Cryogenic Energy Storage

To many people, the term “cryogenic” conjures up images of Walt Disney’s frozen corpse or Woody Allen’s “Sleeper.”  I’m guessing it doesn’t make you think about energy storage, but unlike the aforementioned applications, cryogenic energy storage is technically feasible with current technology, as was recently demonstrated by Highview Power Storage. Highview has a prototype cryogenic energy storage plant that’s been running for over a year. The facility has a 300 kW maximum output and a 2.5 MWh storage capacity. That’s enough to power sixteen houses for eight hours. The company hopes to build a full-scale plant that can output 10 MW with 40 MWh of grid-level storage, which would power up to 250 houses for eight hours.

Grid level energy storage has multiple applications. First, it can provide peak power during times of high demand, reducing the need for natural gas fueled “peaker plants.” Second, grid-level storage is necessary in order to increase the amount of energy that comes from renewables such as wind and solar, both of which are intermittent energy producers.

In 2005, Highview Power Storage began researching the possibility of utility scale energy storage using liquid air. Excess energy (during low-demand times) is used to compress air into a liquid, which can then be stored in insulated low-pressure tanks. When demand exceeds production, the liquid air is warmed and the resulting steam is used to drive the turbine of a generator.

According to Highview, cryogenic energy storage offers the following benefits:

  • It uses proven technology that’s been been around for years.

  • Regulations for cryogenic storage already exist.

  • Storage is at low pressure, making tanks less costly. (Tanks are insulated to keep the liquid air cold, but they’re still less expensive than room-temperature compressed air storage tanks.)

  • Air doesn’t explode and it’s non-toxic.

  • Liquid air has four times the energy density of compressed air.

During the storage process, ambient air is filtered to remove impurities. Water and CO2 are also removed because they’ll freeze solid. The resulting air is refrigerated. Some of the air condenses into a liquid at -196oC. That liquid air is stored in tanks. The remaining unliquified air is very cold, so it’s recycled and used to assist in the cooling process.

During the recovery process, exhaust gas is added to heat the liquid air. When the liquid is gasified, it drives a steam engine that generates electricity. In the process of heating the liquid air, the exhaust gas is chilled to -160oC. The “cold” is stored in a gravel bed and later recovered to help the chilling process used during energy storage. This reduces the amount of work the compressor has to do, making the process more efficient.

Waste heat can also be harvested and used, increasing the efficiency of the gasification process. For example, if this storage plant it located next to a conventional power plant, the waste heat from the power plant can be used to gasify the liquid air.

Highview Power Storage claims a 50% “round trip” efficiency, which they hope to increase to 80%. By comparison, batteries are 60 to 70% efficient, pumped hydro is 75% to 85% efficient, and compressed air energy storage is 45% efficient. Although this system is less efficient than some batteries, it has a virtually unlimited number of charging/discharging cycles with no loss of capacity due to excessive depth of discharge.

This video explains the process:

Image and video: Highview Power Storage

Now that’s a cool way to store energy!

By the way, if you’re wondering about Walt Disney, he wasn’t cryogenically frozen after his death. Quite the opposite, in fact: he was cremated.