testlight
ArboristSite Operative
Maybe you should ask the good people at "Real Goods" They sell all that solar, PV stuff, they would know, or at least have some suggestions
did a little looking around and found this stuff:
cryo-tek February 2007 TM
ANTI-FREEZE for heating and cooling systems
http://www.herchem.com/specs/cryoteck.pdf
rather expensive and you will need to use a jet pump, as stated above, to get it in the system. i believe you have to drain all the water out and fill it completely with this stuff.
ouch.