Question:
Time and again I keep running across the idea that portland cement, and concrete made from it, is environmentally unfriendly. Mostly this is because of the huge amount of energy embodied in it’s manufacture. I wonder how difficult it would be to adapt existing solar thermal technology to provide for the majority of that energy. Has anyone done any research on adapting solar thermal collectors to cement kilns? Since the majority of the energy used to produce portland cement is thermal in nature I would imagine something like a heliostat array could work very nicely. The ordinary burners could be retained to allow the plant to operate all the time while using as much energy from sunlight as possible to offset their no doubt high fuel costs. Also, while I’m asking silly questions, it’s my understanding that old concrete, from demolished buildings, pavements and the like, are a significant amount of waste that is sometimes hard to get rid of. Would it be possible to run old concrete through a cement kiln and get something like portland cement, or perhaps redi-mix, out the other side? And, while I’m at it I’ll throw another question out there. Has anyone looked into using solar thermal for pottery kilns? I do understand that most porcelain and pottery requires firing times longer than daylight hours might provide but it seems to me that a sufficiently large thermal storage running at temperatures higher than those required for the kiln could provide the heat required for nighttime operation. Anthony
Response:
>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. >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. >…it seems to me that a sufficiently large thermal storage running >at temperatures higher than those required for the kiln could provide >the heat required for nighttime operation.
It would be less challenging to build a solar-powered 24-hour pizza oven to start with. Not easy, if reasonably sized. As the thermal mass grows, so does its volume and its heat-losing surface. How do we get the heat into the mass without losing a lot through the "window"? 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 
Nick
Response:
I think I read somewhere that cement kilns burn sometimes rather harmful waste materials which could also be burnt in incinerators. Cement kilns do not have to adhere to strict air pollution requirements of incinerators in most places, so are an escape route for difficult waste.. > Time and again I keep running across the idea that portland cement, > and concrete made from it, is environmentally unfriendly.
<snip>
Response:
Counterpoint: Cement kilns burn perfectly clean (so to speak) waste crankcase engine oil (read into this, no PBC oils or other foreign metal products etc except for dirt) and recover the heat value of such fuel and thus is a great benefit to society for recycling a potential waste product into something useful. Don’t know what the specific rules for air quality is for Cement kilns. I wouldn’t think it is too relaxed since practically every other business is regulated to death. Now ask if the existing rules and regs are enforced and that may be another story. BTW, what is your definition of these so called "rather harmful waste materials…"? Examples?
– Hide quoted text — Show quoted text -> I think I read somewhere that cement kilns burn sometimes rather harmful > waste materials which could also be burnt in incinerators. Cement kilns > do not have to adhere to strict air pollution requirements of > incinerators in most places, so are an escape route for difficult > waste.. > Time and again I keep running across the idea that portland cement, > and concrete made from it, is environmentally unfriendly. > <snip>
Response:
> Counterpoint: Cement kilns burn perfectly clean (so to speak) waste >
crankcase engine oil (read into this, no PBC oils or other foreign metal > products etc except for dirt) and recover the heat value of such fuel
and > thus is a great benefit to society for recycling a potential waste product > into something useful. > Don’t know what the specific rules for air quality is for Cement
kilns. I > wouldn’t think it is too relaxed since practically every other business is > regulated to death. Now ask if the existing rules and regs are enforced and > that may be another story. > BTW, what is your definition of these so called "rather harmful waste > materials…"? Examples?
I understand, but cannot give definite examples, that because the air pollution rules (and this may very from country to country) are less strict for cement kilns than for waste incinerators, in some places chemical waste is added to fuel for cement kilns. Now admittedly these days the term ‘chemical waste’ is used for almost any slightly more complicated waste product, probably for old car tyres too. Again in legislation this varies from country to country. On more than one occasion, old engine oil is mixed with more harmful substances and sold as a fuel. On ships, HFO with ‘additives’ resulted in severe damage to diesel engines and marine engineers. Has anyone else got more definite info?
Response:
Hi all, I am in Kingston, Ontario, Canada, and am looking into adding solar water and space heating to my house. Has anyone else been successful in doing this in southeastern Ontario? I’d be very interested in hearing from you, about your strategies and how well they are working. Replies either by email or posting to the forum would be great. Thanks so much! Jim Before you buy.
Response:
> I wonder how difficult it would be to adapt existing > solar thermal technology to provide for the majority of that energy. > Has anyone done any research on adapting solar thermal collectors to > cement kilns?
I have done a lot of research on this topic. How about glass kilns? Glass is made from sand – there is a lot of this available cheap. Our solar kiln is non-polluting. This is an ideal bootstrapping project. We will bake cement to make more kilns to melt more sand to make more mirrors. I bet we could make huge inexpensive desalinization domes on the coasts. The potential global solar greenhouse boom makes this idea really sizzle. Please contact me and we will talk. - Andy K. A few links… http://www.jrwhipple.com/sr/solheater.html http://www.permapak.net/solarfooddryers.htm http://www.bewellnet.com/rmsi/solar.htm http://www.solarwall.net http://forums.cosmoaccess.net/forum/survival/prep/solheatr.htm http://www.epsea.org/suntemp.html
Response:
Works great in prince edward county. the solar hot water has cut my electric bill by 500 $ per year. As expected most of this 85% is during the summer as I dont get too much sun in the winter. I’m more than happy. Mail me if you want more details. Paul. – Hide quoted text — Show quoted text – > Hi all, > I am in Kingston, Ontario, Canada, and am looking into adding solar > water and space heating to my house. Has anyone else been successful in > doing this in southeastern Ontario? I’d be very interested in hearing > from you, about your strategies and how well they are working. > Replies either by email or posting to the forum would be great. > Thanks so much! > Jim > Before you buy.
