The California Energy Commission's PIER program released a comprehensive report titled "2020 Strategic Analysis of Energy Storage in California." The report discusses the state of technology, policy, barriers to deployment and suggested reforms.
The conclusion of the report is that energy storage involves numerous technologies (see below), applications, regulatory entities, and grid processes. Given California's long-term goals for integrating variable renewable energy into the grid and decreasing greenhouse gas emissions, "policy makers must determine how energy storage can best fit with these future grid needs," the report states. "These leaders should begin by identifying the critical grid needs that energy storage technologies could address, developing a method for valuing the various energy storage technologies in these applications, and evaluating the policy options available to them to increase deployment of energy storage where the technologies, market value, and locational and environmental benefits offer the most value compared to the alternatives. With this framework, policy makers can determine how California can achieve an appropriate and cost-effective deployment of energy storage that meets all of the state's energy and environmental goals."The report also calls out suggested R&D priorities:
- Bulk energy storage demonstrations for variable renewable energy integration (for example, pumped hydro, concentrating solar power, and solar thermal).Field demonstrations of modular energy storage technologies (for example, batteries, flywheels) in various grid applications.
- Evaluation/demonstration of aggregated storage, for example Thermal energy storage HVAC or electric vehicle (EV) batteries, especially in a smart grid scenario.
- Develop simulations, analytical tools, and intelligent control systems for planning, designing, and dispatching energy storage devices for multiple applications and benefits.
- Quantification of costs and benefits of energy storage in grid applications.
- Modeling of the impact of 33 percent renewable energy on California's electricity grid to determine needs for energy storage to support the grid, including sensitivity analysis to address cost variables of storage and other needed energy resources, environmental impacts, and emerging smart grid performance enhancements.
Manufacturers are demonstrating modular technologies, such as flywheels and various forms of batteries, in grid applications. Key challenges relate primarily to cost (although the modularity of the technologies may offer promise for cost reduction through volume production), to the ability to manufacture and deploy on a large-scale basis, to durability, and because of limited experience in grid applications.
Electrochemical Energy Storage
Batteries take in electricity from another producing source, convert the electricity to chemical energy, and store it as a liquid or solution. When operators need energy from the battery, an electric charge chemically converts the energy back into electrons, which then move back into a power line on the electric grid.
Batteries used to store power from renewable energy sources must be reliable, durable, and safe. Ultimately, affordability will be a key to widespread deployment. There are several promising battery technologies for grid energy storage applications including advanced lead-acid, lithium-ion, flow, and sodium-sulfur batteries.
During discharge in a traditional lead-acid battery, sulfuric acid reacts with the lead anode (positive electrode) and cathode (negative electrode) to create lead sulfate. The process reverses during charge. This conversion produces a short, powerful burst of energy, such as needed to jump start a vehicle. Over time, a lead-acid battery can lose its charge due to the gradual crystallization and buildup of lead sulfate within the battery's core. The corrosive acid also can eat away at a battery’s core.
Lead-acid batteries are a mature and proven technology in use in a number of applications including frequency regulation, bulk energy storage for variable renewable energy integration, and distributed energy storage systems. Technology development of lead?acid battery technology is ongoing. Researchers have found that adding carbon to the battery seems to minimize or prevent the detrimental crystallization from occurring, thus improving the life cycle and overall lifespan of the battery. This technology has potential for storing renewable energy, but engineers must work to understand the technology's limitations and to find ways to bring down the cost.
In a Lithium-ion (Li-ion) battery cell, positively charged lithium ions migrate through a liquid electrolyte (fluids that conduct electricity) while electrons flow through an external circuit. Both move back and forth from one side to the other. This movement creates and stores energy. Li-ion batteries store energy in various compounds, composed of layers of different elements, such as lithium, manganese, and cobalt.
Li-ion batteries are most commonly found in consumer products and electric vehicles. The relatively high energy and power capacity offered by Li-ion batteries, when compared to other technologies, has made Li-ion batteries the most promising option for transportation applications such as electric vehicles. Developers are considering and demonstrating Li-ion batteries in the same applications as lead-acid batteries.
Although Li-ion batteries have been a success for small electronics such as cell phones and laptop computers, larger versions are expensive, prone to overheating, and susceptible to electrical shorting. While engineers have made substantial progress over recent years toward improving this technology, they will need to make further advances to extend life, improve safety, and reduce materials cost for this to be an attractive alternative for stationary applications.
A flow battery is a type of rechargeable battery that stores electrical energy in two tanks of electrolytes. When operators need energy, they pump liquid from one tank to another. During this slow and steady process, the technology converts the chemical energy from the electrolyte to electrical energy. When operators need to store energy, they reverse the process. The size of the tank and the amount of electrolyte the battery can hold determine the amount of energy the battery can store.
Flow batteries may be good candidates for backup energy storage up to 12 hours. They may also support integration of variable renewable energy. This technology has potential for grid applications if developers can manufacture it in a variety of sizes and make it portable and more affordable.
The sodium-sulfur battery uses sulfur combined with sodium to reversibly charge and discharge, using sodium ions layered in aluminum oxide within the battery's core. The battery shows potential to store lots of energy in a small space. In addition, its high energy density and rapid rate of charge and discharge make it an attractive candidate for applications that require short, potent bursts of energy.
Sodium-sulfur batteries are a commercial energy storage technology with applications in electric utility distribution grid support, wind power integration, and high-value electricity services. However, materials are expensive, and safety concerns remain with respect to the high operating temperature of the battery. Researchers believe that modifying the shape of the battery can improve efficiency, lower the operating temperature, and reduce cost.
