state of the art hybrid solutions for energy storage … of the art hybrid solutions for energy...
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g GE Power
State of the Art Hybrid Solutions for Energy
Storage and Grid Firming
James DiCampli, P.E.
Donald Laing CEng FIMechE
POWER-GEN & Renewable Energy World
Europe
28 June 2017
State of the Art Hybrid Solutions for Energy
Storage and Grid Firming
Abstract
The impact of COP21 and growing renewable standards are driving new behaviors and
technologies such as two-way power flows, microgrids, digital analytics, and new transmission
and distribution models. Modern power systems must adapt to large fluctuations in both supply
and demand to maintain grid stability.
One solution recently developed by GE and its partners utilizes a hybrid electric gas turbine
(Hybrid EGTTM) to address these challenges. With the increasing proportion of intermittent
resources supplying the grid, battery storage holds significant promise in coming years. Like
renewable technologies before it, manufacturing scale and technical advances will drive costs
lower. Bulk storage will provide backup power, peak shaving, and ancillary services.
Transmission and distribution investments may be deferred as batteries provide congestion relief
during times of peak demand.
The GE Hybrid EGTTM is the world’s first gas turbine and battery storage hybrid, coupling a 10-
megawatt battery with a 50-megawatt (MW) GE LM6000 Gas Turbine, operated by an
integrated digital turbine control system. Key benefits include “spinning reserve” without firing
the gas turbine utilizing near instantaneous battery power through inverters, enhanced primary
frequency response and voltage support, reduced greenhouse gas emissions, and smooth transient
response with less turbine thermal stress, thereby lowering maintenance costs.
This paper reviews advances in storage technology and the state of the art technology that
integrates thermal and battery power to mitigate the challenges of variable generation.
Background
The bulk electric system in Europe is undergoing a significant transformation. Environmental
regulations and the Renewable Energy Directive 2009/28/EC are driving growth in variable
energy resources, notably wind and solar. These renewable technologies are rapidly achieving
grid parity and will likely be cost competitive in the absence of government subsidies, fueling
their growth.
The large-scale integration of variable renewable resources requires the operation of the grid in a
fast, flexible and responsive manner. To maintain power quality and reliability, the thermal
power generation fleet must provide more frequency support, reactive energy for voltage control,
fast dispatch and ramping. These flexible power solutions are required to maintain a stable
generation-load balance. This has typically been met in the past by spinning reserve. However,
operating plants at these lower outputs reduces the power generation efficiency and requires over
capacity during normal operation. Hence there’s a need for a better solution. To address these
requirements, GE has engineered the LM6000 Hybrid EGTTM (electric gas turbine), an integrated
battery and gas turbine solution, that can change output very quickly to support changes in
demand and support overall grid reliability. Well-designed thermal and integrated battery
systems can provide fast ramping, voltage and frequency support, and reduce the frequency and
duration of outages.
Renewable Energy in Europe
Renewable energy in the EU has grown substantially in recent years. This has been prompted by
the legally binding targets for renewable energy enacted by Directive 2009/28/EC. In 2015,
electricity generation from renewable sources, contributed 27.5 % of the EU-28’s gross
electricity consumptioni. The growth in electricity generated from renewable energy sources
during the period 2004 to 2014 largely reflects an expansion in three renewable energy sources;
wind turbines, solar power and solid biofuels. The quantity of electricity generated from wind
turbines in 2014 was 3.3 times higher as in 2004. The growth in electricity from solar power was
rose from just 0.7 TWh in 2004 to 92.3 TWh in 2014.ii
As noted, this variable power generation growth will significantly increase the need for
flexibility in the electricity grid. Storage could help balance electricity supply and demand,
holding the energy produced when the conditions for renewable energy are good but demand
may be low, then using the stored energy when demand (and price) is high. In February 2017,
the European Commission published a Staff Working Document titled 'Energy storage – the role
of electricity.' This document presents different technologies and discusses possible policy
approaches.iii IHS estimates that 640MW of non-traditional storage (storage other than pumped
hydro or compressed air) capacity is currently operating in Europe, with another 190MW of
projects coming on line in 2017.iv
Reliability Impact
Previously noted, large-scale integration of variable renewable resources requires a mix of
solutions to maintain power quality and grid reliability. The most important elements of
reliability on the grid are managing frequency, net demand ramping and fast dispatch, and
voltage support.