Response:
> I think I read somewhere that cement kilns burn sometimes rather harmful > waste materials which could also be burnt in incinerators. Cement kilns > do not have to adhere to strict air pollution requirements of > incinerators in most places, so are an escape route for difficult > waste..
… Cement kilns operate at much higher temperatures than most incinerators so they are able to destroy a much wider variety of materials, or at least reduce them to their basic elements. I think this is why they are chosen for difficult waste more than any pollution controls. Anthony
Response:
> >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? Any idea where to get a hold of this 1996 WREC-IV paper? My thoughts were that if they used a significant amount of solar thermal in the production of their portland cement they might lower their operating costs and could possibly market their product as more environmentally friendly. People might pay a little more for portland cement that was made from sunlight than for ordinary portland cement. Certainly they are paying more for other environmentally friendly products such as sustainably harvested wood and non-genetically engineered foods. >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 lot of energy and therefor a larger collector area? Or it’s a problem in that it takes a long time to heat up and cool down? Or a problem because of the large thermal mass of the kiln and very poor insulation? >…it seems to me that a sufficiently large thermal storage running >at temperatures higher than those required for the kiln could provide >the heat required for nighttime operation. > It would be less challenging to build a solar-powered 24-hour pizza oven > to start with. Not easy, if reasonably sized. As the thermal mass grows, > so does its volume and its heat-losing surface. How do we get the heat > into the mass without losing a lot through the "window"?
As I understood it, the larger the volume the less surface area. That is, 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 ratio of surface to volume of 24/8 or 3/1. It would seem to me that the larger your build your thermal mass the less surface area you have. If, in addition, you choose a thermal storage material with high energy density or use a phase change material, you should be able to store enough energy in fairly reasonable volumes. Most hot air thermal storage is done with rocks in one form or another which does not store very much per cubic foot. Let’s take an example of using say, phase changed lead, or steel (without phase change) for the pizza oven. 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. Steel on the other hand takes about 52.3 Btu per degree F per cubic foot. To get the same 7360 Btu/cubic as lead it would need a temperature rise of some (7360/52.3=) 141 degrees. If we let the temperature swing some 300 degrees from a low of 450 deg F (about what it takes to cook a pizza, though I’m told you can go as low as 300 or so if you don’t mind it taking longer to cook) to a high of 750 deg F we would then have twice the energy storage compared to lead. 700 or 800 deg F should be easily produced with concentrating solar collectors like a parabolic dish, trough, or field of heliostats. How much storage and how big a collector do we need to run a pizza oven? Anyone run a pizza parlor and care to share their gas bill with us? That is, how big is your oven? How many pizza’s you make? How many hours a day does it operate, how many Therms do you use a day? 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, it’s 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. One Therm is 100,000 Btu so this would be some 40 Therms. If they use less than 20 therms a day to run their ovens this would provide a couple of days worth of cooking. 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. That would seem to be a large area, but then I don’t know if a pizza oven requires 20 to 40 therms a day to run. A single therm a day would require an area of some (100,000 / 8523) 12 sq meters (3.46 on a side) or 130 sq feet. So if we knew how many therms a day a pizza oven takes we could just multiply that by 130 to get a rough idea of how big the collector would need to be. Alternatively, if we know how big the pizza place is we can figure how much heat they can collect? Say it’s a little place 10 feet wide by 30 feet long for some 300 sq feet. That’s over 2 therms a day even if they can’t convince the video rental store next door to lend them some roof space. So what would be the heat loss from our 8 foot cube? That would depend on the insulation. Any idea what a couple of feet of fiberglass (or maybe something better but almost as cheap) would get us? > 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
I would imagine that some form of tracking could be performed easily enough so as to let them cook in a single spot. 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. The main problem cooking on something like that would be the intense sunlight focused on the grill could cause serious eye damage. 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. After playing around with parabolic solar cookers, I’ve come to the conclusion that (at least during daylight hours) about 16 sq feet of mirror works as well as a single gas burner. A skylight 200 sq feet would then be about the same as 12 burners. Probably enough for a small burger place. Anthony
Response:
- Hide quoted text — Show quoted text -> I wonder how difficult it would be to adapt existing > solar thermal technology to provide for the majority of that energy. > Has anyone done any research on adapting solar thermal collectors to > cement kilns? > I have done a lot of research on this topic. How about glass kilns? > Glass is made from sand – there is a lot of this available cheap. Our > solar kiln is non-polluting. This is an ideal bootstrapping project. > We will bake cement to make more kilns to melt more sand to make more > mirrors. I bet we could make huge inexpensive desalinization domes on > the coasts. The potential global solar greenhouse boom makes this idea > really sizzle.
I was under the impression it was a little more complicated than just throwing some sand into a furnace. Besides, for large scale thermal projects like this I would think glass would not be the first choice for the reflectors. All the ones I’ve seen have been made out of steel or aluminum. Glass lacks the structural strength and light weight of these other materials. I have heard of people making parabolic mirrors out of concrete coated with a reflective material though. Instead of moving the mirror to track the sun, they would move the receiving unit in front of the mirror. Anthony
Response:
>> >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 8×8′ 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)6×8^2/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 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
Response:
… snip other stuff for the moment … – Hide quoted text — Show quoted text ->> 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: … > 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.
I’ve never really considered liquid lead a good storage medium for cooking. Besides the possible health hazards if there were a leak of some kind where do you go about finding a pump to move liquid lead? I know, someone out there sells an entire line of them. Seems like you can find anything for sale if you look hard enough. … >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.
Certainly steel is pretty inexpensive these days. This is a definite advantage when you need lots of it. >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.
