hydrogen future of energy storage

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HYDROGEN FUTURE OF ENERGY STORAGE 1

DEPT. OF EEE MEA ENGG COLLEGE PMNA

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION

ENERGY conversion from renewable energy (RE) sources, in

particular through wind turbine generators (WTGs) and photovoltaic (PV) arrays with

suitable energy storage can play an important role in the development and operation

of RE systems. The integrated WTG and PV array system, based on long-term

seasonal energy storage as electrolytic hydrogen, is considered a promising

alternative to overcome the intermittence of the RE sources. In comparison to

commonly used battery storage, is well suited for seasonal storage applications

because its inherent high mass energy density leakage from the storage tank is

insignificant and it is easy to install anywhere. A typical self-sufficient RE system

must include both short-term and long-term energy storage. A battery bank is

commonly used for short-term energy storage due to its high round-trip efficiency,

convenience for charging/discharging, and also to take care of the effects caused

by instantaneous load ripples/spikes, electrolyzer transients, wind energy peaks.

However, batteries alone are not appropriate for long-term energy storage because of

their low energy density, self-discharge, and leakage. The combination

Of a battery bank with long-term energy storage in the form of can significantly

improve the performance of stand-alone RE systems. Also, the overall RE system

performance is very sensitive to local weather conditions. Thus, to achieve

an adequate performance from such a complex system, one requires appropriate

components and a well-designed control system in order to achieve autonomous

operation and energy management in the system. The Hydrogen Research Institute

(HRI) has designed and developed a control system with power conditioning devices

to manage the energy flow throughout a RE system to assure continuous supply of

energy to the load. A major emphasis of this work is to test the developed control

system for autonomous long-term operation and technical feasibility of the stand


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alone RE system based on. The system parameters are monitored continuously for

real-time operation and control.

A new methodology, based on the differences between wind power

generation and load variability, was developed in this study to optimize the

technology, energy capacity and power transfer of the hydrogen energy storage

method for specified applications. The dynamics of the complete hydrogen cycle

energy storage and recovery mechanism was investigated, specifically for potential

applications such as power smoothing and peak lopping. A time dependant model of

the efficiency of various hydrogen storage technologies, including high pressure

compression, low temperature liquefaction, metal hydrides and complex hydrides, has

been developed. Based on this study, a practical hydrogen energy storage system for a

5MW micro-grid application was designed

1.2 WHY HYDROGEN?

Why Hydrogen? Hydrogen is one of the promising alternatives that can be

used as an energy carrier. Hydrogen can be stored in vessels for later use as does

electricity in batteries. Many European countries have established hydrogen

transmission pipes, nearly 1600 km, and consider Hydrogen fuels as a prime source

for new energy supply. Electricity is used in electrolysers to split water molecules

into hydrogen and oxygen. Many industrial processes require hydrogen as aningredient, or produce hydrogen as a by-product. Hydrogen is used in refineries, and

also in the ammonia, methanol and metal industry. The majority of hydrogen

production facilities are based on using excess power from fossil and nuclear power

generating plants during low peak demand.

RES based hydrogen production offers a source of domestic and vehicular

energy with safer and lower levels of pollution. Hydrogen as automobile fuel is well

known and has been used in many countries.Recently, the United States, Canada and

13 other nations established the International Partnership for the Hydrogen Economy

(IPHE) to coordinate hydrogen research, development and technology, and have

committed to a roadmap that will put hydrogen vehicles in showrooms within the

next 15 years.


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Fossil fuels, which are non-renewable and will eventually exhausted, produce

large amounts of carbon dioxide, which can not only bring environment pollution, but

also can cause global warming. Therefore, reliable and affordable green energy is a

cornerstone for sustainable development. One potential renewable energy resource is

wind power. However, the intermittency of wind energy limits its penetration in

electricity networks. Although the overall demand could be easily met by energy

generated from wind turbines, there can be significant mismatches between the peak

load and maximum wind power generation. To explore this problem a stand-alone

demonstration power system with a wind turbine (600 kW) and hydrogen energy

storage was launched at the island of Utsira in Norway. The hydrogen storage system

includes water electrolysis (10 Nm/m3), compressed gas storage (2400 Nm3, 200

bar), hydrogen engine (55 kW), and a PEM fuel cell (10 kW) .In this system, a

flywheel, a synchronous generator, and a battery system is employed to ensure the

voltage and frequency. The system can supply 2-3 days full energy for 10 households.

Another example is in Uckermark, Germany where mixed renewable energy

generation such as wind, biogas, and solar energy is being studied with hydrogen

storage system. This plant includes three wind turbines with a total capacity of 6 GW

and abiogas unit producing gas from maize supplied by 21 local farmer


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CHAPTER 2

TECHNIQUE USED FOR H2 PRODUCTION

2.1 Electrolysis of water

Hydrogen can be made via high pressure electrolysis or low

pressure electrolysis of water. Current best processes have an efficiency of 50% to

80%,[21][22][23]so that 1 kg of hydrogen (which has an energy density of 143 MJ/kg,

about 40 kWh/kg) requires 50 to 79 kWh of electricity with traditional methods andcould be brought to 85% efficiency with new proposed methods, although efficiencies

in the order of 100% are theoretically possible. At 8 cents/kWh, that's $4.00/kg,

which is with traditional methods 3 to 10 times the price of hydrogen from steam

reformation of natural gas.[14]The price difference is due to the efficiency of direct

conversion of fossil fuels to produce hydrogen, rather than burning fuel to produce

electricity. Hydrogen from natural gas, used to replace e.g. gasoline, emits more CO2

than the gasoline it would replace, and so is no help in reducing greenhouse gases.

