Turn a glass upside down and plunge it into a sink filled with water.
You’ve just demonstrated the operative principle behind a compressed air storage system being developed by startup Bright Energy Storage Technologies that -- conceivably -- could deliver large amounts of power for 2.5 cents to 6 cents per kilowatt hour.
The company’s core technology consists of a long, flexible polymer/glass bag shaped like a giant sea cucumber. One end of the bag is attached to an air hose on the surface. The other end is open to the sea. Sand from the sea floor anchors the open end of the tube to a sand flat 140 meters below the surface.
To store energy, generators on shore will compress air and funnel it to the bag. While some air can bubble out the open end if overfilled, the majority of it stays inside, in the same way air stays in the upside down glass or when you blow into a straw.
When needed, the air can be quickly dispatched. Sea water (or water from a large lake) fills the void until more air gets sent down.
The potential low cost of the technology derives from the fact that Bright doesn’t need high pressure storage tanks or lots of active mechanical equipment. The body of water does most of the work.
“We design for no moving parts and no metal,” says chief technology officer Brian Von Herzen. “Sand at the sea floor is 100 times less expensive than steel. It is free at the bottom of the ocean. The main expenses for the ballast are the permit and installation.”
Compressed air storage -- the cheapest way to store energy according to many -- barely exists as an industry now. Only two large-scale compressed air facilities currently operate in the world. The concept, however, has uncorked the creativity of engineers and designers all over. Some of the more notable startups include SustainX (compressing air with the help of water vapor), secretive LightSail Energy (founded by an entrepreneur who entered college at age 12), General Compression (cost-effective underground storage) and Isentropic Energy (hot gravel). Some others are also looking at Bright's idea.
At 140 meters below the surface, Bright can store enough air for approximately 1 gigawatt-hour of power in a square kilometer of sea real estate, he said.
The main expense lies in building the tube to refill the bag and to allow for grid connection. Horizontal drilling techniques are employed and the air spout is located close to shore. A self-standing system with its own generator could produce power for under 6 cents a kilowatt hour, he said. If you could latch onto a generator at an offshore wind farm, the costs could potentially drop to 2.5 cents.
Storms? Debris? Sharks? Wave and tidal companies have seen their valuable prototypes shredded by the sea. Those systems, however, mostly float on or near the surface, the most turbulent place in the ocean. At 140 meters down, the ocean is far calmer. Below 70 meters, hurricanes can have little effect, Von Herzen said.
“The deeper you go, the better it gets,” he added.
The air is also only stored at two pounds per square inch of pressure over ambient conditions, so Bright doesn’t have to worry about rips or tears caused by excessively high pressures.
The technology also dovetails well with ocean conservation efforts. Imagine a checkerboard of ocean real estate: one square kilometer is given over to fishing, while the neighboring square kilometer is used for storage. Fish suddenly have protected habitats, but protected environments that are close enough and small enough to placate the fishing industry. Ideally, it could become a model for sustainable aquaculture.
The company will conduct a trial later this year.
The biggest obstacles are permitting and planning and skepticism. It’s a bag in the water. Can it last 25 years? What happens if leaks develop? What if the ballast shifts and the air rushes out the other end? Utilities are accustomed to getting their power from large-scale coal plants.
“Utilities are not used to being entrepreneurial organizations,” he said.