An independent PV power system is not connected to a traditional electric power system and is mostly deployed in remote off-grid areas to meet local demands. PV electrification is possible only during the daytime, while power is required around the clock. To solve this issue, independent PV systems must be provided with ES. Figure 3.1 shows the structure of an independent PV power system, mainly consisting of solar cell array, DC combiner box, controller, battery, off-grid inverter, and AC distribution box. The PV components produce direct current to charge the battery and, after DC to AC conversion by the inverter, serve the AC loads.

A grid-connected PV power system is connected to the grid and injects electricity to the grid. It is the mainstream of PV power systems. Grid-connected PV power systems can be further divided into distributed type and centralized type. The former is a type of DG in a microgrid, in which electricity is directly distributed to users and the surplus or deficit is regulated by the grid. The latter is a PV power system that directly injects electricity to the grid for distribution to users. The structures of these two types are essentially the same.

An independent wind power system is not connected to a traditional electric power system and is mostly deployed in remote off-grid areas to meet local demand. To resolve intermittency of wind-produced electricity, such a system must be provided with ES. Figure 3.3 shows the structure of an independent wind power system, mainly including wind turbine generator, rectifier, controller, battery, off-grid inverter, and AC distribution box. The alternating current from the generator is first converted to direct current by the rectifier to charge the battery and then converted back to alternating current by the inverter to serve AC loads.

A grid-connected wind power system is connected to a grid and injects electricity to it. It is the mainstream of wind power systems. Grid-connected wind power systems can be further divided into distributed type and centralized type. The former is a type of DG in a microgrid, in which electricity is directly distributed to users and the surplus or deficit is regulated by the grid. The latter is a wind power system that directly injects electricity to the grid for centralized distribution to users. The structures of the two types are essentially the same.

The high-speed microturbine is of a simple radial flow design, integrates the circulating reheat technology, and features high reliability, low maintenance cost, low-amplitude vibration, low emissions, and compact structure. It is mainly composed of a single-stage radial compressor, low-emission ring combustor, single-stage radial turbine, compression ratio, and air bearing or bearing with dual lubricating systems.

The high-speed generator and microturbine are mounted on the same shaft. Thanks to its small size, the generator can be placed inside the microturbine to constitute a compact and high-speed turbine AC generator.

The regenerator increases the efficiency of the microturbine, making the microturbine more competitive than a reciprocating generator. It preheats the air to be injected to the combustor and thus reduces fuel consumption.

The high-frequency alternating current produced from the generator must be converted to power-frequency alternating current by the power electronic converter under the control of a microprocessor. The power electronic converter can adjust the speed against load variation of the generator or the grid, or operate as an independent power source. The system facilitates remote management, control, and monitoring, as shown in Figure 3.8.

Figure 3.9 shows the structure of the microturbine control system, in which the regenerator is controlled by adjusting the opening of the active valve and bypass valve to control heat exchange between exhaust gas and clean air (i.e., regenerator effectiveness), so as to control the temperatures of the gas to enter the combustor from the compressor and the exhaust gas and thus improve energy efficiency.

Producing electricity is the major means to utilize geothermal energy. Similar to thermal power generation, in geothermal generation, the thermal energy of steam is converted to mechanical energy in the turbine to drive the generator to produce power. The difference is that geothermal generation does not require a huge boiler and uses geothermal sources instead of fuels. Geothermal power systems may be of flash steam type or binary cycle type, depending on the temperature of the geothermal source. Moreover, total flow generation and dry hot rock generation are currently under study.

In the broad sense, biomass is the organic matter generated from the photosynthesis process of plants and therefore, its energy is converted from solar energy. Biomass energy may come from agricultural, forestry, and fisheries resources such as crops, wood, and seaweed, or industrial organic waste such as pulp waste, black liquid from paper making, and residue from alcoholic fermentation, or common municipal waste such as household garbage and paper scraps, or residual sludge from sewage treatment plants. Biomass power generation comes in three forms: (1) biomass combustion, (2) biomass gasification, and (3) methane combustion.

Ocean energy, a kind of renewable energy generated from the motion of seawater, mainly includes thermal gradient energy, tide energy, wave energy, tidal current energy, sea current energy, and salinity gradient energy. Accordingly, electrification technologies using ocean energy include tide electrification, sea current electrification, tidal current electrification, wave electrification, and thermal gradient electrification.

Pumped storage is a physical ES form. To use this form, an upstream reservoir and a downstream reservoir are required at the storage station. During off-peak hours, the pumped storage device operates as a motor to pump the water from the downstream reservoir to the upstream; and during peak hours, the device operates to produce electricity using the water in the upstream reservoir. A pumped storage station could have a capacity of energy for release for a few hours to a few days, with an efficiency ranging from 70% to 85%. Among all ES forms, this is the most widely used form in power systems, mainly for load shifting, frequency and phase modulation, emergency reserve, black-start, and system reserve. It can also improve the efficiency of thermal and nuclear power stations.

CAES is a physical ES form realized by means of gas turbine power generation and is intended for load shifting. It works like this: during off-peak hours, the excess power of the grid is used to compress the air, which is then stored in a typical 7.5 MPa sealed container and released during peak hours to drive the turbine to generate power. During the power generation, 2/3 of the gas is used to compress air, reducing gas consumption by 1/3, and the gas consumption is 40% less than that of a conventional gas turbine. CAES makes cold start and black-start possible, and can respond immediately. It is mainly for power regulation, load balance, frequency modulation, and distributed ES and reserve of power systems.

Flywheel is a physical ES form. A flywheel ES system is composed of a high-speed flywheel, bearing support system, motor/generator, power converter, electronic control system, and optional equipment including vacuum pump and backup bearing. During off-peak hours, the grid energizes the flywheel to rotate fast, thus realizing conversion from electricity to mechanical energy; during peak hours, the high-speed flywheel works as a prime mover for the generator to produce electricity and output current and voltage via the power converter, thus realizing conversion from mechanical energy to electricity. The flywheel is mainly used for uninterrupted power supply (UPS), emergency power system (EPS), load shifting, and frequency control.

SMES, an electromagnetic ES form, uses superconducting coils to store magnetic energy and requires no energy conversion for power transmission. It has such advantages as quick response (within several ms), high conversion efficiency, high specific capacity/power, and real-time large-capacity energy exchange with and power compensation for the electric power system. It is mainly used for voltage support, power compensation, and frequency regulation of TD networks to improve system stability and transmission capacity.

A supercapacitor, an electromagnetic ES form, is developed based on the electromagnetic double-layer theory and can provide very high pulse power. In the charging process, the charges on the surface of an ideally polarized electrode will attract the ions of opposite polarity in the electrolyte to the electrode surface, forming an electric double-layer and thus an electric double-layer capacitor. As the spacing between the two layers is very small, generally less than 0.5 mm, and the electrode is of a special structure, the surface area of the electrode increases by tens of thousands of times, creating an extremely large capacitance. However, due to the low voltage withstand level of the dielectric medium and occurrence of current leakage, the amount and duration of energy storage are limited, and supercapacitors must be series-connected to increase the volume of the charge and discharge control circuit and the system. The supercapacitor is mainly used for short-time and high-power load shifting, for reliable supply following a voltage drop and instantaneous disturbance, and in situations of high peak power, for example, support for startup and dynamic voltage recovery of a high-power DC motor.

Battery is a chemical ES form and comes in many types, mainly lead-acid battery, nickel-cadmium battery, lithium-ion battery, sodium sulfur battery, and vanadium redox flow battery.