SANDIA REPORT
SAND2008-4247
Unlimited Release
Printed July 2008
Solar Energy Grid Integration Systems
–Energy Storage (SEGIS-ES)
Dan T. Ton, U.S. Department of Energy
Charles J. Hanley, Georgianne H. Peek, and John D. Boyes
Sandia National Laboratories
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2
SAND2008-4247
Unlimited Release
Printed July 2008
Solar Energy Grid Integration Systems
–Energy Storage (SEGIS-ES)
Dan T. Ton
Solar Energy Technology Program
EE-2A / L’Enfant Plaza Building
U.S. Department of Energy
1000 Independence Ave., S.W.
Washington, D.C. 20585-1615
Charles J. Hanley
Solar Systems Department
Georgianne H. Peek and John D. Boyes
Energy Infrastructure & DER
Sandia National Laboratories
P.O. Box 5800
Albuquerque, NM 87185
ABSTRACT
This paper describes the concept for augmenting the SEGIS Program (an industry-led effort
to greatly enhance the utility of distributed PV systems) with energy storage in residential
and small commercial applications (SEGIS-ES). The goal of SEGIS-ES is to develop
electrical energy storage components and systems specifically designed and optimized for
grid-tied PV applications. This report describes the scope of the proposed SEGIS-ES
Program and why it will be necessary to integrate energy storage with PV systems as PVgenerated energy becomes more prevalent on the nation’s utility grid. It also discusses the
applications for which energy storage is most suited and for which it will provide the greatest
economic and operational benefits to customers and utilities. Included is a detailed summary
of the various storage technologies available, comparisons of their relative costs and
development status, and a summary of key R&D needs for PV-storage systems. The report
concludes with highlights of areas where further PV-specific R&D is needed and offers
recommendations about how to proceed with their development.
3
4
Contents
1. Executive Summary............................................................................................. 7
2. Vision .................................................................................................................... 8
3. Program Objective ............................................................................................... 8
4. Program Scope .................................................................................................... 8
5. The Need for Energy Storage in High-penetration PV Systems .................... 10
6. Applications of Energy Storage in High-penetration PV Systems ................ 12
7. Current Electrical Energy Storage Technologies and R&D ........................... 16
8. The Costs of Electrical Energy Storage........................................................... 22
9. Summary of Key R&D Needs for PV-Storage Systems .................................. 23
9.1. Storage Technologies ................................................................................... 23
9.2. Control Electronics ........................................................................................ 24
9.3. Comprehensive Systems Analysis ................................................................ 24
10. Summary – The Path Forward ......................................................................... 25
10.1.Systems Analysis and Modeling .................................................................. 26
10.2.Partnered Industry Research and Development .......................................... 26
10.3.Codes and Standards Development ............................................................ 27
11. References ........................................................................................................ 28
5
6
1.
Executive Summary
In late 2007, the U.S. Department of Energy (DOE) initiated a series of studies to address
issues related to potential high penetration of distributed photovoltaic (PV) generation
systems on our nation’s electric grid. This Renewable Systems Interconnection (RSI)
initiative resulted in the publication of 14 reports and an Executive Summary that defined
needs in areas related to utility planning tools and business models, new grid architectures
and PV systems configurations, and models to assess market penetration and the effects of
high-penetration PV systems. As a result of this effort, the Solar Energy Grid Integration
Systems Program (SEGIS) was initiated in early 2008. SEGIS is an industry-led effort to
develop new PV inverters, controllers, and energy management systems that will greatly
enhance the utility of distributed PV systems.
This paper describes the concept for augmenting the SEGIS Program with energy storage
(SEGIS-ES) in residential and small commercial (≤100 kW) applications. Integrating storage
with SEGIS in these applications can facilitate increased penetration of distributed PV
systems by providing increased value to both customers and utilities. Depending on the
application, the systems can reduce customer utility bills, provide outage protection, and
protect equipment on the load side from the negative effects of voltage fluctuations within
the grid. With sufficient penetration, PV-Storage systems are expected to reduce emissions
related to generation and will be critical to maintaining overall power quality and grid
reliability as grid-tied distributed PV generation becomes more common.
Although electrical energy storage is a well-established market, its use in PV systems is
generally for stand-alone systems. The goal of SEGIS-ES is to develop electrical energy
storage components and systems specifically designed and optimized for grid-tied PV
applications. The Program will accomplish this by conducting targeted research and
development (R&D) on the applications most likely to benefit from a PV-Storage system
(i.e., peak shaving, load shifting, demand response, outage protection, and microgrids) and
developing PV-Storage technologies specifically designed to meet those needs. Designing
optimized systems based on existing storage technologies will require comprehensive
knowledge of the applications and the available storage technologies, as well as modeling
tools that can accurately simulate the economic and operational effect of a PV-Storage
system used in that application.
