Air can be turned into a liquid by cooling it to around -196oC using standard industrial equipment. Around 700 m3 of ambient air becomes one m3 of liquid air which can then be stored in an insulated vessel. When heat (including ambient or low-grade waste heat) is reintroduced to liquid air, it boils and re-gasifies, expanding 700 times in volume. When this is done in a confined space, it creates high pressure. The high pressure derived from this re-gasification can be used to drive a turbine/generator to produce electricity, thereby making it a viable large scale energy storage solution.
An abundant and cost-free fuel supply
Rather than depleting fossil fuel sources and polluting the environment, putting readily abundant air through the liquefaction technology actually cleans the air used. Depending on its contamination levels, the air is taken through a cleaning process, liquefied, and released (after producing electricity) as clean, pure air.
Storage is at low pressure, and there is no fuel combustion risk
Large scale storage and delivery systems are well developed as there are over 300 LNG tankers with a storage capacity in excess of 125,000 m3 each plying the world’s oceans at any given hour. Liquid air, having no fuel combustion or high pressure risks, would make its shipping and handling cost 5 to 15% less than that of shipping and handling LNG. Stored air poses no threat to the environment. Air is safe; it is what we breathe. However, there are some considerations when handling it as a liquid.
Cost competitive to fossil fuel technologies
Renewables have the advantage that their marginal fuel cost is close to zero once the capital expense of developing a grid-connected renewable facility is paid back. Our floating offshore rim-driven wind farm has an LCoE (Levelized Cost of Energy) of 4 cents/KWh. The LCoE of natural gas is around 6.8 cents/KWh. The higher the difference between peak and off-peak cost, the more economical this technology will be. The debate around grid balancing is usually presented in terms of the need for additional gas-fired plants to run when the wind drops or there are shortfalls in energy. There is also a case for additional grid storage to absorb excess ‘wrong time’ energy and store it, allowing power plants to run more efficiently – enter liquid air.
The automotive industry
would have more incentive to produce electric cars if they were charged from renewable electricity. Today’s single biggest constraint facing the use of electric cars is their dependence on fossil fuel to charge their batteries.Liquid air also brings many advantages to the utility industry.
Unlike pumped hydro or large-scale CAES (compressed air energy storage) systems
Grid-based storage has no geographical constraints. Less than 1% of the world’s land (and no ocean) is suitable for pumped hydro or contains caverns large enough for compressed air to be viably transported and stored. However, over 99% of the world’s oceans are suitable for cryogenic transportation and storage of liquefied air. Liquid air would be first used in coastal cities with suitable berthing for cryogenic tankers. Within 5 to 10 years, the LNG industry will have its FSO (floating storage & offshore) facilities coming online which will open much of the world’s coastal ports to cryogenic storage and offloading capabilities.
Cryogenic liquid production, distribution, and equipment are mature infrastructures
LNG (liquid natural gas) is the largest user of cryogenic systems and a mature “end to end” system used on a very large scale. The existence of this system is necessary for the introduction of liquid air as a large-scale stored energy solution.
Advantages to Utilities
The majority of spinning reserves in the U.S. are natural gas turbines. The decreasing price of natural gas that the U.S. has experienced in the last few years has made combined-cycle gas turbines the prime producer of choice. However, 25-30% of the gas feeding these turbines goes into compressing air in an effort to meet air quality standards mandated by the government. If the utilities using these turbines had a source of air on-hand to offset the air produced by the turbine’s compressors, 100% of their fuel could be used to produce electricity. Having pressurized or liquid air onsite meets this need, giving years of service to the turbines and adding value to the plant, while offsetting the need to add new turbines to meet demands.
It is common to decommission natural gas-fired turbines when they fail to meet gas-related safety requirement standards and the costs of turbine reconditioning are no longer justifiable. There are no combustion issues with using a pressurized or liquid air system to spin these machines. Also, they are already connected to the power grid, so using air as fuel would extend their service life and yield considerable savings to the industry.
Most states actually have legal mandates that require IOU’s (Investor-Owned Utilities) to use the least expensive source of electricity available. Fossil fuel historically has been the least expensive, but liquid air could soon out-bid it.
Another utility incentive is to use liquid air to convert low-grade waste heat (such as the heat generated in a power plant) into power. Liquid air is highly efficient because of its low starting temperature (-196oC). Other waste heat recovery technologies’ maximum theoretical yields are usually limited to about 20% because they begin at 100°C. The cold air that comes from the re-gas process could be used to offset the air conditioning needs of the power plant, while the excess power plant heat could be used to re-gas.
Liquid air as stored energy would be well suited for grid reliability. The PUC’s (Public Utility Commissions) of most states will allow energy storage to be a grid reliability device rather than a generating asset. A storage technology that is integrated into the grid architecture and used for reliability can be utility-owned. Peaking and firming can easily be done with air due to its almost instant demand time. Liquid air could phase in for firm capacity as fossil fuel systems age.