preheating the black metal cooking pot and its contents to a full boil, on the kitchen stove, and then taking it outside and placing it inside the clear glass bowl and lid, inside the reflector. Oh puhleeze! What's the purpose of solar cooking, if one has to burn fossil fuels to prepare their meals. Really. But one way to get a legitimate head-start on the cooking process is to use scalding hot water in your recipe, preheated up in a HotPot the day before, and stored in a glass-lined vacuum airpot.
But one still has to respect the heat output of the HotPot reflector. Always wear sunglasses when using the device, and it's sometimes necessary to turn the reflector 180° away from the sun when stirring the pot and making certain adjustments. The raw sunlight radiating out from the reflector sometimes feels like a blast furnace, not to mention the insanely bright sunlight in the face. And never leave the HotPot solar cooker unattended, as children are naturally attracted to it. The HotPot solar oven is an excellent first solar cooker project for the beginner, allowing one to practice the basics of solar cooking safety and precautions, before moving up to larger, and potentially more dangerous solar cookers.
Stay posted for late spring, summer, and early fall boiling tests, as I'm eager to test out some of my tried-and-proven crock-pot dishes on the optimum cooking days, once I find out which days, and weather conditions, are most promising for my area. I have also posted details for a solar powered DC geared motor tracking mechanism that will greatly simplify the solar cooking process.
Step 1: Solar Powered Pan & Tilt DC Motor Drive for the Hot Pot
For the HotPot to be of practical use in the temperate latitudes, I've found it essential to design and build a solar powered DC motor driven base that faithfully tracks the sun, from morning to afternoon, as well as tilts, to concentrate the sun as its altitude rises and sets through the day. Like all solar cookers, the HotPot concentrates and absorbs UV (ultraviolet) radiation from the sun, and internally, converts it to IR (infrared) radiation for cooking. And, due to the relatively small area of the HotPot reflector, everything has to be carefully designed and fine-tuned to work properly.
The heart of the device is a Dayton Model# 2L004 Parallel Shaft Permanent Magnet DC Gear Motor. Powered by up to 12VDC, the 1634:1 gear box turns its 5/16" diameter shaft at a slow and controllable 1.5 RPM, under load, with 46 inch/pounds of torque.
Although the drive motor (a Mabuchi RF-500TB-12560) is designed to run on on 12VDC, with 0.30 Amps draw, it actually has a voltage range of 1.5 - 12 Volts. And to power the tracker, a small 6.3VDC, 120 mA solar panel, 3.125" x 4.25" (80mm x 110mm), was used for the 1/900 horsepower motor, reliably, and with ease, at a no load 1.7 RPM. Unlike the other Dayton 12VDC geared motors, this motor must have been designed to be powered by a relatively weak solar panel, as the 6.3VDC mono-crystalline solar panel, mounted on the advancing side of the HotPot solar cooker reflector, will power the motor, under a no-load condition, even when the panel is turned away, with its back to the sun, since the motor will run with as little as 20 mA. In fact, a much smaller, (55mm x 55mm) solar panel, 2.5V, 85mA, also ran the Dayton gear motor at a no load 0.5 RPM. In comparison, a 12 Volt, 5 Amp battery ran the Dayton gear motor at a no load 3.3 RPM. And with its 1634:1 gear ratio, there is no need for an electronic circuit controller, as the motor easily activates and advances the HotPot solar reflector, to faithfully track the sun.
The 6.3 VDC solar panel is mounted on the advancing side of the reflector, oriented perpendicular to the sun. I also added a 3.25" (83mm) deep corrugated board around all sides, jet-black inside, to block out stray sunlight that sometimes caused the reflector to advance too far ahead of the sun. The north-south ends of the jet-black corrugated board were flared out 0.5" (13mm), since timing wheel is round (my original plans called for a more complicated elliptical timing disk), to compensate for the tilt being off almost 5° at 10:30am and 1:30pm. The center of the solar panel is 5.5" from the edge of the flat reflector base, and 3" forward of the center of the flat reflector base. Of course, make adjustments on your solar panel, as needed. The 6.3 VDC solar panel was mounted on a 5/16" arrow shaft, and snugly inserted into a 5/16" I.D. brass pipe, to allow for quick adjustments.
