The Capture Group’s analyses of wind/compressed air energy storage (wind/CAES) have focused on converting intermittent wind power into baseload power and on the prospects for exploiting good but remote wind resources by sending baseload wind/CAES power via long-distance, high-voltage transmission lines to distant power markets. Since 2005 this research has been led by Samir Succar under Williams’s supervision. During 2007 research exploring the prospects for wind/compressed air energy storage (wind/CAES) systems has been on (i) understanding better the market competition between baseload wind power and baseload coal power, and (ii) getting a better understanding of compressed air energy storage technology.


Baseload wind power vs. baseload coal power

Because the least costly CO2 capture option for coal power is for coal integrated gasifier combined cycle (IGCC) plants (at least for bituminous coals), the wind/CAES systems analysis has focused on the competition between wind/CAES and coal IGCC plants with CCS and coal IGCC plants with CO2 vented.

In a wind/CAES system, a small amount of fuel (typically natural gas) would be burned in the compressed air stream recovered from storage, the combustion products of which would be expanded in a turbine to produce electricity. The GHG emission rate for a natural gas-fired baseload wind/CAES unit would be very small, ~ 1/10 of that for a coal IGCC with CO2 vented or ~ ½ of that for a coal IGCC with CCS. Also, although wind/CAES cannot compete with today’s new coal power plants in the absence of a carbon mitigation policy, the levelized generation cost (in $/MWh) would be very close to that for coal power in the presence of a CO2-equivalent GHG emissions price – about $30//tCO2, the minimum needed to induce by market forces a power generator to build a coal plant with CCS instead of one that vents CO2 – assuming all plants operate at the same “baseload” capacity factor of 85%.

Our research found that the wind/CAES option would have a lower dispatch cost than the coal options more than 90% of the time when the value of CO2-equivalent GHG emissions is $30/tCO2. Adding more and more wind/CAES to the power grid would lead to lower and lower capacity factors for all the competing coal power options—thus driving up the levelized generation costs for the coal power options.

Notably, the wind/CAES option enables both wind and natural gas to compete in baseload power markets in a carbon-constrained world. The intermittency of wind makes it impossible for a “pure” wind system to provide baseload power. Moreover, high natural gas prices exclude natural gas combined cycle power technology from providing baseload power wherever there is a substantial amount of coal power on the grid. But coupling wind to CAES makes it possible for wind to deliver firm power. And the use of wind to provide compressor energy for CAES enables natural gas to be burned at low enough heat rates in CAES units to be competitive with coal in economic dispatch.


CAES technology

The extent of the wind/CAES opportunity in mitigating climate change depends on the availability of suitable geologies for CAES. To better understand the issues involved, a major activity during 2006-2007 was to assess CAES technology for potential applications that include but are not restricted to wind/CAES. The draft final report was widely circulated for review during the second half of 2007, and the report is now being finalized based on feedback received. The report discusses both the turbomachinery of CAES (essentially a gas turbine in which the compressor and expander functions are separated in time) and the geologies of underground air storage.

The major CAES storage options are mined hard rock (including abandoned mines), aquifers, and salt (salt domes or bedded salt). The only commercial plants use salt domes, but the world’s first wind/CAES plant (a 268 MW CAES plant coupled to 75-100 MW of wind capacity at Dallas Center, Iowa, a project being developed by the Iowa Association of Municipal Utilities) will involve aquifer storage and is expected to come on line in 2011. The CAES report focuses on aquifer storage, the dominant geological storage option in most US regions that have high-quality wind resources (see Figure 4).

Figure 4. Distribution in the United States of prospective aquifer, domal salt, and bedded salt compressed air storage resources, and of high-quality wind resources.

The theoretical potential for aquifer CAES in the US is huge. However, the true CAES potential for aquifers cannot be determined without doing detailed studies of different aquifer types, focusing on the implications of air storage. There is considerable experience with underground natural gas storage, but most experience with natural gas storage relates to seasonal storage, whereas CAES coupled to wind would be characterized by charge/discharge cycles of the order of a day. Moreover, air has physical and chemical properties that are different from natural gas (including the fact that storing air introduces oxygen underground that can lead to a wide range of chemical reactions and the introduction of aerobic bacteria) that imply rates of injection and recovery that are not only different from those for natural gas, but which also can change over time as a result of chemical reactions and biological activity.