1、 Overview of hydropower generation
Hydroelectric power generation is to convert the water energy of natural rivers into electric energy for people to use. The energy sources used by power stations are diverse, such as solar energy, water power of 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 undertakings. China is rich in water resources and has excellent conditions. Hydropower plays an important role in national economic construction.
The upstream water level of a river is higher than its downstream water level. Because of the difference between 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 river water surface is called drop, also called water level difference or head. This drop is a basic condition for hydraulic power. In addition, the size of water power also depends on the size of water flow in the river, which is another basic condition as important as the drop. Both drop and discharge directly affect the size of hydraulic power; The greater the water fall, the greater the hydraulic power; If the drop and water volume are relatively small, the output of the hydropower station will be smaller.
The drop is generally expressed in meters. Water surface gradient is the ratio of drop and distance, which can indicate the degree of concentration of drop. If the drop is relatively concentrated, the utilization of water 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 hydraulic turbine.
Flow is the amount of water flowing through a river in a unit time, expressed in cubic meters per second. A cubic meter of water is one ton. The flow of a river changes at any time and anywhere, so when we talk about the flow, we must explain the time of the specific place where it flows. The flow changes significantly in time. In general, rivers in China have large flow in summer, autumn and rainy season, but small flow in winter and spring. The flow varies from month to day, and the water volume varies from year to year. The flow of general rivers is relatively small in the upstream; As the tributaries converge, the downstream flow gradually increases. Therefore, although the upstream drop is concentrated, the flow is small; Although the downstream flow is large, the drop is relatively dispersed. Therefore, it is often the most economical to use water power in the middle reaches of the river.
Knowing the drop and flow utilized by a hydropower station, its output can be calculated with the following formula:
N= GQH
In the formula, N – output, unit: kW, also called power;
Q — flow, in cubic meters per second;
H — Drop, in meters;
G=9.8, is the acceleration of gravity, in Newton/kg
The theoretical power is calculated according to the above formula, and no loss is deducted. In fact, in the process of hydropower generation, water turbines, transmission equipment, generators, etc. 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 generator in hydropower station is called rated power, and the actual power is called actual power. In the process of energy transformation, it is inevitable to lose some energy. In the process of hydropower generation, there are mainly losses of hydraulic turbines and generators (including losses of pipelines). In rural micro hydropower stations, various losses account for 40~50% of the total theoretical power, so the output of hydropower stations can only use 50~60% of the theoretical power, that is, the efficiency is about 0.5~0.60 (including the turbine efficiency of 0.70~0.85, the generator efficiency of 0.85~0.90, and the pipe and transmission equipment efficiency of 0.80~0.85). Therefore, the actual power (output) of the hydropower station can be calculated as follows:
K – efficiency of hydropower station, (0.5~0.6) is adopted for rough calculation of micro hydropower station; The above formula can be simplified as:
N=(0.5 ～ 0.6) QHG actual power=efficiency × flow × Drop × nine point eight
The use of hydropower is to use water to drive a kind of machinery, which is called water turbine. For example, the ancient waterwheel in China is a very simple water turbine. The various hydraulic turbines used now are adapted to various specific hydraulic conditions, so that they can rotate more effectively and turn water energy into mechanical energy. Another machine, the generator, is connected to the water turbine to make the rotor of the generator rotate with the water turbine, and then electricity can be generated. The generator can be divided into two parts: the part that rotates together with the hydraulic turbine and the fixed part of the generator. The part that rotates together with the hydraulic turbine 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, which is called the stator of the generator. The stator is wrapped with many copper coils. When many magnetic poles of the rotor rotate in the middle of the stator copper coil, current will be generated on the copper wire, and the generator is to convert mechanical energy into electrical energy.
The electric energy generated by the power station is transformed from various electrical equipment into mechanical energy (motor or motor), light energy (electric lamp), heat energy (electric furnace), etc.
2、 Composition of hydropower station
The hydropower station consists of hydraulic structures, mechanical equipment and electrical equipment.
（1） Hydraulic structures
It includes weir (dam), intake gate, channel (or tunnel), forebay (or regulating tank), penstock, power house and tailrace, etc.
