Hybrid Systems Review

Hybrid Energy Systems

J. F. MANWELLUniversity of Massachusetts

Amherst, Massachusetts, United States

1. Introduction to Hybrid Energy Systems

2. Characteristics of Hybrid Energy Systems

3. Technology Used in Hybrid Energy Systems

4. Energy Loads

5. Renewable Energy Resource Characteristics

6. Design Considerations

7. Economics

8. Trends in Hybrid Energy Systems

Glossary

biomass Any organic material that is used as a fuel.deferrable load An electrical load that must be supplied a

certain amount of energy over a specified time period inan isolated electrical network, but that has someflexibility as to exactly when.

diesel generator A device for producing electricity throughthe use of a diesel engine.

dump load A device used to dissipate power in order tomaintain stability in an isolated hybrid energy system.

fuel cell A device for continuously producing electricitythrough a reaction between hydrogen and oxygen.

hybrid energy system A combination of two or moreenergy conversion devices (e.g., electricity generators orstorage devices), or two or more fuels for the samedevice, that, when integrated, overcome limitations thatmay be inherent in either.

load (1) An energy consuming device; (2) the cumulativepower requirement due to a number of devicesconnected to an electrical network.

maximum power point tracker A DC-DC converter whosefunction is to maximize the power produced by avariable voltage DC generator, such as PV panels or awind turbine, connected to a fixed voltage bus.

optional load An electrical load which can be supplied byexcess power in an isolated electrical network but neednot be supplied if no excess power is available.

penetration The instantaneous power from the renewablegenerator divided by the total electrical load beingserved in an isolated electrical network.

photovoltaic panel (PV) A semiconductor-based devicethat converts sunlight to electricity.

renewable energy source An energy source that is derivedultimately from the sun, moon, or earth’s internal heat.

supervisory control A central control system whose func-tion is to ensure proper operation of all the deviceswithin a hybrid energy system.

weak grid An electrical network whose operationalcharacteristics can be affected by the presence of agenerator or load connected to it.

wind turbine A device that converts the power in the windto electrical or mechanical power.

The term hybrid energy system refers to thoseapplications in which multiple energy conversiondevices are used together to supply an energyrequirement. These systems are often used in isolatedapplications and normally include at least one renew-able energy source in the configuration. Hybrid energysystems are used an alternative to more conventionalsystems, which typically are based on a single fossilfuel source. Hybrid energy systems may also be usedas part of distributed generation application inconventional electricity grid. The most general defini-tion is the following: ‘‘Hybrid energy systems arecombinations of two or more energy conversiondevices (e.g., electricity generators or storage devices),or two or more fuels for the same device, that whenintegrated, overcome limitations that may be inherentin either.’’ This definition is useful because it includesa wide range of possibilities and the essential featureof the multiplicity of energy conversion. This articlefocuses on stationary power systems, where at leastone of the energy conversion devices is powered by arenewable energy source (which, in the context of thisarticle, is one based ultimately on the sun).

1. INTRODUCTION TO HYBRIDENERGY SYSTEMS

A considerable interest has emerged in combined or‘hybrid’ energy systems. In the context used here,

Encyclopedia of Energy, Volume 3. r 2004 Elsevier Inc. All rights reserved. 215

that refers to an application in which multiple energyconversion devices are used together to supply anenergy requirement. These systems are often used inisolated applications and normally include at leastone renewable energy source in the configuration.Hybrid systems are used as an alternative to moreconventional systems, which typically are based on asingle fossil fuel source.

1.1 Definitions

Hybrid energy systems have been defined in anumber of ways. The most general, and probablymost useful, is the following:

‘‘Hybrid energy systems are combinations of twoor more energy conversion devices (e.g., electricitygenerators or storage devices), or two or more fuelsfor the same device, that when integrated, overcomelimitations that may be inherent in either.’’

This definition is useful because it includes a widerange of possibilities and the essential feature of themultiplicity of energy conversion. Note that this broaddefinition does not necessarily include a renewableenergy based device and allows for transportationenergy systems. In the present discussion, however, wefocus on stationary power systems, where at least oneof the energy conversion devices is powered by arenewable energy source (which is one based ulti-mately on the sun, moon, or earth’s internal heat).

For the purpose of comparison, it is useful toconsider briefly the nature of conventional energysystems that are normally used where hybrid systemmight be used instead. There are basically three typesof conventional systems of interest: (1) large utilitynetworks, (2) isolated networks, and (3) smallelectrical load with dedicated generator.

Large utility networks consist of power plants,transmission lines, distribution lines and electricalconsumers (loads). These networks are based onalternating current (AC) with constant frequency.Such networks are frequently assumed to have aninfinite bus. This means that the voltage andfrequency are unaffected by the presence of addi-tional generators or loads.

Isolated electrical networks are found on manyislands or other remote locations. They are similar inmany ways to large networks, but they are normallysupplied by one or more diesel generators. Generally,they do not have a transmission system distinct fromthe distribution system. Isolated networks do notbehave as an infinite bus and may be affected byadditional generators or loads.

For many small applications, it is common tosupply an electrical load with a dedicated generator.This is the case, for example, at construction sites,highway signs, and vacation cabins. These systems arealso normally AC but have no distribution system.

1.2 Applications for HybridEnergy Systems

There are numerous possible applications for hybridpower systems. The most common examples are (1)remote AC network, (2) distributed generationapplications in a conventional utility network, and(3) isolated or special purpose electrical loads.