High-pressure electrolysis

High pressure electrolysis is the electrolysis of waterby decomposition

ofwater(H2O) into oxygen (O2) and hydrogen gas (H2) by means of anelectric

currentbeing passed through the water. The difference with a standard electrolyzeris

the compressed hydrogen output around 120-200Bar(1740-2900 psi). By pressurising

the hydrogen in the electrolyser the need for an external hydrogen compressoris

eliminated, the average energy consumption for internal compression is around 3%
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High-temperature electrolysis

Hydrogen can be generated from energy supplied in the form of heat and

electricity through high-temperature electrolysis (HTE). Because some of the energy

in HTE is supplied in the form of heat, less of the energy must be converted twice(from heat to electricity, and then to chemical form), and so potentially far less energy

is required per kilogram of hydrogen produced.

While nuclear-generated electricity could be used for electrolysis, nuclear heat

can be directly applied to split hydrogen from water. High temperature (950

1000 C) gas cooled nuclear reactors have the potential to split hydrogen from water

by thermo chemical means using nuclear heat.

Research into high-temperature nuclear reactors may eventually lead to a

hydrogen supply that is cost-competitive with natural gas steam reforming. General

Atomicspredicts that hydrogen produced in a High Temperature Gas Cooled Reactor

(HTGR) would cost $1.53/kg. In 2003, steam reforming of natural gas yielded

hydrogen at $1.40/kg. At 2005 natural gas prices, hydrogen costs $2.70/kg.

High-temperature electrolysis has been demonstrated in a laboratory, at 108 mega

joules (thermal) per kilogram of hydrogen produced, but not at a commercial scale. In

addition, this is lower-quality "commercial" grade Hydrogen, unsuitable for use infuel cells.

Photo electro chemical water splitting

Using electricity produced by photovoltaic systems offers the cleanest way to

produce hydrogen. Water is broken into hydrogen and oxygen by electrolysis

a photo electrochemical cell (PEC) process which is also named artificial

photosynthesis. Research aimed toward developing higher-efficiency multi-junction

cell technology is underway by the photovoltaic industry.

Concentrating solar thermal

Very high temperatures are required to dissociate water into hydrogen and

oxygen. A catalyst is required to make the process operate at feasible temperatures.

Heating the water can be achieved through the use ofconcentrating solar
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power. Hydrosol-2 is a 100-kilowatt pilot plant at the Plataforma Solar de

Almeria in Spain which uses sunlight to obtain the required 800 to 1,200 C to heat

water. Hydrosol II has been in operation since 2008. The design of this 100-kilowatt

pilot plant is based on a modular concept. As a result, it may be possible that this

technology could be readily scaled up to the megawatt range by multiplying the

available reactor units and by connecting the plant to heliostat fields (fields of sun-

tracking mirrors) of a suitable size.

Photo electro catalytic production

A method studied by Thomas Nann and his team at the University of East

Anglia consists of a gold electrode covered in layers of indium phosphide (InP) nano

particles. They introduced an iron-sulfur complex into the layered arrangement,

which when submerged in water and irradiated with light under small electric current,

produced hydrogen with an efficiency of 60%.

Thermo chemical production

There are more than 352 thermo chemical cycles which can be used

forwater splitting, around a dozen of these cycles such as the iron oxide

cycle, cerium(IV) oxide-cerium(III) oxide cycle, zinc zinc-oxide cycle, sulfur-iodine

cycle, copper-chlorine cycle and hybrid sulfur cycle are under research and in testingphase to produce hydrogen and oxygen from water and heat without using

electricity.[33]These processes can be more efficient than high-temperature

electrolysis, typical in the range from 35 % - 49 % LHV efficiency. Thermo chemical

production of hydrogen using chemical energy from coal or natural gas is generally

not considered, because the direct chemical path is more efficient.

None of the thermo chemical hydrogen production processes have been

demonstrated at production levels, although several have been demonstrated inlaboratories.
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CHAPTER3

HYDROGEN STORAGE&SAFETY

3.1 Storage

Although molecular hydrogen has very high energy density on a mass basis,

partly because of its low molecular weight, as a gas at ambient conditions it has very

low energy density by volume. If it is to be used as fuel stored on board the vehicle,

pure hydrogen gas must be pressurized or liquefied to provide sufficient drivingrange. Increasing gas pressure improves the energy density by volume, making for

smaller, but not lighter container tanks (see pressure vessel). Achieving higher

pressures necessitates greater use of external energy to power the compression.

Alternatively, higher volumetric energy density liquid hydrogen orslush

hydrogen may be used. However, liquid hydrogen is cryogenic and boils at 20.268 K

(252.882 C or 423.188 F). Cryogenic storage cuts weight but requires

large liquification energies. The liquefaction process, involving pressurizing and

cooling steps, is energy intensive. The liquefied hydrogen has lower energy density

by volume than gasoline by approximately a factor of four, because of the low density

of liquid hydrogen there is actually more hydrogen in a liter of gasoline

(116 grams) than there is in a liter of pure liquid hydrogen (71 grams).

Liquid hydrogen storage tanks must also be well insulated to minimize boil off. Ice

may form around the tank and help corrode it further if the liquid hydrogen tank

insulation fails.

The mass of the tanks needed forcompressed hydrogen reduces the fuel

economy of the vehicle. Because it is a small molecule, hydrogen tends to diffuse

through any liner material intended to contain it, leading to the embrittlement, or

weakening, of its container.
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Distinct from storing molecular hydrogen, hydrogen can be stored as a

chemical hydride or in some other hydrogen-containing compound. Hydrogen gas is

reacted with some other materials to produce the hydrogen storage material, which

can be transported relatively easily. At the point of use the hydrogen storage material

can be made to decompose, yielding hydrogen gas. As well as the mass and volume

density problems associated with molecular hydrogen storage, current barriers to

practical storage schemes stem from the high pressure and temperature conditions

needed for hydride formation and hydrogen release. For many potential systems

hydriding and dehydrating kinetics and heat management are also issues that need to

be overcome.