Frequency regulation involves second to second balancing of generation and load, and restoring
frequency after an event such as the loss of a major resource. The frequency within an
interconnection will immediately fall upon such an event, requiring a very fast response from
other generating resources to slow the rate of fall. That is, a fast increase in power output (or
decrease in power consumption) to stop the fall and stabilize the frequency, then a more
prolonged contribution of additional power (or reduced load) to compensate for the lost units and
bring system frequency back to the normal level. The kinetic energy extracted from the rotating
mass of the grid’s synchronous machines is critical. Some minimum synchronous inertial
response is needed to keep frequency from reaching a low set point within a given time to
prevent load shedding. With more wind and solar on the grid, frequency will fall more quickly
when there is an upset. Lower inertia increases the rate of change of frequency, giving less time
for primary frequency response to arrest frequency decay above under-frequency load shed set
points.
Capacity ramp rates are important in maintaining frequency during normal operations. Changes
in the amount of non-dispatchable resources, load behaviors and the generation mix can impact
the ramp rates needed to keep the system in balance. As renewables increase as a percentage of
total generation, the flexibility of dispatchable resources will become more important to balance
demand and supply. One of the major challenges of managing power quality is dealing with
steeper ramp rates when, for example, the sun sets and/or the wind diminishes.
The injection or absorption of reactive power to maintain voltage levels in the transmission and
distribution system under normal conditions is referred to as voltage support. This control is
local in nature, at individual transmission substations and the distribution system. The concept
of Voltage/VAR (volt-amp reactive) management is essential to electrical utilities’ ability to
deliver power within appropriate voltage limits so that supplied equipment operates properly,
and to deliver power at an optimal power factor to minimize losses. These concepts are affected
by a variety of factors throughout the distribution network including: substation bus voltages;
length of feeders; conductor sizing; type, size, and location of different loads (resistive,
capacitive, inductive, or a combination of these); and the type, size, and location of distributed
energy resources (photovoltaics, distributed wind, various storage technologies, etc.).v
Utility loads require a combination of real power (watts) and reactive power (VARs). Real
power must be supplied by a generator while reactive power can be supplied either by a
generator or a local VAR supply, such as a capacitor. Delivery of reactive power from a remote
VAR supply results in feeder voltage drop and losses due to increased current flow, so utilities
prefer to deliver reactive power from a local source.
Let’s explore storage options to understand their benefits and uses as related to maintaining
power quality and grid reliability.
Storage Options
With few exceptions, electricity cannot be stored in any appreciable quantities, and thus must be
produced as needed. Further, electricity’s inelastic demand does not move with prices. Lacking
storage and responsive demand, operators must plan and operate power plants and the
transmission grid so that demand and supply continuously match.
This requires close coordination of all utility functions, notably the process of determining which
generating units to use, generally prioritized on efficiency (lowest cost generation is dispatched
first). Operators want to commit just enough capacity to ensure reliability, but no more than is
needed. Reserve power can be provided in multiple ways. These include distribution through
interconnecting grids, operating fossil fuel plants on part power, spinning reserve and energy
storage.
Utility scale storage offers a solution to managing these complexities. There are several
technologies available with varying advantages in terms of cost, capacity, technical maturity,
efficiency cycle life.
Fig. 1 below provides a comparative maturity status. Some technologies are displayed with
respect to their associated initial capital investment requirements and technology risk versus their
current phase of development (i.e. R&D, demonstration and deployment, or commercialization
phases). At one end of the scale pumped hydro storage is well proven and in commercial
operation around the world for over 100 years. At the other end is thermochemical storage;
reversible chemical reactions, a technology at the early R&D stage. In between there is indirect
storage including compressed air storage, batteries, fuel cells and mechanical flywheels, and
direct storage such as supercapacitors and superconducting magnets.
Fig 1: Energy Storage Technology Maturityvi
Let’s briefly review some of these technologies.
Pumped storage
Pumped storage hydro power is the most mature of large scale storage methods. It has a high
efficiency (70 – 80%) and fast response rate. There are relatively few suitable locations and the
local environmental impact can be significant. Still, pumped hydro storage dwarfs all other
forms of electricity storage in Europe, accounting for more than 99% of the total. There is 53.2
GW of pumped storage in Europe as of 2016. vii
Underground Pumped storage
With a limited natural locations suitable for pumped storage, underground pumped storage is an
alternative solution. Typically, mines that are no longer viable for their mineral content can be
converted to provide pumped storage. A reservoir above the mine and another at the bottom
along with the hydro plant can utilize the head of the mine shafts and the excavated underground.
