Energy Storage

A Brief Introduction

Bryan Karney, with Stan Pejovic

with students Xiang Li & Michael Cai Wang

Division of Environmental Engineering and Energy Systems, U. of Toronto

Fundamental Preoccupations• Flux, storage and change• Existence and becoming – is and will be• All around us we see both great change, and

also great stability – robust connections• We can focus on the flows – of heat, energy,

work, mass, money, water, food, oil, etc.• Need to see how inevitable imbalances are

accommodated; inflow/outflow• What storage mechanisms are in play, and

how do they adjust to net flows?

Fundamental Preoccupations• Through storage …

– The land supports growth between rains and the rivers sustain their flow

– The Earth survives the dark and cold of night– Animals and humans live between meals– Bank accounts can be used and balanced– Gas tanks in a car; batteries in a laptop– Systems divide into stable and unstable

• Care is needed to divide the cause: Dubos– “When the tide is receding from the beach it is

easy to have the illusion that one can empty the ocean by removing water with a pail.”

GRADIENTS IN NATURE

• Waves on a rocky shore:– a violent, apparently unfriendly place

yet …– a dynamic environment ideal for motion,

exchange, rich in life and opportunity

TRANSPORT PHENOMENA

• Heat conduction: the transfer of thermal energy from high to low temperature regions

hot cold

QA

x

• Fourier’s law: the time rate of heat flow is proportional to the negative gradient of Temp.

dQ/dt=-k∙A∙(dT/dx)But imbalances cause thermal changes

moderated through heat capacity

Energy Sources• Mechanical

– Potential/gravity– Kinetic

• Elastic strain

• Thermal– sensible heat (T)– latent heat (phase)

• Chemical• stored in chemical bonds• essential electrochemical

• Quantum electro-dynamics• Light/photons • Electrostatics• Electromagnetics

• Nuclear fission and fusion

Energy rate scaling

J.W. Tester, Energy Transfer, Conversion and Storage Sustainable Energy

Energy rate scaling

Energy Fluxes Are Staggering• US energy consumption per year

Q=100,000,000,000,000,000,000 J or 3.5 TW

• Worldwide energy consumption per year Q=400,000,000,000,000,000,000,000 J or 15 TW

ENERGY STORAGE SYSTEMS

Devices can be characterised by the following parameters (rating an “energy bucket”!)

• Overall energy content.• Maximum energy flow during charge and discharge.• Efficiency at high/low cycling rates.• Energy density per mass.• Average life time.• Cost (various measures)

STORAGE• Very Short Term Solutions: (Power Quality

reinforcing)• Superconducting Magnetic Energy Storage Systems• Flywheels Energy Storage Systems • Batteries • Hydraulic Accumulator System • Mini Compress Air Energy Storage • Super capacitors

• Short Term Solutions: (Smoothing of wind power variations due to wind speed turbulence)

• Flywheels Energy Storage System • Batteries• Hydraulic Accumulator System • Mini Compress Air Energy Storage

• Medium Term Solutions: (Local wind fluctuations, Load levelling)

• Batteries• Flow Fuel Cell• Hydrogen (Electrolysis + Fuel Cells)• Compressed Air Energy Storage• Minihydro

• Long Term Solutions: (High capacity credit, daily or seasonal wind power variations)

• Batteries• Hydrogen • Compressed Air • Pumped Hydro Signific

ant for

electricity systems

Superconducting magnetic energy storage (SMESS)

• Based on the zero resistance of some materials to the electrical current when their temperature is below of a critical value (superconductivity)

• Can manufacture a coil with this material and maintain some electrical current flowing indefinitely inside in a closed circuit without losses

• There are different types of superconductors depending on their critical temperature (Low temperature (4ºK) and High temperature (70ºK).

• There are commercial systems for power quality applications (400 kJ and 750 KVA). At present this system is expensive and complex.

Super-conducting Magnetic Energy Storage

• Stores electrical energy in magnetic fields created by the flow of direct current in a coil cryogenically cooled

• Pro: - coil can carry very large current with little power loss almost indefinitely; cryogenic storage = high efficiency of almost 99%

- units respond within milliseconds with high poweroutput

• Con: - high power output can only be maintained for short periods of time

- high cost per unit of energy stored

Compressed Air Energy Storage (CAESS)• This system is based on a gas turbine and a reservoir.

• The gas turbine consists of a compressor, a combustor and an expander.

• This system injecting compressed air in an reservoir.

• The size of this reservoir define the energy storage capacity.

• Most of this systems are designed for bulk energy storage and uses as a reservoir an hard rock cavern or abandoned mine.

• To charge the reservoir power is supplied to the compressor which pumps air at high pressure (a 80 bar or even more).

Compressed Air Energy Storage (CAESS) II• When power is needed, the high pressure air is

withdrawn from the cavern and supplied with fuel to the turbo expander to generate electricity.

• This systems is simple, reliable and inexpensive. It has fast dynamic response and the environmental impact is minimal.

