8/8/2019 EE394V DG Fall2008 Week6 Part2
1/22
1 Alexis Kwasinski, 2008
Energy Storage
In the past 2 classes we have discussed battery technologies and how their
characteristics may or may not be suitable for microgrids.
Batteries are suitable for applications where we need an energy delivery
profile. For example, to feed a load during the night when the only source is PV
modules.
However, batteries are not suitable for applications with power delivery
profiles. For example, to assist a slow load-following fuel cell in delivering
power to a constantly and fast changing load.
For this last application, two technologies seem to be more appropriate:
Ultracapacitors (electric energy) Flywheels (mechanical energy)
Other energy storage technologies not discussed in here are superconducting
magnetic energy storage (SMES magnetic energy) and compressed air (or
some other gas - mechanical energy)
8/8/2019 EE394V DG Fall2008 Week6 Part2
2/22
2 Alexis Kwasinski, 2008
Power vs. energy delivery profile technologies
Ragone chart:
More information and charts can be found in Holm et. al., A Comparison of
Energy Storage Technologies as Energy Buffer in Renewable Energy Sources
with respect to Power Capability.
8/8/2019 EE394V DG Fall2008 Week6 Part2
3/22
3 Alexis Kwasinski, 2008
Power vs. energy delivery profile technologies
8/8/2019 EE394V DG Fall2008 Week6 Part2
4/22
4 Alexis Kwasinski, 2008
Electric vs. Magnetic energy storage
Consider that we compare technologies based on energy density (J/m3)
Plot of energy density vs. length scale (distance between plates or air gap):
Hence, magnetic energy storage (e.g. SMES) is effective for large scale
systems (higher power)
[ ] [ ] [ ][ ] Energy Work F d Nm J = = = =
3 3 2[ ]
J Nm N Energy density Pa
m m m= = = =
University of Illinois at Urbana-Champaign
ECE 468 (Spring 2004)
8/8/2019 EE394V DG Fall2008 Week6 Part2
5/22
5 Alexis Kwasinski, 2008
Ultracapacitors
Capacitors store energy in its electric field.
In ideal capacitors, the magnitude that relates the charge generating theelectric field and the voltage difference between two opposing metallic plates
with an areaA and at a distance d, is the capacitance:
In ideal capacitors:
Equivalent model of real standard capacitors:
QC
V
=
AC
d=
2 2
1w
l
ESR RR C
= +
8/8/2019 EE394V DG Fall2008 Week6 Part2
6/22
6 Alexis Kwasinski, 2008
Ultracapacitors technology: construction Double-layer technology
Electrodes: Activated carbon (carbon cloth, carbon black, aerogel carbon,
particulate from SiC, particulate from TiC) Electrolyte: KOH, organic solutions, sulfuric acid.
Ultracapacitors
http://www.ultracapacitors.org/img2/ultraca
pacitor-image.jpg
8/8/2019 EE394V DG Fall2008 Week6 Part2
7/227 Alexis Kwasinski, 2008
Ultracapacitors technology: construction
Key principle: area is increased and distance is
decreased
There are some similarities with batteries but there are
no reactions here.
Ultracapacitors
The charge of ultracapacitors, IEEE
Spectrum Nov. 2007
Traditional standard
capacitor
Double layercapacitor
(ultracapacitor)
Ultracapacitor with carbon
nano-tubes electrodes
AC
d=
8/8/2019 EE394V DG Fall2008 Week6 Part2
8/228 Alexis Kwasinski, 2008
Ultracapacitors technology: construction
Ultracapacitors
www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultracapacitor_Equivalent_Circuit_Model.pdf
8/8/2019 EE394V DG Fall2008 Week6 Part2
9/229 Alexis Kwasinski, 2008
Some typical Maxwells ultracapacitor packages:
At 2.7 V, a BCAP2000 capacitor can store more than 7000 J in the volume of
a soda can.
In comparison a 1.5 mF, 500 V electrolytic capacitor can store less than 200 J
in the same volume.
Ultracapacitors
www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultracapacitor_Equivalent_Circuit_Model.pdf
8/8/2019 EE394V DG Fall2008 Week6 Part2
10/2210 Alexis Kwasinski, 2008
Comparison with other capacitor technologies
Ultracapacitors
www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultracapacitor_Equivalent_Circuit_Model.pdf
8/8/2019 EE394V DG Fall2008 Week6 Part2
11/2211 Alexis Kwasinski, 2008
Charge and discharge: With constant current, voltage approximate a linear variation due to a very
large time constant:
Temperature affects the output (discharge on a constant power load):
Ultracapacitors
www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultr
acapacitor_Equivalent_Circuit_Model.pdf
8/8/2019 EE394V DG Fall2008 Week6 Part2
12/2212 Alexis Kwasinski, 2008
Aging process: Life not limited by cycles but by aging Aging influenced by temperature and cell voltage Overtime the materials degrade, specially the electrolyte Impurities reduce a cells life.