The classic example of the hybrid energy system isthe remote, diesel-powered AC network. The basicgoal is to decrease the amount of fuel consumed bydiesel generators and to decrease the number ofhours that they operate. The first addition to‘‘hybridize’’ the system is to add another type ofgenerator, normally using a renewable source. Theserenewable generators are most commonly windturbines or photovoltaic panels. Experience hasshown, however, that simply adding another gen-erator is not sufficient to produce the desired results.Accordingly, most hybrid systems also include one ormore of the following: supervisory control system,short-term energy storage, and load management.Each of these will be described in more detail. Anexample of a typical hybrid energy system, in thiscase a wind/diesel system, is illustrated in Fig. 1.

During the 1990s, management of many largeelectrical networks changed so that it is now possible

WindturbineDiesel

gensets

Storagesystem

Primaryload

Dumpload

FIGURE 1 Schematic of wind diesel system with storage.

216 Hybrid Energy Systems

for individuals or businesses to add generation in thedistribution system of the utility. This is known asdistributed generation (DG). Distributed generationcan have a variety of purposes, but in any case, it issometimes desirable to combine a number of energyconversion devices together with the distributedgenerator. This results in a hybrid energy system inthe distributed generation application.

Hybrid systems can be of particular value inconjunction with an isolated or special-purposeapplication. One example is that of using photo-voltaic panels, together with some battery storageand power electronic converters to supply a smallamount of energy to a load in a remote location.Other examples include water pumping or waterdesalination. In these applications, electrical loadsmay take a range of forms. They may be conven-tional AC, DC, or even variable voltage and variablefrequency.

1.3 Impetus for Hybrid Energy Systems

There are a variety of reasons why a hybrid systemmight be used. An important reason is the reductionof fossil fuel use. Fossil fuel can be costly, especiallyin remote locations where the cost of transportingthe fuel to the site must be considered. In manyremote locations fuel oil must be stored, often for anentire winter. Reducing the amount of fuel use canreduce storage costs.

An isolated hybrid power system can be analternative to power line construction or power lineupgrade. Hybrid power systems, because theyinclude so many components, tend to be relativelyexpensive. When they can be used as an alternative topower line construction, the savings can helpcompensate for the cost of the hybrid system.

Under some conditions a power line alreadyexists, but is not capable of carrying the desiredcurrent. This could occur, for example, when a newload is added. In this case, the hybrid system wouldbe a type of distributed generation. The hybridsystem would only need to provide the differencebetween the total load and what the existing linescould carry. The presence of the existing line couldsimplify the design of the hybrid system, in that itneed not necessarily be required to set the power linefrequency and might not have either storage or dumploads (q.v.).

Distributed generation has advantages in a num-ber of situations, and, in some cases, the overallbenefit is enhanced by using a hybrid system. Onecommon application for distributed generation is

combined heat and power (CHP). In this case, wasteheat associated with fossil fuel combustion is usedfor space or process heating, thereby increasing theoverall energetic efficiency of the fuel use.

Use of hybrid systems can result in local environ-mental benefits. In particular, diesel generators (inisolated applications) should run less often and forshorter periods. This will reduce locally produced airpollution and noise, as well as reducing risksassociated with fuel transport.

2. CHARACTERISTICS OFHYBRID ENERGY SYSTEMS

The characteristics and components of a hybridsystem depend greatly on the application. The mostimportant consideration is whether the system isisolated or connected to a central utility grid.

2.1 Central Grid ConnectedHybrid Systems

If the hybrid system is connected to a central utilitygrid, as in a DG application, then the design issimplified to a certain degree and the number ofcomponents may be reduced. This is because thevoltage and frequency are set by the utility systemand need not be controlled by the hybrid system. Inaddition, the grid normally provides the reactivepower. When more energy is required than suppliedby the hybrid system the deficit can be in general beprovided by the utility. Similarly, any excess pro-duced by the hybrid system can be absorbed by theutility. In some cases, the grid does not act as aninfinite bus, however. It is then said to be ‘‘weak.’’Additional components and control may need to beadded. The grid connected hybrid system will thencome to more closely resemble an isolated one.

2.2 Isolated Grid Hybrid Systems

Isolated grid hybrid systems differ in many waysfrom most of those connected to a central grid. First,they must be able to provide for all the energy that isrequired at any time on the grid or find a gracefulway to shed load when they cannot. They must beable to set the grid frequency and control the voltage.The latter requirement implies that they must be ableto provide reactive power as needed. Under certainconditions, renewable generators may produce en-ergy in excess of what is needed. This energy must be

Hybrid Energy Systems 217

dissipated in some way so as not to introduceinstabilities into the system.

There are basically two types of isolated gridhybrid systems which include a renewable energygenerator among their components. These are knownas low penetration or high penetration. In thiscontext, ‘‘penetration’’ is defined as the instantaneouspower from the renewable generator divided by thetotal electrical load being served. Low penetration,which is on the order of 20% or less, signifies that theimpact of the renewable generator on the grid isminor, and little or no special equipment or control isrequired. High penetration, which is typically over50% and may exceed 100%, signifies that the impactof the renewable generator on the grid is significantand special equipment or control is almost certainlyrequired. High-penetration systems may incorporatesupervisory control, so-called dump loads, short-term storage, and load management systems.

Two important considerations in an isolatedsystem are whether the system can at times runtotally on the renewable source (without any dieselgenerator on) and whether the renewable source canrun in parallel with (i.e., at the same time as) thediesel generator. It is most common for one or theother to be possible (and normal). It is less commonthat both modes of operation are possible. This lattersystem offers the greatest fuel savings but is morecomplicated.

