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Lecture 7: Energy Storage
Instructor:
Dr. Gleb V. Tcheslavski
Contact:
gt.lamar@gmail.com
Office Hours:
TBD; Room 2030
Class web site:
http://www.ee.lamar.edu/
gleb/tps/Index.htm
Image from http://www.woosk.com/
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Introduction
The use of intermittent or variable sources of energy – such as solar, wind
energy, and some forms of energy derived from moving water – often
requires some means of energy storage.
Energy storage does not just potentially benefits solar energy systems as
well as other renewable energy resources, but also benefits the
transmission and distribution system since storage applications can be
used to mitigate diurnal or other congestion patterns and, store energy until
the transmission system is capable of delivering it where needed.
By storing energy from variable resources, such as wind and solar power,
energy storage could provide reliable generation from these units, permit
the energy produced to be used more efficiently, and provide supplimentary
transmission benefits.
The adverse impacts of large-scale PV power systems connected to the
power grid and developing output control technologies with integrated
battery storage are still under study.
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Electric utilities, energy service companies, and automobile manufacturers
exhibit a great interest in the possible applications of energy storage in
power systems. For instance, the ability to store large amounts of energy
would permit electric utility companies to have greater flexibility in their
operation, since with this option, the supply and demand do not have to be
instantaneously matched.
The battery technologies are diverse and at different stages of
development. They include a variety of batteries, high-speed flywheels,
supercapacitors, and regenerative fuel cells. Local energy storage would
assist embedded generation from renewable energy by providing a buffer
between the variability of supply and demand. Potential benefits include
capacity reduction, frequency support, standard reserve provision, and cold
start capability. Depending on technical requirements and geographical
settings, a utility company may benefit from one or more of these
techniques.
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Storage systems
Power applications, such as uninterruptable power supply (UPS) backup for
data centers and automotive batteries, represent the largest market for
lead-acid batteries; while laptop batteries and power tools caused
significant growth for lithium ion batteries. For bulk energy storage in utility
grids, pumped hydropower plants dominate, with approximately 100 GW in
service in the world.
Even though many utility companies have pumped storage plants, little
focus is placed on the potential roles that management of load may fill
demand or reduce demand peaks and, therefore, partially decouple energy
production from energy consumption.
Energy storage can serve the same purpose but may also be used as a
generation source, replacing either expensive, low-efficiency storage, or
load scheduling. The generation capacity would be required to meet only
the average electrical demand rather than peak demand. Expensive
network upgrades could be avoided.
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Storage systems
By enabling thermal generating units to operate closer to rated capacity,
higher thermal efficiency is achieved, and both fuel cost and CO2 emissions
are reduced.
As a result, the
balancing cost that
may be associated
with wind variability
can be reduced. Also,
expensive reserve
services – such as
gas turbines, diesel
engines, etc. – can be
reduced, since both
energy storage and
load management
have similar goals.
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Storage technologies in power levels and storage time.
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Storage systems
Power applications are storage systems rated for 1 hour or less; energy
applications are systems rated for longer period.
Power applications of each of the technologies (shown in previous slide)
are found in electrical grid; for instance, in the transmission system for
bulk power storage or in the residential feeder circuits of smaller systems.
Li-ion – lithium-ion battery;
NiCd – nickel-cadmium battery;
NiMH – nickel-metal hydride battery;
CAES – compressed-air energy storage;
SMES – superconducting magnetic energy storage;
VBR – vanadium redox battery;
ZnBr – zinc-bromine battery;
NaS – sodium-sulfur battery;
ZEBRA – high-temperature battery used at substations.
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The conventional technologies include the large hydro, compressed-air,
and pumped hydro storages.
1. Large hydro storage
Image of Hell’s Canyon from pacificenvironment.org
Large hydro is the oldest
renewable source for
power/energy. Small hydro
systems vary from 100 kW
to 30 MW, while microhydropower plants are
smaller than 100 kW. Small
hydropower generators
work at variable speed
since the water, on which
they depend, flows at
variable speed.
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Induction generators are normally used with turbine systems. The turbine
converts the water’s kinetic energy to mechanical rotational energy. The
available power from the water flow can be found as
(7.8.1)
where  is the density of water, kg/m3;
h is the height, m;
r is flow rate, m3/s;
k is the efficiency (ranges from 0 to 1).
Hydroelectric plants typically have fast ramp-up and ramp-down rates,
providing great regulating capability and the generating cost near zero.
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2. Compressed-air storage
Compressed-air storage
involves the storage of
compressed air in
underground cavities, for
instance, in the exhausted
salt mines. Alternatively, an
underground storage
complex can be created
using a network of largediameter pipes. Later, the
compressed air can be
released as part of
generation cycle, providing
a typical cycle efficiency of
approximately 75 %.
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Image from www.scotland.gov.uk
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Storage devices
In an open-cycle gas turbine or combined-cycle gas turbine plant,
incoming air is compressed by the gas turbine compressor before being
ignited with the incoming fuel supply. The exhaust gasses are then
expanded within the turbine driving both the electrical generator and the
compressor.
An open-cycle gas
turbine
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A modern compressed-air energy storage is a peaking gas turbine power
plant consumes less than 40% of the gas used in the combined-cycle gas
turbine (and 60% less gas than is used by a single-cycle gas turbine) to
produce the same amount of electricity. It is accomplished by blending
compressed air to the input fuel of the turbine by compressing air during
peak periods at lower costs than conventional stand-alone gas turbines.
It is required that compressed-air plants should be near proper
underground geological formations, such as salt caverns, mines, or
depleted gas wells. The first CAES plant was a 290 MW unit built in
Germany in 1978. the second one was a 110 MW unit built in Alabama in
1991.
Advanced systems rated for up to 800 MW are under consideration.
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3. Pumped hydro storage
The most widely established large-scale form of energy storage is hydroelectric pumped storage. This is an excellent storage technique, although
only few attractive sites exist and initial investment costs are very high.
Typically, such plant operates on a diurnal basis charging at night during
periods of low demand (and lowpriced energy) and discharging
during periods of high or peak
demand. A pumped storage plant
may have the capacity for 4-8 h of
peak generation with 1-2 h of
reserve, although sometimes the
discharge time may extend to a
few days.
