Start-up compresses air in tanks for energy storage

SustainX, spun out of Dartmouth, is developing a energy production system that uses compressed air stored in portable containers, rather than in natural underground formations.

BOSTON--While hundreds of other companies are trying to make a better battery, start-up SustainX Energy Solutions is trying to find better ways to compress and store air to help utilities take full advantage of intermittent sources of energy like wind.

Dax Kepshire, president of SustainX, sketched out the company's technology and product plans here Thursday at the Fifth Annual Conference on Clean Energy. SustainX was spun out of Dartmouth College last year and received $4 million in funding from Polaris Venture Partners and Rockport Capital in August of this year. It now has 10 employees.

There are already a few compressed-air facilities in the world where off-peak electricity is used to pump air underground for storage. During peak-demand times, the air is released and pushed through a turbine to make electricity.

It's a method that's getting more attention now as a way to store several hours worth of wind power, for example.

Traditional compressed air storage uses underground formations to store compressed air, which is released when needed to make electricity. Click on the image for a photo gallery of other types of energy storage. PG&E

The primary difference with SustainX's approach is that it doesn't need an underground salt dome or limestone cavern to store the compressed air. Instead, it proposes storing the compressed air in off-the-shelf tanks. Its technical goal in two years is to cram 4 megawatt-hours worth of stored energy in a 40-foot long container, said Kepshire. The tank-filled container would be able to deliver 1 megawatt of power.

In the near term, it plans to build a 100 kilowatt hour pilot system to test the efficiency and then to validate the larger model in 2011, Kepshire said.

Its technology is also very different from the existing compressed-air storage facilities. With traditional compressed-air energy storage, a machine called a compressor compacts air and pumps it underground. To make electricity, the air is released and run through special turbines and a generator to make electricity.

SustainX is designing a system that uses a hydraulic piston to compress air. When the air is released, it moves a hydraulic motor which is attached to a generator to make electricity, Kepshire explained.

The key to making the overall system is to reduce the energy loss that happens in the compression and decompression of air, he said. He expects the first pilot system to be about 50 percent efficient but the full system to be more around 70 percent efficient overall.

Compressed air energy storage has a lot of potential because it's relatively inexpensive and because utilities can store many hours worth of electricity. Pacific Gas & Electric is investigating locations for compressd-air storage capable of delivering 300 megawatts of electricity for 10 hours, or 3,000 megawatt-hours. By contrast, utility-scale battery storage systems in use now deliver 1 or 2 megawatts for a few hours.

SustainX doesn't have any customers yet, but Kepshire said the company is targeting utilities looking to use more renewable energy. The company's technology, if it proves efficient enough, can be scaled to stored many hours of energy and deliver large amounts of power, he said.

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