http://www.youtube.com/watch?v=mVHv8lMlNcQ

http://www.youtube.com/watch?v=OE08CloDReY

Yes, a warm slab. That would be nice

You wouldn't want to accidentally kick a football through one of those. Double trouble.

Roger tried filling our windows with water but it just buggered the computer!

Using a fluid that absorbs more heat than water does would enable the same heating effect for less volume. Something like the fluid in freezer packs that go in a picnic box......!

Yes, true. But water has about 4x the thermal capacity of concrete, which isn't bad!
And one aspect these days is the energy needed to transport. Heavy things cost more. Water has the advantage of being heavy but also available locally

I wish I had time to post all the stuff I've found/read.

On further reflection, I think it is essential that the fluid in the windows should be capable of staying liquid (and without expansion/contraction of volume beyond minimal levels) when the outdoor temperatures change from heatwave to severe cold snap. Otherwise there is a strong risk of the windows distorting or fracturing - messy and dangerous!

So to my mind, water is not suitable, but the sort of fluid used in external heat pumps may be.

upside - high thermal capacity..
Downside. Liberal ability to convect at any opportunity.

throw in the 'wetting' ability and the need to use very thick glass (probably around 8mm or more) for the panes, I'd not consider this at all.

Have fun

Owen