Grid-Scale Energy Storage
A Master’s Thesis Project by Daniel Spaizman
What is Grid-Scale Energy Storage?
• Utilizing various technologies to store energy within the electric system (grid) to be used at a later time
• Important because it allows us to use large amounts of energy in a smarter way
What Can Energy Storage Do for Utilities and Consumers?
• Load Shifting/Peak Shaving
• Integration with Variable Energy Resources (VER)
• Voltage Support
• Frequency Regulation
Interfacing with Renewables • Problem with renewables is intermittency
• Even on days with cool temperatures and high irradiance, power output looks like (for fixed tilt system)
Interfacing with Renewables
• Inverter cost ∝ Inverter size
• To save money, smaller inverters are purchased
Interfacing with Renewables
• Two ways of doing so:
• Before power reaches inverter
• After power reaches inverter
Solar Array Strings
Combiner Box
Inverter DC/AC
Electric Grid
Storage System
Solar Array Strings
Combiner Box
Inverter DC/AC
Rectifier AC/DC
Storage System
Electric Grid
Inverter DC/AC
What Features Should Energy Storage Technology Possess?
• High energy density
• High output power
• High cycle/round trip efficiency
• High cycling capability
• Long operating lifetime
• Low capital cost
• Low marginal cost
What Types of Grid-Scale Energy Storage are There?
• Mechanical: • Pumped-Storage Hydro (PSH)
• Compressed Air Energy Storage (CAES)
• Flywheel Storage
• Thermal: • Molten Salt
• Electrochemical: • Battery Storage
• Supercapacitors
Pumped-Storage Hydro (PSH)
Pumped-Storage Hydro
• Global Impact:
• 123+ GW of PSH installed as of 2011
• Pros:
• Can store large amounts of energy
• Can store over long periods of time
• Cons:
• Large amount of space needed
Pumped-Storage Hydro
• How it works:
• Gravitational potential energy
• Pump water from lower to higher reservoir
• Release water from higher to lower reservoir through turbines
Compressed Air Energy Storage (CAES)
• Global Impact:
• 440 MW of CAES installed as of 2011
• Pros:
• Low capital cost for mechanical system
• High output power
• Long lifetime
• Cons:
• Used in conjunction with fossil fuels
CAES • How it works:
Motor and
Compressor
Off Peak Electricity In
Peak Day Electricity
Out
Generator Turbine
Underground Cavern
>1000 ft
Pressure Expanders
Sodium Sulfur (NaS) Batteries
Sodium Sulfur (NaS) Batteries
• 316 MW installed around the world as of 2012
• Pros:
• High energy density
• Can provide local support (close proximity to demand)
• Made from abundant materials
• Cons:
• Operating conditions (high temperature)
What are the Components of a NaS Battery?
• Liquid Sodium (Na) Cathode
• Molten Sulfur (S) Anode
• Electrodes separated by Beta-alumina solid electrolyte (BASE)
• Ceramic material which conducts Na+ ions into the 250+ Celsius range
Components of a NaS Battery
NaS vs Li-Ion
Sodium Sulfur Lithium-Ion
Operating Conditions 600+ degrees Fahrenheit 35 – 120 degrees Fahrenheit
Composition Sodium, Sulfur, Beta Alumina
Lithium, Carbon, Metal Oxide
Energy Density 1000 Wh/L 500 Wh/L
Specific Energy 780 Wh/kg 200 Wh/kg
Yerba Buena Battery Energy Storage System
• Sodium Sulfur (NaS) Rechargeable Battery
• Prated = 4 MW and Erated = 28 MWh
• Comprised of 80 separate 50 kW modules
Vacaville-Dixon Findings
• Two types of tests:
• Idle period between discharge and charge
• No idle period between discharge and charge
• Most important efficiency is 24 hour AC round trip
• 24 hour AC round trip efficiency = Energy Out
Energy In+Heater Energy
Vacaville-Dixon Findings
30
40
50
60
70
80
90
100
0.5 0.75 1 1.25 1.5 1.75 2
Pe
rce
nta
ge
Discharge in MWs
Round-Trip Efficiency Performance Metric for 6 Hour Duration Discharge
DC Efficiency During Charge andDischarge
AC Efficiency During Charge andDischarge
24 Hour AC Roundtrip Efficiency
Vacaville-Dixon Findings
50
55
60
65
70
75
80
85
90
95
3.6 4.6 5.6 6.6
Pe
rce
nta
ge
Duration of Discharge (Hours)
Round-Trip Efficiency Performance Metric At 2-MW Discharge Level
DC Efficiency During Charge andDischarge
AC Efficiency During Charge andDischarge
24 Hour AC Roundtrip Efficiency
Meisei University Findings
• 1 MW system on a campus in Japan
• Employed a load following algorithm to offset school’s usage
• Algorithm considers power and temperature data from:
• one day ago
• two days ago
• one week ago
• one year ago
Meisei University Findings
Meisei University Findings
Meisei University Findings
Meisei University Findings
Driving Algorithm
• Hierarchical system which includes:
• Highest priority: Buy Low, Sell High
• Next Priority: Voltage Support
• Next Priority: Frequency Regulation
• Default Mode: Follow Load, Provide Peak Shaving
References
• Hoshino, and Wada. "Sodium-sulfur Battery. - HITACHI LTD." Free Patents Online. N.p., n.d. Web. 17 Oct. 2013.
• Koritarov, V. (2013, September 3). Grid-Scale Energy Storage. PSERC Webinar.
• Rastler, D. Electricity Energy Storage Technology Options A White Paper Primer on Applications, Costs, and Benefits. Tech. Electric Power Research Institue, Dec. 2010. Web. 17 Oct. 2013.
• SAS, blog.sas.com