An overview of hydroelectric power generation

Hydropower is to convert the water energy of natural rivers into electricity for people to use. There are various sources of energy used in power generation, such as solar energy, water power in rivers, and wind power generated by air flow. The cost of hydropower generation using hydropower is cheap, and the construction of hydropower stations can also be combined with other water conservancy projects. Our country is very rich in hydropower resources and the conditions are also very good. Hydropower plays an important role in the construction of the national economy.
The upstream water level of a river is higher than its downstream water level. Because of the difference in the water level of the river, water energy is generated. This energy is called potential energy or potential energy. The difference between the height of the river water is called the drop, also called the water level difference or the water head. This drop is a basic condition for the formation of hydraulic power. In addition, the magnitude of hydraulic power also depends on the magnitude of the flow of water in the river, which is another basic condition as important as the drop. Both the drop and the flow directly affect the hydraulic power; the larger the water volume of the drop, the greater the hydraulic power; if the drop and the water volume are relatively small, the output of the hydropower station will be smaller.
The drop is generally expressed in meters. Gradient is the ratio of drop and distance, which can indicate the degree of drop concentration. The drop is more concentrated, and the use of hydraulic power is more convenient. The drop used by a hydropower station is the difference between the upstream water surface of the hydropower station and the downstream water surface after passing through the turbine.

Flow is the amount of water flowing in a river per unit of time, and it is expressed in cubic meters in one second. One cubic meter of water is one ton. The flow of a river changes at any time, so when we talk about the flow, we must explain the time of the specific place it flows. The flow changes very significantly in time. The rivers in our country generally have a large flow in the rainy season in summer and autumn, and relatively small in winter and spring. Generally, the flow of the river is relatively small in the upstream; because the tributaries merge, the downstream flow gradually increases. Therefore, although the upstream drop is concentrated, the flow is small; the downstream flow is large, but the drop is relatively scattered. Therefore, it is often most economical to utilize hydraulic power in the middle reaches of the river.
Knowing the drop and flow used by a hydropower station, its output can be calculated using the following formula:
In the formula, N–output, in kilowatts, can also be called power;
Q–flow, in cubic meters per second;
H – drop, in meters;
G = 9.8 , is the acceleration of gravity, unit: Newton/kg
According to the above formula, the theoretical power is calculated without deducting any losses. In fact, in the process of hydropower generation, turbines, transmission equipment, generators, etc. all have inevitable power losses. Therefore, the theoretical power should be discounted, that is, the actual power we can use should be multiplied by the efficiency coefficient (symbol: K).
The designed power of the generator in the hydropower station is called the rated power, and the actual power is called the actual power. In the process of energy transformation, it is inevitable to lose a part of the energy. In the process of hydropower generation, there are mainly losses of turbines and generators (there are also losses in pipelines). The various losses in the rural micro-hydropower station account for about 40-50% of the total theoretical power, so the output of the hydropower station can actually only use 50-60% of the theoretical power, that is, the efficiency is about 0.5-0.60 (of which the turbine efficiency is 0.70-0.85 , the efficiency of generators is 0.85 to 0.90, and the efficiency of pipelines and transmission equipment is 0.80 to 0.85). Therefore, the actual power (output) of the hydropower station can be calculated as follows:
K–the efficiency of the hydropower station, (0.5~0.6) is used in the rough calculation of the micro-hydropower station; this value can be simplified as:
N=(0.5~0.6)QHG Actual power=efficiency×flow×drop×9.8
The use of hydropower is to use water power to propel a machine, which is called a water turbine. For example, the ancient waterwheel in our country is a very simple water turbine. The various hydraulic turbines currently used are adapted to various specific hydraulic conditions, so that they can rotate more efficiently and convert water energy into mechanical energy. Another kind of machinery, a generator, is connected to the turbine, so that the rotor of the generator rotates with the turbine to generate electricity. The generator can be divided into two parts: the part that rotates with the turbine and the fixed part of the generator. The part that is connected to the turbine and rotates is called the rotor of the generator, and there are many magnetic poles around the rotor; a circle around the rotor is the fixed part of the generator, called the stator of the generator, and the stator is wrapped with many copper coils. When many magnetic poles of the rotor rotate in the middle of the copper coils of the stator, a current is generated on the copper wires, and the generator converts mechanical energy into electrical energy.
The electric energy generated by the power station is transformed into mechanical energy (electric motor or motor), light energy (electric lamp), thermal energy (electric furnace) and so on by various electrical equipment.
he composition of the hydropower station
The composition of a hydropower station includes: hydraulic structures, mechanical equipment, and electrical equipment.
(1) Hydraulic structures
It has weirs (dams), intake gates, channels (or tunnels), pressure fore tanks (or regulating tanks), pressure pipes, powerhouses and tailraces, etc.
A weir (dam) is built in the river to block the river water and raise the water surface to form a reservoir. In this way, a concentrated drop is formed between the water surface of the reservoir on the weir (dam) and the water surface of the river below the dam, and then the water is introduced into the hydroelectric power station through the use of water pipes or tunnels. In relatively steep rivers, the use of diversion channels can also form a drop. For example: Generally, the drop per kilometer of a natural river is 10 meters. If a channel is opened at the upper end of this section of the river to introduce river water, the channel will be excavated along the river, and the slope of the channel will be flatter. If the drop in the channel is made per kilometer It only dropped 1 meter, so that the water flowed 5 kilometers in the channel, and the water surface only fell 5 meters, while the water fell 50 meters after traveling 5 kilometers in the natural channel. At this time, the water from the channel is led back to the power plant by the river with a water pipe or tunnel, and there is a concentrated drop of 45 meters that can be used to generate electricity. Figure 2

