hlerpinp Photovoltaic Hardware DeveloDment with
Hvbrid Awlications in the U.S.A.*
Mr. Ward Bower
Senior Member of the Technical Staff
Sandia
NationalLaboratories
-- POBox5800
Photovoltaics Systems Research -- Department 6218
Albuquerque, NM 87185 U.S.A.
Ph (505) 844-5206 FAX (505) 844-6541
Abstract: The use of multi-source power systems, “hybrids,” is one of the fastest growing,
potentially significant markets for photovoltaic (PV) system technology today. Cost-effective
applications today include remote facility power, remote area power supplies, remote home and
village power, and power for dedicated electrical loads such as communications systems. This
market sector is anticipated to be one of the most important growth opportunities for PV over the
next five yearsi The U.S. Department of Energy (USDOE) and Sandia National Laboratories
(SNL) are currently engaged in an effort to accelerate the adoption of market-driven PV hybrid
power systems and to effectively integrate PV with other energy sources. This paper provides
details of this development and the ongoing hybrid activities in the United States. Hybrid systems
are the primary focus of this paper.
Introduction: The USDOE National Photovoltaic Program and SNL are currently engaged in an
effort to accelerate the adoption of market-driven PV hybrid power systems by administering a
“Hybrid Applications Program” and by funding development and improvements of hardware,
referred to as balance-of-system (BOS) hardware, required to integrate PV energy sources into
operational systems. In addition, the Design Assistance Center @AC) at SNL is working to
transfer technology of PV hybrid systems to potential users to encourage the use of PV and
PV/hybridrenewableenergysources.SNL hasidentifiedelectricutility companies,
stateagencies,
and federal agencies as near-term users of cost-effective PV-hybrid energy sources. Through the
Committee on Renewable Energy Commerce and Trade (CORECT), the United States Agency for
International Development (USAID), World Bank, and World Health Organization, international
activities are also under way at SNL. SNL is active in Indonesia, Mexico, Central America, South
America, and the Caribbean Islands to promote the use of cost-effective PV hybrid systems where
they are preferred over engine-generators (E-Gs) or stand-alone renewable systems.
Hybrid PV/engine-generator systems are generally preferred when fuel delivery costs are extreme,
where fuel storage must be limited, where engine-generator noise or pollution can’t be tolerated, or
where PV as the only energy source would be too expensive or intermittent due to clouds. Enginegenerator, PV, wind turbines, and battery storage are typically the energy sources and energy
storage medium used in hybrid systems.
The National Renewable Energy Laboratory (NREL) is also administering a hybrid development
effort under its wind energy program, and is administering the Brazilian electrification program
that is today focusing on stand-alone PV homes, but is expected to include PV hybrid systems for
village electrification in the future. The work at both National Laboratories is enhancing the
market for PV technology in hybrid and other applications, and is providing the impetus to merge
today’s R&D efforts with present-day cost-effective applications.
Hybrid Svstem DescriDtions: Hybrid systems are composed of more than one source of electrical
generation and are configured so that loads can be served directly or indirectly by one or more of
l
This work was supportedby the United StatesDepartmentof Energyunder ContractDE-ACOd-91AL85000
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Merging Photovoltaic Hardware Deveiopment with Hybrid Applications in the U.S.A.
2
these sources. More specifically, most hybrid systems today have an engine-generator, typically
driven by a diesel or propane engine, and a renewable source, such as PV or wind. Battery storage
and a means to convert dc energy into ac energy that is compatible with the distribution system and
the loads complete the energy-delivery side of the system. The dc to ac conversion hardware is
typically a power electronic device, such as an inverter or power processing system,but may be an
electromechanical generator.
Generally, the hybrid system design is
bounded by economic or electrical
Fig 1. “Charge Cycle” System
performance goals and predetermined
needs, but the configuration and
&
‘ii ?%a.
component sizes of the hybrid energy
/i-x
jz
system
generally determine the overall
o i$ 60
%ph
L-J
performance of the system. The hybrid
g g 40”
system design is very different if the
20 :
8
system is designed to provide electrical
ua
0’
backup power or light loads with PV
O~coNaN~~eJco~
sized to keep the batteries charged, if it is
d d 4 ,’
hjeitic4
designed to reduce or augment diesel fuel
Day of Operation
use and engine run time, or if the loads
are periodically
very
heavy or
unpredictable. If the hybrid systemis designedto optimize the diesel contribution to the energy
system,then a charge cycle or pulse charging algorithm is used, with the PV output smaller than
the daily average load, and storage sized to be cycled completely on a periodic basis [1,2]. Figure
1 shows a typical battery state-of-chargefor a charge cycle system, with batteries gradually
discharging, but supplemented by PV each day. The batteries are recharged completely on a
periodic basis each time the state-of-charge drops below a preset value. If a hybrid system is
designed to nearly eliminate the engine-generator, then storage and renewable components typically
are larger.