Seawater pumped-storage hydro system on
Okinawa Island (Source: wastedenergy.net)
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A typical pumped hydro plant consists of two interconnected reservoirs
(lakes), tunnels conveying water from one reservoir to another, valves,
water pump turbine, a generator, transformers, a transmission switchyard,
and a transmission connection.
The amount of
stored energy is
proportional to
the product of
the total volume
of water and the
differential
height between
the reservoirs.
Image from
ww.consumersenergy.com
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For instance, storing 1 GWh (deliverable in a system with an elevation
change of 300 m) determines a water volume of about 1.4 million cubic
meters.
The earliest application of pumped hydro technology was in Zurich,
Switzerland, in 1882. One of the next systems was built in South Carolina
(Hiwassee Dam Unit 2) in 1956 and rated for 59.5 MW.
Today, the pumped hydro storage accounts for about 99% of world-wide
energy storage systems totaling of about 127 GW (Wiki, year 2011).
Approximately 20 GW of pumped hydro systems are operated in the US.
The original intent of these plants was to provide off-peak base loading for
large coal and nuclear plants to optimize their overall performance and
provide peaking energy each day.
Pumped hydro energy storage provides very fast response (< 1 minute) to
start intermittent generation.
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Year
2011
Image from sun.anu.edu.au
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Storage devices
Other, less conventional storage technologies include hydrogen, flywheels,
high-power fuel cells, high-power supercapacitors, superconducting
magnetic energy storage, heat or cold storage systems, and high-power
batteries.
4. Hydrogen
Hydrogen has been proposed as the energy store (carrier) for the future
and the basis for a new transport economy. The reasons for this are
simple: hydrogen is the lightest chemical element offering the best
energy/mass ratio of an y fuel. It can generate electricity efficiently and
cheaply in fuel cells. Indeed, the waste product is water; it can be
electrolyzed to produce more fuel (hydrogen).
Hydrogen can be transported conveniently over long distances using
pipelines or tankers, so that generation and utilization are at distant
locations, while a variety of storage forms are possible (gaseous, liquid,
metal hydrates, etc.)
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Hydrogen can be produced by electrolysis of water using off-peak
electricity or electricity from renewable source. It can be burned later to
generate electricity.
Alternatively,
hydrogen can be
piped as a gas or
liquid to consumers
to be used locally
providing
both electricity and
heat, or it can be
used in transport…
Image from www.oilempire.us
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The combustion of
hydrogen provides
energy and water
with no harmful
emissions or
byproducts. If the
electricity is the final
product, the process
is not very attractive
due to its overall
efficiency of about
50%. Therefore, the
interest in hydrogen
is usually for
transportation needs
Image from sitemaker.umich.edu
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While used for transportation purposes, availability of a proper storage
system is vital.
“The Energy Park and
hydrogen refueling
station in Norway
including wind
turbines, solar panels
and a lead-acid
batteries for energy
storage. Two water
electrolysers are
supplying hydrogen
produced from
renewable energy to
the hydrogen refueling
station on site.”
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It is predicted that hydrogen pipeline infrastructure is likely to be developed
in the future. Such pipelines could carry bulk hydrogen, as the energy
source, to major cities.
For distances greater than 1,000 km, energy transportation by hydrogen
carrier should be more economical than high-voltage electrical
transmission.
Space needed
to store the
energy
contained in a
typical
passenger car
tank.
Table from
www.ika.rwthaachen.de
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Storage devices
5. High-power flow batteries
High-power flow batteries operate similarly to car batteries but without
electrodes. Instead, when the flow cell is used as a “sink”, the electric
energy is converted into chemical energy by “charging” two liquid
electrolyte solutions. The stored energy can be released on discharge. As
with all DC systems connected to AC networks, a bidirectional converter is
needed.
These batteries use electrolyte solutions flowing through a cell stack with
ion exchange through a micro-porous membrane to generate an electric
charge. Several different chemistries were developed for utility power
applications. The main advantage is their ability to scale systems in terms
of power and energy.
Power rating can be increased by including more cell stacks; runtime is
increased by increasing the volume of electrolyte. Also, flow batteries work
at ambient temperatures rhather than at high temperatures.
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6. High-power flywheels
A high-power flywheel is a kinetic energy storage device. The energy is
stored in very fast (up to 75,000 rpm) rotating mass of flywheels. In the
past, flywheels had severe problems with maintenance, losses in bearings,
material strength, and related management problems at high speeds.
Images from www.intechopen.com and www.sciencedirect.com
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Modern flywheels are made of fiber-reinforced composites. The flywheel
motor/generator is interfaced to the main through a power electronic
converter. At present time, this technology is expensive and is only used
for selected applications.
From Wikipedia: “Materials with high strength,
and low density are desirable. For this reason,
composite materials are frequently being used in
flywheels. …values greater than 400 Wh/kg can
be achieved by certain composite materials.”
With use of magnetic bearings, mechanical
efficiency of 97% can be achieved.
Flywheels are used in uninterruptable power
supplies. Costs of a fully installed flywheel UPS
are about $330 per kilowatt (Wiki).
Image of NASA G2 flywheel from http://en.wikipedia.org/
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7. High-power supercapacitors
High-power supercapacitors are also called ultra capacitors or double-layer
capacitors. They consist of a pair of metal foil electrodes, each of which
has an activated carbon material deposited on one side. These sides are
separated by a membrane and then rolled into a package. The operation is
based on an electrostatic effect, where charging and discharging occurs
with the physical (not chemical) reversible movement of ions.
Therefore, fundamental differences between ultra capacitors and battery
technologies lead to considerably longer shelf and operating life od
supercapacitors and their large charge-discharge cycles of up to 500,000.
Supercapacitors are electrochemical capacitors. They may look and
perform similarly to Li-ion batteries.
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From Wikipedia: “The capacitance value of an electrochemical capacitor
is determined by two storage principles, both of which contribute to the
total capacitance:
Double-layer capacitance – electrostatic storage of the electrical energy
achieved by separation of charge in a Helmholtz double layer at the
interface between the surface of a conductive electrode and an
electrolyte. The distance of the static separation of charge in a doublelayer is on the order of a few Angstroms (0.3–0.8 nm).