Weighing in at 1 pound (450g), the geared motor provides much need ballast, to stabilize the reflector against buffeting winds.
Dayton, the manufacturer of the Dayton Model# 2L004 Parallel Shaft Permanent Magnet DC Gear Motor, was bought out by Grainger, who continues to offer the Dayton line, under their name. The geared motor can also be purchased for about $ 70.00 (includes shipping), from Amazon.com:
I actually picked up four of these at a yard sale, for a $ 1.00 each, and I could kick myself for not buying all twenty of those Model# 2L004 geared motors.
The Model# 2L004 gearbox has four mounting screws, all standard 10-32 threads. But to mount the motor/gearbox onto the plywood parallel-o-gram, I removed all 4 screws, careful no to let the gears fall apart, and then drilled out and tapped 3 of the holes for 10-32 threads, but the 4th hole, oddly, was already a 10-32 thread, from the factory. The gearbox was then mounted with four 10-32 thread flat-head screws (1" long), countersunk and flush-mounted to the 5mm plywood.
My plans call for five, round plywood timing disks, 5mm thick. Living at 37.76° latitude, after experimenting, I decided to build one timing disk for the following noon solar altitudes: 65°, 67.5°, 70°, 72.5°, and 75°, since solar cooking with the HotPot on days with a noon solar angle of less than 70° is just not worth it, unless the weather forecast for that day is to be an unusually hot one. I will experience a maximum solar altitude of 76° in my area, and 65° is set as the lowest noon solar altitude that is worthwhile cooking with.
Each tracking timing disk will be from about 4.5" to about 5.25" diameter (subject to change), and will allow the HotPot reflector to pan and tilt, and follow the sun. Each disk is carefully ground down, on a router, to a perfect circle, the circumference will be hollow grooved to accept a black rubber o-ring "tire" with a 1/8" round cross-section. The circumference of each tracking timing disk is determined by the degree sweep of the azimuth of the sun, from 10:00am, to 4:00pm, for a day with a specific high noon solar altitude (65°, 67.5°, 70°, 72.5°, or 75°), and using the 5.34" radius arc travel on a 11.75" diameter tracking base.
To mount the plywood tracking disk onto the 5/16" DC gear motor shaft, I silver brazed a 5/16" allen set screw steel collar onto a 1-5/8" diameter steel washer, drilled and counter-sinked four screw holes, and then mounted the improvised fitting to the disk with flat-head wood screws.
To hold the timing disk on track, as well as hold fast in buffeting winds, a ballast, weighing 4.5 ounces (130 grams) was added to the very center of the disk, and projects out 4.125" (10.3 cm). In such a position, the HotPot reflector will be more stable in gusting breezes, while not placing a heavy load on the geared motor drive. Unfortunately, the eccentric cam action, combined with the ballast, creates excessive drag during its off-center tracking. To greatly reduce this drag, a 28mm diameter wheel (from a toy car, complete with a 5/8" wide rubber tire) was placed directly on the 11.75" diameter plywood tracking base, right under the pivot of the timing disk, on a hinged arm to the lower lazy-susan base, to eliminate the upward tilt drag. It works great, and although the entire parallelogram mechanism will have to be raised 28mm, to keep the HotPot reflector tracking at the optimum 65° solar angle. The 28mm diameter of the wheel is not that important, it's just simply what I found and used, with minimal rolling resistance, keeping the profile of the HotPot solar cooker low enough to stand firm in buffeting winds. I have actually replaced the 28mm wheel with a more robust 40mm (1-9/16") diameter wheel, and will raise the entire parallel-o-gram mechanism 40mm to compensate. Earlier, I had erroneously posted that the clear oven roasting bag cover (details in next step) had somehow changed the optimum solar angle of the reflector, from 65°, to 55°, but later discovered that the low afternoon sun was reflecting off my light-grey cement driveway, bathing the reflector in intense sunlight, causing the HotPot to heat up to its hottest temperature of the day. I just wanted to post this correction. So the optimum solar angle of the HotPot reflector still stands at 65°.
