I was reading an article about using a fuel cell energy storage system for a drone and how it was a high energy density option. This made me wonder how a fuel cell works and what options there are around for the model maker.
What is a fuel cell?
A device that converts the chemical energy obtained from a redox reaction directly into electrical energy. A fuel cell is a battery where reactants are supplied to the cell from an external source.
The most commonly cited example is the hydrogen fuel cell, in which hydrogen and oxygen are combined, producing electric current and water.
The basic types of fuel cell are:
- Alkaline Fuel Cell (AFC)
- Phosphoric Acid Fuel Cell (PAFC)
- Molten Carbonate Fuel Cell (MCFC)
- Polymer Electrode Membrane Fuel Cell (PEMFC) – this is the most common type of fuel cell that we see being used in the automotive industry and in the aerospace world.
- Solid Oxide Fuel Cell (SOFC)
- 1839 Fuel Cell invented by William Grove.
- 1959 First successful fuel cell produced by Francis Bacon.
- 1966 The first fuel cell car – GM Electrovan.
We will need:
- fuel cell
- fuel tank – pressurised hydrogen tank is most common
- battery to absorb and support high peak power requirements
- control system
The most obvious choice of fuel cell for a drone or model is a PEM Fuel Cell as this converts hydrogen gas into electrical power.
Proton Exchange Membrane Fuel Cell
Works with a polymer electrolyte in the form of a thin permeable sheet. This membrane is small and light and works at low temperatures around 80°C.
- Use platinum-containing electrodes
- 10′s of ppm of CO can poison a pure platinum catalyst, this means pure hydrogen is required as the fuel for PEM fuel cells.
- Hydrogen produced by steam reforming light hydrocarbons contains 1 to 3% CO.
- Typical efficiency is in the range 50 to 60%
- 25% load dropping to 30 to 55% at 100% load.
- At low loads (~5%) the efficiency can drop to below 30%.
- Operating temperature in range 50°C to 100°C.
- Most commonly used electrolyte is Perfluoro Sulphonic Acid
- Relies on liquid water humidification of the membrane to transport protons.
- solid electrolyte reduces corrosion and electrolyte management problems
- low temperature
- quick start-up
- expensive catalysts – platinum
- sensitive to fuel impurities – carbon monoxide
- low temperature waste heat – makes heat rejection difficult in high ambient temperature conditions
A 500W PEM fuel cell weighs around 1.3kg and needs around 30s to start up. This will want to operate at a fairly continuous power output and so hence the need for a battery. The stack size will be quite small, one example I found was an Aerostak A-500 that measures just 194 x 105 x 166 mm. The fuel tank will be another challenge altogether.
Whatever the power demand is, drone, radio control car, boat or even something electrical in the workshop the first task is to look at the system sizing. For this we need to know:
- peak power demand
- continuous power
- energy required for the mission or event
- system voltage
- peak discharge current
- peak charge current
From this we can look at the size of the fuel cell, size of the fuel tank and the size of the battery.
Anion – Alkali, molten carbonate and solid oxide fuel cells are “anion-mobile” cells. Anions migrate through the electrolyte toward the anode.
Anode – This is where the fuel reacts or “oxidizes”, and releases electrons.
Atmospheric Pressure – Used to describe a fuel cell system where the only pressure acting on the system is from the atmoshpere; no external pressure is applied.
Balance of Plant – The auxiliary equipment required to ensure the fuel cell operates as a reliable power source. This may include fuel reformers, humidifiers and pumps.
Cathode – Where oxygen, usually taken from the air, “reduction” occurs.
Cation – Phosphoric acid and PEM fuel cells are “cation-mobile” cells. The cation migrates through the electrolyte toward the cathode.
Matrix – The framework within a fuel cell that supports an electrolyte.
Membrane – The separating layer in a fuel cell that acts as electrolyte (an ion-exchanger) as well as a barrier film separating the gases in the anode and cathode compartments of the fuel cell.
Poisoning – The lowering of a fuel cell′s efficiency due to impurities in the fuel binding to the catalyst.
Polarisation Curve – A measure of cell performance that indicates the relationship between current density and voltage across a fuel cell.
Stack – Individual fuel cells connected in a series. For each layer in the fuel cell stack we will get a potential of 0.7V