Explained: Energy Storage
Overview
Batteries play two key roles in the energy sector: maintaining consistent grid voltage, a function called frequency regulation, and multi-hour storage for intermittent electricity harvested from wind and solar sources (read more here).
One battery dominates the current marketplace: lithium ion. The high-energy density (storage capacity per volume) of lithium ion cells makes them a great match for portable electronics, substantiating their widespread use in mobile phones, laptops, and electric vehicles. Though developed for these smaller applications, lithium ion accounts for more than 80% of utility-scale battery storage.
These cells, however, have two major issues. Firstly, operating them in high temperatures severely reduces their battery cycle life, thus temperature controls are needed to keep them cool. Those controls, in turn, create a “parasitic” drain on electricity that reduces overall cell efficiency. The flammability of lithium ion electrolytes is the second, even more serious concern. In addition to highly-publicized Tesla vehicle and Samsung smartphone battery fires, a number of utility-scale battery installations have burst into flames, most recently at Arizona Public Service’s McKicken storage facility in April 2019.
Researchers are developing materials and designs to produce cells that are safer, cheaper, have a longer battery life, and perform better in hot climates than existing lithium ion batteries. Some notable possibilities include lithium-metal, lithium-sulfur, solid-state batteries incorporating ceramics or solid polymers, and “flow batteries” with external tanks that allow for easy expansion of storage capacity.
Prices Drop, Demand Surges
The shortcomings of lithium ion batteries haven’t hindered their exponential growth in the US battery storage market. From just a few megawatts a decade ago, utility-scale battery installations reached 866 megawatts of power capacity by February 2019, and total battery storage is expected to approach 4.5 gigawatts of cumulative capacity by 2024 – a significant leap, but still just a fraction of a percent of overall U.S. generating capacity. To safeguard grid stability against increased consumption and demand uncertainty, deeper investments in energy storage will be needed, for longer-duration, inter-day storage equaling roughly 3-7% of renewable energy-based electricity production.
Though lithium ion prices continue to plummet, as production ramps up. Between 2010 and 2018, the average price of a lithium ion battery pack dropped from $1,160 per kilowatt-hour to $176 per kilowatt-hour – an 85% reduction in just eight years. Within the next few years, Bloomberg New Energy Finance predicts a further drop in price to $94 per kilowatt-hour in 2024 and $62 per kilowatt-hour in 2030.
This huge decline in battery prices has economically enabled solar plants to be paired with storage, particularly in states where high electricity rates coincide with strong policy (like high renewable portfolio standards). A Hawaiian solar-plus-storage plant on the island of Kauai is expected to save 2.8 million gallons of diesel oil annually while supplying 65% of the island’s peak nighttime electric load. It is part of a cohort of new and planned solar-plus-storage facilities that will help Hawaii meet a regulatory mandate requiring 70% renewable energy-based electricity by 2030 and 100% renewable electricity by 2045.
In California, the Los Angeles Department of Water and Power has also committed to making battery storage an integral part of its infrastructure. In September 2019, it approved a power purchase agreement that will provide 400 megawatts of solar power and 1,200 megawatt-hours of battery-stored energy for an astonishingly low price of 3.3 cents per kilowatt-hour, making it a cheaper source of electricity than natural gas. Along with the advantage of favorable economics, this deal was driven by the city’s commitment to deliver customers 100% renewable electricity by 2045.
Microgrids
Along with their utility-scale functions, batteries are emerging as key elements in microgrids – small-scale power systems that can supplement or substitute for grid-supplied electricity. The recent spate of hurricanes and wildfires knocking out grid-supplied electricity has brought significant awareness to microgrids, especially for emergency shelters, hospitals, and similar applications. Creating “energy islands” by pairing battery storage with solar arrays creates a degree of local energy autonomy if grid power is lost (now being planned for Puerto Rico). This architecture is valuable for responding to cyber-threats as well as extreme weather events.
The Next Generation
What technologies are out there to meet our growing demand (25-62% increase by 2050, according to NREL), and replace the hazardous, inefficient lithium ion cell?