Ultracapacitors
Linzen, et al., Analysis and Evaluation of Charge-Balancing
Circuits on Performance, Reliability, and
Lifetime of Supercapacitor Systems
8/8/2019 EE394V DG Fall2008 Week6 Part2
13/2213 Alexis Kwasinski, 2008
Power electronic interface: It is not required but it is recommended
It has 2 purposes: Keep the output voltage constant as the capacitor discharges (a
simple boost converter can be used) Equalize cell voltages (circuit examples are shown next)
Ultracapacitors
8/8/2019 EE394V DG Fall2008 Week6 Part2
14/2214 Alexis Kwasinski, 2008
Model (sometimes similar to batteries)
Ultracapacitors
Mierlo et al., Journal of Power Sources 128
(2004) 7689
http://www.ansoft.com/leadinginsight/pdf/High
%20Performance%20Electromechanical%20Design/Ultracapacitor%20Distributed%20Model
%20Equivalent%20Circuit%20For%20Power
%20Electronic%20Circuit%20Simulation.pdf
Ultracapacitors for Use in Power Quality and
Distributed Resource Applications, P. P. Barker
8/8/2019 EE394V DG Fall2008 Week6 Part2
15/2215 Alexis Kwasinski, 2008
Flywheels
Energy is stored mechanically (in a rotating disc)
Flywheels Energy
Systems
MotorGenerator
8/8/2019 EE394V DG Fall2008 Week6 Part2
16/2216 Alexis Kwasinski, 2008
http://www.vyconenergy.com
http://www.pentadyne.com
Flywheels
8/8/2019 EE394V DG Fall2008 Week6 Part2
17/2217 Alexis Kwasinski, 2008
Flywheels
Kinetic energy:
whereIis the moment of inertia and is the angular velocity of a rotating disc.
For a cylinder the moment of inertia is
So the energy is increased if increases or ifIincreases.
I can be increased by locating as much mass on the outside of the disc as
possible.
But as the speed increases and more mass is located outside of the disc,
mechanical limitations are more important.
21
2kE I=
2 I r dm=
412
I r a =
8/8/2019 EE394V DG Fall2008 Week6 Part2
18/2218 Alexis Kwasinski, 2008
Flywheels
Disc shape and material: the maximum energy density per mass and the
maximum tensile stress are related by:
Typically, tensile stress has 2 components: radial stress and hoop stress.
max/me K =
8/8/2019 EE394V DG Fall2008 Week6 Part2
19/2219 Alexis Kwasinski, 2008
Since
(1)
and
(2)
and
(3)
then, from (2) and (3)
(4)
So, replacing (1) in (4) it yields
max/me K =
2
" " I r m=
2 2 21 1
2 2me r v= =
21
2kE I=
maxmax
2Kv
=
Flywheels
8/8/2019 EE394V DG Fall2008 Week6 Part2
20/2220 Alexis Kwasinski, 2008
However, high speed is not the only mechanical constraint
If instead of holding output voltage constant, output power is held constant,then the torque needs to increase (becauseP = T) as the speed decreases.
Hence, there is also a minimum speed at which no more power can be
extracted
If
and if an useful energy (Eu) proportional to the difference between the disk
energy at its maximum and minimum allowed speed is compared with the
maximum allowed energy (Emax ) then
Flywheels
max
min
r
vV
v=
2
2
max
1u r
r
E V
E V
=
Bernard et al., Flywheel Energy
Storage Systems In Hybrid And
Distributed Electricity GenerationVr
Vr
Eu
/Em
ax
8/8/2019 EE394V DG Fall2008 Week6 Part2
21/2221 Alexis Kwasinski, 2008
Flywheels
In order to reduce the friction (hence, losses) the disc is usually in a vacuum
chamber and uses magnetic bearings.
Motor / generators are typically permanent magnet machines. There are 2
types: axial flux and radial flux. AFPM can usually provide higher power and
are easier to cool.
Bernard et al., Flywheel Energy
Storage Systems In Hybrid And
Distributed Electricity Generation
Bernard et al., Flywheel Energy Storage Systems In Hybrid And
Distributed Electricity Generation
8/8/2019 EE394V DG Fall2008 Week6 Part2
22/2222
Flywheels Simplified dynamic model
Typical outputs
Flywheels Energy
Systems