Commercial ovens most likely run hotter to cook faster so as to have faster throughput. It is possible to cook a pizza at 350 degrees but it takes a while. This is what happens when your solar box oven gets a little less sunlight than you hoped. – Hide quoted text — Show quoted text ->…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 8×8′ 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)6×8^2/R40 = 87K Btu/day (ignoring > door openings.)
I did a quick search on pizza ovens and a single oven, capable of cooking a couple of pizzas at a time, have burners that put out between 40,000 Btu and 60,000 Btu. an hour. Multiply by the number of ovens. I’d say the average pizza joint is probably at the two oven level. Say, for rough guesses, some 100,000 btu an hour or about 1 therm an hour. It actually sounds to me like these things could be designed more efficiently. So figure a 24 hour cooking day and we’re up to 24 therms just to run the oven. Add in 87K btu/day for thermal leakage from the storage mass (heck, call it 1 therm/day) and we’re up to 25 therms a day requirement. >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.
I was thinking more along the lines of something like ball bearings or rough shaped chunks so as to increase surface to air inside the unit. Then to circulate air using fans to transfer heat in and out. The idea being that a small electric circulating fan could be easily thermostatically controlled and the hot air stream could be plumbed into an existing gas appliance with minimal reworking. >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?
With a 5000 sq foot array of mirrors acting as heliostats to focus on a collector which heats air, said air being blown into the 63 tons of steel with a small electric fan that doesn’t mind operating in 800+ deg F environments. I’m thinking perhaps a 1 foot in diameter air duct (or perhaps chimney) style metal, well insulated, attached at both ends into the storage stack with the thermal focus for the heliostats in the middle somewhere (with it’s window, etc.) and some kind of insulated dampers that would close when the fan/collecting isn’t happening. The whole collector being mounted directly above or to one side of those 63 tons of steel with as short a ducting run as possible. Perhaps with the pizza oven built into one side of it. >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?
Um, as above, yeah. I think that about covers it. Besides, with only a couple of ovens we’d probably not need all 40 therms a day, only 25 to run the oven and some extra for rainy days. Unless we use gas on rainy days. Even at 100% efficient we’re still talking thousands of sq feet. But if they could make a solar power station with a couple of acres of heliostats all focused on one tower I’d say it’d be possible to put something like this together. Probably would cost a bundle though. Might loose the occasional pigeon that flew too near the focus. >> 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.
I suppose you could have the BBQ grill, chef and all, on a motorized track along the floor which would move it to be under the hot spot at all times. >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?
I’m always amazed at those huge skylight domes you find in colleges, libraries and malls. They seem to be calling out for sundials, or fancy mirrors to project the hours on the walls, or something as dramatic. All too often they are just foggy holes in the ceiling. >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.
Hmm… if we’re talking thousands of suns of concentrated sunlight even dark surfaces will require welders glasses to look at. I suppose the chef could be surrounded with dark glass windows through which the patrons could peek without risking their vision. If we wanted to make this more dramatic we could have the focus a couple of feet above the cooking surface, with the actual cooking surface receiving a more diffused, unfocused, light, though still plenty to cook with. For a floor show we could have the cook melt beer cans above the grill, bend steel, or make small pieces of lumber burst into flames with a wave of his hand. >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.)
I guess some people like more drama with their meals than others. This would probably go well with the waiters that juggle knives at your table. Anthony
Response:
>I’ve never really considered liquid lead a good storage medium for >cooking. Besides the possible health hazards if there were a leak of >some kind where do you go about finding a pump to move liquid lead?
Check out wave solder machines used for printed circuit electronics. > >Steel on the other hand takes about 52.3 Btu per degree F per cubic foot. >Certainly steel is pretty inexpensive these days.
Our local recycling center tries to sell tin cans for $2 per ton, delivered. > >…let the temperature swing some 300 degrees from a low of 450 deg F > >…to a high of 750 deg F…
Storing 16.5K Btu per (487 lb) solid cubic foot (Would they have handles?) > >700 or 800 deg F should be easily produced with concentrating solar > >collectors like a parabolic dish, trough, or field of heliostats.
I’m picturing our outdoor oven in something like a pyramid shape, with a field of heliostats to the north, and a secondary reflector inside the base of the oven bouncing sun up onto the thermal mass, with the pizzas above that. Kinda like this: The most serious mistake was making the outer container of the receiver of plywood. We thought that the plywood would be sufficiently insulated from the copper panel which was the receiver proper, that it would not get too hot. The copper panel was separated from the plywood by 4" of fiberglass insulation. Nevertheless, the plywood caught fire and the unit was completely destroyed. We suppose this is a success, of sorts… from "A solar collector with no convection losses," (a downward-facing receiver over a 4:1 concentrating parabolic mirror) written by H. Hinterberger and J. O’Meara of Fermilab, ca 1976 > >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 8×8′ shelves, and we cook > 8h/20minx64 = 1536 pizzas per day at 450 F for 20 minutes and evaporate a > pound of water from each 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)6×8^2/R40 = 87K > Btu/day (ignoring door openings.) >I did a quick search on pizza ovens and a single oven, capable of >cooking a couple of pizzas at a time, have burners that put out >between 40,000 Btu and 60,000 Btu. an hour. Multiply by the number >of ovens. I’d say the average pizza joint is probably at the two oven >level. Say, for rough guesses, some 100,000 btu an hour or about 1 >therm an hour. It actually sounds to me like these things could be >designed more efficiently.
The ones I’ve seen seem to have less than 4 inches of insulation. >So figure a 24 hour cooking day and we’re up to 24 therms just to run >the oven. Add in 87K btu/day for thermal leakage from the storage mass
That was leakage from the whole 8′ cube. >and we’re up to 25 therms a day requirement.
