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Isothermal Compressed Air: missing link of renewable energy storage?

by Tina Casey & Nicholas Brown


We don’t expect the wind and solar power naysayers to give up any time soon, but new utility-scale energy storage solutions are beginning to come on line, and they will put to rest the whole notion that intermittent energy sources (namely, wind and solar) can’t provide a significant proportion of reliable power to the national grid.

The latest development comes from a company called SustainX, Inc. The technology is called an isothermal compressed air energy storage system, and SustainX has completed construction of its first utility scale system. It was hooked up to the grid earlier this month and is now in the process of revving up to speed.

Isothermal Compressed Air Energy Storage
SustainX spun out of Dartmouth College in 2009. Their goal was to store four megawatt-hours worth of energy in transportable 40-foot long containers, while achieving a 70% reduction in the amount of energy needed for conventional compressed air energy storage (CAES) systems. Last year, the company took a big leap forward by entering a technology licensing agreement with the University of Minnesota.

Isothermal CAES according to SustainX
“Isothermal” denotes storage of compressed air at a constant temperature, a key element in the improved energy efficiency of the system. The new CAES system is located in New Hampshire, at the SustainX headquarters. It represents a next-generation improvement over earlier CAES systems dating back to the 1970s which are typically located underground and run on fossil fuel. The SustainX system was designed to run on grid-supplied electricity, so depending on the local grid mix it can potentially run exclusively on emission free sources including wind and solar. Furthermore, it is not dependent on caves or other geological quirks for site selection.

Some patented, cutting edge tweaks by SustainX make all the difference, but other than that, the entire system consists of practically nothing but steel, water, and air. A mechanical drivetrain utilizes an electric machine and a crankshaft. This efficient mechanical link powers a two-stage, mixed-phase (water-in-air) heat-transfer process within pneumatic cylinders. During piston strokes, water is sprayed into the air-filled chamber of each cylinder, allowing heat to be transferred from water to air during expansion or from air to water during compression. The same ICAES power unit provides both isothermal compression and expansion, eliminating the cost of separate compressor and expander subsystems.

Advantages & future develpments
If the new facility proves successful, U.S. taxpayers can celebrate because SustainX received a $5.4 million award from the Department of Energy to help accelerate the project, as part of the Obama Administration’s Smart Grid initiatives. The project, which also includes private sector investors, is on  track. Completion of the test phase is due by the end of this year with a final technology report due in 2015.

ICAES and other new storage technologies fit into the Smart Grid concept partly by eliminating the need to construct new peaking plants. Peaking plants, which typically run on natural gas, are designed to come on line quickly to address demand spikes, but most of the time they sit idle, which means that they are a very expensive way to provide for variations in local energy consumption; to say nothing of their dependence on fossil fuel sources.

With respect to the levelized cost of energy (LCOE), a mechanically simple system like the SustainX solution has some clear advantages over building new peaking plants, including the potential for far lower operating, maintenance and repair costs in addition to lower fuel costs.

In the past, CAES systems were primarily sited to take advantage of caves and other geological quirks, so the Smart Grid goal of developing more geographically flexible, above ground systems is also critical if CAES is to play a major role in the national grid.

According to a new report from consultants Navigant Research , CAES growth will be driven by the improvement of scalable, modular CAES technologies such as isothermal and adiabatic CAES. There are a handful of developers in this industry and they have technology which is projected to be commercially validated within the next 12 to 36 months. The report states:

“The unique siting requirements of traditional underground CAES, however, present substantial development risk and limit the technology’s outlook. Higher efficiency, next-generation CAES technologies that are not limited by geological considerations are on the cusp of commercialization, and are well positioned to address the gap in availability of long duration energy storage technology that can be sited where needed. Navigant Research forecasts that worldwide installed capacity of compressed air energy storage systems will exceed 11000 MW by 2023.”

The main applications focused on in this study were wind energy integration, solar energy integration, grid asset optimization, transmission / distribution timing and ancillary services.

The Lightsail system
The basic concept of compressed air energy storage has existed for decades, and has been commercially available for over 30 years. It never became mainstream, as it wasn’t the most cost-effective option in most applications. However, as the need for renewable energy storage has grown greater, CAES has become more attractive, especially with such advancements in technology. LightSail is another company whose energy efficient, mist-cooled system enables above-ground storage. This company has secured $5.5 million in private funding for its CAES project from A-list investors including Bill Gates and PayPal co-founder Peter Thiel. CEO and co-founder Steve Crane explains how CAES crystallizes the transformative potential of alternative energy from a social and political perspective:

“We want to democratize energy – to enable renewable sources to supply energy on demand locally and at a lower cost than centralized fossil-fuel based generation.”

That’s the piece missing from the fossil fuel puzzle, the ability of a democratically organized community to make decisions about its primary sources of energy, just as it makes decisions about roads, zoning, schools, public safety and a host of other issues that are fundamental to the quality of life.

CAES systems can be built without rare materials, to last a very long time. They can be recharged and discharged very quickly. They could even be considered “low-tech” due to their inherent simplicity. That kind of robust simplicity is a great thing!


◊ Publ. here 21.10.2013


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