Pseudocapacitance – Electrochemical storage of the electrical energy
with electron transfer, achieved by redox reactions with specifically
adsorbed ions from the electrolyte, intercalation of atoms in the layer
lattice or electrosorption, underpotential deposition of hydrogen or metal
adatoms (an atom that lies on a crystal surface, and can be thought of as
the opposite of a surface vacancy) in surface lattice sites which result in a
reversible faradaic charge-transfer.”
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More from Wikipedia: “The ratio of the storage resulting from each
principle can vary greatly, depending on electrode design and electrolyte
composition. Pseudocapacitance can increase the capacitance value by
an order of magnitude over that of the double-layer by itself.
Supercapacitors are divided into three families, based on the design of
the electrodes:
Double-layer capacitors – with carbon electrodes or derivates with much
higher static double-layer capacitance than the faradaic
pseudocapacitance
Pseudocapacitors – with electrodes out of metal oxides or conducting
polymers with a high amount of faradaic pseudocapacitance
Hybrid capacitors – capacitors with special and asymmetric electrodes
that exhibit both significant double-layer capacitance and
pseudocapacitance, such as lithium-ion capacitors.”
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From Wikipedia again: “Supercapacitors bridge the gap between
conventional capacitors and rechargeable batteries. They have the
highest available capacitance values per unit volume and the greatest
energy density of all capacitors. They support up to 12,000 Farads/1.2
Volt, with capacitance values up to 10,000 times that of electrolytic
capacitors. While existing supercapacitors have energy densities that are
approximately 10% of a conventional battery, their power density is
generally 10 to 100 times greater. Power density is defined as the product
of energy density, multiplied by the speed at which the energy is delivered
to the load. The greater power density results in much shorter
charge/discharge cycles than a battery is capable, and a greater tolerance
for numerous charge/discharge cycles.” Therefore, supercapacitors can
charge and discharge very quickly!
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Image from
http://en.wikipedia.org/
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Supercapacitors have been applied in blade-pitch control devices for
individual wind turbine generators to control the rate at which power
increases and decreases with changes of wind velocity. This is necessary
if wind turbines are connected to weak utility grids.
A lot of 8 supercapacitors rated
at 350 Farads, 2.7 V, 0.35 Wh,
976 W each; sold for $65 + $10
s/h, June 2013
http://www.ebay.com/
In California, Palmdale water
district uses a 450 kW supercap
to regulate the output of a 950
kW wind turbine to reduce
network congestions.
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8. Superconducting magnetic energy storage
As a result of recent developments in power electronics and
superconductivity, the interest in using SMES units to store energy and/or
reduce power system oscillations has increased. The energy is stored
within a magnetic field created by the flow of direct current in a coil of
superconductive material.
Therefore, SMES can be viewed as a controllable current source, whose
magnitude and phase can be changed within one cycle. The upper limit of
this source is imposed by the DC current in the superconducting coil.
Typically, one coil is maintained in its superconducting state by keeping it
in liquid helium at 4.2 K in a vacuum-insulated cryostat.
A power electronic converter interfaces the SEMS to the grid and controls
the energy flow bidirectionally. With the development of materials
exhibiting superconductivity close to room temperatures, this technology
may become economically viable.
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A typical configuration of
a SMES unit with a
double gate turn-off
(GTO) thyristor bridge.
The superconductive
coil L is coupled to the
transmission system by
two convertors and
transformers. The
convertor firing angles
1 and 2 are
determined by the PQI
controller to modify the
real and reactive power
outputs and the DC
current In of the coil.
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The control strategy is
determined by the modulation
controller of SMES to dump out
power swings in the network.
A 10MW SMES system was
developed in 2010 for
Furukawa Nikko Power
Generation Inc., at Hosoo
Power Plant (Source:
http://www.chuden.co.jp/).
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9. Heat or cold storage
Thermal storage assisting in power system operations is used for many
years. This technology involves modulation of the energy absorbed by
individual consumer electric heating elements and refrigeration systems for
the benefit of the overall system power balance.
An aggregation of a large number of dynamically controlled loads has the
potential of providing added frequency stability and smoothing to power
networks, both at times of sudden increase in demand (or less of
generation) and during times of fluctuating wind or other renewable power.
Such devices could displace some reserve and may cause a substantial
reduction in governor activity of remaining generators. The potential
demand that could be operated under dynamic control is considerable.
Deep-freeze units, industrial and commercial refrigeration, air conditioning,
water heating systems could provide dynamic demand control (DDC). The
potential available in a developed country could be several GW.
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Images from http://earthshelters.com/
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Battery types
Battery systems are quiet and non-polluting. They can be installed near
load centers and existing suburban substations. They have efficiencies
around 85% and can respond to load changes within 20 ms. Lead-acid
batteries as large as 10 MW with 4 h of storage have been used in several
US, European, and Japanese utility companies.
Although both the input and the output energy of a battery are electrical,
the storage is in chemical form. Chemical batteries are individual cells filed
with a conducting electrolyte that, when connected together, form a battery.
Multiple batteries connected together form a battery bank.
Essentially, batteries can be divided into two basic types: primary batteries
(non-rechargeable) and secondary batteries (rechargeable).
While being abundant, primary batteries are outside our discussion.
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Secondary batteries
Secondary batteries are rechargeable batteries. They are divided into two
categories based on the operating temperature of electrolytes.
Ambient operating temperature batteries have either aqueous (flooded) or
non-aqueous electrolytes.
High operating temperature batteries (molten electrodes) have either solid
or molten electrolytes.
Rechargeable lead-acid and NiCd batteries have been used widely by
utilities for small-scale back-up, load leveling, etc.
The largest NiCd battery installation is a 45 MW, 10 MWh system in
Fairbanks, Alaska, built in 2003 and designed to provide a guaranteed 27
MW for at least 15 minutes following local power outages.
For similar reasons, the largest (20 MW, 14 MWh) lead-acid system was
installed by the Puerto Rico Electric Power Authority in 1994.