This paper describes the scope of the proposed SEGIS-ES Program; why it will be necessary
to integrate energy storage with PV systems as PV-generated energy becomes more prevalent
on the nation’s utility grid; and a discussion of the applications for which energy storage is
best suited and for which it will provide the greatest economic and operational benefits to
customers and utilities.
Because selecting and optimizing a storage technology for an application will be critical to
the success of any PV-Storage system, this paper also provides a detailed summary of the
various storage technologies available and compares their relative costs and development
status (e.g., mature, emerging, etc.).
Finally, the paper highlights areas where further, PV-specific R&D is needed and offers
recommendations about how to proceed with the proposed work.
7
2.
Vision
The U.S. infrastructure for electricity generation and delivery is undergoing a revolution that
will lead to increased efficiency, improved reliability and power quality for customers,
‘smart’ communications to match generation and loads, and the development of distributed
generation from local and renewable resources. The high penetration of PV and other
renewable energy technologies into the infrastructure will be enabled by developing
managed, efficient, reliable, and economical energy storage technologies that will eliminate
the need for back-up utility baseload capacity to offset the intermittent and fluctuating nature
of PV generation.
These dispatchable storage technologies will bring added benefits to utilities, homeowners,
and commercial customers through greater reliability, improved power quality, and overall
reduced energy costs.
3.
Program Objective
The SEGIS Program will develop advanced energy storage components and systems that will
enhance the performance and value of PV systems, thereby enabling high penetration of PVgenerated electricity into the nation’s utility grid. Through its RSI initiative, the DOE Solar
Energy Technology Program is identifying needs and developing technologies to facilitate
the high penetration of distributed electricity generation. The need for improved energy
storage has been highlighted as a key factor for achieving the desired level of PV generation.
The electrical energy storage industry is well established and offers a variety of products for
vehicle, uninterruptable power supply (UPS), utility-scale, and other storage applications.
The design and development of storage products specifically for PV applications, however, is
almost nonexistent. Traditional PV-Storage systems have been used for off-grid applications
that required some amount of autonomy at night and/or during cloudy weather.
However, the objective of this Program is to develop energy storage systems that can be
effectively integrated with new, grid-tied PV and other renewable systems, which will
provide added value to utilities and customers through improved reliability, enhanced power
quality, and economical delivery of electricity.
4.
Program Scope
In late 2007, DOE began a series of studies to address issues related to the potentially high
penetration of distributed PV generation systems on our nation’s electricity grid. The RSI
initiative resulted in the publication of 14 reports and an Executive Summary that defined
needs in areas related to utility planning tools and business models, new grid architectures
and PV system configurations and models to assess market penetration and the effects of
high-penetration PV systems.1 As a result of this effort, the SEGIS program was initiated in
early 2008. SEGIS is an industry-led effort to develop new PV inverters, controllers, and
energy management systems that will greatly enhance the utility of distributed PV systems.
SEGIS-ES is closely related to the SEGIS Program, a three-year program with a goal to
develop new commercial PV inverters, controllers, and energy management systems with
new communications, control, and advanced autonomous features.2 The heart of the SEGIS
hardware, the inverter/controller, will manage generation and dispatch of solar energy to
8
maximize value, reliability, and safety, as the nation moves from ‘one-way’ energy flow in
today’s distribution infrastructure to ‘two-way’ energy and information flow in tomorrow’s
grid or microgrid infrastructure.
The applicable markets for the SEGIS Program3 are defined in Table 1, which shows the size
of the PV system in watts, or power output. Storage systems are typically rated in terms of
energy capacity (i.e., watt-hours), which is highly dependent on the application for which the
storage is being used. The applications are discussed later in this document.
Table 1: Target Market Sectors for SEGIS PV Systems
Residential
Less than 10 kW, single-phase
Small Commercial
From 10 to 50 kW, typically three-phase
Commercial
From 50 to 100 kW, three-phase
SEGIS-ES is focused on developing commercial storage systems for distribution-scale PV in
the market sectors shown in Table 1; specifically, PV systems designed for applications up to
100 kW that can be aggregated into multi-megawatt systems.
Integrating electrical energy storage into homes or commercial buildings is also a key focus
of SEGIS-ES. New storage systems developed under the Program will play an important role
in the development of independent microgrids – either individual buildings or communities
of buildings – so microgrid-scale storage, on the order of one megawatt of distributed
generation, is within the scope of this effort.
