Energy Storage Systems
America’s energy grid maintains a real-time balance between energy demand and energy supply. This requires consistent and controllable power sources. The energy produced by most renewable sources is inconsistent; it varies with weather conditions. Consequently, most renewable energy sources need an energy storage device to produce on-demand electricity.
Battery Storage
Mention electricity storage, and batteries immediately come to mind. Lithium-ion batteries are currently being used to store megawatts of electricity. The downside is lithium is toxic, the batteries can overheat and catch fire. Flow batteries appear to be more suitable for utility scale energy storage, but they need further development.
Currently, Tesla is marketing a “Megapack” storage system which uses “units of grid-scale lithium-ion batteries.” A 1-MW unit costs nearly $2 million and can store approx. 3.9 MWh of electricity. The Megapack units can be coupled to store gigawatt-hours of electricity. Tesla is currently advertising a 5 gigawatt-hour (5,000 MWh) system. If we use Tesla’s average home consumption numbers of 9.5 MWh per home/year, 5 GWh can store enough energy to power the equivalent of 530 homes for 1 year. (or 576,315 homes for 8 hours)
Pumped Storage Hydropower
Pumped-storage hydropower facilities pump water from a source to an elevated reservoir. The water flows down from the upper reservoir, driving hydro turbines positioned slightly above the lower water source.
Pumped-storage hydroelectric systems generally use more energy pumping than they can produce. However, they are a green means for storing excess energy.
Thermal Storage
Thermal energy storage systems use mediums, like molten salt, to absorb solar heat and store it in insulated tanks. The heated medium is then used to boil water to drive steam turbines.
Mechanical storage
Mechanical energy storage systems convert surplus electrical power into mechanical power, which is to can be used to drive turbines which can generate electricity.
Flywheel: Surplus electricity is used to spin a flywheel, which can be used to generate electricity as needed.
Compressed air. Surplus electricity is used to compress air into large vessels such as tanks or underground formations. The air is released as needed to drive turbines which can generate electricity.