Peltier Fridge Heat Loss

Question

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?

Any thoughts?

Lance

Answer 1

Welcome to Peltiers. They are convenient and small, not efficient. They are beaten hands down by any compression cycle cooler.

If your Peltier is cycling on and off to maintain your target temperature, then there's some very low hanging fruit to harvest. Heat pumped is proportional to I. Heat generated is proportional to I². This means they run much more efficiently at 100% duty cycle of just the current they need to maintain the temperature. This also prolongs their mechanical life, less expansion and contraction breaking the bonds as their internal temperature cycles. You would ideally implement a PID driving a filtered buck converter to feed the Peltier.

One of the Peltier manufacturer's websites has an online calculator. Given required heat flux and temperature differential, it will give you two solutions for their hardware. One is the smallest solution, the other is the lowest power solution. The first is run near their maximum rated current. The second is typically two or three times as big, and run at a third of max rated current (remember that I vs I² ratio?). Higher initial cost, but much lower running costs. Here's a handy tutorial on the issues.

I would use an alli block, just the size of the Peltier, on either side, before transitioning to a larger heatsink, to allow more insulation thickness than just the Peltier thickness immediately around it. There is of course a tradeoff between conduction path length and insulation thickness.

Every extra degree the hotside runs hotter needs more current, and therefore more waste power dumped into it. Make that hotside sink as large and as low C/W as you can. Maybe even evaporatively cool it.

For the lowest power consumption, you might want to make ice in a freezer, and use that to cool your insulated box, replenishing it every 24 hours, perhaps with a fan or pump to maintain the temperature.