Mechanical Energy Storage
Operators can store energy in water pumped to a higher elevation using pumped storage methods, in compressed air, or in spinning flywheels.
Pumped hydroelectric uses two water reservoirs, separated vertically. During off-peak hours, operators pump water from the lower reservoir to the upper reservoir. The operators reverse the water flow to generate electricity.
Pumped hydroelectric energy storage is a large, mature, and commercial utility-scale technology that utilities use at many locations in the United States and around the world. This application has the highest capacity of the various energy storage technologies that experts have assessed. However, pumped storage plants generally entail long construction times and high capital expenditure for both construction of the plants and needed transmission lines.
Compressed air energy storage technology stores low cost off?peak energy, in the form of compressed air, typically in an underground reservoir. Operators then heat the air with the exhaust heat of a standard combustion turbine and release it during peak load hours. Operators convert the heated air to energy through expansion turbines to produce electricity.
Compressed air energy storage systems suffer from reduced roundtrip efficiency associated with the cooling/reheating process. Air cooling between compression stages, although necessary, results in a loss of heat energy. Compressed air energy storage systems also produce carbon dioxide (CO2) emissions from the reheating process, usually performed by direct combustion with natural gas. Some compressed air energy storage systems under development, such as advanced adiabatic compressed air energy storage, use a thermal energy storage unit that absorbs heat from the hot compressed air and saves the heat energy for later use to reheat the air before expansion, thus avoiding CO2 emissions.
Flywheel energy storage works by accelerating a rotor (flywheel) to a very high speed, maintaining the energy in the system as rotational energy. When operators extract energy from the system, they reduce the flywheel’s rotational speed as a consequence of the principle of conservation of energy. Adding energy to the system correspondingly results in an increase in flywheel speed.
Developers have matured flywheel technology through the advent of strong, lightweight materials, microelectronics, and magnetic bearing systems. Manufacturers are currently developing and demonstrating megawatt?scale flywheel plants with cumulative capacities of 20 megawatts to maintain a uniform quality electricity supply often also termed as frequency regulation applications. Overall, manufacturers have proven flywheels to be an ideal form of energy storage due to their high efficiency, long life cycle, wide range of operating temperature, and higher power and energy density on both a mass and volume basis. FESs still present high costs and technology limitations, including modest energy storage capacity and efficiency losses associated with the bearings.
Thermal Energy Storage
Thermal energy storage comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. Operators can employ them to balance energy demand between daytime and nighttime. Operators maintain the thermal reservoir at a temperature above (hotter) or below (colder) than that of the ambient environment. The applications include concentrating sunlight to produce electricity from solar thermal energy during non-solar periods and the production of ice, chilled water, or salt solution at night, or hot water, which the devices use to cool / heat environments during the day.
Solar Thermal Storage Integration
The integration of thermal energy storage with solar energy offers a direct grid application for thermal energy storage. Unlike solar photovoltaic (PV) generation, concentrating solar power uses the thermal energy of sunlight to generate electricity. An advantage of concentrating solar power is the potential for storing solar thermal energy to be subsequently used during non?solar periods and to dispatch it as needed. Thermal energy storage allows concentrating solar power to achieve higher annual capacity factors from 25 percent without thermal storage up to 70 percent or more with thermal storage. Large concentrating solar power facilities using molten salt energy storage are in construction and/or operation in Spain and the United States. Plans are underway for facilities offering thousands of megawatts of additional generating capacity that will also use this storage technology.
Recent and ongoing improvements in solar thermal generation technologies, coupled with the need for more renewable sources of energy, have caused an increased interest in concentrating solar power. The key challenges lie in further cost reductions and perfecting designs to store solar heat later into the peak electrical period.
Thermal Storage for Heating, Ventilation, and Air Conditioning (HVAC)
In thermal energy storage systems, a device chills a storage medium during periods of low cooling demand and then uses the stored cooling to meet air-conditioning load or process cooling loads. The system consists of a storage medium, such as a water/ice system in a tank, a packaged chiller or built up refrigeration system, and interconnecting piping, pumps, and controls. Heating, ventilation, and air conditioning (HVAC) thermal energy storage systems shift cooling energy use to nonpeak times.
Thermal energy storage for commercial HVAC systems is a mature technology. The key to maximizing the effectiveness of such systems to shift cooling load and thus support the electric grid is appropriate engineering design and implementation.
Hydrogen as an Energy Storage System
Hydrogen as an energy storage system involves four processes. First, a device must produce hydrogen. In a grid energy storage application, the most appropriate production technology is the electrolysis of water using electricity. Second, after electrolysis produces the hydrogen, a device must store it, either in gaseous or liquid form. Third, in many instances, the hydrogen must be transported by truck or pipeline to a distant location. Fourth, to return electric power to the grid, the devices must convert hydrogen to electricity by either a fuel cell or a combustion engine or gas turbine generator.
The primary limitations of hydrogen energy storage systems include the maturity of the fuel cell technology; the durability of fuel cells and electrolyzers; and the capital cost of fuel cells, electrolyzers, and, to a lesser extent, storage vessels. The scale of fuel cells and electrolyzers with respect to grid storage applications and the efficiency of fuel cells and electrolyzers also limits the use of the technology, with roundtrip (electricity into the system to produce hydrogen relative to the electricity produced by the hydrogen fuel cell) energy efficiencies of 31-35 percent.