Hardware Reauirements A hybrid system may be configured in a series topology, a parallel
topology or a switched topology [3]. Each of these topologies is briefly discussed below, using an
engine-generator as one energy source, and each has a set of advantagesand disadvantages.
Figure 2. SeriesHybrid SystemBlock Diagram
Engine
Driver
-
Generator
,
I
Charger
I
Source
I
Battery
Bank
Loads
,
Battery
Renewable
I
-
Jnverter I
Series Topology: The series hybrid
system topology block diagram is
shown in Figure 2. This configuration
passes all of the energy through the
battery by rectifying all ac source
power. DC power is supplied to the
battery either from the renewable
energy source through a charge
controller or a maximum power
tracker/regulator. m of the ac energy
delivered to the load is converted from
Vet-y Large Battery Bank with Frequent Cycling
battery dc to regulated ac with an
Inverter Mud be Rated for Peaks
electronic inverter or motor-generator
Reduced Efiiciency but No Transfer Interruptions
set. This configuration can operate in
a manual mode or an automatic mode with the addition of appropriate controls for sensing battery
voltageand automaticallystartingandstoppingthe engine-generator.
Merging Photovoltaic Hardware Development with Hybrid Applications in the U.S.A.
3
The advantages of this system include the fact that ac switching is eliminated and the output
interface is simplified. There are no power interrupts when the engine-generator is activated. The
inverter for this system can be either a sine-wave inverter or a quasi-sine wave inverter, so long as
the load can tolerate the harmonic distortion associated with the quasi-sine wave power quality.
Inverters for this application are commercially available and relatively low cost, with costs ranging
from $.60 to $1.00 per watt. The engine-generator can be sized to be optimally loaded while
charging the battery bank and will generally be smaller when the series hybrid system is used.
Disadvantages of the series system include the fact that the inverter cannot operate in parallel with
the generator, hence the inverter must be sized to handle the peak load of the system. The battery
bank in this system is exercised extensively. The cycling profile requires a large battery bank to
reduce the depth-of-discharge and allow the engine-generator to operate in a higher performance
regime for a larger percentage of the time when recharging the battery. The series hybrid
configuration exhibits reduced overall efficiency, because all energy must flow through the
inverter. There is no redundancy built into this design, and inverter failure results in loss of power
to the load.
Figure 3. Switched Hybrid System Block Diagran
Switched Topology:- The switched
configuration is shown in Figure 3 and
is the most common installation
It allows operation with
today[3].
either the engine-generator or the
&itch
I
inverter as the generation source. The
Inverter
,
battery bank is charged by either the
engine-generator or the renewable
energy source, but the cycling of the
batteries is reduced because the load is
supplied directly from the engine~Dlrsel & Inverter Must be Sized to Load
generator *when it is running. The
Power is lnterruoted at Transfer
battery charger is a commercially
$0 Optimization’ControI
available component. The inverter
may be either a sine-wave or quasi-sine wave. This configuration can operate in a manual mode or
an automatic mode with the addition of appropriate controls for sensing battery voltage, and
automatically starting and stopping the engine-generator.
The advantages of the switched hybrid system topology include the fact that the inverter for this
system can be either a sine-wave inverter or a quasi-sine wave inverter so long as the load can
tolerate the harmonic distortion associated with the quasi-sine wave power quality. Either source
may power the load directly, and redundancy is a natural feature of the design.
The disadvantages include a power interrupt to the load when ac power sources are transferred.
Both the engine-generator and the inverter are large since they must be sized to handle peak loads.
No significant optimization control is possible.
Parallel Topology: The parallel hybrid energy system block diagram is shown in Figure 4. This
configuration allows either source to supply the loads and allows both sources to be connected to
Merging Photovoltaic Hardware Developmentwith Hybrid Applications in the U.S.A.
