Renewable energy is the fastest-growing energy segment in the world. Renewable energy is expected to reach $1.8 trillion globally by 2027, and $259 billion by 2025. The share of global electricity supply from renewable energy has increased from 20% in 2011 to 28% in 2021, fossil energy has decreased from 68% to 62%, and nuclear from 12% to 10%. However, there are problems with renewable energy. Mainly what provides energy when the sun is not shining, or the wind is not blowing. In this article, we will look at ways being developed to store the energy when it is being made so it can be used when it is not.
There are many different methods of storing energy being produced. They include battery technologies, pumped storage hydropower, thermal and phase transition storage, and mechanical energy storage.
Battery technologies have been around since 1800 when Volta invented the first true battery1. Basically what a battery does is store energy as a chemical interaction and then reverse the reaction to discharge energy. The real push for battery storage is at the grid level, however, which requires new technologies to be developed. The leading technologies in grid-scale batteries are lithium-ion batteries and flow batteries.
Lithium-ion battery development faces challenges. There have been many reports of battery fires when charging Li-Ion batteries and they are relatively small in capacity. By placing many batteries at a site, the capacity can be increased. This is what was done at the Moss Landing Energy Storage Facility in California. They installed a 300-megawatt (comparable or larger than some coal power plants) battery storage system, which took 4500 stacked battery racks.
Flow storage batteries consist of two charged liquids separated by an ion-permeable membrane. Oxidation and reduction reactions take place in the two liquids that release energy when reversed. The advantage of a flow battery is that the liquid storage capability defines the capacity. The more charged liquid is stored, the higher the capacity. This is still experimental technology but major industry leaders are testing prototypes now.
Pumped storage hydropower (PSH) is a type of hydroelectric energy storage. It is configured with two water reservoirs at different elevations that can generate power as water moves down from one to the other (discharge), passing through a turbine. The system recharges by pumping the water back to the upper reservoir when power is available such as during the day for a solar farm. Once the sun sets, the stored energy in the upper reservoir is fed through a turbine to make electricity and the cycle restarts. PSH has grown to enormous popularity with over 96% of all utility-scale energy storage in the United States being PSH2. The obvious disadvantage of PSH is that it requires a height difference and a lot of infrastructure to build.
Thermal and phase transition storage uses different methods to store heat that can then be reclaimed to drive steam turbines. Liquifying rock or superheating sand and water mixtures can be used to store thermal energy. In this method, simple low-grade rocks or sand are liquified by surplus energy (such as from a solar farm during the day) and then the heat is recovered at night to drive steam turbines. In one Finnish research station, the sand is heated to 500°C (930°F) and then used to heat homes during the winter months.
Mechanical energy storage uses surplus energy to store energy using many different methods. One method is energy stored as highly compressed air. When an energy surplus is available, powerful air compressors are used to pump air into a tank (or, in some cases, an in-ground cavern) at higher and higher pressure. When energy is needed, the air is discharged through a turbine to produce electricity. Another method is gravity storage in which a very heavy load is lifted higher and higher into the air when there is a surplus of electricity and then lowered when energy is required. By attaching the load to a gear mechanism and tying it to a generator, electricity is produced on the way down.
Governments around the world have determined that renewable energy is a priority of the future. Until new technologies are developed, the main choices are currently solar and wind energy. Other technologies may show promise with development such as tidal generators and small local power generators on rivers and streams. To be a complete solution, solar and wind energy must have a means of storing the power.
