By Sam Shepard,
CAES Development Co.,
and Septimus van der Linden,
ALSTOM Power Inc.
The electric power industry is on the verge of a new era where many of the rules will be different, but the underlying requirements will be the same: reasonably priced power available on demand and produced in a cost-efficient and environmentally responsible manner. Compressed air energy storage (CAES) technology can play an important role in meeting market demand for electricity, addressing the need for intermediate and peaking power generation, where natural gas-fired turbines are most suitable. CAES technology uses low-cost, off-peak power to pressurize and store air that can later be expanded through a gas turbine during on-peak periods. While many industry participants are not very familiar with this technology, CAES power generation plants are based on proven technology for both compressed air storage and power generation.
Although several niche CAES facilities are operational, large-scale commercial success has not yet been achieved. CAES Development Company (CDC) intends to change this situation with the facility it is developing in Norton, Ohio. CDC develops CAES power-generating plants. CDC has worked with Alstom Power to modify existing proven technology to combine the best features of natural gas and coal-fired power generation to yield cost-competitive, efficiently produced electricity. CDC is owned by its management team and by private equity funds managed by Haddington Ventures L.L.C. Haddington Ventures’ limited partners include affiliates of J.P. Morgan Partners (a J. P. Morgan Chase & Co. affiliate), Vectren Ventures (an affiliate of Indiana Gas and SIGECO), Millenium Energy Holdings (a Tucson Electric affiliate), Travelers Insurance Company, and Prudential Insurance Company.
CDC’s initial project in Ohio is well-advanced in the permitting of the 2,700 MW facility. When completed, Norton Energy Storage (NES) will be the largest CAES facility in the world. The combination of air compression, power generation and subsurface technologies makes this a world-class project. NES will use low-cost, off-peak electricity from underutilized baseload power plants to compress air and inject it into an underground storage reservoir at night. During the day, when electricity demand increases, NES will withdraw the compressed air and combine it with natural gas to fuel up to nine 300 MW intermediate load turbines.
The NES facility will convert an idle man-made limestone mine brown-field site into a productive, tax-paying, job-creating industrial zone in an environmentally responsible manner. The plant will be built in modules over five years of continuous construction. It will use nine Alstom Power Inc. 300 MW turbines. Pending timely regulatory approvals, startup of the first unit is set for May 2003. When completed, NES will provide at least 2,700 MW of power to Ohio and the East Central Area Reliability region (ECAR), or enough electricity to serve nearly 675,000 homes. In addition, the flexibility of CAES technology can provide full electric ancillary services without interruption during both peak and off-peak periods. To ensure that the technology is sound, and to ensure the project’s success, CDC employs leading consultants in mine geology (Sandia National Laboratories and Hydrodynamics Spring) and facility engineering (Sargent & Lundy and CSTI).
CAES vs. CC
While the CAES technology uses many of the same components as in traditional gas turbine (GT) and combined-cycle (CC) technologies, the manner in which the components are configured in the CAES technology allows for more optimal usage. A key distinction between CAES and more traditional GT and CC technologies is in the compression stage. In a CAES configuration, the compression and generation units are completely separated. In the GT and CC applications, both compression and generation are on a single shaft and must work in unison.
Both the CC and CAES technologies strive for thermodynamic efficiency. On a component basis, the CC technology employs a heat recovery steam generator (HRSG) while the CAES technology employs an air-to-air heat exchanger. The GT uses no heat recovery technology outside the gas turbine. A simple way to compare the technologies is to describe the components and outputs when a common base turbine unit, in this case an Alstom 11NM, is used in each. The net generating outputs of the three technologies under ISO (59 F) ambient atmospheric conditions are:
- CAES – 300 MW
- CC – 130 MW
- GT – 87 MW.
In general, a conventional gas turbine works by mixing a stream of air with fuel and igniting the mixture, then flowing this mixture through a low-pressure expander. A gas turbine’s ability to efficiently extract energy from this stream is a function of the mass flow and the post combustion temperature. An air compressor mounted on the same shaft as the low-pressure expander provides the necessary mass flow. The net generation is the power developed by the low-pressure expander less the power required to run the compressor. In the simple-cycle configuration a substantial amount of heat is exhausted to the atmosphere due to the limits imposed by the laws of thermodynamics.
In the combined-cycle configuration the exhaust heat is passed through a heat recovery steam generator where it is converted to steam. A portion of the energy in the steam is then captured by a steam turbine. The net generation from a combined cycle is the power developed by the low-pressure expander plus the power developed by the steam turbine less the power used by the compressor.
