Using hydrogen fuel cells to create electricity is a relatively new process that allows us to fuel electrical devices in a much cleaner way. Depending on the method used to produce the initial hydrogen gas, vehicles of the future could then produce near zero emissions as the only byproducts of generating energy in this way are water and heat. However, to power something as large as a car, the hydrogen cells need to be bundled together as a “stack”. This fuel cell stack allows a large amount of electricity to be produced as long as there is a supply of hydrogen available to use.
But how efficient are hydrogen fuel cells, and how much hydrogen needs to be inputted in order to get a usable electricity output?
According to the US Department of Energy Hydrogen Program, a standard fuel car with a combustion engine runs at around 20 per cent efficiency, whereas vehicles that run using hydrogen fuel cells are around 40 to 60 per cent efficient. This number could vary depending on the size and weight of the car, as well as the kinds of roads being driven on.
The department goes on to state that the maximum efficiency that a combustion engine could achieve is around 58 per cent, however fuel cells have a theoretical maximum efficiency of 85 to 90 per cent. This depends on how the fuel cell is being used and is usually unachievable in real world applications.
If the heat that is produced by fuel cells in a combined heat and power (CHP) system is reused, the efficiency increases even more. Not only is the electricity being put to use, but the heat byproduct can be applied elsewhere too. For example, in a CHP boiler system, the heat that is created can be used to further warm the water in the system for very high energy efficiency. A further bonus is that any excess electricity can be sold back to the grid.
In a typical fuel cell vehicle, you can expect to use around 0.8 kilograms (kg) of hydrogen per 100 kilometres (km). To put this in perspective, around one kg of hydrogen equals nearly five litres of petrol. You could drive for 500 km before needing to refill, making a hydrogen fuel cell car very similar in cost and range to a petrol vehicle. This range is much greater than most electric cars on the market.
Hydrogen costs around £12 per kg, therefore a tank capacity of around five kgs could cost between £40 and £60 to fill.
One hydrogen fuel cell hardly produces any electricity - this is why they need to be stacked together so that the electricity output is usable. A fuel cell will produce just under one volt of electricity and so hundreds or even thousands need to be stacked to run a car.
One of the most common types of fuel cell is an alkali fuel cell. These run using compressed hydrogen and oxygen. It’s thought that their efficiency is around 70 per cent and they can output anything from 300 watts to five kilowatts (kW) of electricity. A proton exchange membrane (PEM) can output between 50 and 250 kWs of electricity and a molten carbonate fuel cell (MCFC) can output a huge two megawatts of electricity. These different kinds of fuel cells can be used depending on the appliance and how much electricity is required. For example, an MCFC is inappropriate for use in the home as it has a very high operating temperature of up to 650 °C. It would be more beneficial in a power plant.
Currently, hydrogen fuel cells are very effective in CHP boilers, however the technology may need to be improved in cars before they become a replacement for fossil fuel engines. They can be difficult to fill up as hydrogen pumps aren’t readily available, and the tanks have been known to crack, causing gas leaks.
Reducing the planet’s carbon emissions is extremely important and fuel cells could potentially be the answer in the future.