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Energy storage optimization
A techno-economic analysis of battery chemistries in hybrid microgrids
October 27, 2015
Rebecca Ciez*, Jay Whitacre*†
*Engineering & Public Policy, †Materials Science & Engineering
Energy storage significant in microgrids• Significant component for both functionality and cost
• How do the inherent characteristics of different storage technologies influence the overall cost of electricity from hybrid systems?
• Which technologies are the most cost-effective, and what economic conditions are necessary for mass adoption?
2
Energy Storage
Solar PV Diesel
Generator
Off-Grid Community (lighting, small electronics)1
Maximize utilization
Account for degradatio
n
Variety of factors influence storage performance
• State of Charge (SOC swing)• Temperature • Operational timeframe
3
ChemistryCost per
kWhCycle life
Sensitivity to SoC Swing
Details
Lead-Acid $200($150-$250)
Poor(~500)
HighMost commonly used battery in hybrid microgrid systems
High Power Density Li-ion
(A123 Systems, LiFePO4)
$680($530-$1000)
Excellent(~10,000)
Low High cost, high power density
High Energy Density Li-Ion
(Panasonic, LiNiCoAlO2)
$250($200-$300)
Average(~5,000)
MediumLow cost, high energy density
Battery cycles depend on SOC swing
4
• Existing data for lead acid [Pesaran & Markel, 2007] and High power density li-ion [Peterson et al 2010]
• Cycle testing for high energy density li-ion
Thanks to W. Wu for SEM images
Operational Model Overview
5
Operational Model
Sola
r PV
Load
Diesel Generator
Battery Storage
Actual Cycling Behavior
Storage Requirement
Power Mix
Operational Parameters
Battery technology
Renewable energy requirements
Maximum SOC swing
Operational lifetime (1, 2, 5, 10, 20 years)
Storage capacity required
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Trends in storage capacity
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Short timeframe: •Falling storage requirement as max SOC swing increasesInflection point: •Deep cycling degradation occurs•Add capacity to extend lifetimeVery long timeframe:•Max. SOC swing reached decreases•Load constant, packs much larger•Never approach specified limit SOC swing
Economic Model Overview
8
Operational Model
Sola
r PV
Load
Diesel Generator
Battery Storage
Actual Cycling Behavior
Economic Model
Cost A
ssum
ptio
ns
Repla
cem
ent
Assu
mptio
ns
LCOEStorage
Requirement
Power Mix
Economic Parameters
Fixed 20 year timeframe
Number of battery replacements (every 1, 2, 5, 10, 20 years)
Cost Assumptions
Discount Rate
Levelized cost of electricity tracks with storage capacity
9
More replacements offer lower costs for higher SOC swings
10
Previous graphs show variation in:•Battery chemistry•Maximum SOC swing•Cost assumptions•Number of replacements
Comparing between technologies
11
• 5% discount rate• 75% min.
renewable energy
requirement
High energy density li-ion slightly cheaper than lead acid
Fixed: discount rate and renewable energy requirement
What about other model parameters?
Renewable energy requirement: •Most optimal combinations exceed the minimum target •Max change was 4¢/kWh
12
Discount rate matters: •Low discount rate: lead acid best•Higher rates, high energy density li-ion •High power li-ion always most expensive
Price changes required increase with discount rate
13
• Diesel prices required to induce a switch track with discount rate
• Larger percentage price decreases on batteries for higher discount rates
• High power li-ion still most expensive in absolute terms
14
Feed-in tariffs comparable to current rates at low discount rates4
Carbon taxes would need to be much higher•Typically $10-30/ton•Highest: $168/ton (Sweden)5
Policy interventions track with discount rate
Conclusions• Optimal number of battery replacements dependent on how
deeply batteries are cycled
• At assumed prices, diesel is still less expensive than hybrid systems
• High performance of high-power li-ion doesn’t offset high capital cost
• Discount rate is a significant factor in determining the lowest-cost storage option
• Low discount rates: Lead-acid best• High discount rates: high energy density li-ion best
• Diesel price changes and policy interventions required to induce a switch to hybrid system track with discount rate
• Larger battery price changes required as discount rate increases
15
Acknowledgements
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1252522 . Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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This research was supported by:•National Science Foundation GRFP •Carnegie Mellon University•Bushnell Fellowship in Engineering•Aquion Energy
Thanks to Inês Azevedo for her thoughtful comments and discussions, and to Wei Wu for his assistance with battery cycle testing & SEM imaging.
References
[1] E.M. Nfah, J.M. Ngundam, M. Vandenbergh, J. Schmid, Simulation of off-grid generation options for remote villages in Cameroon, Renewable Energy. 33 (2008) 1064–1072. doi:10.1016/j.renene.2007.05.045.[2] Pesaran, A. & Markel, T., Battery Requirements and Cost-Benefit Analysis for Plug-In Hybrid Vehicles (Presentation), (2007) 1–22. [3] S.B. Peterson, J. Apt, J.F. Whitacre, Lithium-ion battery cell degradation resulting from realistic vehicle and vehicle-to-grid utilization, Journal of Power Sources. 195 (2010) 2385–2392. doi:10.1016/j.jpowsour.2009.10.010.[4] Feed-in Tariffs and similar programs, US Energy Information Administration. (2013) http://www.eia.gov/electricity/policies/provider_programs.cfm [accessed 27-3-2015].[5] The World Bank, Putting a Price on Carbon with a Tax, 1–4, http://www.worldbank.org/content/dam/Worldbank/document/Climate/background-note_carbon-tax.pdf [accessed 10-12-2014].
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Storage Requirements Converge for different renewable energy requirements in the long-term
19
Frequency of Pack SOC swing as operating time increases
20
Model Specification
21
Discount rate has limited impact on diesel generation costs
22
• Capital costs play a small role in LCOE of diesel-only generation
• Discount both electricity produced & variable costs (fuel)
Value future electricity production less
Model Overview
23
Operational Model
Sola
r PV
Load
Diesel Generator
Battery Storage
Cycling Behavior
Storage Requirement
Power Mix
Economic Model
Cost A
ssum
ptio
ns
Repla
cem
ent
Assu
mptio
ns
LCOE
Operational Parameters
Battery technology
Renewable energy requirements
Maximum SOC swing
Operational lifetime (1, 5, 10, 20 years)
Economic Parameters
Fixed 20 year timeframe
Number of battery replacements (every
1, 2, 5, 10 years)
Cost Assumptions
Discount Rate