Maybe 16, with more insulation. > >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. >I was thinking more along the lines of something like ball bearings >or rough shaped chunks so as to increase surface to air inside the unit.
Is it harder to get the heat into or out of the thermal mass? Conductive heat input favors solid steel, but holes and fast-moving air can help get the heat out (and cook pizzas faster–are there convective pizza ovens?) Evaporating a pound of water from each of the 64 pizzas in 20 minutes requires about 192K Btu/h, about the same as moving 16 therms of solar heat into the mass over 8 hours. An 8×8′ surface supplying 192K Btu/h through a slowly-moving air film conductance of 1.5 Btu/h-F-ft^2 would need to be 192K/(8×8x1.5) = 2,000 F degrees hotter than the air. Radiation helps too. Some copper or high temp heat pipes might help conductive heat input, but that still seems difficult. A foot of steel conducts 26 Btu/h-ft^2-F, while copper conducts 227, with about the same heat storage per ft^3. Putting 200K Btu/h into 4′ of steel through a 1′x1′ hot spot would seem to make the spot 4×200K/26 = 30,769 F degrees hotter than the steel… Old iron radiators or engine blocks might be cheaper than ball bearings, with more space for airflow, but then the volume goes up, the size goes up, getting the heat in becomes harder, and the heat loss or the amount of insulation goes up. >Then to circulate air using fans to transfer heat in and out.
Hmmm. Grainger sells a $300 2,000 CFM fan that will work at 311 F and a $200 280 CFM power venter listed under UL Standard 378 "for use on appliances with flue gas temperatures not exceeding 600 F at the blower inlet." Maybe some kind of automobile turbocharger… >The idea being that a small electric circulating fan could be easily >thermostatically controlled and the hot air stream could be plumbed >into an existing gas appliance with minimal reworking.
Interesting thought. Bigger orifi… A CFM of low-temp air with a 1 F temp difference moves about 1 Btu/h of heat, so moving 200K Btu/h with a 100 F temp difference might requires about 200K/100 = 2,000 CFM, which might move at 1000 FPM (11 mph) through a 2 ft^2 (19" diameter) hole, using a large fan with a reasonable electrical power consumption. > >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? >With a 5000 sq foot array of mirrors acting as heliostats to focus >on a collector which heats air, said air being blown into the 63 >tons of steel with a small electric fan that doesn’t mind operating >in 800+ deg F environments.
Hmmm. >I’m thinking perhaps a 1 foot in diameter air duct (or perhaps chimney) >style metal, well insulated, attached at both ends into the storage stack >with the thermal focus for the heliostats in the middle somewhere (with >it’s window, etc.) and some kind of insulated dampers that would close >when the fan/collecting isn’t happening. The whole collector being mounted >directly above or to one side of those 63 tons of steel with as short a >ducting run as possible. Perhaps with the pizza oven built into one side >of it.
Hmmm. > >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.
Hmmm. >…I think that about covers it. Besides, with only a couple of ovens >we’d probably not need all 40 therms a day, only 25 to run the oven >and some extra for rainy days. Unless we use gas on rainy days.
Maybe on sunny days too. >Even at 100% efficient we’re still talking thousands of sq feet. But >if they could make a solar power station with a couple of acres of >heliostats all focused on one tower I’d say it’d be possible to put >something like this together. Probably would cost a bundle though. >Might loose the occasional pigeon that flew too near the focus.
Pizza ‘N Squab. I wonder what Duane thinks about this. Nick
Response:
>…An 8×8′ surface supplying 192K Btu/h through a slowly-moving air film >conductance of 1.5 Btu/h-F-ft^2 would need to be 192K/(8×8x1.5) = 2,000 F >degrees hotter than the air. Radiation helps too.
So it looks like we need more surface or faster air… >Putting 200K Btu/h into 4′ of steel through a 1′x1′ hot spot would seem >to make the spot 4×200K/26 = 30,769 F degrees hotter than the steel…
OTOH, we might focus the sun onto the whole 8×8′ bottom (rusty iron that absorbs 80% of the sun and radiates with 20% emissivity?) and raise the reflective insulating hinged cover underneath during the night. If the heat only needs to travel an average of 2′ into the 4′x8′x8′ thermal mass, that lowers the temperature rise to 2×200K/(26×8x8) = 240 F. Still large. At 750 + 240 = 990 F (530 R), we’d lose 0.174×10^-8(0.2)(1450^4-530^4) = 1511 Btu/h-ft^2 by radiation to a 70 F outdoors. That’s 96,696 Btu/h for an 8×8′ target which needs to collect 200K+99,696 = 300K Btu/h of sun from a 90% secondary reflector using about 300K/250Btu/ft^2/0.81 = 1,500 ft^2 of heliostats (eg 24 8′x8′ mirrors) with 90% reflectivity. >…engine blocks might be cheaper than ball bearings,
Weld them to the bottom plate? If each contains 2 ft^3 of steel with 16 ft^2 of surface in a 4 ft^3 envelope, we can fit 128 of them into an 8′ cube, with the oven on top. This would lower the discharge temp drop to about 192K/(16×128x1.5) = 63 F. >Grainger sells… a $225 224 watt 280 CFM power venter… for use on >appliances with flue gas temperatures not exceeding 600 F at the blower >inlet… moving 200K Btu/h with a 100 F temp difference might require >about 200K/100 = 2,000 CFM…
We might use 8 power venters to push oven air into the mass, and let warmer air flow back up through some holes into the oven. This could help distribute solar heat into the mass, even with no pizza in the oven. So maybe it’s doable, albeit unwieldy. Nick
Response:
Dear Nick, Just a quick comment. Now I know why so many engineers are lousy cooks. You don’t want to lose a pound of water when you bake a pizza, it would end up with an inedible crust. The trick is to lose as LITTLE water as possible (I suspect one of the purposes of all the grease on the surface of the dough is to seal in the water vapor [as well as make it taste right and give all us health conscious people guilt feelings]). I visited a very large Spanish-made commercial bread oven in Somoto in Nicaragua that was fueled by an amazingly small amount of wood per day to convert a ton of flour into several thousand loves a day. One person could carry the entire bundle of wood in his arms for an entire day’s firing. The main secret is LOTS of insulation, but the other one is that very little actual energy is absorbed during the chemical change in baking. I gave the thermal calculations of this oven as a problem to my students at College of the Atlantic one year and have somewhere the exact data on the wood used, etc. and could dig them up. Hope this helps your discussion, Rich Richard J. Komp, President SunWatt Corporation RR 2 Box 7751 Jonesport ME 04649 [Hmmm, 50 pounds of wood times 12K Btu/lb (at 80% combustion efficiency) = 480K Btu to cook a ton of flour... is only 240 Btu/pound... say 1/4 of a pound of water per 2 lb(?) pizza...]