In contrast, in the CAES machine, a separate compressor train operated during off-peak hours stores high-pressure (1,500 psi) compressed air in a limestone cavern. During on-peak periods, the compressed air is released from the cavern and heated by exhaust heat from the low-pressure expander. Some of the energy from this high-pressure stream of heated air is captured in a high-pressure expander. The resulting air stream is then mixed with fuel and the mixture ignited and flowed through the same low-pressure expander that is part of a traditional gas turbine. A substantial portion of the heat remaining in the exhaust stream is then captured in the heat exchanger to heat the high-pressure air from the cavern as described above. The net power output of the CAES machine is the sum of the power produced by the low-pressure and high-pressure expanders. All of the power required to run the compressor is consumed off-peak. A substantial economic advantage is created by using low cost off-peak electricity to run the compressor, thus allowing all of the power developed by the gas turbine to generate electricity during the valuable peak hours.
Norton Energy Storage site rendering.
The use of a control valve to maintain a constant mass density of the air exiting the cavern provides an additional economic advantage for CAES. The high and low pressure expanders will always see the same air conditions regardless of the ambient conditions, creating a machine that suffers no output degradation as ambient conditions change. This increases the gas turbine’s design output substantially. Alstom has developed a unique approach to create the generating portion of a CAES machine, further reducing costs and increasing operational flexibility. Because compression is external to the turbine, Alstom removed the compressor from the 11NM model, and integrated the air turbine and gas turbine in the same body on a single shaft. Additional economic efficiencies are developed by using highly efficient large industrial air compressors with intercoolers and aftercoolers to minimize power consumption.
CAES technology has been implemented on a smaller scale in two prior projects, one in Germany and the other in Alabama. These projects were designed to fit a very specific market niche. The Huntorf German venture (290 MW) was initially developed to serve as black-start capability for nuclear units near the North Sea, while the McIntosh Alabama project (110 MW) was partially funded by EPRI to develop the technology as a demonstration project. Utilities view neither project as more than a small niche application.
A major difference between the NES plant and the prior CAES plants is the storage cavern. Both Huntorf and McIntosh use solution-mined salt caverns created specifically for these plants. The NES plant uses an existing very large deep limestone mine as an air storage vessel. This mine was created with only two penetrations in the form of very regular vertical elevator shafts used for the transport of personnel and material as well as for ventilation. These shafts will be sealed with impermeable plug materials.
For the CAES cycle, air will be injected and withdrawn from the cavern using air wells created using proven gas well drilling technology. The limestone in the cavern is massive and homogeneous, with few discontinuities that would affect air storage performance. The limestone and the shale cap rock overlying the limestone have extremely low permeability to air. CDC has performed extensive testing to ensure that air migration from the cavern into the surrounding host rock will be extremely low and represents no threat to the surface or the area water supply or to the project economics in terms of loss of air.
Efficiency and Emissions
One of the major challenges of the power business today is to generate electricity using fossil fuels while minimizing the impact of air emissions on the environment. For simple cycle machines, low- NOx burners are the primary emissions control technology. For CC technology, the HRSG enables the use of a selective catalytic reduction (SCR) module that removes most of the NOx from the air stream. CAES technology also permits use of an SCR module, but in this case it is integrated into the air-to-air heat exchanger design, permitting close control of the catalyst temperature. The NES facility is permitted at 3.5 ppm NOx.
The CAES machine also uses much less fuel than either a GT or CC, which results in air emissions per kWh substantially lower than those of a GT or CC. A typical heat rate for the CAES power generation cycle is in the range of 4,000 Btu/kWh (LHV), which is substantially lower than that of even the best combined cycle.
The features of the CAES cycle mentioned above create a physical machine well suited to the demands of today’s changing power business. The CAES cycle turns low-cost energy from underutilized baseload plants into valuable on-peak energy utilizing the least natural gas of any power generating technology. This mix of energy costs allows a CAES plant to operate during the daily peak periods for virtually every day of the year. The lack of a steam cycle minimizes complexity and creates greater flexibility for daily cycling duty compared to a CC or fired steam boiler. The heat recovery aspect provides far better economic performance than a GT.
Because the NES plant will rely on baseload plants for off-peak power – potentially increasing their capacity factors – retrofitting emissions control technology on coal-fired plants in the Midwest becomes more viable. Finally, the combination of generation on the daily peaks and compression during the off-peak period creates ancillary services availability in virtually all hours of the year. CAES is a good example of where technology neglected during cost of service regulation can be encouraged by market forces to increase the efficient use of existing plants while minimizing the capital necessary for additional peaking capacity.
Sam Shepard is chief operating officer of CAES Development Company. He has held a variety of engineering and management positions with Southern Energy International, Southern Company Services, Georgia Power and Amoco Power Resources Corp. Shepard holds a bachelor’s degree in electrical engineering from Mississippi State University, an MBA from the University of Alabama, and is a registered professional engineer.
Septimus van der Linden is vice president of advance marketing with ALSTOM Power Inc., engaged in business development in new market opportunities. He has more than 40 years’ experience in the power generation industry, including 30 years’ experience in gas turbine technology and applications.