University of Colorado at Boulder

Introduction to Energy Storage

The issue of providing energy for our civilization far exceeds the challenge of simply integrating renewable energy sources into the existing utility system. While the challenges ahead include reducing greenhouse gas emissions and pollution, the end of the bountiful supply of fossil fuel energy we now enjoy is drawing closer. Decisive, calculated action to shift how energy is currently harvested and used must be taken to secure mankindā€™s energy future. Renewable energy technologies such as wind and solar generation are near-term options for producing large amounts of inexhaustible, non-polluting energy. At the other end of the spectrum, demand-side management must be comprehensively adopted to limit the amount of energy we use. In the middle of the spectrum, energy efficiency measures, transmission and distribution system updates, distributed energy generation and conditioning, and energy storage provisions are needed.

The enormous amount of effort required to revitalize the energy industry for the future brings with it a valuable opportunity for states and countries to develop useful technologies, build jobs, grow the economy, and provide a more stable and secure energy resource to its people. Political unrest, trade deficits, and negative effects of climate change punctuate the need for this sea change in the energy industry. States or countries who take the initiative to invest in non-fossil fuel energy technology and development will not only set an example for others to follow, but will be positioned to benefit the most economically from continued renewable energy growth. In addition, the level of reliability and security of energy can be enhanced greatly by implementing diversified and distributed generation and energy storage methods.

In the near future, the solution to the energy crisis will involve using all available technologies together in the most beneficial manner. Conventional energy sources using fossil fuels will become more efficient and cleaner. Hybrid systems that use both renewable fuels and fossil fuels will emerge. Optimized transmission and distribution systems will evolve from the existing infrastructure. Solar and wind energy generation will continue to grow. System-wide energy efficiency measures will be taken. Energy use and demand will be optimized through time-of-use management and efficient technologies. Energy storage systems will emerge and evolve that enable renewable energy source deployment, and greatly reduce wasted energy inherent in the current system.

As new technologies and uses increase the value of energy, means of storing excess and waste energy will become increasingly important. Conventional coal or nuclear plants that cannot quickly change their power output end up dumping waste energy when the demand drops off. Transmission and distribution lines are oversized to account for short-term peak demand cycles. Excess energy produced by wind and solar generators is not useable without a means to store it.

Over the years, energy production has developed that strives to match the user demand in the most economical way possible. This has manifested itself in the construction of large coal, nuclear, and hydroelectric base generating plants coupled with fast-response, expensive, peaking gas turbine plants, and in some cases, energy storage plants. Where energy storage plants have been used (mainly pumped hydroelectric), the operators enjoy a flexible energy source that yields considerable revenue. Energy storage serves as a bridge between the limited, variable generation capability of energy sources and the highly variable, cyclical grid demand. Grid demand not only varies substantially minute-to-minute, but also hourly (nighttime vs. early evening), and seasonally (summer vs. winter). Energy storage can be implemented as a buffer to match the available generation to the variable user demand.

The recognized need for electrical energy storage is not new. People have devised many methods of storing energy over the years, however, the problem of storing large amounts of accessible energy in a cost effective and efficient manner has remained one of the most difficult science and engineering problems the world has known. Today, the advent of modern renewable energy sources greatly improves our ability to collect or harvest energy, but not to store what we gather. Modern renewable energy sources intensify the search for robust, cost effective means to store energy. Intermittent energy sources such as solar panels or wind turbines require energy storage capacity if they are to provide consistent, on-demand power to the user, and be able to replace traditional fossil fueled sources. In U.S. patent #1,247,520 titled "System of Storing Power"€¯ filed on June 7, 1907 by R. A. Fessenden and patented on Nov 20, 1917, Fessenden writes:

"...The invention herein described relates to the utilization of intermittent sources of power and more particularly to natural intermittent sources, such as solar radiation and wind power, and has for its object the efficient and practical storage of power so derived. It has long been recognized that mankind must, in the near future, be faced by a shortage of power unless some means were devised for storing power derived from the intermittent sources of nature. These sources are, however, intermittent and the problem of storing them in a practicable way, i.e. at a cost which should be less than that of direct generation from coal, has for many years engaged the attention of the most eminent engineers, among whom may be mentioned Edison, Lord Kelvin, Ayrton, Perry, and Brush..."€¯

Nearly one hundred years have passed and very little progress has been made toward achieving the goals set forth by Fessenden and others. The need to store the available energy from nature still exists, and is even more critical in today's world. Our continued dependence on fossil fuels causes pollution, health problems, climate change, and political unrest. While significant energy storage technology advances have been made in many areas, none have been successfully engineered to meet this storage challenge. No method of long term, high power energy storage has been shown to be cost-effective, efficient and flexible enough to inspire widespread use.

Many advances in electrical energy storage technology and methods have been made in recent times. These advances have come in the areas of batteries, large scale pumped hydroelectric storage plants, compressed air energy storage, flywheels, superconducting magnetic energy storage, and super-capacitors. Chemical energy storage, most commonly applied in batteries, is the world's most prolific form of energy storage. However, there are several drawbacks to batteries for large systems, including cost, short lifetime, and disposal concerns. The next most common form of energy storage is pumped hydroelectric. This method has been successfully applied to large utility scale projects in the 50 MW to 2 GW power range, though it is severely limited by geography. Compressed air energy storage (CAES) is an emerging option for storage, also finding its best application in large utility scale projects. Flywheels, superconducting magnetics, and super-capacitors are generally suitable for lower energy applications, although somewhat high power output can be attained when many devices are combined. These devices are generally quite expensive. No cost-effective and efficient energy storage method for large-scale needs has yet emerged from these advances in technology.