Energy Storage Breakthroughs: 

An Evolving Technology for Managing the Grid   (Overview)

by Michael W. Howard and Haresh Kamath,  EPRI ,   From the earliest days of the electric power system, energy storage has been considered an important technology for managing the grid. Today, the changing ways in which electric power is generated and used are making storage even more attractive than before. The appear­ance of newer, more cost-effective technology options is making it likely that energy storage will finally become a reality in the near future.

The electric grid operates as an enormous just-in-time production and delivery system, with power generated at the same time it is consumed, and with little storage of electrical energy. This means that the transmis­sion and distribution system must be built to accommodate maximum power flow rather than average power flow, resulting in underutilization of assets. Energy storage can improve asset utilization, enhance the network reliability, enable more efficient use of baseload generation, and support a higher penetration of intermittent renewable generation.

Energy storage exists in many electrical power systems. In the United States, about 2.5 percent of electricity that passes through the network has been stored. Pumped hydro facilities, the form of large-scale storage most familiar to utilities, repre­sent most of this storage. Pumped hydro allows the storage of enormous quantities of energy, though it requires a huge initial investment. Compressed air energy storage (CAES) is a less-widely implemented technology that uses off-peak renewable electricity to compress and store air, which can later be used to regenerate the electricity. Such techniques could be used to store renewable energy for convenient dispatch at later times.  There are also various methods of storing energy from solar thermal power plants.

Short-duration storage technologies such as ultracapacitors and flywheels have uses in other applications, such as those in which power and energy requirements are not large but when the storage is expected to see a great deal of cycling. Such technolo­gies can be used to address power-quality disturbances and frequency regulation, applications in which only a few kilowatts to megawatts are required for a few seconds or minutes.

A great deal of effort has gone into the development of electrochemical batteries. Utilities are familiar with lead-acid batteries which are extensively used for backup power in substa­tions and power plants. In larger-scale applications, however, other battery chemistries such as sodium sulfur and vanadium redox flow batteries are more effective. Extensive research and development investments in these technologies have begun to pay off, and several recent installations are demonstrating the viability of battery energy storage in applications such as peak shaving for transmission and distribution asset deferral.

Lithium-ion battery technology has helped enable the portable electronics revolution and promises to do the same in the transportation market by enabling plug-in hybrid electric vehicles (PHEV). The widespread adoption of PHEVs would have significant effects on the utility industry. If 2 percent of vehicles in the United States were plug-in hybrids by 2020, this would mean 2 million PHEVs would exist on the grid. This represents a charg­ing load approaching 40 gigawatts. Assuming such charging would be done primarily at night, PHEVs would reduce the diurnal fluctuation in electrical load. In some concepts, PHEVs might also be used as distributed energy storage systems, discharging energy back to the grid when necessary. Such a development could potentially give utilities access to gigawatts of power on demand.

What might a utility system based on such technologies look like? Large-scale technologies such as pumped hydro and CAES may be used to store large amounts of energy generated from wind farms and other renewable sources, allowing the energy to be dispatched when it is needed. Flywheels might be used to provide minute-to-minute frequency regulation while large-flow batteries provide more large-scale ramping over several hours. Strategically placed sodium-sulfur batter­ies could ease bottlenecks in the distribution system through peak shaving, while reducing demand charges to customers. Ultracapacitors placed at substations could mitigate the effects of momentary interruptions on distribution feeders. The massive aggregation of PHEV batteries could absorb energy produced by nuclear baseload plants during the night and could be used to provide spinning reserve or critical backup power to industrial customers during the day.

The real challenge for energy storage is not whether it is possible, but how it will be used. There is no question that storage represents an opportunity; it will take strategy and understanding of this opportunity to make sure it is exploited to its full potential.

