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1. INTRODUCTION
Making electricity grids “smarter” and modernizing them, so that they can accept large amounts of renewable energy resources is one of the step necessary to achieve a clean and secure electric power industry. The best way to achieve this goal is a topic of debate among power system designers. Although energy storage in utility grids has existed for many decades, the impact of storage in future grids is receiving more attention in these days. The amount of storage in a grid and its value is also a subject of debate.
Our power system will face many challenges in the next several decades. The arrival of plug-in hybrid vehicles, exponential growth in customer-controlled generation, growth of nondispatchable and intermittent renewable sources of energy, increasing demand for higher service reliability and power quality, microgrids—these are just some of the challenges that our system faces now and will continue to confront into the future. While the nature of these challenges is somewhat known, the extent of their impact is not fully understood.
American Electric Power (AEP) views distributed energy storage as a key piece of its future grid. By strategically locating and controlling distributed energy storage systems (DESSs) on a large scale, AEP can potentially reduce adverse impacts on the grid by creating a controllable buffer between utility and nonutility-controlled assets.
Understanding the leading storage technologies and how they can affect grid operations is an important first step in this assessment
2. SMART GRID THE FUTURE GRID
A smart grid delivers electricity from suppliers to consumers using digital technology with two-way communications to control appliances at consumers' homes to save energy, reduce cost and increase reliability and transparency. It overlays the electricity distribution grid with an information and net metering system.
Such a modernized electricity network is being promoted by many governments as a way of addressing energy independence, global warming and emergency resilience issues. Smart meters may be part of a smart grid, but alone do not constitute a smart grid.
The smart grid is made possible by applying sensing, measurement and control devices with two-way communications to electricity production, transmission, distribution and consumption parts of the power grid that communicate information about grid condition to system users, operators and automated devices, making it possible to dynamically respond to changes in grid condition.
A smart grid includes an intelligent monitoring system that keeps track of all electricity flowing in the system. It also incorporates the use of superconductive transmission lines for less power loss, as well as the capability of integrating renewable electricity such as solar and wind. When power is least expensive the user can allow the smart grid to turn on selected home appliances such as washing machines or factory processes that can run at arbitrary hours. At peak times it could turn off selected appliances to reduce demand.
3. APPLICATIONS OF ENERGY STORAGE
In the early 2000s, AEP noticed an alarming rate of increase in the number and accumulated capacity of “customer-controlled” distributed generators (DGs) being connected to its grid. Moreover, the majority of these DGs were either solar or from other intermittent renewable sources that could not be available on demand. It started to become clear that there is a strategic value in having electricity storage act as a “buffer” between variable energy resources and the electric grid—allowing the utility to sustain and even improve the reliability of electric service to customer.
Electricity is generated and consumed instantaneously. Unlike other energy supply systems--oil, natural gas, or coal–the electricity power grid or the generation plants that supply the grid have essentially no storage or “surge” capacity to smooth out peaks and valleys in demand or to provide reserve capacity during sudden spikes in demand.
Fortunately, coal-fired and nuclear power plants, as well as hydroelectric plants, have turbine generators that run continuously so that they deliver firm, continuous, and dispatchable power required by consumers. With such a steady baseload supply, power plants can meet demand shifts and daily cycles either by adding peaking generators or utilizing the available “spinning reserve,” about 13 percent today for the U.S. power industry. Thus, the balancing of power supply and demand, critical for the operation of a safe and reliable grid, is now done “on line in real time.”
Renewable power, whether generated by wind, photovoltaic, or solar thermal, is inherently intermittent, and there is no “spinning reserve.” The sun generates power 10-12 hours a day at best, and clouds can block the sun at any time. The wind changes direction and velocity on a regular basis, and loss of wind can occur at any time. Yet, power must be supplied to match load demand using a mix of baseload and dispatchable generating plants. This is the Achilles’ heel of renewable power.
Massive Electricity Storage (MES) is the critical technology needed by renewable power if it is to become a major source of baseload dispatchable power to eventually replace fossil/nuclear power plants For system stability and load leveling, stored power of multi-MW capacity and multi-hours of application duration are needed to convert the intermittent and fluctuating renewable power that is generated into dispatchable power. Without sufficient MES, accessible online, solar/wind power cannot serve as a stable baseload supplier; it can only piggyback onto baseload fossil/nuclear generators as a small incremental supplier.
