I am not sure how well it will work but , if I were going to try it I would keep the bottles inside the coldframe and I would paint them black . Heat is tranferred out of the water when it freezes and should help to regulate the temp inside the coldframe . I think the downside is that when the sun is shining on the coldframe it will take longer to heat back up due to the thermal mass of the bottles of water . Maybe that's not a real bad thing ...

The water will slightly delay cooling until the temperature hits the freezing point. Then the water will slow the drop in temperature. This is because of the large amount of heat water gives off when it freezes.

In short, a large amount of water will keep plants from freezing; but, it won't keep them warm.

The water will do very little on cloudy days and not all that much on sunny days. Agree that it will help more after the water starts freezing but that's already cold to me.

Last year I set up 2000 gal water in 50 black drums 40 gal each for a severe freeze. My critical temperature was 30F to protect blooming fruit trees. The water was only a small percent of the heat needed. The heater still ran most of the time four nights in a row. Maybe if I could have allowed the water to freeze it would have helped more. But the temperature was dropping at about one degree every two minutes whenever the heater went off at the coldest point, near zero outside. This in a 1700 sqft GH with double inflated poly.

Thank you for all your advice everyone!
Unfortunately I don't think that I'll be trying this little theory of mine. Maybe to extend the growing season a little longer but I probably will pass during the dead of winter in Michigan. Again, thanks for the help. Have a magical night!

With my coldframes I was using gallon jugs filled with water lined up along the northside inside the frames. Added to the water was a variety of food coloring dyes. Here in sunny Colorado the very dark water heated up quite a bit especially on days when I didn't open the tops of my coldframes or days when I got home before 3ish and could close them.

No hard data but I thought it was working out pretty good. The heat from the water would slowly release until about morning when it could start heating up again. Since I have removed the water jugs because I wanted more space for plants. I have an active heating system now which is strings on incandescent Christmas lights which I usually only turn on when it drops to below 15F or lower at night

"I live in Michigan and I was looking on opinions about converting a cold frame into warm by using solar heating?"

I don't know about solar, but this is what I'm considering using in my cold frame.

A link that might be useful:

www.heattrak.com/

Dreamgarden,
Since this thread is about solar heating, we should continue this discussion on a new topic thread. However, in regard to the heattrak you mention, according to the manufacturer, "do not use indoors", and "$0.15/hour" for electricity. Since this is exactly the same electricity use and cost, and heat output, as a $29, 1500W electric heater, what would be the advantage of the heattrak? It would appear that it would cost much more than $29.
I will look for your reply on a new thread. Gary