Method for producing a kinetic energy storage system
A flywheel energy storage system incorporates various embodiments in design and processing to achieve a very high ratio of energy stored per unit cost. The system uses a high-strength steel rotor rotating in a vacuum envelope. The rotor has a geometry that ensures high yield strength throughout its cross-section using various low-cost quenched and tempered alloy steels. Low-cost is also achieved by forging the rotor in a single piece with integral shafts. A high energy density is achieved with adequate safety margins through a pre-conditioning treatment. The bearing and suspension system utilizes an electromagnet that off-loads the rotor allowing for the use of low-cost, conventional rolling contact bearings over an operating lifetime of several years.
1. A method of manufacturing a rotor for a flywheel device, the method comprising:
cutting a cast ingot;
upsetting the cast ingot in an open die at a first temperature;
hot forging the cast ingot into a disc-shaped blank including bosses on opposing surfaces of the disc-shaped blank;
machining the disc-shaped blank to include a rotor disc having an integrally formed shaft therewith;
heat treating the disc-shaped blank including the rotor disc having the integrally formed shaft; and
cooling the disc-shaped blank to form a disc-shaped rotor including the rotor disc having the integrally formed shaft, the rotor disc having a diameter and a substantially constant thickness, the thickness being 15% or less of the diameter; spinning the disc-shaped rotor at a rotational speed that causes a strain on the disc-shaped rotor to exceed a yield strength of a material of the disc-shaped rotor; and responsive to reaching a predefined amount of yielding, slowing the disc-shaped rotor back to a stationary position, wherein the spinning and slowing of the disc-shaped rotor causes an increase in the yield strength of the material of the disc-shaped rotor; and respinning the disc-shaped rotor to the rotational speed, wherein the respinning the disc-shaped rotor to the rotational speed causes a strain on the disc-shaped rotor that does not exceed an increased yield strength of the material of the disc-shaped rotor resulting from the increase in the yield strength of the material of the disc-shaped rotor.
2. The method of claim 1 , wherein the upsetting the cast ingot comprises heating the cast ingot to compress voids in the cast ingot and stretch inclusions in the cast ingot.
3. The method of claim 2 , further comprising machining the disc-shaped blank to reduce a thickness of the disc-shaped blank.
4. The method of claim 1 , wherein the heat treating comprises:
heating the disc-shaped blank to at least 850 degrees Celsius;
cooling the disc-shaped blank in a polymer-modified water bath; and
tempering the disc-shaped blank at a temperature between approximately 210 degrees Celsius and 250 degrees Celsius.
5. The method of claim 1 , wherein the disc-shaped rotor comprises a yield strength of at least about 1,200 megapascals, an ultimate tensile strength of at least about 1,300 megapascals, and a ductility of at least about 6 percent.
6. The method of claim 1 , further comprising finish machining and balancing the cooled disc-shaped blank.
7. The method of claim 1 , wherein the disc-shaped rotor comprises martensitic steel.
8. The method of claim 1 , wherein the cast ingot comprises a steel alloy.
9. The method of claim 1 , wherein the disc-shaped rotor comprises a thickness of 0.25 m or less.
10. A method of manufacturing a rotor for a flywheel device, the method comprising:
cutting a cast ingot;
upsetting the cast ingot in an open die at a first temperature;
hot forging the cast ingot into a disc-shaped blank including bosses on opposing surfaces of the disc-shaped blank;
heat treating the disc-shaped blank;
cooling the disc-shaped blank to form a disc-shaped rotor;
spinning the disc-shaped rotor at a rotational speed that causes a strain on the disc-shaped rotor to exceed a yield strength of a material of the disc-shaped rotor;
responsive to reaching a predefined amount of yielding, slowing the disc-shaped rotor back to a stationary position, wherein the spinning and slowing of the disc-shaped rotor causes an increase in the yield strength of the material of the disc-shaped rotor; and
respinning the disc-shaped rotor to the rotational speed, wherein the respinning the disc-shaped rotor to the rotational speed causes a strain on the disc-shaped rotor that does not exceed an increased yield strength of the material of the disc-shaped rotor resulting from the increase in the yield strength of the material of the disc-shaped rotor.
11. The method of claim 10 , wherein the disc-shaped rotor comprises a thickness-to-diameter ratio of 15% or less.
12. The method of claim 10 , wherein the disc-shaped rotor comprises a yield strength of at least about 1,200 megapascals, an ultimate tensile strength of at least about 1,300 megapascals, and a ductility of at least about 6 percent.
13. The method of claim 10 , further comprising machining the disc-shaped blank to include a rotor disc having an integrally formed shaft therewith.