Other applications of energy storage are triggered peak shaving and dynamic islanding.
Triggered peak shaving does not allow the battery to be discharged unnecessarily during daily peak hours; rather, it discharges the battery when the load of a nearby “bottleneck” on the grid exceeds a certain “trigger” level. This approach allows the battery to offer its peak shaving value while increasing the availability of the remaining stored energy to serve customers in the event of an outage
Dynamic islanding allows a utility to continue serving a group of customers when the rest of the grid loses power: An island is the part of a grid or distribution feeder that remains energized by using the electricity storage during an outage.
“Dynamic” islanding refers to the variable size of the served island based on the energy in the battery and load information (the battery power is limited, and the load, depending on the time of day, would be very variable). Within the island, AEP has installed several sectionalizing devices that allow the battery to choose different groups of customers to serve depending on the actual load.
4. STATE OF MES TODAY
Fig 4.1 energy storage system
In 2006, the U.S. renewable power from non-hydro sources--mostly solar and wind--supplied 2.4 percent of our annual electricity consumption. Hydropower is a renewable power source and it supplied an additional 7 percent as dispatchable power; but this resource is essentially “maxed out”. The 2.4 percent renewable power is simply piggybacked onto our fossil/nuclear power grid by direct connection. America’s huge grid can absorb this small intermittent incremental amount without encountering grid instability.
However, as renewable power increases its grid penetration to 15 percent or more, grid instability will rise to a level that must be controlled. For smaller and weaker grid systems on islands, such as Hawaii or Ireland, which cannot be tied into large continental grids, the sensitivity to instability is much more acute. No U.S. renewable power generator has MES today. Among battery storage technologies, only one is operating at above 1 MW power level, the 1.2 MW NaS (sodium-sulfur) battery system with 7 hours of storage capacity at an American Electric Power facility. The purpose for this installation is to reduce peak loads for improved distribution service, and not yet for renewable power stabilization.
In America today, there is an almost total absence of public awareness of the need for MES. The prevailing public view is that renewable power can replace fossil/nuclear as power sources if enough wind farms and solar generators are built. All attention and R&D support are focused on improving the cost and performance of wind/solar electricity generators. As a result, it is not surprising that MES is not even recognized as a top priority critical technology deserving sustained attention and support by both the U.S. Department of Energy (DOE) and Congress.
Because our power grid is the backbone of our society, its reliable and safe operation is absolutely essential to our nation’s well-being. The MES devices to be deployed are very large, both in size and investment. More importantly, they must be tested on a commercial scale under a wide range of operating conditions and for extended periods of time before MES can be accepted and included in our grid system. The main energy storage techniques used nower days are¬-
PUMPED HYDRO POWER PLANTS
Utility system designers have seen the benefits of massive amounts of energy storage in the form of pumped hydro power plants. A typical pumped hydro plant consists of two interconnected reservoirs (lakes), tunnels that convey water from one reservoir to another, valves, hydro machinery (a water pump-turbine), a motor-generator, transformers, a transmission switchyard, and a transmission connection. The product of the total volume of water and the differential height between reservoirs is proportional to the amount of stored electricity. Thus, storing 1,000MWh (deliverable in a system with an elevation change of 300 m) requires a water volume of about 1.4 million m3.During non peak hours water is pumped to the elevated reservoir. This water is stored there, and during peak hours water in this reservoir is used to produce the electricity.
COMPRESSED AIR ENERGY STORAGE
Fig 4.2 compressed air energy storage
Compressed air energy storage (CAES) is a peaking gas turbine power plant that consumes less than 40% of the gas used in a combined-cycle gas turbine (and 60% less gas than is used by a single-cycle gas turbine) to produce the same amount of electric output power. This is accomplished by blending compressed air to the input fuel to the turbine.
By compressing air during off-peak periods when energy prices are very low, the plant’s output can produce electricity during peak periods at lower costs than conventional standalone gas turbines can achieve.
