© a. kwasinski, 2014 ece 2795 microgrid concepts and distributed generation technologies spring...

© A. Kwasinski, 2014

ECE 2795

Microgrid Concepts and Distributed Generation Technologies

Spring 2015

Week #4


Energy Storage• Distributed resources (DR) and distributed generation (DG):

• DG can be defined as “a subset of DR” [T. Ackermann, G. Andersson, and L. Söder, “Distributed

generation: A definition.” Electric Power Systems Research, vol. 57, issue 3, pp. 195-204, April 2001]• DR are “sources of electric power that are not directly connected to a bulk power transmission system. DR includes both generators and energy storage technologies” [T. Ackermann, G. Andersson, and L. Söder, “Distributed generation: A definition.”

Electric Power Systems Research, vol. 57, issue 3, pp. 195-204, April 2001]• DG “involves the technology of using small-scale power generation technologies located in close proximity to the load being served” [J. Hall, “The new distributed generation,” Telephony Online, Oct. 1, 2001

http://telephonyonline.com/mag/telecom_new_distributed_generation/]• Microgrids are electric networks utilizing DR to achieve independent control from a large widespread power grid

• Prevailing technologies: • Batteries• Flywheels• Ultracapacitors


Energy Storage

• Uses of energy storage devices in DG (focus is on elements with electrical output):

• Power buffer for slow, bad load followers, DG technologies.• Energy supply for stochastic generation profiles.• Improved availability

• Power vs. Energy

• Power delivery profile: short, shallow and often energy exchanges.• Flywheels• Ultracapacitors

• Energy delivery profile: long, deep and infrequent energy exchanges.• Batteries

• For the same energy variation, power is higher during short exchanges.

dEP

dt


Battery technologies• Batteries stores energy chemically. •Main technologies:

• Lead Acid• Nickel-Cadmium• Nickel-Metal Hydride• Li-ion


Battery technologies


Lead-acid batteries• Lead-acid batteries are the most convenient choice based on cost. The technology that most of the users love to hate.

• Lead-acid batteries are worse than other technologies based on all the other characteristics. Disposal is another important issue.

• In particular, lead-acid batteries are not suitable for load-following power buffer applications because their life is significantly shortened when they are discharged very rapidly or with frequent deep cycles.

http://polarpowerinc.com/info/operation20/operation25.htm


Lead-acid batteries life• Lead-acid batteries are very sensitive to temperature effects. It can be expected that battery temperature exceeding 77°F (25°C) will decrease expected life by approximately 50% for each 18°F (10°C) increase in average temperature. [Tyco Electronics IR125 Product Manual]


Lead-acid batteries• Positive electrode: Lead dioxide (PbO2)

• Negative electrode: Lead (Pb)

• Electrolyte: Solution of sulfuric acid (H2SO4) and water (H2O)

PbPbO2

H2O

H2O

H2O

H2O H2O


Lead-acid batteries

PbO2

H2O

H2SO4

• Chemical reaction (discharge)

Pb2+

O22-

2H+

SO42-

2e-

Pb2+

PbSO4

2-

2H+

H2SO4

PbSO4

2e-

PbSO4

H2O

H2O

2H2O

H2OH2O


Lead-acid batteries

PbO2 + 4H+ + 2e- Pb2+ + 2H2O


• Negative electrode

• Electrolyte

• Positive electrode

•Overall

• The nominal voltage produced by this reaction is about 2 V/cell. Cells are usually connected in series to achieve higher voltages, usually 6V, 12 V, 24 V and 48V.

Pb Pb2+ + 2e-

2H2SO4 4H+ + 2SO42-

Pb2+ + SO42- PbSO4

Pb2+ + SO42- PbSO4v

Pb + PbO2 + H2SO42- 2PbSO4 + 2H2O


Lead-acid batteries• As the battery discharges, sulfuric acid concentration decreases.

• At the same time, lead sulfate is deposited on the electrode plates.

• Charging follows the inverse process, but a small portion of the lead sulfate remains on the electrode plates.

• Every cycle, some more lead sulfate deposits build up on the electrode plates, reducing the reaction area and, hence, negatively affecting the battery performance.

• Electrode plates sulfatation is one of the primary effects that affects battery life.

• To avoid accelerating the sulfatation process, batteries need to be fully charged after every discharge and they must be kept charged at a float voltage higher than the nominal voltage. For lead acid batteries and depending their technology the float voltage is between 2.08 V/Cell and 2.27 V/cell. For the same reasons, lead-acid batteries should not be discharged below 1.75 V/cell


• All models imply one issue when connecting batteries of different capacity in parallel: since the internal resistances depend on the capacity, the battery with the lower capacity may act as a load for the battery with the higher capacity.