2.3 Isolated or Special PurposeHybrid Systems

Some hybrid systems are used for a dedicatedpurpose, without use of real distribution network.These special purposes could include water pumping,aerating, heating, desalination, or running grindersor other machinery. Design of these systems isusually such that system frequency and voltagecontrol are not major issues, nor is excess powerproduction. In those cases where energy may berequired even when renewable source be temporarilyunavailable, a more conventional generator may beprovided. Renewable generators in small isolatedsystems typically do not run in parallel with a fossilfuel generator.

3. TECHNOLOGY USED IN HYBRIDENERGY SYSTEMS

A wide range of technology may be used in a hybridenergy system. This section describes some of them in

more detail than was done in previous sections.Devices to be discussed include energy consumingdevices (loads), rotating electrical machinery, renew-able energy converters, fossil fuel generators (often,but not always, diesels), energy storage devices,power converters, control systems, and load manage-ment devices. Some of the various possible devicesand arrangements that may be found in hybridenergy system are illustrated in Fig. 2. The primaryfocus here is on isolated network hybrid systems, butmuch of the technology applies to other types ofhybrid systems as well.

3.1 Energy Consuming Devices

Hybrid energy systems typically use the same types ofenergy consuming devices that are found in conven-tional systems. These include lights, heaters, motors,and electronic devices. The combined energy require-ment of all the devices is know as the total load, orjust load. The load will typically vary significantlyover the day and over the year. An example of a loadvarying over a year on an isolated island is shownin Fig. 3.

3.2 Rotating Electrical Machinery

Rotating electrical machinery is found in manyplaces in a hybrid energy system. Most suchmachines can function as either motors or genera-tors, depending on the application. This sectionfocuses on the generating function.

3.2.1 Induction GeneratorsThe induction generator has been the most commontype of generator used in wind turbines. They arealso occasionally used with other prime movers, suchas hydro turbines or landfill gas fueled internalcombustion engines. More information on inductionmachines is provided elsewhere in this encyclopedia,so they will not be discussed in detail here. As far ashybrid systems, however, there are two importantconsiderations. First of all, induction machinesrequire a significant amount of reactive power. Thisis not a fatal problem, but it does affect the design ofthe system in a number of ways.

The second consideration is starting. When aninduction machine is brought on line from stand still,it requires much higher current than when operatingnormally. Provision must be made to ensure that thehybrid system has the capability of starting anyinduction motors or generators that it connectedto it.


3.2.2 Synchronous GeneratorsSynchronous generators (SG) may also be used witha variety of prime movers. These generators arediscussed in detail elsewhere in this encyclopedia. In

hybrid system applications SGs are of one of twotypes: those with electromagnetic fields and thoseusing permanent magnets. The first type, operatingwith a voltage regulator, can maintain the voltage on

AC

busDC

busAC generators

Wind turbine

PV array with MPPT

Diesel generators

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Primary

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Optional

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Rectifier

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converter

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PV array

Diesel generators

Bidirectional

converter

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Primary

Deferrable

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Dump

DC storage

AC machineDiesel engine

Clutch

Shaft busDC machine

FIGURE 2 Devices and arrangements in hybrid energy systems.

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FIGURE 3 Daily electrical loads on Cuttyhunk Island, Massachusetts.


the electrical network and provide reactive powerrequired by other devices in the system. Permanentmagnet SGs are often used in small wind turbines.Such generators cannot maintain voltage and so arenormally used in conjunction with power electronicconverters.

A synchronous machine can be brought on line sothat it will run in parallel with other generators.When this is done, particular attention must be takenso that the various generators are in phase witheach other.

3.3 Renewable Energy Generators

Renewable energy generators are devices that con-vert energy from its original form in the renewableenergy source into electricity. Renewable energygenerators that are most likely to be found in hybridenergy systems include wind turbines and photo-voltaic panels. Some hybrid energy systems usehydroelectric generators, biomass fueled generators,or fuel cells. It should be noted that many renewableenergy generators include rotating electrical ma-chines acting in the generating mode, which is alsocalled a generator. It should be clear from the contextwhat is meant.

3.3.1 Wind TurbinesWind turbines are devices that convert the energy inwind into electricity. A typical wind turbine is shownin Fig. 4. The main parts of a wind turbine are therotor, the drive train (including the generator), mainframe, tower, foundation, and control system. Therotor consists of the blades and a hub. The bladesserve to convert the force of the wind to a torque thatultimately drives the generator.

The two most important features of a windturbine as far as a hybrid energy system is concernedare the type of generator and the nature of the rotorcontrol. Most wind turbines use induction genera-tors, although some use synchronous generators. Ineither case the generator may be connected directlyto the electrical network or it may be connectedindirectly through a power electronic converter.

There are two main forms of rotor control onwind turbines: stall control and pitch control. Themost important function of rotor control is to protectthe wind turbine from high winds. In hybrid systems,rotor control is also important because it affects onhow energy flows are regulated within the hybridsystem. Under some conditions (with pitch control),the rotor can be controlled to facilitate start up or toreduce production when full output is not required.

Details on rotor control may be found elsewhere inthis encyclopedia or in Manwell et al.

The power generated by a wind turbine varies inrelation to the wind speed. That relation is summar-ized in a power curve, a typical example of which isillustrated in Fig. 5.

