The Department of Energy’s dream to build a superconducting power infrastructure in the U.S. has been dealt a symbolically serious, if not unexpected blow. General Electric has quit its $27 million 100MVA high temperature superconducting (HTS) generator program. The generator was envisioned as an opportunity to eventually introduce large generators (in the 100 to 500MVA range) that higher power densities and increased efficiency. The heart of the DOE’s superconductivity program, HTS power cables, remains on track.
There were major problems, or at least challenges, with the program from the outset. The first was that conventional generators, which use copper windings in both the stator and rotor, are extremely efficient, and the introduction of a HTS rotor meant only about a 0.35 to 0.55% increase in electrical efficiency. While this does not sound like much, over the course of the machine’s life it meant considerable electricity savings that were anticipated to offset the technology cost of the superconducting rotor, and cryogenic refrigeration system.
GE says that another factor contributing to the decision to terminate the program was a shift in the energy market: “GE has observed that in the present energy marketplace, where the spot market price of natural gas has recently been greater than $10/MM BTU, operators of combustion turbines are shifting larger units to cyclic operation and supporting the base load with less expensive fuels such as coal and nuclear. This shift in generation dispatch undercuts the economic benefit of the HTS generator.”
GE has over 5,500 wind and 3,600 hydro turbines, for an installed capacity of renewable energy of 160,000MW, but this is a microscopic fraction of the capacity provided using fossil fuels, so its safe to say the drive for renewable energy is not negatively impacting R&D on large HTS generators.
The other major factors involved the technical and economic hurdles surrounding HTS. The BSCCO-2223 HTS wire (currently produced primarily by Sumitomo Electric and American Superconductor) was too fragile, too expensive, and required too much cooling. GE had repeatedly said it wanted the wire to cost well below $25 per kiloAmp-meter (this is the cost for an amount of HTS material sufficient to transport one 1000 Amps across a length of one meter). At present, that target is a somewhat remote prospect. (An OP-ED piece in the latest issue of Superconductor Week (v20 n02) provides an interesting analysis of these costs.)
Cooling the HTS rotor is another issue. Today’s HTS wire does not work sufficiently well in the strong magnetic fields present with a device such as a motor without substantial refrigeration. Whereas devices such as HTS cables may be able to operate in a liquid nitrogen environment at 77 degrees Kelvin, motors must run closer to 30K, or even cooler, using liquid neon or gaseous helium coolant.
The cancellation of this program leaves the superconducting generator concept squarely in the hands of the military, which is effectively indifferent to costs, and is focused on increasing power density (the ability to cram a huge amount of generating capacity, or in the case of motors, horsepower and torque, inside a small, light-weight package) for applications where space/weight is restricted, such as ships and aircraft.
It is likely that if these military programs are successful, the technology will eventually trickle back into the commercial sector. For now, we will have to focus on the promise of HTS elsewhere.