One of the biggest challenges with using renewable energy for electricity generation—specifically wind and solar power—is intermittency. The wind doesn’t always blow and the sun doesn’t always shine. Affordable, reliable, and deployable storage is seen as the holy grail of renewable energy integration, and recent advances in storage technology are getting closer to finding it.
The current electricity grid has virtually no storage—pumped hydropower is the most prevalent, but is largely location dependent. As higher levels of solar and wind energy are added to the grid, however, storage will become increasingly fundamental to ensuring that the power supply remains stable and demand is met. Utilities and businesses around the globe are starting to use large-scale batteries to complement their renewable energy generation: in Texas, for example, Duke Energy installed a 36 megawatt lead-acid storage system to balance its wind power.
Storage technologies not only provide utilities with grid reliability for renewable integration, but also offer additional benefits such as ancillary services, ramp rate control, frequency regulation, and peak shaving, which can lower costs and improve the performance of the transmission system. Power system operators have always had to match electricity demand with supply, and energy storage is an additional tool in their grid-management toolbox.
Storage technologies vary by application, but most research and development is occurring in the realm of batteries, which compete mainly on cost, battery life, and safety. Lead-acid batteries are the oldest type of rechargeable battery and have a very low cost, but they suffer from a short cycle life. Newer batteries, such as lithium-ion options, have very high energy densities and efficiencies, but are hampered by high costs. Supercapacitors, meanwhile, have very long cycle life and high efficiencies, but lower energy densities, needing more space for the same capacity.
Different battery types have distinct discharge times and power capacities that lend them to different applications. Assuring continuity of quality power requires fast response times; assuring continuity of service when switching energy generation sources requires high levels of flexibility. Decoupling generation and consumption of electric energy—that is, charging storage when the cost is low and consuming energy on demand—requires high cycle life.
In addition to batteries, energy storage technologies include flywheels, pumped hydro storage, and compressed air energy storage. Flywheels store energy in the form of motion via a rotating mass that has very low frictional losses, making them best suited for high-power, low-energy applications that require frequent cycling. Pumped hydro storage is able to store energy in the form of water at a higher elevation by pumping water up while supply is high (and electricity is cheap) and using gravity to transport water down when demand is high (and electricity is expensive). Compressed air energy storage (CAES) is similar to pumped hydro storage in application, output, and storage capacity, but instead of using water as a storage medium, it uses ambient air.
CAES technology has been growing in popularity as a competitor to pumped hydro, since it offers large-scale storage without the geographic restrictions. In CAES, ambient air is compressed and driven into storage tanks or underground caverns, or stored underwater. When electricity is needed, the compressed air is expanded, driving a motor and producing power. CAES technology is not new—it has been used since the 1970s—but improvements in efficiency, air storage methods, and type of fuel used in compression has given it renewed attractiveness.
One California-based company, LightSail Energy makes CAES more efficient by storing both the mechanical energy and the thermal energy created in compressing air through the addition of water mist to the air. Water captures the generated heat and returns the thermal energy when it is re-infused into the air, heating the air and thus delivering more power.
Similarly, the New Hampshire company SustainX captures the heat from compression in water and stores it until it is needed for expansion. By carrying out isothermal–constant temperature-compression and expansion in situ, this system yields higher round-trip efficiencies and lower capital costs.
A Canadian company, Hydrostor, uses an innovative air storage method. While most new applications of CAES use above-ground metal containers, Hydrostor stores the compressed air 100-500 meters underwater in large polyester bags or cement cavities, keeping the air at the pressure to which it was compressed. When energy is needed, the flow of the system is reversed and the weight of the water forces the air to the surface, where it can be expanded to produce electricity.
Islands are the perfect candidates for CAES systems, making this technology especially relevant to the Worldwatch Institute’s efforts to expand renewable energy use in the Caribbean. The region’s high fuel import costs and rich renewable resources make renewable energy competitive in price and the logical solution when socioeconomic and environmental factors are taken into account. High levels of renewable power, however, would require either oversized systems for which there is insufficient space or funding; backup diesel generation, which is expensive and dirty; or energy storage.
As the technology matures, CAES systems are becoming low-cost energy storage solutions with high scalability and negligible environmental impacts. Underwater CAES has lower capital cost requirements than other CAES systems and smaller onshore footprints, and it requires deep waters to store air, a characteristic that is well suited to the waters surrounding islands in the Caribbean. The sun can be used as a heat source to further increase expansion efficiency.
CAES has many advantages over other forms of energy storage, and if ongoing technological innovations succeed in making it as efficient as pumped hydro storage, it may well become the cheapest type of large-scale storage. The ambient air used is free; the materials and technologies used are abundant and well understood, and there are no siting restrictions. Moreover, CAES has scalable capacity—you can just add more storage containers.