Making the CAES concept work depends on locating plants near appropriate underground geological formations, such as mines, salt caverns, or depleted gas wells. The first commercial CAES plant was a 290-MW unit built in Handorf, Germany, in 1978, and the second commercial site was a 110-MW unit built in McIntosh, Alabama, in 1991. These units are fast-acting plants and typically can be in service in 15 min when called upon for power.
5. BATTERY ENERGY STORAGE
Advancements in battery technology over the last 20 years have been driven primarily by the use of batteries in consumer electronics and power tools. Only in the last ten years—with efforts to design better batteries for transportation—have possible uses of battery technology for the power grid emerged. One driver that has helped make potential utility applications possible is more efficient cost-effective power electronics. For battery technologies to be practically applied in the ac utility grid, reliable power conversion systems (PCSs) that convert battery dc power to ac were needed.
These devices now exist and have many years of service experience, which makes a wide range of battery technologies practical for grid support applications in the future.
A large variety of battery types are being used for the grid support Various battery technologies are
5.1) Sodium Sulphur Batteries
5.2) Flow Battery Technology
5.3) Lithium-Ion Batteries
5.4) Nickel-Cadmium Batteries
5.5) Lead-Acid Batteries
5.6) Electro chemical capacitors.
5.1 SODIUM SULPHUR BATTERIES
Fig 5.1.1 sodium sulphur batteries
The sodium sulphur (NaS) battery is a high-temperature battery system that consists of a liquid (molten) sulphur positive electrode and a molten sodium negative electrode separated by a solid beta alumina ceramic electrolyte (Figure 6). The electrolyte allows only positive sodium ions to pass through it and combine with the sulphur to form sodium polysulphides
During discharge, positive sodium ions flow through the electrolyte and electrons flow in the external circuit of the battery, producing about 2 V. This process is reversible since charging causes sodium polysulphides to release the positive sodium ions back through the electrolyte to recombine as elemental sodium. The battery operates at about 300 °C. NaS battery cells are efficient (about 89%). This battery system is capable of six hours of discharge time on a daily basis.
NaS battery technology was originally developed in the 1960s for use in early electric cars, but was later abandoned for that application. NaS battery technology for large-scale applications was perfected in Japan. Currently, there are 190 battery systems in service in Japan, totalling more than 270 MW of capacity with stored energy suitable for six hours of daily peak shaving. The largest single NaS battery installation is a 34-MW, 245-MWh system for wind power stabilization in northern Japan. The battery will allow the output of the 51-MW wind farm to be 100% dispatch able during on peak periods.
5.2. FLOW BATTERY TECHNOLOGY
Fig 5.2.1 flow battery technology
Flow batteries perform similarly to a hydrogen fuel cell. They employ electrolyte liquids flowing through a cell stack with ion exchange through a micro porous membrane to generate an electrical charge. Several different chemistries have been developed for use in utility power applications. An advantage of flow battery designs is the ability to scale systems independently in terms of power and energy. More cell stacks allows for an increase in power rating; a greater volume of electrolytes translates to more runtime. Plus, flow batteries operate at ambient (rather than high) temperature levels
Zinc-bromine flow batteries are being used for utility applications. The battery functions with a solution of zinc bromide salt dissolved in water and stored in two tanks.
The battery is charged or discharged by pumping the electrolytes through a reactor cell. During the charging cycle, metallic zinc from the electrolyte solution is plated onto and the metallic zinc plated on the negative electrode is dissolved in the electrolyte solution and available for the next charge cycle.
One of the advantages of flow batteries is that their construction is based on plastic components in the reactor stacks, piping, and tanks for holding the electrolytes. The result is that the batteries are relatively light in weight and have a longer life. The typical flow battery can be used in any duty cycle and does not have self-discharge characteristics that can cause damage like other battery technologies can.
Flow battery manufacturers are using modular construction to create different system ratings and duration times.
