Active solar energy refers to systems that use mechanical and electrical equipment to convert sunlight into a usable form of energy, primarily electricity or heat. These systems include Photovoltaic (PV) systems, which convert sunlight directly into electricity, and Concentrated Solar Power (CSP) systems, which use mirrors to focus sunlight to generate heat. Storing this energy is necessary because sunlight is intermittent; generation peaks during the day and drops to zero at night, creating a mismatch with electricity demand. Energy storage ensures a reliable, continuous power supply and is a requirement for the widespread adoption of solar power as a primary energy source.
Storing Electricity Chemically
The most common and flexible method for storing solar electricity involves electrochemical devices, predominantly batteries. Lithium-ion batteries dominate this sector due to their high energy density and efficiency, making them suitable for both residential and utility-scale applications. When a solar panel generates excess direct current (DC) electricity, it forces lithium ions to move through an electrolyte from the cathode to the anode. This movement stores the electrical energy as chemical potential energy within the battery cell.
During discharge, the process reverses: the lithium ions move back to the cathode, and the freed electrons flow through an external circuit to generate electricity. Lithium-ion batteries exhibit a high round-trip efficiency, often between 90% and 95%, meaning minimal energy is lost during the charge and discharge cycle. Common chemistries include Lithium Iron Phosphate (LFP) and Lithium Nickel Manganese Cobalt (NMC), which offer a high depth of discharge, maximizing the amount of stored energy that can be used.
For larger, utility-scale needs, flow batteries are used for long-duration storage. Unlike lithium-ion cells, flow batteries store energy in liquid electrolytes contained in external tanks. Charging involves pumping these electrolytes through a central cell where electrochemical reactions change the chemical composition of the liquids.
The size of the storage tanks determines the total energy capacity, while the size of the central cell determines the power output, allowing for independent scaling of both factors. This design offers advantages like long cycle life and enhanced safety because the electrolytes are non-flammable. Flow batteries, such as those based on vanadium or iron, can provide power for significantly longer durations than lithium-ion systems, sometimes up to 12 hours.
Storing Energy Mechanically
Large-scale, grid-stabilizing storage relies on mechanical systems that convert electrical energy into potential or kinetic energy. Pumped Hydro Storage (PHS) is the most established mechanical method, accounting for the vast majority of utility-scale energy storage capacity globally. A PHS system consists of two water reservoirs at different elevations connected by a tunnel.
When solar generation exceeds demand, surplus electricity powers pumps to move water uphill from the lower reservoir to the upper one, storing the energy as gravitational potential energy. When power is needed, the water is released back down through turbines, which spin a generator to produce electricity. This method is highly effective for large volumes of energy but is limited by the availability of suitable geographical locations, such as mountains or deep valleys.
Compressed Air Energy Storage (CAES) is another mechanical technique for utility-scale storage. Excess solar electricity powers large compressors that force air into an underground storage vessel, often a large, airtight geological formation like a salt cavern. The energy is stored as pressure within the compressed air.
When the grid requires power, the pressurized air is released and expanded through a turbine, which drives a generator to produce electricity. CAES plants offer a high system power rating and can support the grid for several hours, similar to PHS. However, CAES efficiency can be affected by the heat generated during the compression phase and the need to heat the air again before expansion.
Storing Energy Thermally
Thermal storage is primarily linked to Concentrated Solar Power (CSP) plants, which use mirrors to focus sunlight onto a receiver. Unlike PV panels that generate electricity, CSP systems generate intense heat. This concentrated solar energy heats a heat transfer fluid, often a mixture of molten salts like sodium and potassium nitrate, to very high temperatures, sometimes exceeding 565°C.
This superheated fluid is pumped into large, insulated storage tanks, effectively creating a thermal battery that retains heat with minimal loss for many hours. Molten salt is effective because it is abundant, relatively low-cost, and has a high heat capacity. The stored thermal energy can be accessed even after the sun has set.
To generate electricity on demand, the hot molten salt is circulated through a heat exchanger. The heat is used to boil water, creating high-pressure steam. This steam drives a conventional turbine-generator set, allowing the CSP plant to provide dispatchable, reliable power to the grid long into the night. This process allows the solar plant to mimic a traditional fossil fuel power station.
Chemical Conversion for Long-Term Storage
For extremely long-duration or seasonal storage, converting solar electricity into a stable chemical fuel is a promising approach. This concept, known as Power-to-Gas (P2G), uses surplus solar electricity to produce a gaseous energy carrier. The primary method involves electrolysis, where electricity splits water molecules (\(\text{H}_2\text{O}\)) into hydrogen (\(\text{H}_2\)) and oxygen (\(\text{O}_2\)).
The hydrogen gas produced can be stored under high pressure in tanks or in existing natural gas infrastructure, offering a pathway for storing large quantities of energy for extended periods with minimal loss. Hydrogen has a high energy density, making it suitable for seasonal storage needs that exceed the capabilities of battery systems.
When the stored energy is needed, hydrogen can be used in a fuel cell to generate electricity, with the only byproduct being water, or it can be used directly as a clean fuel. Hydrogen can also be chemically converted into synthetic methane, which is fully compatible with the existing natural gas pipeline system. P2G technology represents the frontier of solar storage, providing a solution for decoupling solar generation from energy demand by weeks or months.