monte carlo simulation

What about this:

You buy a small aquarium, fill it with water, make to it an inlet and an outlet.

from the outlet connect a flexible pipe or hose, and then connect it to a black paint coated copper coil (many turns and 1 in diameter, at least), and then the exit of the coil connect it to the entrance of the aquarium. Cover the aquarium, and isolate it well.

Then take the part of the coil outside under the sun. but place it in a lower height level than the aquarium.

When the water inside the coil gets heated by the sun, it will push up the cold water at the entrance of the aquarium, and some of the fresh water of the aquarium will get pushed to the coil. you will have a discontinuous flow rate of water (mass) getting in and going out from your system.

If you get to define your system as the water tank (not including the walls)

Later you can place some thermocouples inside the tank and a termocouple placed just right next to the cover on the outside Wall of the tank, and calculate how many energy (in the form of heat) you are loosing for convection from the inside of the tank to the walls.

:)

Interesting question