Solar Parabolic Trough Charcoal Oven

[redgreenblue]

The thought did occur to me to insulate the top portion of the receiver tube that lay above the plane of the trough. Any thoughts on what material? Fiberglass wool? Asbestos?:confused: Cogitating.:confused:

 

Perhaps you could even use a layer of clay to act as a thermal mass (with fiberglass wool on the outside of this?).

[Turtle]

Perhaps you could even use a layer of clay to act as a thermal mass (with fiberglass wool on the outside of this?).

Clay is a good idea, but I thought more on the fiberglass & decided a layer thick enough to make much difference would stick up & throw a big shadow on the reflector.

Maybe some of those formed clay tiles they sell in garden shops?

I also considred a layer of pumice between a couple pieces of screen/hardware cloth. Use the dark pumice they sell for bonsai & make a 3/4" thick shield that covers the top 30° of the receiver.

That's all I have for now; still no reply from my enquiry to the commercial builder. Anyone else written any enquiries? Keep us posted. :D

[cwes99_03]

Might I recommend a simple test before going to the next step of actually insulating.

Find a way to shade just the tube from the sunlight above it and after an hour in the shade feel the tube. If it is still hot, then you have a good reason for insulating. If it is cooler to the touch than the underside of the tube, then insulating will actually hurt your end result (because the black paint on the tube is doing more to warm it than insulation will do).

Even black stone keeps the ground under it cool. Rake away the stone and the ground underneath it is generally cool. Five minutes later once the sun has been active on it, the once covered ground is hot.

[Turtle]

Construction notes:

I left my reflector outside a couple nights & it is now nicely rimpled. Nice rimpling=diminished efficiency. Fortunately my corrugated paperboard structure only needed to establish some baseline data points for trough width to temperature ratio.

 

Since gost is using plywood & masonite for a trough 2 times the width of mine, I recommend paint or varnishing the exposed surfaces.

I think the most practical design once the optimum size is determined is an all aluminium structure. The plywood cutouts you use as a form for the trough supports, and thin sheet alumnium for the reflector surface. Polish the reflector to as high a sheen as possible.

Make it collapsible & modular so you can transport it. Then take it on site (garden, forest, farm, etc.) bake up the charcoal from on site materials, discharge the charcoal, collect your bill, & then load up & head to the next job.:eek_big:

[Michaelangelica]

The problem with trapping heat inside of the trough is that it would then require the entire contraption to be made from non-combustible materials. The temperature we're trying to reach is around 470F.

Temperatures of 350-400C would be optimum for Terra preta.

A random thought:-

Could you modify this design to make a solar water still?

[Turtle]

Temperatures of 350-400C would be optimum for Terra preta.

A random thought:-

Could you modify this design to make a solar water still?

 

That is a higher temp than I (we?) set as a mark, i.e. 450° -500°F (232°-260°C).

Yes, the design is adaptable to a solar still. It's a matter of changing the receiver setup. Here is a link to a group using them to make dry ice:

http://www.thesustainablevillage.com/servlet/display/microenterprise/display/14

 

:cup:

[Michaelangelica]

See pages 34 -38?

of

http://www.energy.gatech.edu/presentations/dday.pdf

 

Sorry thought you were talking C not F

200C might be OK look at graphs from page 34 on

Temperature

The last website I posted had an interesting graph:-

275-400c best to maximise carbon storage for mankind and optimal zone for energy extraction

400-600C best for microbial life but complete devloatilization -requires addition of energy &/or oxygen

400C chars very gradually released its ammonia

I can't see how Amazonian Indians could control temperature anyway. Perhaps they had a mixture of all temps?

 

Thanks for the tip about solar stills.

I am very interested in low tech designs

I might need one soon if the drought here gets any worse!

[SolarFreak]

this is great work.

 

is there a forumal for how much surface area your parabolic needs to be reach the temp needed?

