1. 1 © A. Kwasinski, 2015 Cyber Physical Power Systems Fall 2015 Microgrids and Smart Grids (part 2)

2. 2 © A. Kwasinski, 2015 Microgrids Energy Storage

3. 3 © A. Kwasinski, 2015 Energy Storage Uses of energy storage devices in microgrids: Power buffer for slow, bad load followers, power sources. Energy supply for renewable energy sources. Stability support Increased 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 in short exchanges.

4. 4 © A. Kwasinski, 2015 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.”

5. 5 © A. Kwasinski, 2015 Power vs. energy delivery profile technologies

6. 6 © A. Kwasinski, 2015 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.

7. 7 © A. Kwasinski, 2015 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]

8. 8 © A. Kwasinski, 2015 Lead-acid batteries Positive electrode: Lead dioxide (PbO 2 ) Negative electrode: Lead (Pb) Electrolyte: Solution of sulfuric acid (H 2 SO 4 ) and water (H 2 O) PbPbO 2 H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O

9. 9 © A. Kwasinski, 2015 Lead-acid batteries PbO 2 H2OH2O H 2 SO 4 Chemical reaction (discharge) Pb 2+ O 2 2- 2H + SO 4 2- 2e - Pb 2+ Pb SO 4 2- 2H + H 2 SO 4 PbSO 4 2e - PbSO 4 H2OH2O H2OH2O 2H 2 O H2OH2OH2OH2O

10. 10 © A. Kwasinski, 2015 Lead-acid batteries PbO 2 + 4H + + 2e - Pb 2+ + 2H 2 O 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 Pb 2+ + 2e- 2H 2 SO 4 4H + + 2SO 4 2- Pb 2+ + SO 4 2- PbSO 4 Pb 2+ + SO 4 2- PbSO 4v Pb + PbO 2 + H 2 SO 4 2- 2PbSO 4 + 2H 2 O

11. 11 © A. Kwasinski, 2015 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

12. 12 © A. Kwasinski, 2015 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. http://polarpowerinc.com/info/operation20/operation25.htm

13. 13 © A. Kwasinski, 2015 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. http://polarpowerinc.com/info/operation20/operation25.htm

14. 14 © A. Kwasinski, 2015 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.

15. 15 © A. Kwasinski, 2015 Lead-acid batteries efficiency Consider that during the charge you apply a constant current I C, a voltage V C during a time ΔT C. 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: E in = I C V C ΔT C Now the battery is discharged with a constant current I D, a voltage V D during a time ΔT D. The final state of charge coincides with the original state of charge. Then the energy delivered by the battery during this process is: E out = I D V D ΔT D 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

16. 16 © A. Kwasinski, 2015 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

17. 17 © A. Kwasinski, 2015 Li-ion batteries Positive electrode: Lithiated form of a transition metal oxide (lithium cobalt oxide-LiCoO 2 or lithium manganese oxide LiMn 2 O 4 ) Negative electrode: Carbon (C), usually graphite (C 6 ) Electrolyte: solid lithium-salt electrolytes (LiPF 6, LiBF 4, or LiClO 4 ) and organic solvents (ether) http://www.fer.hr/_download/repository/Li-ION.pdf discharge

18. 18 © A. Kwasinski, 2015 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

19. 19 © A. Kwasinski, 2015 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)

20. 20 © A. Kwasinski, 2015 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

21. 21 © A. Kwasinski, 2015 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

22. 22 © A. Kwasinski, 2015 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

23. 23 © A. Kwasinski, 2015 Battery technologies Cobasys: “Inside the Nickel Metal Hydride Battery”

24. 24 © A. Kwasinski, 2015 Electric vs. Magnetic energy storage Consider that we compare technologies based on energy density (J/m 3 ) 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) University of Illinois at Urbana-Champaign ECE 468 (Spring 2004)

25. 25 © A. Kwasinski, 2015 Ultracapacitors Capacitors store energy in its electric field. In ideal capacitors, the magnitude that relates the charge generating the electric field and the voltage difference between two opposing metallic plates with an area A and at a distance d, is the capacitance: In ideal capacitors: Equivalent model of real standard capacitors:

26. 26 © A. Kwasinski, 2015 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

27. 27 © A. Kwasinski, 2015 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 layer capacitor (ultracapacitor) Ultracapacitor with carbon nano-tubes electrodes

28. 28 © A. Kwasinski, 2015 Some typical Maxwell’s 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

29. 29 © A. Kwasinski, 2015 Comparison with other capacitor technologies Ultracapacitors www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultracapacitor_Equivalent_Circuit_Model.pdf

30. 30 © A. Kwasinski, 2015 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

31. 31 © A. Kwasinski, 2015 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 cell’s life. Ultracapacitors Linzen, et al., “Analysis and Evaluation of Charge-Balancing Circuits on Performance, Reliability, and Lifetime of Supercapacitor Systems”

32. 32 © A. Kwasinski, 2015 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

33. 33 © A. Kwasinski, 2015 Flywheels Energy is stored mechanically (in a rotating disc) Flywheels Energy Systems Motor Generator

34. 34 © A. Kwasinski, 2015 Flywheels Kinetic energy: where I is the moment of inertia and ω is the angular velocity of a rotating disc. For a cylinder the moment of inertia is So the energy is increased if ω increases or if I increases. 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.

