Author Topic: Wich system is best ?  (Read 37697 times)

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Onthero

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Wich system is best ?
« on: March 07, 2010, 07:01:44 AM »
I see a lot of diy projects that capture the sun like this:



Thus, some copper hoses and black fins, you all know the system.


But less often i see something like this to heat water:

http://www.iwilltry.org/b/projects/build-a-heliostat-for-solar-heating-and-lighting/

or


How come, that (parabolic) reflectors are not much used in diy projects ?

I know that parabolic mirrors are very expensive, but a diy solution could be the use of shiny, polished metal, like the inside of thni cans for example !

Maybe because it needs a -hard to build- tracker to aim ?


And why not combine the first system with the second ? Thus putting mirrors on your collector (only in the winter so it doesnt get to hot) for extra incoming sunlight.

(Yeah, mounting the mirrors on each winter will be some work :D )

« Last Edit: March 07, 2010, 07:01:44 AM by (unknown) »

Airstream

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Re: Wich system is best ?
« Reply #1 on: March 07, 2010, 07:12:48 AM »
Anything with an exposed mirror as pictured requires a climate without trees (sap, pollen, critters) and low dust, smog, dew fall out. Sure, its possible but hardly worth the effort in 99% of locations.
« Last Edit: March 07, 2010, 07:12:48 AM by Airstream »

Jon Miller

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Re: Wich system is best ?
« Reply #2 on: March 07, 2010, 08:00:00 AM »
Parabolic is only really good in direct sunlight but worth a go.


If you have a CAD package such as Solidworks they normally have parabolic curves in them and so a DIY parabolic tough with a concentration ratio of 20:1 can be made with a little engineering.  The collector pipe really needs to be insulated which is commercial done with evacuated glass.  I think using one evacuated tube from a standard panel would be a good option.


Re tracking its not to hard as its only in one axis that needs to be tracked and can be pre programmed rather then actively tracked.  


Regards


 

« Last Edit: March 07, 2010, 08:00:00 AM by Jon Miller »


wdyasq

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Re: Wich system is best ?
« Reply #3 on: March 07, 2010, 08:23:12 AM »
A great loss in solar collectors - or any heat system is the 'Delta' between the working fluid and the outside temperature. A simple flat plate collector using water has a Delta of less than 100C and probably closer to 50C as 50C water/air is very warm.


Concentrating collectors have a much higher Delta and will have a higher heat rejection per area, greater insulation problems and the additional heat is harder to handle, needing special fluids.


The science is all available. If you feel you want to experiment go for it. Your cost will be higher and the efficiency lower.


Ron

« Last Edit: March 07, 2010, 08:23:12 AM by wdyasq »
"I like the Honey, but kill the bees"

Onthero

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Re: Wich system is best ?
« Reply #4 on: March 07, 2010, 10:04:17 AM »
I once saw a documentairy on National Geographic Channel, about the parabolic mirror system, and they used molten salt (!) (=the salt keeps fluid because of the high temps of course) as fluid in their pipes.

Salt melts at 801°C :D

Also i though that salt can capture very much heat. More than water (i think).


That was also the reason i was thinking of storing the heat in a vat/tank, filled with salt. Have been searching on this, but couldnt find many.


Ok, very interesting comments already. I'll do some further reading for now, so i can better understand the basic principles.


The only thing i would be glad to have figured out here is this:

if (if!) you can transport the heat fast enough, away from your concentrated collector, why do you still need fluids that can handle the higher temps ?

It seems to me that, if you 'cool' your concentration point fast enough, temperature wont get to high. Or isn't that possible ?


Yeah i'll do some more reading now :)

« Last Edit: March 07, 2010, 10:04:17 AM by Onthero »

Jon Miller

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Re: Wich system is best ?
« Reply #5 on: March 07, 2010, 10:30:18 AM »
The high temperature differences is only a problem if you want a high temperature out.  I assume you are wanting to heat water and not drive a steam turbine, if so then the temperature difference will be nothing to write home about.  I am suggesting using one evacuated tube as the collector to avoid the engineering challenge the collector would otherwise pose.


The use of salts in electrical generating parabolic arrays is to allow for a buffer as when the liquid salts undergo a phase change back to a solid they reduce a large amount of heat energy.  A large buffer tank of liquid salts can allow for continued operation of the steam generation when the sun goes behind a cloud and, if there is a large enough buffer tank, all night.  I am not sure if adding salts to water gives it a higher specific heat capacity but I am sure for the purpose of water heating it is an added cost and would require stainless pipes due to the increase corrosion levels.  Water being as cheap as it is just use some more.


