Friday, January 6, 2012

Large Thermosyphon Collector Updates and Developments

I've been using a very simple solar thermosyphon collector to heat my shop for several years.  It is large and puts out a lot of heat -- its also simple, cheap, and low maintenance.  Where we live it has a kind of amazing one year payback on the cost of materials to build it.
The large thermosyphon shop heating collector
The collector has been getting some attention lately and that coupled with some new testing and 9 years of accumulated experience makes me think its time to do an update.  This update covers a lot of territory:
  • A new commercial version of the collector 
  • A video featuring the collector
  • Potential changes and refinements
  • Maintenance and upkeep over 9 years
  • Testing effect of reducing vent size to make construction easier
  • More accurate collector efficiency measurements

A New Commercial Version 
Paul House  of Choice Energy here in Bozeman  is offering a commercial version of the collector.  He will build one on your wall for about $2000.  In our climate, the collector offers about a 5 year payback (rebates could make it even shorter).  This is the best rate of return I know of for commercially installed collectors by a wide margin.  For example, if you compare it to the popular SolarSheat commercial collector, it is about 5 times more cost effective on a dollars per BTU basis.

A New Video on the Collector
There is a new video in works on the collector being done by Peter and Dana.  This is the first in a series of videos they are doing that will feature renewable energy projects in the SW Montana area.  They were out capturing video and audio before Christmas, and the final product should be out in a month or two -- I'll post a notice when its available.   I think this series of videos is going to be very interesting as there is a lot going on the renewable energy front in this area.

Paul and Gary on video day

Potential Changes and Refinements
The collector has performed very well, but, as always, there are a few things I would do differently if starting over.  So, this section lists a few small changes and one large change I would make.

The small changes have to do with details of the absorber installation, flashing, glazing...   just small refinements.

Collector cross section (from Home Power article)
The absorber change has to do with the tilt of the screen layers.

The one big possible change would be to revise the design to allow the collector to pass more daylighting into the shop area.  On my other shop, I use this glazed door arrangement, and I have to say I just love the solar daylighting it provides -- it not only helps when doing work, but it also just gives the place a more pleasant feeling.  I'd like to incorporate some some glazed area in the back wall of the thermosyphon collector that would add this solar daylighting feature.   If you have any ideas on good ways to do this, please let me know.

Testing Smaller Vents
Paul suggested the idea of using a large hole saw to cut the inlet and outlet vents for the collector.  This would save some work and look nicer.  The potential downside is that a 6 inch or even 8 inch hole saw results in a smaller vent size than the design guide lines for these collectors specify.  

The two adjacent bays of the collector used to test
effect of smaller vents.
In order to get an idea what the performance penalty for the smaller vents might be, I masked down the vents on one bay, and then compared the heat output from this bay to the adjacent bay with the full sized vents.  Unfortunately, there is a pretty big penalty for the 6 inch round vents, and even the 8 inch round vents show a drop in heat output.  It looks like a 9 inch circular vent would be about right.

Efficiency and Flow Rate Testing
While doing the small vent testing, I also had another go at testing the efficiency of the collector.  I have somewhat improved instrumentation and accounted for more of the odds and ends that effect efficiency.  Its difficult to get good efficiency measurements on any solar air heating collector because measuring airflow accurately is hard.  Getting good airflow measurements on the thermosyphon collector is even more difficult due to the large vents with low vent velocities.    But, I do think these new measurements are pretty good.  

Bottom line is that for the full sun, moderate winter day I tested on, the efficiency comes out between 62 and 65%  -- this is really very good, especially considering how simple and inexpensive the collector design is.

Logger plot used for vent size and efficiency estimates.
There are also some test results on collector flow rate, dust filter effects on flow rate, and on using the back surface of the collector as a radiant room heater.

Through the first 9 years, the collector maintenance has been very minimal.  

The poly back draft dampers have been replaced once at a cost of 2 cents per damper.

I painted the outside of the collector frame last year.

The fact that the collector does not have a fan or controller eliminates the two most maintenance prone items on solar air heating collectors. 

After 9 years, the polycarbonate glazing still looks very good.

If you have any thoughts or suggestions on the collector, please let me know.

Collector parts diagram (from Home Power article)

January 6, 2012

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