Fuel cells are now beginning to emerge as a viable power source for applications big and small. In addition to them being an environmentally friendly solution from an emissions point of view, they are sought after for their efficiency, portability and quietness. While many fuel cells do use a carbon-based fuel such as butane, propane or natural gas, they output only electricity, heat and water, as a result of an electrochemical reaction. There is no combustion or resulting carbon emissions.
On the small end of the spectrum, micro-fuel cells are being developed to recharge laptops, cell phones, PDAs and other portable devices without requiring access to ac power. Portable fuel cells have a wide range of uses from recharging military field equipment, eliminating the need for vast supplies of batteries to be carried, to quietly powering RVs at campgrounds instead of loud, noise-polluting gas generators. Large stationary fuel cells are being deployed to generate power in remote areas where there is no access to the electric grid in addition to being used as back-up power for both commercial and residential buildings. Other residential systems are being developed to power electric appliances in areas where grid power is very expensive, and for residential cogeneration systems in which the hot water output is sent to a heat exchanger for heating and hot water (see diagram).
To make the fuel cell as efficient as possible, it is important to accurately control the fuel usage and air intake. Mass flow sensors are often employed for these functions. MEMS Mass Flow Sensors offer highly accurate, compact and stable measurement performance. Internal temperature compensation circuitry maintains accurate readings over a wide range of operating temperatures. The MEMS Flow Sensor chip is extremely sensitive at its low end, down to a few cm3/min and can be packaged for flows over 200 LPM. Omron Electronic Components LLC has standard products available from 1 to 50 LPM and has produced custom products for fuel cell applications for other flow ranges with special space constraints.
For systems with larger flow requirements, space and cost savings can be gained by using a small flow sensor in a bypass setup. A mass flow sensor used in bypass is similar to using a differential pressure sensor, in which two ports are installed on either side of a flow restrictor in the main flow path. The restriction in the main flow path causes the flow to follow the path of least resistance into the bypass channel and through the flow sensor. The pressure drop over the sensor needs to be greater than or equal to that between the bypass ports. The system is then calibrated so the portion of the flow pulled off the main pipe is measured and scaled-up for the full flow reading. Omron's engineering team assists customers with their bypass design by providing a basic CFD (computational fluid dynamics) analysis to optimize the orifice size and port spacing of the bypass system.