AirMobile - 1 - Experimenting with Peltier cell

Experimenting with the power source: Peltier cell

In 1821, Seebeck found by using two different metals that are connected by two separate junctions, they will develop very small voltage if the two junctions at maintained at different temperatures.

In 1834, Peltier discovered the opposite of this, he found that if you apply a voltage to the same setup that it caused a different temperature at each junction, allowing you to generate both heat and cold from the voltage. Although what’s actually happening is heat transfer, the heat is transferred from one side to the other, making this a solid state heat pump.

You may also find they are referred to as TEC’s – ThermoElectric Coolers or in some cases TEG’s – ThermoElectric Generators. Essentially the Peltier Element is a combination of lots of very small thermocouples, junctions between 2 different metals or semi conductors and these are sandwiched between 2 ceramic plates and then encased in silicon.

Energy Scavenging with Peltier cells

OK, so you can’t get a lot out of these, but the point is by combining them in systems that produce a lot of wasted heat, we could minimize the waste and reclaim this. Granted, this is not going to amount to much, but scale it up and you can see why car manufacturers such as BMW are beginning to combine them around the exhaust – some of that wasted heat from the engine can be converted to electricity. So if you’re going to waste heat, why not get the most out of it?

Imagine an oven lined with these, or a device that could cook your food and chill something at the same time. Of course, it’s much harder than that, unfortunately Peltiers aren’t able to transfer much heat and because they work by creating a temperature differential, you need a way extract the heat and keep the other side cool at the same time. So just sticking them out in the sun or on the side of your oven isn’t going to generate electricity, it works on a car exhaust because of the air flow when the car moves cools one side.

Cell design

Before proceed, I need a rough estimation of the power requirements of the sensor.

The problem comes with the Figaro TGS 2442 CO sensor, that has a very high consumption (203 mA) during the heating cycle. The heating cycle last 14 ms according to the datasheet, so it's better to keep the Peltier cell as small as possible and use a super capacitor to store enough energy to drive the heating cycle.

The power required during the heating cycle is

            P = V * I = 5V * 203 mA = 1.015 W

The energy consumption during the heating cycle is

            E = P * t = 1.015 W * 14 ms = 15 mJ

So I can compute the capacitance of the supercapacitor

            C = (2* E) / (V * V) = (2 * 15 mJ) / (5 V * 5V) = 1.2 mF

So 0.1 F super-capacitor should suffice all the power requirements and provide enough energy during the heating cycle of the sensor

That said, and assuming that not all the sensor will be switched on at the same time, the maximum power requirement will be of about 40 mA @ 3.3V, that is to say 0.132 W

The specifications of the Peltier cell state the following

The Peltier cell with a hot side temperature of 85 °C will provide 341 mW. This value provides a good margin in case the delta T between the hot side and cold side decreases (when, for example, the car is in queue)