December 19, 2012 | 18
Over the next several weeks, we’ll be joined by Robert Fares, a graduate student at The University of Texas at Austin researching the benefits of grid energy storage as part of Pecan Street Inc.’s ongoing smart grid demonstration project. Robert will be contributing a series of guest posts discussing grid storage technologies, and how storage could benefit the electric grid.
It may come as a surprise that essentially no electric energy is stored between the point of generation and the point of delivery in today’s electric grid. Each kilowatt-hour you consume in your home is generated, transmitted, and delivered to your particular electrical outlet in real time. In other words, the present electric grid operates totally on demand. This seemingly inefficient operating paradigm stems from the high cost of electricity storage technologies; it is less expensive to oversize every component of the grid to serve peak energy demand than it is to store electric energy.
Adding energy storage to the electric grid would improve its reliability and permit the widespread utilization of intermittent renewable energy. However, the present cost of grid-scale storage technologies remains prohibitive. To overcome this barrier, the U.S. Department of Energy has set an ambitious long-term cost goal of $150/kWh for battery energy storage—about one third the estimated cost of existing commercial battery systems.
Despite the present prohibitive cost of storage, large-scale grid storage has been used in the United States since 1930, when the Connecticut Electric Light and Power Company built the first American pumped-hydroelectric storage facility. The Connecticut pumped-hydro plant used excess electricity to pump water from the Housatonic River into an enormous water reservoir 230 feet above. During peak electric demand hours, the stored water was fed downhill through a hydroelectric turbine to produce electricity.
Today, pumped-hydro dominates the U.S. energy storage landscape: 95% of U.S. grid storage capacity was pumped-hydroelectric in 2011. Nevertheless, energy storage is still just a tiny part of the grid system, making up just 2% of U.S. electric generation capacity. So, what’s standing in the way of further pumped-hydro development? Pumped-hydro requires a large elevation difference between suitable water reservoir sites, like the site of the Connecticut storage plant. Furthermore, a huge volume of water is required to store energy. These difficulties don’t make further development of pumped-hydro impossible—just impractical and expensive.
Consider the illustrative example of West Texas. With its enormous swath of wind farms, West Texas could benefit from some energy storage capacity. However, pumped-hydro is a not a feasible option in drought-prone and geographically flat West Texas. Incidentally, the state of Texas has invested nearly $7 billion in new transmission lines to pump West Texas wind energy to eastern cities in real time.
While pumped-hydro is not the solution to the electricity storage problem, there are a number of emerging technologies that could one day break our electricity-on-demand paradigm. I’ll describe three of the most promising technologies: compressed-air energy storage, flywheel energy storage, and battery energy storage.
Compressed-air energy storage is similar to pumped-hydro in many ways. Both technologies borrow components from traditional methods of generating electricity, and both technologies store energy in the potential energy of a working fluid. While pumped-hydro stores energy in the gravitational potential energy of water, compressed-air technology stores energy in the pressure potential energy of air. To store electricity, an air compressor pushes high-pressure air inside an underground cavern, such as a vacant salt dome, or a depleted gas well. Electricity is extracted from the high-pressure air by burning it with natural gas and allowing the hot combustion gases to expand inside an ordinary gas turbine generator. Compressed-air storage is less water-intensive than pumped-hydro, and only requires a suitable underground cavern to operate effectively. Because of its advantages over pumped-hydro, the U.S. Department of Energy estimates compressed-air provides cost savings of nearly 40% over the long-established pumped-hydro technology.
While compressed-air storage is less expensive and less water-intensive than pumped-hydro, it still requires an underground cavern to store electricity. Flywheel and battery energy storage, on the other hand, operate as a “box” for electricity. They could be installed almost anywhere on the electric grid.
Flywheel storage converts electric energy into the kinetic energy of a rotating mass. To store electricity, an electric motor spins up a massive flywheel. To discharge, the flywheel’s kinetic energy is used to spin an electric generator and output electric current. Flywheel storage devices require complex technology like magnetic bearings and a vacuum enclosure to operate most effectively. Grid-scale flywheel technology is still nascent, but has the potential to act as a buffer between intermittent renewable energy and the grid in the future.
Battery energy storage is unique in that it requires no moving parts, is relatively compact, and can be sized for diverse grid applications. Batteries utilize electrochemical reactions to convert electric energy into materials. A battery consists of two materials (a cathode and an anode) that “want” to react together due to their electrochemical potential difference. An electrolyte separates the materials inside the battery cell, so that reaction is blocked unless an external electric load connects the positive and negative sides of the battery.
One of most prominent battery technologies, lithium-ion, has grown commonplace from its widespread use in portable electronics. Lithium-ion is just one member of a diverse population of battery chemistries that could one day transform the grid’s operational nature. High-temperature, molten-sodium batteries have been commercialized in Japan by NGK Insulators for grid applications. A Texas company, Xtreme Power, is working to commercialize an advanced lead-acid battery for the grid. Other companies are working to develop proprietary chemistries, or unique liquid-metal designs to overcome the limits of existing technology. Still other companies are developing advanced “flow batteries,” which store energy in liquid chemical solutions.
The battery energy storage field is exciting, and batteries are the focus of the Department of Energy’s energy storage program. Over the coming weeks, I will write more about where storage is needed in the present grid, how storage can make the grid more resilient, and some of the barriers limiting adoption of grid energy storage. While there are a number of challenges that must be overcome before we can store electricity on a massive scale, storage has the capability to fundamentally transform the way we deliver electricity from the point of generation to the point of consumption. It will be exciting to witness how storage and other innovative energy technologies transform the aging grid over the coming years.
Robert Fares is Ph.D. student in the Department of Mechanical Engineering at The University of Texas at Austin. As part of Pecan Street Inc.’s ongoing smart grid demonstration project, Robert’s research looks at how energy storage models can be used with large-scale data and optimization for economic operational management of battery energy storage. Robert hopes to develop novel operational methods and business models that help to integrate distributed energy generation and energy storage technologies with restructured electricity markets and retail electric tariffs. Through his research, he hopes to demonstrate the marketability and technical compatibility of these new technologies.
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