Power Transmission Cable for Electric Aircraft using Bio LNG for Cooling and Thermal Management

The key problem for electric power distribution cables for double-aisle passenger aircraft is the weight of Cu or Al cables which is too high to be practical, even when the voltage is increased to kilo-volts to reduce the amperage. To carry the same power, the reduced amperage and increased voltage helps to reduce the amount of ohmic waste heat. Thus, for room temperature cables, the two issues are: 1) dealing with high voltage dielectric and 2) dissipating heat at high altitude with the accompanying low air pressure. However, even if the ohmic heat is removed locally from the cable with a coolant at room temperature, the heat in the conductor during cruise has to be removed at high altitude to and from a heat exchanger. That heat exchanger has to be large when considering the air density at high altitude is less than 1/10 what it is at ground elevation. We propose to use Bio-LNG whose room temperature boiling point is 111K (but higher under pressure) for dissipating ohmic heat loss in our electrical conductor. This deals well with overall thermal management of an airplane that at best has difficulty dumping heat and as we pursue electric powered aircraft, the problem is worse. This program is compatible with the ARPA-E, REEACH and ASCEND programs in the desired use of carbon neutral fuels. Hyper Tech has already been selected under the ACCEND program to develop and demonstrate a high power density motor and drive using Bio-LNG for cooling the motor and drive. The intent is that the somewhat warmed Bio-LNG will go on to burn in a turbine generator or fuel cell. Also, a vacuum cryostat that necessarily surrounds our cryogen transport cable further increases the voltage standoff. Inside the vacuum boundary, the dielectric of a liquid or gas under pressure becomes the primary high voltage insulator. The only exposure to high voltage at high altitude is at the terminations where the leads emerge from the cryostat to the outside environment; the cryostat shields the cable from the low pressure air at altitude along the entire cable length. At cryo-temperatures around 120K, we decrease the resistivity of our Cu clad Al conductor by a factor of 3 in the aluminum, which allows for the current density to be increased due to the decreased cross section. Thus, the overall effect is to dramatically reduce the conductor size, cryostat size, volume, and weight which ultimately lowers the mass-per-unit length (kg/m). Our present estimate less than 1 kg/m several times less than room temperature cables and close to superconducting cables. In addition, this dramatically increases the power carrying capacity (kW/m) compared to room temperature cables. Ultimately, this further leads to having a much lower cost, $/kW compared to room temperature cables. In addition, we should be able to provide better fault protection than superconductor cables, especially ReBCO superconductors which are very difficult to detect a quench and mitigate before damage develops. During the program we will look at the additional option of using a secondary helium gas loop (from 120K down to 40K) to cool the conductor to 40K where the resistivity of high purity Al is 10-15 times lower at 40K than room temperature. In the same way as with Bio-LNG, we will be dumping the ohmic heat to Bio-LNG by way of a cryocooler that cools the helium gas. We will determine if this is a better option for the transmission cables than using just the Bio-LNG only at 120K for further lowering the kg/m, raising the kW/m and lowering the $/kW. For both options we will also be determining the weight of the terminations and comparing costs for these two Bio-LNG systems verses using room temperature cables or superconducting cables.