4
loads at the sametime. The inverter for this topology is bidirectional and can act as a battery
chargeror as a dc to ac converter[4]. Th? inyerter must be ableto perform either as a stand-alone
or as a utility-interactive inverter. The bidirectional inverter may provide peak shavingwhen the
engine-generator
is overloadedaspart of an optimizationcontrol.
Figure 4. Parallel Hybrid SystemBlock Diagram
Engine
-
Generator
0 IO
I
DXiVfX
I
Synchrjmized
*
Loads
co&Q1
Invelter ,
II
Renewable
solute
-
Battery
Bank’
’
SYControl
’
The advantagesof the parallel hybrid
energy systemtopology include the fact
that optimum system performance is
achievableby selecting the appropriate
source of energy and then optimizing
that source by providing battery
chargingat the appropriatepower level.
This configuration allows a smaller
engine-generator,a smaller inverter, and
a smallerbatterybank to be used.
III
The disadvantagesinclude the fact that
the control systemis now complex and
not commercially available. The bidirectional inverter is commerciallyavailableat this time, but integratedcontrols need additional
developmentfor implementationinto a parallel hybrid energysystem. The synchronizedcontrol is
still custom-designed
hardwareandneedsadditionaldevelopment.
Inverter is Bi-Directional, Sinewave
Smaller Diesel, Jnverter, & Ba!kry Possible
Optimum Performance is Achievable
Hybrid control methodologiesand algorithms
used for the control strategiescan be optimized for each type of application [2,3]. The most
commonapplicationis that of engine-generator
augmentationwherethe PV is sizedto augmentthe
engine-generator’byreducing enginerun time and optimizing the loading on the generator. This
type of system generally receivesbetween40% to 70% of the system energy from PV. The
augmentationtype of system is generally used in applicationssuch as village power where fuel
delivery andmaintenancefor the engine-generator
are economicallyaccessible.
Hvbrid Control MethodoloPies and Alporithms:
A displacementtype of systemis designedso that PV would displacethe engine-generatorand the
fuel it useswith 70% to 90% of the energycomingfrom PV. The engine-generatoris mainly used
to equalizethe battery banks or to act as a back-up generator. Displacementsystemsare used
whereboth fuel delivery or storageandmaintenancearevery costly or difficult.
The remainderof this paper discussesthe elementsof the work
under way at the USDOE and SNL to promote the use of PV in cost-effectiveapplications.
Cooperativeparticipation with utilities, federal and stateagencies,and other institutional usershas
developedinto a highly leveragedprogram with cost-shareddollars. The elementspresentedhere
focus on hardwaredevelopmentand evaluation,applicationsprograms,and activities in the public
andprivate marketsectors.
Hvbrid Propram Discussion:
Improved (BOS) componentssuch as static power
converters(SPCs),power processingcenters(PPCs),control hardware, and systemsoftware are
neededfor almostall of today’sphotovoltaichybrid applications. DC load-breakhardware,ground
fault detectors and interrupters, fieldable dc-rated interconnects, improved battery charge
controllers, user-friendly componentsizing computersoftware programs, control methodologies,
Hardware Development and Evaluation:
Merging PhotovoltaicHardware Developmentwith Hybrid Applications in the U.S.A.
5
.
and modelingprogramsare also neededfor hybrid systemapplications. Many of theseelements
are commerciallyavailabletoday, but modularity, reliability, and systemcompatibility are often
compromisedbecauseof operational incompatibilities. Several evolutionary developmentsfor
controllers,power processing,and dc switch hardwareareunder way at SNL, through the USDOE
National PhotovoltaicProgram,to develophardwarefor near-termapplications.
Hardwareidentification and developmentalsupportof evolutionarychangesin existing and/or nonexisting BOS hardwarefor hybrid applicationsare the bridges necessaryto merge today’s R&D
activities with tomorrow’s marketableapplications,and are a key aspectof this program. Control
algorithms and methodologiesfor componentssuch as battery storage, engine-generatorand
specific loads are also being developedtoday for specific near-term applications. Follow-up
feedbackwith designersand the manufacturersis resulting in improvementsin systemperformance
and is helping to provide marketablehardwareat SNL andthe Florida Solar Energy Center[s].