5.3. LITHIUM-ION BATTERIES
Fig 5.3.1 lithium-ion batteries
The battery technology with the broadest base of applications today is the lithium-ion battery. This technology can be applied in a wide variety of shapes and sizes, allowing the battery to efficiently fill the available space, such as a cell phone or laptop computer. In addition to their packaging flexibility, these batteries are light in weight relative to aqueous battery technologies, such as lead-acid batteries. Lithium-ion batteries have the highest power density of all batteries on the commercial market on a per-unit-of-volume basis. Safety issues with lithium-ion batteries in laptop computers have been a recent concern, but continued development of the technology for PHEV application has resulted in newer types of lithium-ion cells with more sophisticated cell management systems to improve performance and safety. The leading lithium-ion cell design being applied in new PHEV designs is a combination of lithiated nickel, cobalt, and aluminium oxides, referred to as an NCA cell. The designed life characteristics on float and cycling duty have made NCA cells the primary choice for the next generation of PHEVs. Two lithium-ion designs that are starting to be used in higher-power utility grid applications are lithium titanate and lithium iron phosphate.
The lithium titanate approach uses manganese in the cathode and titanate anodes. This chemistry results in a very stable design with fast-charge capability and good performance at lower temperatures. The batteries can be discharged to 0% and appear to have a relatively long life..
The lithium-ion battery using iron phosphate cathodes is a newer and safer technology. In this chemistry, it is much more difficult to release oxygen from the electrode, which reduces the risk of fi re in the battery cells. This design is more resistant to overcharge when operated in a range of up to 100% state of charge. As mentioned previously, lithium-ion batteries are used in a wide variety of applications and will benefit from economy of scale in production over the next decade. As volume production increases, the future cost of lithium-ion battery systems will play a key role in how fast they penetrate utility power application
5.4. NICKEL-CADMIUM BATTERIES
As shown in Figure 5, which depicts the exponential growth in the power density of batteries, nickel-cadmium (Ni-Cad) batteries represented a substantial increase in battery power in the middle of the last century. The Ni-Cad battery quickly gained a reputation as a rugged, durable stored energy source with good cycling capability and a broad discharge range. Ni-Cad batteries have been applied in a variety of backup power applications and were chosen to provide “spinning reserve” for a transmission project in Alaska. This project involves a 26-MW Ni-Cad battery rated for 15 min, which represents the largest battery in a utility application in North America. Ni-Cad batteries are still being used for utility applications, such as power ramp rate control for “smoothing” wind farm power variability in areas with weak power grids (such as island power systems).
5.5. LEAD-ACID BATTERIES
The lead-acid battery is the oldest and most mature of all battery technologies. Because of the wide use of lead-acid batteries in a wide variety of applications, including automotive starting and UPS use, lead-acid batteries have the lowest cost of all battery technologies. For utility power application, a 40-MWh lead-acid battery was installed in the Southern California grid in 1988 to demonstrate the peak shaving capabilities of batteries in a grid application. The battery demonstrated the value of stored energy in the grid, but the limited cycling capability of lead acid made the overall economics of the system unacceptable. For backup power sources in large power plants, lead-acid battery plants are still used as “black start” sources in case of emergencies. Their long life and lower costs make them ideal for applications with low duty cycles.
Advanced Lead-Acid Batteries
The high volume of production of lead-acid batteries offers a tremendous opportunity for expanded use of these batteries if their life could be significantly extended in cycling applications. Adding carbon to the negative electrode seems to be the answer. Lead-acid batteries fail due to sulphating in the negative plate that increases as they are cycled more.
Adding as much as 40% of activated carbon to the negative electrode composition increases the battery’s life. Estimates of a cycling life improvement of up to 2,000 cycles represent a three to four times improvement over current lead-acid designs. This extended life coupled with the lower costs will lead storage developers to revisit lead acid technology for grid applications.
5.6. ELECTROCHEMICAL CAPACITORS
Commonly called “supercapacitors,” electrochemical capacitors look and perform similar to lithium-ion batteries. They store energy in the two series capacitors of the electric double layer (EDL), which is formed between each of the electrodes and the electrolyte ions. The distance over which the charge separation occurs is just a few angstroms. The extremely large surface area makes the capacitance and energy density of these devices thousands of times larger than those of conventional electrolytic capacitors.
