Grainger CEME

Balachandran Thanatheepan with adviser K. Haran

The Center for Cryogenic High-Efficiency Electrical Technologies for Aircraft (CHEETA) is a NASA-funded project that aims to design an ultra-efficient (99%) electrical system with high specific power (>25 kW/kg) required for commercial electric aircraft. The system uses liquid hydrogen both as the energy source for the fuel cells and the cryogen for the electrical system. This eliminates additional cryogenic- system weight and opens up the propulsion system design space for various motor topologies which leverage superconducting (SC) technologies. Operating the electrical system under the cryogenic system also enables advantages, such as low resistivity of “conventional” conductors like copper or aluminum.
This project considers a 40 MW regional airplane operating for six hours and analyzes the suitability of three motor topologies for its electric propulsion: (1) fully SC machine with active-shield and iron-shield configurations, (2) partially superconducting machine with cryocooled AL conductor, and (3) permanent magnet machine with SC armature coils and cryocooled conventional conductors. Pros and cons of each topology at the required power level will be analyzed, and the optimal motor topology for enabling commercial electric aircraft will be identified. Because the system provides free cryogenic cooling for electrical components, an ultra-efficient electrical system could be obtained if the required cryogenic power to remove generated heat losses in the electrical system is maintained below the available free cooling. Available total power and estimated cooling capacity are provided in Table 1 for a six-hour regional flight.

Table 1: Fully SC motor cooling budget

Table 1: Fully SC motor cooling budget
Specification                 Value
Total Energy                      1068 GJ
Propulsive power              40 MW
Mission length                   8 hrs
Amount of fuel (LH2)      14833 kg
Number of motors            16
Motor Power                      2.5 MW
Motor Speed                      4500 rpm
Mass flow rate at cruise   0.23 kg/s
Cooling budget                  4.3 W
Although SC machines are attractive for this application, they have significant practical challenges introduced by the SC armature coils. When alternating fields and currents are introduced to SC materials, they generate losses in the form of heat, known as ac losses. These are cyclical and increase with both applied frequency and peak applied flux density squared. Motors for electric propulsion operate at high speeds and therefore experience high electrical frequencies; thus losses in the armature coils are significant and determine a design’s feasibility.
A series of experimental studies are conducted to validate the enabling technologies for an SC motor in a hydrogen-powered electric aircraft. MgB2 and high-purity Aluminum stator windings will be tested under applied external fields and alternating transport current to validate performance stability. A rotating cryocooler experiment shown in Figure 1 will be conducted to examine alternate ways to cool the rotating field coils. These experimental results will be used to determine the feasibility of SC machines for aircraft propulsion.

Figure 1: Rotating cryocooler experimental setup