A third approach is to absorb molecular hydrogen into a solid storage

material. Unlike in the hydrides mentioned above, the hydrogen does not

dissociate/recombine upon charging/discharging the storage system, and hence does

not suffer from the kinetic limitations of many hydride storage systems. Hydrogen

densities similar to liquefied hydrogen can be achieved with appropriate absorption

media. Some suggested absorbers include MOFs, nanostructure carbons

(including CNTs) and clathrate hydrate.

The most common method of on board hydrogen storage in today's

demonstration vehicles is as a compressed gas at pressures of roughly 700 bar

(70 MPa).

Underground hydrogen storage is the practice ofhydrogen storage in

underground caverns, salt domes and depleted oil and gas fields. Large quantities of

gaseous hydrogen are stored in underground caverns by ICI for many years without

any difficulties. The storage of large quantities of hydrogen underground can function

as grid energy storage which is essential for the hydrogen economy.
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3.2 Hydrogen safety

Hydrogen has one of the widest explosive/ignition mix range with air of all

the gases with few exceptions such as acetylene, silane, and ethylene oxide. That

means that whatever the mix proportion between air and hydrogen, a hydrogen leak

will most likely lead to an explosion, not a mere flame, when a flame or spark ignites

the mixture. This makes the use of hydrogen particularly dangerous in enclosed areas

such as tunnels or underground parking. Pure hydrogen-oxygen flames burn in

the ultraviolet color range and are nearly invisible to the naked eye, so a flame

detectoris needed to detect if a hydrogen leak is burning. Hydrogen is odorless and

leaks cannot be detected by smell.

Hydrogen codes and standards are codes and standards for hydrogen fuel cellvehicles, stationary fuel cell applications and portable fuel cell applications. There are

codes and standards for the safe handling and storage of hydrogen, for example

the Standard for the installation of stationary fuel cell power systems from

the National Fire Protection Association.

Codes and standards have repeatedly been identified as a major institutional barrier to

deploying hydrogen technologies and developing a hydrogen economy. To enable the

commercialization of hydrogen in consumer products, new model building codes andequipment and other technical standards are developed and recognized by federal,

state, and local governments.

One of the measures on the roadmap is to implement higher safety standards

like early leak detection with hydrogen sensors. The Canadian Hydrogen Safety

Program concluded that hydrogen fueling is as safe as, or safer than, CNG

fueling. The European Commission has funded the first higher educational program

in the world in hydrogen safety engineering at the University of Ulster. It is expectedthat the general public will be able to use hydrogen technologies in everyday life with

at least the same level of safety and comfort as with today's fossil fuels.
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3.3 Fuel cells as alternative to internal combustion

One of the main offerings of a hydrogen economy is that the fuel can replace

the fossil fuel burned in internal combustion engines and turbines as the primary way

to convert chemical energy into kinetic or electrical energy; hereby eliminating

greenhouse gas emissions and pollution from that engine.

Although hydrogen can be used in conventional internal combustion engines, fuel

cells, being electrochemical, have a theoretical efficiency advantage over heat

engines. Fuel cells are more expensive to produce than common internal combustion

engines, but are becoming cheaper as new technologies and production systems

develop.

Some types of fuel cells work with hydrocarbon fuels, while all can be operated on

pure hydrogen. In the event that fuel cells become price-competitive with internal

combustion engines and turbines, large gas-fired power plants could adopt this

technology.

Hydrogen gas must be distinguished as "technical-grade" (five nines pure), which is

suitable for applications such as fuel cells, and "commercial-grade", which has

carbon- and sulfur-containing impurities, but which can be produced by the much

cheaper steam-reformation process. Fuel cells require high purity hydrogen because

the impurities would quickly degrade the life of the fuel cell stack.

Much of the interest in the hydrogen economy concept is focused on the use of fuel

cells to power electric cars. Current Hydrogen fuel cells suffer from a low power-to-

weight ratio.[40]Fuel cells are much more efficient than internal combustion engines,

and produce no harmful emissions. If a practical method ofhydrogen storage is

introduced, and fuel cells become cheaper, they can be economically viable to

powerhybrid fuel cell/battery vehicles, or purely fuel cell-driven ones. The economic

viability of fuel cell powered vehicles will improve as the hydrocarbon fuels used in

internal combustion engines become more expensive, because of the depletion of

easily accessible reserves or economic accounting of environmental impact through

such measures as carbon taxes.
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Currently it takes 2 times as much energy to make a hydrogen fuel cell than is

obtained from it during its service life.[41]

Other fuel cell technologies based on the exchange of metal ions (i.e. zinc-air fuel

cells) are typically more efficient at energy conversion than hydrogen fuel cells, butthe widespread use of any electrical energy chemical energy electrical energy

systems would necessitate the production of electricity.
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CHAPTER 4

SYSTEM DESCRIPTION

Fig:1 system description


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The stand-alone RE system based on hydrogen production has been

successfully tested at the HRI. The system consists of a 10-kW WTG and 1-kW

(peak) PV array as primary energy sources. The excess energy with respect to the

load requirement has been stored as electrolytic hydrogen through a 5-kW

electrolyzer and utilized to produce electricity as per energy demand through a 5-kW

fuel cell (FC) system.The electrolyzer and the FC system are major components of

the RE system for energy storage as H2 and its re-utilization. Their performance

(polarization) characteristics depend mainly on their voltage, current and temperature.

The RE system components have substantially different voltage-current

characteristics and are integrated through the developed power conditioning devices

on a 48-V dc bus, which allows power to be managed between input power, energy

storage and load.

The dc-dc buck and boost converters are connected for power conditioning

between the electrolyzer and the dc bus, and between the FC and the dc bus,

respectively. The schematic of the RE system is shown in Fig. 1 and the system

components specifications are given in Table I. To simulate any type of electrical

load profile, dc- and ac-programmable loads are used. The HRI developed RE system

has also a 10-kW programmable power source connected on the dc

Bus and can be used to test the system, when there is not enough power available

from RE sources. The programmable power source can simulate any type of

intermittent power output.

