Solar Parabolic Trough Charcoal Oven

[freeztar]

Originally Posted by Turtle

with the Fresnel or parabolic dish any increase in size of the 'roundish' shaped receiver just puts it outside the focus and so requires multiple load/process/extract cycles.

I'm not sure I understand you here.

In any case, I think a practical home/farm solar furnace ought to at least have interchangeable receivers so one can make char, heat water, or make electricty.

This is not necessary with a dish or fresnel, but would definitely be useful for a trough.

While there would be more losses, the ideal seems to me to just make electrity, store it, and use it to heat water, make char, light lights, pump pumps etc.

That's a very valid argument. One problem I see though is batteries. They would not last forever and are potentially caustic to the environment. Also, the manufacturing of the batteries would tip the carbon scale back the other way. I'm not saying it's unethical, but that battery storage is probably not the best method.

Of course there are other ways to store the energy. The same friend I mentioned before came up with a clever idea (it's what we do for fun, he comes up with ideas and I try my hardest to make them unfeasible ). He suggested using the electricity produced by the dish to run a pump that would pump water from a lower elevation pond into an upper elevation pond. His reasoning is that you could have a water wheel between the two ponds and by releasing water from the elevated pond, you could have energy on demand, night or day, rain or shine. I'm sure you can come up with some of the arguments I made against the system he proposed , but it's still a pretty novel idea.

The losses in conversion are offset by the decrease in labor to change receiver setups. That's all i got.

Again, the "labor to change receiver setups" () is only a big deal if you are using a trough. Keep in mind that the "losses in conversion" are higher when you compare a parabolic trough versus a 'Stirling dish', and I imagine a fresnel as well.

Originally Posted by Wiki on Parabolic Troughs

The overall efficiency from collector to grid, i.e. (Electrical Output Power)/(Total Impinging Solar Power) is about 15%, similar to PV(Photovoltaic Cells) and less than Stirling dish concentrators.[1]

Parabolic trough - Wikipedia, the free encyclopedia

[Turtle]

Originally Posted by freeztar

...
Of course there are other ways to store the energy. The same friend I mentioned before came up with a clever idea (it's what we do for fun, he comes up with ideas and I try my hardest to make them unfeasible ). He suggested using the electricity produced by the dish to run a pump that would pump water from a lower elevation pond into an upper elevation pond. His reasoning is that you could have a water wheel between the two ponds and by releasing water from the elevated pond, you could have energy on demand, night or day, rain or shine. I'm sure you can come up with some of the arguments I made against the system he proposed , but it's still a pretty novel idea.

I'll go just with this one as I have some chores. In short, good idea. This is in fact what all city water departments do and why we have/need water towers. The pumps only pump into the tower when the level in the tank drops. The supply pressure to users is entirely a result of the hydraulic head in the tower. gotta run.

[Turtle]

Originally Posted by freeztar

I'm not sure I understand you here.

OK; I'll try to kill as many birds with one throw as possible.
First the one at the top of the pecking order, which is a affirmative secular "the devil is in the details."
The detail I meant to convey above. Suppose you have a dish and a trough which have a focus point on a plane that is the same in one dimension (call it height). The dimension of the focus plane width for the dish is also the same as its height (circular) wheras the width of the focal plane for the trough is as long as the trough.(rectangular) All else being equal, the trough can accomodate a larger receiver and so expose a larger area to focussed Sun. Let me know if that's still not clear or if you think it's mistaken.
The rest of the flock is one detail after another, because all things are not equal in much of this at all. All of the plans put forward here appear feasible to me given the proper 'this material' and the correct 'that measure' in the such-n-such a 'given circumstance'...well, you get the idea.
The upshot is, if you have an idea then give it a try and let us know how it works out.
PS The water storage idea will work, but even small power generation at night I think would require a very large reservoir. I think a water tower tank in every yard is a good idea for many reasons outside the scope of this thread.