Response:
Hi Anthony; > I would imagine that some form of tracking could be > performed easily enough so as to let them cook in a > single spot. 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. The main problem cooking on something like > that would be the intense sunlight focused on the grill > could cause serious eye damage.
Just a technical detail here. While the concentration zone will be highly concentrated and contain appear quite bright the intrinsic brightness can be no greater than the sun by itself. The effect of concentration is to make the light subtends a wider angle than that of the sun. The human eye is designed to tolerate the intrinsic brightness of the sun without damage. We easily can look directly at the sun for an instant and then avert our eyes. This reflex is easily done when looking at the receiver. One just looks away. Of course the mirror absorbs some of the light. My mirrors reflect 85% of the incoming light. This in effect reduces the intrinsic brightness by 85%. I’m not saying that one wouldn’t need the goggles. They are needed if one wants to continuously look at the bright spot. I’m just pointing out that the intrinsic brightness does not change with increasing concentration factors. > 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.
Here is a picture of a cooker that works in a similar way that you describe: http://www.redrok.com/main.htm#cooker Click on the 3rd and 4th icon. > After playing around with parabolic solar cookers, I’ve > come to the conclusion that (at least during daylight > hours) about 16 sq feet of mirror works as well as a > single gas burner.
Sounds like about 1400W of power. > A skylight 200 sq feet would then be about the same as > 12 burners. Probably enough for a small burger place.
Say about 17000W. Of course there won’t really be 200′^2 as the skylight is horizontal. > Anthony
Have fun! Duane — CUL8ER Receiver Powered by [*] Thermonuclear SolarEnergyfrom the Sun /////| Energy(the Sun) / / // / /| / / / / / / | WA0VBE / / / / / / /| Ziggy / / / / / / | / / / / / / / | "Red Rock Energy" === === / / === / === Duane C. Johnson, Designer=== === === / | 1825 Florence St Mirrors,Heliostats,Controls & Mounts| White Bear Lake, Minnesota / | USA 55110-3364 | (651)635-5O65 work / | (651)426-4766 home | (413)556-659O Fax copyright / | (651)583-2O62 Red Rock Energy Site (C)980907 === | http://www.redrok.com/index.htm (My New Web site) | These are my opinions, and not that of Unisys Corp. ===
Response:
Hi Anthony; – Hide quoted text — Show quoted text -> > I wonder how difficult it would be to adapt existing > > solar thermal technology to provide for the majority > > of that energy. > > Has anyone done any research on adapting solar > > thermal collectors to cement kilns? > I have done a lot of research on this topic. How about > glass kilns? Glass is made from sand – there is a lot > of this available cheap. Our solar kiln is non-polluting. > This is an ideal bootstrapping project.> We will bake > cement to make more kilns to melt more sand to make more > mirrors. I bet we could make huge inexpensive > desalinization domes on the coasts. The potential global > solar greenhouse boom makes this idea really sizzle. > I was under the impression it was a little more complicated > than just throwing some sand into a furnace. Besides, for > large scale thermal projects like this I would think glass > would not be the first choice for the reflectors.
Glass is my first choice. Its also the first choice of a lot of heliostat projects. One of the detail characteristics needed in heliostat mirrors is surface flatness. Glass mirrors are very smooth and flat. While the mirrors I use have about 85% reflective efficiency they are quite cheap at about $1.50/ft^2. Metal backed mirrors are hard to get smooth and flat. They may be covered with aluminized plastic or polished stainless steel. These have an initial cost many times more than the glass mirrors. > All the ones I’ve seen have been made out of steel or aluminum.
Well maybe if you are looking at dishes or troughs. Also, dishes and troughs generally have short focal lengths. However, heliostats generally have long focal lengths and need flatter surfaces. > Glass lacks the structural strength and light weight of > these other materials.
I find the total package weight of the glass mirrors with equal surface quality similar in lighter than the metal types. The brittleness, ( I think that is what its called ), is a plus. It equates to stiffness. Metal surfaces are easily bent and dented. These bends and dents become permanent. Glass mirrors come flat out of the box. I just haven’t had any problems with my 1/8" thick float glass mirrors. I do passivate the aluminized side with trailer house aluminized roofing tar. > I have heard of people making parabolic mirrors out of > concrete coated with a reflective material though. Instead > of moving the mirror to track the sun, they would move the > receiving unit in front of the mirror.