35. 35 © A. Kwasinski, 2015 Still, 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 (because P = Tω ) 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 ( E u ) proportional to the difference between the disk energy at its maximum and minimum allowed speed is compared with the maximum allowed energy ( E max ) then Flywheels Bernard et al., Flywheel Energy Storage Systems In Hybrid And Distributed Electricity Generation VrVr VrVr E u /E max

36. 36 © A. Kwasinski, 2015 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

37. 37 © A. Kwasinski, 2015 Power Electronic Interfaces

38. 38 © A. Kwasinski, 2015 Power electronic interfaces Power electronic converters provide the necessary adaptation functions to integrate all different microgrid components into a common system.

39. 39 © A. Kwasinski, 2015 Power electronic interfaces Integration needs: Component with different characteristics: dc or ac architecture. Sources, loads, and energy storage devices output. Control issues: Stabilization Operational issues: Optimization based on some goal Efficiency (e.g. MPPT) Flexibility Reliability Safety Other issues: Interaction with other systems (e.g. the main grid)

40. 40 © A. Kwasinski, 2015 Power electronics basics Types of interfaces: dc-dc: dc-dc converter ac-dc: rectifier dc-ac: inverter ac-ac: cycloconverter (used less often) Power electronic converters components: Semiconductor switches: Diodes MOSFETs IGBTs SCRs Energy storage elements Inductors Capacitors Other components: Transformer Control circuit

41. 41 © A. Kwasinski, 2015 Power electronics basics Types of interfaces: dc-dc: dc-dc converter ac-dc: rectifier dc-ac: inverter ac-ac: cycloconverter (used less often) Power electronic converters components: Semiconductor switches: Diodes MOSFETs IGBTs SCRs Energy storage elements Inductors Capacitors Other components: Transformer Control circuit Diode MOSFET IGBT SCR

42. 42 © A. Kwasinski, 2015 Power electronics basics dc-dc converters Buck converter Boost converter Buck-boost converter

43. 43 © A. Kwasinski, 2015 Power electronics basics Rectifiers RectifierFilter tt t v v v

44. 44 © A. Kwasinski, 2015 Power electronics basics Inverters dc to ac conversion Several control techniques. The simplest technique is square wave modulation (seen below). The most widespread control technique is Pulse-Width-Modulation (PWM).

45. 45 © A. Kwasinski, 2015 Smart grid basics

46. 46 © A. Kwasinski, 2015 Smart grids There are two similar but not equal approaches to the smart grid concept. EU-led vision (customer and environmentally driven): Europe’s electricity networks in 2020 and beyond will be: Flexible: Fulfilling customers’ needs whilst responding to the changes and challenges ahead; Accessible: Granting connection access to all network users, particularly for renewable energy sources and high efficiency local generation with zero or low carbon emissions; Reliable: Assuring and improving security and quality of supply, consistent with the demands of the digital age; Economic: Providing best value through innovation, efficient energy management and ‘level playing field’ competition and regulation. “European Technology Platform SmartGrids. Vision and Strategy for Europe’s Electricity Networks of the Future” European Commission KI-NA-22040-EN-C EUR 22040

47. 47 © A. Kwasinski, 2015 Smart grids US led vision (security and consumer driven) - Motivated by needs in availability improvements “The NETL Modern Grid Initiative A VISION FOR THE MODERN GRID”, US DOE

48. 48 © A. Kwasinski, 2015 There are many views of what is In reality, a smart grid is not a single concept but rather a combination of technologies and methods intended to modernize the existing grid in order to improve flexibility, availability, energy efficiency, and costs. Smart Grid 1.0: Smart meters Smart Grid 2.0 (“Energy Internet” enabler): advanced autonomous controls, distributed energy storage, distributed generation, and flexible power architectures. Future smart grids: Integration with other infrastructures, IoT The smart grid concept

49. 49 © A. Kwasinski, 2015 Centralized operation and control Passive transmission and distribution. Lack of flexibility Vulnerable Smart grid evolution: Past

50. 50 © A. Kwasinski, 2015 Still primarily centralized control. Limited active distribution network (distributed local generation and storage). Use of virtual storage (demand- response) Addition of communication systems More efficient loads Flexibility issues Somewhat more robust Smart grid evolution: Present/immediate future

51. 51 © A. Kwasinski, 2015 Distributed operation and control Active distribution network (distributed local generation and storage). Integrated communications Advanced more efficient and “smarter” loads Flexible More robust Integration with other infrastructures (e.g. roads) Smart grid evolution: Future

52. 52 © A. Kwasinski, 2015 Smart grids Technologies and concepts: Distributed energy resources (generation and storage) are fundamental parts. They provide the necessary active characteristics to an otherwise passive grid. Advanced and distributed communications. All the grid components (including loads) are able to communicate. The grid operates like a power-Internet (distributed, multiple-redundant, interactive and autonomous). I.e. a Power-Net. Intelligent metering. Policies and regulatory actions. Necessary to achieve integration of all the parts. Inadequate pricing models is a significant barrier to introduce service-based business models (vs. energy-based). Grid modernization.

