Today, passenger aircrafts are almost exclusively fueled by fossil energy. The industry emits about 3 % of the global CO2 omissions. Based on the projected growth in global airline traffic the emissions can be expected to double toward 2050 unless radical measures are taken to switch to more sustainable energy sources.
Electrification of the passenger aircraft fleet has the potential to enable climate neutral aviation. It all depends on the source of electrical energy. If the electricity is harvested from renewable energy sources, the aircrafts will be practically emission free. Some few electrical aircrafts do already exist today. These aircrafts can only carry a limited number of passengers over relatively short distances, and huge challenges remain to be solved before electric aircrafts can constitute any significant share of the passenger fleet. Weight is a key concern here. If batteries are to be used for energy storage, it is not feasible to transport a larger amount of people over longer distances.
Liquid hydrogen could be an attractive alternative to batteries. 1 kg of liquid hydrogen has the same energy content as more than 100 kg of batteries. It must be pointed out that it is a false equivalence to compare these weights directly. The reason is that while the energy stored in a battery is readily available as electricity, it requires a fuel cell to convert the energy stored in the hydrogen into electricity. Fuel cells add significant weight to the aircraft.
In a hydrogen-based propulsion system much of the weight is transferred from the energy storage system to the energy conversion system, as compared to a battery-based propulsion system. Why such a transfer can be a good idea can be illustrated by a simple thought experiment where we analyse what it would require to double the range of an electric aircraft. In course terms, it would either require doubling the number of batteries or doubling the amount of stored hydrogen, depending on the plane type. In this thought experiment, the added mass for the battery plane would be more than 100 times the added mass for the hydrogen plane. This means that the hydrogen plane has huge advantages over the battery plane in terms of scalability.
Liquid hydrogen does not only have a very high energy density. It is also extremely cold. The temperature of the hydrogen inside the storage tanks is minus 250 degrees Celsius. This temperature is far too low for the hydrogen to be utilized directly in the fuel cell. First, it must be evaporated and heated to 80 degrees Celsius. From this it may seem that storing the hydrogen at such a low temperature is only an unfortunate necessity. Luckily, this is not the full story. The low hydrogen temperature opens for some very convenient synergies.
If we instead of copper wire use high-temperature superconductors in an electrical motor, it can be made extremely compact. One of the big challenges of superconductors are that they do not become superconducting before they are cooled to almost minus 200 degrees Celsius. Normally, such cooling requires bulky refrigerators, which almost nullify the benefit of having ultra-compact superconducting motors.
I hydrogen-planes we already have access to the liquefied hydrogen. Since this must be heated to 80 degrees Celsius before it can be consumed in the fuel cells, we can obtain large synergies by channelling the hydrogen through the cooling channels of the superconductors on its way to the fuel cells. In this way we can heat the hydrogen and cool the superconductors and at the same time significantly reduce the requirement for refrigerators.
It is not only the the motors that benefit from hydrogen cooling. Power electronics converters and transmission cables can be made much more compact if they are cooled with hydrogen.
The main objective of my PhD study is to investigate how the propulsion motors best can be designed to utilize hydrogen cooled superconductors. This investigation comprises both theoretical and experimental studies. The complete propulsion architecture is also studied to understand the interaction between the components and the hydrogen.
Thus far in the PhD study I have done a detailed study of the heat balance in the propulsion system, where the main objective has been to study the hydrogen temperature profile over the different components through all phases of an aircraft mission. The article was published in IEEE Transactions on Transportation Electrification in 2022. We have also started building cryogenic lab facilities at the Institute for electric power engineering at NTNU. We have also built a test unit of a superconducting propulsion motor that will be tested in the cryogenic lab.
Significant efforts have been put into modelling the superconductors in finite element software with emphasis on superconductor loss and critical current models.
I have also nearly finished my obligatory courses.
The PhD study is on track per December 2022.