 

 

is only the very bottom edge of your pipe in focal point?

 

I would think that lowering the pipe just a little, would give it more surface area for the sunlight to hit.

 

also on the top side of the pipe I would use some type insolation to keep the heat from radiating. or cover the top though with glass so the pipe is not cooled by a breaze.

 

I know that in building solar ovens to cook food the key to getting them up to temp was keeping the wind and open air off the cooking area. most times this best results I have gotten is to make the area inside air tight.

[Turtle]

this is great work.

 

is there a forumal for how much surface area your parabolic needs to be reach the temp needed?

If there is, no one has offered it & I haven't found it. I wrote to a commercial manufacturer to ask & have as yet received no reply. [Perhaps they are too busy designing/making charcoal ovens now?:hyper: ]

is only the very bottom edge of your pipe in focal point?

No; roughly 2/3 is immersed.

 

also on the top side of the pipe I would use some type insolation to keep the heat from radiating. or cover the top though with glass so the pipe is not cooled by a breaze.

Acknowledged.

 

As my oven is too small for the required temp., I have just been waiting to hear from gost on his larger trough. Thanks for your interest SolarF.:doh:

[SolarFreak]

If there is, no one has offered it & I haven't found it. I wrote to a commercial manufacturer to ask & have as yet received no reply. [Perhaps they are too busy designing/making charcoal ovens now?:doh: ]

 

Yeah, I have looked around for a while for that information too.

 

No; roughly 2/3 is immersed.

That is about the same as I was thinking too.

 

 

I think to get more surface reflective area you are going to need to move the focus point up.

[Turtle]

I think to get more surface reflective area you are going to need to move the focus point up.

Agreed; but only as a condition of increasing the width (breadth?) of the trough, that is to say the distance across/between the "horns" of the profile of the parabola.

The comercial ovens used in electric power production look about 12 to 16 feet across in the photos; as I say I can't find any specs for them. I expect we need a similar size to make the charcoal we want.

I planned some insulation experiments for my 4 foot project, but the foiled paper-board is too badly wrimpled...well, I suppose wrimpled with & without insulation is at least some comparitive measure eh?:doh:

That is all.:hyper:

[Michaelangelica]

I did a bit of a web search with Google on solar charcoal making but ended up with very little. . .

Except this site! Which appears somewhat unique!!

 

You would think the solar oven society would be helpful but it does not look like it. An email might be worth the effort?

http://www.solarovens.org/

 

This is not solar but may spark an idea:

http://sleekfreak.ath.cx:81/3wdev/VITAHTML/SUBLEV/EN1/CHARCOAL.HTM

 

This guy wants you to buy his book (looks interesting) but not a lot of help

http://standeyo.com/Our_Books/DTP.html

 

Wood transformed into charcoal is more efficient as a fuel

 

Table 15-5

CALORIFIC POWER OF SELECTED COMBUSTIBLE SUBSTANCES

Combustible Substance

Calorific Power (Kcal/kg)

 

Paraffin

10,400

 

Diesel Petroleum

9,800

 

Charcoal

7,100

 

Dried Wood

4,700

 

Lignite

4,000

 

Wood (25-30% Moisture)

3,500

 

Most web sites seem to want to get rid of charcoal and just use solar.

But the FAO says an intermediate technology is in order

But again no ideas on what that is.

FROM:

http://www.fao.org/documents/show_cdr.asp?url_file=/docrep/p3350e/p3350e02.htm

 

Wood, charcoal and agricultural wastes provide almost 100 percent of the household energy in the Sahel.

 

Solar energy. Sunshine, the renewable energy par excellence, is more than abundant in the Sahel countries. Many people and even the women of the region place great hopes in its domestic use. Research undertaken in the past 20 years, especially in Niger,

 

Upper Volta and Mali, has led to the development of a solar cooker offering undeniable advantages:

 

· The energy it uses is totally free and it does not pollute the environment.