Storage systems developed through SEGIS-ES will interface with SEGIS products to further
enhance PV system value and economy to customers. Products to be developed through
SEGIS-ES include, but are not limited to:
•
•
•
•
Battery-based systems using existing technologies that are enhanced or specifically
designed for PV applications, including the development of PV-Storage hybrid
systems;
New energy storage system controllers that interface with SEGIS hardware to
optimize battery use in order to obtain the highest possible system efficiency and
battery life;
Non-battery storage systems (e.g., electrochemical capacitors [ECs], flywheels)
designed specifically for PV applications; and
New devices that integrate with building infrastructure.
SEGIS-ES does not address:
•
•
•
•
•
Development of PV modules;
Development of new battery technologies (although collaboration with the DOE
Office of Basic Energy Sciences Energy Frontier Research Centers’ Funding
Opportunity is encouraged);
Utility-scale storage systems or storage at the level of large distribution feeders
(Although these efforts are key to achieving high penetration of distributed generation,
they will be addressed through other Program activities);
PV inverters or related power conditioning devices; and
Non-solar-related storage system development, smart appliances, and utility portals.
9
5.
The Need for Energy Storage in High-penetration PV
Systems
PV systems are a small part of today’s electricity infrastructure and have little effect on the
overall quality or reliability of grid power. Nevertheless, state and federal efforts are
currently underway to greatly increase the penetration of PV systems on local and regional
utility grids to achieve goals related to emissions reduction, energy independence, and
improved infrastructure reliability. However, when PV penetration reaches sufficiently high
levels (e.g., 5 to 20% of total generation), the intermittent nature of PV generation can begin
to have noticeable, negative effects on the entire grid.
3000
420
2500
350
2000
280
1500
210
1000
140
500
70
0
dc Voltage (V)
Power (W)
Figure 1 illustrates the transient nature of PV generation as clouds pass over a typical
residential system during the course of a day. Both the magnitude and the rate of the change
in output are important: in mere seconds, the PV system can go from full output to zero
(essentially), and back again. At high levels of PV penetration, this intermittency can wreak
havoc on utility operations and on load-side equipment, due to fluctuations in grid voltage
and power factor. Fluctuations at this scale simply cannot be allowed.
0
3/8/08 4:33 3/8/08 6:33 3/8/08 8:33 3/8/08 10:33 3/8/08 12:33 3/8/08 14:33 3/8/08 16:33 3/8/08 18:33
Date and Time
Pmp Modeled
measured DC Power
measured AC Power
Vmp Modeled
measured Vdc
Figure 1: Measured and modeled PV system output on a day with frequent passing clouds.
To some degree, the distributed nature of PV can help mitigate negative consequences of
high PV penetration; over large regions, the effects of intermittent generation on the grid will
be less noticeable. Nevertheless, utilities must continue to address worst-case possibilities.
When transients are high, area regulation will be necessary to ensure that adequate voltage
and power quality are maintained. When PV generation is low, some type of back-up
generation will be needed to ensure that customer demand is met. Additionally, because most
utilities require an amount of ‘spinning reserve’ power that typically is equal to the power
output of the largest generating unit in operation, the amount of spinning reserve necessary
will increase with the amount of distributed PV generation that is brought online. Without
such measures, the benefits of high PV penetration are partially lost—carbon and other
10
emissions are offset through PV-produced electricity; but, utility infrastructure is not reduced
and power quality is not necessarily improved.
As the graph in Figure 2 illustrates, high PV penetration might reduce intermediate fossil fuel
generation; but, without storage, PV will do little, or nothing, to reduce a utility’s overall
conventional generation due to the higher requirements for spinning reserve.
Figure 2: The need for additional spinning reserve or storage to back up
intermittent PV generation at increasing levels of penetration.
As a whole, the utility grid must evolve in several ways to accommodate any increased
penetration of PV and other distributed and intermittent electricity generation sources;
including improved flexibility, better load management, integration of storage technologies,
and even limited curtailment for extreme events. Several efforts are underway to define the
next-generation grid infrastructure, which will include those characteristics.4
A recent study that specifically focused on the current grid and high-penetration PV called
energy storage the ‘ultimate solution’ for allowing intermittent sources to address utility
baseload needs. The report stated that “a storage system capable of storing substantially less
than one day’s worth of average demand could enable PV to provide on the order of 50% of a
system’s energy.”5 This paper focuses on incorporating storage as part of the overall
‘systems’ solution.
