1. turbine installation
Please be sure to follow the manufacturer’s installation instructions to ensure that the turbine and generator are firmly fixed in place. Please be aware that the water level will rise in the flood season. If the turbine is not submergible, the generator must be installed above the water level in the flood season.
2. type of hydraulic turbine
There are two types of turbines to choose from, and the final choice will depend on the actual available net head, actual available water flow and power demand.
(1). Impulse turbine
The impulse turbine is impacted by one or more jets on the turbine impeller. The impeller can rotate freely in the air, and the water flow falls into the wake area after impacting the turbine. In this case, the actual available net head is the distance from the reservoir water level to the horizontal plane at the location of the turbine multiplied by a factor considering the friction loss of the penstock.
Such turbines cannot be immersed in water, so they must be installed above the flood season water level of the flow. Because they must be installed at a sufficiently high position above the tail water level, part of the potential head is wasted. The Pelton turbine and the oblique impact turbine belong to this kind of turbine. Please note that micro turbines and generators may emit excessive noise. When selecting the site for the micro hydropower system, the noise should be minimized as far as possible, and barriers, trees, etc. can be used to insulate the sound.
(2). Reaction turbine
The impeller of the reaction turbine is completely immersed in water and rotates in a sealed spiral case. After the water is powered by the turbine, it flows into the tailrace area through the draft tube. The gravity of water will generate negative pressure on the discharge side of the impeller. When the installation position of the water turbine is higher than the water level in flood season, the water potential energy below the position of the water turbine will not be wasted. The net head includes the head above the turbine and the head below the turbine; In practice, the water head below the turbine may be higher than that above the turbine. Francis turbine, Kaplan turbine and axial-flow turbine all belong to this kind of turbine, and their performance is acceptable when the head is low.
Generally speaking, the pipe diameter should be as large as possible to reduce the friction loss along the pipe; The pipeline should also be as straight as possible and continue to decline. The high point of the inlet pipe will cause air to mix into the flow channel, thereby reducing its effective diameter and the effective water flow through it. The downward rate of the pipeline should not increase gradually, because there will be suction space at the position where the rate increases. This may cause air to be sucked into the pipeline, causing the vibration of the turbine, and the water hammer effect will impact the turbine, and may even damage the turbine and pipeline. Exhaust holes must be set near the pipe inlet. If the water inlet is blocked and there is no exhaust hole, the rapidly falling water flow will form a vacuum behind it, and then the pipe may be deformed. In the independent power supply system, when the battery charging capacity is small, the water turbine is generally small, and the flow required is also quite small. Therefore, PVC (polyvinyl chloride) or polyethylene pipe is usually applicable. The grade of penstock is also different according to the different static pressure head. For example, grade 6 PVC pipes are suitable for hydrostatic heads up to 60m.
The water turbine is connected to the generator, which can be AC or DC type. If the generator is 240V AC type, it can be directly connected to the load. In addition, if the long-term internal water flow is constant, there is no need to use inverter and energy storage battery. Few micro hydropower systems can provide enough 240V, 50Hz AC energy for ordinary families.
In the independent power supply system, the generator is generally preferred to charge the battery. Due to the constancy of water flow in a day, small generators can charge considerable electric energy into the battery in a day. This means that the capacity of the generator can be quite small and can operate normally at low flow rates or heads. For example, a 100W hydraulic generator can output 2.4kw · h of electric energy within 24 hours.
Of course, the biggest advantage is that due to the regularity of power production, a small battery capacity can meet the requirements. In other words, the storage capacity of the battery does not need to be able to independently complete the power supply for 5 days, but generally 2 days is enough, which has taken into account the reasonable battery charge and discharge rate, discharge depth and cycle life.
- Control equipment
If the generator outputs AC to directly supply power to the load, the speed of the generator must be controlled. The governor can be used for speed control. The governor can distribute excess power to auxiliary loads (such as water heating or indoor heating), or control the flow of water through the turbine by using mechanical devices.
The DC generator may also need water flow control to make the output electric energy suitable for charging the battery. The control equipment can automatically start the water flow when there is load in the system. For hydropower stations supplied by water storage dams, this can save hydraulic resources.
- Power transmission
Because long-distance transmission is easier and cheaper than water transmission, the installation location of hydraulic turbines is usually far away from users. This will result in very long cable lines and corresponding line losses. For this reason, the battery pack can be installed near the water turbine, and then converted into 240V power through the inverter to transmit it to the user. Otherwise, it is necessary to increase the cross-sectional size of the cable to adapt to the transmitted current, so as to maintain the voltage loss on the cable within an acceptable range.
As/nzs4509.2-2010 “independent power supply system Part 2: system design guidelines” suggests that the maximum voltage drop from micro hydro generator to storage battery should be less than 10%.
A characteristic of micro hydropower system is that when the rotating speed of the turbine is half of the water flow speed, its output torque is the largest. If the load of the turbine decreases, its speed will increase. The maximum power point tracker with built-in DC-DC voltage conversion can overcome the line loss problem of long-distance transmission by increasing the transmission voltage. In order to solve the problem of voltage drop in long-distance transmission, some hydraulic turbines generate AC voltage to transmit power, and then rectifier it through the battery.
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