Current from the dc bus bar keeps batteries (short-term energy

storage) charged, feeds power to the load bank via an inverter and also supplies

power to the electrolyzer via power-conditioning device. Sensors are critical for

proper functioning of the entire system. The different sensors are used to record real

time voltages and currents of WTG, PV array, dc bus/batteries, electrolyzer, FC, load,

electrolytic H2 flow rate from the electrolyzer, consumption rate and oxidant

consumption rate of the FC, H2 and oxidant pressure of the FC, FC stack

temperature, electrolyzer cell temperature, dc-dc converter (boost and buck) duty

ratio,H2 detectors.


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The control system is complex and was a real challenge to design, because the

sensor data are required for decision-making and continuous real time autonomous

operation, and the same control algorithm sends signals to the power conditioning

devices on a real time basis for effective operation of the RE system.

4.2 System Control

The control system manages the energy flow among the different components of

the RE system. The control system consists a master controller for the overall energy

management, and secondary micro controllers, which manage the energy, flow

through power conditioning devices [10]. The control system has been designed to

maximize the direct energy flow from the RE sources to the electrolyzer and the load

in order to avoid losses in the buffer energy storage i.e., batteries. The dc bus

voltage depends on the operating conditions of the system.

Due to the intermittent nature of the RE sources; it varies instan instantaneously.

It also changes during battery charging/discharging, load peaks, electrolyzer ripples.

The dc bus voltage alone cannot be considered as a decision variable to control the

operation of RE system. The energy level at dc bus i.e., batteries energy level

[state-of-charge (SOC)] plays an important role for operation and control of the REsystem and it depends on the available energy from the primary sources, the load

requirement and the FC system output power. It allows effective energy management

among them. With respect to the energy level at the dc bus and pre-defined limits of

energy levels in the control algorithm, the master controller sends the conditioned

signal (duty ratio) to the secondary controllers for on/off operation of the electrolyzer

and the FC. As the specified energy levels at the dc bus are

reached, the control algorithm sends a conditioned signal (duty ratio) through micro-

controllers to the buck/boost converters for effective operation of the electrolyzer/FC

system. The secondary micro-controllers manage the power flow of electrolyzer

and FC, with respect to the energy availability at the dc bus through the digitally

controlled dc-dc converters. The dc-dc converters use multiphase technique to

generate pulse width modulation signals to control the power flow.


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The limits of energy levels in the control algorithm have been

managed through the double hysteresis strategy. The control algorithm has been

developed in such a way that the FC and the electrolyzer do not operate

simultaneously.

4.3 Boost converter

A boost converter (step-up converter) is a power converterwith an output DC voltage

greater than its input DC voltage. It is a class of switching-mode power supply

(SMPS) containing at least two semiconductorswitches (a diode and a transistor) and

at least one energy storage element. Filters made of capacitors (sometimes in

combination with inductors) are normally added to the output of the converter to

reduce output voltage ripple.

Fig:2 boost converter

The switch is typically a MOSFET, IGBT, orBJT.

Overview

Power can also come from DC sources such as batteries, solar panels, rectifiers and

DC generators. A process that changes one DC voltage to a different DC voltage is

called DC to DC conversion. A boost converter is a DC to DC converter with an

output voltage greater than the source voltage. A boost converter is sometimes called
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a step-up converter since it steps up the source voltage. Since power (P= VI) must

be conserved, the output current is lower than the source current.

History

For high efficiency, the SMPS switch must turn on and off quickly and have

low losses. The advent of a commercial semiconductor switch in the 1950s

represented a majormilestone that made SMPSs such as the boost converter possible.

Semiconductor switches turned on and off more quickly and lasted longer than other

switches such as vacuum tubes and electromechanical relays. The major DC to DC

converters were developed in the early 1960s when semiconductor switches had

become available. The aerospace industrys need for small, lightweight, and efficient

power converters led to the converters rapid development.

Switched systems such as SMPS are a challenge to design since its model depends on

whether a switch is opened or closed. R.D. Middlebrook from Caltech in 1977

published the models for DC to DC converters used today. Middlebrook averaged the

circuit configurations for each switch state in a technique called state-space

averaging. This simplification reduced two systems into one. The new model led to

insightful design equations which helped SMPS growth.

Applications

Battery powered systems often stack cells in series to achieve higher voltage.However, sufficient stacking of cells is not possible in many high voltage applications

due to lack of space. Boost converters can increase the voltage and reduce the number

of cells. Two battery-powered applications that use boost converters are hybrid

electric vehicles (HEV) and lighting systems.

The NHW20 model Toyota Prius HEV uses a 500 V motor. Without a boost

converter, the Prius would need nearly 417 cells to power the motor. However, a

Prius actually uses only 168 cells and boosts the battery voltage from 202 V to 500 V.

Boost converters also power devices at smaller scale applications, such as portable

lighting systems. A white LED typically requires 3.3 V to emit light, and a boost

converter can step up the voltage from a single 1.5 V alkaline cell to power the lamp.

Boost converters can also produce higher voltages to operate cold cathode fluorescent

tubes (CCFL) in devices such as LCD backlights and some flashlights.
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A boost converter is used as the voltage increase mechanism in the circuit known as

the 'Joule thief'. This circuit topology is used with low power battery applications, and

is aimed at the ability of a boost converter to 'steal' the remaining energy in a battery.

This energy would otherwise be wasted since the low voltage of a nearly depleted

battery makes it unusable for a normal load. This energy would otherwise remain

untapped because many applications do not allow enough current to flow through a

load when voltage decreases. This voltage decrease occurs as batteries become

depleted, and is a characteristic of the ubiquitous alkaline battery. Since (P= V2 /R)

as well, and R tends to be stable, power available to the load goes down significantly

as voltage decreases.

