The output power of a hydroelectric generator mainly depends on two factors: the water supply pressure to the turbine and the flow of water through the turbine. Therefore, in order to determine the theoretical hydroelectric power, the water flow and head at the turbine inlet need to be known. Generally speaking, the water head refers to the height difference between the starting point of the water source and the generator. According to the water head, it is possible to calculate the water pressure at the location where the water flow reaches the generator, and then it can be calculated whether the actual available power can meet the design load requirements.
- Flow measurement
The unit of measure for flow is L/s. Turbine manufacturers can generally assist in providing methods for determining water flow. An easy way to determine the flow of water in a creek or pipe is by measuring the time it takes to fill a container of known volume. Alternatively, weirs of known size can be used to calculate the flow of small streams.
The flow of water depends on rainfall and hydrological conditions, so the flow of water in the same river at different points in time and at different locations is different. Where conditions permit, water flow measurements should be carried out regularly at the proposed location for at least one year. However, as recommended by the AS4509.2 standard, if a nearby site has similar rainfall and hydrological conditions, and the water flow data at that site is measured and recorded, the ratio of the measured ground water flow to the water flow at the adjacent point is equivalent to that of the two sites. ratio of water area.
Weirs are weirs that traverse the flow of water, with regular grooves of known size through which water flows. There is a direct relationship between the depth of water flowing through the weir and the flow of water. Temporary weirs can be constructed with planks and star piles, and a large sheet of plastic sheeting is used to address leaks. The weir must be at right angles to the direction of water flow, and the top of the weir should be sharp, preferably made of metal such as aluminum or stainless steel.
The ideal location of the weir should be in the low velocity section at the tail of the straight stream. For the fast water flow section, we should consider using the velocity method. This can be done by measuring the height of the water level at a distance in front of the weir where the water flow is not affected by the acceleration effect over the weir.
The most common groove shape for weir weirs is rectangular, but other shapes such as trapezoid and triangular are also used. According to the shape of the groove, the flow rate can be determined by consulting the water flow chart. Temporary weirs may become permanent water collection points in front of turbines. For larger rivers, the flow cross-sectional area can be determined using the method shown in Figure 1.
The cross-sectional area of the water flow can be determined by measuring the depth of the water flow at regular intervals (depending on the width of the water flow, the depth can be measured at most every half-meter interval). Flow rate, and finally multiply the flow rate and the area. At this time, the unit of flow is m²/s, 1m²=1000L, so the flow rate can be multiplied by 1000 to get the flow value in L/s.
- Head Measurement
The pressure of water can be characterized by the water head. Head refers to the vertical height between the reservoir water surface and the turbine inlet. Different from the above measurement principles, for some fully submerged turbines, the measurement of the water head also needs to include the distance from the turbine to the tail water level, as shown in Figure 2.
In order to accurately measure the water head, a fixed mirror level or theodolite can be used. Both instruments, although more expensive, can be rented. Since the above two instruments require a wide field of view, they may not be available in densely forested areas. Another method is to use an observation level. Through the telescope, pick out objects that are at the same level as your line of sight, and then multiply your own line of sight height by a certain multiple to get the final water head, as shown in Figure 3. A simple observation level can also be made with a stand and a level. Other methods that may be applicable are to use a water fill tube, either as a simple level, or with a pressure gauge, or with a precision altimeter. When using a water tube, make sure there are no air bubbles in the tube or the readings will be inaccurate. A good altimeter has an accuracy of ±1m, but is affected by changes in temperature as well as changes in air pressure, so it must be used with caution, especially at low water heads.
Once the flow and head are determined, the available hydraulic power can be read from the graph shown in Figure 4. Stream flow should be measured during dry periods (be sure to wait at least two days after heavy rain).
- Actual power generation
After the theoretical available power of the water flow is determined, the actual generated power can be calculated. In any system, there are efficiency losses that must be accounted for. The two main factors of efficiency loss in micro-hydro systems are pipe losses and turbine efficiency. Pipeline losses can be obtained from manufacturer-supplied charts, as can turbine efficiencies. In addition, please keep in mind that the total flow of the water source cannot be fully utilized. It is recommended to use 50% or less of the minimum total flow.
The formula for calculating the power of the micro-hydrogenerator is:
In the formula, P is the power generation of the micro-turbine, W;
- the efficiency of the turbine;
- Density of water, constant 1000kg/m;
g——gravitational acceleration, constant 9.81m/s²;
H – water head, m.
[Example] The flow rate of a water stream is 5L/s (0.005m²/s), the water head is 16m, and the turbine efficiency is assumed to be 65%. According to the hydroelectric power calculation formula, the calculation process is as follows
P=0.65×1000×9.81×0.005×16=510(W) (pipeline loss is small)
Another rough calculation method is that the actual generated power is 5QH (but the flow rate in this formula must be calculated in L/s, not m²/s). This calculation takes into account most of the losses and constants, and the result is a concrete number, for the example above, the actual power is calculated to be 400W. If this water flow can continuously generate electricity 24 hours a day, the total electricity generation per day is about 9.6kW·h. It can be seen from this example that a small micro hydropower system can output a large amount of electric power at a relatively low price.