The electrodes are often made with porous carbon material. The electrolyte is either aqueous or organic. The aqueous capacitors have a lower energy density due to a lower cell voltage, but are less expensive and work in a wider temperature range. The asymmetrical capacitors that use metal for one of the electrodes have a significantly larger energy density than the symmetric ones do and also have a lower leakage current. Compared with lead-acid batteries, electrochemical capacitors have lower energy density, but they can be cycled hundreds of thousands of times and are much more powerful than batteries (fast charge and discharge capability). Supercapacitors have been applied for blade-pitch control devices for individual wind turbine generators to control the rate at which power increases and decreases with changes in wind velocity. This functionality is desirable if wind turbines are connected to weak utility power grids.
6. THERMAL STORAGE
All of the energy storage technologies discussed are targeting ways to help the utility grid cope with balancing generation and load in the most optimal ways possible. Traditionally, utility grids have been designed to deal with the highest load peaks that typically occur less than a few hours per day for only a few days per year. Just like batteries and peaking generators, any storage device that helps meet this objective should be considered in utility system planning. Thermal storage devices that can be deployed at the residential and commercial level should be given more attention. Modular ice storage systems can generate ice during off-peak power periods to power air-conditioning systems for several hours each day during the peak afternoon load times. Similarly, in cold climates, modular heat storage systems can capture electric power during offpeak periods and use that energy to store heat in a ceramic heatsink to be dispatched during higher peak periods in the winter. As more utilities consider real-time pricing of energy based on actual cost, all forms of energy storage will provide more value and contribute to lowering the overall peak demand. This concept is not limited to small applications. In Europe, a very large thermal storage system (up to 10,000MWh) is being proposed.
7. HYDROGEN-BASED FUEL CELLS
The development of hydrogen-based fuel cells as clean energy sources continues around the world. In the transportation arena, PHEVs appear to be developing a commanding lead over fuel cell-powered vehicles as the clean energy choice. Proponents of a hydrogen economy argue that large wind farms could be used to power hydrogen-processing facilities and that pipelines—in lieu of large electrical transmission lines—could carry bulk hydrogen—as the energy source—to major population centers. Like today’s large natural gas pipeline networks that store gas conveniently in the system to match customer demand, hydrogen would be stored as necessary to match the demand for fuel cells for electricity and hydrogen- powered cars. Critics question the overall effi ciencies of creating large quantities of hydrogen to power fuel cells to create electricity. Large-scale adoption of hydrogen would require a signifi cant paradigm shift in the overall energy delivery strategy in major world markets.
8. CONCLUSIONS
AEP views energy storage as a strategic investment for its future grid. It has recognized that, in the long run, storage has the unique ability to act as a buffer between the grid and generation that is either intermittent or not controlled by the utility
Besides its strategic value, electricity storage offers many more tangible values that, if added up, would exceed the cost of deployment. Although electricity storage technologies have changed substantially over the past decade, making them economically feasible remains the greatest challenge for utilities. As countries around the world continue to increase their renewable energy portfolio—namely, wind power— the participation of storage in the success formula needs attention.
The future of electric grids will be impacted by a growing penetration of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs), which will represent a new dimension for grid management; vast amounts of energy storage will be present in the grid in the form of millions of electric cars. Gigawatts to kilowatts, electricity storage devices will change the grid dramatically.
8. REFERENCES
[1] Ali Nourai,Chris Schafer “changing the electricity game,” Elect. Perspect, vol. 33, no. 5, pp. 30–47, Sept./Oct. 2009.
[2] C.Vartanian, “The coming convergence, renewables, smart grid and storage,” IEEE Transaction on Energy 2030, Nov.2008.
[3] Bradford Roberts, “capturing grid power” IEEE power and energy magazine, july/august 2009.
[4] A. Nourai, V. I. Kogan, and C. M. Schafer, “Load leveling reduces T&D line losses,” IEEE Transaction on Power Delivery., vol. 23, no. 4, pp. 2168–2173, Dec.2008.
[5] A. Nourai, “Installation of the first distributed energy storage system at AEP,” Sandia National Laboratories,Albuquerque, NM and Livermore, CA, Sandia Rep. SAND2007-3580.