Circuit analysis

Operating principle

The key principle that drives the boost converter is the tendency of an

inductor to resist changes in current. When being charged it acts as a load and absorbs

energy (somewhat like a resistor); when being discharged it acts as an energy source

(somewhat like a battery). The voltage it produces during the discharge phase is

related to the rate of change of current, and not to the original charging voltage, thusallowing different input and output voltages.

The basic principle of a Boost converter consists of 2 distinct states

in the On-state, the switch S (see figure 1) is closed, resulting in an increase inthe inductor current;

in the Off-state, the switch is open and the only path offered to inductorcurrent is through the fly back diode D, the capacitor C and the load R. These

results in transferring the energy accumulated during the On-state into the

capacitor.

The input current is the same as the inductor current as can be seen in figure 2.So it is not discontinuous as in the buck converterand the requirements on the

input filter are relaxed compared to a buck converter.
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Continuous mode

When a boost converter operates in continuous mode, the current through the inductor

(IL) never falls to zero.

During the Off-state, the switch S is open, so the inductor current flows through the

load. If we consider zero voltage drop in the diode, and a capacitor large enough for

its voltage to remain constant

As we consider that the converter operates in steady-state conditions, the amount of

energy stored in each of its components has to be the same at the beginning and at the

end of a commutation cycle.

4.4 BUCK CONVERTER

A buck converter is a step-down DC to DC converter. Its design is

similar to the step-up boost converter, and like the boost converter it is a switched-

mode power supply that uses two switches (a transistorand a diode), an inductor and

a capacitor.

The simplest way to reduce the voltage of a DC supply is to use a linear

regulator (such as a 7805), but linear regulators waste energy as they operate by

bleeding off excess power as heat. Buck converters, on the other hand, can be

remarkably efficient (95% or higher for integrated circuits), making them useful for

tasks such as converting the 1224 V typical battery voltage in a laptop down to thefew volts needed by the processor.

Theory of operation

The operation of the buck converter is fairly simple, with an inductorand

two switches (usually a transistorand a diode) that control the inductor. It alternates

between connecting the inductor to source voltage to store energy in the inductor and

discharging the inductor into the load.

Continuous mode

A buck converter operates in continuous mode if the current through the inductor (IL)

never falls to zero during the commutation cycle. In this mode

When the switch pictured above is closed (On-state, top of figure 2), thevoltage across the inductor is VL = Vi Vo. The current through the inductor
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rises linearly. As the diode is reverse-biased by the voltage source V, no

current flows through it;

When the switch is opened (off state, bottom of figure 2), the diode is forwardbiased. The voltage across the inductor is VL= Vo (neglecting diode drop).

Current IL decreases.

Discontinuous mode

In some cases, the amount of energy required by the load is small enough to be

transferred in a time lower than the whole commutation period. In this case, the

current through the inductor falls to zero during part of the period. The only

difference in the principle described above is that the inductor is completely

discharged at the end of the commutation cycle (see figure 5). This has, however,

some effect on the previous equations.

Output voltage ripple

Output voltage ripple is the name given to the phenomenon where the output

voltage rises during the On-state and falls during the Off-state. Several factors

contribute to this including, but not limited to, switching frequency, output

capacitance, inductor, load and any current limiting features of the control circuitry.

At the most basic level the output voltage will rise and fall as a result of the outputcapacitor charging and discharging:

During the Off-state, the current in this equation is the load current. In the

On-state the current is the difference between the switch current (or source current)

and the load current. The duration of time (dT) is defined by the duty cycle and by the

switching frequency.

Qualitatively, as the output capacitor or switching frequency increase, the magnitude

of the ripple decreases. Output voltage ripple is typically a design specification for the

power supply and is selected based on several factors. Capacitor selection is normally

determined based on cost, physical size and non-idealities of various capacitor types.

Switching frequency selection is typically determined based on efficiency

requirements, which tends to decrease at higher operating frequencies, as described


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below in Effects of non-ideality on the efficiency. Higher switching frequency can

also reduce efficiency and possibly raise EMI concerns.

Output voltage ripple is one of the disadvantages of a switching power supply, and

can also be a measure of its quality.

Effects of non-ideality on the efficiency

A simplified analysis of the buck converter, as described above, does not

account for non-idealities of the circuit components nor does it account for the

required control circuitry. Power losses due to the control circuitry are usually

insignificant when compared with the losses in the power devices (switches, diodes,

inductors, etc.) The non-idealities of the power devices account for the bulk of the

power losses in the converter.

Both static and dynamic power losses occur in any switching regulator.

Static power losses includeI2R (conduction) losses in the wires or PCB traces, as well

as in the switches and inductor, as in any electrical circuit. Dynamic power losses

occur as a result of switching, such as the charging and discharging of the switch

gate, and are proportional to the switching

4.5 HYDROGEN STORAGE

Hydrogen has a very high enthalpy of 120MJ/kg [5], which is about 3times that of Gasoline. Therefore, hydrogen is a good candidate as an energy carrier

and methods for its storage have been investigated intensively. Five basic methods

are proposed in the literature for hydrogen storage: compressed and stored in a

pressure tank; cooled to a liquid state and kept cold in an insulated tank; physisorpted

in carbon; metal hydrides and complex compounds. In order to choose the optimize

method to integrate in the wind power system; the hydrogen storage capacity of each

method has been compared, see Figure 2. As can be seen, metal hydrides and

complex compounds occupy a smaller volume to store the same amount of hydrogen;

however, this method is not suitable for this application due to its high ad/absorption

temperature. Both liquid hydrogen and compressed gas at high pressure were better

candidates for suitable methods for this project, however, liquid hydrogen requires

more expensive equipments and very low temperature. Figure there are various
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inefficiencies involved with storage and recovery of electrical energy via the use of

