SolSwitch

Its been a few weeks since the last post, mainly because I have been on a business trip and a vacation.  I am back, and there are some developments!  The most significant development is that I have discovered a competing idea.

A couple weeks ago I was browsing this article.  On page 4 I came across this picture:

 On further googling I found a published paper on the idea.  I cant display it here or reproduce the images for fear of copyright infringement, but I'll summarize.  You can glean almost all the information you need from the above picture.  After some more googling, I found a website for the idea.

The concept is wonderful.  When the panel is off, a chamber between the plastic "saw tooth" glazing and the black absorber plate is filled with air, or perhaps more accurately, it is drained of water.  If the sun light enters the panel, the difference in the index of refraction of the plastic (~1.4) and air (~1.0) will cause total internal reflection, a phenomena that all high school physics student learn about.  A real-world example is if you are in a swimming pool just under the surface.  If you look straight up you can see outside, but if you look out at an angle, the water is acts like a mirror and you just see the bottom of the pool.  However, if the chamber is filled with water, the index of refraction increases to 1.33 and...the total internal reflection goes away!  Thus, when the water is not being pumped the panel is reflective/non-absorbing.  When the water is flowing it is absorbing. The idea is that you use the property of total internal reflection to create a "light switch". 

The first problem I thought of is that for this idea to work the sun must be normal to the panel.  However, the suns position in the sky changes.  In the winter (when we really need the heat), the sun is low on the horizon.  In the summer it is high in the sky.  If we are going to use these panels to heat my house (that's all I really care about at the moment), I will want to angle the panels so they are normal to the sun in the winter to maximize the energy I can capture.  In the summer the sun will be almost vertical in the sky, like this:

 The panels will work in the winter, but they fail in the summer from my thinking.  Interested readers  should check out this website, which includes a nice little interactive application.

After reading the published paper, it turns out that a double-layer configuration does work for all seasons: 

The above image is from the sol-switch website set up by the inventor.

Problem solved?  Well, sort of.  On first blush, there is no obvious problems.  The panels can turn on and off by pumping water, and that's really a solution to the overheating problem.  However, when I started to think about the design as it will look like in a completed solar panel, including the absorption plate, insulation and glazing, the problems start to appear.  When I compare SolSwitch to ParticlePanels, I appreciate how simple the particle panel concept actually is.

SolSwitch is a mechanism for controlably reflecting away or transmitting light onto an absorption plate.  Its an added layer, between the absorption plate and the glazing.  ParticlePanels solve the overheating problem by controlling the absorption plate directly.  There is no added layer, because the mechanism is both the absorption plate and the light switch.  


For SolSwitch to be economical, the design must use a small amount of plastic.  As the size of the grating is reduced (reducing the plastic), the channel for the water is also reduced, and at some point pumping water into it is a problem.  A read of the paper, however, indicates they are thinking about using a phase-transition of the liquid.  In other words, if water is in the channels, if the temperature gets above boiling then the water goes to a gas and the panel shuts off.  This seems very problematic to me.  Along with the transition to a gas, there will be a huge pressure spike.  This is going to cause the plastic to deform, which in turn will cause the angles that the light hits to all change, and this could mess everything up.  

I would be very interested in comments from readers.  On the one hand I think this idea is remarkably clever.  One the other hand, when I think about reducing it to a workable solar thermal panel, a whole lot of issues start to be raised.  Can these problems be overcome?  That's what innovation is all about!  If you dont have a problem, you cant innovate.  Hey, we all have problems, and Particle Panels are no exception.  The only difference is that I am telling the world about mine.  I emailed the inventor to see if I could get any information about the idea and possible commercialization.  He was very cagey, only saying that "they are working on optimizing the structure".  Given the seemingly simplicity of the idea, and the fact that they have stated "the idea is ready for commercialization", I find it curious that they are still optimizing the structure and not working with a company to make some panels.  Perhaps they are, but I just don't know.  Perhaps they just cant figure out a problem.  If they opened up the idea to the world and let everybody think about it, I wonder how much more efficiently the problem would get solved?

In any case, its good to have a competing idea.  It helps to highlight design advantages and weaknesses and focuses the development effort.  Competition is always a good thing.