• Efficiency is characterised by two parameters: heat rate and energy ratio.

• There are several ways to increase the efficiency of CAES systems. CAES with steam injection and CAES with humidification.

• Main drawback is to have adequate geological conditions closed to the consumers (wind park.)

• Combustion turbines (CT) have become the preferred means of supplying electricity to meet peak loads.

• Their attributes are low first cost, rapid installation compared to other options, low operating and maintenance costs (other than fuel) and relatively low emissions.

• Problems: relatively low efficiency and consume expensive fuel - as a result operate only when market prices for electricity are high enough to justify their dispatch.

NEW COMPRESSED AIR ENERGY STORAGE CONCEPT IMPROVES THE PROFITABILITY OF EXISTING SIMPLE CYCLE, COMBINED CYCLE, WIND ENERGY, AND LANDFILL GAS POWER PLANTS, Nakhamkin M., Ronald H., Linden S., Hall R., Patel M., ASME Turbo Expo 2004, GT2004-54278

Chemical ReactionsHydrogen production• H2 O + electricity = H2 + ½ O2 – electrolysis of water• CH4 + H2 O = CO + 3H2 – steam reforming of methane• CO + H2 O = CO2 + H2 – water gas shift reaction

Hydrogen fuel cell• H2 + ½ O2 = H2 O + electricity – overall reaction• H2 + ½ O2 = H2 O + work + heat – overall reaction

Fuel combustion• CH4 + 3 O2 = CO2 + 2 H2 O – natural gas• C8H12 + 11O2 = 8 CO2 + 6 H2 O – gasoline• C6H12 O6 + 6O2 = 6 CO2 + 6 H2 O– cellulosic biomass

Electrolysis of waterBy providing energy from electricity grid (battery), water (H2 O) can be dissociated into the diatomic molecules of hydrogen (H2 ) and oxygen (O2 ).

Hydrogen Fuel Cell Hydrogen and oxygen in a fuel cell produce electrical energy.

Fuel cell uses a chemical reaction to provide an external voltage.

Fuel is continually supplied in the form of hydrogen and oxygen gas.

Wind Power Plus Stored Energy• Iowa Stored Energy Plant will be the first plant to use

energy from a wind farm plus supplemental off peak electricity to produce compressed air to be stored in an underground aquifer.

• When demand for electricity is high, the air will be released and used in combination with a small amount of natural gas to drive combustion turbines to generate electricity.

• This method will save one-third to one-half the natural gas that would otherwise be needed.

Iowa Stored Energy Plant to deliver Wind Energy on Demand, The Messenger, Fort

Dodge, Iowa, Nov 24, 2005

ISEP Project Review and Update, May, 2005

Iowa Stored Energy Project (ISEP), website

Transforming Wind-power into a Reliable Resource, ISEP fact sheet, February 2003

Wind Power Plus Stored Energy• This has been used in Alabama and Germany, but at

these locations the energy for storage does not come from wind.

• A separate section of the underground aquifer will also be used for storing natural gas.

• Gas storage will allow the facility and other gas utilities to buy natural gas when prices are lower.

• This type of gas storage is widely used in the U.S.

Iowa Stored Energy Plant to deliver Wind Energy on Demand, The Messenger, Fort

Dodge, Iowa, Nov 24, 2005

ISEP Project Review and Update, May, 2005

Iowa Stored Energy Project (ISEP), website

Transforming Wind-power into a Reliable Resource, ISEP fact sheet, February 2003

Hydro pumping• Large scale storage has in the main usually been in the

form of pumped hydro because in this case it is really cost effective.

• Sites for constructing new pumped hydro storage are becoming limited by topography and the environmental impact of these schemes is of increasing concern.

• The operation is simple, a pump/turbine is driven by the surplus wind (any) energy to raise water to a higher level.

• When power is needed, the flow can be inverted.

Pumped Hydro• Artificial or natural reservoirs• Same principle as hydroelectric damn except with the

addition of round-trip inefficiencies• 240 installations worldwide• Development trend in underground reservoirs

(integration with compressed air technology; i.e. variable volume comp. air storage)

• Pro: -large energy density-relatively quick responsiveness

• Con: -cost of excavation may be high-geographical, environmental constraints-potential losses in evaporation and leakage

pumped storage:• have ability to absorb power from the grid • can absorb surplus output at night• This is crucial to nuclear plants; they can only

change load very slowly. • Maintenance costs

of certain types of power

stations increase sharply

if they are forced to reduce load at night; so they benefit from storage plant attached to the grid.

• All other generators can be operated closer to their optimum efficiency

(and thus reduced

gaseous emissions)

Pumped Storage Basics

• Two water reservoirs, with vertical separation.

• During off-peak hours, water is pumped from the lower to upper reservoir.

• When energy is required, flow is reversed to generate electricity through turbines

27

• Load Leveling – original reason why pumped storage plants were built– Both daily and seasonal adjustments

• Pumped hydro first used in Italy and Switzerland in 1890's.

• By 1933 reversible pump-turbines with motor- generators were available.

