How does the raspberryHy fuel cell system work?

The raspberryHy system consists of four main components: A PEM fuel cell, that produces electrical energy out of hydrogen, fluidic parts that regulate the pressure for the fuel cell, a Li-Ion battery for starting the system and bridging power drops and a circuit board that loads / unloads the battery, converts the fuel cell and battery voltage to 5V and powers the Raspberry Pi. The board additionally controls the purge valve to exhaust residual hydrogen out of the fuel cell in certain intervals.

The Figure shows a schematic overview of the first system, developed at Fraunhofer ISE:

raspberryHy scheme

A polymer electrolyte membran fuel cell is an electrochemical device that generates energy directly out of the reaction between hydrogen and oxygen. Its electrical efficiency is not limited by the carnot cycle, therefore higher efficiency than with combustion engines is possible at lower temperatures. Compared to a battery, in a fuel cell system, energy storage and conversion are seperated, leading to higher energy density if long rumtimes are needed.

Hydrogen is a colourless, odourless gas with the lowest density of all elements. It has a high gravimetric energy density (33,3 kWh/kg) but a low volumetric density (3 kWh/m3) under standard conditions. It is not more or less dangerous than other fuels. Experiments with a burning hydrogen tank compared to a gasoline one even show a better behaviour as hydrogen dilutes very fast in open air while gasoline stays in place (external link)