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The Principals
 
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Compressed Air Energy Storage (“CAES”)

Viewed simplistically, the CAES cycle can be described as a means to move inexpensive off-peak electricity to peak periods, when it can command a higher price.  In some markets, this may offer a sufficient economic impetus to make a CAES project successful.

Layout of a typical CAES plant

As a process, CAES borrows from the well-understood fundamental technologies of combustion turbine power generation and high deliverability gas storage.  It complements those fundamentals with the enhancements of exhaust heat recovery and emissions controls.  CAES incorporates all of the functional elements of a combined cycle plant without the complications of the water and steam cycles but adds air storage, thus time shifts, between compression and expansion.  This strategic approach enables CAES to use off-peak electricity to do the compressor work, resulting in reduced primary fuel consumption, lower total energy costs, and better utilization of the host utility’s best and most efficient base load units.  The selected design offers superior flexibility and regulation service capabilities, which are expected to result in lower energy supply and transmission/distribution system costs and improved grid system reliability.

EASE’s CAES system is designed around available CAES turbine units supplied by manufacturers who have existing CAES plants in their portfolios with many years of operating experience.  EASE uses a cavern or reservoir suitable for large volume air storage.  At Norton, Ohio the CAES project owned by Norton Energy Storage LLC will use a limestone mine about 2,200 feet deep with a displacement volume of 338,000,000 cubic feet.  The McIntosh, Alabama CAES plant (1991) owned by Alabama Electric Cooperative and the Huntorf CAES plant (1978) owned by RWE utilize cavities in geological salt for air storage.  At other locations geological salt, depleted gas formations, aquifers or mines would work well for CAES service.

In very general terms, the CAES air compressors compress air into the storage cavity during off-peak periods at night and on weekends.  The high efficiency industrial compressors are driven by electric motors rather than by the fuel driving the combustion turbine.  This separation of the compressor from the turbine is a key to the technical and economic viability of CAES. Return to the top

When generating, the CAES unit uses air from storage.  By contrast, a combustion turbine must compress the atmospheric air it uses, and it directs about 60% of the turbine work to the compressor.  The remaining 40% is directed to the generator.  All of the power from the CAES turbine, however, is directed to the generator, so the same base turbine delivers approximately 2˝ times as much power to the system.  In the CAES cycle, air from storage is heated in an exhaust heat recovery heat exchanger called a “recuperator.”  Much like a heat recovery steam generator in a combined cycle unit, the recuperator captures heat that would otherwise be vented to the atmosphere and lost; it returns the heat to the turbine to increase efficiency and output.  Unlike the combined cycle unit, the CAES unit uses no water or steam in the cycle. This recovered-heat portion of the cycle generates nearly as much power as the base combustion turbine that is at the heart of the cycle thus increasing the entire output to 3˝ times the base turbine capacity in simple cycle service.

For more general applicability, however, it is important to understand the dynamics of the electricity market, the operating objectives of multi-unit generating systems and the operating dynamics of the CAES machine and system.  It is important, also, to understand the operating costs and limitations of the combustion turbine and combined cycle units that compete with CAES.  When operated at full load – the best operating condition -- each design has a place in the market to call its own, but no other technology offers the same advantages as CAES.

Combustion turbines, in simple cycle configuration, offer low capital cost and short lead times.  They are burdened by relatively high heat rates.  Thus combustion turbines have “all-in” capital and operating cost advantages at relatively low capacity factors or very low fuel costs.  These units can be started rather quickly and can be ramped up or down rapidly if necessary.  Under adverse conditions, like high ambient temperatures, heat rate increases significantly and output suffers significant derating. Return to the top

Combined cycle units have much higher capital costs, but offer much lower heat rates.  In fact, if the units are expected to run at high capacity factors, their average “all-in” capital and operating costs in $/MWh will be lower than those of simple cycle combustion turbines, perhaps low enough to overcome the increased investment.  These units take longer to start, and they incur heat rate degradation and output derating at high ambient temperatures, much like simple cycle units.

The CAES cycle offers capital costs that may be higher than simple cycle unit costs, but are generally lower than the capital costs of combined cycle units.  The increased capacity of the equipment and the savings from omitting the water and steam system more than offset the additional costs of the external compressors and the cavern.  The amount of reduction depends on the cost of the storage elements of the cycle.  For example, a new cavern or mine in hard rock can be expected to cost more than a new solution mined cavern in geological salt.

Accordingly, CAES has an “all-in” capital and operating cost advantage in general, but can have a very significant advantage if cavern costs are low, charging energy costs are low, fuel costs are high, or if operation at capacity factors between 15% and 45% is desirable.  Low cavern costs make CAES relatively inexpensive to build.  Low charging costs make CAES operating costs lower than those of competing gas-fired units, because expensive fuel is not used for compression.  If fuel prices are high, CAES has an advantage because it uses much less premium-priced fuel than competing units. Return to the top

Current CAES systems can vary in output from 110 MW to 300 MW, the smaller units offering increased flexibility of operation.  Because the CAES unit is designed for frequent cycling, it is a very durable machine.  It can be kept “hot” to accommodate fast starting and steep ramping comparable to a simple cycle gas turbine.  The air injected into storage is cooled to ensure stable conditions and maximum storage capacity.  This consistent conditioning of the air, even at adverse atmospheric ambient conditions, means that the mass flow through the turbine can be maintained at the optimum level causing the heat rate and output of the CAES unit to remain almost the same through the entire range of ambient temperatures experienced at the site.   

The heat injection designs of CAES units offer combustion control that combustion turbines cannot achieve.  The ability to control both the air flow rate and fuel flow rate enables the CAES machines to operate at all times and all load levels, with stoichiometrically desirable fuel mixtures.

Recent developments in the Gas Turbine industry have led several manufacturers to offer machines which take advantage of some of the features that have historically separated the CAES designs from the mainstream of generating unit markets. It may be possible to adapt these new generation machines to CAES service with less modification than would have been necessary in earlier versions, thus increasing the CAES options available.  If so, as many as four manufacturers may soon offer equipment for the CAES market

 

For more information on CAES contact Michael McGill

Telephone      (281) 376-2817

or  e-mail at     mailto:mcgill@easellc.net

 

© Electricity and Air Storage Enterprises, LLC, 2004

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