Back in 2008 I wanted to learn something about solar thermal devices, the simplest (and cheapest) thing I could think to mess with was a bottle of water. I chose to do this in winter because even under an English sky, solar devices will do something useful in summer, but winter is more of a challenge.
The first attempt was just a randomly selected bottle with a thermometer pushed through the cork, this was left lying around in the garden and on a sunny day there was a 10 deg. C temperature rise and on a cloudy one, cold water was still cold water. My understanding of the heating process is that the sun’s rays warm the glass bottle and the bottle warms the water it contains. I never got my head around the extent to which the sun’s rays acted directly on the water.
The next attempt involved three different coloured wine bottles; clear, green and dark brown with the temperature being measured indirectly with a thermistor. The 2nd December produced cold fingers and this set of curves:
Dark objects absorb radiant heat more readily then light coloured ones and this is reflected in the graph, the dark brown bottle produced a greater temperature rise than the clear one.
The bottles were uninsulated and the temperature stabilised when the the heat gain and loss balanced. I tried putting a glass cylinder around a bottle, the effect of this was not dramatic, possibly because the effect of the insulation was offset by by the reduced energy gain. My perception of solar thermal design is that it is a balancing act between maximising the radiative energy gain and minimizing the convective and conductive losses.
When looking at the results I was struck by how easy it is to get a 10 – 15 deg. C temperature rise. A domestic gas boiler typically takes in water at 10 deg. C and heats it to 50 deg. C a rise of 40 deg. C. Maybe a role for thermal devices in a seasonal, cloudy climate is as a feed water heater, if the boiler feed water temperature is raised from 10 deg. C to 20 deg. C there is a 25% saving in gas.
Somewhere in the dim and distant past I had read that the performance of solar stills might be increased by adding charcoal (I have no idea if that is correct). This made some sort of sense, a carbon particle suspended in liquid might act as a microscopic solar panel warming the water around it. I created a lot of mess trying to make carbon black with a candle and even more mess trying to get the stuff to go into suspension (a little knowledge of chemistry would have helped). After messing with several household substances (ink, edible colourings etc.), I opted for gravy browning which is not ideal, but it is blackish, unlikely to harm the dog and cheap.
On the 6th January, I produced this curve which suggests the dyed water water warms up faster than clear water (at some time in the future, I would like to see if this result is repeatable). A couple of years later, my son bequeathed me a leaking tube of black ink which appeared to be carbon based and would have been ideal, however, there is a limit on the amount of bizarre behaviour one family can tolerate.