[freeztar]

Originally Posted by Turtle

OK; I'll try to kill as many birds with one throw as possible.
First the one at the top of the pecking order, which is a affirmative secular "the devil is in the details."
The detail I meant to convey above. Suppose you have a dish and a trough which have a focus point on a plane that is the same in one dimension (call it height). The dimension of the focus plane width for the dish is also the same as its height (circular) wheras the width of the focal plane for the trough is as long as the trough.(rectangular) All else being equal, the trough can accomodate a larger receiver and so expose a larger area to focussed Sun. Let me know if that's still not clear or if you think it's mistaken.

Roger that. But getting back to what Nitack stated, the surface area of the dish/trough should determine output, not the surface area of the receiver. If a 42" fresnel lens can melt glass, then there is something to be said for a 'circular' approach. A 42" fresnel pointed at a steel drum (slightly off focal) should produce very high temperatures even at the 'antipode' of the surface being heated.

The rest of the flock is one detail after another, because all things are not equal in much of this at all.

Agreed.

All of the plans put forward here appear feasible to me given the proper 'this material' and the correct 'that measure' in the such-n-such a 'given circumstance'...well, you get the idea.
The upshot is, if you have an idea then give it a try and let us know how it works out.

I hope to build a dish at some point. If I can bring the DIY cost below the cost of buying a prefabricated parabola, then I will be more motivated. I still might do it anyway, for kicks, but at present it is a 'top-of-the-shelf' pursuit.

PS The water storage idea will work, but even small power generation at night I think would require a very large reservoir. I think a water tower tank in every yard is a good idea for many reasons outside the scope of this thread.

Indeed. Perhaps we should start a new thread, or multiple threads that can be spawned from this one.
I agree that you would need a very large elevated reservoir and a substantial turbine to reclaim the energy on demand. Batteries would still be efficient in this setup, but not as necessary as in a Stirling-dish. It would be a good setup for a landowner with lots of unused land, which met the topology requirements. I don't see potential for large scale applications of this principle, outside of dams of course.

What do you think about the PVC idea for making a parabolic trough/dish?

[Turtle]

Originally Posted by freeztar

Roger that. But getting back to what Nitack stated, the surface area of the dish/trough should determine output, not the surface area of the receiver. If a 42" fresnel lens can melt glass, then there is something to be said for a 'circular' approach. A 42" fresnel pointed at a steel drum (slightly off focal) should produce very high temperatures even at the 'antipode' of the surface being heated.

roger roger the experimental approach seems the right one.

Originally Posted by freezy

Perhaps we should start a new thread, or multiple threads that can be spawned from this one.

build it & they will come.

Originally Posted by freezter

What do you think about the PVC idea for making a parabolic trough/dish?

I think the resulting curve will be a catenary and not a parabola. Here's another thread we have on the DIY dish/mirror scheme. >> building a parabolic with mirrors.

[Nitack]

Originally Posted by freeztar

Roger that. But getting back to what Nitack stated, the surface area of the dish/trough should determine output, not the surface area of the receiver. If a 42" fresnel lens can melt glass, then there is something to be said for a 'circular' approach. A 42" fresnel pointed at a steel drum (slightly off focal) should produce very high temperatures even at the 'antipode' of the surface being heated.

Whether you are reflecting the energy using a parabola or concentrating using a lens the amount of energy you start with is absolutely the same per area squared. What we are trying to do is refine how much energy is lost in the transfers and possibly increasing the surface area (extra lenses and mirrors) in order to increase the starting energy.

By spreading out the foci of the energy (parabola) we lose more heat energy because there is more area to lose is from. By focusing the heat energy into specific spots there is less surface area for which to lose the energy to the air and more will be transfered to the inner kiln where our carbonization will take place.

[freeztar]

Originally Posted by Turtle

I think the resulting curve will be a catenary and not a parabola. Here's another thread we have on the DIY dish/mirror scheme. >> building a parabolic with mirrors.

I think you are right. While reading up on catenaries, I came across this tidbit:

Free-hanging chains follow the curve of the hyperbolic function above, but suspension bridge chains or cables, which are tied to the bridge deck at uniform intervals, follow a parabolic curve, much as Galileo originally claimed (derivation).