Nacelle cover

Rot

or

Hub

Control

Drive train Generator

Main frame/yaw system

Tow

er

Balance ofelectrical system

Foundation

FIGURE 4 Components of typical wind turbine.

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50454035302520151050

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Sample power curve

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d tu

rbin

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, kW

FIGURE 5 Typical wind turbine power curve.


3.3.2 Photovoltaic PanelsPhotovoltaic (PV) panels are used to produceelectricity directly from sunlight. PV panels consistof a number of individual cells connected together toproduce electricity of a desired voltage. Photovoltaicpanels are inherently DC devices. To produce AC,they must be used together with an inverter.

Most PV cells are made from crystalline silicon.PV cells produce current in proportion to the solarradiation level (up to a certain voltage). The current/voltage relation of a typical silicon cell at a fixed levelof solar radiation is shown in Fig. 6. Since power isproportional to the product of current and voltage,the power from a PV cell will continue to increaseuntil the current begins to drop.

Because the maximum voltage from individualcells is less than 1 V, multiple cells are connectedtogether in series on a PV panel.

The actual radiation level at any given time at aparticular spot on the earth’s surface will varysignificantly over the year and over the day. SeeSection 5 for more details. More information on PVpanels themselves is provided elsewhere in thisencyclopedia.

3.3.3 Hydro TurbinesHydropower is one of the oldest forms of electricityproduction and is used in large electricity networksas well as small ones. Small hydroelectric plants areused in isolated systems in many parts of the world.Hybrid systems, which include hydroelectric plantswith other forms of generation, are relatively rare,but they do occur. The most common of these are inlocations where the resource varies significantly

over the year, and there is not enough water incertain seasons. A diesel generator is then usedinstead of the hydroelectric generator during thoseseasons. The primary requirements of a hydro-electric plant are a continuous source of water and achange in elevation. More information on hydro-electric generators is provided elsewhere in thisencyclopedia.

3.3.4 Biomass Fueled GeneratorsBiomass fueled generators are occasionally used inhybrid energy, though most often when they areemployed in isolated networks they are the onlypower plants used. Biomass fueled generators arequite similar to conventional coal or oil firedgenerating plants, except for the combustors them-selves and the fuel handling equipment.

Biomass is any organic material which is used as afuel. For purposes of power generation, the mostcommon sources of biomass include wastes fromwood products or the sugar cane industry. Otheragricultural wastes are sometimes used as well.

Biomass generators are sometimes fueled withlandfill gas. Such generators most commonly employan internal combustion engine as a prime mover.Occasionally fuel cells have been used.

3.3.5 Fuel CellsFuel cells can be used in hybrid power systems, andare expected to become more common as their costsdrop. Fuel cells ultimately run on hydrogen, but inmany cases the input fuel is some other gas, such asnatural gas or landfill gas.

Fuel cells can be thought of as a continuousbattery. The fundamental reaction is between hydro-gen and oxygen, the product of which is water andelectric current. The reaction takes place in thevicinity of a membrane, which serves to separate thevarious components of the electrolyte and theelectrodes. The electrodes are the terminals of thefuel cell and carry the current into an external circuit.Like batteries, fuel cells are inherently DC devices.Fuel cells can be used in an AC network if theiroutput is converted to AC via an inverter.

When natural gas or landfill gas is used as the fuel,it must first be used to produce hydrogen. This isdone in a reformer.

There are a number of different designs for fuelcells. They include proton exchange membrane(PEM), solid oxide, and molten carbonate. Moreinformation on fuel cells is presented elsewhere inthis encyclopedia.

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FIGURE 6 Photovoltaic cell current versus voltage.


3.4 Fossil Fuel Generators

Fossil fuel generators are commonly used in hybridenergy systems. In fact, most isolated power systemsat the present time are based on fossil fuel, usinginternal combustion engines as prime movers. Mostmedium-sized and larger isolated systems use dieselengine/generators. The smallest systems sometimesuse gasoline. Some very large isolated power systemssometimes use conventional oil-fired steam powerplants. They will not be discussed here, however.

3.4.1 Diesel Engine/GeneratorsDiesel engine generators are discussed elsewhere inthis encyclopedia, so they will only be summarizedhere. Emphasis is given to those aspects of relevanceto hybrid energy systems.

Diesel generators typically consist of three mainfunctional units: a diesel engine, a synchronousgenerator with voltage regulator, and a governor.

The fuel injection system is an important part ofthe diesel engine. Its function is to inject the properamount of fuel into the proper cylinder at theappropriate time in the cycle. The timing of theinjection is determined by the design of the engineitself; the amount of fuel is determined by thegovernor.

The diesel engine is normally connected directly toa synchronous generator. A voltage regulator ensuresthe proper voltage is produced. The frequency of theAC power is directly proportional to the enginespeed, which in turn is controlled by the governor.

Historically diesel generators have employed‘‘droop’’ type governors. A set of flyballs is drivenby the engine, so the speed varies in proportion to theengine speed. Through a set of linkages, the amountof fuel that can be injected at any time is variedaccording to how far the operating speed differs from(or ‘‘droops’’) from nominal. As the electrical load onthe generator increases, the droop increases andmore fuel is injected. Diesel generators are now morelikely to have electronic governors without droop,but the function is the same.

An important consideration regarding diesel gen-erators is their fuel consumption, both at full loadand part load. Diesel fuel consumption is frequentlydescribed in terms of electricity produced per gallonof fuel consumed. Full load values ranging from alow of 8 kWh/gallon to 14 kWh/gallon have beenreported.