In Germany, the 600 meter (1,969 foot) deep Prosper-Haniel coal mine is about to be turned into
a 200 MW pumped-storage hydroelectric reservoir to help provide uninterrupted power in
support of the renewables.viii
Compressed Air Storage
Compressed air energy storage (CAES) can store large amounts of energy like pumped storage
and has potential for large-scale, cost-effective storage. It has a fast rate of response but requires
sealed caverns. In CAES, excess power from the grid is used by an electric motor to drive a
compressor. The compressed air is cooled and stored at pressures of typically 60-70 bar. At
times of high electrical demand, the air is drawn back from the store, heated and then supplied to
a modified gas turbine. The energy from this high-pressure air, along with some thermal input,
drives the turbine generator to supply electrical energy to the grid. The first CAES plant with
290MW capacity has been operating in Huntorf, Germany, since 1978 and another 110MW plant
has also been operating in the US since 1991. New developments in adiabatic and underwater
CAES look promising. Underwater storage involves a balloon-like vessel made of stretched
fabric, anchored to a sea- or lakebed. When energy is needed, its compressed air can be released
to drive turbines.
Thermal storage
Thermal storage entails storing energy in form of heat. Storing large amounts of heat can be
achieved by simply heating an insulated mass or through phase changing of materials. Some
materials can hold large energy amounts when changing from one phase into another. Molten
salt storage (a combination of sodium and potassium) offers such capability. Gemasolar in
Seville, Spain is the first commercial scale solar thermal power plant. It can produce 19.9 MW
for 24 hours a day. While it has the benefit of high efficiency, high life cycle and low cost, it
needs to be coupled with concentrated solar power which is expensive.ix
Flywheels
A flywheel is a rotating mechanical device that is used to store rotational energy. The amount of
energy stored in a flywheel is proportional to the square of its rotational speed. For power
generation, a flywheel releases stored energy by applying torque to a generator. Flywheels are
well suited for shorter-duration frequency regulation.
Electromagnetic Energy Storage
Electromagnetic energy storage systems store electricity “directly” in the electromagnetic fields
without transformation. The two main technologies are supercapacitors and superconducting
magnetic energy storage (SMES) technologies.
Supercapacitors
The capacitive energy storage is based on the parting of positive and negative electrical
charge carriers. Supercapacitors are basically double-layered versions of normal
capacitors but with considerably higher electrode surfaces and a fluid electrolyte.
Compared to lead-acid batteries, supercapacitors have lower energy density but they have
longer cycle lives and faster charge and discharge capabilities than batteries. However,
large-scale commercial supercapacitors are still under development, with cost being the
greatest hindrance to date.
Superconducting Magnetic Energy Storage (SMES)
Superconducting magnetic energy storage (SMES) systems store electrical energy in the
magnetic field of a coil. They typically consist of a superconducting coil, a power
conditioning system, a refrigeration system for cooling of the coil and a cryostat/vacuum
vessel. The superconducting material itself has very little resistive losses, but must be
cooled down by the cryogenic system to extremely low temperatures (~ 5 K). This
cooling is mainly done with liquid helium, which leads to considerable high operational
costs. The major advantages of SMES systems are their high efficiencies (~ 95%) and
their ability to provide very high output and fully recharge in minutes. SMES systems
are relatively new technologies with only a few prototypes developed.
Batteries
The battery storage industry is growing quickly, particularly Lithium-ion (Li-ion). Electric cars
are largely responsible, with increasing demand driving a large-scale manufacturing. As
manufacturing capacity grows, prices for lithium-ion batteries have fallen fast—by almost half
just since 2014. Three massive battery storage plants—built by Tesla, AES Corp., and Altagas
Ltd. are being built in southern California. Combined, they amount to 15 percent of the battery
storage installed planet-wide last year. x China leads the world in Li-ion production, followed by
South Korea, the U.S., and Poland.xi Some of the competing battery technologies:
Lead acid
Lead-acid remains the least-cost battery technology and has a well proven track record,
but it’s very low cycle life keeps it from being competitive in most grid applications.
Lithium ion
Lithium-ion is a higher-cost option today, but as noted, costs are falling rapidly. These
batteries use lithium as an electrolyte and have a very high energy density and both cycle
life and efficiency are superior to lower-cost options.