Dont be put off, read more around the design of it all, have you an email address?


Regards  

« Last Edit: March 07, 2010, 10:30:18 AM by Jon Miller »


Onthero

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Re: Wich system is best ?
« Reply #6 on: March 07, 2010, 11:38:30 AM »
I sent you a mail (Fieldlines 'at' otherpower 'dot' co 'dot' uk) Interested to hear if more from it !

(btw www.otherpower.co.uk  isnt working)
« Last Edit: March 07, 2010, 11:38:30 AM by Onthero »

Onthero

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Re: Wich system is best ?
« Reply #7 on: March 07, 2010, 11:46:41 AM »
Very interesting to read more about it, and indeed the plan is to heat water.

« Last Edit: March 07, 2010, 11:46:41 AM by Onthero »

TomW

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Re: Wich system is best ?
« Reply #8 on: March 07, 2010, 12:05:42 PM »
Indeed the website redirect for www.otherpower.co.uk is hosed:


Not Found


The requested URL /website/index.htm was not found on this server.


Apache/2 Server at www.otherpower.co.uk Port 80


Time for an upgrade or some maintenance I guess?


Tom

« Last Edit: March 07, 2010, 12:05:42 PM by TomW »

Jon Miller

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Re: Wich system is best ?
« Reply #9 on: March 07, 2010, 12:45:42 PM »
It is Tom, its finding time.
« Last Edit: March 07, 2010, 12:45:42 PM by Jon Miller »


GaryGary

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Re: Wich system is best ?
« Reply #10 on: March 07, 2010, 05:20:16 PM »
Hi,

One downside to parabolic collectors is that you have to track them to the sun.  But, once you have invested in the tracking system they will take in more solar input over a day than a flat plate collector -- at least on a sunny day.


George has done a nice job (I think) on a ganged set of parabolic collectors:

http://www.ffwdm.com/solar/solar-index.htm


and

http://georgesworkshop.blogspot.com/


Gary

« Last Edit: March 07, 2010, 05:20:16 PM by GaryGary »

Onthero

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Re: Wich system is best ?
« Reply #11 on: March 08, 2010, 11:38:40 AM »
Nice system, but his tubes arent insulated ?

What if you'd make a glass chamber around george's design ?
« Last Edit: March 08, 2010, 11:38:40 AM by Onthero »

dboller

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Re: Wich system is best ?
« Reply #12 on: March 08, 2010, 10:06:46 PM »
This has been said in this thread but I'll say it another way.  If you want very high temperatures to do something like run a turbine then the cost of concentrators and tracking and cleaning them is worthwhile and the evacuated tube collectors that minimize radiant losses are necessary.


 http://www.fieldlines.com/images/scimages/5560/collector_small.jpg


Here (if the picture loads alright) is a conical collector I built with an evacuated tube collector.  It makes superheated steam but inaccuracies in the mirrors also cause hot spots on the collector that melt the inner glass tube.


I have not had much time to work on this for a few months and frankly it's been difficult and discouraging but what I am learning is that concentrating collectors take some serious accuracy in all of their components to work properly.  Yes, superheated steam from solar is fun (and dangerous) but melted glass is not all that useful (unless of course you are trying to melt glass).  So my opinion is that if you want hot water then look at flat plates or evacuated tubes.  Just my opinion based on some experience.  It might be interesting to mount some plate plates or arrays of tubes on a passive tracker like the zomeworks units.  That would up your daily output without getting into the kind of tracking accuracy that concentrators require.


Damian

« Last Edit: March 08, 2010, 10:06:46 PM by dboller »

Onthero

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Re: Wich system is best ?
« Reply #13 on: March 09, 2010, 12:19:28 AM »
Interesting. I once read that correct tracking of PV-panels can boost your efficiency up to 50%, I'll try to refind the original article.
« Last Edit: March 09, 2010, 12:19:28 AM by Onthero »

bob golding

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Re: Wich system is best ?
« Reply #14 on: March 11, 2010, 05:05:20 PM »
before you go any further try to find out how  much cloud cover you get over a year. parabolic s will work fine in a desert but not where there is any significant  cloud cover. if i were me and i just wanted to heat water i would go for flat panels every time. as ron has pointed out the delta t is the important figure. water is cheap so unless you are very restricted in space go for a larger area of lower efficiency panels. no point in going to all that trouble getting it hot unless you can use the heat fairly quickly such as in a steam turbine,  if you are just heating domestic hot water you dont need all that much heat anyway.

.
« Last Edit: March 11, 2010, 05:05:20 PM by bob golding »
if i cant fix it i can fix it so it cant be fixed.