With the advent of renewable energy as a significant fraction of our energy generation, the need for energy storage has given impetus to efforts for exploring many alternative schemes. What makes a good choice for an energy storage system depends on the specific requirements, eg location, size, reaction time, funding, etc., but in general, capacity, efficiency, life cycle cost per MW, scalability, easy locatability, availability of mature technology, off-the market components, environment friendliness, and many other factors are desirable to various degrees. Liquid Air Energy Storage (LAES) is one of the more attractive options.
The Concept of Liquid Air Energy Storage
Excess available energy is used in cooling clean air to -196 C. That is the temperature at which air gets liquefied completely. The volume decreases by about seven hundred times which is a major advantage. This liquid is stored in insulated vessels at low press. When power is required, the liquid gas heated and compressed to generate a high pressure gas at a temperature suitable to run a turbine and generator. Because of a quick reaction time, the system is said to act like a giant battery. Such a demonstration plant built by HighView is already functioning at Pilsworth, UK since summer 2018. The plant possesses a capacity of five megawatts at 15 megawatt hours (MWh) of electricity. That is sufficient to feed about 5,000 homes of average size for nearly three hours. A full commercial scale plant could store something like 50 MW.
The entire operational cycle can be divided into three stages.
Air is drawn from the atmosphere at no cost and cleaned of dust and other non-gaseous pollutants. The energy required to be stored is used to cool this air. The water content freezes at 0 C and is easily separated. The volume also reduces. Further cooling to -78.5 C reduces volume and liquefies the carbon dioxide content. Cooling is continued and volume keeps decreasing. At -183 C oxygen liquefies. Finally at -196C, liquefaction is complete when nitrogen also becomes liquid. All this is done without applying pressure. The volume reduces by 700 times, that is, 700 liter of air at standard temperature and pressure (STP) reduces to just one liter of liquid air.
That makes the system space efficient. Cooling down to this temperature is necessary because nitrogen, the biggest component of air, does not become liquid at higher temperatures. Liquefaction is necessary because the change of state absorbs a lot more energy than mere cooling. Further cooling is not cost effective for a similar reason.
This liquid air is kept stored in thermally insulated tanks at low pressure. Since the volume is small storage cost is not very high. Further, the storage technology is mature as such equipment is already used for bulk storage of liquid nitrogen, oxygen and LNG etc. all over the world. GWHs of energy can be stored in commercially available equipment.
When required, liquid air at is heated and compressed. Addition of heat results in air at pressure and temperature suitable for driving a turbine to turn the electric generator.
The basic Liquid Air Energy Storage system described above has low efficiency. However, efficiency can be greatly improved with proper handling of the heat extracted during cooling. This heat is stored in a heat store and used during the recovery process for warming the liquid air. Similarly, the cooling obtained during warming of the gas can be stored in a cold store and used to cool the air in a subsequent charging operation. In addition, waste heat from many industrial processes eg, thermal power generation and steel manufacture, can be utilized. Similarly waste cold is available in may other industrial processes and can be gainfully utilized. Efficiency as high as 70% is quoted.
At large scale, the Liquid Air Energy Storage scheme is cheaper than batteries . The plant can last for about four decades. Another advantage that the plant can be located anywhere where energy is available. A special advantage is that it will comprise of components which are already available in the market and very reliable.