Wednesday, May 15, 2013

Is Solar Thermal Too Expensive?

Recently we were invited to visit with Patrick Spearing and Suni Ball at Enerworks in Woodstock.  These guys do nothing but solar thermal.  We were pretty much blown away by their knowledge, professionalism, and by how much they taught us about solar thermal in one afternoon.  I'd like to recount some of the knowledge transfer as a way of note taking.

Narva Solar Thermal evacuated tubes and heat pipes

Solar Thermal Collectors:
Enerworks uses Narva glass tubes for their evacuated tubes.  Narva is long established as a UV-light glass tube manufacturer in Germany.  Making high quality tubes for evacuated solar thermal was a natural extension for them, and they've done some interesting things to address two critical issues:  Vacuum and stagnation.  Apparently you will find it very difficult to find any warranties on the vacuums of evacuated tubes.  This is because the borosilicate glass usually used is actually more permeable to helium than regular soda-lime glass.  Why use borosilicate glass, then?  I think it is because the clarity of the glass may be better than regular glass, and perhaps this is cheaper than manufacturing the low-iron soda-lime glass that Narva uses. Anyway, Narva tubes are evacuated to 10 to the -6 torr, (pretty deep vacuum), and the vacuum is guaranteed for 10 years, so this is a big deal.  The vacuum greatly improves the efficiency of the collector, and it also protects the materials inside the heat pipe from freezing.  In addition, the Narva tubes are single-wall, which makes them very robust.  The double-wall tubes are highly susceptible to breaking due to the long moment-arm that multiplies the stresses on the part of the tube that supports the whole inner glass wall.

The other thing is stagnation.  Somebody actually did a bunch of research and analysis and designed a heat pipe which self-limits at ____ temperature.  We asked how this was achieved, and the answer was thus:  by controlling all the physical factors affecting heat pipe design, one can actually make a heat pipe that self-limits its heat transfer and therefore its temperature.  These heat pipes are carefully manufactured to control:
  1. internal volume
  2. chemistry of working fluid
  3. volume/mass of working fluid
Without tight control of these parameters, there can easily be run-away temperatures and significant system stresses which can cause vapour-block, glycol break-down, etc.  It appears the Narva heat pipes contain a lot less working fluid than other heat pipes.  This one fact is sensible to me - it means they can completely vaporize their working fluid, which can limit the heat-pipe's upper temperatures, as well as reduce damage due to any freezing.  It also seems to me that heat-pipe technology is one area in which lower-cost and copycat products may be hard pressed to perform in.

Another Issue:  SRCC ratings for solar panels are somewhat skewed or inaccurate for solar evacuated tube panels.  They count the whole area of the panel without subtracting the spaces between the tubes.  Apparently the Solar KeyMark metric is much better to go by.

Will continue this another day as there is much more to tell.

1 comment:

  1. Borosilicate glass may be preferred because of its heat resistance. Glass cookware, for example, is borosilicate. The heat induced stresses in the tubes is probably a significant issue.