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Secondary batteries
However, considering the toxic nature of materials involved, low efficiency
(70% - 80%) and the limited life and energy density, secondary batteries
based on other technologies are researched for utility applications.
Batteries in electric vehicles are the secondary rechargeable batteries
utilizing other technologies. Such batteries have to satisfy certain
performance goals that include quick charge and discharge capabilities,
long cycle life, low cost, recyclability, high specific energy (the amount of
usable energy measured in watt-hours per kilogram), high energy density
(amount of energy stored per unit volume), high specific power (that
defines the potential for acceleration), and the ability to work in extreme
heat or cold.
However, at present time, no such battery technology exists that would
satisfy all the criteria above.
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Secondary batteries
Battery performance
seems to improve
exponentially over years…
A large variety of battery
types are used in electric
power systems for grid
support.
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Secondary batteries (Wiki)
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Secondary batteries: Sodium-Sulfur
Sodium-Sulfur battery is a type of molten salt high-performance battery
with the electrolyte operating at temperatures around 300 C (572F). It
consists of a liquid (molten) sulfur positive
electrode and a molten sodium negative
electrode separated by a solid beta
alumina ceramic electrolyte. The
electrolyte permits only positive sodium
ions to pass through it and, while
combining with sulfur, to form sodium
polysulfides.
The sodium component of this battery
explodes on contact with water (or air),
which raises certain safety concerns.
Image from www.masterresource.org
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Secondary batteries: Sodium-Sulfur
The materials of the battery need to be capable of withstanding the high
internal temperatures they create, as well as freezing and thawing cycles.
Sodium-sulfur batteries have a very high specific energy of 150 Wh/kg.
during discharge, positive sodium ions flow through the electrolyte and
electrons flow in the external circuit of the battery providing about 2 V.
This technology was perfected in Japan for large-scale applications.
Presently, there are 190 battery systems in service in Japan, totaling more
than 270 MW of capacity with stored energy suitable for 6 h of daily peak
cancelling. The largest single NaS battery installation is a 34 MW, 245
MWh system for wind power stabilization in northern Japan. The battery
will permit the output of the 51 MW wind farm to be 100% dispatchable
during the on-peak periods.
In the US, 9 MW of NaS batteries are installed for peak cancelling, backup
power, firming wind capacity, etc.
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Secondary batteries: Sodium-Sulfur
1 MW NaS battery
in Minnesota.
The battery was
installed in 2008
and tested for
storing wind
energy.
The battery is in
cold shutdown
since 2011 (as for
2012) waiting for
design
modifications
addressing
potential fire
hazard.
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Source: http://www.energystorageexchange.org/
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Secondary batteries: ZEBRA
ZEBRA battery is another molten salt high-temperature battery and is
based on sodium nickel chloride chemistry. It is used for electric
transportation in Europe and is considered for utility applications as well.
From Wikipedia: “This battery operates at 245 °C (473 °F) and utilizes
molten sodium aluminumchloride (NaAlCl4), which has a melting point of
157 °C (315 °F), as the electrolyte. The negative electrode is molten
sodium. The positive electrode is nickel in the discharged state and nickel
chloride in the charged state. This battery was invented in 1985 by the
Zeolite Battery Research Africa Project (ZEBRA) group in Pretoria, South
Africa. The technical name for the battery is Na-NiCl2 battery.
The ZEBRA battery has a specific energy of 90 Wh/kg and a specific
power of 150 W/kg. For comparison, lithium iron phosphate batteries store
90–110 Wh/kg and the more common lithium ion batteries store 150–200
Wh/kg. The ZEBRA's liquid electrolyte freezes at 157 °C (315 °F), and the
normal operating temperature range is 270 °C (518 °F) to 350 °C (662 °F).”
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Secondary batteries: ZEBRA
Iveco and Think
City offer electric
vehicles with
ZEBRA batteries
option.
When not in use,
ZEBRA batteries
are usually kept
hot…
Image from
www.cleanbreak.ca
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Secondary batteries: Flow Battery
technology
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The performance of flow batteries is similar to the performance of a
hydrogen fuel cell. They use electrolyte liquids flowing through a microporous membrane to generate an electric charge. They store and release
electrical energy through a reversible electrochemical reaction between
two liquid electrolytes.
The liquids are separated by an ion-exchange membrane, allowing the
electrolytes to flow into and out from the cell through separate manifolds
and to be transformed electrochemically within the cell. For their utility
applications, various chemistries have been developed. In standby mode,
the batteries have a response time of the order of milliseconds to seconds,
making them suitable for frequency and voltage maintenance.
One of the advantage of this flow battery design is the ability to scale
systems independently in terms of power and energy. For instance, more
cell stacks permit for an increase in power rating, a greater volume of
electrolytes provides more runtime. Additionally, flow batteries operate at
ambient temperatures.
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Secondary batteries: Flow Battery
technology
46
1. Zinc-Bromine flow batteries
ZnBr flow battery operates with a solution of zinc-bromide salt dissolved in
water and stored in two tanks. The battery is charged or discharged by
pumping the electrolytes through a reactor cell.
During the charging cycle, metallic zinc from the electrolyte solution is
plated onto the negative electrode surface of the reactor cell. The bromide
is converted to bromine at the positive surface of the electrode in the
reactor cell and then is stored in the other electrolyte tank as a safe
chemically complex oily liquid.
During the discharge of the battery, the process is reversed, and the
metallic zinc plated on the negative electrode is dissolved in the electrolyte
solution producing zinc-bromide for the next charging cycle.
To create batteries with different ratings, flow batteries have modular
construction, such as a ZnBr package rated for 500 kW for 2h.
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From Wikipedia: “The primary features of the zinc bromine battery are:
• High energy density relative to lead-acid batteries;
• 100% depth of discharge capability on a daily basis;
• High cycle life of > 2,000 cycles at 100% depth of discharge, at which
point the battery can be serviced to increase cycle life to over 3,500
cycles;
• No shelf life limitations since zinc-bromine batteries are non-perishable,
unlike lead-acid and lithium-ion batteries, for example;
• Scalable capacities from 10 kW·h to over 500 kW·h (1.8 GJ) systems;
• The ability to store energy from any electricity generating source.”