Sodium -Sulphur
Sodium-sulfur batteries have historically been the least-cost battery option. They are cost
competitive with lithium ion, but are currently only available with a six/seven hours’
duration, operate at high temperatures (300 – 350oC) and costs have not declined much in
over a decade of deployment.
Zinc
Two zinc-based batteries on the cusp of commercialization are the next-lowest-cost
option. Their materials are abundantly available; however, they must still build up a track
record and are challenged by both cycle life and efficiency.
Flow Batteries
Flow batteries have a much longer cycle life and lower cost than lithium-ion but lack
efficiency and to date are much more expensive.
The Li-ion battery was selected for the GE Hybrid EGTTM due to its high efficiency, high energy
density and rapidly falling costs. Combining a large-scale battery system and a gas turbine
combines the benefits of both, the first of which has been deployed in California, USA. Let’s
examine the case study there and how the benefits apply in Europe.
EGTTM Case Study
Grid Requirements
The California Independent System Operator (CAISO) is a nonprofit public benefit corporation
and manages wholesale electricity markets through much of California, centrally dispatching
electricity generation facilities. In managing the grid, CAISO provides open access to the
transmission system and performs long-term transmission planning. CAISO’s peak load is
around 50MW. System capacity is ~60% gas fuel powered, with much of the balance nuclear
and renewables. CAISO manages a day ahead market, a real-time spot market and ancillary
services. The real-time market uses final day-ahead schedules for resources within the network
as a starting point, then operates a fifteen-minute market to adjust resource schedules, and then a
five-minute market to balance generation and loads.xii CAISO procures four ancillary services in
the day-ahead and real-time markets, including:
• Regulation up: units providing regulation up must be able to move quickly above their
scheduled operating point in response to automated signals from the ISO (equivalent to the
Transmission System Operator (TSO) in Europe) to maintain the frequency on the system by
balancing generation and demand.
• Regulation down: units providing regulation down must be able to move quickly below their
scheduled operating point in response to automated signals from the TSO.
• Spinning reserve: Resources providing spinning reserves must be synchronized with the grid
(online, or spinning) and can respond within 10 minutes. This is more reliable than non-
spinning reserves because generating capacity is already online and synchronized.
• Non-spinning reserve: Resources providing non-spinning reserves must be able to
synchronize with the grid and respond within 10 minutes.
Regulation up and regulation down are used continually to maintain system frequency by
balancing generation and demand. Spinning and non-spinning resources are used to maintain
system frequency and stability during emergency operating conditions (such as unplanned outage
of generation or transmission facilities) and major unexpected variations in load. Spinning and
non-spinning resources are often referred to collectively as operating reserves.xiii
Supply and Demand Challenge
Within CAISO, Southern California Edison (SCE) is the primary electricity supply company
for much of Southern California. It provides 14 million people with electricity across a service
territory of approximately 50,000 square miles. SCE selected GE to partner on a new battery
energy storage system (BESS), to help avert grid issues related to gas shortages caused by a
gas leak at the Aliso Canyon storage facility, the second largest of its kind in the United
States. The leak, discovered on October 23, 2015, resulted in shutdown of the facility, and the
unavailability of gas storage in the region resulted in insufficient delivery of gas to power plants,
leading to a strained electricity grid. Load pockets that are chronically constrained include San
Diego, Los Angeles Basin, and North Coast/North Bay area (San Francisco).
Even prior to the leak, California was leaning forward on energy storage. In 2013 the
California Public Utilities Commission enacted the nation's first energy storage mandate,
directing investor-owned utilities to buy 1.325 GW of energy storage capacity by 2020.
Storage is expected to play an important role in meeting the state's 50% renewable energy
mandate and a recently passed goal to cut greenhouse gas emissions 40% from 1990 levels
by 2030.
The Equipment
The EGTTM project utilizes a 10 MW battery energy storage system supplied by GE Current,
and an existing LM6000PC gas turbine with control system upgrades provided by GE’s
Power Services (Fig. 2).
Fig 2: LM6000 Hybrid EGTTM
Gas Turbine
The CF6 family of GE Aviation’s aircraft engines is packaged by GE Power for industrial use as
the LM6000 in power generation or mechanical drive applications. The LM6000 gas turbine is a
dual-rotor, concentric drive-shaft, gas turbine capable of driving a load from the front and/or rear
of the low-pressure (LP) rotor. Introduced in 1961, there have been more than 1,100 LM6000’s
in operation around the world, adapted to dynamically demanding applications using a broad
range of liquid and gaseous fuels. Combustor configurations include steam or water injection for
emissions control, or a Dry Low Emissions (DLE) option. Wet compression power
augmentation is also offered through Sprint™.
