Thermal Flow Sensors and Thermal Conductivity Sensors for Fuel Cell Applications

Xiangzheng Tu

A fuel cell generates electricity by the chemical reaction of hydrogen and oxygen. It is different from a battery in that it requires a continuous source of hydrogen and oxygen to sustain the chemical reaction, whereas in a buttery the chemicals to be reacted for generating electricity are stored in the buttery. The fuel cell can produce electricity continuously for as long as these inputs are supplied.

Fuel cell cars have been on the road for many years. The first commercial fuel cell cars are being sold in California by Toyota. Fuel cells are being developed and tested in buses, boats, motorcycles and bicycles, among other kinds of vehicles. Fuel cells have many more applications including portable generators, battery replacements, residential back power, and so on.

As shown in the above picture, the heart of the fuel cell is the polymer electrolyte membrane. It is sandwiched by the anode layer and the cathode layer. At the anode layer, the input hydrogen is converted into the hydrogen ions and the electrons. The hydrogen ions are transferred to the cathode through the conductor and the electrons are transferred through the outer circuit. At the cathode the transferred hydrogen ions together with the transferred electrons and the oxygen of the input air react to produce the water vapor. The water vapor is carried away by the unused input air.

It can be seen that in order to regulate the chemical reaction of the fuel cell the amount of the reacted hydrogen, the reacted oxygen and the produced water vapor should be measured in real-time. The fuel cell converts chemical energy into electrical energy and also, as a by-product of this process, into heat energy. This makes capacitive and resistive humidity sensors no useful for measuring the water vapor. Fortunately, POSIFA Microsystems provide thermal conductivity humidity sensors with greater resolution at higher temperature. It measures the absolute humidity by quantifying the difference in thermal conductivity of the input air and the output air containing water vapor. The accuracy of the sensors can reach at least +3 g/m3; this converts to about ±5% RH at 40°C and ±0.5% RH at 100°C. It is worth to note that the unused hydrogen also contains water vapor, which moves through the polymer electrolyte membrane from cathode to anode by diffusion. Similarly, the absolute humidity in the unused hydrogen can be measure by quantifying the difference in thermal conductivity of the input hydrogen and the unused hydrogen containing water vapor.

The thermal flow sensors provided by POSIFA Microsystems have been used for fuel cells successfully. The sensor consists of a heater and two symmetric thermopiles positioned on a porous silicon membrane. The heater is made of polysilicon, whereas the thermopiles are made from a combination of polysilicon and aluminum. The sensor is operated by a constant power circuit which provides constant power to the heater during the measurement. Power consumption and response time of the sensor are 30mW and 5ms, respectively. The noise of the sensor is very low and has the same amount of the noise as the equivalent resistor of the thermopile.

Flow rate and humidity are critical parameters associated with the performance of fuel cells. The product of the chemical reaction that proceeds in a fuel cell is water, which lowers the working efficiency of the cell and affects the fuel flow rate. With POSIFA thermal conductivity sensors any water vapor flooding in the fuel cell can be diagnosed easily and correctly. When the chemical reaction is not balanced well a microcontroller can modify the hydrogen flow rate and air flow rate automatically, which is based on the data collected by the thermal flow sensors. It should be no doubt that POSIFA flow sensors and POSIFA thermal conductivity sensors is a powerful combine for promoting fuel cell applications in fuel cars, fuel boats, fuel smart phones, etc.