A flywheel is a device for storing energy on momentum in a rotating mass. As energy is drawn from the system, the rotating components slow down, and as the system is charged, they speed up. To accommodate this changing rotational rate, it is necessary to leave a variable speed drive as part of the system. A flywheel power system has a DC output and this is similar to a chemical battery. The addition of an inverter allows direct connection to standard AC circuitry. Douglas Thorpe, Thortek, received a DOE/DOC grant of $99K in 1995 to continue development of his flywheel design.
Overview:
•Thortek has developed a new flywheel energy storage technology with transportation & telecommunication applications
•NASA awarded Thortek a SBIR Phase II grant in 2002 to develop an electric motor that will be used in Thortek’s flywheel systems.
•DOD awarded Thortek a SBIR Phase I grant in 2004 & 2005 to develop mag-bearings and electric motor.
Applications:
•Aerospace
•Electric vehicles
•Industrial Applications
•City Buses
Capabilities:
•Increased energy density (wk/kg)
•Reduced mass for space applications
•Replacement of conventional batteries at less life-cycle cost
Benefits:
•Robust energy storage system with nearly unlimited charge/discharge cycles and impervious to ambient conditions.
•Increase power output per mass of flywheel
•Reduced life-cycle cost of flywheel system
Flywheels vs. Chemical Batteries
For the applications related to this paper, only lead-acid gel cells and wet cells should be considered when referring to chemical batteries. Gel cells require no maintenance, but don’t usually last longer than 3 years and are easily killed by heat and cold. Wet cells can last 15 years, but require monthly adjustments to electrolyte level. Flywheels are superior to chemical batteries because they can be rapidly recharged as fast as they can be discharged and for 100,000’s of cycles. Flywheels are not effected by temperature and can lasts more than 20 years with minor maintenance (bearings, seals, and lubrication replacement, if any). Lead-acid batteries do have the advantage of having lots of management inertia.Conventional Technology (Steel) Flywheels
Steel Flywheels are currently being sold in some quantities in UPS systems. However, weight restrictions will permanently forbid their use in Bus Applications. As previously shown in the Table above, the Thortek Hybrid flywheel will be lighter than Steel Flywheels and cheaper because of their VST.Modern Technology (Composite) Flywheels
Until now, Composite Flywheels have been too expensive for all but military, space applications, or where a country has subsidized the project (Germany). Not only is the cost of the flywheel expensive, but the support hardware of special servo controllers, high-speed motors, and magnetic bearings greatly add to the cost of the Flywheel System.
The biggest problem with composite flywheels is that their lightweight and great composite strength produces a large flywheel tip speed, Vt. A large Vt will translate into a large rotational speed and rotational speeds greater than 12,000 rpm will forbid the use of ball bearings, shaft seals, and geared transmissions (some flywheel competitors speak of 60,000 rpm or even 180,000 rpm flywheels). Since ball bearings can't be used, flywheel designers are force to use expensive magnetic bearing systems. However, all magnetic bearing systems are very expensive and require a ball bearing system as a back up (a.k.a., touchdown bearings).Thortek Hybrid Flywheel w/ Variable Speed Transmission
Contemporary electric vehicle and UPS system designs typically depict an energy storage device, such as chemical batteries, flywheels, ultra-capacitors, SMES, etc., which stores energy from a wall-socket or generated by diesel, turbine, fuel cells, etc. Energy is discharged by the energy storage device to a load through a Very Expensive electronic motor controlling device. Modern Technology Flywheel designers are force to use high speed servo motors and controllers that take 3 phase electricity, rectify it to DC, and inverted the DC into a variable frequency, variable voltage, 3 phase AC; a very expensive process costing $25/kw
To quantify Very Expensive: The electronic power systems (electric motors, invertors, and rectifiers) for a 150 kW Brushless DC Motor propulsion system have been proposed to cost $25/kw retail ($3,750) in large quantities. However in the real world, the typical retail price of a 75 kW, variable frequency, AC motor drive is $11,000 ($147/kw). Controllers for Brush-Type motors used in forklifts are an order of magnitude simpler in function and are readily available for $10/kw retail by Curtis/PMC of Dublin, CA.Thortek will ELIMINATE the electronic systems via a Variable Speed Transmission that will directly spin a synchronous generator at a constant speed (or turn wheels on a bus) without going through an electrical converter.
