Adding value to the grid

The energy storage technologies that help make our electricity infrastructure more intelligent. By Brad Roberts

Modernizing electricity grids around the world to accept large amounts of renewable energy resources plus adding more intelligence to make grids “smarter” is fairly universally accepted as a necessity 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 by system designers, grid operations and regulators. The amount of storage in a grid and its value is a subject of debate. Understanding the leading storage technologies and how they can affect grid operations is an important first step in this assessment.

In April 2003, the Department of Energy convened a meeting of 65 senior executives representing the electric utility industry, equipment manufacturers, information technology providers, federal and state government agencies, interest groups, universities and national laboratories to discuss the future of the North American electrical system. The goal of the meeting was to establish 'Grid 2030', a national vision for electricity's second 100 years.

From that meeting energy storage emerged as one of the top five concerns for the future grid. Since that meeting more attention has been given to storage in the grid at all levels, from large-scale bulk-storage systems to small units at or near the point of load. Other nations are ahead of the US with regard to bulk storage, as the value to grid operations was recognized sooner. The future of electric grids will be impacted by growing penetration of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs), which will represent a new dimension for grid management with vast amounts of energy storage present in the grid in the form of millions of electric cars. From gigawatts (GWs) to kilowatts (kWs), electricity storage devices will change

the grid dramatically.

Spectrum of electricity storage

Nearly every person in industrialized countries depends on some form of energy storage every day. Every electronic device, from cell phones to laptop computers, depends on battery power to function properly. The evolution of these storage energy devices continues to grow as newer applications are introduced.

One application having a great impact on potential utility grid applications is electric cars. The technologies that have worked in electronic devices are being scaled up for higher power use in cars and the electric grid. Figure 1 is a storage technology chart published by the Electricity Storage Association (ESA), which shows various technologies in terms of total power (kW) and energy capacity (time).

Power applications such as uninterruptible power supply (UPS) backup for data centers and automotive starting batteries represent the largest market for lead acid batteries, whereas laptop batteries and power tools have fueled incredible growth for lithium-ion. For bulk energy storage in utility grids, pumped hydropower plants dominate with approximately 100 GWs in service around the globe.

In general terms, power applications would be storage systems rated for one hour or less and energy applications would be for longer periods. Each of these technologies is finding applications in the electric grid. The location in the grid will vary from the transmission system for bulk storage systems to the residential feeder circuit for smaller systems based on the economics of each technology.

Types of storage

Pumped hydro

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 1000 MWh (deliverable in a system with an elevation change of 300 m) requires a water volume of about 1.4 million cubic meters.

Compressed air

The basic concept for compressed air energy storage (CAES) is a peaking gas turbine power plant that consumes less than 40 percent of the gas used in a combined cycle gas turbine and 60 percent less than 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 plants output can produce electricity during peak periods at lower costs than conventional stand-alone gas turbines

Battery

Advancements in battery technology over the last 20 years have been driven primarily by their use in consumer electronics and power tools. Only in the last 10 years have efforts to design better batteries for transportation resulted in possible uses for power grid application. One driver that has helped make potential utility applications possible is more efficient cost-effective power electronics. 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 make a wide range of battery technologies practical for grid support applications.

Flywheel

Spinning a weighted mass on the end of the shaft of an electrical motor or generator to provide 'ride-through' energy during short input power sags or outages has been around for decades. Slow speed (up to 8000 RPMs) steel flywheels have been used as 'battery substitutes' in the uninterruptible power supply (UPS) market for many years. These devices are practical for ride-through times up to 30 seconds. Achieving longer storage times at high power levels requires significant changes to the flywheel design and choice of materials.

Electrochemical capacitors

Commonly called 'supercapacitors', these electro-chemical capacitors look and perform in a similar way 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 conventional electrolytic capacitors.

New battery technology

The interest in energy storage for greater use in transportation and renewable energy research activities are increasing in private industry, universities and national laboratories Congress mandated increased funding for R&D in energy storage. Major universities like the Massachusetts Institute of Technology (MIT) have work underway to design new storage technologies. MIT is investigating ways to create very large-scale batteries capable of storing enormous amounts of power in the utility grid.

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. Utility grids have been traditionally designed to deal with the highest load peaks that occur typically less than a few hours per day for only a few days per year. Any storage device that helps meet this objective should be considered in utility system planning just like batteries and peaking generators.

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 off-peak periods and use that energy to store heat in a ceramic heatsink to be dispatched during higher peak periods during 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 lower the overall peak demand.

Hydrogen

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 power vehicles as the clean energy choice. Hydrogen economy proponents argue that large windfarms could be used to power hydrogen processing facilities and pipelines could carry bulk hydrogen to major population centers as the energy source in lieu of large electrical transmission lines. 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 of fuel cells for electricity and hydrogen powered cars.

Conclusions

Education about the value of energy storage in operating electric power grids has been lacking for a long time. As revealed in the 2003 conference on establishing a vision for the future smart electric grid, storage was identified as playing a vital role in managing new and more complex networks. Since that time more attention is being given to benefits storage can provide. The infrastructure stimulus bill passed by the US Congress provided increases in funding for storage in the electric grid and significant monies to advance storage devices for PHEVs.

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. Like wind power, storage can benefit from financial stimulus to support the growth and demonstrate the value in actual performance. The US, Japan and Germany currently benefit from having fairly large amounts of storage (pumped hydro) in their grids.

Recognizing the value of storage in dealing with variability of renewable resources is essential to harnessing the maximum potential of wind and solar power. Fortunately, storage systems used in grid applications will benefit from the huge investment in electric-based transportation. In fact, growth of electric vehicles to 50 million units (45 kW capacity average) by 2030 would dwarf the installed capacity of major renewable energy sources. The real technology challenge will be making all of the new electric power resources function in a fully integrated "smart grid".

Bradford Roberts is the Power Quality Systems Director for S&C Electric Company and Executive Director of the Electricity Storage Association. He is a member of the US Department of Energy Electricity Advisory Committee and Chairman of that group's subcommittee on energy storage.

FIGURE

Figure 1 - Electricity storage by technology

Battery power

Comparing battery storage

Sodium sulfur

The sodium sulfur (NaS) battery is a high-temperature battery system consisting of a liquid (molten) sulfur positive electrode and molten sodium as the negative electrode separated by a solid beta alumina ceramic electrolyte The electrolyte allows only positive sodium ions to pass through it and combine with the sulfur to form sodium polysulfides.

Flow battery technology

Flow batteries are class of batteries that perform similarly to a hydrogen fuel cell employs 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. The advantage of flow battery designs is their ability to scale systems independently in terms of power and energy.

Lithium-ion

The battery technology with the broadest base of applications today is lithium-ion. 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. This packaging flexibility is accompanied by light weight relative to aqueous battery technologies such as lead acid.

Lead acid

Lead-acid 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 from automotive starting to uninterruptible power supply use, lead acid is the lowest cost of all technologies. Lead acid battery plants are still used for back-up power sources in large power plants as 'black start' sources in case of emergencies. Their long life and lower costs are ideal for applications with low duty cycles.

Advanced lead acid

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 sulfation in the negative plate that increase as they are cycled more. Adding as much as 40 percent of activated carbon to the negative electrode composition increases the battery's life.

Nickel cadmium

Nickel-cadmium (Ni-Cad) batteries represented a substantial increase in power in middle of the last century. Ni-Cad batteries quickly gained a reputation as a rugged durable stored energy source with good cycling capability and a broad discharge range.