Exciting news for the energy storage industry, in an article published in IEEE spectrum. A team in Eindhoven University of Technology passed a first industrial test for iron powder as an energy storage.
For more details on the technical process of iron as an energy storage please refer to the original article: Iron Powder Passes First Industrial Test as Renewable, Carbon Dioxide-Free Fuel
But as an overview, iron can be combusted to produce heat and the heated gas can be utilized fuhrer for electricity production. The combination of heat and electricity is the yielded energy production. Unlike fossil fuel, combusting iron does not emit any Co2, the only byproduct is iron oxide in the form of dust. This dust is easily captured. This captured dust can be recycled by splitting oxygen from the iron oxide(reduction) and the isolated iron can be used again to produce energy by combustion. The oxygen reduction and separation of iron and oxygen can be done by a chemical reaction with hydrogen which combines with the oxygen in the rust and produce iron and water. This separation process can be thought of as recharging the energy storage, because we need electricity to split iron oxide. The electricity needs to be taken from a renewable source such as solar or wind in order for the whole process to be green.
Iron as an energy storage could be ideal as a renewable Co2-free energy source for industrial use. Whenever we are introduced to any energy source there are 3 big questions that must be asked, What is the energy density? What is the specific energy? And what is the efficiency?
Energy Density
Energy density is how much energy is contained per unit volume. This is very important in may applications such as in the automobile industry. We always have a very limited space available for fuel tanks, therefore the energy source we should use must be very dense. Iron, has an energy density of 11.3 KWh/L which is very close to petrol’s energy density (12.9 KWh/L), and when compared to the pressurized hydrogen at 70 MPa (used Toyota Mirai) we find out that at this pressure hydrogen has 2.3 KWh/L.
Specific Energy
Specific energy is how much energy we have per unit mass. Going back to the automobile example, if the car is gets too heavy because of the mass of the fuel used then we might need an even more energy to move the car. So, the more the specific energy the better. Iron has a specific energy of 5.2 MJ/Kg. This is unfortunately a very low specific energy, compared to petrol which has an specific energy of 46.4 MJ/Kg. Does that mean Iron has no place in automotive? The answer is no, even though Iron is not high in specific energy, but electric cars nowadays use lithium-ion batteries with specific energy of 0.9 MJ/Kg. This tells us that batteries are almost an order of magnitude heavier yet it pulled it off. The caveat would be how heavy would the combustion engine for iron be? Electric cars use very light induction motors which compensate for their heavy batteries. The last major contender is Hydrogen which has a great specific energy of 141 MJ/Kg. Hydrogen is the clear winner when it comes to specific energy.
Efficiency
Efficiency is a measure of how much of the available intrinsic energy can be converted to another form of energy. Iron powder presents itself as a renewable source, which means we should also count of both the efficiency of combusting iron and the efficiency of reverting the iron-oxide(rust) back into iron. Since, this method is still in the research and early adaption phase, I couldn’t find figures for those efficiencies, but we know that combustion engines have a theoretical efficiency limit (carnot efficiency) of 83% efficiency. But the gap between this theoretical efficiency and practical efficiency found in modern gasoline cars, we find that gasoline cars go upto 45% efficiency, and if we subtract all the energy loss in oil pumping, refining, and distribution we end up with almost half of that efficiency. Hydrogen, has a better efficiency than gasoline. Toyota mirai has an efficiency of 66% but we can also subtract from it the efficiency of the water splitting process for hydrogen production and we end up with a net efficiency of around 20%. Batteries are by far the most efficient energy storage we have to this day, reaching up to 85%.
When it comes to the automobile industry, Iron most likely will never be the fuel of choice due to it’s reliance on combustion engine along with its low specific energy. But when it comes to industrial use where weight of fuel is not an issue then Iron will be of great use as a co-generation method used in heat production for a factory along with a sterling engine for electricity production.
Alawi Midher 