After successfully experimenting with several sizes of timing disks, throughout the season, I plan to redesign the pan&tilt mechanism (yet again) featuring one elliptical disk only, designed for 85° high-noon solar angle, mounted on a extendable, but locking arm, progressively marked for high-noon solar angles of 60° through 85°, to allow more flexibility and greater ease of use. Also a much larger diameter tracking base will be needed, to accommodate the extended locking arm, for those low solar angle days and latitudes. Stay posted.
The solar powered DC geared motor shaft / timing disk turns, on a given day, about 324° in 6 hours. But the HotPot reflector needs to aim much higher in the sky, at high noon, than it does at 10:00am, or 4:00pm.
To accomplish this, an off-center axis was bored into the plywood timing disk. This disk design enabled the pivot base to track the morning and afternoon sun, as well as fully tilt the reflector into the sun. At solar noon, the timing wheel pivot will be at its highest point, turning the reflector 1.75x to 4.33x faster than it does at 9:00am or 3:00pm, as well as reducing the tilted reflector to a more level attitude. The exact position of the timing disk pivot is determined by plotting the overhead speed of the sun, at noon, and comparing it to the travel of the sun at 9:00am, or 3:00pm (these two will always be the same). As the original timing disk, just described, was designed for a windup clock drive, but the details will soon be posted for a timing disk, slightly re-designed for the solar powered geared motor drive.
The DC geared motor drive, with timing disk, is mounted on a simple hinged parallel-o-gram, 4.75", top-to-bottom, and 5.125" span.
For those of you who don't want to bother with building this sun tracking mechanism, the simple parallelogram pan & tilt base will work just fine without it. Built with two 4" lazy-Susan bases (ball bearing) and 10 small hinges, everything pivots on a standard 3/8" threaded rod. It should be a good platform to later add a geared motor tracking device, as going outside every hour or so, to adjust the HotPot reflector, will easily monopolize one's day.
A 11.75" diameter plywood disk serves as the tracking base, marked off in degrees, for a total maximum sweep of 180°, where the timing disk is carefully set down at a specific azimuth, at 9:00am (but starting out at later times will also work), to begin the solar cooking process. In my latitude, the mechanism will do a 140° sweep (on 65° noon solar angle days) to a 163° sweep (on 75° noon solar angle days). As my HotPot DC motorized tracker / oven roasting bag solar cooking rig is intentionally designed for cooking temperature overkill, the 11.75" diameter plywood base will also serve for temperature adjustment: Simply lifting up the tracking timing disk, and re-positioning it forwards, 10 degrees, or so, will skew the vertical tracking angle and effectively position the reflector slightly out of direct sunlight, allowing one to easily, accurately, reduce the cooking temperature of the HotPot, in a controlled manner.
To facilitate the setup and accurate positioning of the timing wheel, a 3-position, "Advance-Off-Reverse" DPDT electrical switch (Radio Shack #275-664) was mounted and wired to just below the sun-facing edge of the solar panel. This streamlines the setup of the HotPot cooker: Removing the solar panel from its holder, it's held up to the sun and the Advance-Off-Reverse switch is activated to turn the timing disk to the high noon position, and then the switch is set to the "Off" position. The timing disk is then lifted up and moved to the 270° high noon position (the 40mm wheel will automatically follow), with the axis of the 40mm wheel directly over the 270° mark on the 11.75" diameter tracking base. Again, holding the solar panel up to the sun, the "Reverse" switch is activated to slowly turn the HotPot reflector into the morning sun, and then turned off when aligned and positioned at the correct angle to the morning sun. This can to be accurately done by standing behind the HotPot reflector, and, using the "Advance-Off-Reverse" electrical switch, turning the reflector until the sun, reflector, and its shadow are in alignment, and then switched to the "Off" position. But actually, it's easier to simply turn the reflector past the solar alignment point, and let the solar-motor bring itself back into correct alignment. The solar panel is then placed back in its holder, and switched to the "Advance" position, to begin automatically tracking the sun.
