I stumbled across an interesting message board thread today:
Lots of good pictures and diagrams of someone's experiments with solar and building their own solar regulator.
One thing that particularly caught my attention was later on down the thread they start using a compressor fridge as a dump load for the experiments. They found the built in fridge thermostat drew a large quintessent load when the fridge was not running. They crated their own thermostat which monitored the battery levels and adjusted the temperature depending on what was available in the battery. This meant if they had 'excess' energy coming from the panel then they would chill the fridge to a lower temperature, and if less power available would run the fridge a bit warmer.
I have a compressor fridge in my van and have wondered about doing something along the same lines. My original plan was to have a temp sensor in the fridge and controlled by a microprocessor so that I could set the temperature to a known digital level. Currently you turn a knob inside to set the temperature and I never know exactly what it is set to. So I probably have it set too cold or too warm most of the time which is no doubt in-efficient.
Setting it so that it detect when we are driving and cools the fridge down to a lower temperature when there is excess power coming (from the engine) and maybe runs it slightly warmer at night when there is no power from the solar panel and we are likely to have less power in the battery bank. I could also maybe have different modes depending on what was in the fridge. ie. if I know we have meat int here then then I need to keep it to the correct temperature. If I just have drinks in there then it can afford to run a bit warmer.
The Danfoss compressor in my fridge also has a resistor in the controller in the back that controls how fast the compressor motor runs. I need to check what it is set to, but I think by default it is set to run the compressor as fast as possible. Whilst this means the fridge cools down quicker, it also means that the fridge draws maximum current and probably maximum in-efficiency. It also leads to uneven cooling in the fridge, with some areas frozen with others still warm. If I could control the fridge with a microprocessor then I could adjust the speed of the compressor depending on how the temperature changes. Ie. if the door opens and the temp increases rapidly then run the compressor fast to cool it down and then drop down to low speed to keep it at that temperature.