Solar, tube in tube collector.

[cyclops2]

As I am electronic drives designer I need a lot of help.
Seat of the pants and past experience responses are close enough.
Using a 3/4" o.d. copper tube with circulating 50/50 % antifreeze solution, centered in a 2" to 4" o.d. glass tube. How many feet would be required to generate the equivalant BTUs of 5 KW / hour?
Would a larger pipe & tube be required to to operate in the real world?
Does evacuating the glass tube to a low vacuum help or hinder overall efficiency?

Many thanks.

 

[gwolf2]

Is this a home-made solar collector?

 

[cyclops2]

Of course.
I finally found the correct terminology of " evacuated tube collectors " & the " Combined Gas Law calculator ".
I am now learning to calculate my sizing needs.

Thank you.
Rich

 

[PaoloPemi]

not an easy work...
the sun's radiation on the earth's is approximately 1.4 kW/m2 , around midday on summer in temperate climates, about 1 kW/m2 of power reaches Earth's surface, std. conventional solar panels capture 50-70% of this power, evacuated-tube collectors can capture 80-95%, to get high values you need special anti-reflection coatings, angles about 90 etc. also consider that evacuated-tube collectors get hot easily and the temperatures they produce can exceed the boiling point of water, in addition glass tubes are very fragile...

 

[gwolf2]

Evacuating the tube will help a lot, but you will find it very difficult in the DIY environment to both generate and hold a vacuum better than about 0.1 mbar.

0.1 mbar will cut out convection and reduce losses to about 1/3 of that of non evacuated. The real savings are to be had at 1e-4 mbar or lower which is where the commercial solar vacuum tubes operate. The technology for this is beyond DIY.

You should also consider aluminium plates with 'selective solar coatings' which are not too expensive to purchase from the mfr if you speak nicely too them.

The biggest thing you need to deal with is that in northern temperate latitudes the incoming solar radiation varies by a factor of almost 10 from winter to summer. This means you need to make some awkward decisions. You can mitigate this a little by having more collectors than you need for summer but mounting them at a relatively high angle like 60 degrees so that they are low sun winter biased and present only a fraction of their area to the mid day sun in summer.

Insulation is EVERYTHING.

Have fun!

gwolf.

 

[cyclops2]

Thank you everyone.
The 1970 house is now at R 44 ceiling & R 22 basement. R 12? in the walls. Total electric. AC ducts hand fitted with 1" Dow Styrofoam & all seams taped. All trim work sealed everywhere. All ceiling, outlet & switch plates sealed with blue closed cell foam gaskets.
So far this fall the house is very even temperatured everywhere. Will find out on November 22 if there is a noticeable drop in KWH consumption based on Degree Days.
I live in N J.
I did not like the solar people not giving me a BTU / hour generating rate at midday or for 3 hours either side of it.
Me thinks it will take a unbelievable amount of roof tubes to generate the equivalant of 5.5 KW / H with roof tubes in great weather.
Am I beating a dead horse?

Rich

 

[racookpe1978]

Problem you have is that the 1.0 /meter^2 is ONLY good near the equator, in clear weather, with no haze nor humidity, no clouds, and no dust or dirt on the collectoer. No shade either obviously.

Then, you'll find that this heat load only occurs around local noon. Figure from 11:00 local sun time to 1:00 local sun time. Before that period, or after that noon period, there's still a lot of energy coming in from the sun, but not as much: rule of thumb I found from solar-voltaic is to count on the sun's energy ONLY from 9:00 am to 3:00 pm local sun time. Before or after that 6:00 hour window, you get essentially nothing.

This leaves you 3/4 of the day when you'll only be loosing heat out of your collectors, so you'll need some way to automatically isolate the collectors from the system/heat storage media. Otherwise, you'll only be cooling off the system fluid.

 

[gwolf2]

Adding to the above 6 hour rule, think roughly 100w/m2 winter, 300 w/m2 spring and 500 w/m2 summer on average at your lattitude. Also the reality is more like 4 hours winter to 8 hours summer - hence my 10:1 summer/winter rule of thumb.

You sound quite practical so I would suggest the cheapest option is to buy evacuated solar tubes and assemble them into your own arrays. There are mail order/DIY companies who will sell you the tubes alone.

If you can get the price in at less than 400 dollars US/net square metre of collecter area you will be doing well. By net area I mean the actual absorber area not the tube diameter x length.

If you go ahead you need to consider lots of other stuff, particularly over-temperature in summer and freeze protection in winter.

Interesting project though.

gwolf

 

[cyclops2]

I was getting very leery about solar hot water replacing a electric 4 stage furnace.

I will not beat the " dead horse " any longer.

Many thanks for the constructive past experience with how solar claims come about.

I have a back up plan ( maybe ? } for myself & some social friends. Water to water Ground Source Heat Pump. I will start a post in the HVAC section.

Rich