The use of diversion channels, tunnels or water pipes (such as plastic pipes, steel pipes, concrete pipes, etc.) to form a hydropower station with a concentrated drop is called a diversion channel hydropower station, which is a typical layout of hydropower stations.
(2) Mechanical and electrical equipment
In addition to the above-mentioned hydraulic works (weirs, channels, forecourts, pressure pipes, workshops), the hydropower station also needs the following equipment:
(1) Mechanical equipment
There are turbines, governors, gate valves, transmission equipment and non-generating equipment.
(2) Electrical equipment
There are generators, distribution control panels, transformers and transmission lines.
But not all small hydropower stations have the above-mentioned hydraulic structures and mechanical and electrical equipment. If the water head is less than 6 meters in the low-head hydropower station, the water guide channel and the open channel water channel are generally used, and there is no pressure forepool and pressure water pipe. For power stations with small power supply range and short transmission distance, direct power transmission is adopted and no transformer is required. Hydropower stations with reservoirs do not need to build dams. The use of deep intakes, dam inner pipes (or tunnels) and spillways eliminates the need for hydraulic structures such as weirs, intake gates, channels and pressure fore-pools.
To build a hydropower station, first of all, careful survey and design work must be carried out. In the design work, there are three design stages: preliminary design, technical design and construction detailing. In order to do a good job in the design work, it is first necessary to carry out thorough survey work, that is, to fully understand the local natural and economic conditions – ie topography, geology, hydrology, capital and so on. The correctness and reliability of the design can be guaranteed only after mastering these situations and analyzing them.
The components of small hydropower stations have various forms depending on the type of hydropower station.
3. Topographic Survey
The quality of the topographic survey work has a great influence on the engineering layout and the estimation of the engineering quantity.
Geological exploration (understanding of geological conditions) in addition to general understanding and research on the geology of the watershed and along the river, it is also necessary to understand whether the foundation of the machine room is solid, which directly affects the safety of the power station itself. Once the barrage with a certain reservoir volume is destroyed, it will not only damage the hydropower station itself, but also cause huge loss of life and property downstream.
4. Hydrological test
For hydropower stations, the most important hydrological data are records of river water level, flow, sediment content, icing conditions, meteorological data and flood survey data. The size of the river flow affects the layout of the spillway of the hydropower station. Underestimating the severity of the flood will cause the damage of the dam; the sediment carried by the river can quickly fill the reservoir in the worst case. For example, the inflow channel will cause the channel to silt up, and the coarse-grained sediment will pass through the turbine and cause wear of the turbine. Therefore, the construction of hydropower stations must have sufficient hydrological data.
Therefore, before deciding to build a hydropower station, we must first investigate the direction of economic development in the power supply area and the future demand for electricity. At the same time, estimate the situation of other power sources in the development area. Only after research and analysis of the above situation can we decide whether the hydropower station needs to be built and how big the scale should be.
In general, the purpose of hydropower survey work is to provide accurate and reliable basic information necessary for the design and construction of hydropower stations.
5. General conditions for site selection
The general conditions for selecting a site can be explained from the following four aspects:
(1) The selected site should be able to utilize water energy in the most economical way and comply with the principle of cost saving, that is, after the power station is completed, the least amount of money is spent and the most electricity is generated. It can usually be measured by estimating the annual power generation revenue and the investment in the construction of the station to see how much time the invested capital can be recovered. However, the hydrological and topographical conditions are different in different places, and the electricity needs are also different, so the construction cost and investment should not be limited by certain values.
(2) The topographic, geological and hydrological conditions of the selected site should be relatively superior, and there should be possibilities in design and construction. In the construction of small hydropower stations, the use of building materials should be in accordance with the principle of “local materials” as much as possible.
(3) The selected site is required to be close to the power supply and processing area as much as possible to reduce the investment of power transmission equipment and the loss of power.
(4) When selecting the site, the existing hydraulic structures should be used as much as possible. For example, the water drop can be used to build a hydropower station in an irrigation channel, or a hydropower station can be built next to an irrigation reservoir to generate electricity from the irrigation flow, and so on. Because these hydropower plants can meet the principle of generating electricity when there is water, their economic significance is more obvious.

Post time: May-19-2022

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