I am looking to solve the problem of our Freezer door popping open, and nobody noticing until all our food has defrosted. Is it possible to power an Arduino board and a buzzer using a peltier, and the heat energy coming from the "Heatsink" on the back of the Freezer?
The idea is to have a reed switch and a magnet on the door and freezer body respectively. Having code in the Arduino cause a wait of 60 seconds if door remains open, then sound a buzzer. I want to have continuous power so that I can't have the situation of forgetting to replace the batteries and find the freezer door open and no alarm. I could just plug it into the mains via a USB adapter, but that seems wasteful when there is so much heat energy kicking out the back of the freezer.
Is it possible to power all this from a peltier attached to the back of the Freezer, where it outputs most of the heat energy?
Peltier modules put out such a low voltage you need a lot of support electronics to make it usable.
You'd really be better off using a wall wart or batteries. If you sleep the micro appropriately you should be able to get a very long life from just a couple AAs. If your concern is dead batteries you can always monitor the battery voltage and alarm when appropriate.
It seems possible to replace a solar panel with peltier elements (and cooling the backside). The generated electricity is in the same range as solar panels.
How would you transfer the hot tubes from the Freezer to the flat surface of a peltier element ?
I agree with Chagrin, it is better to use a wall wart or batteries.
You are trying to run an Arduino on the energy from the freezer. That is not free energy.
The amount of energy you are trying to safe could be less than 1% of what the freezer is using. Perhaps the cooling of the freezer on the backside gets worse with your peltier element and wires and perhaps the freezer needs to work 5% harder.
A typical buzzer takes 35mA. Maybe you want a louder one, so let's assume 100mA.
A typical lithium AA battery is good for low temperatures. It retains almost its full capacity at freezer temperature. The capacity might be 3000mAH.
So the buzzer can run continuously for 30 hours. If your duty cycle is 30 minutes every week, that's about 0.3% duty cycle. At that rate, the lithium battery should last for about 420 days = well over a year. Do you think you could replace the battery once per year?
I haven't allowed for the power consumption of the Arduino itself. With some effort, that can be reduced to nano-amps and its overall consumption may be about the same as the buzzer on its own.
In theory you could - you'd have to see what kind of voltage you can get from your
heat source + sink, probably add a boost DC-DC converter. But the freezer will cycle
on and off so you will lose power and regain it frequently I suspect, you'd have to
choose a DC-DC converter that cuts out cleanly on undervoltage and restarts
cleanly, most won't I fear.
To get any kind of efficiency out of a peltier you'd need a good thermal interface on
both sides and decent sized heatsink on the cold side - you need a good temperature
difference.
When used as heat-pumps peltiers are only about 30% efficient I think, so don't expect
anything like 12V coming out when used as a pyroelectric element.