Handbook of Energy Storage for Transmission or ... - W2agz.com
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Handbook of Energy Storage for Transmission or ... - W2agz.com

Handbook of Energy Storage for Transmission or ... - W2agz.com Handbook of Energy Storage for Transmission or ... - W2agz.com

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EPRI Proprietary Licensed Material • For porous rock formations, the storage zone must be sufficiently porous to provide the required storage volume at the desired pressure and sufficiently permeable to permit the desired airflow rates. In addition, the over-burden and adjacent geological formations must have sufficient structural integrity to contain the air vertically and laterally; that is, the storage zone must be overlain by an impermeable rock layer to prevent the air from leaving the storage zone and escaping to the surface. All of these types of characteristics are the same as those used for over 80 years in the aquifer-based natural-gas storage industry. • Porous rock formations need to possess a mineralogy that does not result in rapid chemical consumption of the oxygen in the stored air through oxidation reactions. Geologic opportunities for CAES plants in the U.S. are shown in Figure 8, which indicates that over 80% of the U.S. territory has geological formations suitable for the underground storage. Figure 8 Geologic Opportunities For CAES Plant Sites In The U.S. Deep underground caverns may be operated with or without hydraulic compensation. With hydraulic compensation, water at the bottom of the storage cavern is connected to a surface reservoir. Thus, the storage pressure is always at or near the hydrostatic pressure of the water column to the surface. For caverns operated without hydraulic compensation (e.g., salt caverns), the air pressure varies between the two design pressure levels associated with the CAES plant. In addition, it is generally better to operate the surface turbomachinery at a constant pressure that is slightly lower than the lowest pressure in the cavern. The Hybrid Plant Concept As conceived by Dr. Michael Nakhamkin in 1998 under EPRI sponsorship, a hybrid CAES plant can be operated in a variety of modes [12]. The concept allows the plant to operate continuously Compressed Air Energy Storage (CAES) Page 9

EPRI Proprietary Licensed Material as a base-load combustion turbine and, when necessary, to operate at increased power during peak hours to supply intermediate/maximum peak power as needed. This plant concept is particularly well suited for distributed power generation applications. At present, the only hybrid plant configuration developed is based on the Rolls Royce Allison Company’s KM7 combustion turbine [13]. As such, the following is a brief description of the major operating modes of the hybrid concept sized using the KM7 turbine: • Base load operation – The plant is operated as a conventional combustion turbine with 100% of the expander flow provided by the compressor. The turbine supplies a net power output of 4.8 MW e at 11,700 Btu/kWh. • Intermediate peak load operation -- The expander flow and power are increased because the expander receives compressed air flow from the storage reservoir in addition to the full airflow from the main compressor. It is estimated that, when 20% additional airflow comes from the storage reservoir, the net output power will be 7.9 MW e for 3 hours at 8,300 Btu/kWh. • Maximum peak load operation -- The compressor is disengaged, and the full flow of the compressed air from the storage reservoir goes into the expander. The power output is approximately 16 MW e at a heat rate of approximately 4,000 Btu/kWh. • Storage-charging mode of operation --The off-peak power feeds both the motor-driven main compressor and the separate motor-driven boost compressor. For the KM7, 12.2 MW e of off-peak power is required to drive the compressors. • Self-charging operation -- 78% of the main compressor’s flow is sent to the expander to generate electric power to drive the booster compressor to fill the storage reservoir. This requires about 3.5 hours of charging time, with no power going to or from the grid. • Synchronous condenser mode of operation – By opening the clutch between the compressor and the motor/generator, and between the expander and the motor/generator, the motor/generator is synchronized to the grid and is operated as a synchronous condenser, providing power factor correction. In this mode, the motor/generator works to stabilize line voltage and frequency, ease grid power transitions, and provides reactive power to assist in providing high quality electrical power to the grid. Key Features and Limitations The key features of compressed air energy storage offer several advantages over alternative energy storage technologies. • The CAES plant is the only technology that can provide significant energy storage (in the thousands of MWhs) at relatively low costs (approximately $400 to $500/kW e ). The plant has practically unlimited flexibility for providing significant load management at the utility or regional levels. • Commercial turboexpander units range in size from 10 MW e (Rolls Royce-Allison) to 135 MW e (Dresser-Rand) to 300-400 MW e (Alstom). Compressed Air Energy Storage (CAES) Page 10

EPRI Proprietary Licensed Material<br />

• F<strong>or</strong> p<strong>or</strong>ous rock <strong>f<strong>or</strong></strong>mations, the st<strong>or</strong>age zone must be sufficiently p<strong>or</strong>ous to provide the<br />

required st<strong>or</strong>age volume at the desired pressure and sufficiently permeable to permit the<br />

desired airflow rates. In addition, the over-burden and adjacent geological <strong>f<strong>or</strong></strong>mations must<br />

have sufficient structural integrity to contain the air vertically and laterally; that is, the<br />

st<strong>or</strong>age zone must be overlain by an impermeable rock layer to prevent the air from leaving<br />

the st<strong>or</strong>age zone and escaping to the surface. All <strong>of</strong> these types <strong>of</strong> characteristics are the<br />

same as those used <strong>f<strong>or</strong></strong> over 80 years in the aquifer-based natural-gas st<strong>or</strong>age industry.<br />

• P<strong>or</strong>ous rock <strong>f<strong>or</strong></strong>mations need to possess a mineralogy that does not result in rapid chemical<br />

consumption <strong>of</strong> the oxygen in the st<strong>or</strong>ed air through oxidation reactions.<br />

Geologic opp<strong>or</strong>tunities <strong>f<strong>or</strong></strong> CAES plants in the U.S. are shown in Figure 8, which indicates that<br />

over 80% <strong>of</strong> the U.S. territ<strong>or</strong>y has geological <strong>f<strong>or</strong></strong>mations suitable <strong>f<strong>or</strong></strong> the underground st<strong>or</strong>age.<br />

Figure 8<br />

Geologic Opp<strong>or</strong>tunities F<strong>or</strong> CAES Plant Sites In The U.S.<br />

Deep underground caverns may be operated with <strong>or</strong> without hydraulic <strong>com</strong>pensation. With<br />

hydraulic <strong>com</strong>pensation, water at the bottom <strong>of</strong> the st<strong>or</strong>age cavern is connected to a surface<br />

reservoir. Thus, the st<strong>or</strong>age pressure is always at <strong>or</strong> near the hydrostatic pressure <strong>of</strong> the water<br />

column to the surface.<br />

F<strong>or</strong> caverns operated without hydraulic <strong>com</strong>pensation (e.g., salt caverns), the air pressure varies<br />

between the two design pressure levels associated with the CAES plant. In addition, it is<br />

generally better to operate the surface turbomachinery at a constant pressure that is slightly lower<br />

than the lowest pressure in the cavern.<br />

The Hybrid Plant Concept<br />

As conceived by Dr. Michael Nakhamkin in 1998 under EPRI spons<strong>or</strong>ship, a hybrid CAES plant<br />

can be operated in a variety <strong>of</strong> modes [12]. The concept allows the plant to operate continuously<br />

Compressed Air <strong>Energy</strong> <strong>St<strong>or</strong>age</strong> (CAES) Page 9

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