Image Credit: Antilo | Shutterstock.com
As one of the cleanest sources of energy in the world, solar energy offers a completely renewable and environmentally friendly way to generate electric power. While certain limitations in its ability to be utilized at night are present, solar energy technologies are easy to install, and require little maintenance compared to other energy sources.
Within the United States alone, over 22,700 megawatts (MW) of cumulative solar energy is currently being operated, which is enough to power more than 4.6 million average American homes.1 As the solar industry continues to grow around the world, ways in which this energy can be effectively stored is a challenge engineers are striving to overcome.
Thermal Energy Storage Systems
Current technologies that allow the storage of solar energy include thermal storage, compressed air energy storage (CAES), hydrogen storage, pumped hydroelectric storage, flywheels, and batteries.2 Since its first successful implementation in 1985, thermal energy storage technologies generate electricity by capturing heat from the sun and storing this energy in water, molten salts, or other fluids.2
A thermal energy storage system (TES) typically consists of a storage medium in a reservoir or tank, a built-in refrigeration system, piping, pump(s) and controls.
Depending on its operating temperature, TES systems can be classified as either low- or high-temperature TES, where low-temperature TES normally uses cold water and reheating processes, whereas high-temperature TES systems are based on latent and thermochemical heat storage.3 With a minimal daily self-discharge loss of 0.05-1%, the TES system is able to store large quantities at a relatively low capital cost, while also avoiding the production of any major hazards.
Compressed Air Energy Storage
A similar generation storage technology is Compressed Air Energy Storage (CAES), in which the elastic potential energy of compressed air is stored until its release to produce electricity. As solar energy enters the CAES system, an electric motor drives an air compressor in which compressed ambient air is then stored under pressure in an underground cavern until its desired release.4
Due to the high pressures being applied to the air, heat generation within the CAES system occurs, which can lead to unwanted energy discharge. In an effort to reduce discharge temperatures to 300/350 °F, inter and after coolers are present within the CAES system, in which heat is either extracted during the compression process or removed completely by an intermediate cooler.
Salt caverns are often the preferred locations for CAES due to its desirable properties of high flexibility, minimal pressure losses, as well as the complete absence of oxygen reactions between the air and host salt rock.4
Hydrogen Gas as a Storage System
As one of the largest energy components of any fuel, hydrogen gas is the ideal vehicle for the storage and distribution of energy. Through a reproducible cyclic process, where hydrogenation is followed by dehydrogenation, the hydrogen storage system manipulates the properties of cyclohexane for energy production. The hydrogenation process forms cyclohexane (C6H12) through the addition of six hydrogen atoms from the abundant hydrocarbons to benzene (C6H6) present in the hydrogen storage system following solar exposure.5
Dehydrogenation processes occur following the removal of the six carbons from cyclohexane, allowing this chemical to become available for use in energy storage devices and other applications.5
Platinum-based nanoparticles are an essential aspect of the dehydrogenation reaction, in which these nanoparticles act as photocatalysts by providing a temporary donation of their photoexcited electrons to existing cyclohexane molecules. This donation breaks the carbon-hydrogen bonds, releasing hydrogen atoms without causing excess heat to be released.
While most suitable for stationary applications, such as solar energy and wind turbines, this strategy allows for up to 97% of benzene to be converted back to cyclohexane, providing one of the most efficient options for energy storage.
Future research projects are looking towards further applications in the storage and transportation of hydrogen generated from other natural sources, such as water splitting and water electrolysis, in order to continue efforts towards the preservation of renewable energy.
Pumped Hydroelectric Storage Systems
Image Credit: Vladislav Gurfinkel | Shutterstock.com
Pumped hydroelectric storage systems store solar energy by pumping water into the upper reservoir when supply exceeds demand. In an event where demand exceeds supply, water within this initial reservoir is released by running downhill into a lower reservoir through turbines, allowing for the generation of electricity to occur.2
A similar transmission energy storage technology is referred to as a flywheel, in which this cylindrical-shaped device contains a large rotor inside a vacuum. After drawing in power from its energy source, the rotor accelerates to very high speeds, storing electricity as rotational energy within the device.2
Energy can later be distributed once the rotor is switched to “generation mode,” which slows down the rotor and returns electricity to the grid for consumer use. Batteries, like flywheels, can be located anywhere, and are often seen as similar storage systems for the distribution of energy. For large-scale energy storage potential, batteries can vary from sodium-sulfur, metal air, lithium ion, and lead-acid batteries, depending on its energy source and application.
The Future of Solar Energy Storage
While solar energy storage systems can be expensive, research organizations such as the Department of Energy’s Energy Storage Program and the Federal Energy Regulatory Commission (FERC) in the United States are looking towards enhancing the development and deployment of current and future energy storage technologies.
For example, FERC has created a pricing structure called Pay-for-Performance, in which the agency regulates the electricity grid by paying storage technologies and other resources a higher price for their services, allowing for more cost-effective options to be available for commercial purposes.
References and Further Reading
- Solar Industry Facts and Figures
- How Energy Storage Works
- Overview of Current Development in Electrical Energy Storage Technologies and the Application Potential in Power System Operation
- Concentrating Solar Power Thermal Storage System Basics
- Simple Hydrogen Storage Solution Is Powered by Solar Energy