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