Abstract: Three different lids were compared for use on greenhouse
pots in solar panel cookers: a simple black steel lid, a two-part
glass-and-steel greenhouse lid, and a simple transparent glass lid.
To compare two lids, water was heated under them simultaneously in similar
greenhouse pots in side-by-side cookers. The thermal performances
of the three lids were found to be substantially equal.
Plastic oven bags are often used as greenhouse enclosures in solar panel cookers. [Kerr1] The plastic oven bag encloses the pot and its lid, denying easy access to the pot, and retaining cooking vapors which fog the greenhouse bag and reduce its solar heat gain. A greenhouse pot does away with the plastic bag and its problems by integrating a greenhouse into the pot. Roger Bernard [Bernard1] first described the use of a glass bowl as a greenhouse that encloses the bottom of the pot, giving the cook free access to the lid, and allowing cooking vapors to escape without fogging the greenhouse.
A previous article [Delaney1] described the integration of a pot and greenhouse bowl into an easily handled assembly. The same article described a two-part greenhouse lid. A greenhouse lid consists of a transparent top and a metal bottom, or pan, separated by an enclosed air space. The top, or inner, surface of the pan has a black surface to absorb solar energy received through the glass top. The air space serves to reduce heat loss through the lid.
This work seeks to determine whether the greater complexity of the greenhouse lid is justified by a thermal advantage over simpler lids. To this end, I ran a series of trials to compare a greenhouse lid, a black steel lid, and a transparent glass lid.
I compared the three lids in pairs. In each comparison, two different lids were placed on nearly identical pot/stand combinations in similar solar panel cookers, and exposed simultaneously to nearly identical solar conditions for timed periods. Each pot of the pair contained 2 liters plus or minus 10 ml of cold water at the same known initial temperature (usually about 10 C) . I measured the temperature of the water in each of the two pots at nearly equal intervals (about once per hour). My thermometer was the kind used in high school science labs. It was glass, and had a Celsius scale from 0 to 110 degrees. I could read it reliably to 0.5 degrees. I simply lifted the edge of the lid, inserted the thermometer, waved it in the water, keeping the lid in place and as low as possible, until I had a stable reading, then removed the thermometer, closed the lid, and promptly repeated the procedure on the other pot.
Each time I took the temperatures, I re-oriented both cookers to point approximately a half hour ahead of the sun.
Each comparison experiment continued until flattening of the higher temperature curve indicated that its pot had reached a final temperature. The resulting temperature profiles give a measure of the relative thermal effectiveness of the different lid types.
At the beginning of each day of trials, I removed accumulated dust and grime from the reflective surfaces of the cookers by wiping with vinegar applied from a spray bottle.
The same two cooker configurations were used for all experiments of the series. Cooker configuration 1 consisted of pot 1, stand 1, and solar panel cooker 1. Cooker configuration 2 consisted of pot 2, stand 2, and solar panel cooker 2. (The stand is the glass bowl used as the lower greenhouse.) Although the two configurations were constructed as nearly identically as possible, the experimental procedure was intended to prevent remaining differences from affecting the conclusions. Thus each comparison of lid x to lid y was carried out in multiple paired experiments. In the first experiment of a pair, lid x was tested in cooker configuration 1, and lid y in cooker configuration 2. In the second experiment of a pair, lid x was tested in cooker configuration 2, and lid y in cooker configuration 1.
In fact, there was a noticeable systematic difference between the two cooker configurations. Configuration 1 consistently out-performed cooker configuration 2, achieving a final water temperature a few degrees higher than configuration 2 would in the same circumstances. Because of the sequence and nature of the trials, I did not notice this difference until the sequence of trials was well under way. Because no trials were run with identical lids in both cookers, the performance difference between the cookers was deduced from the observations collected over multiple experiments. The magnitude of the difference was comparable to the difference produced by the differences between the lids.
The two cooker configurations seem to me to be displaying consistent relative performance with different lid types throughout the trials.
A typical pair of trials with lids x and y would go as follows. With lid x in configuration 1 and lid y in configuration 2, there would be a final temperature difference (dT) of, say, 9 C, with the water in cooker 1 reaching the higher temperature. A subsequent trial, with the lids exchanged between the cookers would then produce an insignificant, or much smaller dT, of, say, 1 C, with either cooker achieving the higher temperature.