If, later in the day, an unusually strong wind blows the HotPot reflector upwards and out of alignment, it is repositioned, again, from the 270° high noon reference position, and then Advanced or Reversed into correct solar alignment. I've found this method to be much quicker and reliable than the initial confusing reference marks I scribed on the timing disk.
The rotating and tilting base for the HotPot reflector has a 19" long x 1/8" wide x 1/8" deep groove, for the center hinge of the aluminum reflector base to nest in, to make accurate positioning quick and sure, as well as four wide-head aluminum screws to hold the reflector in place, as it steeply angles and to also hold fast in buffeting winds.
In an effort to initially cut costs on the tracker, I unsuccessfully experimented with using the gears / generator, from a hand-cranked LED flashlight, using the generator as a DC motor. Powered by the small 6.3VDC, 0.12 Amp solar panel, 3.125" x 4.25" (80x110mm), its 94:1 gear ratio struggled, but still sometimes stalled when it began the rapid upward tilt, towards high noon, caused by the off-center torque (creating drag) from the o-ring tire on the 11.75" plywood tracking base. Even with all those reduction gears, it's still not geared down enough. Even when the tiny generator/motor was replaced with a small 3VDC high-torque motor salvaged from an old VCR camera, but it still tended to stall on the rapid upward tilt, towards high noon. But I suggest checking out another Instructable that features a simple capacitor-based circuit board that should provide enough intermittent torque to do the job, if combined with the LED flashlight gearbox and "motor":
And I also unsuccessfully experimented with an Intermatic™ 6-hour wall timer, Model# FD6H, windup clock drive. While it worked perfectly on my workbench, it couldn't stand up to the buffeting winds in an outdoor environment. Also, while converting the HotPot over to a solar powered geared motor drive, I discovered that the lower lazy-susan base had become contaminated with airborne dirt and grit, and no longer rotated freely, completely explaining the problem in getting the original clock driven mechanism to function properly during the outdoor testing phase. I cleaned out the greased ball bearings of both upper and lower lazy-susan bases with solvent, and then lubricated them with an aerosol graphite lubricant, commonly used for lock mechanisms. And as a further precaution, loosely tied a pipe cleaner around both the upper and lower lazy-susan bases, to prevent any further contamination. The tracker now freely rotates, as it did during the initial indoor tests. For you experimenters out there, it's also important to note that these Intermatic™ clock timers are now in short supply, as the Intermatic™ website no longer seems to be running and hardware stores no longer stock these reliable and well-made devices. There are other brand(s) of household windup clock drive wall timers on the market, but they are not well made and are unreliable, and are, simply, false economy. I'd really prefer to run this rig with the 6-hour windup clock drive, and will make more attempts to eventually get it to work. Call me sentimental.
But this DC motor-driven tracker is still not enough for solar cooking in the temperate latitudes, as a clear plastic oven roasting bag needs to cover the reflector to bring the HotPot quickly and reliably up to proper cooking temperatures.
The HotPot, full and quite heavy with 3-4 quarts of water or food, resting on top of the 3/8" threaded rod, is completely stationary, and stable, throughout the entire process, as everything, reflector and all, revolves around it.
See the next part of this Instructable for the oven roasting bag details.
Step 2: Fashioning an Oven Roasting Bag for the Hot Pot Reflector
As mentioned earlier, the HotPot reflector, to function in the temperate latitudes, needs to be covered with a clear plastic oven roasting bag. At my latitude (37.76°), one is needed, as the HotPot is just too unpredictable and unreliable without it.
The clear plastic oven roasting bag is designed to hold up in oven temperatures of up to 400°F, but temperatures in the HotPot reflector area will never get that high. And only time will tell how long it will hold up to direct sunlight.
For the first oven roasting bag test, I purchased Reynolds® Turkey Size oven roasting bags, two to a box, 19" x 23.5" (482mm x 596mm). Both bags were slit open to 38" x 23", and then clear taped together to form a 38" x 46" sheet. The HotPot reflector was completely covered with the oven roasting bag, taped it in place, with promising results.
