Increasing capacity of Solar Storage Tank

Hello,

Does anyone have an opinion of adding steel to say a 1000 gallon storage tank. Steel is about 8 times heavier than water per cubic foot so should hold 8 times the BTU storage per cubic foot. A steel fabricating company has tons of scrap plate cutouts available. They would have to be sandblasted and painted with a high heat epoxy.

Thoughts?? I don't want to create a rust/corrosion source.

thanks

Alex

My opinion is that it won't work the way you think it will. I can't really explain why.

Heat capacity of steel --per pound-- is much lower than water. So its no dice.

Thanks for sharing your knowlege.

Aled

Yep, water has one of the greatest heat capacities out there: http://en.wikipedia.org/wiki/Heat_capacity#Table_of_specific_heat_capacities
You will be hard pressed to find anything better.

there are phase change materials, mostly various kinds of salts, that might be interesting to the OP. As they are slushy solids, heat transfer can be hard. One could imagine sealed bags of these salts inside your water tank?? You would want a formulation that changes at your hydronic supply temp. Hmmm.

I hated thermodynamics in college. Heat transfer wasn't bad, but throw in Cp and Cpk and all those other things and it gets easier and easier to get confused.

Alex, At first glance it sounds like a real interesting idea until you compare the specific heat of the two materials. The specific heat is the amount of energy it takes to raise a gram of the material by 1degC. Water has a specific heat of 4.18Joule/g degC. Carbon steel is only 0.486 J/gdegC.
Here's a ref. on specific heats of a few materials: http://invsee.asu.edu/nmodules/engmod/propsh.html

So a gram of water will absorb or store 8.6 times as much energy as gram of steel for each degree of temp rise! Strange, huh?

Interesting thought.
These salts, aka "latent heat of fusion" salts, for one, have been used in solar thermal air based systems. The salts chosen solidify near the temps you want coming out of your HVAC vents for heating. These are widely used in the chemical hand warmer packs that can be boiled to return them to a liquid state after use.
The idea of using bags of them in a water bath is something I've not heard of before.

Thank You for the info,

Alex

Figuring steel is 7.5 times heavier, it's still not quite as good as water. But it's close.
Guess it depends on what the operating temp is. If we're talking 130 or so you could use a phase change material like wax. At melting (freezing) it'll store 50 times more than when stable.
If anyone wants to make a billion buck just invent a water like substance that melts at 70F