Burning issue

Tiny fibre-optic temperature sensors inside fuel cells could stop burnout failure and allow designers to improve power efficiency, according to their US inventors.

Researchers at Oak Ridge National Laboratory have developed miniature diagnostic sensors that sit inside fuel cells as they function and monitor hot spots. The sensors work in all types of fuel cell, including low-temperature proton-exchange membrane fuel cells used in vehicles and also solid-oxide cells for power plants at over 1,000ËšC, the team claimed.

Dr Stephen Allison of ORNL’s Engineering Science and Technology division said the sensors have a temperature response time of one-tenth of a second.

‘Temperature is a clear indicator of malfunction in a fuel cell. One potential problem is when the membrane dries out and you get burn-through that stops the cell functioning,’ he said. Diagnostic sensors would allow fuel cell designers to increase stack power density and match real data to their models.

The sensors combine fibre-optic and luminescence technology. An LED injects a pulsed light into a fibre that illuminates phosphor powder or a ruby sphere inside the fuel cell. The luminescence decay time of the phosphor or ruby in between pulses is an indicator of the temperature of fuel cell. The hotter the fuel cell, the shorter the decay time.

The fibre-optic sensors can also be adjusted to measure humidity to within one per cent accuracy. ‘The humidity in a fuel cell needs to be very high for the membranes to function,’ said Allison.

The diameter of the fibre is only 200-300 microns. ‘The goal was to develop a temperature measurement tool as thin as possible that doesn’t interfere with the other channels in the cell,’ he said. ‘The biggest challenge was in the fragility of the fibre, which is made from tiny glass strands.

‘Fibre sensors are usually fairly robust but we’ve removed the usual protection to get them smaller. And the smaller you make it, the less light you get into the fibre.’ The team used two fibres side-by-side, one to illuminate the phosphor sensor, the other to measure the decay. They also experimented with a single fibre.

Conventional thermocouple sensors would interfere with fuel cell channels and be significantly less accurate than the team’s sensors, said Allison. ‘You have high current in fuel cells and the inductive fields affect the electrical signal in thermocouple sensors,’ he said. ‘There is also a safety issue, because they give you a path to ground.’