I would conclude from such a pair of trials, that lid x performed better than lid y, with a characteristic dT of roughly half the observed difference between the trials.
A combined difference between the two cookers and two lids may be seen most clearly in trials 7 and 12, shown in the figure.
The difference between the cookers may have been due to a difference in the reflectance of the aluminum foil incorporated in each. Although the geometry, dimensions, and method of construction were identical, the cookers were constructed several weeks apart with rolls of aluminum foil of slightly different characteristics. Once the difference had become manifest in the experimental data, I looked carefully at the two cookers standing side by side. I fancied that I was able to detect that cooker 1 was slightly more reflective, although the visibility of the difference was slight. The difference was not due to using the shiny side of the foil in one cooker and the dull side in the other, which would have produced a greater difference.
I did not have time to relaminate one of the cookers to bring their
performance closer together and then re-run the tests performed before
the discrepancy was discovered. It did not occur to me until the
end of the period of this series of experiments to measure the difference
between the cookers directly by running experiments with identical lids,
and so at least have an accurate characterization of the difference.
In the end, I did not have time to do this, either. In spite of these
deficiencies in the experimental procedure, it was possible to draw
conclusions from the data about the relative performance of the lids tested.
The glass bowls for the lower greenhouses (there is an upper greenhouse only when there is a greenhouse lid on the pot) were a pair of transparent glass mixing bowls. The metal cooking pots were a pair of stainless steel mixing bowls of a kind that has a slightly flaring edge and no handle or thumb ring. The stainless steel bowl was of a size to hang suspended from its rim inside the glass bowl. I painted the outer surface of the steel bowls flat black.
It was a challenge to obtain suitable components for pots and lids.
I spent several days in kitchen shops in Ottawa, looking for components
that would fit each other. I had to buy glass bowls in one shop and steel
bowls in another shop. Pans that might fit the steel bowls as lids were
only to be found in yet other shops. As I shopped I carried a ruler,
and went from shop to shop and back, carrying a list of measurements of
components available in the different shops. At one point I bought
a glass and steel bowl that went together particularly well, and carried
them around looking for a suitable pan to serve as a lid. In the
end, I discarded that first set of bowls in favor of a completely different
set, because I could not find compatible lid components. In the end,
I accepted, for lack of a better fit, steel and glass bowls that did not
fit perfectly with each other, the steel bowl projecting an unnecessary
1 cm above the glass bowl when nested in it.
for photo of glass bowls, steel bowls with water, and meter stick.
The greenhouse lid was constructed from a piece of glass and a steel pan stuck together with silicone sealer. The bottom, or pan, of the greenhouse lid was a shallow steel cake pan, very similar to a pie plate, but slightly deeper, and with steeper sides. The pan was of a size to jam neatly into the top of the steel bowl. Three of these cake pans were used, one as the bottom of greenhouse lid, another as the simple black metal lid, and a third, with its bottom cut out, as the rim of the transparent lid.
I painted the tops of the pans with the same flat black paint I used
on the steel bowls. The top element of the transparent lid
was a piece of 3 mm window glass cut to size and circular form by a local
hardware store, and attached to a metal rim cut from a cake pan.
An identical piece of glass attached to the top of a whole cake pan served
to construct the greenhouse lid. Various drawings in this article
show the dimensions and construction of these assemblies.
The use of a metal rim for a transparent top is not, in general, a good idea. A transparent lid designed for manufacture would be formed from a single piece of glass with no metal rim. A metal rim reduces the performance of the transparent lid by increasing heat loss at the edge, and adds to its cost. I provided a metal rim for the transparent top in these experiments to ensure a fair comparison between the different lid types. The greenhouse lid and the simple metal lid had rims which protruded above the pot to an extent which was unnecessary in principle, but which was unavoidable in this case because of the limited choice of components from which to assemble the pots and lids. By cutting the bottom out of a pan identical to the one used for the greenhouse lid and the simple metal lid, I created a rim for the simple transparent lid which should have reduced its thermal performance in a comparable way, and put it on a fair footing with the other two lids. Thus all three lids had identical unnecessary metallic protrusions above the pot at the edge of the lid, reducing their thermal performances equally.
