Solar kiln

April 29, 2005 8:12 AM

Alternative-energy junkies : help! Solar / kiln question...

I would like to fire a kiln using solar energy. The kiln needs 240 volts, draws 60 amps and runs for 13 hours (approximately). The energy needs are not constant during a firing. Any suggestions? Is this possible? Thanks!

posted by Slothrop to Technology (12 answers total)

Probably not, unless you charged up a LOT of batteries. 240V * 60A = 14.4 kW. A 160W solar cell is approx. 18 sq. feet, so you'd need 1620 sq. feet of solar cells, approximately a whole house's roof-full in bright sunlight to power it. And that's assuming no loss in conversion to 240VAC.
posted by zsazsa at 8:18 AM on April 29, 2005

Well, a wood-fired kiln is solar. Strictly speaking gas is solar too. I read years ago (Ceramics Monthly?) about someone in the SW who fired pots with a (huge) parabolic reflector. One at a time. When I ran a pottery studio in rural New England we had a hard enough time firing electric (Cone 7) in a similar kiln because we were too far from the sub-station. So good luck.
posted by TimeFactor at 8:47 AM on April 29, 2005

Tales of a Solar Kiln for Ceramics has some interesting ideas.
posted by Floydd at 9:55 AM on April 29, 2005

Does the kiln actually draw that 60A for the full 13 hours? That's an aweful lot of power. What kind of temperatures do you need inside the kiln and how big (volume) is it.

You don't need a huge solar panel to supply even this much power if you don't need to fire the kiln everyday. For example with batteries if you need only fire the kiln every 10 days you could get away with ~162 sq. of panel using zsazsa's numbers.

I don't know much about insolation value for Detroit, do you have a clear view south during daylight hours?
posted by Mitheral at 10:17 AM on April 29, 2005

Pottery is fired to "cones" which are ultimately a measure of work, not of temperature, but based on Slothrop's kiln's specs and firing time, I assume he's firing in the 1000-1300C range (roughly cones 04-10). My glaze firings consumed the maximum wattage (15k) to reach final temperature (~1200) and only slightly less to hold that temperature ("soaking"), and only slightly less than that to bring down the temperature slowly to ~900. And our kilns were extra-insulated. You should have seen our electric bills. I don't really understand the physics of the situation but electricity is a really inefficient way to bring things to very high temperatures. I do know that switching from three 10 cu-ft 15kW electric kilns to one ~24 cu-ft gas kiln cut our energy costs by a factor of more than 10 yet our pottery output more than doubled.
posted by TimeFactor at 11:07 AM on April 29, 2005

You shouldn't be bothering with electrical conversion at all, it will be very inefficient and very expensive.

Set up a bunch of mirrors all reflecting the sun to the same spot. You'll need a control system to keep them pointing at the right place...
posted by Chuckles at 11:33 AM on April 29, 2005

Just as an aside to Mitheral et al., when my dad got his kiln, he had to have the power company run a new circuit from the street pole. The cable must have been an inch thick or so. When I was helping him shop for the kiln, we were warned that the cost of purchasing the kiln would be about half of what we'd spend after the electrical work. They use a lot of power.
posted by MrMoonPie at 12:05 PM on April 29, 2005

Problem with that Chuckles is the temperature he is going to need (assuming he has the same needs as TimeFactor). 1300F is really hot in solar collector terms. If he needs 15000 watts of power (as in TimeFactor's 10 cuft kiln) he'll need 15 sq metres of of reflector material at noon and two to three times that in the morning and evening. That's a lot of mirror to keep focused on a collector.

TimeFactor your single 24cuft was so cheap in comparison to your three 10 cuft kilns because your surface area was so much less. A 10ft^3 kiln has a surface area(if a perfect cube) of 27.8 sq ft. Times that by three and you have 83.4 square feet of surface area vs just 49.8 sqft for the 24cuft kiln. Plus most places gas is way cheaper than electricity. If your 10s weren't perfect cubes the surface area goes up. Plus you have 25.2 vs 11.52 of gasket. If you account for wall thickness the smaller units perform even worse. A change in brick material, say from fire brick to insulated refractory material could probably account for the rest.

Thanks MrMoonPie I've done some solar projects but haven't messed with a kiln since 10th grade.
posted by Mitheral at 12:43 PM on April 29, 2005

The take-home lesson here is that making pottery just isn't a very energy-conscious pastime. If saving energy is your objective then consider macrame. Or drag-racing.

Mitheral: I did the same calculations at the time and did expect significant savings from the reduced surface area (not to mention the blessing of only firing three days a week instead of seven nights - we had to fire the electrics at night because if any of our neighbors used their electric ovens or dryers during the firing the kilns often failed to reach temperature within the programmed time and the entire load would be lost). However, the per energy unit cost of gas was actually higher than electricity (this was in the boondocks so we had propane trucked in and electricity was highly regulated in the state at the time). Plus the electric kilns were actually better insulated (high quality cast refractory vs. cheap firebrick laid by hand) and gas firing pottery (even in an oxidizing atmosphere) is inherently wasteful (flames shot out the 10' chimney during most of the ramp up). Yet the savings were still much better than I expected. That's why I said I didn't understand the physics. Our electric kilns shouldn't have performed so poorly but with that kind of power and at those temperatures the calculations ceased working and a lot of electricity got consumed by gremlins or something. Note also that we continued to use the electric kilns for bisque (cone 06) firings without problem.
posted by TimeFactor at 5:11 PM on April 29, 2005

Response by poster: Wow! Who knew there were so many people thinking about this out there. Sounds like solar fired kilns aren't really feasible yet, bummer. I do know of a few folks who fire to cone 1 using wood and then switch to biodiesel for the rest of the firing- a great solution, but one I can't use in my present situation. I feel the not-ecologically-sound vibe, but hey, when you have to work in dirt, you have to work in dirt. I just make sure nothing goes through the firing cycle that I don't mind becoming part of the archaeological record!

Thanks for all the help!
posted by Slothrop at 8:19 PM on April 29, 2005

Any particular reason you settled on solar? I would have thought wind (or hydro) would be better for a 13-hour duty cycle. (Geography presumably rules out hydro, but I actually know a guy who has a mini hydro generator in a shed out the back - you'd swear his house was on the grid it has so much power available, it was pretty cool).

If self sufficincy is not a concern, and you haven't already done so, you could also check whether your grid supplier offers a green power option - where you pay a slightly higher power bill in order to buy your power from renewable source powerplants like wind farms instead of coal plants.
posted by -harlequin- at 2:04 AM on April 30, 2005

It sounds like you are going to have to take the entire south wall of your house and make a reflector out of it. When you are not firing pottery you can use it for a barbecue or hot water pre-heater. Perhaps if this was used to supplement a gas or wood or biodiesel or electric system???
posted by Ken McE at 5:16 AM on April 30, 2005