Flywheels represent a purely kinetic means of mechanical
energy storage that leverages the rotational inertia embodied in a large
spinning flywheel. Motors often spin the flywheel until they reach their
operating angular velocity (Figure
1).
At this point small amounts of energy are required to keep the flywheel
spinning. To produce electricity the flywheel spins the motor (which now acts
as a generator) until power is brought back into the flywheel or the flywheel
stops spinning. Flywheels were historically characterized by massive wheels
spinning on shafts physically connected to bearings. Traditionally they were
prone to catastrophic, sometimes fatal failures with somewhat short term
output. Modern flywheels feature more compact designs with greater capacity.
They are constructed with compact composite rotors housed in vacuum enclosures,
spinning on frictionless magnetic bearings (Figure
1) (Abele,
Elkind, Intrator, & Washom, 2011) . Despite the
improvements in the safety and performance of a modern flywheel, they are still
often covered with a protective shield designed to contain the unit in case of
a failure.
Flywheels have been used for
thousands of years to store energy. Some early examples include potter’s
wheels, grinding stones and the large flywheels on steam engines. In all of these
applications, the energy from variable or pulse sources of power is balanced to
provide a continuous output using the inertia of the spinning mass of the
flywheel. This steady or continuous nature of the output power make modern
flywheels a natural fit for power quality applications and UPS applications (Abele, Elkind, Intrator, & Washom, 2011) . The ability to charge using highly variable pulses of power,
in the form of kinetic or electrical energy, makes flywheels an ideal candidate
for regenerative power applications such as industrial cranes and light rail (Baxter,
2006) .
Flywheels are
tolerant of a wide range of environmental conditions and can be deeply (even
completely) discharged over a large number of cycles. They are also
characterized by a long service life with corporations producing flywheels
offering product warranties in excess of 20 years. The power of a flywheel is
directly related to the mass of the rotor; however the power increases to the
square of the rotational surface speed of the rotor. For this reason modern flywheels
have lighter composite rotors that spin at very high speeds, potentially up to
100,000 RPM, but more commonly in the range of 20,000 to 60,000 RPM (Baxter,
2006) . With
a direct connection between the motor/generator, lighter rotors, spinning at
high speed, give flywheels a very fast response time that rivals that of
batteries. These high speed rotors also make flywheels more appropriate for
applications requiring short duration bursts of power. Individual flywheels can
produce power in the 100kW range with a discharge duration measured in minutes.
A common strategy is to combine a large number of flywheels to increase the
power output. Flywheels are estimated to have a cost of $459/kW, however the
life cycle costs of a flywheel can be very low (Baxter,
2006) .
The
reliability, fast response times and low operating costs of flywheels make them
very appealing; however the low discharge duration is considered a significant
limitation of flywheel technology. This short discharge duration has the effect
of limiting the applicability of flywheels to support renewable energy
generation. For this reason recent research activities have centered on
increasing the energy output of flywheels to allow them to be used for
applications such as renewable firming, arbitrage or even load shifting. One
example is the 2010 ARPA-E contract signed with Beacon Power (a Massachusetts
based flywheel manufacturer) to develop long discharge duration flywheels with
the goal of supporting renewable energy installations (GlobeNewswire, 2010) . In the following
year Beacon Power applied for bankruptcy protection, forcing Beacon Power to
shift from a flywheel research and development role to a company that owns and
operates flywheels to provide power quality applications to utilities. Recently,
on September 11, 2013, Beacon Power began providing frequency regulation
services to Pennsylvania’s PJM Interconnection with a flywheel energy storage
plant currently rated at 4 MW. The plant is expected to be built out to reach a
power rating of 20 MW by the second quarter of 2014 (Beacon Power, 2013) . Though recent investment
is clearly focused on power applications, it should be noted that other
companies such as Temporal Power, in Ontario Canada, are continuing to innovate
to develop flywheels that can be used for energy applications.
Works Cited
Abele, A., Elkind, E., Intrator, J., & Washom, B.
(2011). 2020 Strategic Analysis of Energy Storage in California. Los
Angeles: California Energy Commission. Publication Number: CEC-500-2011-047.
Baxter, R. (2006). Energy Storage; A Nontechnical
Guide. Tulsa, Oklahoma: PennWell Corporation.
Beacon Power. (2013, September 18). Beacon Power
LLC Begins Commercial Operation at Flywheel Energy Storage Plant in
Pennsylvania. Retrieved October 16, 2013, from www.beaconpower.com:
http://www.beaconpower.com/files/pr/Hazle4_MW_release_FINAL.pdf
GlobeNewswire. (2010, September 27). Beacon Power
Signs ARPA-E Contract to Begin Developing Long-Duration Flywheel for Renewable
Energy Integration. Retrieved January 25, 2013, from NASDAQ OMX,
GlobeNewswire: http://globenewswire.com/news-release/2010/09/27/430303/202351/en/Beacon-Power-Signs-ARPA-E-Contract-to-Begin-Developing-Long-Duration-Flywheel-for-Renewable-Energy-Integration.html
Molina, M. G. (2010). Dynamic Modelling and
Control Design of Advanced Energy Storage for Power System Applications.
San Juan: Argentinean National Research Council for Science.
No comments:
Post a Comment