The Artist and the Engineer - Tales of a Solar Kiln for Ceramics

Introduction

This is an evolving report. Although I know that it is possible to build a solar powered kiln for the firing of ceramics, I don't know how practical it is. The outcome of this investigation may turn out to be negative.

I am intensly interested in solar energy. I am an engineer (of the Electrical variety, though having perhaps a more eclectic mix of interests than many of my colleagues). My other half is a potter. One day I was idly wandering and wondering about various subjects and posed the question "I wonder if I could build a solar kiln?". Judy asked if that was possible, I said I thought so, and she said she thought it would make a bit of splash in the ceramics world. As I considered the basic heat transfer problem of building a low cost concentrating device capable of sustaining a temperature of 2000 degrees F, Judy wondered about the types of glazing effects that might result from exposure to extreme light levels while firing.

It has been done before. C.A.Sanger posted the following to a web discussion group in '98, and was kind enough to copy the articles for us:
"They're in the winter 75-76, volume 4, no. 2 Studio Potter. The first is Experimental Solar Kilns by Zeljko Kjundzic, Pg. 70-73. The second is Firing With Sunlight by Tom Fresh, pg. 74. If you can't get the journal, they also appear in a hardback book collecton of Studio Potter's first 6 years, called Studio Potter Book, edited by Gerry Williams, Peter Sabin, and Sarah Bodine; a Daniel Clark Book, 1978. The articles are on pages 164-168. Neither is a ready-to-go kiln plan. But they could really get the right potter going!"

The first article describes a kiln using a precision parabolic reflector, 62 inches in diameter (about 21 square feet), made of bronze, and chrome plated. The kiln vessel, mounted at the focal point, was a 1 pound coffee can lined with ceramic fiber insulation. Firing chamber temperatures of 2800 F were reached. Mechaical details were not given.

The second article describes an arrangement that used a 47 square foot collector to focus sunlight on a 6 x 18 inch rectangle. At the focal point was a kiln vessel with a Pyrex window and 1 inch of ceramic insulation. It appears that things had to be moved around occasionally to keep the heat going.

While these designs were effective, they have a way to go before they could be considered "production" ready. In particular, they seem to fire a single item at once, and require considerable attention.

There has been some other discussion of solar kilns on the web. Many point to giant solar furnaces that generate thousands of degrees. But, nothing of fairly low power, suitable for day to day use.

It must be recognized that firing with the sun presents problems that are out of our control. Even assuming a design that can generate the required temperature, clouds do happen, so the sun is not completely reliable. Probably the best defense against this is to do this in a desert climate.

Thermal Balance

Before building anything, it is important to consider the overall thermal balance of the kiln. Assume that the kiln is basically an insulated box with a small hole through which concentrated sunlight is projected. The box has energy gains and energy losses. The energy losses increase as the temperature in the box increases, so the temperature climbs until the energy in equals the energy being lost.

Energy is lost via three mechanisms. The first is the conduction loss through the walls of the box. This heat loss is equal to the temperature difference between the inside and outside walls multiplied by the thermal conductance of the walls. The second heat loss is the radiant heat transfer through the hole that admits the concentrated solar radiation. For this purpose, we can consider our kiln to be a black body at the temperature of the kiln's interior. The energy emitted from this source is proportional to the 4th power of the absolute temperature, times the area of the aperture.

The third loss is from convection. Heated air in the kiln cavity can possibly circulate out of the cavity, resulting in heat loss. We are going to ignore this effect for now, and assume that it can be minimized by placing the radiation entry point at the bottom of the cavity, so that stratification results.

My initial thought is that I would have an insulated box with a hole at the bottom. I would arrange a sun tracking mirror to send light in a constant direction regardless of sun angle, and then focus this back up through an aperture in the bottom of the box, using either the combination of a big fresnel lens and a mirror, or a section of a parabolic reflector.

Insulation

Kiln temperature is determined by the balance between energy gain and energy loss. To minimize solar collection area, we need to make a kiln box with effective insulation. Common insulations are not useable at the temperatures we are seeking. Unfortunately, materials that can withstand the high temperatures of a kiln, so called refractory materials, are not especially good insulators. It appears that there are three types of refractory materials commonly used for kiln walls, firebrick, insulating firebrick, and ceramic fiber blankets (or boards). Firebrick is simply brick made of clay that has been found to withstand high temperatures. Insulating firebrick is like fire brick, except that it has been mixed with an organic material that burns away when it is fired, leaving voids. Insulating firebrick is also very light weight. The third option is ceramic fiber blankets or boards (a popular brand name is Fiberfrax). Of the three the ceramic fiber materials have the lowest thermal conductivity, about 1/2 that of insulating firebrick. At 1000F, this is about 0.85 BTU-in/hr-ft2-F.

For a sanity check, let's do some rough calculations on a straw man design. Let's assume that we build a cube to be the firing chamber, with interior dimension of a foot each way, of ceramic fiber board, that it has 3" thick walls, and that we want it to operate at 2000F.

Let's estimate the heat loss through the walls of the cube. The heat loss will be proportional to the area of the walls. The geometry is a little complicated, but for now, let's assume the 6 square feet of the interior of the cube. At 2000F, The thermal conductivity will have increased to about 2 BTU-in/hr-ft2-F. Given interior temperature of 2000F and exterior temperature of 150F, let's assume an average temperature of 100F, so our heat loss through conduction will be about 0.85 * 2000-150 * 6 / 3 = 3145 BTU/hr ~= 921 watts.

That's less than one square meter of sunlight, assuming we can collect and concentrate it all. Might we do better on the insulation? One thing that will make it worse is the effective area increase as we go outward from the center of the vessel. Another thought is that a composite insulation could be used, say 2 inches of ceramic fiberboard, then another inch of something with a much higher insulation value, like rock mineral wool, which can still withstand temperatures up to 1200F. The combination would give a higher overall insulation value. It seems that modeling this will be a little complicated. Still, it seems sane that we might achieve our desired temperature with less than a kilowatt of incoming energy.

The next question is what the blackbody radiant losses through the hole in the bottom of the vessel would be. This depends on the area of the hole, which in turn depends on how precisely we can focus the sunlight. About all we can do now is make a guess. Say, one inch across. My handbook says that such a hole would only radiate about 20 watts/cm2. Our 1" hole has an area of about 5 cm2, so the total blackbody loss will be about a hundred watts. We can probably ignore this for now.