Saturday, June 23, 2007

Building a Solar Cooker

At a recent gathering of family and friends, the topic of conversation started with the current hot conditions. From there, it naturally progressed to solar energy and cooling. From there to the possibility of a solar cooker.

In places like Florida, where part of the family lives, the idea has natural appeal. By cooking inside the home you pay for the energy twice... first, the energy to heat the oven and second, the energy to remove the heat from your house with the air conditioner. And, besides heating by electric resistance, as most ovens do, is an inefficient application.

This got me thinking it might be worth discussing. Building a solar oven is relatively easy to do. The website, has a great selection of plans for building solar ovens. I particularly like the "Minimum" Solar Box cooker. Obviously, the appeal of the design is their use of simple materials like cardboard and foil.

But, for more practical, everyday use, while keeping it simple, a few changes might be worthwhile. Also, it might be worthwhile to dig into the theory so we can understand the process and obtain more customized results.

First, practicality... if the cardboard box is left out during a typical Florida afternoon thundershower you would soon have a pile of ruined cardboard. So, I'd suggest using a sheet of Polyisocyanurate covered on both sides with aluminum foil. This material is relatively weather resistant and rigid. It can be obtained at pretty much any building supply store for about $10 per 4'X8' sheet and has good insulating properties, R value of approximately 4 per half inch.

That leads us to some theory...the authors don't say what temperatures can be attained with the simple ovens, but with a little understanding of the theory, it is possible to estimate temperatures and see how adjustments can effect it.

For any space, the equation "Heat in = Heat out" represents the equilibrium, or steady condition. This allows us to estimate the temperatures which can be obtained and to make adjustments to obtain the desired results.

"Heat in" is a function of the solar rays entering the box. It is generally accepted that for most subtropical locations the radiant heat of the sun is somewhat above 2oo BTU/sq ft/hr.

"Heat out" is a function of the insulation around the space, represented by the equation

Heat out = UxAxdT/R
U= heat transfer coefficent. This depends of the surfaces and the fluid on each side, but generally for thin, smooth surfaces with air on both sides is about 1.5 BTU/sq ft/degree F.
A= area in sq ft
dT= difference in temperature, or (T inside - T outside)
R = Resistance to heat transfer of insulation, generally referred as to R value.

So, let's build an oven and estimate the temperature which can be obtained. Assume the box is a 1 foot cube, built of 1/2" Polyisocyanurate board, with the inside painted flat black, so absorption is close to 100%. Let's have a 1 foot clear film on the top to allow entrance of the sun. And let's have a somewhat oversize reflector on the back side to reflect more sunlight into the clear film area. Assume we can get 1.5 sq ft of sunlight into the box. We would probably want to raise the pot off the floor of the oven with a canning ring or other pedestal so it is heated from the bottom as well. I picked this general design because the discussion was around a slow cooking "Crock Pot Type" cooking style where food could be put on in the morning and ready to eat for dinner with minimum attendance.


Heat in = 200 x 1.5 = 300 BTU/hr

Heat out is equal to the heat escaping through the 5 insulated walls with an R value of about 4, plus the heat escaping through the clear film, with an R-1. Therefore, heat out is represented by:

Heat out = (1.5 x 5 x dT/4) + (1.5 x 1 x dT/1) BTU/hr = (1.9 x dT + 1.5 x dT) BTU/hr = 3.4 x dt BTU/hr.


300 BTU/hr = 3.4 x dT BTU/hr


dT = 300/3.4 = 88 degrees temperature difference

So, this oven would obtain a temperature difference with the outside air of about 88 degrees. Assuming 90 degrees outside, the inside temperature would be about 178 degrees.

Disappointing, you say? Well, use the above theory to build a better oven! Since we have plenty of material left over from our sheet of foam board, let's make a slightly larger box to put the first box inside of, with 1.5" of wadded newspaper in the space between the boxes. Overall the walls and floor now have an R value of 10. Also, I like the "Simple" box cooker idea of using a turkey cooking bag so you have double film over the opening, doubling the insulating value of the film. Also, let's design a reflector which increases the area of sun into the box to 2 sq ft. Now,

2 x 200 = (1.5 x 5 x dT/10) + (1.5 x 1 x dT/2) = .75 dT + .75 dt = 1.5 dt

dt = 400/1.5 = 267 degree difference

Again, assuming outside temperature of 90 degrees, your oven temperature would approach 357 degrees. Oops, better start thinking about the melting and ignition temperatures of the foam or some type of insulating liner!

Keep in mind, these are the equilibrium temperatures, which the empty oven could be expected to approach pretty quickly assuming a tight enclosure and good sun. Any reflective pot would decrease the heat captured, and the mass of pot and food, plus the energy absorption of moisture would substantially slow the approach to these temperatures.

So, there you have it. For less than $15, you can cook outside for free, rather than endure the double whammy to your utility bill of cooking in the kitchen in the summer.

Tuesday, June 19, 2007

Solar Cooling Prototype

Time for an update on my efforts to generate cooling from solar heat. For those who have not been following this blog, I set up a prototype to test some concepts for air conditioning the house using solar heat. The prototype is pictured above, and the details of how it works are included in my previous post. The results so far have been a bit disappointing, but as Thomas Edison said, everytime I fail I find out another thing that won't work.
I ran another test yesterday, after adding a fan and some finer spray nozzles. I was able to generate 77 degrees Fahrenheit, in ambient conditions of 95 degrees and 55% relative humidity. Since the wet bulb temperature is 80.5 degrees at these conditions, I got 3.5 degrees cooler than theoretically possible with a simple evaporative cooler. Even so, I expected a lower temperature. Meanwhile, I'm learning that popcorn (my trial dessicant) is probably not a good choice. The humidity absorption rate seems to be too slow, but most of all, the popcorn seems to deteriorate quickly in outside conditions. The is a bit of a surprise, since the experiments I did prior to setting up the prototype seemed to indicate the popcorn would stand up to the expected conditions and repeated regeneration. But, it has happened twice now in a matter of just a few days. The first time, I thought it might have been wetted from rain or overflow, but the second time there was no rain and no evidence of overflow from the exchanger.
Other lessons learned:
  • If I want to use natural convection to drive the process, I'll need to have taller columns to generate the needed flow.
  • I need greater contact area with the desiccant to facilitate air drying.

Since these changes would take some time and I'm up against a deadline on my lease, I'm going to switch to a different setup to try an ammonia/water adsorption setup.

Below is the schematic for this arrangement. An ammonia/water solution would be in the section of pipe in the solar collector. When the solar collector heats up, it will boil off the ammonia, which will then condense in the cooler evap section, which is cooled by pumping heat medium to the heat storage. Then, when the solar collector cools off, the water will attract the ammonia, evaporating it from the evap section. In this stage, heat will be added to the ammonia by the heat medium, which is circulated to cool storage. By this mechanism, I expect to obtain, alternatively, both heat and cooling from my solar collector. Below is a schematic. Keep tuned for results from this trial.