Power Electronics for Grid Scale Energy Storage

Transcription

1 University of Oxford Power Electronics for Grid Scale Energy Storage Getting the most out of your cells Dr Dan Rogers Senior Research Fellow, Department of Engineering Science UKES 2016, Birmingham 1 st December 2016

2 Overview What is power electronics? A quick look at how today s grid-scale storage systems are normally connected to the grid How more power electronics can improve Reliability Accessible system capacity The challenges of control and management of very large numbers of cells Some hardware and results 2

3 What is power electronics? The use of power semiconductor devices to control and convert electrical energy DC source Input filter Power transistors Output filter AC source 3

4 Power electronics for grid storage Battery pack DC-DC converter (optional) DC-AC converter Step up transformer Grid Vdc 700 Vdc 400 Vac 11 kvac Suitable for relatively small systems: essentially a scaled up EV 2.5 V, 20 Ah cells (50 Wh): A 1 MW / 1MWh system needs 20,000 cells If pack voltage is 500 V then pack is 200s100p and DC bus current is 2 ka 10 cm 2 cross section copper wiring and large I 2 R losses throughout the system Monolithic systems of this scale start to become impractical 4

5 Statistical modelling of packs Notation N = number of cells in system c i = capacity of cell i C = capacity of system Ideally we would like N c i C = i=1 But for a naïve series string design N c i C = N min i=1 c i varies from cell to cell and from moment to moment* c i t = c i 0 d i t e i t initial capacity degradation rate (d i > 0) catastrophic failure (1 0 at some t) c i, d i and e i vary from cell-to-cell: we assume normal distributions or parameters in order to model large systems *modelling framework developed in collaboration with Matthias Troffaes (Durham) and Louis Aslett (Oxford) 5

6 The problem with large series packs Cell capacities are fairly tightly distributed at SoL 95% of cells within ±0.1 of mean Cell degradation is relatively slow 10% decrease in mean capacity over lifetime ~90% have more than 0.8 capacity left at EoL Even at SoL, only ~80% of total cell capacity is available to the system It s quite likely we will have only 60% of capacity accessible at EoL end: c EoL end: C EoL start: c 0 start: C 0 6

7 Packing cells into modules A solution to this problem is to break the system up into small modules that are managed individually power electronics provides control over the average current flowing in each module M cells in a module System contains MN cells But module voltage is only MV cell modules should be connected in series N modules in the system 7

8 Benefits of modularising a system The depth of modularisation strongly influences lifetime behaviour Start of life If M = 1 we achieve complete capacity utilisation Choosing M = 5 gives ~20% gain in EoL capacity Modularisation gives more predictable EoL capacity This assumes no random complete failure mechanism! End of life From no modularisation to using M = 5 modules 8

9 More benefits of modularisation Adding random failure makes modularisation even more attractive Here, probability of cell failure is i.e. we expect about 20 cells to fail over the lifetime of the 20,000 cell system Of course, in a real system, maintenance will replace failed cells But system must not suffer downtime as a result of a single cell (or power electronics) failure Modularisation of some sort is required Start of life End of life M > 500 produces very unpredictable system EoL capacity 9

10 Option 1 Conventional battery pack Power block Np Np Cell 1 Ns.1 Ns.Np Step-up transformer Cell 2 Grid Cell Nc Power block 2 Power block M 10

11 Option 2 Intelligent battery pack Np Mediumvoltage converter Np Grid Ns.1 Ns.Np 11

12 Option 3 Cascaded H-Bridge Phase A Grid 1 2 Phase B Phase C Nhb N 12

13 Markov models for reliability modelling Model a system as composed of many components The model includes all possible system states coupled by transitions that occur as components fail Chain terminates at system failure E.g. inability to delivery the rated power or energy Driven by loss of cells themselves, or loss of access to cells (e.g. because of power converter failure) Assumptions and limitations Failure rate of components is constant in a state But failure rates can change between states E.g. as components fail, other components work harder 13

14 Comparing reliability of options 1, 2 and 3 System MTTF (hours) Constant temperature Including thermal model Higher cell reliability Higher relative electronics reliability (or cells age) Cell failure rate/base failure rate 14

15 Hardware Lithium titanate 20Ah cells (50 Wh) 12 cells in a 3U case (600Wh) 144 cells in a rack (7.2kWh) 15

16 System architecture 16

17 Output waveforms Output voltage Grid current 17

18 Distributed balancing The CHB circuit provides free balancing at the cell level as well as DC to AC conversion Balance cells in a module Balance modules in a bank Balance banks in a system This limits the exchange of information at the cell level c.f. trying to manage 20,000 cells from a central controller Power hardware is flat but communication and management functions are layered 18

19 Balancing video 19

20 Conclusion Cell-based energy storage systems need a power electronic system of one form or another to exchange power with the grid Fundamentally to provide bidirectional DC-AC conversion Perform balancing and deal with cell failure [typical cell voltage] <<< [grid connection voltage] Massive series strings are a bad idea There are more intelligent ways to organise a system that give better cell capacity utilisation and much high reliability Some power electronics circuits provide direct AC synthesis Distributed rather than centralised (or monolithic) converters There are lots of interesting algorithmic challenges attached to large storage systems 20

21 Related publications E. Chatzinikolaou; D. J. Rogers, "A Comparison of Grid-connected Battery Energy Storage System Designs," in IEEE Transactions on Power Electronics, 2016 (in press). doi: /TPEL E. Chatzinikolaou and D. J. Rogers, "Cell SoC Balancing Using a Cascaded Full-Bridge Multilevel Converter in Battery Energy Storage Systems," in IEEE Transactions on Industrial Electronics, vol. 63, no. 9, pp , Sept doi: /TIE E. Chatzinikolaou and D. J. Rogers, "Electrochemical cell balancing using a full-bridge multilevel converter and pseudo-open circuit voltage measurements," 8th IET International Conference on Power Electronics, Machines and Drives (PEMD 2016), Glasgow, 2016, pp doi: /cp C. A. Ooi, D. J. Rogers, and N. Jenkins, Balancing control for grid-scale battery energy storage system in Proceedings of the ICE Energy, vol. 168:2, pp , doi /ener Whitepaper: UK Research Needs In Grid Scale Energy Storage Technologies (Brandon et al.), 21