Storage Plant Controller
GE's Energy Storage Plant Controller (ESPC) is a supervisory control and data acquisition
(SCADA) system for energy storage plants and renewable hybrid plants. The ESPC is built on
GE’s MarkVIe Control System, a mature platform that is used worldwide to monitor and control
GE's gas turbine, wind, and solar energy fleets. The MarkVIe controller is installed in over 400
global locations with more than 16 million hours of combined operation. GE's ESPC monitors
and controls inverter-and plant-level functions and provides real-time and historical operational
data for performance analysis. It includes a variety of grid-friendly controls for managing plant-
level active power, reactive power, voltage, and frequency. These controls can be configured to
meet operating requirements under a variety of conditions such as grid connect mode and
islanded mode, and to allow GE's Battery Energy Storage System (BESS – Fig. 3) to connect to
the grid at one point rather than at several inverters.
Battery Enclosure
Purpose Built Enclosures for the batteries are low maintenance and modular. They are designed
for 20 years of life and offer more than 25 percent higher power and energy density than
standard shipping containers. Aisle widths of 48 inches meet National Electrical Code (NEC)
requirements. Integrated Fire Suppression System, cooling and insulation are also included.
Brilliance Inverter
GE's Brilliance Inverter includes features such as voltage ride-through, frequency response, and a
variety of reactive power configurations. There are more than 23,000 GE Renewables Inverters
installed on GE's wind turbines and solar installations around the world. Each 1.25 MW inverter
is capable of charging and discharging at 1.25 MW, has 50/60 Hz options and a peak conversion
efficiency >98%. It includes an integrated 3000 amp dc disconnect and 1600 amp ac circuit
breaker and provides zero, low and high voltage ride through capabilities.
Fig 3: Battery Energy Storage System
Lithium-Ion Battery
This battery technology was chosen for several reasons. It can reach the minimum on-line load
commitment without burning fuel or adding wear on the plant. It is greenhouse gas free, it
reduces the fuel expense for peaking and mid-merit power operation and it also increases the
plants utilization for peaking and mid-merit operation. Previously noted, Lithium-ion costs are
falling rapidly. These batteries have a very high energy density and both cycle life and
efficiency are superior to other battery options.
GE partners with several of the world’s Tier 1 Li-ion battery manufacturers to provide fully
tested and integrated solutions. Each manufacturer and candidate cell must pass stringent
performance and quality inspections. GE Energy Storage performs a comprehensive Safety Risk
Assessment for our equipment based on ISO 12100, the Recommendations for Safety Risk
Assessments and ISO 13849, Functional Safety.
Operation and Benefits
Reduce System Costs to Create Ratepayer Value
The largest value is typically captured by load serving entities or transmission system operators
(TSO), with savings that can be passed to ratepayers (Fig 4a and 4b). The GE LM6000 EGTTM
Hybrid system provides transient response and provides high-fidelity frequency regulation with
GT fuel burn, and 50MW of rapid response contingency reserve with zero fuel burn, including
primary frequency response and voltage support services.
In the United States, this has been made possible by the Federal Electric Regulatory Commission
(agency that regulates interstate electric transmission) rules that allow energy storage systems to
qualify as spinning reserve. Key sections of the rule:
• FERC Order 789 Section 48: BAL-002-WECC-2 R2
o “The Commission determines that non-traditional resources, including electric
storage facilities, may qualify as ‘Operating Reserve—Spinning’ provided those
resources satisfy the technical and performance requirements in Requirement R2.”
• R2 Requirements:
o 2.1 Reserve that is immediately and automatically responsive to frequency
deviations through the action of a governor or other control system;
o 2.2 Reserve that is capable of fully responding within ten minutes.
The EGTTM battery system provides the immediate response. Within ten minutes, the gas turbine
can be on line at full power. This frees up resources lower in the bid stack (e.g., curtailed solar
and negative priced assets such as CCGT otherwise used for capacity) to generate power at
higher efficiencies. Since more energy is generated by these base assets, fewer expensive peak
resources need to be called upon, both saving fuel burn and lowering the overall market clearing
price for energy.