Michael W. Howard is vice president of research at the Electric Power Research Institute and Haresh Kamath is project manager at EPRI.©2006-2007

Storing Green Energy,  
September 12, 2008

By making a $20 million investment in energy storage, Public Service Enterprise Group hopes to breathe new life into green energy. The New Jersey-based utility has entered into a joint venture with a key inventor of the technology. It will market the compressed air energy storage tools needed to generate power when demand is highest

Energy storage could advance the cause of wind and solar power, which are often criticized as they are intermittent fuels. That would give utilities, power marketers and large commercial or industrial customers the flexibility of how they respond to shortages, price spikes or brownouts. Utilities, for instance, must precisely measure their load generation with the demands of their end users. Without adequate generation capacity, all wholesale buyers of electricity would be subject to the whims of the market.

"We believe this technology is an important component of a broad effort to combat climate change, an effort that must include increased conservation, expanded renewable energy and new clean central power," says Stephen Byrd, president and CEO of PSEG Energy Holdings.

Two major compressed air energy storage plants exist worldwide: an Alabama plant, which was built in 1991 and which is 110 megawatts and a German facility, which was constructed in the late 1970s and which is 290 megawatts. Over the next 5-10 years, experts say that the technology has the potential to get a firm foothold in the market.

Palo Alto, Calif.-based EPRI has estimated that more than 75 percent of the U.S. has geological characteristics that will accommodate underground compressed air energy storage. Those reservoirs will allow utilities to use off-peak electricity to compress air and store it in airtight underground caverns. In urban areas where space is at a premium or where bedrock makes deep drilling cost prohibitive, developers say that ground level pipelines can be used instead.

When the air is released from storage, it is heated in combustors by natural gas and sent through expanders to power a generator to create electricity. Nearly two-thirds of the natural gas in a conventional power plant is consumed by the natural gas turbine because the gas is used to drive the machine's compressor. In contrast, a compressed-air storage plant uses low-cost heated compressed air to produce off-peak electricity, conserving some natural gas.

In the case of PSEG, it has formed a joint venture with Michael Nakhamkin, who created the compressed air site in Alabama that can fill a cavern with enough energy to generate 110 megawatts for 26 hours. The venture, called Energy Storage & Power, would try to extend those benefits to other power providers by using standardized parts.

"The technology has evolved to the point where it can be critical to helping this nation meet its growing energy needs while helping decrease carbon emissions from the electricity sector," says Nakhamkin.

Strong Fundamentals

To be sure, compressed air energy storage has its pitfalls. The disadvantage is that energy is lost when it is "pumped" into the cavern and then re-extracted as compressed air. Some estimates say that it could be as high as 80 percent. That, in effect, means that the selling price must accommodate that shortcoming, which may drive up rates for consumers.

Also, building storage can be pricey, which might make some prospective projects infeasible. But with gas prices estimated to be as high as $14 per million BTU, an investment in underground storage could pay for itself over time.

Costs could also be defrayed if compressed air energy storage replaced expensive "peaking" units that provide power during the hottest summer days or the coldest winter nights. Air is stored in the form of compressed air energy during off peak hours and then released during the periods of highest demand, which will also lower the prices that consumers pay for power. At the same time, compressed air energy storage units can reduce the stress on base load plants that would otherwise have to ramp up and down.

The Iowa Association of Municipal Utilities believes in the cause. It has selected a reservoir near Des Moines to build a 300-megawatt plant that is comprised of a 100 megawatt wind farm and a 200 megawatt compressed air energy storage facility. Most distributors will continue to use coal-fired steam generating power plants to serve their "base-load" needs but will use energy from the new facility to meet intermediate needs. It is anticipated this electricity will be available to utilities and their customers in 2011.

"We see strong market potential for compressed air energy storage in the traditional power industry as well as for the growing renewable energy industry," says Roy Daniel, chief executive of the PSEG-led venture, Energy Storage & Power. "Energy storage is the missing piece of the puzzle for a green, affordable and reliable electric grid for the 21st century."

If the technology becomes cost effective and wins wider market acceptance, it would cause the energy paradigm to shift. By storing wind and solar energy and discharging the air when it is needed, suppliers could reduce their need for fossil fuels and give utilities a powerful tool in their quest to meet their expected future energy needs.