ENERGY STORAGE TYPES
Energy storage devices can be categorized as mechanical, electrochemical, chemical, electrical, or thermal devices, depending on the storage technology used (Figure 1.1). Mechanical technology, including pumped hydropower generation, is the oldest technology. However, a limitation of this technology is its need for abundant water resources and a different geographic elevation, as well as the construction of power transmission lines to households that consume electricity. Recently, transmission-line construction cost has surpassed the cost of installing a pumped hydropower generation facility.
In addition to the recent spread of mobile information technology (IT) devices and electric vehicles, the increased mass production of lithium secondary batteries and their lowered costs have boosted demand for energy storage devices using such batteries. Lithium secondary batteries convert electric energy to chemical energy, and vice versa, using electrochemical technologies. Such technologies also include lead storage batteries and sodium–sulfur batteries. Chemical technologies include energy storage technologies such as fuel cells, and mechanical technologies include electric double-layer capacitors.
The performance of energy storage devices can be defined by their output and energy density.Their use can be differentiated by place and duration of use, as defined by the technology adopted.In Figure 1.2, the applications (in the tan-colored boxes) are classified according to output, usage period, and power requirement, and the energy storage devices (in the amber-colored boxes) according to usage period, power generation, and system and/or network operation.
Energy storage devices can be used for uninterruptible power supply (UPS), transmission and distribution (T&D) system support, or large-scale generation, depending on the technology applied and on storage capacity. Among electrochemical, chemical, and physical energy storage devices, the technologies that have received the most attention recently fall within the scope of UPS and T&D system support (Figure 1.3). Representative technologies include reduction–oxidation (redox) flow, sodium–sulfur (Na–S), lead–acid and advanced lead–acid, super-capacitor, lithium, and flywheel batteries. Lithium batteries are in common use today.
Battery technologies for energy storage devices can be differentiated on the basis of energy density, charge and discharge (round trip) efficiency, life span, and eco-friendliness of the devices (Figure 1.4). Energy density is defined as the amount of energy that can be stored in a single system per unit volume or per unit weight. Lithium secondary batteries store 150–250 watt-hours per kilogram (kg) and can store 1.5–2 times more energy than Na–S batteries, two to three times more than redox flow batteries, and about five times more than lead storage batteries.
Charge and discharge efficiency is a performance scale that can be used to assess battery efficiency. Lithium secondary batteries have the highest charge and discharge efficiency, at 95%, while lead storage batteries are at about 60%–70%, and redox flow batteries, at about 70%–75%.
One important performance element of energy storage devices is their life span, and this factor has the biggest impact in reviewing economic efficiency. Another major consideration is eco-friendliness, or the extent to which the devices are environmentally harmless and recyclable.
Technological changes in batteries are progressing toward higher energy density (Figure 1.5). Next-generation battery technologies—lithium-ion, zinc–air, lithium–sulfur, lithium–air, etc.—are expected to improve on the energy density of lithium secondary (rechargeable) batteries, and be priced below $50 per kilowatt (kW).
Energy storage device applications vary depending on the time needed to connect to the generator, transmitter, and place of use of energy, and on energy use. Black start, a technology for restarting generators after blackouts without relying on the external power grid, is installed in the generating bus and supplies energy within 15–30 minutes. Power supply for maintaining frequency is provided within a quarter-hour to an hour of system operation. Power supply for maintaining voltage level is provided within a shorter operating interval. Grid storage needs are categorized in Figure 1.6 according to network function, power market, and duration of use. Table 1.1 compares the various battery technologies according to discharge time and energy-to-power ratio.