But after stretching the Reynolds® oven roasting bags over my hinged wooden frame and trying it for a second test run (see below), I was not satisfied with the results, and removed the oven roasting bag covering from the wooden frame. The Reynolds® Turkey Size oven roasting bags are, simply, not clear. They are more of a translucent beige, in appearance, which not suitable for solar cooking.
I have just purchased a pack of Heuck® jumbo oven roasting bags Item# 00124, two to a pack (www.heuck.com), from a local hardware store, and installed them on the wooden frame. These oven bags are crystal clear, also designed to hold up in oven temperatures of up to 400°F, they are about .0006" thick (versus .001" for the Reynolds® oven roasting bags), about the same size as the Reynolds® bags, and have more dimensional stability than the Reynolds® bags.
But the Heuck® roasting bags are a little more difficult to install in that the bag has a welded seam on both sides, with a fold at the bottom (which will be slit open). While the weld-seam will hold quite well during the stretching process of installing on the wooden frame, the weld-seam can easily be pulled apart from the end and zipped open, if not careful. After installing (stretching and stapling), a clear 3/4" wide plastic tape was applied over all the staples (underneath the wooden frame) and overlapping to the sides, to prevent the Heuck® roasting bags from tearing out from its staples. And, while setting up for a HotPot test run, I accidentally poked my finger through a seam, but quickly repaired it with a .25" x 1.5" piece of clear adhesive tape.
But I have not experienced reflector temperatures of more than 160°F, so there are many clear plastic, durable films out there that should do just fine, instead of using flimsy oven roasting bags. I'm checking on other plastic films that are crystal clear, thicker, with resistance to abrasion, can withstand temperatures of up to 200-250°F, and are UV resistant. Stay posted for the results.
I have a real problem with the idea of using "disposal" items for solar cooking, so I urge you to also build this wooden frame, to make the oven roasting bag covering a permanent and practical feature. So I have built a lightweight wooden frame, hinged and grooved, to fit snugly onto the HotPot solar reflector. The thin, horizontal wooden spanners are carefully angled at 65° (also the optimum solar cooking angle), so as to block as little direct sunlight as possible. Carefully sanded to a smooth finish, the frame will prevent snags and tears, greatly extending the service life of the "disposable" oven roasting bags.
But the wooden frame will also provide much needed strength and rigidity to the flimsy aluminum reflector, allowing it to stand up to gusting and buffeting winds. Those delicate, thin aluminum plates, with aluminum hinges... and aluminum hinge pins... and aluminum rivets, simply cannot not stand up to buffeting winds without being easily damaged and destroyed, if not reinforced.
The wooden frame, for the oven roasting bag covering, is hinged at the front, and is required for the frame to fit onto the odd-shaped reflector, as well as occasionally opening up the top, or front, to stir and inspect the progress of the cooking. Held together with 4 small hinges, the 29.5" long hinge joint also allows for convenient stowing and transporting.
The wooden frame is secured to the HotPot reflector by four long rubber bands, each attached to an eyescrew at the front, rear, and left and right sides (at the hinge joint), and the other end of the rubber bands hooked onto an eyescrew at each of the corners of the 5mm plywood base that the reflector is mounted to. This provide additional strength and rigidity to the rig, as well as making it much easier to open up the top of the wooden frame, to inspect and stir the HotPot contents.
I'll soon upload some more photos on this, as well as my cooking test results.
With this clock drive/Reynolds® Turkey Size oven roasting bag setup, I was able achieve decent results, for the latitude. For example, on a sunny day, no clouds, a very light breeze, May 5th, northern hemisphere, 37.5° latitude, I poured a gallon of 65°F water into the HotPot, the ambient temperature was 75°F. I chose to test one gallon of tap water, just to put the HotPot through a demanding test:
I'm not exactly wild about the morning HotPot temperatures, as the real star of the show was actually the 75°-85°F ambient temperatures during the day. But there was a problem achieving a required 20° tilt of the HotPot reflector at 9:00am, since it was the first test run for the exotic device. But a simple position modification of the plywood base, which the reflector is fastened to, should allow it to now tilt up to 29°.
