Ins and Outs of Compressed Air Energy Storage
The growing interest in renewables such as wind and solar energy has created a challenge for conventional generators and grid operators who must balance changing loads with variable supplies. Wind farms typically generate more electricity at night when there already is a surplus of electricity. The ability to store this energy for daytime use (when it is most valuable) is an attractive possibility. Likewise, solar power from sunny, but sparsely populated regions could be sent through low-loss, high-voltage DC transmission lines, stored and converted to AC close to the densely populated areas where it will be consumed.
Concerns about distribution system limitations and supply imbalances also present an opportunity for facility operators who are prepared to help stabilize the grid by cutting their consumption or absorbing oversupplies. Methods and technologies range from demand response and in-house generation to pumped water storage for hydroelectricity and massive battery systems. A less known technology is compressed air energy storage (CAES), which offers the potential to store large amounts of power more cost-effectively than batteries and without pumped hydro’s geographical limitations.
“With the increasing use of renewables around the world and the intermittency of their delivery – the sun shines during the day, the wind blows at night – utilities may even pay you to take that power,” says Jim Heid, senior vice president and a member of the executive leadership staff for Dresser-Rand, which engineers and installs CAES systems. “Japan has decreed that every new kW of capacity will be accompanied by a kW of storage.”
CAES is not a pure energy-storage technology like batteries: it requires additional energy in the form of heat, typically supplied by fossil-fuel burners, to achieve its high efficiency. And to be cost-effective, the energy to be stored must be low-cost or free, like excess solar, wind or hydroelectric power. But under the right conditions, CAES technology gives utility operators the means to operate their base load plants more efficiently and provides a solution for balancing the grid.
In industrial facilities, CAES can be used to reduce peak loads, provide back-up power and create an additional revenue stream through demand management. “CAES can strengthen a weak grid at a remote location by handling peak loads, provide reliability, and allow you to bid into a demand reduction,” says Heid. And it enables green technologies such as solar cells and wind turbines to be matched with daily and weekly demand requirements for electricity.
A CAES Primer
In a CAES plant (see figure), available off-peak electricity is used to power a motor that drives compressors to force air into tanks or into an underground storage reservoir at high pressures. This process (called charging) usually occurs at night, and during weekends when utility system demands and electricity costs are low.
“It’s a load when you want it – a controllable load at 65% to 115% of design when compressing, and from 10% to 100% when generating,” says George Lucas, PE, senior engineering consultant, Dresser-Rand. “It allows owners to participate in the grid services market, for example, reserve capacity.”
During intermediate electrical demand periods, the air is released from the reservoir, and without further compression is heated and expanded through gas- or fuel oil-fired combustion turbines to drive a generator to produce electrical power.
Compressed air may be stored in above-ground tanks or in below-ground reservoirs created by solution mining bedded or domed salt formations, conventionally mining solid rock, or in aquifers and depleted natural gas fields. These formations can be found around the world, on land or sea.
The compressed air is stored at pressures ranging from 450 psi to 3,000 psi. Underground storage capacities are only limited by the geological formation. Above-ground capacities are usually designed for short durations but have been built for high capacity. For example, [a facility in McIntosh, Alabama][link to PDF “Dresser-Rand CAES Technology”] is rated for 110 MW and can store 2600 MWh.
For the utility operator, CAES can optimize base load units and minimize load swings to maximize efficiency and extend unit life. Storing energy lets you use off-peak power to meet peak demand. This is less expensive than using traditional gas turbine peaking units or purchasing power from other sources.
When it’s not generating power, the CAES system is available for compression duty and can be configured for daily, weekly, or extended cycles. This allows owners to balance the grid by using inexpensive power for air storage (charging).
CAES has environmental advantages compared to conventional gas turbines because its combustors use as little as two-thirds the fuel. Furthermore, CAES can be an attractive alternative to the costly modifications required to make coal-burning plants comply with increasingly stringent fossil fuel emissions requirements.
A CAES generator is designed to be started and brought to full load in as little as 10 minutes, eliminating the need for intermediate-load plants and providing a cost-effective way to meet spinning reserve requirements. CAES generators also have excellent load-following capability and very good part-load efficiency. Compressors can be engaged quickly to absorb load rather than reducing base load generation.
“We custom-engineer each CAES train to provide you with a system designed specifically to meet your site’s operating and geologic requirements,” says Lucas. “We select and fine-tune standard Dresser-Rand components for your project, then we make sure that all components work together to maximize efficiency, and reduce installation and start-up times.”
Dresser-Rand offers long-term service agreements (LTSA) to clients who require personnel to supplement or replace their maintenance organizations. A typical LTSA includes project management, technical services, field crews, and support from the company’s OEM technical resource network. Lucas says, “Our field teams are OEM-trained, fully equipped, committed to safety, and logistically prepared to provide professional and timely services to keep your critical equipment on-line.”
Learn more about compressed air energy storage (CAES).
Additional information about Dresser-Rand.
Dresser-Rand CAES brochure.
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