Successfully integrating energy storage with distributed PV generation in grid-connected
applications involves much more than selecting an adequately sized system based on one of
the many, commercially available technologies. Optimal integration of storage with grid-tied
PV systems requires a thorough understanding of the following:
•
The application for which the storage is being used and the benefits integrated storage
provides for that application;
11
•
•
•
•
The available storage technologies and their suitability to the application;
The requirements and constraints of integrating distributed generation and electrical
energy storage with both the load (residential, commercial, or microgrid) and the
utility grid;
The power electronics and control strategies necessary for ensuring that all parts of
the grid-connected distributed generation and storage system work; and
The requirements to provide service to the load and to maintain or improve grid
reliability and power quality.
The complexity of an integrated PV-Storage system is illustrated in Figure 3, which shows
SEGIS-based generation integrated with electrical energy storage for a residential or small
commercial system.
Figure 3: The relationship between SEGIS, electric energy storage,
the customer, and the utility in an optimal configuration.6
6.
Applications of Energy Storage in High-penetration
PV Systems
Integrated PV-Storage systems provide a combination of operational, financial and
environmental benefits to the system’s owner and the utility through peak shaving and
reliability applications.7
Peak Shaving, Load Shifting, and Demand Response are variations on a theme—supplying
energy generated at some point in time to a load at some later time. The rate structure and
12
interactions between the utility and the customer determine which application is being
addressed.
Peak Shaving: The purpose of this application is to minimize demand charges for a
commercial customer or to reduce peak loads experienced by the utility. Peak shaving using
PV-Storage systems requires that the PV provide all required power above a specified
threshold and, if PV is not available, then provide adequate energy storage to fill the gap.
Failure to peak shave on one day can have severe economic consequences in cases where
customers’ rates are based on monthly peak demand. Thus, reliability of the PV-storage
system is a key element. If PV is unavailable to meet the load, the system controller must be
able to dispatch power from an energy storage system in order to implement peak shaving.
Load Shifting: Technically, load shifting is similar to peak shaving, but its application is
useful to customers purchasing utility power on a time-of-use (TOU) basis. Many peak loads
occur late in the day, after the peak for PV generation has passed. Storage can be combined
with PV to reduce the demand for utility power during late-day, higher-rate times by
charging a storage system with PV-generated energy early in the day to support a load later
in the day.
Pacific Gas and Electric (PG&E) offers the experimental rate structure shown in Table 2 for
residential customers. In this schedule, peak rates apply between 2 p.m. and 7 p.m. on
weekdays and super-peak rates apply between 2 p.m. and 7 p.m. for no more than 15 days in
a calendar year during critical events (as designated by the independent system operator, or
ISO) and emergencies. Thus, customers with a PV-Storage system could use PV to charge
the storage device earlier in the day (i.e., during peak insolation) and then use the storage
system to supply all or part of the load when peak or super-peak rates are in effect. With rate
structures such as these in effect, a PV-Storage system could potentially provide significant
economic benefits to residential and small commercial customers.
Table 2: PG&E Rate Schedule E-3 – Experimental Residential
Critical Peak Pricing Service8
Total Energy Rates ($/kWh)
Super Peak
Peak
Off Peak
Summer Baseline Usage
0.67439
0.23096
0.08039
Winter Baseline Usage
0.50997
0.31197
0.10497
Demand Response: Demand response is rapidly becoming a viable load management tool for
electric utilities. During high-demand periods, demand response allows the utility to control
selected high-load devices, such as heating, ventilation, and air conditioning (HVAC) and
water heating, in a rolling type of operation. Utility rate structures are currently changing to
accommodate this new operational strategy by reducing rates for customers who choose to be
included in the demand response program. For both residential and small commercial
customers, using an appropriately sized PV-Storage system should allow the implementation
of demand response strategies with little or no effect on local operations.
While both residential and commercial PV-Storage systems have the inherent capability to
manage demand response requirements, control systems capable of reacting to demand
response must be developed. Specifically, control systems must dispatch the PV-Storage
system, as necessary, to manage the loads curtailed by the demand response program.
Consequently, at least one-way communication with the utility might be required.
13
Outage Protection, Grid Power Quality Control, and Microgrids increase the reliability of
the electricity grid and are not as subject to regulatory and rate-based actions.
Outage Protection: An important benefit of a PV-Storage system is the ability to provide
power to the residential or small commercial customer when utility power is unavailable
(i.e., during outages). To provide this type of protection, it is necessary to intentionally island
the residence or commercial establishment in order to comply with utility safety regulations
designed to prevent the back-feeding of power onto transmission and distribution (T&D)
lines during a blackout. Islanding requires highly reliable switching equipment for isolating
the local loads from the utility prior to starting up local generation.