With the clock drive/clear Heuck® jumbo oven roasting bag setup, I was able achieve the best results possible, for the latitude. It was on a bright, sunny day, no clouds, UV Index of 7 (= high), gusting breezes, May 9th, northern hemisphere, 37.5° latitude, I poured a gallon of 63°F water into the HotPot, the ambient temperature was 61°F. I chose to test one gallon of tap water, just to put the HotPot through a demanding test:
* The windup clock drive ran out at 3:00pm, but continued to face the sun, the reflector frozen at a 19° tilt. But the low afternoon sun, reflecting off my light-grey cement driveway, bathed the reflector in intense sunlight, causing the HotPot to heat up to its hottest temperature of the day. So note that the optimum solar angle of the HotPot reflector still stands at 65°. Also, the final HotPot temperature of 186°F, at 5:00pm, is important since one of the drawbacks of solar cookers is that the cooking periods do not mesh well with lunch and supper: Most solar cooking devices don't really start cooking until about noon, and, by mid-afternoon, the solar cooking window is usually closed. But, with the above HotPot rig, supper will be served hot on the table.
The downside of all my research, tweaks, and experimenting is that my unique HotPot rig will probably not be predicable and reliable on days with a noon solar altitude of less than about 70°. Also, a minimum UV Index of 7 is required, since it's the UV radiation the actually does the cooking. And, for my location, that means reliable solar cooking for the months of May, June, and July, with maybe a few hot days in April, or August. This means that my HotPot will probably be useful on as many days as my old fashion ice cream freezer. Sure, successful cooking may be done on, say, a 65° noon solar altitude day, but there is no reserve cooking leeway, in case the weather turns out to be not as good as predicted. But the above test notes were for a gallon (4 quarts) of water, so one can easily opt for a 3-quart, or a 2-quart recipe and have more control of the cooking process. This information is transferable to those who live in other latitudes, as noon solar altitude is really where it's at.
Glancing over the above notes, it's not a good idea to start cooking at 9:00am, as the HotPot temperature, by 11:00am, is only 142°, and food spoilage occurs within two hours if the food fails to reach a temperature of 140°F. And 142°F is just pushing it too close. It's much safer to start cooking at 10:00am, with a starting minimum solar altitude of 57°. But starting at 9:00am can be useful for simply getting a head-start on heating up water, or baking bread.
Compare the above HotPot temperatures to a standard 5-quart plug-in electric Crock-Pot™ slow cooker. Like the HotPot™, it cooks continuously. After starting off, for about an hour, with a 250 watt "high" setting, the contents warm up to about 142°F, the electric cooker is then switched to the 165 watt "low" setting, slowly heating up to a maximum of 185°F at the end of the day (about 12 hours of cooking). Likewise, with the HotPot, the trick is to get above 70°C (158°F), but stay below 100°C (212°F), as steam is just a waste of energy, as well as a critical loss of water from the cooking process.
But, unlike an electric Crock-Pot slow cooker, the HotPot doesn't have the luxury of extended cooking times.
But the HotPot+relector+oven-roasting-bag combination, plus careful pan and tilt tracking, raises its solar energy efficiency, and compares favorably to the performance of the Crock-Pot slow cooker on the 250 watt "high" setting (170-205°F). We now have a solar cooker that will reliably function in the temperate latitudes.
Also, the "Global Effective Solar Exposure Chart (pictured below) which illustrates the daily HotPot+relector solar cooking hours, on the best summer month of the year (June, in the northern hemisphere): The 3.0 - 3.9 solar cooking hours, a day, for my area, are basically doubled, if using the HotPot+relector+oven-roasting-bag combination, plus careful pan and tilt tracking.
So, for now, guys, it looks like we're cooking with nuclear fusion.
Step 3: Cooking With the HotPot
The first point I want to make is that the HotPot solar cooker does not saute vegetables, nor brown meat. This is a slight disappointment, since an empty HotPot can quickly and easily be brought up to a temperature of 350-400°F. And I had hoped that simply adding a meager 7 ounces (200g) of chopped onions and 2 fluid ounces of cooking oil to a hot, empty HotPot, would allow for a high-temperature quick stir-fry, until slightly browned. But the process, believe it or not, took 5 hours. Not exactly worthwhile.
