Liquid Air Combined Cycle
Liquid Air Combined Cycle
LACC is based on proven equipment that has been assembled based on sound principles:
- Air is plentiful, free and non-toxic, and is safely stored at low pressure as a cryogenic liquid.
- Liquid air can be produced in high volume using cryogenic refrigeration processes and equipment already proven in the industrial gas and liquefied natural gas industries.
- Liquid air has high energy density, enabling tens of gigawatt-hours to be stored within
- Energy conversion requires large temperature differences, so we use the synergy of high temperature exhaust from combustion turbines and cryogenic temperature of the stored liquid air to maximize efficiency.
During charging, air is drawn from the atmosphere, filtered, cooled to a liquid for storage. During discharge, the liquid air is pressurized, evaporated, heated and use to produce power before being returned to the atmosphere.
- Cryogenic Refrigerator. During charging, a cryogenic refrigeration system uses low-cost or excess renewable energy to cool air to the liquid state. Various processes have been developed and perfected, but all rely on compressors, heat exchangers, and expanders to cool gas, and reject heat to the environment. In an LACC application, the compressors are motor driven to consume electricity.
- Storage Tank. Liquid air is stored at atmospheric pressure in an insulated tank. Losses are low, less than 0.1% per day, which permits long-term storage of weeks or months.
- Cryopump. A conventional cryopump, such as used for LNG send-out, pressurizes liquid air from the storage tank.
- Regasification. The pressurized liquid air is converted back to vapor by absorbing heat rejected from an Organic Rankine Cycle (ORC).
- Pre-heating. The pressurized regasified air is warmed by a closed cooling loop that circulates anti-freeze. The anti-freeze in heated by Air used for energy discharge.
- Exhaust gas heating. The pressurized air is then heated to high temperature using exhaust gas, from a combustion turbine, steam boiler, or other heat source.
- Air Turbine Generator. Pressurized hot air flows through a turbine generator, similar to a steam turbine, to produce power. The clean, dry air is exhausted to atmosphere.
During discharge, ambient air is heated to drive the discharge process. Typically, the exhaust heat from a thermal power plant is used, such as combustion turbine which starts quickly and produces exhaust heat at a useful temperature.
- Inlet Air Cooling. Ambient air is cooled, increasing the air density to allow the combustion turbine to operate at its maximum power point and maximize the exhaust heat available for extracting stored energy. Heat absorbed from the inlet air is transferred via a cooling loop, filled with anti-freeze, to pre-heat the regasified air.
- Combustion Turbine Generator. Any suitable combustion turbine can provide the exhaust heat used to drive the discharge cycle. By controlling the inlet air temperature via cooling loop, this turbine can always operate at optimum conditions to maximize the cycle efficiency.
- Exhaust Heat Recovery. Exhaust heat from the combustion turbine is extracted by two heat exchanger coils. The first coil heats pressurized air from the Cooling Loop pre-heater; the second coil heats the ORC working fluid.
The ORC uses refrigerant, such as R-290 (propane), in a closed cycle to extract additional power to improve the efficiency of the LACC.
- Pump. Liquid refrigerant is pumped to pressure.
- Pre-heating. The pressurized liquid refrigerant is preheated in a recuperator by low pressure refrigerant exhausted from the ORC Turbine.
- Exhaust gas heating. The refrigerant is evaporated and superheated by combustion turbine exhaust.
- ORC Turbine Generator. The pressurized hot refrigerant produces power in a turbine, and exhausts as low-pressure refrigerant.
- Pre-cooling. The low-pressure refrigerant vapor is cooled in the recuperator.
- Condenser. Finally, the ORC loop is closed in the condenser, where refrigerant transfers its latent heat to the liquid air, which absorbs latent heat.
LACC can be deployed cost-effectively at a range of sizes. It is ideally suited for large-scale renewable time-shifting applications where many GWh of storage need to be shifted by days, weeks, or months. LACC is easier to site than Pumped Storage Hydro or Compressed Air Energy Storage, since it does not require any special geological features.
LACC is also well suited to handle natural gas reliability concerns, such as from a Polar Vortex in the U.S. northeast or Aliso Canyon in Southern California. Liquid air is colder than LNG, so it is straightforward and inexpensive to use cold liquid air to liquefy natural gas for long-term storage in separate tanks.
The Liquid Air Combined Cycle employs proven equipment from top-tier suppliers.
The cryogenic equipment (liquefaction trains, storage tanks, and cryo-pumps) is similar in scope to an LNG liquefaction facility, which range in size from hundreds of tons to 100 thousand tons per day of liquid production.
The discharge equipment employs customary power plant equipment (combustion and air turbine generators), plus heat exchangers and refrigerant equipment from the process industries.
Like other hybrid energy storage systems with two energy inputs, performance is measured by:
- Electric-rate – the ratio of electricity stored per unit of electricity delivered.
- Fuel Heat Rate – the ratio of fuel consumed per unit of electricity delivered.
In typical applications, the Electric-Rate is about 85% (118% electric round trip), which is enabled by the fuel consumed in the combustion turbine at a fuel Heat Rate of about 5000 Btu/kWh LHV.
New metrics are being developed by professional associations to support advances in storage technology. The key performance metrics of Energy Storage Systems (ESS) defined in the ASME PTC-53 Performance Test Code (currently available in draft form) are:
- Discharge Power Output, for example MegaWatts (MW)
- Discharge Energy, for example MegaWatt-hours (MWh)
- Charge Energy, for example Megawatt-hours (MWh)
- Storage Efficiency.
Hybrid storage technologies use two energy inputs – electricity during charging and fuel or waste heat during discharging and can discharge more electric energy than was stored. Since an efficiency greater than 100% is potentially confusing, PTC-53 follows the practice used in conventional power generation and expresses the efficiency as the energy input per unit of electrical energy output.
- Fuel Heat Rate, expressed as Btu/kWh, is the ratio of fuel energy consumed per electricity produced. This is the customary efficiency for thermal power plants; for pure electric storage systems, like Pumped Hydro or batteries, this is zero.
- Electrical Rate, expressed as kWh/kWh, is the ratio of electrical energy consumed per unit of electricity produced (the inverse of Round-Trip Efficiency used for single input storage systems).