Lead-acid batteries models

“A New Battery Model for use with Battery Energy Storage Systems and Electric Vehicles Power Systems”

H.L. Chan, D. Sutanto

“A New Dynamic Model for Lead-Acid Batteries”N. Jantharamin, L. Zhangt


Lead-acid batteries models

• Most circuit parameters depend on:• State of charge

• Charge / Discharge rate

• Temperature

“Internal Resistance and Deterioration of VRLA Battery - Analysis of Internal Resistance obtained by Direct Current

Measurement and its application to VlRLA Battery Monitoring Technique”

Isamu Kurisawa and Masashi Iwata

http://www.mhpower.com.au/images/tecfig23.gifSONNENSCHEIN

http://www.mhpower.com.au/images/tecfig23.gif


Lead-acid batteries capacity• Battery capacity is often measured in Ah (Amperes-hour) at a given discharge rate (often 8 or 10 hours).

• Due to varying internal resistance the capacity is less if the battery is discharged faster (Peukert effect)

• Lead-acid batteries capacity ranges from a few Ah to a few thousand Ah.



Lead-acid batteries capacity• Battery capacity changes with temperature.

• Some manufacturers of battery chargers implement algorithms that increase the float voltage at lower temperatures and increase the float voltage at higher temperatures.



Lead-acid batteries discharge

• The output voltage changes during the discharge due to the change in internal voltage and resistances with the state of charge.

Tyco Electronics 12IR125 Product Manual

Coup de Fouet

Patent 6924622 Battery capacity measurement Anbuky and Pascoe


Lead-acid batteries charge• Methods:

• Constant voltage• Constant current• Constant current / constant voltage

• Cell equalization problem: as the number of cells in series increases, the voltage among the cells is more uneven. Some cells will be overcharged and some cells will be undercharged. This issue leads to premature cell failure

• As the state of charge increases, the internal resistance tends to decrease. Hence, the current increases leading to further increase of the state of charge accompanied by an increase in temperature. Both effects contribute to further decreasing the internal resistances, which further increases the current and the temperature….. This positive feedback process is called thermal runaway.


Lead-acid batteries efficiency

• Consider that during the charge you apply a constant current IC, a voltage VC during a time ΔTC. In this way the battery goes from a known state of charge to be fully charged. Then the energy transferred to the battery during this process is:

Ein = ICVC ΔTC

• Now the battery is discharged with a constant current ID, a voltage VD during a time ΔTD. The final state of charge coincides with the original state of charge. Then the energy delivered by the battery during this process is:

Eout = IDVD ΔTD

• So the energy efficiency is

• Hence, the energy efficiency equals the product of the voltage efficiency and the Coulomb efficiency. Since lead acid batteries are usually charged at the float voltage of about 2.25 V/cell and the discharge voltage is about 2 V/cell, the voltage efficiency is about 0.88. In average the coulomb efficiency is about 0.92. Hence, the energy efficiency is around 0.80

D D DE V C

C C C

V I T

V I T


Lead-acid batteries calculations

• Most calculations are based on some specific rate of discharge and then a linear discharge is assumed.•The linear assumption is usually not true. The nonlinearity is more evident for faster discharge rates. For example, in the battery below it takes about 2 hours to discharge the battery at 44 A but it takes 4 hours to discharge the battery at 26 A. Of course, 26x2 is not 44.• A better solution is to consider the manufacturer discharge curves and only use a linear approximation to interpolate the appropriate discharge curve.• In the example below, the battery can deliver 10 A continuously for about 12 hours. Since during the discharge the voltage is around 12 V, the power is 120 W and the energy is about 14.5 kWh

Discharge limit

Nominal curve

10 A continuous discharge curve approximation


Li-ion batteries• Positive electrode: Lithiated form of a transition metal oxide (lithium cobalt oxide-LiCoO2 or lithium manganese oxide LiMn2O4)

• Negative electrode: Carbon (C), usually graphite (C6)

• Electrolyte: solid lithium-salt electrolytes (LiPF6, LiBF4, or LiClO4) and organic solvents (ether)

http://www.fer.hr/_download/repository/Li-ION.pdf

discharge


Li-ion batteries• Chemical reaction (discharge)



•Overall

• In the above reaction x can be 1 or 0

• With discharge the Co is oxidized from Co3+ to Co4+. The reverse process (reduction) occurs when the battery is being charged.

LiCoO2 Li1-xCoO2 + xLi+ + xe-

xLi+ + xe- + 6C LixC6

Through electrolyte

Through load

LiCoO2 + C6 Li1-xCoO2 + C6Lx


Li-ion batteries

• Contrary to lead-acid batteries, Li-ion batteries do not accept well a high initial charging current.• In addition, cells in a battery stack needs to be equalized to avoid falling below the minimum cell voltage of about 2.85 V/cell.• Thus, Li-ion batteries need to be charged at least initially with a constant-current profile. Hence they need a charger• Typical float voltage is above 4 V (typically 4.2 V). Lower than nominal float voltages reduce capacity but improves lifetime.

