Paul, I have to ask; why didn't you paint the barrels flat black? Is that color blue more retentive?Good questions!
Like Michael, I was wondering about the color of the barrels. Also, do you have a way to close things up on a cold night to minimize heat loss back through the glass? Can you monitor the actual temperature of the barrels?
In summer of 2007 I painted the south side of one barrel flat black. The barrel was positioned with similar barrels at the same initial temperature and in the same relative position to the sun. At the end of the day the black barrel was two degrees warmer than the blue barrels.
Let's approximate the physics and . . . on second thought, let's skip the details. A black barrel would absorb a few hundred BTUs above what a blue barrel would absorb. In my situation, it's not significant. I did one test on one day and at the end of the day decided I didn't like looking at black. Rationally black is better but emotionally blue is my choice.
Here's a quote from the Whole Building Design Guide website. The color of the mass surface is less important than originally thought; "natural" colors (e.g. colors in the 0.5 to 0.7 absorption range) are quite effective. (http://www.wbdg.org/resources/psheating.php)
I don't have a way to reduce heat loss at night. I researched options and couldn't find a good solution that balanced initial cost, installation labor, function, aesthetics and daily operation.
On the web I've seen photos of external shutters that are hinged on the bottom. In the morning they are opened and lay flat on the ground. They have a reflective surface and increase the sunlight entering the glazing. At night the insulated shutters are closed to reduce heat loss. This would work but seems like an excessive solution for my situation.
In the end I decided to do nothing and accept heat loss. If this becomes a problem then I'll find a solution in the future.
I have an infrared thermometer and do measure the temperatures of the surfaces of the barrels. The thermometer enables me to measure the temperatures at the top and bottom of the barrels and determine the stratification. Also, I can measure surfaces around the room. On a cold morning before the sun enters the room the warmest spot is near the barrels. The ceiling above the windows is the coolest spot.
Here's a prior post about the infrared thermometer and colors.