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re: has anyone researched using solar for cement or pottery kilns?
2 jun 2000
anthony matonak   wrote:

>> >time and again i keep running across the idea that portland cement,
>> >and concrete made from it, is environmentally unfriendly... i wonder
>> >how difficult it would be to adapt existing solar thermal technology
>> >to provide for the majority of that energy.
 
>> tough to do economically, as i recall from a 1996 wrec-iv paper.
>> practical-sized solar furnaces tend to make tiny hot spots. otoh,
>> some cement kilns are heated by burning tires at high temps.
>> adding the tire steel to the cement is a plus.
 
>i don't know the economics of it. i've no doubt burning coal, oil, 
>gas, used tires and hazardous waste is all cheaper. perhaps the 
>economic dynamics have changed in the last 4 or 5 years enough to
>make it a viable proposition?

but oil's getting cheaper and cheaper, adjusted for inflation. 

>any idea where to get a hold of this 1996 wrec-iv paper?

i have a copy. not sure how much detail, compared to the talk. or
you might try to interlibrary loan volume 1 of the proceedings of
the june 1996 world renewable energy congress held in denver. (our
solar closet paper's in vol. 2, and on my web page.)

>> >has anyone looked into using solar thermal for pottery kilns?
 
>> i've heard the thermal mass of the kiln and its insulation
>> is the problem. pricey low-thermal-mass space insulation
>> made with sapphire fibers could help.

>thermal mass is the problem... in that it takes a long time
>[and a lot of energy] to heat up...

pe howdy reichmuth says the kiln itself (walls and insulation)
ends up having a lot of thermal mass, even when empty.

>> it would be less challenging to build a solar-powered 24-hour pizza oven
>> to start with. not easy, if reasonably sized...

>as i understood it, the larger the volume the less surface area.

the surface grows too, altho s/v falls, as you show: 

>a cube 1 sq meter on a side has 6 sq meters of surface area and
>a volume of 1 cubic meter and a ratio of surface area to volume of 6/1,
>while a cube 2 sq meters on a side has a surface area of 24 sq meters
>and a volume of 8 cubic meters with a [s/v] ratio of 3/1. 
 
>it would seem to me that the larger your build your thermal mass the
>less surface area you have.

the ratio decreases, and the time constant increases, given the same
proportions. so big thermal stores are more efficient, but it can be
harder to get the heat into them. their thermal resistance and "window
size" tends to increase.

>...let's take an example of using say, phase changed lead, or steel
>(without phase change) for the pizza oven.

pizza with lead? not a local option :-) just pineapple, peppers, pickles,
pepperoni, etc. phase change could make easier heat transport from
collector to storage. 

>the numbers i looked up a while ago showed it would take some 7360
>btu/cubic foot to change lead from solid to liquid and it melts at
>about 620 degrees f.

sounds nice.

>steel on the other hand takes about 52.3 btu per degree f per cubic foot.

close to water (0.113btu/lb-fx487lb/ft^3 = 55 vs 64 btu/f-ft^3
for 1% carbon steel), but capable of higher temps...

>to get the same 7360 btu/cubic as lead it would need a temperature rise
>of some (7360/52.3=) 141 degrees.

sounds doable.

>if we let the temperature swing some 300 degrees from a low of 450 deg f
>(about what it takes to cook a pizza...

i've heard warmer.

>...to a high of 750 deg f we would then have twice the energy storage
>compared to lead.

ok.

>700 or 800 deg f should be easily produced with concentrating solar 
>collectors like a parabolic dish, trough, or field of heliostats.

ok.

>how much storage and how big a collector do we need to run a pizza oven?

let's say the oven holds 64 2' pizzas on 4 8x8' shelves, and we cook 
8h/20minx64 = 1536 pizzas per day at 450 f for 20 minutes and evaporate a
pound of water in your 8' cube with r40 insulation (a foot of fiberglass)
in a 70 f room. that's about 1.5 million btu to evaporate water, plus the
conductive thermal loss of 24h(450-70)6x8^2/r40 = 87k btu/day (ignoring
door openings.) 

>for arguments sake, say our storage unit is some 8 feet cube, with half 
>of the internal volume composed of air for easy heat exchange...

maybe the bottom half, since hot air rises.

>its volume would be some 512 cubic feet and would have 256 cubic feet of 
>material (steel). 256 cubic feet, with 300 deg f swings, could store 
>some (256 x 300 x 52.3) 4,016,640 btu.

nice. now how do we get the heat into that 63 tons of steel? 
 
>how big a collector to collect 40 therms in one day? figuring a 
>parabolic reflector setup is maybe 50% efficient (wild guess) we could 
>collect 500 watts per sq meter. say an average day of 5 sun hours would
>make (5 x 500) 2500 watt-hours a day per sq meter (2.5 kwh) or (x 3409)
>8523 btu a day. we'll need (4,000,000 / 8,523) 470 sq meters or an area
>21.6 meters on a side (71 feet), or over 5000 sq feet.

well, maybe half that much, but that's still pretty big, and
how to get the heat into the steel? conduction through a window?
and how to get the heat out into the oven, airflow through holes?

>> otoh, a restaurant in some sunny place might have a chef making burgers
>> under a concentrating skylight, tracking the focus around with an unlit
>> portable barbeque, in a central ring surrounded by ropes and diners :-)
 
>...some form of tracking could be performed easily
>enough so as to let them cook in a single spot.

more complicated and less dramatic.

>either a very large tracking mirror on the roof acting as a heliostat
>to focus on the fixed point of his grill or some form of movable prisms
>or such under the skylight.

i was thinking about this while contemplating the new skylight at the
local fossil-fueled college (ursinus), a dome-shaped double-glazed net-
heat-losing thingy about 30' in diameter in the center of the round
cafeteria building. why not line the north half with aluminized mylar?

>the main problem cooking on something like that would be the intense
>sunlight focused on the grill could cause serious eye damage.

a dark concrete floor and dark tools and surfaces. 

>i would think it would be better to focus the energy on a collector that
>is shielded from view and duct hot air, steam, oil, or other heated
>fluid to the grill. not as dramatic as watching your fry cook go up
>in flames after accidentally waving his hand over the grill, but
>probably safer. 

welding gloves and a flameproof suit (including the tocque.)

nick

nicholson l. pine                      system design and consulting
pine associates, ltd.                           (610) 489-1475/0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email: nick@ece.vill.edu

computer simulation and modeling. high performance, low cost, solar heating
and cogeneration system design. bsee, msee. senior member, ieee. registered
us patent agent. web site: http://www.ece.vill.edu/~nick 




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