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These battery systems
have the potential to
provide energy storage
solutions at a lower
overall cost than other
energy storage systems
such as lead-acid,
vanadium redox, sodiumsulfur, lithium-ion and
others (Wiki).
RedFlow Zinc Bromine
Module (ZBM) Gen 2.5
module: 5 kW and
10 kW·h. Source:
http://en.wikipedia.org/
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2. Vanadium Redox flow batteries
Vanadium Redox batteries (VRB) is another type of flow batteries. During
its charge and discharge cycles, positive hydrogen ions are exchanged
between the two electrolyte tanks through a hydrogen-ion permeable
polymer membrane. Vanadium ions in different oxidation states are used to
store chemical potential energy.
Specific energy
10–20 Wh/kg
Energy density
15–25 Wh/L
Charge/discharge
efficiency
75-80%
Time durability
10–20 years
Cycle durability
>10,000 cycles
Nominal cell voltage
1.15–1.55 V
Source: http://en.wikipedia.org/
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Images from www.flickr.com and www.ericom.com.tr
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Currently installed VRBs (Wiki):
• A 1.5 MW UPS system in a semiconductor
fabrication plant in Japan.
• A 275 kW balancer on a wind power
project of Hokkaido.
• A 250 kW, 2 MWh load leveler in use at
Castle Valley, Utah.
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Secondary batteries: Lithium-Ion
Lithium-Ion batteries have the greatest applications among the available
battery technologies. It can be implemented in a large variety of shapes
and sizes, permitting the battery to efficiently fill the available space, such
as cell phone or laptop computer.
Lithium-Ion batteries are also lighter in weight compares to other aqueous
battery technologies, such as lead-acid batteries. They have the highest
power density (up to 250 Wh/kg) of all batteries on the commercial market
on a per-unit-of-volume basis.
The leading Li-ion cell design is a combination of lithiated nickel, cobalt,
and aluminum oxides, referred to as an NCA cell. Two Li-ion designs that
are starting to be used in high-power utility grids are lithium titanate and
lithium iron phosphate.
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1. Lithium Titanate batteries
Lithium titanate batteries use manganese in the cathodes and lithiumtitanate nano-crystals instead of conventional carbon in the anodes. This
chemistry produces a very stable design with fast-charge capability and
good performance at lower temperatures.
From Wikipedia: “Use of lithium-titanate gives the anode a surface area of
about 100 m2/g compared to 3 m2/g for carbon, allowing electrons to enter
and leave the anode quickly. This makes fast recharging possible and
provides high currents when needed.
The disadvantage is that lithium-titanate batteries have a lower inherent
voltage, which leads to a lower energy density than conventional lithiumion battery technologies”.
The batteries can be discharged to 0% and have a relatively long life. They
are used in a utility power service applications for frequency regulation.
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Secondary batteries: Lithium-Ion
Titanate batteries are used in Mitsubishi's i-MiEV electric vehicle and
Honda uses them in its EV-neo electric bike and Fit EV.
Toshiba released a lithium–
titanate battery named Super
Charge Ion Battery (SCiB). The
battery is designed to offer 90%
charge capacity in just 10
minutes. The life cycle of the
battery is more than 10 years.
SCiB is also said to be safer than
early models with their potential
fire hazard.
These batteries are installed in
Honda Fit EV and in various
electric bikes.
Source: http://www.electricvehiclesresearch.com/
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Secondary batteries: Lithium-Ion
2. Lithium Iron Phosphate batteries
Lithium Iron Phosphate (LFP) is a newer and safer technology that is more
difficult to release the oxygen from the electrode, which reduces the risk of
fire in the battery cells. It is more resistant to overcharge when operated in
a range of up to 100% state of charge. They are also used in a utility power
service applications for frequency regulation.
From Wikipedia: “LiFePO4 is used as a cathode material. These batteries
have somewhat lower energy density than the more common LiCoO2
design found in consumer electronics, but offers longer lifetimes, better
power density, and are inherently safer.”
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Secondary batteries: Lithium-Ion
Selected specifications (Wiki):
Cell voltage:
Min. discharge voltage = 2.8 V
Working voltage = 3.0 ~ 3.3 V
Max. charge voltage = 3.6 V
Gravimetric energy density =>
90 Wh/kg
Number of cycles to 80% of
original capacity = 2,000-7,000
48 V, 20 Ah LFP battery:
Max charging/discharging current: 25 A
Approximate weight: 12 kg
Dimensions: 32 cm x 12 cm x 14.5 cm
Source: http://sealedenergysystems.tradeindia.com/
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Secondary batteries: Lead-Acid
Lead-acid batteries, invented in 1859 by French physicist Gaston Planté,
are the oldest and most mature among the all battery technologies. Due to
their large applications, lead-acid batteries have the lowest cost.
This battery operates at an ambient temperature and has an aqueous
electrolyte. Even though the lead-acid battery is relatively inexpensive, it is
very heavy, with a limited usable energy by weight (specific energy). These
batteries should not be discharged by more than 80 % of their rated
capacity. Exceeding 80 % of the depth discharge shortens the life of the
battery.
From Wikipedia: “In the discharged state both the positive and negative
plates become lead(II) sulfate (PbSO4) and the electrolyte loses much of
its dissolved sulfuric acid and becomes primarily water.
In the charged state, each cell contains negative plates of elemental lead
(Pb) and positive plates of lead(IV) oxide (PbO2) in an electrolyte of
approximately 33.5% sulfuric acid (H2SO4).”
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Secondary batteries: Lead-Acid
Lead-acid: the charging speed problem (from Wiki)
The capacity of a lead-acid battery is not a fixed quantity but varies
according to how quickly it is discharged.
When a battery is charged or discharged, this initially affects only the
reacting chemicals, which are at the interface between the electrodes and
the electrolyte. With time, the charge stored in the chemicals at the
interface, (interface charge) spreads by diffusion of these chemicals
throughout the volume of the material.
If a battery has been completely discharged (leaving the car lights on
overnight) and then is given a fast charge for only a few minutes, the
battery plates charge only near the interface between plate and electrolyte.