Islanding capability, whether utility or customer-controlled, is mutually beneficial to both the
utility and the customer, because it allows the utility to shed loads during high demand
periods while protecting the customer’s loads if the utility fails. To realize the full benefit of
these capabilities, however, new controllers are needed to respond to both utility and
customer needs. Additionally, new regulations will be needed to define how these controllers
will be managed safely to benefit both the utility and the customer.
Grid Power Quality Control: In addition to outage protection, power quality ensures
constant voltage, phase angle adjustment, and the removal of extraneous harmonic content
from the electricity grid. On the customer side, this function is currently supplied by UPS
devices. A UPS must sense, within milliseconds, deviations in the AC power being supplied
and then take action to correct those deviations. A common deviation is a voltage sag in
which the UPS supplies the energy needed to return the voltage to the desired level.
UPS functions can be added to PV-Storage systems in the power conditioning system by
designing it to handle high power applications and including the necessary control functions.
UPS functionality can be combined with peak shaving capability in the same system.
Microgrids: Microgrids have the potential to significantly increase energy surety,9 and their
incorporation into the larger grid infrastructure is expected to become an increasingly
important feature of future distribution systems. Renewable generation and energy storage
are essential to achieving highly sustainable, highly reliable microgrids. When operating
separately from the local utility (i.e., when ‘islanded’), microgrids with PV-Storage systems
will use PV-generated electricity to supply power to the load.
Energy storage is essential to ensure stable operation of the Microgrid, through management
of load and supply variations, and for keeping voltage and frequency constant. Successful
integration into the larger utility grid infrastructure of microgrids that include PV-Storage
systems will provide many operational benefits to utilities and customers. However,
microgrids will require a high level of system control, a detailed knowledge of the load(s)
being served, and thoughtful design of the PV-Storage system.
Table 3 summarizes current and future applications that can be addressed by integrating
energy storage with distributed PV generation.
14
Table 3: Applications for Storage-integrated PV
Residential
Homeowner-owned Systems
Current:
• Save solar energy for
evening use in TOU
operations
• Back-up power (UPS)
Utility-owned Systems
Future:
• With time of day
residential rates, load
shifting
• Lower cost than utility
• Smart grid interface
Current:
• Solar community –
ride-through during
cloud cover
• Distributed generation
• Congestion reduction
Future:
• Smart grid applications
(e.g., distributed
energy management,
microgrid islanding,
peak shaving/shifting.)
• High penetration ramp
control (short-term
spinning reserve)
• Emission reduction,
carbon credits (with
high penetration)
Commercial
Business-owned Systems
Current:
• Peak shaving to
reduce TOU and/or
demand charges
• Power quality and
UPS
Utility-owned Systems
Future:
• Carbon credits
• Microgrid generation
and islanding
• Smart grid/building
management
interfaces
Current:
• Distributed generation
• Congestion reduction
• Improved power quality
Future:
• Microgrid generation
and islanding
• Emission reduction,
carbon credits (with
high penetration)
The economic benefits that can be realized from PV-Storage systems are a function of the
application, the size of the system, the sophistication of the system’s electronic control
equipment, the customer’s rate structure, and the utility’s generation mix and operating costs.
Systems that include UPS features are expected to mitigate the costs of power quality events
and outages.
Results of a recent study also suggest that adding PV generation to a planned UPS
installation is attractive because of the synergy between PV and storage in the UPS market.
In other words, sites where customers have already decided to purchase load protection via
energy storage might be an attractive near-term target for PV developers.10
In general, however, most financial benefits will result from reduced peak-demand and TOU
charges for consumers and the avoided costs of maintaining sufficient peak and intermediate,
power generating capability plus spinning reserve for utilities. By facilitating an optimal mix
of generation options, it is expected that the cost to the utility of adding additional generating
capacity and the associated T&D equipment can be reduced, as can the costs associated with
upgrading existing T&D equipment to meet new demand.
In the future, additional financial benefits could accrue to the end user by selling power back
to the utility and to the utility by selling carbon credits realized by aggregating PV generation
as a market commodity. Ideally, rate structures for PV-Storage systems could be designed to
benefit both system owners and utilities. To fully realize all of the potential, economic
benefits will, however, require an advanced control system that includes communications
between the utility, the PV-Storage system, and (possibly) the customer.
Finally, at high levels of penetration, PV systems offer significant environmental benefits.
