Production process and operating characteristics of nuclear power plants and other energy power plants

Production process and operating characteristics of nuclear power plants and other energy power plants
  1. Nuclear power plants

A nuclear power plant is a power plant that uses the energy released by nuclear fission to generate electricity, and the nuclear fuel used is the isotope of U-235.
The production process of a nuclear power plant is shown in Figure 1.

Figure 1 - Schematic diagram of the production process of a nuclear power plant
Figure 1 – Schematic diagram of the production process of a nuclear power plant

Nuclear power plants have two circulation systems:
(1) The primary loop system consisting of 1-2-3-4 (ie, the coolant circulation system) The role of the coolant is to cool the reactor, and as a working medium for heat transfer, it releases heat in the steam boiler to generate steam. Most of the nuclear power plants built in China use light water (H2O) as the coolant, and a few use heavy water (D2O) as the coolant. The fission of nuclear fuel in a nuclear reactor releases heat. The coolant flows in the system driven by a circulating pump, is heated as it flows through the nuclear reactor, and transfers heat to the secondary loop system as it passes through the steam boiler. The regulator is used to regulate the pressure of the primary loop system to ensure that the loop operates under a stable pressure.

The primary loop system of a nuclear power plant consists of multiple parallel loops. At present, the power of a single loop can be described as 300-600 MW, and the power of one reactor is about 1000 MW.

(2) Secondary circuit system consisting of 3-5-6-7 If the steam boiler is regarded as the boiler of a conventional thermal power plant, the secondary circuit system is similar to the steam power cycle system of the thermal power plant. The location of the primary circuit system equipment is called the nuclear island, and the location of the secondary circuit system equipment is called the conventional island.

The reaction intensity of a nuclear reactor is controlled by control elements that can significantly absorb neutrons and the direct addition of neutron-absorbing substances in the core.
The operation characteristic of a nuclear power plant is that it operates with a fixed load and does not participate in the regulation of the system, and its purpose is to ensure the safety of the nuclear reactor.
The total installed capacity of nuclear power plants is relatively large, which brings inconvenience to the operation and dispatch of the power system.

  1. Other energy power plants
    In order to protect the natural environment and consider sustainable development strategies, the world is vigorously developing renewable and clean energy power generation, the most important ones are wind, solar, geothermal and tidal power generation.

2.1 Wind power generation
Converting wind energy into electrical energy is called wind power generation. Figure 2 shows the structure of a wind turbine.

Figure 2 - Wind Turbine Structure
Figure 2 – Wind Turbine Structure

Wind turbines use their blades to absorb wind energy to generate electricity. Generally, 3~24m/s is the working wind speed of the fan. The ideal fan can keep the output power approximately constant within the range of wind speed of 12~24m/s. Since the rotor of the asynchronous generator has no external excitation system, the structure is simple and the operation is reliable, and it has been widely used in wind power generation.

The wind turbine control system implements normal operation, regulation and protection of the entire wind turbine. The control system of the wind turbine is a very complex system, which is realized by the computer. The single-unit capacity of wind turbines is not large (currently, it is generally about 1000kW, and the maximum capacity is about 3000kW), so a group of wind turbines are built in an area, which is called a wind farm.

2.2 Solar power generation
Converting sunlight energy into electricity is called solar power generation. FIG. 3 is a schematic diagram of a solar cell power generation system.

Figure 3 - Schematic diagram of solar cell power generation
Figure 3 – Schematic diagram of solar cell power generation

Devices that can generate electromotive force under sunlight are called photovoltaic devices. Semiconductor PN junction devices are photovoltaic devices with high photoelectric conversion efficiency, and the photoelectric conversion units composed of such devices are called solar cells.

The smallest unit of a solar cell is called a solar cell. The working voltage of the monomer is only 0.45~0.5V, and the working current density is about 20~25mA/cm2. A solar cell module is formed by connecting multiple monomers in series, parallel and encapsulation, and the power can reach several hundred watts. A solar cell array is formed by connecting multiple modules in series and parallel. A solar power station is formed by connecting a plurality of square arrays in parallel. Such a power station that directly converts solar energy into electrical energy is often called a photovoltaic power station. The role of the charge controller is to prevent overcharge and overdischarge of the battery.

Solar cells were first used in space flight systems. After the 1970s, it has also been widely used on the ground, especially to solve the problem of power supply in remote areas. The advantages of solar cell power generation are no noise, no pollution, low failure rate, easy maintenance, and unattended operation. The biggest hurdle at the moment is high cost, which is expected to drop significantly by the mid-21st century, making it competitive with conventional energy sources.

2.3 Geothermal power generation
A power station that uses high-temperature geothermal resources to generate electricity is called a geothermal power station. Figure 4 shows a common production process of a geothermal power station.

Figure 4 - Schematic diagram of the production process of a geothermal power plant
Figure 4 – Schematic diagram of the production process of a geothermal power plant

The hot water pump pumps the underground hot water into the expansion container. Since the pressure in the expansion container is lower than the pressure of the hot water, the hot water suddenly expands and evaporates, and part of it becomes steam, which is called flash evaporation. The generated steam enters the steam turbine to generate power. The cold water pump beats the cold water into the condenser, so that the steam after work becomes condensed water, which ensures a lower temperature and pressure at the end of the steam turbine, and improves the conversion efficiency of the thermal energy of the steam in the steam turbine to the mechanical energy. The wastewater is returned to the ground through the recharge well.

Tibet Yangbajing Geothermal Power Plant is the first commercial geothermal power plant built by China itself, with an installed capacity of 25.18MW (of which the first power plant is 10MW and the second power plant is 15.18MW).

2.4 Tidal power generation
A power plant that builds a dam at the entrance of a bay or an estuary to form a reservoir and uses the water level difference on both sides of the dam when the tide rises and falls is called a tidal power plant.
Tidal power plants are divided into one-way power generation and two-way power generation. Only when the tide is low, power generation is called one-way power generation, which is a more commonly used method.

The operating conditions of the one-way power generation tidal power plant in a tidal period are divided into four stages: water filling, waiting for ebb tide, power generation, and waiting for high tide.
(1) Water filling The rising tide enters the reservoir through the sluice gate and closes the gate to store water when the water level on both sides of the dam is level.
(2) Wait for the ebb tide Wait for the water level outside the dam to recede.
(3) Power generation When the water level outside the dam drops and reaches the optimum water head for power generation, the hydro-generator unit starts to generate electricity until the water level difference between the two sides of the dam is less than the minimum water head required for the unit to generate electricity.
(4) Waiting for high tide Wait for the next high tide to fill with water.

Read more: Power Transmission Design Principles

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