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alternative firings (solar) - long, technical, possibly boring,

updated wed 12 may 99


Bruce Girrell on tue 11 may 99

This subject keeps coming up from time to time. I've been resisting the urge
to jump in, but just can't stand it any longer.

Solar kilns are indeed possible. It's a matter of how much work and money
you want to put into one for what you get.

The Advanced Ceramic Manual by John Conrad shows a couple of sketches for
mirror and fresnel lens kilns, but don't rush out to buy the book to get the
sketches - they are very basic and show principle only - you wouldn't really
want to build it the way they are shown (for example, the mirror version
shows the focal point up in the air with the opening facing downward.
Anything that you put in the kiln would fall out!)

Probably more significant is this comment:

"Solar firing has limited use, but it can be an interesting challenge. Paul
Soldner has done several experiments using both mirrors and fresnel lenses.
Mirrors are positioned in such a way that the sunlight is reflected into the
chamber of a small kiln, and every few minutes the mirrors are moved to
maintain the light in the chamber. A fresnel lens is placed the proper
distance from the kiln chamber facing the sun and is moved every so often to
maintain light in the chamber. Temperatures up to 871 deg C are possible in
a six inch cube chamber to fire small pots."

I'm not sure where the 871 degree figure came from. The temperature is
pretty much limited only by how much solar energy you can concentrate on
your target. Some solar furnaces can produce very high temperatures.

The largest solar furnace that I know of is operated by The Sciences and
Materials and Process Engineering Institute of the French National Center
for Scientific Research (CNRS) at Odeillo France. For a nice picture of its
concentrator see

The Odellio furnace is rated at 1000KW and can reach temperatures over 3000
deg C.

In the US, the largest installation is at Sandia Labs in Albuquerque, New
Mexico. Check out the following site for a picture of the installation.

Other large facilities in the US are at Georgia Tech and at White Sands.
There is also a very large solar installation called Solar Two in the Mojave
desert. It is an electrical power generation installation, though, producing
up to 10 megawatts of power. See

Finally, there is a facility operated by the National Renewable Energy
Laboratory, Golden Colorado, called the High Flux Solar Furnace. I don't
know if it is completed yet, but when complete it is estimated to produce
the heat of 50,000 suns and will be the hottest solar furnace on Earth,
though its output will be only 1 kW. See

I doubt that any of us are monetarily up to matching those facilities, but
it does show what can be done, given the resources.

Somewhere between these giant facilities and Conrad's (or is it Soldner's?)
871 degree six-inch cube, I think that there may be a reasonable
compromise. Solar furnaces typically have three parts, four if you count the
target. The first part is called a heliostat. It is a flat mirror or array
of flat mirrors with drive motors and bearings to allow them to rotate with
the Earth's movement, thereby keeping the sun's light pointed at the next
major subsystem, the concentrator.

The concentrator can be a concave mirror (a parabaloid) or a convex lens. In
a lens system fresnel lenses are almost always employed because of their
light weight and lower cost. The concentrator does not have to be a single
unit. It, like the heliostats, can be an array of mirrors or lenses.

The third part is some means of controlling the solar energy, usually a
shutter or venetian blind like array set far enough in front of the focal
point that it does not melt.

The final part is the target, which is your kiln. How to best make use of
the heat that you have collected (how do you distribute it so that the back
side of an object is heated in the same manner as the front, for example)
will be an interesting engineering challenge.

Back to the heliostats for a second. The following site is run by a guy who
is trying to produce heliostats at a reasonable price and contains a lot of
information and links about solar energy products:

I would think that the design approach would be to start at the target and
determine your solar flux needs at that point, taking into account losses
incurred in redistributing your concentrated heat. That, then, should
determine how much mirror area you need in your concentrator and heliostats
(there are losses at each of those points, as well). Then you can calculate
the cost of the subsystems and see if it will be practical for you. Don't
forget to add a small computer for running the heliostats and a
photovoltaic/battery system to power the controls. I would expect that you
could get a small kiln operating for about US$5000.

Costs can be reduced by keeping the target area small and well insulated -
the same as any kiln. Also, a design temperature of cone 06 would require
much less mirror area than a design for cone 10.

I do not currently have the time or money to apply to a project like this. I
do, however, have an engineering background, I am a computer programmer, and
I have had an interest in solar energy for many years. If someone else is
interested in pursuing such a project and needs help with the math or design
work, I can help. Otherwise, it will have to wait until Lynne and I can move
back to Arizona.

Bruce Girrell
who would love to have nothing more to do than build one of these things