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This forum like many others has had lively discussions of the merits of the new evacuated tube collectors versus the old flat panel collectors and this dialog is still ongoing.  The author’s favouritism for Evacuated Tube collectors is well known.

Recently I ran some simulations with my licenced copy of the T*SOL simulation software.  This excellent package is available at valentin.de and yes if you are a plumber I recommend you get it.  I have found some interesting results.

My scenario is a laundromat where northern sun access is available.  The laundry’s demand is 2000 litres per day of 50°C water.  The usage peaks in the morning and again in the afternoon like so:

Now the monthly demand is assumed to be constant over the year.  We have a flat roof of only about 5m x 6m.

Evacuated Tube

Now using T*SOL to optimise this with a 500l stratified storage tank (e.g. Rotex or Latento), and inline gas boosting of 17kW, we should use four 30-tube evacuated tube panels (e.g. from Jiangsu Sunrain) giving us a gross surface area of approx 20 sq.m.  The solar contribution graph is then as follows:

The Solar contribution is a little over half of the energy demand.  Now how do flat panel collectors stack up?

Flat Panel Collectors

How to make the comparison meaningful?  The four ET collectors have cost us approx. $8000 or $500 per sq.m.   For that you could get say 10 flat panel collectors from Solahart and again the gross surface area is around 20sq.m.

So as you can see solar contribution for Evacuated Tube only 92% of what the FP collectors give us.  But given that this is a dollar-for-dollar comparison, it is wrong to say that the ET panels cost significantly more.  In fact they cost only 9% more (1/0.92 = 1.09).

So given that ET collectors cost a whopping 9% more than flat panel collectors, why are they taking over the world?

So why ET?

Secondly, longevity.  ET collectors just don’t degrade over time the same way FP collectors do.  Many FP collectors just out of warranty will suffer some moisture ingress into the absorber cavity, which will cause the characteristic white tin oxide corrosion you see.  The system still works, but the efficiency is degraded.  How much I’m not sure but I bet it’s more than 9%.  In comparison the ET collectors degrade less, and they degrade differently.  Typically one of the tubes will lose its vacuum for some reason (perhaps someone threw a brick at it!)…This is a $30 end-user replacement and no plumber is required.  In the meantime the system still performs almost as before, with 96% of the original efficiency.  Throw a brick at a FP panel, and you will need a new panel – and a plumber!

Frost-tolerance is far better.  The ET panels from Hills Solar are rated to -15°C here in Australia covering ALL country and alpine areas.  A flat panel collector would require a glycol solar loop in such areas.  This requires  a heat exchanger in the tank and an expansion vessel.  Not to mention ethylene glycol, which is not so eco-friendly.

Installation is easier.  The installer can carry the 15kg manifold piece up the ladder and attach it to the frame.  Then the tubes can be carried up and installed separately.   For a FP collector, two installers or a crane are required.

Lastly, the cost differential between ET and FP is declining.  Anecdotally I have been told that there are approx 1200 manufacturers of evacuated tube collectors in China.  Personally, I find this hard to believe.  I can believe there would be 1200 brands globally, all made by perhaps 20 manufacturers, most of them willing to brand their products.  The point is still valid, that evacuated tube collectors for hot water are the future, and the flat panel type are a thing of the past.

This entry was posted on Thursday, September 3rd, 2009 at 11:43 am and is filed under Economics, News, Technology. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.