Diesel engine generators are often called on tofollow the load. That means that their output mustbe equal to the system load (or to the system load less

the production of any other generators that might beon; this is called the net load). As the load may go upand down, so must the electricity generated. This isknown as part load operation. Generally, theconversion efficiency is less at part load than at fullload. Fuel consumption over the full range ofoperation is summarized in fuel curves. In thesecurves, fuel consumption is graphed against power.Figure 7 shows a typical example.

Regardless of efficiency considerations, manufac-turers normally recommend that diesel generatorsnot be run below some specified minimum powerlevel, known as the minimum load. Typically, theminimum recommended load is between 25% and50% of rated. Engines run for long periods at levelsbelow the minimum recommended can experience anumber of problems.

3.4.2 Gasoline GeneratorsGasoline generators are sometimes used for verysmall hybrid energy systems applications. Thesegenerators have the advantage that they are readilyavailable throughout the world and are relativelyexpensive. They are similar in many ways to dieselgenerators, except that use spark ignition, and havelower compression ratios. They also typically usecarburetors rather than fuel injection. The maindisadvantage of gasoline generators is that they areless efficient than diesel.

3.5 Energy Storage

Energy storage is often useful in hybrid energysystems. Energy storage can have two main func-tions. First of all, it can be used to adapt to amismatch between the electrical load and the renew-able energy resource. Second, it can be used tofacilitate the control and operation of the overallsystem. There are basically two types of energy

7.0

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15 kW diesel enginefuel consumption

Engine load, kW

FIGURE 7 Typical diesel engine generator fuel curve.


storage, convertible and end use. Convertible storageis that which can readily be converted back toelectricity. End-use storage can be applied to aparticular end-use requirement but may not readilybe converted back to electricity.

3.5.1 Convertible StorageThere are a number of convertible storage media,although only a few of them have been usedfrequently in hybrid energy systems. The mostcommonly used form of convertible storage is thebattery. Less commonly used, but frequently dis-cussed, forms include pumped hydroelectric, fly-wheels, compressed air, and hydrogen.

3.5.1.1 Batteries Batteries are the most com-monly used form of convertible storage for hybridenergy systems. They have been used both for shortterm (less than 1 hour) and long term (more than1 day) storage. A number of types of batteries havebeen developed. The most common type of storagebattery for hybrid applications is the lead acidbattery. Nickel cadmium has also been used occa-sionally. Batteries are discussed elsewhere in thisencyclopedia, so only those aspects most relevant tohybrid energy systems are summarized here.

As far as hybrid energy systems are concerned,there are five important performance characteristicsof batteries: (1) voltage, (2) energy storage capacity,(3) charge/discharge rates, (4) efficiency, and (5)battery lifetime.

Batteries by their nature are DC. Individualbatteries are made up by a number of cells in series,with each cell nominally two volts. Completebatteries are typically 2, 6, 12, or 24 Volts. Theactual terminal voltage will depend on three factors:(1) state of charge, (2) whether the battery is beingcharged or discharged, and (3) the rate of charge ordischarge.

The energy storage capacity is primarily a func-tion of battery voltage and the amount of charge itcan hold and then return. Charge is measured inunits of current times time (Ampere-hours). Theamount of charge that is stored in a battery at anyparticular time is often described with reference to itsfull state by the term ‘‘state of charge’’ (SOC).Discharging and charging back to a given level(normally fully charged) is referred to a cycle. Thetotal amount of charge that a battery can hold isprimarily a function of the amount of material usedin the construction.

Battery capacity is normally specified with refer-ence to a specific discharge rate. This is because the

apparent capacity of batteries actually differs withcharge and discharge rate. Higher rates result insmaller apparent capacities.

As energy storage media, batteries are not 100%efficient. That is, more energy is expended incharging than can be recovered. Overall efficienciesare typically in range of 50 to 80%.

An important characteristic of batteries is theiruseful lifetime. Experience has shown that theprocess of using batteries actually decreases theirstorage capacity until eventually the battery is nolonger useful. The important factors in battery lifeare the number of cycles and the depth of dischargein the cycles. Depending on the type of battery, thenumber of deep cycles to which a battery can besubjected ranges from a few thousand down tohundreds or even tens of cycles. The cycle life of atypical battery is illustrated in Fig. 8.

3.5.1.2 Pumped Hydro One form of convertiblestorage that has been applied in some hybrid energysystems is pumped storage. In this case, water ispumped from one reservoir at a low elevation up toone at a higher elevation. The amount of energy thatcan be stored is a function of the size of the reservoirand the difference in elevation. The overall efficiencyof the storage is a function of the efficiency of thepumps and turbines (which may be the same devices)and the hydraulic losses in the pipes connecting thetwo reservoirs. The use of pumped storage is limitedby the lack of sites where such facilities can beinstalled at a reasonable price.

3.5.1.3 Flywheels Flywheels can be used to storeenergy in a hybrid system. A flywheel energy storagesystem consists of the following components: (1) theflywheel itself, (2) an enclosure, usually evacuated tominimize frictional losses, (3) a variable speedmotor/generate to accelerate and decelerate the

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FIGURE 8 Battery cycle life.


wheel, and (4) a power electronic converter. Energyis absorbed when the wheel is accelerated up to itsmaximum speed and is released when the wheel isdecelerated to some lower speed. A power electronicconverter accompanies the flywheel and motor/generator because the input and output power toand from the motor/generator is typically variablevoltage and variable frequency AC. Flywheelstypically store relatively small amounts of energy,but they can absorb or release the energy at highrates. Thus in hybrid power systems they can be usedto smooth short term fluctuations in power (on theorder of seconds or minutes) and they can facilitatesystem control.

