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Energy storage systems are enabling technologies; their goal is to enhance and extend the operating capabilities of other assets on the grid.
Although electricity cannot be (cheaply) stored directly, it can be easily stored in other forms and converted back to electricity when needed.
By decoupling the production and consumption of electricity, resources such as solar and wind energy that may normally not be cost-effective can be made competitive and viable solutions to a far wider set of energy needs.
Electric power has a tremendous weakness; it must always be used precisely when it is produced. Based on this tenuous balance of supply and demand, its inherent monetary value also changes by the hour.
Storage not only helps marginally competitive resources get around these limitations, but can also improve the economic efficiency and utilization of the entire system.
By optimizing the existing generation and transmission assets in the market, less capital is needed to provide a higher level of service - while giving energy sources such as renewables more opportunities for development.
Energy storage facilities can interact in the electric value chain within three 'business models' which correspond to the market areas where they will interact: wholesale, retail and renewable.
Within the wholesale market, large-capacity storage facilities are able to arbitrage power generation from night- to daytime peak prices. These facilities also provide ancillary services to the grid, to promote stability and provide for power transfers across the grid.
Technologies include pumped hydro, compressed air energy storage (CAES), superconducting magnetic energy storage (SMES) and flow batteries (fuel cells).
In the retail market, small-scale energy storage facilities provide energy management, power quality and power reliability services to end-use consumers. By providing 'clean' and reliable power and a ride-through capability in the event of a power outage, a manufacturer can get back to his business with peace of mind - and a lower utility bill. Technologies include batteries, flywheels and thermal storage.
Renewable energy storage strategies include both wholesale and retail strategies that leverage the strengths of renewable resources.
Three general strategies correspond to the level of integration to a transmission system: off-grid, distributed generation support and baseload wind concepts.
One of the greatest challenges facing the electric power
industry worldwide is how to harness the immense renewable energy resources
and deliver them in a useable form as a higher-value product. By storing
the power produced from renewable sources off-peak and releasing it during
on-peak periods, energy storage can transform this low-value, unscheduled
power into schedulable, high-value 'green' products. Developing these
resources will not only lessen environmental impacts but also increase
each country's domestic energy security (lowering payments for imported
energy).
Two challenges in particular are well known to renewable energy proponents. First, many of the potential power generation sites are located far from transmission facilities. Although the costs of connecting these sites to the transmission system will delay these resources from being tied into the local grid for a long time, it leaves the possibility for off-grid applications.
The second challenge is the timing of resource itself. Generally, renewable energy sources are intermittent or vary in intensity throughout the day, with much of the potential for generated power not coincident with the peak demand. Renewables - especially wind - suffer from lower prices in the wholesale market due to the inability to guarantee delivery levels.
Besides these challenges with the renewable resources, technical hurdles must also be overcome - even with mature storage media such as lead-acid batteries.
According to Steve Drouilhet, President of Sustainable Automation, LLC (Denver, Colorado, US), system integration must be thought through properly to create a viable system.
Two issues make or break most projects: first, 'properly sizing and dispatching the energy storage to achieve an economically optimal system (lowest lifecycle cost of energy)'; and secondly, 'designing the power converter interface between the AC power bus and the energy storage to be efficient, reliable and robust'.
No storage technology is suitable for all applications. Each technology stores energy in a different form, giving it inherent properties that tailor it for one role rather than another. To rank the technologies for each application on technical grounds, they are evaluated on five issues.
Real power is the MW output of generation facilities, and is used for commodity power sales and peak shaving strategies.
Reactive power maintains the electric field of AC equipment and is required for the proper operation of the grid; it is measured in megavars (MVAR).
The length of time a storage facility can discharge energy. Generally, longer endurances tend to be real power, with shorter times, reactive power.
Some applications, like grid support, require discharges to commence less than a second after beginning; others, like power sales, can be scheduled allowing for a reaction time of a few minutes.
Some applications require that the storage facility be housed inside, taking up valuable floor space and requiring additional space-conditioning costs.
Pumped-hydro storage is the oldest and largest of all of the commercially available energy storage technologies, with facilities up to 1000 MW. Pumped storage projects differ from conventional hydroelectric projects in that they normally pump water from a lower reservoir to an upper reservoir when demand for electricity is low.
Pumped-hydro facilities consist of two large reservoirs; one located at a low level and the other situated at a higher elevation. During off-peak hours, water is pumped from the lower to the upper reservoir, where it is stored. To generate electricity, the water is then released back down to the lower reservoir, passing through hydraulic turbines and generating electrical power.
For example, in the summer water is released during the day for generating power to satisfy the high demand for electricity for air conditioning. At night, when demand decreases, the water is pumped back to the upper reservoir for use the next day.