On second thought, those simple extra culinary steps we take to make our soups and stews taste better on our natural gas stove (preliminary stir-fry, oven roasting, caramelizing, etc.) are, in fact, extremely energy intensive, often using more energy than is used for the main extended simmering process. So we have decided to scale back our flavor expectations, somewhat, with our HotPot cooking. And, if possible, develop new recipes that will work within the limitations of solar cooking, without compromising on flavor. We always like a challenge.
I also tried baking a one pound loaf of yeast-raised bread in the off-the-shelf HotPot-Reflector, but it took so long to bake that the finished loaf was dried out, crumbly, and not very tasty. But I'll try again with my new hacked HotPot tracking rig and see if a faster cooking time will produce a better loaf of bread.
A pleasant surprise is that the double-walled HotPot keeps the prepared meal hot on the table, through the entire meal, with its insulating properties.
But I decided to prepare and cook a HotPot full of chicken soup, since a favorable forecast was predicted, for July 30, 2011, of a 67°F solar noon temperature, a bright and clear sunny sky, and a noon UV index of 10 (very high). But I got carried away with the soup preparation and violated my 3 quart (2.85 liter) rule for cooking, and made up a 1.04 gallon (3.95 liters) recipe. Weighing 8.77 pounds (3.99 kilograms), the mix of refrigerated chicken, frozen vegetables, fresh and canned vegetables, the contents of the HotPot had a temperature of 37°F (2.8°C), I knew that I would be starting out at a serious cooking disadvantage. And if the weather forecast turned out not to be as favorable to solar cooking, cooking more than 3 quarts would leave me with no latitude, and no margin of error.
As the noon solar altitude would be 71° for that day, I installed the 70° timing wheel (4.92" diameter, with 1.22" off-center axis).
And, unfortunately, the morning weather turned out to be not as forecast, and was cloudy to partly-cloudy all morning. And even with my enhanced HotPot rig, with pan-and-tilt solar tracker, and oven roasting bag covered frame, I knew I was really pushing the limits of my solar cooking setup. But, by noon, the sky turned bright and sunny, with no clouds, UV Index of 10 (= very high), and with light gusting breezes.
Here are the results, for July 30th, northern hemisphere, 37.5° latitude:
* As the timing wheel was initially designed for a 6-hour windup clock drive, the solar powered geared motor tracker was turned off at 3:00pm, leaving the solar reflector at its lowest tracking angle, but continued to face the sun, the reflector frozen at a 20° tilt. But the low afternoon sun, reflecting off my light-grey cement driveway, bathed the reflector in intense sunlight, causing the HotPot to heat up to its hottest temperature of the day. One could probably achieve the same end-of-day results if the HotPot was setup on the edge of a lake.
Also, the final HotPot temperature of 176°F, at 6:00pm, is important since one of the drawbacks of solar cookers is that the cooking periods do not mesh well with lunch and supper: Most solar cooking devices don't really start cooking until about noon, and, by mid-afternoon, the solar cooking window is usually closed. But, with the above HotPot rig, supper was served hot on the table.
But, as mentioned earlier, I was really pushing things, as the potatoes were still not quite done at 5:00pm. But everything fell into place by 6:00pm, and I served up the fully cooked meal to my appreciative wife.
Also, in the early cooking process, it took about 3 hours for the contents of the HotPot to rise from 65°F to 140°F. That's not safe to practice as a habit, as food can spoil if the temperature rise takes longer than 2 hours, to rise from ambient to 140°F. In fact, the United States Department of Agriculture goes one step further and states that internal food temperature should not remain between 40°F and 140°F for more than two hours. A more conservative, 3-quart soup preparation would have kept the required cooking temperature rise on track.
A sound cooking practice, from now on, is to place the black steel inner bowl, which weighs 1.5 pounds (0.68 kilogram), on a scale, and add the contents of the recipe until the bowl + food weigh no more than about 7.9 pounds (3.59 kilograms) = about a 3-quart (2.85 liters) meal. Also, those who live at the more northern latitudes, 2-quart meals may be a safer option.