The battery voltage may rise to be close to the charger voltage so that the
charging current decreases significantly. After a few hours this interface
charge will spread to the volume of the electrode and electrolyte, leading to
an interface charge so low that it may be insufficient to start the car.
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On the other hand, if the battery is given a slow charge, which takes
longer, then the battery will become more fully charged. During a slow
charge the interface charge has time to redistribute to the volume of the
electrodes and electrolyte, while being replenished by the charger. The
battery voltage remains below the charger voltage throughout this
process allowing charge to flow into the battery.
Similarly, if a battery is subject to a fast discharge (such as starting a car,
a current draw of more than 100 amps) for a few minutes, it will appear
to go dead, exhibiting reduced voltage and power. However, it may have
only lost its interface charge. If the discharge is halted for a few minutes
the battery may resume normal operation at the appropriate voltage and
power for its state of discharge. On the other hand, if a battery is subject
to a slow, deep discharge (such as leaving the car lights on, a current
draw of less than 7 amps) for hours, then any observed reduction in
battery performance is likely permanent.
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Secondary batteries: Lead-Acid
A version of lead-acid battery, the deep-cycle lead-acid battery, is widely
used in golf carts and forklifts.
From Wikipedia: “They are designed to be regularly deeply discharged
using most of its capacity. In contrast, starter batteries (e.g. most
automotive batteries) are designed to deliver short, high-current bursts for
cranking the engine, thus frequently discharging only a small part of their
capacity. While a deep-cycle battery can be used as a starting battery, the
lower "cranking amps" imply that an oversized battery may be required.
The structural difference between deep cycle batteries and cranking
batteries is in the lead battery plates. Deep cycle battery plates have
thicker active plates, with higher-density active paste material and thicker
separators. Alloys used for the plates in a deep cycle battery may contain
more antimony (stibium, Sb) than starting batteries. The thicker battery
plates resist corrosion through extended charge and discharge cycles”.
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Secondary batteries: Lead-Acid
Images from primeproducts.in
engg-learning.blogspot.com
and www.asia.ru
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Secondary batteries: Lead-Acid
Advanced Lead-acid batteries
Carbon can be added to the negative electrode to significantly extend the
life of lead-acid batteries. However, lead-acid batteries fail due to sulfation
in the negative plate that increases as the battery is cycled more.
Adding 40 % of activated carbon to the negative electrode composition
increases the battery life up to 2,000 cycles. This represents a three- to
four-time improvement over regular lead-acid batteries.
This extended life together with the low cost may lead storage developers
to reconsider lead-acid technology for grid applications.
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Secondary batteries: Nickel-Cadmium
Nickel iron (Edison cell) and NiCd pocket and sintered plate batteries have
been in use for many years. Both of these batteries have a specific energy
of approximately 55 Wh/kg that is higher than advanced lead-acid
batteries. They can be discharged to 100 % depth of discharge without
damage.
The primary trade-off with NiCd batteries is their higher cost and the use of
cadmium. This heavy metal is an environmental hazard, and is highly toxic
to all higher forms of life (Wiki).
In power systems, NiCd batteries have been used in a variety of backup
applications and were chosen to provide “spinning reserve” for the
transmission project in Alaska. The project involved a 26 MW NiCd battery
rated for 15 min that is the largest battery in a utility application in North
America (as for 2011). NiCd batteries are still used for utility applications,
such as in power ramp-rate control for smoothing in weak power grids.
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NiCd “memory effect” (from Wiki).
“NiCd batteries may suffer from a "memory effect" if they are discharged
and recharged to the same state of charge hundreds of times. The
apparent symptom is that the battery "remembers" the point in its charge
cycle where recharging began and during subsequent use suffers a
sudden drop in voltage at that point, as if the battery had been discharged.
The capacity of the battery is not actually reduced substantially. Some
electronics designed to be powered by NiCd batteries are able to withstand
this reduced voltage long enough for the voltage to return to normal.
However, if the device is unable to operate through this period of
decreased voltage, it will be unable to get enough energy out of the
battery, and for all practical purposes, the battery appears "dead" earlier
than normal.
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The original paper describing the memory effort was written by GE scientists
and later retracted by them, but the damage was done. It is unlikely to be a
real phenomenon, but has taken on a life of its own as an urban myth.
An effect with similar symptoms is the so-called voltage depression or lazy
battery effect. This results from repeated overcharging; the symptom is that
the battery appears to be fully charged but discharges quickly after only a
brief period of operation. In rare cases, much of the lost capacity can be
recovered by a few deep-discharge cycles, a function often provided by
automatic battery chargers. However, this process may reduce the shelf life
of the battery. If treated well, a NiCd battery can last for 1,000 cycles or
more before its capacity drops below half its original capacity. Many
consumer chargers claim to be "smart chargers" that will shut down and not
damage the battery, but this seems to be a common problem. For instance,
Black and Decker 18 volt drill can be bought with 3 different chargers, only
one of which (so-called 9 hour charger) is safe to leave on the battery for an
extended time. Faster chargers will damage the battery if left on.”
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Secondary batteries: Nickel-Cadmium
Images from www.ustudy.in
www.drillspot.com
inventors.about.com
and http://www.ebay.com/
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Operational problems in battery usage.
The PV systems with integrated battery storage can recover the energy
that would have been lost when the voltage is over the limitation value.
Since the risk of over-voltages is higher when the reverse power flow is
greater, the state of charge of the storage battery should not be full at
around noon.
Therefore, only part of the surplus power that is greater than the load
demand should be applied to the storage battery for the efficient operation
of the battery.
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Secondary batteries
The following problems should be considered when operating the storage
battery integrated in a PV system:
1. The storage battery must be at a discharge state in the morning to
prepare for the charging around the noon.
2. If the lead-acid battery is left in the discharge state, it may deteriorate
and shorten its lifetime.
3. The frequency of use of the storage batteries may be varied by the
impedance of the distribution line and by a power-flow condition.
4. “Round-trip” energy losses of the storage battery and power
conditioning systems increase when charging and discharging larger
amounts of energy.
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Fuel cells
Fuel cells were developed in 1839 but were only put in to practical use in
1960s by NASA to generate fuel for their Apollo spacecraft.
