I have built a micro fridge to store a breadmaking sourdough starter at 9-10C. The cold compartment is 10x10x10cm, so size is 1 litre with removable lid. Insulation is 50mm polyiso foam all round. Cold side heatsink is a 10x10cm plate of 3mm anodised aluminium. There is a small 12v turbo fan in the cold chamber to assist with temperature uniformity.
The hot side heatsink is an aluminium finned extrusion with a 3.5" 12v fan bolted to it. The 12706 peltier is mounted behind the hot heatsink and then I have a piece of square aluminium bar 1.5 inch square and length 47mm that connects to the cold heatsink. I had to think long and hard about this spacer as most designs don't really explain how to maintain a good insulation thickness between the cold and hot sides.
Temperature control is via a thermistor probe in the cold compartment feeding an LAE on/off fridge controller.
Currently the peltier and both fans are all running off a 5 volt supply.
Everything is working OK but I am rather disappointed with the projected power consumption of the set up. I have just measured the electrical energy used over a 24hr period and I reckon this will equate to 60kWh per annum. Not a lot, I know, but if I compare my micro fridge to a Russel Hobbs 17l mini fridge, it only uses 82kWh pa for 17l and a more challenging temperature differential - presumably 25C/4C vs my current 18C/10C.
So (finally!) my question: why is the performance of my micro fridge so much worse? And what can I do to improve it?
The area I am looking at is what I will call "back conduction": when the cooler is not running, cold must be leaking back from the cold compartment via my ali block to the peltier and then through the peltier which is only a few mm thick to the hotside heatsink.
Surely this must be a major problem with all peltier systems; what can be done to minimise it? I am thinking that perhaps both heatsinks should be small so there isn't a lot of heat transfer without the fans running. And perhaps the hot side heatsink should be inside a totally enclosed insulated housing with the fan blowing in via a duct?