What ever happened to "Ra". His link has been down for quite a while. > Anthony
– CUL8ER Receiver Powered by [*] Thermonuclear SolarEnergyfrom the Sun /////| Energy(the Sun) / / // / /| / / / / / / | WA0VBE / / / / / / /| Ziggy / / / / / / | / / / / / / / | "Red Rock Energy" === === / / === / === Duane C. Johnson, Designer=== === === / | 1825 Florence St Mirrors,Heliostats,Controls & Mounts| White Bear Lake, Minnesota / | USA 55110-3364 | (651)635-5O65 work / | (651)426-4766 home | (413)556-659O Fax copyright / | (651)583-2O62 Red Rock Energy Site (C)980907 === | http://www.redrok.com/index.htm (My New Web site) | These are my opinions, and not that of Unisys Corp. ===
Response:
Hi Nick and Anthony; >> >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.
50% isn’t to far off with reradiation losses and all. > Hmmm. >…I think that about covers it. Besides, with only a couple of ovens >we’d probably not need all 40 therms a day, only 25 to run the oven >and some extra for rainy days. Unless we use gas on rainy days. > Maybe on sunny days too. >Even at 100% efficient we’re still talking thousands of sq feet. But >if they could make a solar power station with a couple of acres of >heliostats all focused on one tower I’d say it’d be possible to put >something like this together. Probably would cost a bundle though.
Might be less than you think. I suspect it would cost about $20/ft^2 With 5000 ft^2 this would be $100,000. Ok, that is a lot of power though. In the neighborhood of 250KW thermal input. I have a gut feeling that this is way over sized. >Might loose the occasional pigeon that flew too near the focus. > Pizza ‘N Squab. I wonder what Duane thinks about this.
I like the concept. It takes a lot of mirror area doesn’t it. > Nick
This project is perfect for a Back Woods Pizzeria. — CUL8ER Receiver Powered by [*] Thermonuclear SolarEnergyfrom the Sun /////| Energy(the Sun) / / // / /| / / / / / / | WA0VBE / / / / / / /| Ziggy / / / / / / | / / / / / / / | "Red Rock Energy" === === / / === / === Duane C. Johnson, Designer=== === === / | 1825 Florence St Mirrors,Heliostats,Controls & Mounts| White Bear Lake, Minnesota / | USA 55110-3364 | (651)635-5O65 work / | (651)426-4766 home | (413)556-659O Fax copyright / | (651)583-2O62 Red Rock Energy Site (C)980907 === | http://www.redrok.com/index.htm (My New Web site) | These are my opinions, and not that of Unisys Corp. ===
Response:
Hi Andy; > I wonder how difficult it would be to adapt existing > solar thermal technology to provide for the majority > of that energy. > Has anyone done any research on adapting solar thermal > collectors to cement kilns? > I have done a lot of research on this topic. How about > glass kilns? Glass is made from sand – there is a lot of > this available cheap. Our solar kiln is non-polluting.
I have a technical question. I thought that the temperatures within the kiln needed to be controlled to fairly close tolerances. Especially the tempering furnace. Where the temperatures need to be maintained for several days with gradually decreasing temperatures. How do you accomplish this regulation? > This is an ideal bootstrapping project. We will bake > cement to make more kilns to melt more sand to make more > mirrors. I bet we could make huge inexpensive > desalinization domes on the coasts. The potential global > solar greenhouse boom makes this idea really sizzle. > Please contact me and we will talk. > - Andy K.
Do you have any pictures of your kiln? > A few links… > http://www.jrwhipple.com/sr/solheater.html > http://www.permapak.net/solarfooddryers.htm > http://www.bewellnet.com/rmsi/solar.htm > http://www.solarwall.net > http://forums.cosmoaccess.net/forum/survival/prep/solheatr.htm > http://www.epsea.org/suntemp.html
Have fun! Duane — CUL8ER Receiver Powered by [*] Thermonuclear SolarEnergyfrom the Sun /////| Energy(the Sun) / / // / /| / / / / / / | WA0VBE / / / / / / /| Ziggy / / / / / / | / / / / / / / | "Red Rock Energy" === === / / === / === Duane C. Johnson, Designer=== === === / | 1825 Florence St Mirrors,Heliostats,Controls & Mounts| White Bear Lake, Minnesota / | USA 55110-3364 | (651)635-5O65 work / | (651)426-4766 home | (413)556-659O Fax copyright / | (651)583-2O62 Red Rock Energy Site (C)980907 === | http://www.redrok.com/index.htm (My New Web site) | These are my opinions, and not that of Unisys Corp. ===
Response:
> Hi Anthony;
Hi Duane; – Hide quoted text — Show quoted text -> Just a technical detail here. > While the concentration zone will be highly concentrated > and contain appear quite bright the intrinsic brightness > can be no greater than the sun by itself. > The effect of concentration is to make the light subtends > a wider angle than that of the sun. > The human eye is designed to tolerate the intrinsic > brightness of the sun without damage. We easily can look > directly at the sun for an instant and then avert our > eyes. > This reflex is easily done when looking at the receiver. > One just looks away. > Of course the mirror absorbs some of the light. My mirrors > reflect 85% of the incoming light. This in effect reduces > the intrinsic brightness by 85%. > I’m not saying that one wouldn’t need the goggles. They are > needed if one wants to continuously look at the bright spot.
I’m not exactly sure of the details here but my gut feeling is that any spot of light bright enough to fry burgers with is going to be bright enough to burn out your eyes. I suppose this might not be as bright as the sun, but it’s going to appear in a place (a burger grill) entirely different in position and behavior than the sun does so peoples built in reactions aren’t guaranteed to protect them from it. Telling your customers ‘just don’t look at the cook’ isn’t going to protect you from getting sued when little johnny goes blind watching his burger. > I’m just pointing out that the intrinsic brightness does > not change with increasing concentration factors.