Saft Intensium 3 Li-ion battery“Advanced Lithium Ion Battery Charger”

V.L. Teofilo, L.V. Merritt and R.P. Hollandsworth


http://www.gpbatteries.com/html/pdf/Li-ion_handbook.pdf

• Effects of temperature:

Li-ion batteries


Li-ion batteries

Saft Intensium 3 Li-ion battery“Advanced Lithium Ion Battery Charger”

V.L. Teofilo, L.V. Merritt and R.P. Hollandsworth

“Increased Performance of Battery Packs by Active Equalization”Jonathan W. Kimball, Brian T. Kuhn and Philip T. Krein

• Controlled charging has 2 purposes:• Limiting the current• Equalizing cells


Li-ion batteries

• Factors affecting life:• Charging voltage.• Temperature• Age (time since manufacturing)

• Degradation process: oxidation


Li-ion batteries• Advantages with respect to lead-acid batteries:

• Less sensitive to high temperatures (specially with solid electrolytes)• Lighter (compare Li and C with Pb)• They do not have deposits every charge/discharge cycle (that’s why the efficiency is 99%)• Less cells in series are need to achieve some given voltage.

• Disadvantages:• Cost


Ni-MH batteries

Cobasys batteries

• Negative electrode: Metal Hydride such as AB2 (A=titanium and/or vanadium, B= zirconium or nickel, modified with chromium, cobalt, iron, and/or manganese) or AB5 (A=rare earth mixture of lanthanum, cerium, neodymium, praseodymium, B=nickel, cobalt, manganese, and/or aluminum)

• Positive electrode: nickel oxyhydroxide (NiO(OH))

• Electrolyte: Potassium hydroxide (KOH)


Ni-MH batteries



•Negative electrode

•Overall

• The electrolyte is not affected because it does not participate in the reaction.

NiO(OH) + H2O + e- Ni(OH)2 + OH-

MH + OH- M + H2O + e-

Through electrolyte Through load

NiO(OH) + MH Ni(OH)2 + M


Ni-MH batteries

• It is not advisable to charge Ni-MH batteries with a constant-voltage method. Ni-MH batteries do not accept well a high initial charging current.• Float voltage is about 1.4 V• Minimum voltage is about 1 V.

Saft NHE module battery

Cobasys Nigen battery


http://www.panasonic.com/industrial/battery/oem/images/pdf/panasonic_nimh_overview.pdf


Ni-MH batteries

Saft NHE module battery


Ni-MH batteries• Advantages:

• Less sensitive to high temperatures than Li-ion and Lead-acid• Handle abuse (overcharge or over-discharge better than Li-ion bat

• Disadvantages:• More cells in series are need to achieve some given voltage.• Cost


Ni-Cd batteries

Saft batteries

• Negative electrode: Cadmium (Cd) – instead of the MH in Ni-MH batteries

• Positive electrode: nickel oxyhydroxide (NiO(OH)) – the same than in Ni-MH batteries

• Electrolyte: Potassium hydroxide (KOH) solution


Ni-Cd batteries




•Overall

• The electrolyte is not affected because it does not participate in the reaction.

2NiO(OH) + 2H2O + 2e- 2Ni(OH)2 + 2OH-

Cd + 2OH- Cd(OH)2 + 2e-

Through electrolyteThrough load

2NiO(OH) + Cd + 2H2O 2Ni(OH)2 + Cd(OH)2


Ni-Cd batteries

• It is not advisable to charge Ni-Cd batteries with a constant-voltage method. Ni-Cd batteries do not accept well a high initial charging current, but they can withstand it sporadically.• Float voltage is about 1.4 V• Minimum voltage is about 1 V.

http://www.saftbatteries.com/doc/Documents/telecom/Cube788/tel_tm_en_0704.26962445-6b1b-44fb-aea7-42834c32be40.pdf

Saft Ultima plus



Ni-Cd batteries

http://www.saftbatteries.com/doc/Documents/telecom/Cube788/tel_tm_en_0704.26962445-6b1b-44fb-aea7-42834c32be40.pdf


Ni-Cd batteries

• Due to their better performance at high temperatures, Ni-Cd batteries are replacing Lead-acid batteries in outdoor stationary applications. But, they do not resist hurricanes very well, yet……(AT&T’s DLC at Sabine Pass CO, Saft NCX batteries)


Ni-Cd batteries• Advantages:

• Less sensitive to high temperatures than all the other batteries• Handle some abuse (overcharge or over-discharge better than Li-ion bat)

• Disadvantages:• More cells in series are need to achieve some given voltage.• Cost


Portable NiCd- and Ni-MH-Batteries for Teiecom ApplicationsJ. Heydecke and H.A. Kiehne

Ni-Cd batteries• Comparison with Ni-MH batteries (not much of a difference)


Battery technologies

Cobasys: “Inside the Nickel Metal Hydride Battery”

© a. kwasinski, 2014 ece 2795 microgrid concepts and distributed generation technologies spring...

Documents

energy storage technologies

energy power delivery

energy variation

energy supply

new distributed generation

energy storage devices

sources of electric

infrequent energy exchanges