My Parabolic Trough Water Heater

[brianc4]

I just completed my first parabolic concentrator trough for solar hot water!

This unit is only a prototype made out of marine plywood 2x4s plexiglass & plastic mirror.

The final product will have a aluminum frame,bows & housing with a polished stainless steel mirror & tempered glass cover.

I am using a deep focus trough with a glass cover & insulation between mirror & back in order to keep ambient air off of collecter tube.

In test results on the partly cloudy day that these pictures were taken the trough raised the temperature of 5 gallons of water 30 degrees in the first hour & continues to climb as I am posting this message, last check 160 degree water.

You can see the clouds in the reflection of the mirror in the picture above. You can also see some distortion of the plastic mirror in the above picture ie . you should see the black pipe all the way across the parabola. The distortion of the plastic mirror gets worse as the temperature in the trough rises. Hopefully the polished stainless sheet will behave better!

This final picture is of the thermostat in the end of the trough that keeps trak of air temperature inside the trough 150 degrees at last check.

I am very pleased with the prototype & will test it this winter for heating my battery storage shed before I sink the money into the final version with sun tracking.

[Stonebrain]

Looks good.

I think it will be quit expensif compared to 'ordinary' solar collectors.

(for the construction but also because you need some kind of sun-tracking).

The only advantages I see is that you can raise the watertemp much higher..and that you have much more fun.

cheers,

stonebrain

[oztules]

Interesting design, do you think that you should insulate the stainless mirror from behind to keep the ambient inside temperature up (the plastic did it for you here I think).

Neat example of heating up above the passive ones.

.........oztules

[phil b]

Nice work!

I can see a definate advantage with your parabolic heater. Flat panels continue to heat up when not in use, and are difficult to shade.You can simply turn the collector out of the sun.

Thanks for sharing!

[MAXial dreamer]

I bet with a smaller pipe you could distill water

[brianc4]

oztules,

   On the prototype in this post I used 3-1/2" fiberglass batt insulation between the back of the mirror & the plastic back on the trough.

   For the final unit I may have a local contractor spray in polyisolene insulation.

   Also considering double pane glass on the front for a dead air gap & better insulation.

   I think that the parabolic style solar water heater can work well even in a cold climate if the collector tube is protected from ambient air.

Any thoughts on insulation or glass for front cover would be appreciated.

Brian

[brianc4]

Phil b,

  On the final unit I want to put 4 troughs on a tracking unit tied to a thermostat & a plc. Which like you said can be rotated out of the sun if temperature gets to hot.

  I also think with a little planning the tracking unit can be made to rotate out of harms way in a storm to avoid hail damage.

   The tracker can also be made to rotate the troughs down at night to avoid snow accumulation in the winter.

   Lots of posabilities!  Still in the R&D stage & having lots of fun!

Brian Clark

[brianc4]

MAXial dreamer,

Even with the 2" aluminum pipe we used if you cut down the flow rate or just simply fill the tube with water & let it sit, it will boil water & create steam.

Could also be used as a solar still for alcohol!

Brian Clark

[dsmith1427]

Nice work! I played around with a similar design but never got it off the drawing board.  If you had a closed system with a pressure releif valve (for safety), do you think higher temperatures can be acheived due to thermal expansion?  Also, how did you determine your parabolic shape?  I put together a spread sheet to play around with different sizes but considered the following my limiting factors: plywood size, pipe size, and location of focus point) the limiting factor.  Thanks for sharing!

[mkseps]

If you are looking for a fool proof way of designing a parabola, go to:

http://home.germany.net/100-441770/amsi-model.html

You start out by setting the focal point and the rest falls into place.

Gene

[brianc4]

dsmith,

   There are numerous ways of calculating & laying out parabolas! I have seen excel spread sheets ,layout with a T square and many others.

   I downloaded a small freeware computer program from one of the solar cooker sites that gives a quick width / depth / focal point calculation with X & Y coordinates in a text file output that can be imported onto excel or into autocad.

   Unfortunatley what I have found out is that none of the spreadsheet type output programs output a true acurate parabola!!!

   They are handy to get the rough width depth & focal point but from there I draw the parabola in autocad where I can maintain accuracy within a thousandth of an inch!

  Once I have a true parabola I draw the bows & end plates & cut a Steel template with My CNC Plasma Table & used the Templates to cut the pieces out of plywood with a jigsaw. I know I lost some accuracy cutting with the jigsaw on the prototype but all parts on the final version will be cut on the Plasma Table & should hold a tight tolerance.

   I learned how to draw a perfect parabola in autocad based on one of My most prized possesions, which is an old 1940s  drafting & sheet metal layout book.