Click for photo of pot assemblies with greenhouse lid and transparent lid.
The flat black paint that I used was Beauti-Tone High Heat Barbecue
Paint, in a spray can, available from Home Hardware, (bearing a Home Hardware
logo) and described as a water-based silicone enamel. I recommend
it, at least for experiments like these. Its ability to resist very high
temperatures (538 C) is not required in this application, but it
was inexpensive, convenient, and had no troubling solvent odor. The can
contains far more paint than you are likely to need, even for multiple
projects of this kind. Although it carries no health warnings, I would
not apply it to surfaces intended to contact food. It adhered well
to the outer surfaces of the stainless steel bowls, and to the top surfaces
of the steel cake pans. It produces what seems to be a very effective
flat black finish. I have no idea of the numerical value of its absorbtance,
but it seems to work well as a surface to absorb solar energy.
For my two solar cookers, I adopted a design by Roger Bernard [Bernard1] He called this design the Reflective Open Box Cooker, or ROB. My cookers were constructed from double-thickness corrugated cardboard taken from new packing boxes ( "china crates") 18"x18"x28" in size (46cm x 46cm x 71cm).
Click for close-up photo of ROB cooker from above, with meter stick (40 KB)
Click for photo from right of two ROB cookers, with meter stick (37 KB)
Click for photo from left of stool and two ROB cookers, with meter stick (44 KB)
The aspect ratio of these boxes made it impossible to follow the construction procedure used by Bernard. I was not able to use the box structure of my boxes. I had to cut the boxes into pieces, which I re-assembled into cookers with the aid of spray contact cement. I used 3M Super 77 spray contact cement. Contact cement permits freehand assembly of structures that would otherwise require complicated clamping. The dimensions of the resulting cookers are given in the accompanying drawings, but not the details of the components and the construction sequence.
I formed the reflective surfaces by laminating aluminum foil to the cardboard. I used Loblaw's No Name Foil for Barbecue for cooker number 1, and Loblaw's No Name Aluminum foil for cooker number 2. (A mistake, as described above.). The barbecue foil is somewhat thicker than the usual supermarket foil, making it easier to handle. It also seems to have better reflectivity.
I attached the foil to the cardboard with Krylon spray glue, which has the advantage that it takes several hours to set, allowing time to correct fumbles and remove wrinkles. The Krylon glue does not seem to cause the cardboard backing to warp, although this may be due to the double thickness of the backing in this case. Even with the flexibility provided by the Krylon glue, and considerable effort, I found it impossible to produce wrinkle-free reflective surfaces by my methods. The Krylon will not form a good bond unless allowed to cure completely, in a horizontal position for 24 hours. Even then, I had problems with delamination after a few weeks of use, requiring the foil to be pressed down before each use. This problem of progressive delamination may be caused by repeated heating of air trapped under the foil in small bubbles and wrinkles.
The cookers work well in spite of the wrinkles. Each of the two cookers had a total reflective area (sum of areas of all reflective surfaces) of 1577 square inches (11.95 square feet, 1.11 square meters.)
for photo of cooker on table, with wrinkles (50 KB)
Preliminary tests of these cookers indicated that the ROB is a highly effective design. Several meals were cooked in one of the cookers, including several cakes, rice, beans, and a large (two liters) beef stew. In preliminary tests, one of these cookers brought two liters of water (in one of the pots described above, with a greenhouse lid) from 11 C to 99 C in three hours on a clear June day in Carleton Place, Ontario, Canada (just west of Ottawa, 45.3 degrees north latitude, elevation approximately 50 meters), the location where all of the tests described here were conducted.
I build my own panel cookers because I wanted the experience, and because I wanted to try cookers of larger capacity than the designs I had seen. You may wish simply to purchase commercially available panel cookers, such as the Cookit, available from Solar Cookers International.
Solar Cookers International
1919 21st St., Suite 101
Sacramento, California 95814
Here are the data for the 12 trials that were formally recorded. A number of informal trials and cooking experiments were also performed, but are not reported in detail.