Control concepts,hardwareperformance,requiredmaintenanceand reliability, andultimately costeffectivenesscan be assessedonly by the installation, operation,and analysesof fielded systems,
andthoseanalysesof recentlyinstalledsystemsare currently under way by SNL and the Southwest
TechnologyDevelopmentInstitute. Laboratory testing and analysisare also an important part of
this effort, andtestsarebeing conductedon batteries,chargecontrollers,power processingcenters,
inverters,and other importantcomponentsmakingup the balance-of-system
hardwarefor hybrids.
The performanceof hardware, a comparisonof software simulations and theory, and detailed
performanceof commerciallyavailablehardwareand systemsare being compiledinto a databaseas
part of this work. The effort includes remote hybrid system monitoring, site evaluationsand
comparisonsof the results to performancepredictions. Better modeling and sizing software are
anticipatedas the new databaseinformation becomesavailablefor detailed modeling inputs and
performancepredictions.
Stmtegic Envimrunenfal Research & Development Pmgmm (SERDP):
0
.
The United States
Departmentof Defense(DOD) and SNL are collaboratingto provide for the designand installation
of PV/engine-generatorhybrid systemsthroughout DOD facilities where engine-generatorsare
currently being usedfor power generationfor remotefacilities. This work is associatedwith the
DOD’s StrategicEnvironmentalResearch& DevelopmentProgram(SERDP). The addition of PV
energysourcesand energystorageto the existing engine-generators
will augmentthosegeneration
systemsby optimizing their performance,by reducing fuel storagerequirementsand fuel usage,
and by reducingengine-generator
run time, therefore,maintenanceof the engine-generators.This
is an effective applicationof PV in remote applicationswhere fuel or maintenanceis expensive,
and it providesadditionalenergysecurityfor the sites.
Although hybrid systemsutilizing PV or wind havebeenin operationfor nearly ten years, most of
the installed systemstoday are small, with generatorssmaller than 6 kW, and PV arrays or wind
generatorssmaller than 3 kW. Most of theseinstalled systemshave been custom designs,and
often their controls are manual, with no automaticcontrol set-pointsor functions. The SERDP
programus& systemsin the 50 kW to 250 kW size range. The addition of automaticcontrolsand
the useof larger componentsis requiring hardwaredevelopment,characterizationand evaluationto
assurecompatibleperformancewith non-linearor reactiveloads and with unbalancedthree-phase
distribution grids. Many of the power processing inverters for hybrid applications are
modifications of commercially available hardware, used for uninterruptible power supplies or
Merging Photovoltaic Hardware Developmentwith Hybrid Applications in the U.S.A.
6
frequency changers, and have not been fully characterized in hybrid applications. Many of the
controls and control algorithms must be considered prototype designs at this time. SNL is
planning to provide hardware and systems evaluation before the hardware is deployed in the field
as part of this program. The first three-phase power processor for hybrids is on hand for testing.
Minimum performance, safety and institutional issues, standards and codes have been established
by various utilities, code writing agencies, standards groups and testing labs. These criteria will be
usedfor theinstallation
of hardware
in theSERDPprogram.Themanufacturers
areresponsible
for meeting these minimum requirements but often do not have the facilities or budget to perform
the necessary testing for verification. A set of essential measurements, tests, conditions for
operation, and guidelines minimum requirementsand suitability power processinghardware for
hybrid systems are being developed for this program at SNL in collaboration with BOS industry.
UtzWest Hybrid Activities: One immediate result of the hardware development and technology
transfer efforts of this program is electric utility companies use of hybrid systems today to supply
grid-quality power to customers where extension of power grids is economically impractical.
Pacific Gas & Electric, Southern California Edison, LaPlata Electric Company, and Idaho Power
are a few of the utilities in the U.S.A. that have recently designed and installed hybrid systems to
economically meet customers’ needs.
A common application is PV/engine-generator power systems for remote areas where grid
extensions are too expensive and where just engine-generator is too costly because of maintenance
costs or fuel delivery or storage problems. The Desert Studies Center at Zzyzx, CA, until
recently, received electricity from propane engine-generators, causing noise and air pollution. The
new system uses a IO-kW PV source with 5280 W-hr of battery storage to supply over 95% of the
center’s energy.