3.5.1.4 Compressed Air Compressed air can beused for storage in hybrid systems, and someexperimental prototypes have been built. Efficiencyis relatively low, however, and this method of storagehas not been widely used.

3.5.1.5 Hydrogen Hydrogen can be produced bythe electrolysis of water, using a renewable energysource for the electricity. The hydrogen can be storedindefinitely and then used in an internal combustionengine/generator or a fuel cell to generate electricityagain. This method of storage appears to have a lotof potential, but it is still quite expensive.

3.5.2 End-Use StorageEnd-use storage, as opposed to convertible storage,refers to the situation in which some product is createdthrough the use of electricity when it is available. Theproduct is then stored and made available when it isneeded. Load management schemes often incorporatesome end-use storage. Three examples are given next.

3.5.2.1 Thermal Energy A common form of end-use storage is thermal energy, most often in the formof hot water. Hot water can be used for space heatingapplications, domestic hot water, swimming pools,and so on. Hot water can be stored relativelyinexpensively in insulated water tanks. Dependingon the location of the water tanks relative to the enduse, the efficiency of the storage can be quite high.

3.5.2.2 Pumped Water Pumped water is anotherform of end-use storage. This form of storage has along history of use in conjunction with windmills. Inthis application, water for any plausible purpose ispumped into a reservoir or storage tank, usuallyelevated, from which it can be released as needed.When the storage is elevated, then the water can flow

by gravity to the point of use, so no further input ofexternally produced energy is required.

3.5.2.3 Pure Water Another, less common formof end-use storage is the production and then storageof pure water from salty or brackish water. Depend-ing on the quality of water at the input of theprocess, ultrafiltration, reverse osmosis, or vaporcompression evaporation may be used to produce thepure water. All of these are energy intensive, so theimplicit energy density of the storage is high.

3.6 Power Converters

For any hybrid energy system to function properly, itis common that one or more power converter beincorporated into the system. These are eitherelectromechanical or electronic devices.

3.6.1 Electromechanical Power ConvertersThere are at least two types of electromechanicalpower converters that have been used in hybridenergy systems. They include the rotary converterand the synchronous condenser.

3.6.1.1 Rotary Converter A rotary converter isan electromechanical device that converts AC to DCor vice versa. When it is converting AC to DC it is arectifier. When operating the other way it is aninverter. The rotary converter consists of tworotating electrical machines that are directly con-nected together. One of them is a DC machine; theother is an AC machine. Either of them can run as amotor or generator depending on the intendeddirection of power flow. The AC machine can beeither an induction machine or a synchronousmachine. Which is used will depend on the require-ments of the system.

Rotary converters have the advantage that theyare a rugged and well-understood technology thathas been around for many years. Their disadvantageis that their costs are high and their efficiencies arelower than are electronic devices that can serve thesame purpose.

3.6.1.2 Synchronous Condenser Synchronouscondenser is the name given to a synchronousmachine that is connected into an electrical networkto help in maintaining the system voltage. Thesynchronous machine in this case is essentially amotor to which no load is connected. A voltageregulator is connected to the synchronous machineand functions in the same way as described


previously. Synchronous condensers are used inhybrid energy systems when there are, for at leastsome of the time, no other synchronous machinesconnected. This is often the case in those systems thatinclude diesel generators but that are intended toallow all the diesels generators to be turned off undersome circumstances. For example, a synchronouscondenser could be used in a wind/diesel system inwhich the wind turbines used induction generatorsand could at times supply all of the electrical load.The diesel could then be turned off, but thesynchronous machine would be needed to supplythe required reactive power to the induction gen-erators and maintain system voltage in the process.

3.6.2 Electronic Power ConvertersA wide range of electronic devices has been devel-oped and adapted for use in hybrid energy systems.Many of them serve similar functions to theelectromechanical devices described previously, buthave a number of advantages, such as lower cost,higher efficiencies, and greater controllability. Thedevices of greatest interest include rectifiers, inver-ters, dump loads, and maximum power pointtrackers.

3.6.2.1 Rectifiers A rectifier is a device thatconverts AC to DC. The primary functional elementsare diodes, which only let current pass one way. Bysuitable layout of the diodes AC in a single or three-phase circuit is converted to a rippling, but singledirection, current. Capacitors may be used to smooththe resulting current.

3.6.2.2 Inverter An inverter is a device thatconverts DC to AC. The primary switching elementsare either silicon controlled rectifiers (SCRs) orpower transistors (IGBTs). They are arranged in abridge circuit and switched on (and off, in the case oftransistors) in such a way that an oscillating wave-form results. Some inverters operate in conjunctionwith other devices that set the system frequency.These are referred to as line commutated. Otherinverters have the capability to set frequencythemselves. These are called self-commutated.

3.6.3 Maximum Power Point TrackersAnother electronic device that may be used in hybridenergy systems, particularly ones with photovoltaicpanels is the maximum power tracker. This device isDC-DC converter than can be partially thought of asa DC transformer. Its function to provide a particulardesired output voltage to the rest of the system, while

adjusting the voltage at the input to allow themaximum power production by the generator that isconnected to it. In the case of photovoltaic panels,the voltage at the input will be the maximum powerpoint voltage corresponding to the incident solarradiation level. Maximum power point trackers havealso been used with small wind turbines.

