1. Microgrid Concepts and Distributed Generation Technologies
ECE 2795
Microgrid Concepts and Distributed Generation Technologies
Spring 2017
Week #6
© A. Kwasinski, 2017

2. 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 , 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 , 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,
Microgrids are electric networks utilizing DR to achieve independent control from a large widespread power grid
Prevailing technologies:
Batteries
Flywheels
Ultracapacitors
© A. Kwasinski, 2017

3. 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.
© A. Kwasinski, 2017

4. Battery technologies Batteries stores energy chemically.
Main technologies:
Lead Acid
Nickel-Cadmium
Nickel-Metal Hydride
Li-ion
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5. Battery technologies
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6. 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.
© A. Kwasinski, 2017

7. 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]
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8. Lead-acid batteries Positive electrode: Lead dioxide (PbO2)
Negative electrode: Lead (Pb)
Electrolyte: Solution of sulfuric acid (H2SO4) and water (H2O)
H2O
PbO2 Pb
H2O
H2O
H2O
H2O
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9. Lead-acid batteries Chemical reaction (discharge) 2H2O H2SO4 2e- 2H+
Pb2+
O22-
2H+ Pb
PbO2
H2SO4
PbSO4
Pb2+
SO42-
2e-
PbSO4
H2O
H2O
H2O
H2O
H2O
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10. Lead-acid batteries Chemical reaction (discharge) 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-
Pb2+ + SO PbSO4
2H2SO H+ + 2SO42-
PbO2 + 4H+ + 2e Pb2+ + 2H2O
Pb2+ + SO PbSO4v
Pb + PbO2 + H2SO PbSO4 + 2H2O
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11. 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
© A. Kwasinski, 2017

12. “A New Dynamic Model for Lead-Acid Batteries”
Lead-acid batteries models
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.
“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
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13. Isamu Kurisawa and Masashi Iwata
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
SONNENSCHEIN
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14. 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.
© A. Kwasinski, 2017

15. 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.
© A. Kwasinski, 2017

16. Tyco Electronics 12IR125 Product Manual
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.
Coup de Fouet
Patent
Battery capacity measurement Anbuky and Pascoe
Tyco Electronics 12IR125 Product Manual
© A. Kwasinski, 2017

17. 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.
© A. Kwasinski, 2017

18. 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 In average the coulomb efficiency is about Hence, the energy efficiency is around 0.80
© A. Kwasinski, 2017

19. 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
10 A continuous discharge curve approximation
Discharge limit
Nominal curve
© A. Kwasinski, 2017

20. 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)
discharge
© A. Kwasinski, 2017

21. Li-ion batteries Chemical reaction (discharge) Positive electrode
Negative electrode
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.
LiCoO Li1-xCoO2 + xLi+ + xe-
Through electrolyte
Through load
xLi+ + xe- + 6C LixC6
LiCoO2 + C Li1-xCoO2 + C6Lx
© A. Kwasinski, 2017

22. 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.
“Advanced Lithium Ion Battery Charger”
V.L. Teofilo, L.V. Merritt and R.P. Hollandsworth
Saft Intensium 3 Li-ion battery
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23. Li-ion batteries Effects of temperature:
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24. Li-ion batteries Controlled charging has 2 purposes:
Limiting the current
Equalizing cells
“Increased Performance of Battery Packs by Active Equalization”
Jonathan W. Kimball, Brian T. Kuhn and Philip T. Krein
“Advanced Lithium Ion Battery Charger”
V.L. Teofilo, L.V. Merritt and R.P. Hollandsworth
Saft Intensium 3 Li-ion battery
© A. Kwasinski, 2017

25. Li-ion batteries Factors affecting life: Charging voltage. Temperature
Age (time since manufacturing)
Degradation process: oxidation
© A. Kwasinski, 2017

26. 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
© A. Kwasinski, 2017

27. Ni-MH 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)
Cobasys batteries
© A. Kwasinski, 2017

28. Ni-MH batteries Chemical reaction (discharge) Positive electrode
Negative electrode
Overall
The electrolyte is not affected because it does not participate in the reaction.
NiO(OH) + H2O + e Ni(OH)2 + OH-
Through electrolyte
Through load
MH + OH M + H2O + e-
NiO(OH) + MH Ni(OH)2 + M
© A. Kwasinski, 2017

29. Saft NHE module battery
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.
Cobasys Nigen battery
Saft NHE module battery
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30. Saft NHE module battery
Ni-MH batteries
Effects of temperature:
Saft NHE module battery
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31. 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
© A. Kwasinski, 2017

32. Ni-Cd 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
Saft batteries
© A. Kwasinski, 2017

33. Ni-Cd batteries Chemical reaction (discharge) Positive electrode
Negative electrode
Overall
The electrolyte is not affected because it does not participate in the reaction.
2NiO(OH) + 2H2O + 2e Ni(OH)2 + 2OH-
Through electrolyte
Through load
Cd + 2OH Cd(OH)2 + 2e-
2NiO(OH) + Cd + 2H2O Ni(OH)2 + Cd(OH)2
© A. Kwasinski, 2017

34. 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.
Saft Ultima plus
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35. Ni-Cd batteries Effects of temperature:
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36. 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)
© A. Kwasinski, 2017

37. 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
© A. Kwasinski, 2017

38. Ni-Cd batteries
Comparison with Ni-MH batteries (not much of a difference)
Portable NiCd- and Ni-MH-Batteries for Teiecom Applications
J. Heydecke and H.A. Kiehne
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39. Cobasys: “Inside the Nickel Metal Hydride Battery”
Battery technologies
Cobasys: “Inside the Nickel Metal Hydride Battery”
© A. Kwasinski, 2017