[Greenbuilding] Phase change heat storage for solar houses

[Nick Pine]

George Lane is 75 years old and used to work for Dow Chemical and
has about 150 patents, including US 4,613,444, "Reversible phase change
compositions..." issued on 9/23/1986, now expired, which describes
adding 1-3% sodium chloride, potassium chloride, and strontium chloride
hexahydrate to help ensure phase change stability, ie lots of cycles.
Doerken's DeltaCool 24 German product costs about 5 Euros/kg plus
shipping and stores 158 kJ/kg, like a pound of water cooling from 150
to 75 F, but it freezes and thaws from 26 to 30 C. They say don't heat
their mix above 60 C. George says don't heat his mix much above 40 C,
because that will melt the nucleation centers, which might not return
until the mix is brought to 0 F.

He says the containers need to be strong to avoid being scratched
or punctured by the mechanical actions of freezing and thawing.
He had trouble with aluminized mylar film developing leaks from
scratching. He successfully used soup cans, aerosol cans, and
HDPE with a min 35 mil wall thickness. Cheap thinwall 3" diameter
black layflat agricultural water supply pipe might work with RH control.
He used hard round black HDPE pipe with great success. There's
a 10% volume change with phase, so the containers have to be elastic
or pleated or have some air fill to reduce the peak pressure. Air fill
enables metal corrosion, but that stops when the oxygen is used up.

The containers want to be well sealed to avoid losing or gaining
water from surrounding air, but he said this can be avoided by
keeping the air at an RH of about 35%, with something like
the vapor pressure of CaCl2+6H20 at 90 F (a humidistat might
open a solenoid valve with a water mist nozzle inside something
like a large wine storage cabinet with a glass front near a south
window.) George says you can't add too much sodium or potassium,
mere saturation is fine, but too much or too little strontium gives
the freezing curve a slope vs a plateau. He says 1-liter soda bottles
could work well in a cabinet full of 90 F air at 35% RH, or less. We
might look at the water level and top up 1000 bottles every 10 years...

He made calcium chloride hexahydrate from DowFlake and PellaDow
road salt, at about 5 cents a pound (the strontium salt used in road flares
costs twice as much), by adding the salt to water to make it about 50%
water by weight and stirring. (Add salt to water for safety, since this is
exothermic.) The salt starts as a di- or tetrahydrate, with desirable
impurities. When I suggested baking it in an oven to drive off all the 
water,
then adding water to double the weight, he said that couldn't be done
without disassociation, but a university chem lab could analyze
the water content by various simple means.

George is happy to answer further questions or hear about something
that works, after all these years and a few spectacular failures.
He's a big solar energy fan.

Nick