3.7 Dump Loads

A dump load is device that is used to dissipate powerin order to maintain stability in an isolated hybridenergy system. Dump loads are used primarily tomaintain power balances. They may also be used tocontrol frequency. The most common use of a dumpload is in isolated wind/diesel systems where thepenetration of the wind energy generation is suchthat instantaneous power levels sometimes exceedthe system load less than minimum allowed dieselpower level. The dump load control can sense theexcess power and dissipate whatever is required toensure that the total generated power is exactly equalto the actual system load plus that which isdissipated. The dump load is virtually a requirementin high penetration wind/diesel systems using stallcontrolled wind turbines, because adjusting thepower is impossible. When pitch controlled turbinesare used it may be possible to dispense with the dumpload, since power can be limited by pitchingthe blades.

3.8 Supervisory Controller

Many hybrid systems, especially the more complexones, have a supervisory controller to ensure properoperation of all the devices within the system. Thepossible functions of a supervisory controller areillustrated in Fig. 9. The controller itself consists ofthree main functional units: (1) sensors, (2) logicalunit, and (3) control commands.

Diesel generators

Inverter

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FIGURE 9 Supervisory controller functions.


Sensors are distributed throughout the system.They provide information on power levels, operatingconditions, and so on. The information gatheredfrom the sensors is directed to the logical unit. Thelogical unit is based on a computer or microproces-sor. It will make decisions based on an internalalgorithm and the data from the sensors. Algorithmscan be fairly simple or quite complex.

The decisions made by the logical unit are referredto as dispatch decisions, since their function relatesto dispatching of the various devices in the system.Dispatch in this sense refers to turning a device on oroff, or in some cases to setting its power level. Thedispatch instructions from the supervisory controllerare sent to controllers of the various devices inthe system.

4. ENERGY LOADS

The energy supplied by a hybrid system can becategorized in a variety of ways. The first has to dowhether the energy supplied is electricity or heat.Within the electrical category, electricity loads areoften divided into primary and secondary loads.Primary loads are those that must be servedimmediately. Secondary loads are associated withload management, and they may be further dividedin what are known as deferrable and optional loads.Deferrable loads have some flexibility in when theyare served, while optional loads are those which are

only served if there happens to be sufficient excessenergy available to do so. Regardless of type, it maybe noted that loads frequently vary significantly fromone season to the next as well as over the week andover the day, as was discussed earlier.

5. RENEWABLE ENERGYRESOURCE CHARACTERISTICS

It is worth considering briefly the time varying natureof the various renewable resources that might beused in a hybrid system, because that nature willaffect the design and operation of the system.

5.1 Wind

The wind resource is ultimately generated by the sun,but it tends to be very dependent on location. Overmost of the earth, the average wind speed varies fromone season to another. It is also likely to be affectedby general weather patterns and the time of day. It isnot uncommon for a site to experience a number ofdays of relatively high winds and for those days to befollowed by others of lower winds. The daily andmonthly average wind speed for an island off thecoast of Massachusetts, illustrating these variations,is illustrated in Fig. 10. The wind also exhibits short-term variations in speed and direction. This is knownas turbulence. Turbulent fluctuations take place overtime periods of seconds to minutes.

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FIGURE 10 Daily and monthly wind speeds on Cuttyhunk Island, Massachusetts.


5.2 Solar Radiation

The solar radiation resource is fundamentally deter-mined by the location on the earth’s surface, the date,and the time of day. Those factors will determine themaximum level of radiation. Other factors, such asheight above sea level, water vapor or pollutants inthe atmosphere, and cloud cover, decrease theradiation level below the maximum possible. Solarradiation does not experience the same type ofturbulence that wind does, but there can be varia-tions over the short term. Most often, these arerelated to the passage of clouds. Figure 11 illustratesthe solar radiation over a 5-day period in Decemberin Boston, Massachusetts.

5.3 Hydropower

The hydropower resource at a site is a function of theamount of flowing water available (discharge) in ariver or stream and the change in elevation (head).The head is usually relative constant (affected onlyby high water during storms), but the amount ofwater available can vary significantly over time. Theaverage discharge is determined by rainfall and thedrainage area upstream of the site on which the rainfalls. Discharge will increase after storms anddecrease during droughts. Soil conditions and natureof the terrain can also affect the discharge. Short-term fluctuations are normally insignificant.

5.4 Biomass

Sources of biomass are forest or agricultural pro-ducts. The resource is ultimately a function of suchfactors as solar radiation, rainfall, soil conditions,temperatures, and the plant species that can begrown.

6. DESIGN CONSIDERATIONS

The design of a hybrid energy system will depend onthe type of application and the nature of theresources available. The primary consideration iswhether the system will be isolated or grid con-nected. Other important considerations include (1)how the various generator types will be intended tooperate with each other, (2) excess power dissipa-tion, (3) use of storage, (4) frequency control, (5)voltage control, (6) possible dynamic interactionsbetween components. Many of the possible technol-ogies were discussed previously and will not berepeated here.

6.1 Load Matching

In the design of an isolated hybrid system withrenewable energy source generators one matter ofparticular concern is known as load-matching. Thisrefers to coincidence in time (or lack thereof) of therenewable resource and the load. The match betweenthe two will affect the details of the design and willalso determine how much of the energy that can begenerated from the renewable energy source canactually be used. When too much is available, someof it may have to be dissipated. When too little isavailable, either storage or a conventional fueledgenerator will be needed.