 

· There is neither smoke nor heat radiation, the heat being entirely concentrated on the appliance.

 

· The appliance and its accessories are easy to care for.

 

However, against these advantages we must list the following limitations:

 

· The high cost of the solar cooker, from F CFA 17000-25000.

 

· The space it requires, due to the diameter of the reflector (1.5 m) and the support framework. Many housewives do not have a separate kitchen and cannot store these large appliances in their already small living quarters.

 

· The powerful reflections of the sun's rays which can damage their eyes.

 

· The lack of stability for certain kinds of cooking pots.

 

· The required use of metal pots for cooking instead of the traditional terracotta "canards."

 

· The very slow cooking process.

 

· The limited period during which the solar cooker can be used on cloudy or rainy days.

Solar energy researchers and technicians must proceed very cautiously because any haste and error can harm future decisions but they are, nevertheless, pursuing their efforts with the Human Sciences Research Institutes and the national women's organizations of the various countries.

They have already had some unquestionable successes with solar water-heaters which are now often planned for or installed in public buildings such as hospitals and schools.

Improving charcoal-making.

Charcoal is obviously preferred by many urban housewives in Mauritania, the Gambia and Senegal. In Senegal, fuelwood production. . .

 

This is why there are major efforts everywhere to improve the methods and train the charcoal makers.

For example, two USAID and UNDP/FAO projects in Senegal are seeking to improve charcoal-making and simultaneously promote a better use of this fuel through improved stoves such as the cast-iron sealed oven, under testing in the Gambia, or the "Malagasy oven" which is being encouraged in Senegal.

[SolarFreak]

That is a higher temp than I (we?) set as a mark, i.e. 450° -500°F (232°-260°C).

Yes, the design is adaptable to a solar still. It's a matter of changing the receiver setup. Here is a link to a group using them to make dry ice:

http://www.thesustainablevillage.com/servlet/display/microenterprise/display/14

 

:rolleyes:

 

hmm makeing ice from solar that is sweet!!

[Kayra]

hmm makeing ice from solar that is sweet!!

 

Making Icecream would be sweeter :rolleyes:

 

And if you need to keep it cool...

 

http://www.chinatech.com/solar.htm

 

I wonder if you could use your waste heat to cool your house...

[SolarFreak]

By the way, for you calculus whizzeesss; given the dimensions earlier, what is the surface area of the inside of my trough in square inches?:eek: :D

 

until some calculus ninja steps up.

 

the low tech way is to take a string and measure the length of your parabola curve and take that # * width.

 

Area is length * width.

 

you said your width is 20" so your halfway there

 

 

if your parabola baseline is 42" and your using 10" focal point. I calc'd if you measure your parabola curve it would be 50" long. so you would have area of 1000sqr"

 

I graph'd it out by x^2 = 4py started y at -21 - 21 to get 42units. I measured the line it was 50units

 

I made a quick php program http://forum.teamfc3s.org/parabola/ to calc a parabola using your specs

[Turtle]

until some calculus ninja steps up.

 

the low tech way is to take a string and measure the length of your parabola curve and take that # * width.

 

Area is length * width.

 

you said your width is 20" so your halfway there

 

 

if your parabola baseline is 42" and your using 10" focal point. I calc'd if you measure your parabola curve it would be 50" long. so you would have area of 1000sqr"

 

Just got the heads up & took my measures:

20" x 48.5" = 970 in^2 = 6.736 feet^2

:)

[SolarFreak]

ok so .645 W/in^2 solar power constant

 

970in^2 * .645 = 625.5W Thats is if could reflect 100% of the light. from what I have looked up online mirrors reflect around 85% - 95% of the light.

 

I searched around the net to find what the reflection of alum roll is but didnt find it.

 

I did find this 3m film thats say its 98.5% reflective.

http://cms.3m.com/cms/US/en/2-136/cRFFrFP/view.jhtml