The stored hydrogen can be converted to electricity using an open-cycle
gas turbine. However, the efficiency tends to be low in this case, even if
the losses associated with transportation and converting the electricity to
hydrogen were ignored.
Fuel cells are quiet, clean, and highly efficient onsite generators that use
the electrochemical process (reverse electrolysis) to convert fuel to
electricity. This process produces very little waste heat or gas. In addition
to generating electricity, fuel cells can also serve as a thermal energy
source for water and space heating or for cooling absorption.
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Fuel cells
Fuel cells consist of an electrolyte (liquid or solid) membrane sandwiched
between two electrodes.
A block diagram of a fuel cell is shown
below.
Proton-conducting fuel cell.
Source: http://en.wikipedia.org/
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Fuel cells
A single fuel cell produces output voltage less than 1 V. Therefore, fuel
cells are stacked on top of each other and connected in series forming a
fuel cell system to produce higher voltages. Electrical efficiencies of fuel
cells are between 36% and 60% depending on the type and system
configuration. Using the conventional heat recovery techniques, the overall
efficiency can be increased to approximately 85%.
Steam reforming of liquid hydrocarbons (CnHm) is a potential way of
providing hydrogen-rich fuel. This is a preferred method since storage of
hydrogen is hazardous and expensive. Reformers facilitate a continuous
supply of hydrogen without having to use bulky pressurized hydrogen
tanks or hydrogen vehicles for distribution.
The endothermic (i.e., the energy is absorbed from the environment)
reaction takes place in the reforming process in the presence of a catalyst:
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Fuel cells
The endothermic (i.e., the energy is absorbed from the environment)
reaction takes place in the reforming process in the presence of a catalyst:
(7.71.1)
Also, carbon monoxide combines with steam to produce more hydrogen
through the water-gas shift reaction
(7.71.2)
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The flows and reactions in
a fuel cell:
Fuel cells are classified
according to the nature of
electrolyte used and the
operating temperature. Each
type requiring particular
materials and fuels.
Fuel cells are classified according to the nature of electrolyte used and the
operating temperature. Each type requiring particular materials and fuels.
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Fuel cells
Polymer
electrolyte
membrane
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Fuel cells
The electrochemical efficiency tends to increase with fuel cell temperature.
The operating temperature is often determined by the nature of the
membrane. The expensive catalyst, such as platinum, may be needed to
increase the rate of electrochemical reactions. In general, fuel cells may
use hydrogen, natural gas, methanol, coal, or gasoline.
In addition to these fuels, more environmentally friendly fuels, such as
biogas and biomass, may be used. For most fuel cells, such fuels must be
transformed into hydrogen using a reformer or coal gasifier.
However, high-temperature fuel cells can generally use a fossil fuel
(natural gas, coal gas, etc.) directly. In this case, polluting products may be
produced.
The hydrogen molecules are supplied to one electrode (anode), where
they produce ions moving to the cathode. At the cathode, these ions
combine with oxygen to form water. The oxygen may be extracted from air
or supplied from the water electrolysis as a stored product.
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Fuel cells
For large-scale utility storage applications, the selection of technology will
depend on the ability to use pure hydrogen (electrolyzed from water) as
the fuel, on the electrical efficiency of conversion, and on the load-following
capacity of the fuel cell (that may provide some regulation from fluctuating
wind or other sources). Out of various options available, SO and PEM
seem most likely to succeed.
The efficiency of conversion of fuel to electricity can reach 65%, which is
almost twice the efficiency of conventional power plants. Small-scale fuel
cell plants are as efficient as the large ones, whether they operate at full
load or not. Because their modular nature, fuel cells can be placed at or
near load centers; therefore, saving on transmission network expansions.
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A fuel cell power plant consists of three subsystems (sections):
1. The natural gas or other hydrocarbon fuel is converted to hydrogen-rich
fuel in a fuel-processing section. This process is known as a steam
catalytic reforming process.
2. The fuel is fed to the power section, where it reacts with oxygen in a
large number of individual fuel cells to produce DC electricity.
3. The DC electricity is converted to the power utility-grade AC electricity
in the power-conditioning subsystem.
Also, heat can be produced as a byproduct in the form of usable steam or
hot water. The number of fuel cells at a power plant may vary from several
hundreds (for a 40 kW plant) to several thousands (for a multi-MW plant).
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In the power section of a fuel cell that has the electrodes and the
electrolyte, two separate electrochemical reactions happen: an oxidation
half-reaction (at the anode) and a reduction half-reaction (at the cathode).
The electrodes are separated from each other by the electrolyte. During the
oxidation half-reaction at the anode, gaseous hydrogen produces hydrogen
ions that travel through the ionically conducting membrane to the cathode.
At the same time, electrons travel through an external circuit to the cathode.
In the reduction half-reaction at the cathode, oxygen supplied from air
combines with the hydrogen ions and electrons to form water and excess
heat.
Therefore, the products of the overall reaction are electricity, water, and
excess heat.
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Types of fuel cells
Fuel cells are categorized based on their electrolyte type, since the
electrolyte defines the key properties of fuel cells; specifically, the
operating temperature. The main types are:
1. Polymer electrolyte membrane (PEM);
2. Alkaline fuel cell (AFC);
3. Phosphoric acid fuel cell (PAFC);
4. Molten carbonate fuel cell (MCFC);
5. Solid oxide fuel cell (SOFC).
These fuel cells operate at different temperatures and each of them suits
best specific applications.
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Fuel cells: types
1. Polymer electrolyte membrane
The electrolyte in PEM fuel cell is a polymer that is usually referred to as a
membrane (thus the name). PEM’s electrolyte is somewhat unusual in
that, in presence of water (that membrane readily absorbs), the negative
ions are rigidly held within their structure. Only the positive (H) ions
contained within the membrane are mobile and are free to carry positive
charges through the membrane in one direction only, from anode to
cathode.
At the same time, due to its organic nature, the PEM structure acts as an
electron insulator, forcing it to travel through the outside circuits providing
electric power to the load. Each of the two electrodes is made of porous
carbon with very small platinum particles bounded. The electrodes are
slightly porous so that the gasses can diffuse through them to reach the
catalyst. Since both carbon and platinum conduct electrons well, they can
move freely through the electrodes.