One such benefit is that they create no emissions while generating electricity. Another is that
they can be installed on rooftops and on undesirable real estate, such as brown fields, which
15
can reduce a utility’s need to acquire land for construction of new, large-scale generating
facilities, not to mention the associated local opposition to such acquisitions, and the
environmental consequences of large-scale industrial construction. Adding electrical energy
storage to distributed PV generation also produces no emissions and, by allowing PVgenerated electricity to be used at times when PV would normally not be available (e.g., at
night or when it is cloudy), allows greater benefits to be realized than with PV systems alone.
7.
Current Electrical Energy Storage Technologies and
R&D
Energy storage devices cover a variety of operating conditions, loosely classified as ‘energy
applications’ and ‘power applications’. Energy applications discharge the stored energy
relatively slowly and over a long duration (i.e., tens of minutes to hours). Power applications
discharge the stored energy quickly (i.e., seconds to minutes) at high rates. Devices designed
for energy applications are typically batteries of various chemistries. Power devices include
certain types of batteries, flywheels, and ECs. A new type of hybrid device, the lead-carbon
asymmetric capacitor, is currently being developed and is showing promise as a device that
might be able to serve both energy applications and power applications in one package.
Figure 4 illustrates several battery and capacitor technologies in relation to their respective
power/energy capabilities.11 The traditional lead-acid battery stands as the traditional
benchmark. The plot shows that significantly greater energy and power densities can be
achieved with several rechargeable battery technologies.
.
Figure 4: Specific power vs. specific energy of several energy storage technologies.12
To date, the advantages of lead-acid technology, such as low cost and availability, have made
it the default choice for energy storage in most PV applications. Indeed, new developments in
16
valve-regulated lead-acid (VRLA) technology might revolutionize this well-established
technology.
A number of lead-acid battery manufacturers, such as East Penn in the U.S. and Furukawa in
Japan, are manufacturing prototype batteries for hybrid electric vehicles (HEVs) that promise
to overcome the main disadvantages of VRLA batteries by using special carbon formulations
in the negative electrode. The added carbon inhibits hard sulfation, which minimizes or
eliminates many common failure mechanisms (e.g., premature capacity loss and water loss).
In cycling applications, the new VRLA technology could dramatically lower the traditional
battery energy costs by increasing cycle life, efficiency, and reliability.
Traditionally, nickel-cadmium (NiCd) batteries have been the replacement for lead-acid; but,
due to various operational and environmental issues, industry is moving away from this
technology as newer and better technologies are developed. Indeed, even in the portable
electronics market, lithium-ion (Li-ion) batteries are rapidly replacing NiCd.
Additionally, a new Li-ion technology, the Li-iron phosphate (Li-FePO) cell, is rapidly
becoming a prime contender for the next generation of HEV batteries, replacing existing
nickel-metal hydride (Ni-MH) technology. This Li-ion technology is proving to be much
safer than the previous generation and is capable of higher power levels, which makes it a
better candidate for HEV applications.
A lesser known technology, sodium/nickel-chloride (Na/NiCl), has been developed by Zebra
Technologies in Europe for motive applications, and is currently being considered for some
stationary applications, such as peak shaving, in the U.S. Other advanced battery
technologies (e.g., sodium/sulfur, or Na/S) are currently targeting utility-scale (> 1MW)
stationary applications.
Although these technologies are not currently being considered for use in the smaller
applications discussed here, future advances in their developments might increase the
technical and economic viability for such applications. Table 4 summarizes the battery
technologies that have been identified as potential candidates for integration with grid-tied
distributed PV generation in residential and small commercial systems.
Table 5 provides a summary of non-battery technologies that can be integrated with grid-tied
distributed PV generation. Although they are still in the early commercial stage of
development, hybrid lead-carbon asymmetric capacitors are also targeting the peak-shaving
market and low-speed flywheels are currently being used in many UPS applications.
ECs are ideal for high-power, short-duration applications because they are capable of deep
discharge and have a virtually unlimited cycle life. Due to these advantages, a great deal of
research is being focused on developing ECs that can be used for small-scale stationary
energy storage. Any of these battery or non-battery technologies can be appropriate for
residential and small-commercial integrated PV and storage systems in the near future.
In addition to the efforts of the technology manufacturers, DOE (through several program
offices) is conducting research and executing pilot programs to improve the utilization of
electrical energy storage for stationary applications. In particular, since the late 1970s, the
DOE Energy Storage Systems Program (DOE/ESS) has worked with the utility industry to
develop stationary energy storage systems for utility applications.