When suspension bridges are constructed, the suspension cables initially sag as the catenaric function, before being tied to the deck below, and then gradually assume a parabolic curve as additional connecting cables are tied to connect the main suspension cables with the bridge deck below.

Catenary - Wikipedia, the free encyclopedia

So it seems possible still, but requires more connection points to be winched down. Now where can I find some cheap wenches...

[freeztar]

Originally Posted by Nitack

What we are trying to do is refine how much energy is lost in the transfers and possibly increasing the surface area (extra lenses and mirrors) in order to increase the starting energy.

I like your idea of using additional mirrors and lenses, but is it really necessary? How much focused heat will be needed to char a steel barrel full? Remember, we don't want it TOO hot. Of course, that would be a good problem to have at this point eh?

[Nitack]

Originally Posted by freeztar

I like your idea of using additional mirrors and lenses, but is it really necessary? How much focused heat will be needed to char a steel barrel full? Remember, we don't want it TOO hot. Of course, that would be a good problem to have at this point eh?

I thought about that, about possibly having too much heat. My thinking was that we want more than one point of heat.

First, even with an insane temperature like 900 °F , it will still take quite a while for that heat to build up inside if it is only coming from one spot. By having multiple spots it would raise the inner temperature faster.

We also saw that even though Turtle was able to focus the energy of his parabolic trough, it did not capture enough heat to carbonize his materials. We don't know if it was an issue of energy loss or not enough reflecting area. This will ensure that we can capture enough energy to heat it up to 500+.

Finally, if we captured too much heat what would be our product? My understanding in charcoal making is that the hotter the kiln the more impurities are burned off, leaving you with purer charcoal. I checked and the melting point of carbon is 8000 °F. No way we will get up to that temp, meaning that most likely we will just have incredibly pure charcoal.

Added bonus, I have been thinking about this solar capture method and it could also be converted to form a very simple electric generator using some piping and a cars water pump and an alternator. In this case more heat = better. If we were able to generate electricity from this method we could power a solar tracker which would then allow this kind of generator to run all day long. If it were closed loop and we used something non-corrosive (not water) it could run continuously. In essence a solar generator without the costly silicon panels.

My mind is really flying on these ideas

[freeztar]

Originally Posted by Nitack

I thought about that, about possibly having too much heat. My thinking was that we want more than one point of heat.

That would certainly be ideal.

First, even with an insane temperature like 900 °F , it will still take quite a while for that heat to build up inside if it is only coming from one spot. By having multiple spots it would raise the inner temperature faster.

I agree. Though perhaps we could use something like copper coils around the barrel to conduct the heat more evenly?

We also saw that even though Turtle was able to focus the energy of his parabolic trough, it did not capture enough heat to carbonize his materials. We don't know if it was an issue of energy loss or not enough reflecting area. This will ensure that we can capture enough energy to heat it up to 500+.

I get the impression that Turtle thinks the area needed to be increased, but I'll let him speak to this.

Finally, if we captured too much heat what would be our product? My understanding in charcoal making is that the hotter the kiln the more impurities are burned off, leaving you with purer charcoal. I checked and the melting point of carbon is 8000 °F. No way we will get up to that temp, meaning that most likely we will just have incredibly pure charcoal.

While (nearly) pure charcoal (activated charcoal) is ideal for consumption purposes, it is not ideal for a soil ammendment. From what I've read, leaving a little bit of impurities in the final product increases the amount of wee beasties that take advantage of it. So there is an advantage to keeping a controlled temperature if you plan to use the char as a soil ammendment. Otherwise, I suppose it doesn't matter much.

Added bonus, I have been thinking about this solar capture method and it could also be converted to form a very simple electric generator using some piping and a cars water pump and an alternator. In this case more heat = better. If we were able to generate electricity from this method we could power a solar tracker which would then allow this kind of generator to run all day long. If it were closed loop and we used something non-corrosive (not water) it could run continuously. In essence a solar generator without the costly silicon panels.

This is something my friend and I have pondered in depth. We like the simplicity of the aeolipile. I can't seem to find efficiency numbers on it now, but it is quite efficient with super heated steam IIRC.

My mind is really flying on these ideas

I'll try to keep up.