A BBQ thermometer is the perfect application for energy harvesting. There is a lot of energy available, and the high temperatures are challenging for batteries (and other electronics). It should not be difficult to harvest enough energy with a peltier device. The bigger challenge will be designing a cost effective system that will work reliably in this extreme environment. The system will generate electric power by creating a thermal gradient across a peltier device by putting it between the BBQ and a heat sink to the ambient air. The temperatures of the BBQ can get too high for a common inexpensive peltier device, so there will need to be a thermal circuit to manage the temperature across the peltier. The voltage out of the peltier will need to be conditioned and regulated to power the system. There will be a micro-controller to manage measuring the temperature and reporting it. The temperature sensor will also need to be capable of withstanding the extreme temperatures.
Here is the basic concept for the thermal circuit:
The heat source feeds a heat pipe to a Peltier based thermo electric generator. There are two heatsinks: one on the Peltier and one in between the Peltier and the heat source to prevent extreme temperatures from reaching the Peltier device. A fan turns on when the temperature starts getting too hot as determined by the voltage across the Peltier.
The low input voltage LTC3106 buck-boost regulator converts the voltage from the Peltier to 3.3V to power the PIXL.js board which monitors the BBQ temperature by reading a thermocouple with the MAX31855K.
I want the device to start as soon as possible, so a low input boost regulator is needed, but at high temperatures, the input voltage may get higher than 3.3V. I need to see how well the adaptive thermal circuit works to see if we can keep the voltage below 3.3V, but for now I will plan on a buck-boost regulator that can start up with less than 1 volt, like the LTC3106. That is probably overkill for this application and I may later try to replace it with an adaptation of the joule thief and a linear regulator.
To generate power with a Peltier device, you need to apply a thermal differential across the device. You need to make one side hot and one side cold like this:
The BBQ is much hotter than the air, so generating a differential is not difficult. The difficulty comes in trying to manage the extreme temperatures of the BBQ. Lets take a closer look at the elements of the thermal circuit:
Point A is contacting the BBQ, which could be several hundred degrees Celsius. The air in the vicinity of the BBQ could be a little higher than typical room temperature, the standard commercial temperature range of 0 to 70 degrees Celsius is probably a reasonable approximation for point D. Points B and C are the interfaces to the Peltier device. The TEC1-12706 like this one ( http://peltiermodules.com/peltier.datasheet/TEC1-12706.pdf ) is rated for a maximum temperature of 138 degrees Celsius, so the biggest challenge will be dissipating enough heat from the BBQ before it reaches the Peltier device. A common heatsink like 655-53AB from Wakefield should be sufficient for the cold side. For the hot side we will need some sort of heat pipe. Since we want to dissipate heat before it gets to the Peltier, we actually do not want a high performance heat pipe and a simple slab of metal, like aluminium should fit the bill. To ensure that enough heat is removed before the Peltier, we can add another heatsink on the heat pipe between the heat source and the Peltier. This will take some trial and error to determine the right amount of heat to remove (it would probably be a lot quicker in a thermal simulation tool, but I don't have access to that, and it sort of takes the fun out of it anyway). The basic thermal circuit I envision is something like this:
I am concerned with the wide variation of temperatures we could see from the BBQ. The max temperature is quite high, but I also want it to turn on quickly, so I need it to start working at lower temperatures. If only there was a way to actively adapt the thermal circuit as the temperature rises...
Adaptive Thermal Circuit
If you look at the datasheet for the Wakefield 655-53AB, one thing that you will notice is that the effectiveness of the thermal transfer is very dependent on the airflow across the device. WIth natural convection, there is a much larger temperature differential between the air and the hot side, than there is when air is forced through the fins. When the temperature starts rising, we will see a higher voltage across the Peltier device, we can use this voltage rise to turn on a fan to force air through the fins and remove more heat from the heat pipe. Thanks to the popularity of drones, there are some very cost effective, small, low power motors readily available. Fortunately when the voltage rises, it also means we have extra energy available to power the fan. If we happen to cool it too much, the voltage will drop so it should self regulate, but we should tune the gain to avoid listening to the fan cycle too often.
The temp sensor needs to be economical and work at high temperatures. A BBQ is too hot for thermistors so thermocouples are probably the next most cost effective. There are chips that can digitize thermocouples like the LTC2485 and the MAX31855. The price seems similar, but the MAX31855 is tuned for the specific type of thermocouple so there will be less software processing. The K type covers the temperatures of interest so I'll add the MAX31855KASA+ to the component list.
The first step is to find a suitable peltier module to use. Let's try to scope the requirements. To measure and report temperature should take well under 100mW. We don't have any significant size restrictions. I don't have a big prototype budget and one of the goals is to find a cost effective solution, so price and availability are important.
A little searching online makes it pretty clear that thermo-electric-coolers (TEC) are more common than thermo-electric generators (TEG) and so the price and selection of TEC is significantly better than TEG. There are many examples of people using the TEC devices for generation, even though their generation capabilities are not usually characterized very well. Most of the people trying to use these for power generation are trying to extract much more power than I need, so I feel pretty comfortable that the TEC1-12706, which seems to be a popular low price module, will be enough for this project.
The maximum working temperature for this module is 150C. This is significantly less than the maximum temperature inside the BBQ, so we need to be careful to shield it from the heat. There are higher temp devices, but the cost is significantly higher. We may need to revisit this selection, but it should be good enough for the initial prototypes.