COMPONENTS OF AN ENERGY STORAGE SYSTEM
The various components of a battery energy storage system are shown in the Figure 1.7 Schematic.
ESS components (Figure 1.8) are grouped according to function into battery components, components required for reliable system operation, and grid connection components..
However, high penetration creates power transmission instability challenges, thus Grid Operators require stringent dynamic and static grid support features for Power Conversion Systems. For a stable transmission and distribution, the power grid operators need a real-time match between electricity supply and consumption.
The battery system consists of the battery pack, which connects multiple cells to appropriate voltage and capacity; the battery management system (BMS); and the battery thermal management system (B-TMS). The BMS protects the cells from harmful operation, in terms of voltage, temperature, and current, to achieve reliable and safe operation, and balances varying cell states-of-charge (SOCs) within a serial connection. The B-TMS controls the temperature of the cells according to their specifications in terms of absolute values and temperature gradients within the pack.
The components required for the reliable operation of the overall system are system control and monitoring, the energy management system (EMS), and system thermal management. System control and monitoring is general (IT) monitoring, which is partly combined into the overall supervisory control and data acquisition (SCADA) system but may also include fire protection or alarm units. The EMS is responsible for system power flow control, management, and distribution. System thermal management controls all functions related to the heating, ventilation, and air-conditioning of the containment system.
The power electronics can be grouped into the conversion unit, which converts the power flow between the grid and the battery, and the required control and monitoring components—voltage sensing units and thermal management of power electronics components (fan cooling).
BATTERY CHEMISTRY TYPES
Lead–Acid (PbA) Battery
This type of secondary cell is widely used in vehicles and other applications requiring high values of load current. Its main benefits are low capital costs, maturity of technology, and efficient recycling.
Lithium-Ion (Li-Ion) Battery
Li-ion battery chemistries have the highest energy density and are considered safe. No memory or scheduled cycling is required to prolong battery life. Li-Ion batteries are used in electronic devices such as cameras, calculators, laptop computers, and mobile phones, and are increasingly being used for electric mobility. Their advantages and disadvantages are summarized in Table 1.7.
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.