Build a weir (dam) in the river to block the river, raise the water surface and form a reservoir. In this way, a concentrated drop is formed from the water surface of the reservoir on the weir (dam) to the water surface of the river under the dam, and then water is introduced into the hydropower station through water pipes or tunnels. In the steep river channel, the use of diversion channels can also form a drop. For example, the drop of a natural river is 10 meters per kilometer. If a channel is opened at the upper end of this section of river to introduce water, the channel will be excavated along the river, and the gradient of the channel will be flat. If the drop in the channel is only 1 meter per kilometer, the water will flow 5 kilometers in the channel, and the water will fall only 5 meters, while the water will fall 50 meters after walking 5 kilometers in the natural river. At this time, the water in the channel is led back to the power house by the river with water pipes or tunnels, and there is a 45m concentrated drop that can be used to generate electricity.
A hydropower station that uses diversion channels, tunnels or water pipes (such as plastic pipes, steel pipes, concrete pipes, etc.) to form a concentrated drop is called a diversion channel type hydropower station, which is a typical layout of hydropower stations.
（2） Mechanical and electrical equipment
In addition to the above hydraulic works (weir, canal, forebay, penstock and powerhouse), the hydropower station also needs the following equipment:
(1) Mechanical equipment
There are hydraulic turbines, governors, gate valves, transmission equipment and non power generation equipment.
(2) Electrical equipment
There are generators, distribution control panels, transformers, transmission lines, etc.
However, not all small hydropower stations have the above hydraulic structures and mechanical and electrical equipment. If the low head hydropower station with water head less than 6 meters generally adopts the way of diversion channel and open channel diversion chamber, there will be no forebay and penstock. The power station with small power supply range and short transmission distance adopts direct transmission without transformer. Hydropower stations with reservoirs do not need to build dams. The deep water inlet is adopted, and the inner pipe (or tunnel) and spillway of the dam are not required to use hydraulic structures such as weir, intake gate, channel and forebay.
To build a hydropower station, careful survey and design should be carried out first. There are three design stages in the design: preliminary design, technical design and construction details. In order to do a good job in design, we must first carry out thorough survey, that is, fully understand the local natural and economic conditions — that is, topography, geology, hydrology, capital, etc. The correctness and reliability of the design can only be guaranteed after mastering these conditions and analyzing them.
The components of small hydropower stations have various forms according to different types of hydropower stations.
3、 Topographic survey
The quality of topographic survey has a great influence on the project layout and estimation of quantities.
Geological exploration (understanding of geological conditions) requires not only general understanding and research on the basin geology and riverside geology, but also understanding whether the machine room foundation 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 losses of life and property in the downstream. Therefore, the geological selection of the forebay is generally put in the first place.
4、 Hydrometry
For hydropower stations, the most important hydrological data are records of river water level, flow, sediment concentration, icing, meteorological data and flood survey data. The size of the river flow affects the layout of the spillway of the hydropower station, and the severity of the flood is underestimated, which will lead to the destruction of the dam; The sediment carried by the river can fill the reservoir quickly in the worst case. For example, inflow into the channel will cause channel siltation, and coarse sediment will pass through the hydraulic turbine and cause wear of the hydraulic turbine. Therefore, the construction of hydropower stations must have sufficient hydrological data.
Therefore, before deciding to build a hydropower station, it is necessary to investigate and study the direction of economic development and the future demand for electricity in the power supply area. At the same time, estimate the situation of other power sources in the development area. Only after studying and analyzing the above conditions can we decide whether the hydropower station needs to be built and how large the construction scale should be.
In general, the purpose of hydropower survey is to provide accurate and reliable basic data necessary for the design and construction of hydropower stations.
5、 General conditions of selected station site
The general conditions for selecting the station site can be described in the following four aspects:
（1） The selected station site shall be able to make the most economical use of water energy and conform to the principle of cost saving, that is, after the completion of the power station, the minimum cost will be spent and the maximum power will be generated. Generally, it can be measured by estimating the annual revenue from power generation and investment in station construction to see how long the invested capital can be recovered. However, due to different hydrological and topographic conditions and different demands for power, the cost and investment should not be limited by certain values.
（2） The selected station site should have superior topographic, geological and hydrological conditions, and be possible in design and construction. The construction of small hydropower stations shall conform to the principle of “local materials” as far as possible in terms of building materials.
（3） The selected station site shall be as close to the power supply and processing area as possible to reduce the investment in transmission equipment and power loss.
（4） When selecting the station site, existing hydraulic structures shall be used as much as possible. For example, water drop can be used to build hydropower stations in irrigation channels, or hydropower stations can be built near irrigation reservoirs to generate electricity using irrigation flow, etc. Because these hydropower stations can comply with the principle of generating electricity when there is water, their economic significance is more obvious.
Post time: Oct-25-2022