   The book was written based on hand drafting tecniques but is a breeze to adapt to autocad using automatic guidelines & snap points.

   I use a resolution of a 1/16" of an inch to set all of the points along the parabola which for the proto type trough was 384 intersecting points & guidelines. That many guidelines makes your eyes about cross when drawing the final parabola by using the fitted curve drawing function in my cad program but it is the only accurate way I have found to generate a perfect parabola.

   If you or anyone else is interested in how to layout a parabola by this method let me know and I will send them a copy of the instructions from the book along with my crib notes on how to adapt them to autocad.

Brian Clark

[BigBreaker]

The math of a parabola is pretty easy, y = a * x * x.  To find the focus just put 2 units (inches, feet, whatever) in for "x", find y and then the reciprocal, IE 1 divided by "y".  That's your focal length.  Alternatively the take your desired focal length, take the reciprocal and multiply by 4 to find "a" and plot your curve.

http://en.wikipedia.org/wiki/Parabola

I think graph paper and a hand calculator would be plenty to draw a near perfect template.  Also there are drafting aids like "soft" rulers that you can use to fit a curve to multiple points.

I wonder if there is a mechanical method of drawing it like there is for an ellipse, and obviously a circle.  A parabola is a conic section like an ellipse and a hyperbola, so there is definitely a good shot.  Ok there is a method but it isn't very practical.  You'd need some very inelastic string as a parabola is basically an ellipse with one focus out in infinity.

[TomW]

BB;



I wonder if there is a mechanical method of drawing it like there is for an ellipse, and obviously a circle.  A parabola is a conic section like an ellipse and a hyperbola, so there is definitely a good shot.



mkseps posted this link above in #9 I think:

http://home.germany.net/100-441770/amsi-model.html

It looks easy just a T square, string, a pencil and a couple pins. No math just draw it.

Tom

[BigBreaker]

Very neat link - thank you.  I was going to suggest approximating a parabola with a very eccentric ellipse.  You can use just a string to make an ellipse so it requires very little in the way of equpiment.

Adding in a t-square is no great burden and using it in the way suggested on that website looks easy and accurate - no approximations.  I like it.

[spinningmagnets]

Heres's the link (good pics!):

http://www.iedu.com/DeSoto/Projects/Stirling/Heat.html

I must confess I hadn't thought too much of a solar parabolic trough. I figured "if you had the free materials laying around, why not?", but...

700F?  YIKES !! I'm now a believer. I think a double paned sliding glass door would make a good and affordable lens.

Concerning your design brianc4 (Brian, are you really experienced with c4?) I like the configuration, the only thing I might change is making the pipe the pivot point for rotating. If the back/front weight bias is fairly balanced, it doesn't take much effort for an auto tracker to move it.

[fcfcfc]

Hi:

Looks nice... I think you would have better results from a lot of different perspectives though, if you would insulate the pipe rather than the whole unit. What you need is a piece of borosilicate glass one inch bigger in diameter than the receiver pipe. This will give you a 1/2" "dead" air space for insulation. It will solve you expansion issues with the mirrors and the whole exo-structure thermal stress problem. The tube will get hotter and get hotter faster as well. Just make sure your reflector material is up to the "weather" challenge....

A good material for the exposed reflector is this:

http://www.reflectechsolar.com/index.html

.....Bill

[brianc4]

Bill,

   Thanks for the link to the reflective material it may be just the stuff I am looking for!

   I have looked at the borosilicate glass tube designs on the net and they are neat compact & efficient but also very expensive. I would love to use the design but can't afford it at this time so I was playing with other designs I can afford.

Brian Clark

[brianc4]

spinningmagnets,

   The c4 in the user name is by accident only there seems to be 3 other brianc's on the net and they must all have similar interests to my own! Plain ole brianc is always taken times 3 about every web site I visit!

   As for the temperatures that can be achieved check out Edmund Scientifics web site they offer a 2 foot diameter perfect mirrored parabola that will hold a quarter of an inch focal point & achieve 2,500 deg. F temperature!

   That is a good thought on using the pipe as the pivot point. Very elegant and simple and well worth incorporating into my final design.

   I have my first tracking system designed using some very small but powerfull pv cells driving relays hooked up to dc gearmotors that has proven to work out very well in the prototype stage.

   The first working model should be done next week and will be installed on a dual axis parabolic dish solar cooker.

   I will post pictures of unit in the coming week.