In these 12 trials, both pots contained 2 liters of water at the same initial temperature. All ambient temperatures were between 20 C and 35 C, often increasing over a substantial part of this range during a trial. Variations in the ambient temperature were not thought to be significant. I give a rough indication of cloud conditions, determined by cursory visual inspection of the sky, for each trial.
In trial 10, a fourth type of lid, a transparent lid with no rim, was compared with the rimmed transparent lid which served in all other comparisons of a transparent lid. The rimless lid was just a flat piece of glass identical to the one used as a component of the transparent lid. There was no significant difference in performance.
I believe these trials show that there is no substantial difference in thermal performance between the lids tested. After ruminative contemplation of the data, I also believe that the transparent lid performs slightly better than the other two. A more careful experiment might demonstrate such differences more clearly, but I doubt that they will much widen them. All three lids are effective for cooking.
There may be a question of the transferability of results obtained by the heating of water to the cooking of food. One could argue that white rice would reflect more light than the clear water, favoring one of the opaque lids. On the other hand, darker food might favor a transparent lid.
The equal, and perhaps slightly superior, thermal performance of the simple transparent lid is fortunate. A transparent lid is intrinsically simpler (less costly) than a greenhouse lid, and provides more visual information to the cook than either a greenhouse lid or a simple black steel lid.
The next time I perform this kind of comparative experiment, I will ensure more equal performance of the pair of cookers by the use of more uniform reflective surfaces. I will also calibrate the similarity of the two cookers by performing simultaneous experiments with identical lids and pots.
The next time I want to construct a reflective surface on corrugated cardboard or wood in a home workshop, I will follow the recipe given by Roger Bernard in his little book "La Cuisson Solaire Facile" [Bernard2] (p.81) He first laminates foil onto a lot of small rectangular pieces of very smooth thin cardboard -- pieces somewhat larger than a playing card. Working with small pieces of smooth thin cardboard enables a very uniform lamination. He then covers a larger surface by tiling it with the small laminations.
The ROB cookers were robust, convenient, and durable, apart from the previously noted tendency to delaminate, which was due to the inadequate lamination technique used. They could be picked up and moved casually and frequently without changing shape or fatiguing the cardboard. Their strength was due in part to the triangular bracing of the vertical reflectors, and in part to the use of 2-ply cardboard.
The unnecessary protrusion of the metallic edges of the lids above the pots, must have reduced both the performance of the lids and the apparent differences between them. The magnitude of this effect was tested by comparing the transparent lid with the metallic rim with a lid consisting of a single piece of glass (Trial 10). Although the lid consisting of a single piece of glass must theoretically have performed better than the two-part glass lid with the metal rim, the performance difference was not observable, so the heat loss due to the metallic rim was small in comparison with other heat losses. This result probably applies to the unnecessary protrusion of the steel bowls above the rim of the glass bowls.
I live in downtown Ottawa, in an apartment with no southern exposure. All the trials and cooking experiments were performed at my sister's rural home near Carleton Place, one hour west of Ottawa. The travel time and organizational impact of commuting to the site of the experiments significantly reduced the time available for the experiments.
I did not have a table large enough to hold both cookers, so they rested
on grassy ground during the comparison tests. A pot collected
insects almost every time I lifted its lid. When the cooker was elevated
on a table, no insects got in. Conclusion: cook on a table
if you are going to lift the lid!
This work would not have been possible without the enthusiastic support and encouragement provided by my sister, Elizabeth Delaney.
[Bernard2] Bernard, Roger. "La Cuisson Solaire Facile", Editions Jouvence, 1999, ISBN 2-88353-172-2. 91 pages.
[Delaney1] Delaney, David. Pot with Integrated Greenhouse for Solar Panel Cooker, http://www.geocities.com/~dmdelaney/intgrnhouse/conv-pot.htm
[Kerr1] Kerr, Barbara. Barbara Kerr tests the solar panel cooker, in Journal of Solar Box Cooking No 16.), http://www.solarcooking.org/spc.htm. The web reference is to a file that contains two articles. The first article is the description by Bernard of his first solar panel cooker. The second article in the file is the referenced article by Kerr.