One possible way of electric energy storage is the flywheel electrical drive, which stores the energy in kinetic form.
The flywheel storage uses the EL kinetic energy of a mass with qL inertia rotating with wL angular speed. The maximal kinetic energy corresponds to the maximal speed:
(12.1)
If the kth part of the ELmax energy should be utilised, then:
(12.2)
(12.3)
Usual practical values are: k=0.75, wLmin=0.5wLmax.
Fig.12.1. Modern flywheel drive. a. Block scheme, b. Operation range.
Fig.12.1. Modern flywheel drive. c. Fully utilising the limits.
The kinetic energy can be modified by the mL torque of the flywheel‟s drive, i.e. by its pL power:
(12.4)
During deceleration (decreasing wL, discharging) energy is withdrawn, during acceleration (increasing wL, charging) energy is supplied to the flywheel. In a modern flywheel drive (Fig.12.1.a.) L is the flywheel, Á is the gearbox, VG is the electrical driving machine, TE is the power electronic circuit, H is the electric grid, q is the
Applications
power flow. The VG electric machine is in motor mode at pL>0 (charging), and in generator mode at pL<0 (discharging). The modern, low-loss applications are gearless, they use direct drive.
The usual operation range of the TE-VG electric drive is given in Fig.12.1.b. on the wL-mL plane. In the wLmin£ωL£wLmax operation range the maximal power is +PLmax at charging and –;PLmax at discharging. The maximal driving torque of the drive is MLmax=PLmax/ωLmin, the maximal brake torque is –MLmax. It can be established, that the flywheel drive is a mono-directional two quadrant drive, and its normal operation range is the field weakening.
The nominal point of the drive should be selected to point 2: MLn=MLmax, wLn=wLmin and PLn=MLnwLn=PLmax. Fig.2.1.c. shows that case, when the limits are fully used, when the pL power pulsates in the range ±PLmax with 2DT periodicity and the energy is changing between ELmin and ELmax linearly. It can be derived for DT:
(12.5.a,b)
Where TLin is the nominal stating time of the drive.
The principal task of the flywheel drive is to compensate (smooth) the pulsating electric power. The control of the cage rotor induction machine (Fig.7.7) driven flywheel drive is described as an example. One possible block scheme of the control loop to compensate the power pulsation is presented in Fig.12.2.
Fig.12.2. The block scheme of the control loop of the cage rotor induction machine driven flywheel drive connected to the three-phase grid.
The pulsating power of the G consumer or generator should be compensated. From the measured pG
instantaneous value the SZ filter provides the mean value (pGk) and the difference of these two powers sets the electric power reference of the flywheel drive (pLGa):
(12.6)
From pLGa and wL unit MA provides a torque reference:
(12.7.a)
(12.7.b)
Where pLGa-pLv is the mechanical power of the drive, pLv is the wL dependent loss of the drive, mLv=pLv/ωL is the corresponding torque. The mLv motor mode torque is necessary to keep the wL angular speed constant. Instead of the MA torque set point element power controller also can be used, but the power of the flywheel drive (pL) must be measured too in this case. From the wL and mLa signals the FA block provides the rotor flux reference of AL machine. It mainly depends on the speed:
Applications
(12.8)
Where Yrn is the nominal rotor flux. The machine-side SZÁLG current vector controller controls the torque and the flux of the AL induction machine by the ÁLG converter. The control can be implemented by field-orineted control (see chapter 5.3.1.). From the flux reference (yLa) and the torque reference (mLa) the current components references can be calculated in the field reference frame:
(12.9)
These are constrained by the SZÁLG current vector controller.
The grid-side voltage controller (SZULE) controls the DC voltage (uLe) by its active power reference (pLHa). The reference of the reactive power (qLHa) is determined by external grid demands. The SZÁLH current vector controller controls the active and reactive power of the flywheel drive by the ÁLH converter. The line-oriented current vector control can be implemented according to chapter 7.1.1. From the active and reactive power references (pLHa and qLHa) the current components references can be calculated:
(12.10)
These are constrained by the SZÁLH current vector controller.
The structure of a permanent magnet synchronous machine driven flywheel drive is similar. The block scheme of the double-fed induction machine driven flywheel drive is different because of the missing field weakening possibility.
The block scheme of the control (Fig.12.2.) does not contain the initial charging part (the starting and acceleration form zero speed).
As an example, the compensation of a sinusoidally pulsating pG power is demonstrated in Fig.12.3. in per-unit system. The amplitude of the pulsation is set to such a value for k=0.75, which results in reaching the speed (wLmin and wLmax) and power (±PLmax) limits. At the beginning of the power pulsation compensation the speed of the flywheel is set to such a value, which results in symmetrical compensation reserve. The corresponding values are (wLn=wLmin):
(12.11)
For k=0.75: , ELk=2,5ELmin, ELmax=4ELmin.
Applications
Fig.12.3. Perfect compensation with reaching the speed and power limits. a. The current vector of the AL induction machine in field reference frame.
Fig.12.3. Perfect compensation with reaching the speed and power limits. b. The torque and speed of AL.
Fig.12.3. Perfect compensation with reaching the speed and power limits. c. The power of the flywheel drive (pL) and the resultant power (pG+pL).
Fig.12.3. Perfect compensation with reaching the speed and power limits. d. The magnitude of the rotor flux vector in AL.
Fig.12.3. Perfect compensation with reaching the speed and power limits. e. The compensation process on the ωL-mL plane.
Applications
The example drive can not compensate perfectly larger amplitude or larger period power pulsation.
Among the practical implementations, the product of Beacon Power can be mentioned (Smart Energy 25). In this product the flywheel is driven by permanent magnet synchronous machine, it rotates in vacuum with magnetically levitated bearing, with 8000-16000rpm (k=0,75). It can provide PLmax=100kW power for 15min, i.e.
ELmax-ELmin=25kWh.