Pumped-Storage Hydropower: Pumped-storage hydro (PSH) facilities are large-scale energy storage plants that use gravitational force to generate electricity. Water is pumped to a higher elevation for storage during low-cost energy periods and high renewable energy generation periods. When electricity is needed, water is released back to the lower pool, generating power through turbines. Recent innovations have allowed PSH facilities to have adjustable speeds, in order to be more responsive to the needs of the energy grid, and also to operate in closed-loop systems. A closed loop PSH operates without being connected to a continuously flowing water source, unlike traditional pumped-storage hydropower, making pumped-storage hydropower an option for more locations.
In comparison to other forms of energy storage, pumped-storage hydropower can be cheaper, especially for very large capacity storage (which other technologies struggle to match). According to the Electric Power Research Institute, the installed cost for pumped-storage hydropower varies between $1,700 and $5,100/kW, compared to $2,500/kW to 3,900/kW for lithium-ion batteries. Pumped-storage hydropower is more than 80 percent energy efficient through a full cycle, and PSH facilities can typically provide 10 hours of electricity, compared to about 6 hours for lithium-ion batteries. Despite these advantages, the challenge of PSH projects is that they are long-term investments: permitting and construction can take 3-5 years each. This can scare off investors who would prefer shorter-term investments, especially in a fast-changing market.
Compressed Air Energy Storage (CAES): With compressed air storage, air is pumped into an underground hole, most likely a salt cavern, during off-peak hours when electricity is cheaper. When energy is needed, the air from the underground cave is released back up into the facility, where it is heated and the resulting expansion turns an electricity generator. This heating process usually uses natural gas, which releases carbon; however, CAES triples the energy output of facilities using natural gas alone. CAES can achieve up to 70% energy efficiency when the heat from the air pressure is retained, otherwise efficiency is between 42-55%.
Thermal (including Molten Salt): Thermal energy storage facilities use temperature to store energy. When energy needs to be stored, rocks, salts, water, or other materials are heated and kept in insulated environments. When energy needs to be generated, the thermal energy is released by pumping cold water onto the hot rocks, salts, or hot water in order to produce steam, which spins turbines. Thermal energy storage can also be used to heat and cool buildings instead of generating electricity. For example, thermal storage can be used to make ice overnight to cool a building during the day. Thermal efficiency can range from 50 percent to 90 percent depending on the type of thermal energy used.
Flow Batteries: Flow batteries are an alternative to lithium-ion batteries. While less popular than lithium-ion batteries—flow batteries make up less than 5& of the battery market—flow batteries have been used in multiple energy storage projects that require longer energy storage durations. Flow batteries have relatively low energy densities and have long life cycles, which makes them well-suited for supplying continuous power.
Solid State Batteries: Solid state batteries have multiple advantages over lithium-ion batteries in large-scale grid storage. Solid-state batteries contain solid electrolytes which have higher energy densities and are much less prone to fires than liquid electrolytes, such as those found in lithium-ion batteries. Their smaller volumes and higher safety make solid-state batteries well suited for large-scale grid applications.
However, solid state battery technology is currently more expensive than lithium-ion battery technology because it is less developed. Fast-growing lithium-ion production has led to economies of scale, which solid-state batteries will find hard to match in the coming years.
Hydrogen: Hydrogen fuel cells, which generate electricity by combining hydrogen and oxygen, have appealing characteristics: they are reliable and quiet (with no moving parts), have a small footprint and high energy density, and release no emissions (when running on pure hydrogen, their only byproduct is water). The process can also be reversed, making it useful for energy storage: electrolysis of water produces oxygen and hydrogen. Fuel cell facilities can, therefore, produce hydrogen when electricity is cheap, and later use that hydrogen to generate electricity when it is needed (in most cases, the hydrogen is produced in one location, and used in another). Hydrogen can also be produced by reforming biogas, ethanol, or hydrocarbons, a cheaper method that emits carbon pollution. Though hydrogen fuel cells remain expensive (primarily because of their need for platinum, an expensive metal), they are being used as primary and backup power for many critical facilities (telecom relays, data centers, and credit card processing).
Flywheels: Flywheels are not suitable for long-term energy storage, but are very effective for load-leveling and load-shifting applications. Flywheels are known for their long-life cycle, high-energy density, low maintenance costs, and quick response speeds. Motors store energy into flywheels by accelerating their spins to very high rates (up to 50,000 rpm). The motor can later use that stored kinetic energy to generate electricity by going into reverse. Flywheels are commonly left in a vacuum so as to minimize air friction, which would slow the wheel.
Primary Source: Yale Climate Connections