Instructables

Hot Pot is a great solar cooker, but was designed for the Tropics

Step 2: Fashioning an Oven Roasting Bag for the Hot Pot Reflector

Picture of Fashioning an Oven Roasting Bag for the Hot Pot Reflector
HotPotOvenWithRoastingBagCover.jpg
GlobalEffectiveSolarExposureChart.jpg
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:

_solar____ambient__________HotPot________reflector______solar
_time:____temperature:______temperature:___temperature:___altitude:
_9:00am____75°F_____________65°F__________75°F________45°
10:00am____79°F____________110°F_________131°F________56°
11:00am____81°F____________149°F_________138°F________65°
12:00pm____84°F____________174°F_________151°F________69°
_1:00pm____84°F____________188°F_________135°F________65°
_2:00pm____85°F____________197°F_________137°F________56°
_3:00pm____84°F____________199°F_________148°F________45°

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°.

Finally!

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:

_solar____ambient__________HotPot________reflector______solar_______wind_speed
_time:____temperature:______temperature:___temperature:___altitude:_____gusting:
_9:00am____61°F_____________63°F_________102°F________46°_______to__4mph
10:00am____67°F____________108°F_________124°F________57°_______to__4mph
11:00am____67°F____________142°F_________132°F________66°_______to__5mph
12:00pm____69°F____________164°F_________123°F________70°_______to__9mph
_1:00pm____71°F____________172°F_________123°F________66°_______to_11mph
_2:00pm____71°F____________182°F_________119°F________57°_______to__9mph
_3:00pm____69°F____________186°F_________119°F________46°_______to__9mph
*4:00pm____68°F____________191°F_________137°F________34°_______to__8mph
*5:00pm____67°F____________186°F_________136°F________22°_______to__6mph

* 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.
 
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