hydrogen. Energy is consumed to place the hydrogen in storage. This varies with

the different energy storage approaches, the efficiency of each method is summarized

in Table 1. From this we see that the energy lost to compress the gas is relatively low

and therefore yields higher conversion efficiency. Activated carbon also has a high

efficiency; however, a very low temperature is required during the process. During

the storage period, the hydrogen leakage rate should also be considered because it is

part of systems dynamic efficiency. As shown in Table.1, the compressed gas method

has a very low leakage rate compared to the other methods. Without considering the

operational losses, the main los

The excess produced energy has been stored in the form of Electrolytic H2

through the electrolyzer unit, which consists of a control unit, a compressor, and

purification and drying process. The electrolyzer input power consists of the cell

and the parasitic power consumption of the H2 production process. The parasitic

component consists of the power for the process control and the power for the gas-

handling unit, i.e., the compressor. s in compressed gas is permeation.

4.6 FUEL CELLS

A single fuel cell consists of an electrolyte sandwiched between two

electrodes, an anode and a cathode. Bipolar plates on either side of the cell help

distribute gases and serve as current collectors. In a Polymer Electrolyte Membrane

(PEM) fuel cell, which is widely regarded as the most promising for light-duty

transportation, hydrogen gas flows through channels to the anode, where a catalyst

causes the hydrogen molecules to separate into protons and electrons. The membrane

allows only the protons to pass through it. While the protons are conducted through

the membrane to the other side of the cell, the stream of negatively-charged electronsfollows an external circuit to the cathode. This flow of electrons is electricity that can

be used to do work, such as power a motor.


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Fig:3 fuel cell

On the other side of the cell, oxygen gas, typically drawn from the outside air,

flows through channels to the cathode. When the electrons return from doing work,

they react with oxygen and the hydrogen protons (which have moved through the

membrane) at the cathode to form water. This union is an exothermic reaction,

generating heat that can be used outside the fuel cell.

The power produced by a fuel cell depends on several factors, including the

fuel cell type, size, temperature at which it operates, and pressure at which gases are

supplied. A single fuel cell produces approximately 1 volt or less barely enough

electricity for even the smallest applications. To increase the amount of electricity

generated, individual fuel cells are combined in series to form a stack. (The term fuel

cell is often used to refer to the entire stack, as well as to the individual cell.)

Depending on the application, a fuel cell stack may contain only a few or as many as

hundreds of individual cells layered together. This scalability makes fuel cells ideal


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for a wide variety of applications, from laptop computers (50-100 Watts) to homes (1-

5kW), vehicles (50-125 kW), and central power generation (1-200 MW or more).

Types of Fuel Cells

Fuel cells are classified primarily by the kind of electrolyte they employ. This

classification determines the kind of chemical reactions that take place in the cell, the

kind of catalysts required, the temperature range in which the cell operates, the fuel

required, and other factors. These characteristics, in turn, affect the applications for

which these cells are most suitable. There are several types of fuel cells currently

under development, each with its own advantages, limitations, and potential

applications. Learn more about:

Polymer Electrolyte Membrane (PEM) Fuel Cells Direct Methanol Fuel Cells Alkaline Fuel Cells Phosphoric Acid Fuel Cells Molten Carbonate Fuel Cells Solid Oxide Fuel Cells

Regenerative Fuel Cells Comparison of Fuel Cell Technologies

Polymer Electrolyte Membrane (PEM) Fuel Cells
http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#pemhttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#methanolhttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#alkalinehttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#phosphorichttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#moltenhttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#oxidehttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#fchttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#comparisonhttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#comparisonhttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#fchttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#oxidehttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#moltenhttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#phosphorichttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#alkalinehttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#methanolhttp://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html#pem


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Fig:4 PEM fuel cell

Polymer electrolyte membrane (PEM) fuel cellsalso called proton exchange

membrane fuel cellsdeliver high-power density and offer the advantages of low

weight and volume, compared with other fuel cells. PEM fuel cells use a solid

polymer as an electrolyte and porous carbon electrodes containing a platinum

catalyst. They need only hydrogen, oxygen from the air, and water to operate and do

not require corrosive fluids like some fuel cells. They are typically fueled with pure

hydrogen supplied from storage tanks or on-board reformers.

Polymer electrolyte membrane fuel cells operate at relatively low

temperatures, around 80C (176F). Low-temperature operation allows them to start

quickly (less warm-up time) and results in less wear on system components, resulting

in better durability. However, it requires that a noble-metal catalyst (typically

platinum) be used to separate the hydrogen's electrons and protons, adding to system

cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it

necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen

is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are

currently exploring platinum/ruthenium catalysts that are more resistant to CO.

PEM fuel cells are used primarily for transportation applications and some

stationary applications. Due to their fast startup time, low sensitivity to orientation,


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and favorable power-to-weight ratio, PEM fuel cells are particularly suitable for use

in passenger vehicles, such as cars and buses.

A significant barrier to using these fuel cells in vehicles is hydrogen storage.

Most fuel cell vehicles (FCVs) powered by pure hydrogen must store the hydrogen

on-board as a compressed gas in pressurized tanks. Due to the low-energy density of

hydrogen, it is difficult to store enough hydrogen on-board to allow vehicles to travel

the same distance as gasoline-powered vehicles before refueling, typically 300400

miles. Higher-density liquid fuels, such as methanol, ethanol, natural gas, liquefied

petroleum gas, and gasoline, can be used for fuel, but the vehicles must have an on-

board fuel processor to reform the methanol to hydrogen. This requirement increases

costs and maintenance. The reformer also releases carbon dioxide (a greenhouse gas),though less than that emitted from current gasoline-powered engines.

