re: a stupid question SOLAR.KnowledgePublications.com

re: a stupid question
9 feb 2004
rich komp (sunwatt@juno.com) writes:

>     a couple of short comments on your re-invention of the pv-thermal
>hybrid.  mylarª is polycarbonate and will stand up to the weather just as
>well as the second piece of basically the same material.

really? as i recall, duane johnson says his mylar/plywood heliostat mirrors 
deteriorate sooner. perhaps it's aluminized on both sides, and the exposed side
quickly oxidizes to a chalky white surface. i have some cheap one-sided mylar.

>i have had pv-hybrid fins covered with mylarª outdoors in the weather for
>over 20 years and although the material has gotten a bit brittle and slightly
>fogged, the pv output is still close to what it was when we made the fin
>at a workshop on long island, ny in 1983.   remember each layer of
>glazing loses about 8% of the light by reflection (if both sides are
>agianst air).

i think the fresnel loss is about ((1-1.5)/(1+1.5))^2 = 0.04 per side.

>polyurethane varnish will eventually yellow and is not necessary in this
>application.

3m's "outdoor mylar" has a 1/4 mil layer of polyurethane...

>    inexpensive mirrors: we have been gluing pieces of shiny plastic
>tortia chip bags (opened out so the aluminum side is facing out) onto
>boards to make reflectors for our village-built solar ovens, and also
>using the same material on cardboard as reflectors on the 15 watt
>fluorescent light fixtures in the campesino's homes to inhance the
>illumination from the tiny pv systems they can afford.
>     i have an excel spreadsheet that i had done years ago (as a lotus
>123 one) that does all the calculations for the performance of a
>pv-hybrid, starting from the geonetric equations in farrington daniel's
>book.  if you would like a copy, just drop me an e-mail.
>     rich