Compressed air energy storage (CAES) systems use off-peak power to pressurize air into an underground reservoir (salt cavern, abandoned hard rock mine, or aquifer) which is then released during peak daytime hours to be used in a gas turbine for power production. Facilities are sized in the range of several hundred megawatts. In a gas turbine, roughly two thirds of the energy produced is used to pressurize the air. The idea is to use low-cost power from an off-peak baseload facility in place of the more expensive gas turbine-produced power to compress the air for combustion. Since CAES facilities have no need for air compressors tied to the turbines, they can produce two to three times as much power as conventional gas turbines for the same amount of fuel. Developers include CAES Development Company, Ridge Energy Storage, Alstom Power and Dresser-Rand.
Flow batteries - also known as regenerative fuel cells - are capable of storing and releasing energy through a reversible electrochemical reaction between two salt solutions (electrolytes). These systems are excellent at storing real power (MW), but poor at delivering reactive power (MVAR) quickly.
Designs exist around the use of zinc bromide (ZnBr), vanadium bromide (VBr), and sodium bromide (NaBr) as the electrolytes. Charging of the facility occurs when electrical energy from the grid is converted into potential chemical energy. Release of the potential energy occurs within an electrochemical cell, with a separate compartment for each electrolyte, physically separated by an ion-exchange membrane.
The technology is a closed loop cycle, so there is no discharge of the regenerative electrolyte solutions from the facility. The scale of the facility is based primarily on the size of the electrolytic tanks. Developers include Regenesys, Vanteck, Powercell and ZBB Energy.
A number of battery technologies exist for use as utility-scale
energy storage facilities. Primarily, these installations have been lead-acid,
but other battery technologies such as sodium sulphur (NaS) and lithiumion
are quickly becoming commercially available.
All batteries are electrochemical cells. They are composed of two electrodes separated by an electrolyte. During discharge, ions from the anode (first electrode) are released into the solution and oxides are deposited on the cathode (second electrode). Reversing the electrical charge through the system recharges the battery. When the cell is being recharged, the chemical reactions are reversed, restoring the battery to its original condition. Developers include SEI and SAFT.
Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a coil of cryogenically cooled, superconducting material. A SMES system includes a superconducting coil, a power conditioning system, a cryogenically cooled refrigerator and a cryostat/vacuum vessel.
SMES are highly efficient at storing electricity (greater than 95%), and provide both real and reactive power. These facilities are used to provide grid stability in a distribution system and power quality at manufacturing plants requiring ultra-clean power, such as microchip fabrication facilities. Developers include American Superconductor.
A flywheel energy storage system works by accelerating a rotor to a very high speed and maintaining the energy in the system as inertial energy. Advanced composite materials are used for the rotor to lower its weight while allowing for the extremely high speeds; energy is stored in the rotor in proportion to its momentum, but the square of the angular momentum. The flywheel releases the energy by reversing the process and using the motor as a generator. As the flywheel releases its stored energy, the flywheel's rotor slows until it is discharged. Developers include Active Power, AFS Trinity and Beacon Power.
Not generally thought of as one of the new, high-tech energy storage technologies, thermal energy storage systems already exist in widely used applications. Thermal systems can either be ice-based (for peak-shaving commercial and industrial cooling loads), or heliostat-based (mirror-based) using molten salt for electric power production (still in the development phase).
Off-grid applications for renewable resource and storage
systems are well understood. Remote or simply self-generation applications
of solar have been used throughout the world for decades - and so, more
recently, have wind applications - to ensure sufficient supply of electrical
power when needed. In these off-grid systems, the decision to go forward
isn't necessarily the cost of the electricity produced, but rather the
value of running electrical equipment.
It is likely that the cost of stringing out power lines to these remote locations will prohibit electrification for years, if not permanently. Many such locations are in environmentally sensitive locations, and this tips the scale in favour of renewables, rather than small generators, which in themselves would require a fuel delivery infrastructure. Utilizing renewable energy and a storage facility obviates the need for the fossil technology with its accompanying supply infrastructure.
One development from 1997, typical of many small-scale rural projects, is found in the village of Yuxquen in Guatemala. The village does not have electricity service due to its remoteness from transmission power lines. The Guatemalan government considered renewable resources to provide power, due to the cost of adding the lines necessary for transmission; the local cloudy and windy conditions ensured that wind was a preferable power source to solar.
A 1500 W wind generator, coupled with a battery storage system, was designed to provide power for battery charging, lighting, and radio and TV service. The wind turbine is located on a hill near the village. The initial capital cost of the system was $17,500, and a monthly charge paid by each family allows the creation of a fund to be used for spare parts and to maintain the battery bank.