17
In the 1990s, DOE/ESS shifted the focus of its development of advanced storage
technologies to include an emphasis on integrating storage devices with power electronics
and communications equipment for use in specific applications. Over the past decade, the
Program has gained valuable practical experience by partnering with storage technology
manufacturers, power electronics and monitoring equipment manufacturers, systems
integrators, electric utilities, and their customers to demonstrate integrated electric energy
storage systems of all types and sizes. Lessons learned from the Program’s demonstrations
and research provide uniquely applicable experience for successfully incorporating electrical
energy storage with distributed PV generation.
The DOE Vehicle Technologies Program, in partnership with the automotive industry,
manages and conducts research on battery technologies for EVs and HEVs (e.g., lithiumaluminum-iron-sulfide, Ni-MH, Li-ion, and lithium-polymer). Li-ion systems come closest to
meeting all of the technical requirements for vehicle applications; but, they face four barriers:
calendar life, low-temperature performance, abuse tolerance, and cost. Technology advances
that address these barriers will have direct applicability to PV-Storage systems for stationary
applications.
Finally, the DOE Office of Basic Energy Sciences conducted a comprehensive workshop on
April 2-4, 2007, that set R&D priorities for improving the energy density of several storage
technologies. Principal barriers identified at the workshop were related to reducing cost,
increasing power and energy density, lengthening lifetime, increasing discharge times,
improving safety, and providing reliable operation through one-to-ten thousand rapid
charge/discharge cycles.
Several associated R&D efforts are underway. One such effort is the announcement of a
funding opportunity to establish Energy Frontier Research Centers (EFRCs) specifically
focused on “addressing fundamental knowledge gaps in energy storage.”13 The SEGIS-ES
program will be closely coordinated with any developments to come from these programs.
18
Na/S
Li-FePO
Li-ion
NiMH
NiCd
VRLA
4
Flooded
Lead-acid
Technology
• Safer than traditional Li-ion
• High power density
• Lower cost than traditional
Li-ion
• High energy density
• No emissions
• Long calendar life
• Long cycle life when deeply
discharged
• Low maintenance
• Integrated thermal and
environmental management
• High energy density
• High efficiency
• Good energy density
• Low environmental impact
• Good cycle life
Good energy density
Excellent power delivery
Long shelf life
Abuse tolerant
Low maintenance
• Relatively high cost
• Requires powered thermal
management (heaters)
• Environmentally hazardous
materials
• Rated output available only
in 500-kW/600-kWh
increments
• High production cost
• Scale-up proving difficult
due to safety concerns
• Lower energy density than
other Li-ion technologies
• Expensive
• Moderately expensive
• “Memory Effect”
• Environmentally hazardous
materials
•
•
•
•
•
• Cost effective
• Mature technology
Disadvantages
• Low energy density
• Cycle life depends on
battery design and
operational strategies when
deeply discharged
• High maintenance
• Environmentally hazardous
materials
• Traditionally have not cycled
well
• Have not met rated life
expectancies
Advantages
• Cost effective
• Mature technology
• Relatively efficient
19
• Recently commercial
(2002) in Japan
• Estimated $0.4B in
utility/industrial
applications worldwide
• High-volume production
began in 2008
• Globally commercial for
small electronics
• Emerging market for
larger applications
• 50% of global small
portable market
• Globally commercial
• Over $40B in all
applications
• Estimated $1B in utility
applications worldwide
• Globally commercial
• Over $1B in all
applications
• Over $50M in utility
applications worldwide
Commercial Status
• Globally commercial
• Over $40B in all
applications
• Estimated $1B in utility
applications worldwide
Focused on increasing
manufacturing yield and
reducing cost.
Batteries for use in EVs
and HEVs are currently
being developed.
Focused on improving
performance and safety
systems.
Bipolar design.
Improving cycle-life and
extending operating life,
such as using carbonenhanced negative
electrodes.
None identified.
Current R&D
Focused on reducing
maintenance
requirements and
extending operating life.
