Solar Panel Design.


Recently, after a rather large heating bill, I became interested in solar heat.  I are a engingeer so I should be able to figgur it out.

One of the things that troubled me on looking around the web was the lack of available science.  If you are heating your house with solar, or anything for that matter, you have to consider how much power is available.  I suppose this is why most of the WWW is the color comics section of the Internet (sigh).

Think, how many space heaters would you need to turn off your furnace?  Each of those is likely around 1000 to 1,500 Watts. Mind you, they don't run continuously.  (reminder to self convert this to btu's, liters of oil, and cubic liters of gases)

Solar Panel Discussion

My buddies and I started an email conversation.  This is the summary:

One of the initiating notes is here. By the way the note says "I obtained a fan in Taiwan".  Really!  I flew to Taiwan and bought a $US 3 fan.  Pinch me am I living like this?

After creating the Mark I panel at copious expense. (hmm $10) I took a few pictures.  Very difficult to  make out the design with the black paint.  BTW this version has no collector plate. It's actually remarkable how it performed given the construction of basically black paint on 3mm plywood with a 24mm panel thickness around 35cm square covered in a clear plastic bag I found in my cupboard. (also from Taiwan and obtained on the same trip BTW)

Solar Panel Front Covered Solar Panel Back Panel Front Uncovered

Richard wrote me back with a few suggestions.  The Mark II won't have all of this but some of the ideas will bet incorporate some of this stuff.

Perry added some helpful input and links.  It's worth considering how much total power is really available when we want it!  Well it's not MegaWatts!!

It turns out that on further reading Richard is on to something that's backed by some science see below.  Most recently another friend involved in this discussion.  He did the math to try to sort out what kind of power we might be getting off this VERY small panel.  

The Mark 2 Panel is under construction.  I had a running version but the fan blew up.  The truth is the next panel is more like a Mark 1.5.  The Mark 2 panel general arrangement is more like the image here.

Mark 2 Panel General Arrangement

Solar Science

This must be the third time I've tried to write this!  (grumble backups grumble rsync... snark!)

I've developed a working model or theory from reading.  My main thinking has been driven by a chapter out of Marks Handbook.

I think elements of this model apply to "solar" cooling (ok radiating heat to free space) as well as solar concentrators and both water and air working fluid solar collectors.  In short this should be helpful for any thermal application.

Focusing on solar collection, we want the energy radiated by the Sun that strikes the earths surface to pass through an perfectly transparent to short wavelength (0.5 - 2.5 microns ) surface (the glass), strike an absorber surface and be 100% converted to heat energy.  This heat energy is then transfered to a working fluid by convection, conduction and radiation.  

Solar Radiation Striking the Earth
Fig 1: Solar Radiation Striking the Earth
Source: http://solardat.uoregon.edu/SolarRadiationBasics.html

The "glass" surface needs to transparent to the wavelengths striking it from the sun and opaque and reflective to the radiation coming back out of the collector radiated by the absorber surface.  The glass also needs to be a good insulator so that little heat is lost by conduction and convection on the outside and inside surfaces.  It turns out that ordinary window glass gets pretty close to the ideal however some sources indicate that borosilicate glass is better.  What I have found so far neither supports nor contradicts this as I don't have transmission graphs for Soda Lime or Borosilicate Glass for wavelengths out to 700 or 800 microns.

Soda Lime Glass Transmittance
Fig 2: Soda Lime Glass Transmittance
Source: http://en.wikipedia.org/wiki/Soda-lime_glass#Typical_optical_properties


borosilicate glass transmittance
Fig 3: Borosilicate Glass Transmittance
Source: http://www.sinclairmfg.com/datasheets/borosilicatecurve.htm


According to Mark's Handbook good old flat black paint can get pretty close to "black body" in terms of behavior.  So we can get a good estimate of what might get radiated out of the collector by studying that ideal case.

Black Body Raditation @ 300K
Fig 4: Black Body Radiation at 300K
Source: http://www.spectralcalc.com/spectralcalc.php

The Final part of this working model is that we need to keep Delta T small for efficiency.  Which Delta T? Well, all of them.

The idea is to keep the inside collector temperature as close to the outside temperature as possible, and to keep the inlet and outlet temperatures of the working fluid as small as possible.  This may seem counterintuitive however its more a matter of high fluid flow with low temperature rise (= efficient) or smaller fluid flow with larger temperature rise (= less efficient).

Some discussion on this topic can be found here: http://www.apricus.com/html/solar_collector_efficiency.htm.  


Last updated 2007-06-03 IGJ