One group likes water and air. The other prefers heat and earth.
It sounds like a panel that Aristotle might have moderated 2,500 years ago, but actually it marks the difference between two different approaches in modular compressed air energy storage.
SustainX, a well-funded spin-out from Dartmouth College, has come up with a modular energy storage system that relies on water to prepare a mass of compressed air that can later be delivered to a turbine. A 40-kilowatt prototype exists, and in 2012, a 1-megawatt system for AES should be ready.
LightSail, a somewhat secretive startup funded by Khosla Ventures, is taking a similar approach.
England’s Isentropic Energy compresses gases for turbines too, but instead of water, the active ingredients are two gravel-filled tanks: a cold one with rocks at minus 160 degrees Celsius and a hot one with rocks at 500 degrees Celsius.
In Isentropic’s system, the gas only gets compressed to around 174 psi, but it radically changes temperature. When it comes out, it is at a high pressure and temperature, said CEO Mark Wagner.
In the SustainX and LightSail systems, the gas stays at close to room temperature throughout the cycle, but it gets compressed to 3,000 psi so it comes out at an extremely high pressure.
Who’s right? Utilities will ultimately have to determine that through rigorous testing, but here are the basic differences. Compressed air remains far cheaper than batteries or other means for storing energy, according to EPRI and others. Modular units like these have an advantage over geologically-based storage in that they can be made in different sizes and be placed virtually anywhere on the globe. (General Compression, which just raised $54.5 million, concentrates on geological compressed air storage that uses wind turbines to compress air. So it's mechanical compression, but different in its market approach than these three.)
SustainX. The key intellectual property behind the company is a proprietary compression/expansion motor powered by a hydraulic transmission compresses the air.
The transmission functions essentially the same way as a portable hydroelectric dam: a large volume of water with the help of gravity is harnessed perform mechanical work. Dax Kepshire, co-founder and general manager, wouldn’t describe it much beyond that, but agreed loosely with the hydroelectric analogy.
The motor can also be used for functions other than compressing air.
While getting compressed, water vapor is employed to cool the air. Cooling is important during compression. Gases heat up as they get compressed, and that increases the amount of energy required to compress the substance. In the few underground compressed air storage facilities in the world today, the energy required to run the gas turbines compress the air comes roughly to about 50 percent of the energy stored. SustainX, he said, has an energy density of seven times more than conventional systems.
“We are spraying water directly into the cylinders,” he said.
The heat extracted with the vapor is subsequently stored in, yes, another body of water. Water is employed as a heat sink because “Water has the best specific heat of any substance on the planet,” he said. The compressed air, meanwhile, is stored in the same sort of tanks used by fossil fuel companies to store compressed natural gas: pressures of 3,000 psi do not present a problem, he said.
When the air gets released, the heat from the reservoir gets injected back into the air as it expands so that when it gets to the turbine, it is a rush of fast-moving air at ambient temperatures. In a sense, air is just a medium for energy created by water.
“We want to maintain the temperature constant in compression and expansion,” he said.
The company, he added, has also figured out techniques for removing the water vapor from the compressing air to eliminate the possibility of internal corrosion. Additionally, SustainX may not only be seen at modular facilities: the hydraulic pumps and other aqueous tools can be used at cave storage facilities, he added.
Isentropic Energy. By contrast, Isentropic thrives on the heat generated in the compression process. Its proprietary compression/expansion engine compresses captured argon to around 12 bars of pressure. The compression heats it up to 500 degrees Celsius. The hot air is then passed through gravel, transferring the heat to the gravel in the same way steam heats rocks in a sauna.
The heat returns to ambient temperatures but stays at 12 bars. It is then passed to another chamber where it expands. The temperature drops to minus 160 degrees Celsius and the pressure goes back to normal.
When a utility needs energy, the air goes through the process in reverse, delivering 500-degrees-Celsius, 12-bar air to a generator. Discharge takes a few seconds.
Isentropic’s compression engine runs about $500 a kilowatt, but the gravel only runs about $25 a kilowatt-hour. In total, an Isentropic system will cost $87.50 per kilowatt-hour installed, claimed CEO Mark Wagner.
“Temperature difference is the cheapest way to store energy,” he said.
Storing the heat in gravel also reduces the level of pressure required, he added, which in turn reduces the cost of the storage tanks.
Kepshire wouldn’t give numbers, but reiterated that his company would be competitive with gas turbines when it came to delivering power.
The results should be interesting.