Hmmm… Maybe I’m completely wrong here, but I thought that if you take the same amount of light that falls on 200 sq feet and focus it onto 1 (one) sq foot you have some 200 times concentration and the light is therefor brighter. Bright enough certainly to do serious eye damage if one was to be incautious though perhaps not as bright as the surface of the sun where it originated. Anthony
Response:
- Hide quoted text — Show quoted text -> > Has anyone done any research on adapting solar thermal > > collectors to cement kilns? > I have done a lot of research on this topic. How about > glass kilns? Glass is made from sand – there is a lot of > this available cheap. Our solar kiln is non-polluting. > I have a technical question. > I thought that the temperatures within the kiln needed > to be controlled to fairly close tolerances. > Especially the tempering furnace. Where the temperatures > need to be maintained for several days with gradually > decreasing temperatures. > How do you accomplish this regulation?
… Well, I’m no expert in ceramics but I would assume that a similar method as the 24 hour pizza ovens we’ve been discussing elsewhere in this same thread could be used. You would have a large thermal mass which is heated to temperatures higher (probably much higher) than those you need inside the kiln and tempering furnace. This thermal mass would operate like a battery, storing heat in the day, releasing heat at night. You control the heat inside the kiln or furnace by controlling the transfer of heat from this storage mass to your kiln. My suggestion was through the use of heated air via electric fans but I’m sure you could use an oil that can be heated to high temperatures, liquefied metals or some such thing too. Maybe even just dampers that open and close air passages. The temperature would be controlled by thermostats of course, just like they use now. Anthony
Response:
Hi Anthony; – Hide quoted text — Show quoted text -> Just a technical detail here. > While the concentration zone will be highly concentrated > and containly appear quite bright the intrinsic brightness > can be no greater than the sun by itself. > The effect of concentration is to make the light subtend > a wider angle than that of the sun. > The human eye is designed to tolerate the intrinsic > brightness of the sun without damage. We easily can look > directly at the sun for an instant and then avert our > eyes. > This reflex is easily done when looking at the receiver. > One just looks away. > Of course the mirror absorbs some of the light. My mirrors > reflect 85% of the incoming light. This in effect reduces > the intrinsic brightness to 85% that of the sun directly. > I’m not saying that one wouldn’t need the goggles. They are > needed if one wants to continuously look at the bright spot. > I’m not exactly sure of the details here but my gut feeling > is that any spot of light bright enough to fry burgers with > is going to be bright enough to burn out your eyes.
You are mixing up two attributes, brightness and power. 1. Power is the total quantity of light received. 2. Brightness is the apparent intensity when looking at a light source. The sun has an apparent angle of about 1/2 deg of arc in diameter. A solar concentrator takes the suns brightness and increases its angle and apparent diameter. In the 200X example: If you look back into a dish the sun now would appear to be about 7 degrees in diameter. While the brightness would be the same, less the 15% loss, the power in the light would be about 200 times greater than the sun alone. Another way of looking at this is to think of the concentrator as an array of 200 1ft^2 mirror all reflecting on the same location. If you look back at the array there will be 200 images of the sun. Each image will be at a different location in the field of view. Each image will be on a different location on the eye’s retina. Clearly the brightness of each image can be no brighter than any other image and they don’t overlap. – Hide quoted text — Show quoted text -> I suppose this might not be as bright as the sun, but > it’s going to appear in a place (a burger grill) > entirely different in position and behavior than the sun > does so peoples built in reactions aren’t guaranteed to > protect them from it. Telling your customers ‘just don’t > look at the cook’ isn’t going to protect you from > getting sued when little johnny goes blind watching his > burger. > I’m just pointing out that the intrinsic brightness > does not change with increasing concentration factors. > Hmmm… Maybe I’m completely wrong here, but I thought > that if you take the same amount of light that falls on > 200 sq feet and focus it onto 1 (one) sq foot you have > some 200 times concentration and the light is therefor > brighter. Bright enough certainly to do serious eye > damage if one was to be incautious though perhaps not > as bright as the surface of the sun where it originated.
Eye damage can occurs in two different ways: 1. If the intensity of light hitting a particular rod or cone cell is above a certain level the cell is overloaded and can be damaged. I understand this can happen in an instant. However, since the source is the sun this intensity is not high enough to do this type of damage. 2. If the total power entering the eye is to high the eye is above a certain level the eye can’t dissipate the heat fast enough and will over heat. This type of damage is happens at a much slower rate. I understand that the eye, generally, doesn’t feel pain due to increased heat. Furthermore you will also burn your skin which is more sensitive to this and you will turn away. This is all kind of academic as these damages will only happen if your head is at the point of focus. Our pizza oven will be at the point of focus and not our heads. Lastly: Since the receiver for the pizza oven should have a black coating to efficiently absorb the light and convert it to heat it will not reflect much light. Lets sat that the receiver only reflects 10% of the incoming light. Then the intrinsic brightness will be 85% * 10% or only 8.5% of the brightness of the sun. Clearly this is not bright enough to do damage. Also since one will be away from the receiver by at least some distance the concentration factor will be much less at the patrons pizza bar seat. The thermal damage will not happen. One might argue that the hot receiver can become a source of infrared light due to black body radiation. It can be proven that this source of light can never be more intrinsically bright than the power source driving the system. In conclusion: I would always suggest the use of goggles when working around highly concentrated light. This is just prudent. I only bring up to show that no mater what the concentration factor is the brightness is never higher than the brightness of a black body radiation source. The concentration of power is the thing to watch out for. This can cause severe burns. I can attest to this myself. Eye damage is not very likely. > Anthony