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The chemical reactions inside a PEM fuel cell are:
At anode:
(7.80.1)
At cathode:
(7.80.2)
Net reaction:
(7.80.3)
Here, hydrogen gas diffuses through the polymer electrolyte until it meets
a platinum particle in the anode. The platinum catalyzes dissociation of the
hydrogen molecule into two hydrogen atoms (H) bonded to two
neighboring platinum atoms.
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Only then can each H atom release an electron to form a hydrogen ion
(H+) that travels through the membrane, at the same time as the free
electron through the outside circuit. At the cathode, the oxygen molecule
interacts with the hydrogen ion and the electron from the outside
circuit to form water. The
performance of PEM fuel
cells is limited mainly by the
slow rate of the oxygen
reduction half-reaction at
the cathode, which is 100
times slower than the
hydrogen oxidation halfreaction at the anode.
1 KW portable PEM fuel cell
By Ballard
Source: www.fuelcell.no
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2. Alkaline fuel cell
From Wikipedia: “The alkaline fuel cell (AFC), also known as the Bacon
fuel cell after its inventor, is one of the most developed fuel cell
technologies. NASA has used alkaline fuel cells since the mid-1960s, in
Apollo-series missions and on the Space Shuttle. AFCs consume
hydrogen and pure oxygen producing potable
water, heat, and electricity. They are among the
most efficient fuel cells, having the potential to
reach 70%.
The two electrodes are separated by a porous
matrix saturated with an aqueous alkaline
solution, such as potassium hydroxide (KOH).
The world's first Fuel Cell Ship HYDRA used an
AFC system with 6.5 kW net output.
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Fuel cells: types
Aqueous alkaline solutions do not reject carbon dioxide (CO2) so the fuel
cell can become "poisoned" through the conversion of KOH to potassium
carbonate (K2CO3). Because of this, alkaline fuel cells typically operate on
pure oxygen, or at least purified air and would incorporate a 'scrubber' into
the design to clean out as much of the carbon dioxide as is possible.
Because the generation and storage requirements of oxygen make pureoxygen AFCs expensive, there are few companies engaged in active
development of the technology. There is, however, some debate in the
research community over whether the poisoning is permanent or
reversible. The main mechanisms of poisoning are blocking of the pores in
the cathode with K2CO3, which is not reversible, and reduction in the ionic
conductivity of the electrolyte, which may be reversible by returning the
KOH to its original concentration. An alternate method involves simply
replacing the KOH which returns the cell back to its original output.
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Because of this poisoning effect, two main types of AFCs exist: static
electrolyte and flowing electrolyte. Static, or immobilized, electrolyte cells
(used by NASA) typically use an asbestos separator saturated in potassium
hydroxide. Water production is managed by evaporation out the anode,
which produces pure water that may be used. These fuel cells typically use
platinum catalysts to achieve maximum efficiencies.
Flowing electrolyte designs use a more open matrix that allows the
electrolyte to flow either between the electrodes (parallel to the electrodes)
or through the electrodes in a transverse direction.”
The chemical reactions inside an AFC are:
At anode:
(7.84.1)
At cathode:
(7.84.2)
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3. Phosphoric acid fuel cell
Phosphoric acid fuel cell technology has already moved from the
laboratory to the first stages of its commercial applications. Today, 200 kW
plants are available built at more than 70 sites in the US., Japan, and
Europe. Operating at approximately 200 C, the PAFC plant also produces
heat for water and space heating; its electrical efficiency is close to 40%.
Its high cost is the main obstacle that stops this technology from its wide
commercial acceptance. Presently, capital cost of PAFC plant is
approximately $2,500-$4,000 per kW. It is assumed that this technology
may be accepted by the power industry if its cost reduces to $1,000$1,500 per kW.
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The chemical reactions inside a PAFC are:
At anode:
(7.86.1)
At cathode:
(7.86.2)
PAFC have been used for
stationary power generators with
output in the 100 kW to 400 kW
range; they are also used in large
vehicles such as buses.
Major manufacturers of PAFC
technology include ClearEdge
Power and Fuji Electric.
PureCell system 400 CEP
Source: http://en.wikipedia.org/
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4. Molten carbonate fuel cell
Molten carbonate fuel cell technology is attractive since it offers several
potential advantages over PAFC. Carbon monoxide is indirectly used in the
MCFC. The higher operating temperature of approximately 650C makes
the MCFC a better candidate for combined-cycle applications, where the
fuel cell exhaust can be used as input to the intake of a gas turbine or the
boiler of a steam turbine.
The total efficiency can approach 85%. Capital costs are expected to be
lower than PAFC. Molten carbonate fuel cells were (as for 2007) tested in
full-scale demonstration plants.
The chemical reactions inside a MCFC are:
At anode:
(7.87.1)
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Fuel cells: types
and
(7.88.1)
At cathode:
(7.88.2)
February 2009: Molten Carbonate
Fuel Cell with bio-methane fuel is
used to power part of Munich data
center of the company T-Systems.
MCFC generates 245 kW.
Source: http://www.datacenterknowledge.com/
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5. Solid oxide fuel cell
Solid oxide fuel cell an electrochemical conversion device that produces
electricity directly from oxidizing fuel and was (as for 2007) demonstrated
at a 100 kW plant. This technology requires very significant changes in the
structure of the cell. A solid ceramic electrolyte is used, so the electrolyte
does not need to be replenished during the operational life of the cell.
This leads to simplifications in design, operation, and maintenance;
potential cost reductions are predicted. Additionally, such electrolytes offer
the increased stability, reliability of all solid-state constructions and permit
higher operational temperatures.
The ceramic composition of the cell leads to cost-efficient fabrication
techniques. Its tolerance to impure fuel streams makes SOFC systems
especially attractive for utilizing H2 and CO from natural gas steam-forming
and goal gasification plants.
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Fuel cells: types
The chemical reactions inside a SOFC are:
At anode:
and
(7.90.1)
(7.90.2)
At cathode:
(7.90.3)
Delphi SOFC
auxiliary power unit
being currently (as for
June 2013) field
evaluated.
Source: http://delphi.com/
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