Direct Methanol Fuel Cells

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell

system directly or can be generated within the fuel cell system by reforming

hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct

methanol fuel cells (DMFCs), however, are powered by pure methanol, which ismixed with steam and fed directly to the fuel cell anode.

Direct methanol fuel cells do not have many of the fuel storage problems

typical of some fuel cells because methanol has a higher energy density than

hydrogenthough less than gasoline or diesel fuel. Methanol is also easier to

transport and supply to the public using our current infrastructure because it is a

liquid, like gasoline.

Direct methanol fuel cell technology is relatively new compared with that of

fuel cells powered by pure hydrogen, and DMFC research and development is

roughly 34 years behind that for other fuel cell types.


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Alkaline Fuel Cells

Fig:5 alkaline fuel cell

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies

developed, and they were the first type widely used in the U.S. space program to

produce electrical energy and water on-board spacecrafts. These fuel cells use a

solution of potassium hydroxide in water as the electrolyte and can use a variety of

non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs

operate at temperatures between 100C and 250C (212F and 482F). However,

newer AFC designs operate at lower temperatures of roughly 23C to 70C (74F to

158F)

AFCs' high performance is due to the rate at which chemical reactions take

place in the cell. They have also demonstrated efficiencies near 60% in space

applications.

The disadvantage of this fuel cell type is that it is easily poisoned by carbon

dioxide (CO2). In fact, even the small amount of CO2 in the air can affect this cell's

operation, making it necessary to purify both the hydrogen and oxygen used in the

cell. This purification process is costly. Susceptibility to poisoning also affects the

cell's lifetime (the amount of time before it must be replaced), further adding to cost.


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Cost is less of a factor for remote locations, such as space or under the sea.

However, to effectively compete in most mainstream commercial markets, these fuel

cells will have to become more cost-effective. AFC stacks have been shown to

maintain sufficiently stable operation for more than 8,000 operating hours. To be

economically viable in large-scale utility applications, these fuel cells need to reach

operating times exceeding 40,000 hours, something that has not yet been achieved

due to material durability issues. This obstacle is possibly the most significant in

commercializing this fuel cell technology.

Phosphoric Acid Fuel Cells

Phosphoric acid fuel cells use liquid

phosphoric acid as an electrolytethe acid iscontained in a Teflon-bonded silicon carbide

matrixand porous carbon electrodes containing a

platinum catalyst. The chemical reactions that take place in the cell are shown in the

diagram to the right. Fig:6 PAFC

fuel cell

The phosphoric acid fuel cell (PAFC) is considered the "first generation" of

modern fuel cells. It is one of the most mature cell types and the first to be used

commercially. This type of fuel cell is typically used for stationary power generation,

but some PAFCs have been used to power large vehicles such as city buses.

PAFCs are more tolerant of impurities in fossil fuels that have been reformed

into hydrogen than PEM cells, which are easily "poisoned" by carbon monoxide


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because carbon monoxide binds to the platinum catalyst at the anode, decreasing the

fuel cell's efficiency. They are 85% efficient when used for the co-generation of

electricity and heat but less efficient at generating electricity alone (37%42%). This

is only slightly more efficient than combustion-based power plants, which typically

operate at 33%35% efficiency. PAFCs are also less powerful than other fuel cells,

given the same weight and volume. As a result, these fuel cells are typically large and

heavy. PAFCs are also expensive. Like PEM fuel cells, PAFCs require an expensive

platinum catalyst, which raises the cost of the fuel cell.

Molten Carbonate Fuel Cells

Molten carbonate fuel cells

(MCFCs) are currently being

developed for natural gas and coal-

based power plants for electrical utility,

industrial, and military applications.

MCFCs are high-temperature fuel cells

that use an electrolyte composed of amolten carbonate salt mixture

suspended in a porous, chemically inert

ceramic lithium aluminum oxide

(LiAlO2) matrix. Because they operate at extremely high temperatures of

Fig:7 molten carbonate fuel cell

650C (roughly 1,200F) and above, non-precious metals can be used as catalysts atthe anode and cathode, reducing costs.

Improved efficiency is another reason MCFCs offer significant cost

reductions over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel cells,

when coupled with a turbine, can reach efficiencies approaching 65%, considerably


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higher than the 37%42% efficiencies of a phosphoric acid fuel cell plant. When the

waste heat is captured and used, overall fuel efficiencies can be as high as 85%.

Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel

cells, MCFCs do not require an external reformer to convert more energy-dense fuels

to hydrogen. Due to the high temperatures at which MCFCs operate, these fuels are

converted to hydrogen within the fuel cell itself by a process called internal

reforming, which also reduces cost.

Molten carbonate fuel cells are not prone to carbon monoxide or carbon

dioxide "poisoning" they can even use carbon oxides as fuelmaking them more

attractive for fueling with gases made from coal. Because they are more resistant to

impurities than other fuel cell types, scientists believe that they could even be capable

of internal reforming of coal, assuming they can be made resistant to impurities such

as sulfur and particulates that result from converting coal, a dirtier fossil fuel source

than many others, into hydrogen.

The primary disadvantage of current MCFC technology is durability. The high

temperatures at which these cells operate and the corrosive electrolyte used accelerate

component breakdown and corrosion, decreasing cell life. Scientists are currentlyexploring corrosion-resistant materials for components as well as fuel cell designs

that increase cell life without decreasing performance.

Solid Oxide Fuel Cells


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Fig:8 SOFC fuel cell

Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as

the electrolyte. Because the electrolyte is a solid, the cells do not have to be

constructed in the plate-like configuration typical of other fuel cell types. SOFCs are

expected to be around 50%60% efficient at converting fuel to electricity. In

applications designed to capture and utilize the system's waste heat (co-generation),

overall fuel use efficiencies could top 80%85%.

Solid oxide fuel cells operate at very high temperaturesaround 1,000C

(1,830F). High-temperature operation removes the need for precious-metal catalyst,

thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables

the use of a variety of fuels and reduces the cost associated with adding a reformer to

the system.

SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate

several orders of magnitude more of sulfur than other cell types. In addition, they are

not poisoned by carbon monoxide (CO), which can even be used as fuel. This

property allows SOFCs to use gases made from coal.

High-temperature operation has disadvantages. It results in a slow startup and

requires significant thermal shielding to retain heat and protect personnel, which may

be acceptable for utility applications but not for transportation and small portable

applications. The high operating temperatures also place stringent durability

requirements on materials. The development of low-cost materials with high

durability at cell operating temperatures is the key technical challenge facing this

technology.

Scientists are currently exploring the potential for developing lower-

temperature SOFCs operating at or below 800C that have fewer durability problems

and cost less. Lower-temperature SOFCs produce less electrical power, however, and

stack materials that will function in this lower temperature range have not been

identified.


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Regenerative Fuel Cells

Regenerative fuel cells produce electricity from hydrogen and oxygen and

generate heat and water as byproducts, just like other fuel cells. However,

regenerative fuel cell systems can also use electricity from solar power or some other

source to divide the excess water into oxygen and hydrogen fuelthis process is

called "electrolysis." This is a comparatively young fuel cell technology being

developed by NASA and others.

4.7 STORAGE SYSTEMS TO ADDRESS ELECTRICITY SUPPLY

INTERMITTENCY

Storage elements are integrated with RES systems in

many ways using power electronic switching converters [6-7]. Figure 2 shows how

one storage system is incorporated to regulate the voltage and power of transmission

lines carrying power from RES energy stream. The storage element is incorporated

within a unified power flow controller (UPFC) made of controlled switching


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4.8 SIMULATION OF THE BUFFERING PROCESS

The imbalance (or difference) between (e.g. wind power) supply and demand

can vary significantly. Here it is assumed that (with the help of permanently updated

weather and load forecasts) the mean imbalance can be predicted sufficiently well for

timescales of about a minute (in less favorable cases it may be a few minutes). But

momentary fluctuations of the imbalance can still be significant, especially during

phases of strong wind power supply. Arbitrary 24-hour imbalance curves

which also include windless phases, were created. Simple control algorithms for the

hybrid energy storage system were tested as regards their usefulness for buffering

these imbalances.

The forecasted mean imbalance is used to define the operating conditions for

the slow EES which consists of modular electrolyzer and fuel cell blocks. For a

positive imbalance the electrolyzer blocks produce H2 whereas for a negative one H2

is used-up in the fuel cell blocks. If the EES system is

operated too far away from this mean imbalance, then the H2 system can be

adapted by switching an additional block on or off. The algorithm also takes into

account the current charging status of the SMES i.e. whether the currently stored

energy is below or above certain thresholds (20% and 80% of the storage capacity).

Below the lower threshold, the EES operating level is increased by either increasingthe number of active electrolyzer blocks by one (for a positive imbalance), or by

decreasing the number of active fuel cell blocks by one (for a negative imbalance).

Accordingly, the EES operating level the operational level of the H2 part is then

compensated by the faster (and more efficient) SMES. The control of the thresholds

then ensures that the SMES can take up or deliver short term power at any time.


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CHAPTER 5

COMPARISON

5.1Cost comparison

The costs of energy storage technologies for distributed generation (DG)

are displayed in Table I. In a system, there are many kinds of costs, used for setting

up, running, fixing, replacement, revenue, etc. The capital cost is the setting up

fee.

table:1 cost comparison

5.2 Other Compar isons

Flywheels, NaS batteries, and hydrogen storage methods including water

electrolysis, compressed gas, and fuel cell, are compared in Table II from several

aspects in terms of energy losses, efficiencies, costs, response times, and

lifetimes, etc, to find out an optimum way for micro-grid application. Flywheels are

mature technologies that are used commercially. Hydrogen storage is quite a new

technology in this area. It is a good candidate chosen in this application for

it is suitable for large scale, long storage time and reduced environmental impact.

In the hydrogen storage application, efficiency is the whole hydrogen storage

systems, the loss only stands for the compressed gas process, and the other aspects for

it presented in the table below are the lowest level during the three


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Unitswater electrolysis, compressed gas, and fuel cell. From Table II, we can only

see the hydrogen storage method has the lowest efficiency, shortest replacement

period and high cost, etc. Actually, only the fuel cell part contributes it. Hydrogen

combustion cell instead of it will be investigated to improve it in later research.


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Table:2 other comparison


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CHAPTER6

CONCLUSION

A new wind power design methodology that identifies the optimal use of

hydrogen energy storage in order to balance the electricity production to load demand

has been described. Different hydrogen storage methods were carefully compared

and the compressed gas approach was chosen as the best solution for this study due to

its relatively high conversion efficiency, easy operation and low leakage rate. The

Methodology was tested using a case study based on the wind and load data for

University of Bath, UK.

The results showed that the electricity demands can be met entirely locally by

the equivalent of a 48.4 m radius wind turbine in conjunction with a compressed

hydrogen energy storage and recovery system with a 2000 m3 capacity. The storage

size was calculated using the minimum of the integrated energy balance curve for a

complete annual cycle of data and identifying the maximum depth of storage capacity

from this. The size of the wind turbine required depends on the size of storage size. It

has been shown that, for this case study, a wind turbine of 48.4 m ensures that the

micro-grid becomes self sufficient with hydrogen storage. There are further studies

that need to be performed in the future work for this research. The method needs to be

extended to include more dynamic storage models, where the energy leaks over time

as shown in Figure 4.

The method should also use multi-year data to get a more sustainable and

accurate design for the renewable energy installation. Neither of these extensions to

the work is incompatible with the approach taken so far. Hydrogen energy storage toprovide off-grid renewable energy smoothing certainly seems practical when

optimized using the methodology described within this pape


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hydrogen future of energy storage

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