   The great thing about the design is that the relays will handle 7 amps of current at 30vdc so you can drive some big torque dc gearmotors if necessary. From the input side the relays only require 1.2 milliamp current at 12v dc. The small PV cells I am using to drive the relays are epoxy encapsulated and produce 8v at 44 milliamps in full sunlight. They have been very reliable at tripping the relays.

Thanks for the input!

Brian Clark

[fcfcfc]

I may be able to help you there... What is the diameter of the receiver pipe you are using..??..

.....Bill

[brianc4]

Bill,

   For The prototype I used a 2" schedule 40 aluminum pipe 2-3/8" outside diameter.

   I can change pipe size if needed.

Brian Clark

[fcfcfc]

Hi: I might be able to get you 2 lengths of 70mm borosilicate glass from broken single wall evac tubes, around 5' long each. I would just have to get them cut off smooth. That is around 2.75 inches outside. The cost to cut the glass at the glass shop should not be more than around $20 plus the shipping and packaging to send it to you where ever you are, if interested.....

[spinningmagnets]

The Massachusetts Institute of Technology designed a solar parabolic trough to run an ORC "Freon Steam" engine to generate electricity, with the side benefit that the condenser cooling water was hot after it passes through the cycle.

This was for a poor village in Lesotho, Africa that did not previously have electricity or hot water. Here's the curve they thought was best:

http://www.polskifiat.net/images/ORC.jpg

Good pic of MIT/Africa tracking parabolic trough collector

The pic was around 800 X 600, I tried to resize and then post, but I am still learning...

[spinningmagnets]

I stumbled across a link that presents an odd-shaped compound parabola, similar to a rounded "W".

Harder to make than the simple one-curve that is proven to work well, but the claim is that you get full sun through most of the day without tracking.

http://www.fossilfreedom.com/increase-output.html

compound parabola solar trough collector doesn't require tracking

[brianc4]

spinningmagnets,

   I have read all about the Letotho project and I have already been collecting parts to build such a unit. I liked that they used all off the shelf components. I already have a brand new large turbo charger and I have several good friends who are good with hvac stuff so I have them thinking about the freon and expansion valve setup for the unit.

   I like the link you sent me in the second post about the double parabola design and it has alot of merit. I am not worried about the tracking aspect of the array as I have a tracking unit I have built from off the shelf components that is proving to work very well. There should be some signifigant improvements to effiency if you track one of the double parabola designs and I will be thinking about that!

Brian Clark

[spinningmagnets]

Brianc4, Shoot me a PM, would like to share info. Don't want to post anything until I have working hardware. (must also learn how to resize pics)

velmis1450bc@aol.com

[brianc4]

spinningmagnets,

I sent you my email address bclark@blueriver.net Let me know what you need.

I use a shareware program called photoelf to resize my pics there are many out there. This one lets you batch pictures to physical size & filesize & works well I think it was about $20.00 and available for immediate download.

Brian Clark

[clarsondd]

Not only can a higher temperture be reached, it is also more likely to reach the minimum useable temperture.  as an example, if your using hot water storage to use the heat at a later time and the water stored is at 120 degrees.  on a cold partly cloudly day, a flat plate collector might never get to 120 degrees.  So the flat plate collector doesn't supply any heat on such a day.  Where the trough can still reach the 120 degree minimum even under partly cloudly conditions.  albeit, the trough's output will be lower then on a sunny day.  Still, some heat is better then no heat.

[clarsondd]

as the previous poster mentioned, putting the pivot point where the trough balances is the best, but that probably isn't going to be at the collector pipe.  that will be determined by the materials used and how you construct the trough, as the trough will balance at it's center of mass.

You mentioned that  your using a deep trough design.  Such a design uses its material inefficiently.  if the edges of the trough are twice the height from the bottom of the trough as the focal point, the edges are at 60 degrees.  at the edge for an added inch of materials, you only add 1/2 an inch of sunlight.

for the opposite situation, the focal point is twice the height from the bottom of the trough as the edge of the trough, the edges are at 30 degrees. at the edge for an added inch of material, you add .8 inch of sunlight.  With the focal point so high it is hard to put on a lid.  So, in this case, you'll probably use a "glass" evacuated collector.

If you going to put a lid over your trough, to make the best use of your materials, put the focal point at the same height as the edges of the trough.

here is a link to more pics on the mit lesothos project: http://www.solarturbinegroup.org/media/

here is a pic of the pipe they used.  From the picture, it seems like their pipe is only about 1 1/2 inch at the most.  And, their trough is HUGE compared to yours.  I would think you could get away with a pipe of only 1".

Not sure what they are  using to mount the "glass" tube.  what is that grey stuff and the red stuff.  And why are they using a clamp?