I would post the above chicken soup recipe, but it was not as palatable as our usual chicken soup recipe, with heavy portions of diced and stir-fried onions, and oven roasted chicken, which greatly enhances the flavor of the soup. But I will experiment with some of my wife's absolutely delicious Crock-Pot recipes, and post them as soon as we try them out, as they show great promise.
As mentioned earlier in this Instructable, hot water can be force-fed through the HotPot, up to 2 or 3 times throughout a hot day. And I store the hot water in 0.75 gallon (2.84 liter) glass-lined vacuum airpots. A stainless steel-lined vacuum airpot, while far more tough and rugged, simply does not hold their heat for more than a couple of hours. But a glass-lined vacuum airpots will keep my hard-earned hot water, hot: Scalding hot 24 hours later, hot 48 hours later, and warm 72 hours later. A 3/4 gallon airpot of warm water may not sound like much of a commodity, but someone in a developing country would be able to completely bathe, shave, and rinse off, and also hand wash and rinse their clothes from the day before. 3-day old warm water can also be much more quickly brought back up to boiling temperature by pouring it back into the HotPot and reheating it. A real convenient feature of airpots is that they have a convenient push-button, or lever-pump, top that allows it to function as a portable hot water faucet, to be placed throughout a summer cabin in the kitchen, dining room, bathroom, or generously loaned to a neighbor.
With a favorable forecast predicted for August 2: A 68°F solar noon temperature, a bright and clear sunny sky, and a noon UV index of 10 (very high), I decided to give this promising recipe a try. I successfully cooked it in a charcoal dutch oven a couple of years ago, but lost the recipe.
But here's a close approximation:
Boston Market Style Zucchini-Squash Casserole
8 ounces (0.5#) water, 170°F (pre-heated the day before in HotPot solar cooker, and stored in a thermos).
1/2 stick (2 ounces / 0.13#) margarine, room temperature.
12 ounce can (0.75#) evaporated milk.
39 ounces (2.47#) of yellow squash and zucchini squash, cut up into bite size chunks.
6 ounce (0.38#) chopped onion.
8 ounces (0.50#) grated sharp cheddar cheese.
6 ounce (0.38#) of package of dried seasoned corn bread stuffing.
= 6.61 pounds total - 1.5 pound black steel liner pot = 5.11 pounds net
Melt the stick margarine in the hot water, add the rest of the ingredients, and stir.
The mixture had a starting out temperature of 90°F, before being placed in the HotPot reflector.
Weighing in at well under the 6.32 pound (2.79 kilograms)/3 quart (2.85 liter) rule for HotPot solar cooking, the high-bulk casserole actually occupied about 1 gallon (3.80 liters) in the HotPot, but for solar cooking, weight is more relevant than volume. This time I knew that I would be starting out with a favorable cooking advantage. And if the weather forecast turned out not to be as favorable to solar cooking, I would still have ample latitude, and a wide margin of error.
As the noon solar altitude would be 70° for that day, I installed the 70° timing wheel (4.92" diameter, with 1.22" off-center axis).
Although the morning weather started out with cloudy skies, the day was clear and sunny by 10:00am solar time, with no clouds, a noon UV Index of 10 (= very high), and with light gusting breezes.
Being a casserole, the mixture was not stirred after 10:00am, and the internal HotPot temperature was measured in 2 places, and noted below, as center-edge measurements.
Here are the results, for August 2, northern hemisphere, 37.5° latitude:
The casserole began to simmer at about 1:00pm, and began to brown around the edges at 2:00pm. By 3:00pm, the top of the casserole was lightly browned. Sampled each hour, the casserole was actually fully cooked by 2:00pm. And, unlike the chicken soup recipe, this one was absolutely delicious. I continued to cook the casserole as I was clearly excited about the newly-discovered browning capability of this solar oven, with this recipe at least. The volume of the finished casserole was considerably less than a gallon, due to evaporation, as well as my uncontrolled sampling.
But really, for the beginner, a simple, off-the-shelf HotPot can still make an impressive showing at any Earth Day event: Just open up four to six 15 ounce cans of a high quality soup, chili, clam chowder, or baked beans, and quickly heat it up in the HotPot. And you'll be a local hero.