Other utilities, such as DelMarVa (Delaware, Maryland, Virginia) Power and Idaho Power, are
installing or planning remote hybrid systems for homes or remote recreational sites. Off-grid tariff
plans are under consideration by the utilities for the sales of hybrid systems for remote customers
in Idaho and California. However, independent system designers in California are lobbying for an
independentand competitivemarket, free from subsidies.
Government-supported applications for
Staie & Federal Government-Supported Activities:
PV/hybrid systems are occurring on the state and federal level in many locations across the U.S.A.
Most of the sitesare in remoteareassuchasnationalforest sites, stateparks, and recreationalareas
where grid power is not feasible. One village power system is currently being developed for
Wales, Alaska, where fuel and maintenance costs for engine-generators are becoming prohibitive.
This system combines PV, wind, battery storage and engine-generators. The Wales, Alaska
Village Power System design is based on earlier feasibility assessmentsperformed by SNL and
SWTDI [6].
A PV-powered airport has been opened at the Cal Black Memorial Airport at San Juan County,
Utah (71. The airport serves an increasing volume of tourist traffic to Lake Powell National
Recreation Area. A 5-kW PV system charges a 100-kWhr battery bank with an inverter supplying
ac power to normal operational loads. An engine-generator serves mostly as a backup and to
equalize batteries. Also, in the Lake Powell area, the Dangling Rope Marina is under
consideration to receive a hybrid system to serve boaters on the lake. The Dangling Rope Marina
is approximately 45 miles up lake from Glen Canyon Dam and Wahweap Marina, at a very remote
Merging Photovoltaic Hardware Developmentwith Hybrid Applications in the U.S.A.
7
location. It is a critical refueling stop for small boats. It is powered from diesel-generators today
and the cost for electricity is one of the highest for a park service facility, at $.38/kWhr.
Pn’vate Sector Activities: Private sector activities are not supported directly by the USDOE
National Photovoltaic Program or by state programs, however, the hardware developed for those
government-supported programs is finding its way into cost-effective public sector applications. In
Hawaii, cost-effective PV/engine-generator systems are being installed in homes. One home on
Maui is offered as a photovoltaic powered vacation home, where visitors are offered the
opportunity to experience living in a solar/wind-powered home. The home is outfitted with
entertainment center, high-efficiency lights, a refrigerator, and an inverter to power all the
appliances carried by the visitor.
California private sector businesses are installing numerous off grid PV/hybrid systems to remote
homes in mountain areas and for communications systems. The California private sector is actively
lobbying to keep the installation of PV stand-alone and hybrid systems out of the hands of the
utilities to maintain a competitive and independent market. Colorado private sector installations of
PV/hybrids are common and generally are handled by electricians and electrical contractors
working through local distribution houses.
Anh’cQm!edHybrid Activifies: The Amundsen-ScottSouth Pole Station and remote Antarctic
Scientific Sites have been assessedfor renewable hybrid applications [8]. Both applications have
extremely high fuel costs, are extremely remote, and have excellent resources for summer
activities. PV has been shown to be technically feasible; however, the cost at the scientific sites is
not favorable since those sites are used only 90 days per year. Installations of PV hybrids at these
sites would be based more on environmental impact issues for storing and transporting fuel. The
Amundsen-Scott South Pole Station is a site chosen for both PV and wind hardware evaluations
during the 1993-1994 austral summer to determine feasibility with the environment.
Summarv: The USDOE National Photovoltaics Program and Sandia National Laboratories’ effort
to accelerate the adoption of PV hybrid power systems through development of hardware,
technology transfer to user organizations, cooperative programs, design assistance, and
evaluation/analysis of hardware and installed systems is making inroads to establish cost-effective
designs and to provide reliable, modular hardware for the industry. Utilities, state and federal
agencies, domestic private sector, and international agencies are installing cost-effective systems in
remote areas where utility grids are inaccessible. The program is still in its infancy, and numerous
future needs for PV/engine-generator hybrid or PV/wind/engine-generator hybrid systems are
anticipated.
Acknowledpments: The author wishes to express thanks to many representatives from utilities
and private enterprise for their useful input for this paper. Special thanks to Rick Chapman,
Manager to the SNL/DOD SERDP program, and to the staff at SNL for their work in testing,
analysis, and interfacing with the users and designers to make this program successful.
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