The battery storage system is located in a small house underneath the wind tower. The batteries' energy (DC) is converted to AC, and then stepped up in voltage to transmit it to the village, where it is stepped down in voltage for distribution. Inside each house, a security system was installed to prevent any problem in one house from affecting the rest of the users. The battery system was crucial in extending the usefulness of the wind energy for the village: although normally strong, the wind power would be intermittent on daily and seasonal cycles.
Renewable energy storage can also support distributed generation facilities, significantly lowering their operation costs in small grid environments.
Without storage power generators, or gensets, on a small grid will run sporadically to match peak demands. Many times, the peak usage will sometimes last only a few minutes, necessitating multiple starts and stops for the genset, which dramatically raises the operating costs and shortens the unit's life.
Reducing the number of generator starts by providing the first few minutes of power to the grid from the wind turbine-charged storage system can significantly increase fuel savings in the generators.
To maximize the savings from the hybrid system, the duration of the energy storage system discharge was found to be a crucial component.
Discharges from the system could last from simply a ride-through for diesel generator starts to load shifting, storing power to allow service without the generator being in operation.
Based on a study the US National Renewable Energy Laboratory conducted in 1997 on a system in Deering, Alaska, the largest fuel saving came from relatively short-term storage. Evidence from this test indicated that a storage capability of 10 minutes reduced the fuel use by 18%, the diesel running time by 19%, and the number of diesel starts by 44%.
Northern Power Systems (Waitsfield, Vermont, USA) recently installed a 50 kW hybrid power system for the US-Brazilian Renewable Energy Rural Electrification Project, in the Brazilian community of Joanes. Power demand was growing in the village, and traditional measures were proving uneconomical.
Prior to installation of this system, residents of the town relied upon a 1 MW diesel plant to supplement system power, which came from a long, weak and unreliable power line. Unfortunately, this resource was expensive and insufficient for the village's highly variable loads. The area did, however, contain favourable wind and sun conditions, sufficient for incorporating wind and solar energy.
To take advantage of these renewable resources and cover the peak power demand needs, four 10 kW wind turbines and a 10 kW photovoltaic array were coupled with a 228 kW battery storage system. These renewable resources can either deliver power directly to the grid, or can charge the battery bank to dispatch its full 50 kW capacity to the grid during times of peak demand.
One of the most exciting market opportunities lies in
enabling renewable energy to become more competitive in the wholesale
electric power market.
Coupling storage (of 100 MW or more) with a large-scale wind project is a key part of this strategy to minimize the total cost of power delivered. By storing the power from renewable sources during off-peak periods, and releasing it at on-peak times, coincident with periods of peak consumer demand, energy storage can transform this low-value, unscheduled power into schedulable, high-value products.
Current large-scale merchant wind developments must contend with utility push back towards these facilities. Some utilities support wind projects while others do not; their response generally depends upon the level of constraint on the local transmission system.
In Europe, utilities are integrating weather forecasts on the supply side as well as demand, in an attempt to anticipate the level and timing of the wind resources for grid management.
A way around this is to rethink the production of power from wind resources. Instead of smoothing energy production from wind turbines, the maximum energy production would be produced and then stored on-site for release later. Not only does this strategy allow for the guaranteed delivery of 'green' power during peak power costs, it also increases the potential wind energy produced from the wind resources.
Baseload wind developments will still need transmission connections, but by increasing the value (monetarily for the developers and strategically for the grid operators) of the wind resources through storage, the number of potential sites effectively increases.
Discharges would normally occur once per business day (250 times per year), allowing the sale of a block of power into the peak daily market. In addition to these weekday sales, the storage facility could also operate at weekends, but the peak pricing would lower the revenue from operations - possibly below the cost of production.
Once the storage facility was fully charged, it would be able to act as an emergency back-up power source for the grid, providing additional revenue to the project.
No project is currently under development, although a number have been discussed.
Thinking up ideas to get around the problems of rural renewable energy production is not new. Many concepts exist for the conversion of wind power to hydrogen gas, which would be collected by a tanker on a set schedule, to be brought to a central station to power fuel cells connected to the grid. This concept also prevents the need for expensive transmission facilities. See Hydrogen Storage of Wind Energy for further ideas.
Energy storage technologies can enable renewable resources to either compete effectively in a competitive market, or make a local renewable resource a valuable asset to a rural area. Additional development is still needed to advance the state of the art for these technologies, but that does not mean that they are not ready in their present form to help make renewable resources a far more prevalent component of the world's electric power industry.
Helping developers utilize renewable resources generally piques their interest; save them money from their existing operations, and they'll beat a path to your door.