– CUL8ER Receiver Powered by [*] Thermonuclear SolarEnergyfrom the Sun /////| Energy(the Sun) / / // / /| / / / / / / | WA0VBE / / / / / / /| Ziggy / / / / / / | / / / / / / / | "Red Rock Energy" === === / / === / === Duane C. Johnson, Designer=== === === / | 1825 Florence St Mirrors,Heliostats,Controls & Mounts| White Bear Lake, Minnesota / | USA 55110-3364 | (651)635-5O65 work / | (651)426-4766 home | (413)556-659O Fax copyright / | (651)583-2O62 Red Rock Energy Site (C)980907 === | http://www.redrok.com/index.htm (My New Web site) | These are my opinions, and not that of Unisys Corp. ===
Response:
– Hide quoted text — Show quoted text ->Time and again I keep running across the idea that portland cement, >and concrete made from it, is environmentally unfriendly. Mostly >this is because of the huge amount of energy embodied in it’s >manufacture. I wonder how difficult it would be to adapt existing >solar thermal technology to provide for the majority of that energy. >Has anyone done any research on adapting solar thermal collectors to >cement kilns? Since the majority of the energy used to produce portland >cement is thermal in nature I would imagine something like a heliostat >array could work very nicely. The ordinary burners could be retained >to allow the plant to operate all the time while using as much energy >from sunlight as possible to offset their no doubt high fuel costs. >Also, while I’m asking silly questions, it’s my understanding that >old concrete, from demolished buildings, pavements and the like, are >a significant amount of waste that is sometimes hard to get rid of. >Would it be possible to run old concrete through a cement kiln and >get something like portland cement, or perhaps redi-mix, out the other >side? >And, while I’m at it I’ll throw another question out there. >Has anyone looked into using solar thermal for pottery kilns? I do >understand that most porcelain and pottery requires firing times >longer than daylight hours might provide but it seems to me that >a sufficiently large thermal storage running at temperatures higher >than those required for the kiln could provide the heat required >for nighttime operation. >Anthony
It’s been a while since I’ve read the newsgroup, glad to have a good thread here to reply to. I was working on a simple system for generating steam a couple years ago and had to deal with the unique problems of solar systems. After a lot of brainstorming I came up with a system that appears to be a winner and is adaptable to many applications. Except in the desert a system is going to have to deal with potentially long periods of cloud – this makes it very hard to use the solar heat directly, especially if high temperatures are needed. My line of reasoning ran — if I need to burn fuel anyway, why not use the solar heat indirectly and just preheat the combustion air to 500 – 1,000 degrees F. The hot combustion air increases the efficiency of the burners greatly and means little fuel is actually burnt during bright days. If cloud or storms shut down the collectors completely for a couple days it just means I need to burn more fuel. Wes
Response:
Hi Wes; – Hide quoted text — Show quoted text -> Time and again I keep running across the idea that portland > cement, and concrete made from it, is environmentally > unfriendly. Mostly this is because of the huge amount of > energy embodied in it’s manufacture. I wonder how difficult > it would be to adapt existing solar thermal technology to > provide for the majority of that energy. > Has anyone done any research on adapting solar thermal > collectors to cement kilns? Since the majority of the energy > used to produce portland cement is thermal in nature I would > imagine something like a heliostat array could work very > nicely. The ordinary burners could be retained to allow the > plant to operate all the time while using as much energy from > sunlight as possible to offset their no doubt high fuel costs. > Also, while I’m asking silly questions, it’s my understanding > that old concrete, from demolished buildings, pavements and > the like, are a significant amount of waste that is sometimes > hard to get rid of. > Would it be possible to run old concrete through a cement > kiln and get something like portland cement, or perhaps > redi-mix, out the other side? > And, while I’m at it I’ll throw another question out there. > Has anyone looked into using solar thermal for pottery kilns? > I do understand that most porcelain and pottery requires > firing times longer than daylight hours might provide but it > seems to me that a sufficiently large thermal storage running > at temperatures higher than those required for the kiln could > provide the heat required for nighttime operation. > Anthony > It’s been a while since I’ve read the newsgroup, glad to have > a good thread here to reply to. > I was working on a simple system for generating steam a > couple years ago and had to deal with the unique problems > of solar systems. After a lot of brainstorming I came up > with a system that appears to be a winner and is > adaptable to many applications. > Except in the desert a system is going to have to deal > with potentially long periods of cloud – this makes it > very hard to use the solar heat directly, especially if > high temperatures are needed. > My line of reasoning ran — if I need to burn fuel > anyway, why not use the solar heat indirectly and just > preheat the combustion air to 500 – 1,000 degrees F. > The hot combustion air increases the efficiency of the > burners greatly and means little fuel is actually burnt > during bright days. If cloud or storms shut down the > collectors completely for a couple days it just means > I need to burn more fuel. > Wes
This has been experimented with a number of times. Its the the equivalent of a hybrid vehicle only solar. I’d call it hybrid solar. The United Stirling, I think it was them, solar power plant used auxiliary fuel gas to run the stirling at night and when in cloudy weather. I agree with you that a solar assisted burner will work very well. We have been working on just such a unit ourselves. Keep up the good work! Duane — CUL8ER Receiver Powered by [*] Thermonuclear SolarEnergyfrom the Sun /////| Energy(the Sun) / / // / /| / / / / / / | WA0VBE / / / / / / /| Ziggy / / / / / / | / / / / / / / | "Red Rock Energy" === === / / === / === Duane C. Johnson, Designer=== === === / | 1825 Florence St Mirrors,Heliostats,Controls & Mounts| White Bear Lake, Minnesota / | USA 55110-3364 | (651)635-5O65 work / | (651)426-4766 home | (413)556-659O Fax copyright / | (651)583-2O62 Red Rock Energy Site (C)980907 === | http://www.redrok.com/index.htm (My New Web site) | These are my opinions, and not that of Unisys Corp. ===
If you like this post and would like to receive updates from this blog, please subscribe our feed.