• Utility grid-integrated renewable
generation support
• Utility T&D system optimization
• Commercial/industrial peak shaving
• Commercial/industrial backup power
• Small consumer goods and tools
• EVs, HEVs
• Small consumer goods
• Limited motive power applications
(e.g., electric wheelchairs)
• Back-up power
• Short-duration power quality
• Short-duration peak reduction
• Aircraft cranking, aerospace, military
and commercial aircraft applications
• Utility grid support
• Stationary rail
• Telecommunications back-up power
• Low-end consumer goods
• EVs, HEVs
• Small, low-current consumer goods
Applications
• Motive power (forklifts, carts, etc.) and
deep-cycling stationary applications
• Back-up power
• Short-duration power quality
• Short-duration peak reduction
Table 4: Battery Technologies for Electric Energy Storage in Residential and Small-commercial Applications
Zinc/bromine
(Zn/Br)
Vanadium Redox
Zebra Na/NiCl
Technology
•
•
•
•
•
•
•
•
•
•
•
•
Advantages
High energy density
Good cycle life
Tolerant of short circuits
Low-cost materials
Good cycle life
Good AC/AC Efficiency
Low temperature/low
pressure operation
Low maintenance
Power and energy are
independently scaleable
Low temperature/low
pressure operation
Low maintenance
Power and energy are
independently scaleable
Disadvantages
Only one manufacturer
High internal resistance
Molten sodium electrode
High operating temperature
Low energy density
• Low energy density
• Requires stripping cycle
• Medium power density
•
•
•
•
•
20
• Emerging commercial
products
• Commercial production
since 2007
Commercial Status
• Globally commercial for
traction applications.
Focused on system
integration.
Current R&D
Focused on cost
reduction and systems
for stationary
applications.
Focused on cost
reduction.
•
•
•
•
•
Back-up power
Peak shaving
Firming capacity of renewables
Remote area power
Load management
• Firming capacity of renewable
resources
• Remote area power systems
• Load management
• Peak shifting
Applications
• EVs, HEVs, and locomotives
• Peak shaving
Flywheels
Electrochemical
Capacitors
Lead-carbon
asymmetric
capacitors
(hybrid)
Storage
Type
Rapid recharge
Deep discharge
High power delivery rates
Long cycle life
Low maintenance
Extremely long cycle life
High power density
• Low maintenance
• Long life
• Environmentally inert
•
•
•
•
•
•
•
Advantages
21
• Commercialized in US,
Japan, Russia, and EU,
emerging elsewhere
• Over $30 million in all
applications
• $5 million in utility
applications by 2006
• Commercialized in US,
Japan, Europe,
emerging elsewhere
• Projected to sell over
1,000 systems per
year, estimated rated
capacity of 250 MW
• Retail value exceeding
$50 million by 2006
• Low energy density
• Expensive
• Low energy density
• High cost
• Non-commercial
prototypes
Commercial Status
• Lower energy density than
batteries
• Lower power density than
other ECs
Disadvantages
Focused on low cost
commercial flywheel
designs for long duration
operation.
Devices with energy
densities over
3
20 kWh/m are under
development.
Current
R&D
Laboratory prototypes
Field demonstration
planned FY08 in NY.
•
•
•
•
•
•
•
•
•
•
Aerospace
Utility power quality
T&D stability
Renewable support
UPS
Telecommunications
HEVs
Portable electronics
Utility power quality
T&D stability
• Peak shaving
• Grid buffering
Applications
Table 5: Non-battery Technologies for Electric Energy Storage in Residential and Small-commercial Applications
8.
The Costs of Electrical Energy Storage
Both current and projected costs for battery and other storage systems are related to
capital costs (first costs) and are based on the overall energy capacity of those systems.
Table 6 shows the current and projected first capital costs of energy storage systems
based on technologies identified as suitable for residential and small-commercial
PV-Storage systems.
Table 6 was compiled from the results of a literature review and discussions with
technology leaders at national laboratories and in industry. Recent increases in the prices
of materials, such as lead, for existing battery technologies have led to increased system
costs. These trends are likely to continue, possibly driving the prices for established
technologies even higher.
Unless noted, the system costs include the storage device and the power conditioning
system necessary for turning DC output from the storage device into 60-Hz AC power
suitable for delivery to the load. For these systems, capital costs will be lowered by
combining the power electronics for both the PV and storage components.
Table 6: Energy Storage System Capacity Capital Costs14 15 16 17 18 19 20 21 22
Current Cost
($/kWh)
10-yr Projected
Cost ($/kWh)
Flooded Lead-acid Batteries
$150
$150
VRLA Batteries
$200
$200
NiCd Batteries
$600
$600
Ni-MH Batteries
$800
$350
Li-ion Batteries
$1,333
$780
Na/S Batteries
$450
$350
Zebra Na/NiCl Batteries
$8001
$150
20 kWh=$1,800/kWh;
100 kWh =$600/kWh
25 kWh=$1,200/kWh
100 kWh=$500/kWh
$250/kWh plus
2
$300/kW
Technology
Vanadium Redox Batteries
Zn/Br Batteries
$500
Lead-carbon Asymmetric Capacitors (hybrid)
$500