Under ideal circumstances, the energy requirementwill match the available resource. Often, however,that is not the case. In many islands of the world, forexample, the largest loads are during the summertourist season, but the wind resource is greatest in thewinter. Figures 3 and 10 above illustrated a typicalexample of this for the load and wind, respectively.

When there is a mismatch between the resourceand the load, the system must be designed to functionproperly under all conditions. For example, a wind/diesel system designed for the situation shown inFigs. 3 and 10 would most likely be designed so thatwind energy could supply most of the load in thewinter, keeping most or all of the diesel generatorsoff for long periods, but in the summer, the dieselswould continue to run and the wind turbines wouldserve as fuel savers. Figure 12 illustrates the possibleenergy flows for a 50-second period for a hypothe-tical wind/diesel system on an isolated island. Thesystem illustrated has a primary load, an optionalheating load, a dump load, a wind turbine, andmultiple diesel engine/generators, at least one ofwhich must always remain on.

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FIGURE 11 Hourly solar for 5 days radiation in Boston,

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7. ECONOMICS

Whether or not a hybrid energy system is actuallyused, and what it looks like, is strongly related to theeconomics of the system. In particular, will the costof energy from the hybrid system be lower than thatof a more conventional alternative? The cost ofenergy of a hybrid system is determined primarily bytwo factors: the cost of the system and the amount ofuseful energy that is produced. Other factors that arealso important include the value of the energy, thecost of conventional energy, lifetime of the system,maintenance costs, and financial costs.

The cost of a hybrid energy system is first of allaffected by the cost of the individual components thatmake up the system. Installation of the componentsand integrating them into a functioning unit will alsocontribute to the total cost. Generally, the renewableenergy generators themselves are the most expensiveitems. For example, at the present time, wind turbinescost on the order of $1000 to $2000/kW, dependingon the size. Photovoltaic panels cost on the order of$6000 to $8000/kW. Diesel generators are also costly,but considerably less so than wind turbines. Costs of$250 to $500/kW are typical. Lead acid batteries cost$100 to $200/kWh.

The energy that can be produced by a renewableenergy generator will depend on the type ofgenerator, its productivity at different levels of the

resource, and the distribution of the occurrences ofthose levels. For example, a wind turbine willproduce different amounts of power depending onthe wind speed. If the average wind speed each hourover a year is known, this information can becombined with the data in the turbine power curveto predict the total energy that the turbine couldproduce. Similar curves describe the output ofphotovoltaic panels. These curves, combined withhourly data on the solar resource, can be used topredict the total annual energy from the panels.

When a renewable energy generator is operatingin a hybrid energy system, predicting the usefulenergy is not as simple as it would appear, based onthe previous discussion. First of all, because of alikely mismatch between the available resource andthe load, it is may be that not all of the energy can beused when it is available, except in very lowpenetration systems. When not all of the load issupplied by the renewable generator, the rest is madeup a conventional generator. Because of the difficultyin determining the amount of useful energy producedby the renewable energy generators in a hybridsystem and predicting the actual fuel savings,detailed computer models are frequently used tofacilitate the process.

The value of energy in a hybrid energy system isrelated to the nature of the energy produced, the costof the alternatives, and the vantage point of the

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FIGURE 12 Power flows in hypothetical wind/diesel system.


operator of the system. For example, the operator ofan island diesel power system, considering installinga wind turbine, would want to know that the cost ofthe turbine would be offset by the decrease in dieselfuel purchased.

Economics of hybrid energy systems are typicallyevaluated by a technique known as life cycle costing.This method takes account of the fact that hybridenergy systems have relatively high initial costs butlong lifetimes and low operating costs. These factors,together with various financial parameters, are usedto predict present value costs, which can then becompared with costs from the conventionalalternatives.

8. TRENDS IN HYBRIDENERGY SYSTEMS

Hybrid energy systems are still an emerging technol-ogy. It is expected that technology will continue toevolve in the future, so that it will have widerapplicability and lower costs. There will be morestandardized designs, and it will be easier to select asystem suited to particular applications. There willbe increased communication between components.This will facilitate control, monitoring, and diag-nosis. Finally, there will be increased use of power

electronic converters. Power electronic devices arealready used in many hybrid systems, and as costs godown and reliability improves, they are expected tobe used more and more.

SEE ALSO THEFOLLOWING ARTICLES

Biomass for Renewable Energy and Fuels � Cogen-eration � Economics of Energy Supply � Flywheels �

Fuel Cells � Geothermal Power Generation � Hydro-gen Production � Hydropower Technology � Photo-voltaic Energy: Stand-Alone and Grid-ConnectedSystems � Solar Thermal Power Generation � Waste-to-Energy Technology � Wind Farms

Further Reading

Hunter, R., and Elliot, G. (1994). ‘‘Wind-Diesel Systems.’’

Cambridge University Press, Cambridge, United Kingdom.

Lundsager, P., Binder, H., Clausen, H-E., Frandsen, S. Hansen, L.,and Hansen, J. (2001). ‘‘Isolated Systems with Wind Power.’’

Riso National Laboratory Report, Riso-R-1256 (EN), Roskilde,

Denmark, www.risoe.dk/rispubl/VEA/veapdf/ris-r-1256.pdf.

Manwell, J. F., Rogers, A. L., and McGowan, J. G. (2002). ‘‘WindEnergy Explained: Theory, Design and Application.’’

John Wiley & Sons, Chichester, United Kingdom.


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