• Adjustable speed machines introduced (1990)– Improve efficiency (10%) and – Reduce operation costs (50%).

Pump, turbine, and generator/motor on a single shaft

Qp

Qp

By Contrast, Pumped Storage• Can effectively absorb power from the grid

– absorbing surplus output at night• This is crucial for many generating mixes:

– Nuclear plants can only change load slowly – Maintenance costs

of some power stations

increase sharply

if they are forced to reduce load at night

– All other generators can be operated closer to their optimum efficiency

• Thus, pumped storage leads to reduced emissions and improved overall system performance.

Ontario and Canada have many suitable locations

Rivers, lakes, seas-shores and a hill nearby

Lake on the top of a hill and tens of m of shore

Supercapacitors (S-CP)

• Useful device for power quality reinforcing.

• It present a fast dynamic response and competitive cost.

• There are commercial systems (660 kJ energy storage capacity and rated power up to 2 MW)

• Interesting tradeoffs – get much better economies with large surface area and plates very near each other, but this increases losses and internal resistance

Significant fo

r

electricity storage

ENERGY STORAGE SYSTEMS

Fuel CellsFlywheel: a Mechanical AlternativeUnderground Thermal Energy StoragePumped Hydroelectric Energy StorageCompressed Air Energy Storage (CAES)Superconducting Magnetic Energy StorageSupercapacitors: replacing batteries

Ener

gy s

tora

ge

in g

ener

al

Energy storage in general

J.W. T

este

r, En

ergy

Tra

nsfe

r, Con

vers

ion

and

Stor

age

Susta

inable

Ene

rgy

Storage Characteristics

Compressed Air Energy Storage

Superconducting Magnetic Energy Storage

J.W. Tester, Energy Transfer, Conversion

and Storage Sustainable Energy

for comparing storage technologies

J.W. Tester, Energy Transfer, Conversion

and Storage Sustainable Energy

Energy transformationLaws of Thermodynamics provide limits

• Heat and work are not the same

• Maximum work output (or minimum work input) only occurs in idealized reversible processes

• All real processes are irreversible

• Losses always occur to degrade the efficiency of energy conversion and storage

Transformation• Laws of Thermodynamics provide performance limits for

reversible processes (Carnot cycle)

• Thermodynamics characterizes equilibrium and quasi- static processes but tells us nothing about rates

• Energy balances

input – output = accumulation

Production = Prime energy + stored energy

Thermodynamics bottleneckSecond Law says that no heat engine can use all heat to generate work

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw2.html#c1

Carnot cyclesets limits

Real machine efficiencies

are much less.

Energy Storage and Transformation

“Differentiating” Energy

• High Quality Energy VS. Low Quality Energy

• focus on storage of high quality energy- i.e. electricity

• address thermal storage later

The Need for (Utility Level) Energy Storage

• Relieve transmission congestion• Adaptability of renewable, distributed

generation• Energy security; power-levelling, load

balancing, frequency control• Alternate energy solutions for

transportation

Primary VS. Secondary

• Primary Energy:-energy that HAS NOT been subjected to any conversion or transformation process

i.e. solar, fossil fuels, nuclear, geothermal, wind, etc

• Secondary Energy:-energy that HAS been subjected to any conversion or transformation process

i.e. electricity

Not a rigorous definition!

Benefits & Stakeholders

• Utility Power Quality Control-demand-side management-supply-side management

• Rural Areas-reliant power supply for off-grid areas

• Independent Operators-profit from spot pricing

• End Consumers-clean, affordable, and reliable electricity

Integration

• Storage integration can be very flexible- sited near the generator, transmission, substation, end-user-UPS (uninterrupted power supply)-reduce/optimize utility of existing infrastructural capacity-defer costly expansion of existing T&D infrastructure

• Overall goal: – optimize production, distribution, and use of electricity by

storing off-peak power for use during peak demand

Integration: Conventional Sources

• Reduce load following generation and spinning reserves (peak shaving)

• Reduce overall installed capacity• Exploit the characteristic high efficiency operation of fuel

and nuclear plant at full load• Savings in capital investment, operation, maintenance

Integration: Renewable Sources

• Facilitate larger degree of penetration of intermittent renewable sources

• Renewable sources suffer from low prices due to unreliable delivery levels (generation does not coincide with peak demands)

• Storage allows ability to schedule power supply to tremendously increase its value

• Serves to reduce environmental impact • Increases each country’s own energy independence

Forms of Storage

• Innate form VS. Conversion to another form• Potential Energy

– Chemical– Electromagnetic– Gravitational– Mechanical

• Kinetic Energy– Rotational

Storage Types

• Pumped Hydro• Flywheel• Compressed Fluid• Chemical Cells• Synthetic Fuel• Super-conducting Magnet• Super-capacitors

Selection Criteria

• High energy density• High power density• Reliability

- i.e. Power Quality• Responsiveness• Cost• Safety• Mass & Volumetric Constraints• Environmental Footprint• Social Impact