53. 53 © A. Kwasinski, 2015 DOE view for a smart grid: - “An electrical grid is a network of technologies that delivers electricity from power plants to consumers in their homes and offices.” A Power-Net expands this view based on paradigms from the Internet, thus, being a cyber-physical system. Some features compared with conventional power grids: more reliable, efficient, and flexible. The Power-Net

54. 54 © A. Kwasinski, 2015 The Power-Net Like the Internet, the Power-Net involves diverse and redundant path for the power to flow from distributed generators to users. Its control resides in autonomous distributed agents. Power is generated in distributed generators, usually from alternative or renewable energy sources. Power buffers are included to match generators and loads dynamics. Energy buffers are added to make variable sources dispatchable. Contrary to the Internet, the Power-Net involves a local approach for power interactions.

55. 55 © A. Kwasinski, 2015 Desired Internet features: distributed and autonomous control, diverse information routing and redundant data or application storage, performance degradation instead of full failure, link transmission rate control through temporary data storage in buffers. The Internet Buffer size Link bandwidth Maximum (delay) time

56. 56 © A. Kwasinski, 2015 Key aspect: add distributed generation (fuel cells, microturbines, PV modules, small wind, reciprocating engines) close to the load to make power grids distribution portion an active electric circuit. Autonomous and distributed controls can be implemented with DG. Power vs. Energy buffers: Extending the Internet into Smart grids Ultracapacitors or flywheels (power buffer) Batteries (Energy buffer) Predicted solar radiation on PV module

57. 57 © A. Kwasinski, 2015 Control and communication issues Coordination is needed in order to integrate variable generation sources (such as PV modules) in the grid. Centralized control requires significant communication resources (i.e., large bandwidth spectrum allocation) which in general is not available. The alternative is to provide all nodes with an autonomous control that allows controlling power interactions with the grid without dedicated communication links. These more intelligent nodes become agents. VS.

58. 58 © A. Kwasinski, 2015 In the past, several issues were identified in conventional power grids that affect their availability, particularly during natural disasters. Conventional power grids were shown to be very fragile systems. Some of the issues found in conventional power grids include: Primarily centralized control and power distribution architecture. Passive power distribution grid Lack of redundancy in most sub-transmission and distribution paths. Difficulties in integrating meaningful levels of energy storage. Power supply issues during disasters is a grid’s problem transferred to the load. Power Supply Resilience

59. 59 © A. Kwasinski, 2015 Smart grid planning for disaster resiliency must consider disaster impact on lifelines. During disasters special attention should be paid to dissimilar ways in which disasters affect different distributed generation (DG) technologies. Renewable sources do not have lifelines but they are not dispatchable, they are expensive, and they require large footprints. Most DG technologies have availabilities lower than that of the grid. DG needs diverse power supply in order to achieve high availabilities. DG provides a technological solution to the vulnerable availability point existing in air conditioners power supply. DG provides the active component to grid’s distribution portion, essential for advanced self-healing power architectures. Loads are a valuable asset that can by used to improve resilience. Power Supply Resilience

60. 60 © A. Kwasinski, 2015 Lifeline dependencies can be reduced by extended local energy storage. Lifeline’s effects on availability can be mitigated with diverse local power generation. PVs and wind do not require lifelines but their variable profile leads to added DG or extensive local energy buffers. Performance degradation: voltage regulation or selective load shedding. Advanced (active) distribution through power routing interfaces Extending the Internet into Smart grids

61. 61 © A. Kwasinski, 2015 A hybrid ac (solid lines) and dc (doted lines) architecture with both centralized and distributed generation resources. Advanced Power Architectures Monitoring points Power routers

62. 62 © A. Kwasinski, 2015 Problem: Typical home peak power consumption is below 5 kW. An electric vehicle may require 1 kW to be charged in 8 hrs. or up to 8 kW for shorter charging profiles. Also, PEV and PHEV penetration is not uniform (higher for neighborhoods with higher economical household income). Hence, grid’s distribution transformers can be easily overloaded PEV and PHEV even if charging is done during nighttime. DG avoids overloading distribution transformers but economical issues still need to be addressed Combination of DG and energy storage may be a suitable solution. Need for calculating a spatially moving demand. Smart grids: PHEV and PEV integration

63. 63 © A. Kwasinski, 2015 FUEL CELL ENERGY STORAGE PV MODULES AIR CONDITIONER REFRIGERATOR (LOAD) WIND GENERATOR MAIN DC BUS EPA 430-F-97-028 Loads control In the IoT loads can be controlled to improve efficiency, but how to (globally) optimize this and how to control this? Other issues: protection coordination. LED LIGHTS (DC) ELECTRIC VEHICLE