solar reflector

[bj]

     First and foremost, thanks to all that replied to what I thought

was an off the wall thought.  Everything has been tried before by

someone.  I need to remember that.  Lots of good info, and again

thanks.

     Seems like something I have to try, soo, so far, have two working

linear actuators, (24v) and my system so far is 24 volt.

     Enough pipe to build the mount.  Enough shafts, bearings to build

three or four mounts.

     Found one perfect dish, on the back of a truck with a bunch more aluminum, but when I asked, I heard a faint ca-ching and the price was

more than new.  Left my name and # with the comment if he reconsidered

before heading to scrap meatal, to give me a call. Greed is out there.

     Regardless, there used to be lots of the big dishes around here,

as we are quite far north of the 49nth.  

     My only question at this point IS-- I was intending to cast the

focul point out of aluminum, with an internal copper coil, but from

metalurgy classes, I know that alumin/copper is not good long term.

Galvanic reaction, combined with heat, and rain etc. will eventually

cause aluminum oxide to build up between the metals.  Not good long

term.  I have the ability to make it from titanium,which is good heat

transfer wise, but IF it gets too hot, titanium goes to a pretty metal

that has no strenght.  I am intending to make steam.  At whatever temp

a 50/50 azio boils.  Heating.  

     With zero experience in a small system like this--Is  there any experience with this?   Have searched, but am not as experienced

on this site as a lot of you are.

     Thanks for your patience

     bj

[PHinker]

BJ,

   I looked closely at this a while ago and have a couple pointers for you to chase if you're intent on heading down this path.  First, I would suggest reading through this document.  

http://www.ida.net/users/tetonsl/solar/page_iii.htm

They actually built a large version of what you're looking at and did a fair bit of testing concerning the focal point.  They ended up with a clever unit that you can build and will survive any number of failures in terms of the target going dry.

   Second, Mike Brown (a fellow who builds steam engines) has some calculations concerning a solar driven steam engine which show a medium sized (e.g. 9 foot diameter) solar concentrator won't create enough steam to run even a 1 hp steam engine.

http://www.mikebrownsolutions.com/solarblr.htm

   Now, there's a discrepancy between the two articles.  Mike talks about a 9 foot concentrator and the plans (from the first link) are for a 13 foot concentrator.  The 13 foot dish should be able to put out around 20,000 btu of thermal energy in unobstructed sunlight.  That's still 3-4x smaller than required by one of Mike's 1 hp steam engines.

   I'd love to build one of these homemade solar concentrators but it's so far down my list I'm sure I won't get to it for years.  Good luck with your efforts.

Paul

[tecker]

Those Mike Brown pages are very interresting

[coldspot]

 This is a project I've thought about and

 someday will try, I hope!

 "My only question at this point IS"

 "cast the focal point out of"

 This, I think, should be cast with and from

 "ceramic" around copper tubing.

 I don't know how this would work or if it can even be done?

 But, if a ceramic block could be fired with

the copper tubing inside I think it would be a great way to capture the heat and transfer it to the working fluid inside the tubing.

 My plan is to make steam for distilling the water into potable water.

$0.02

[joestue]

Don't be discouraged, Mike Brown is very inconsistent with much about what he says about solar steam power.

Couple things here, you need to undersize the turbine if you intend to build such, if it is underpower it by 50%, the efficiency will also decrease about 40-50%, (similar to a gas engine). Similarly if you have an 80% efficient turbine, and you overpower it by 20-30%, the efficiency does not drop off more than 10-20%.

If you only intend for this to be a source of low grade heat then most of this don't matter. I have thought about building a boiler for a high pressure organic cycle from a stainless steel pipe and a cast copper cavity boiler. They both have the same thermal expansion coefficient, and it would be hard to melt copper, whereas many people have demonstrated melting aluminum with solar heat.

Don't overlook the temperature gradient that appears across a piece of aluminum when you have an energy density of kilowatts per square decimeter. It is easy to calculate a 100C+ temperature difference from the outside surface to the coil.

[joestue]

Please take 5 minutes and calculate the thermal gradient across such a ceramic block.

[bj]

     Paul--lots of great info there.

     Thank you

     bj

[bj]

    Coldspot--this is an old thread, so I doubt rhat you will

see this, but thanks.  Ceramics is kind of out of my knowledge

but will keep in mind, and in spite of some of the comments, I

think is worth considering.

   bj