Fig 4a: Rate Payer Savings Illustration without GT
Fig 4b: Rate Payer Savings Illustration with GT
The GT can reach max power in a minimum of 5 minutes if needed. The hybrid EGTTM delivers
the commanded net-output on a blended basis from the Battery and/or Gas Turbine at the most
optimal ratio given both internal and external system requirements. The desired hybrid power
demand is calculated automatically by the EGTTM control system based on net power demand
and required frequency control contribution subject to the BESS max power limits, ramp rate
limits and stored energy capacity.
When the GT is not running, the BESS system supplies all demanded power until the demand
exceeds the BESS capacity. Once the GT is running, the GT power demand is calculated using a
combination of EGTTM net power demand, hybrid frequency droop power demand and hybrid
charge power demand subject to GT MW and ramp rate constraints.
The BESS operates across the complete MW range providing high-fidelity frequency regulation.
Figure 5 indicates an actual example of a highly variable power output the hybrid EGT will to
produce through a blended combination of the BESS and the GT. The GT provides the overall
power with its output increasing and decreasing gradually to meet the overall demand. At the
same time the BESS provides the highly variable and transient output, thereby evening out the
load on the GT. Thus, there’s considerably less stress on the GT and consequently there’s a
considerable reliability and availability benefit.
Fig. 5: EGT Response
Each battery is equipped with a battery management system (BMS) that monitors and maintains
the battery for the optimal performance per system guidelines. The BMS controls and protects
the battery, and relays information on temperature of the cells, voltage and current parameters,
and reports on the health of the battery. The BMS can open contactors to electrically isolate a
battery module from the rest of the battery rack as necessary. When conditions improve or when
the battery status returns to normal, the BMS reconnects the battery module to the rest of the
rack. In addition, each BMS also includes a fuse that can protect against over-current events if
the BMS’s control of the battery is compromised.
The ESPC is connected to these devices through a communication protocol. It serves as the main
interface for retrieving all system information, communicating with remote locations, performing
grid services, providing data for HMI and SCADA units, and monitoring the system.
Conclusions
The bulk electric system in Europe is undergoing a significant transformation. Growth in
renewable energy capacity will require flexible thermal power systems that will maintain grid
reliability and power quality. Reserve dispatching favors low cost providers and often requires
assets to run at non-optimum, minimum loads. Additionally, the market needs fast transient
response and low emissions; meeting this dichotomy poses new challenges to grid operators and
generators alike.
To address these challenges, GE has developed the LM6000 Hybrid EGTTM, coupling a 10 MW
battery with a 50 MW GE LM6000 Gas Turbine, operated by an integrated digital control
system. The first plant was made operational 17 April 2017 in California, USA. Key benefits
include “spinning reserve” without firing the gas turbine utilizing near instantaneous battery
power through inverters, enhanced primary frequency response and voltage support, reduced
greenhouse gas emissions, and smooth transient response with less turbine thermal stress,
thereby lowering maintenance costs.
i Gross national electricity consumption includes the total gross national electricity generation
from all fuels (including auto-production), plus electricity imports, minus exports. Auto-
production is defined as a natural or legal person generating electricity essentially for his/her
own use. Gross electricity generation is measured at the outlet of the main transformers, i.e. it
includes consumption in the plant auxiliaries and in transformers. ii "Energy from Renewable Sources." Energy from Renewable Sources - Statistics Explained.
European Commission, Mar. 2017. Web. 18 Mar. 2017. iii European Commission - Fact Sheet, http://europa.eu/rapid/press-release_MEMO-15-
5181_en.htm iv Mann, Deborah. European Grid Storage, Technology Options Beyon Batteries. Rep Houston:
HIS, 2017. Print. v Web: https://www.nema.org/Policy/Energy/Smartgrid/Documents/VoltVAR-Optimazation-
Improves%20Grid-Efficiency.pdf vi Web:
https://www.iea.org/publications/freepublications/publication/TechnologyRoadmapEnergystorag
e.pdf vii IBID, part 2, page 8 viii Web: https://www.bloomberg.com/news/articles/2017-03-17/german-coal-mine-to-be-
reborn-as-giant-pumped-hydropower-battery ix Web: https://en.wikipedia.org/wiki/Gemasolar_Thermosolar_Plant x Web: https://www.bloomberg.com/news/articles/2017-01-30/tesla-s-battery-revolution-just-
reached-critical-mass xi Web: http://www.visualcapitalist.com/china-leading-charge-lithium-ion-megafactories/ xii Energy Primer: A Handbook of Energy Market Basics, November 2015. Rep. Washington,
DC: Federal Energy Regulatory Commission, 2015. Print. xiii IBID