The amount of instantaneous waterflow of a river system depends on the rains, which is dependant on the season. The instantaneous waterflow varies from day to day with a minimum therof, located usually at the end of the dry season if it is marked. The concept of average flow has no interest in powerplants "along the waterstream", however, it does allow to better estimate the potential energyoutput of an infrastructure if an accumulation is envisaged. Low water flow, ie the minimum flow of the river during 24h states the minimal poweroutput potential of an installation. If the hydrological observations (measures of the flow of the river) are done for several years, it is possible to know the average minimum waterflow attained annually, or it is possible to observe it every 5 years, or -even more rare-, every 10 years. Indeed, the severity of the drought is variable depending on the year. A flow measure during 365 days can not indicate whether the observed minimum is an exceptional speed (either low or high) or rather an average minimum.
The hydrological data may be essential for the design of the proposed small hydroelectric plant. A lack of flow and thus availability of water will lead to disillusionment when the installation is working due to the large gap between the expected power output and true available power. There is of course no need to seek accurate hydrological data if the power output of the proposed installation is well below the maximum power of the site chosen for the project. Given that the turbine is to be placed near the river, it is highly desirable to know the variations of water level, to avoid seeing water invading the facilities during floods.
===Listing of the main elements of an installation===
An installation, no matter how small, always includes the following elements (following the flow of water):
# a main water intake which ensures that as long as there is water in the river, it is directed towards the power plant. This intake is fitted with a valve that can cut the water supply in case of failure of the turbine or in case of repairs of the infrastructure.
# an intake channel more or less long will lead water from the main water intake to the head intake or startup room. This channel can also create a greater drop height when choosing a suitable route.
# the intake of the startup room ensures filling the pipeline linking the intake to the turbine. The correct creation of the startup room allows to keep the penstock submerged. The intake is foreseen of a emergency release to allow the evacuation of excess water by reducing the water needs of the turbine.
# the penstock (pipe between the intake of startup room and turbine) creates the water column which allows the startup of the turbine. This pipeline usually of steel for conventional plants, but can be carried out in polyethylene for pico-power. Its diameter is calculated to avoid loss of charges in the flow of water. A large pipe is expensive in purchase and placement but reduces the loss. When the height of the water column is low, there is no penstock but a concrete construction in which water flows to supports the water column.
# the turbine room of the facility, is a kind of wheel that is rotated by the flow of water flowing through it. The type of wheel is to be chosen depending of the given instantaneous waterflow, but also depending of the water pressure. Some of these turbines resemble the old bucketwheels are particularly suitable for plants that need to operate with significant waterflow variations. The turbines may be vertical or horizontal axis.
# The speed regulator controls the speed of the turbine. In times of normal operation, speed is a balance between the hydraulic load that turns the turbine and the resistive load of the electricity production. In case of a ruptured electrical circuit, the resistive electrical is canceled and the speed of the turbine will accelerate because the hydraulic load remains unchanged. Too much speed can be fatal to the equipment and the presence of the regulator corrects the beginning of the acceleration also called the runaway.
# The electricity generator, is another essential part as it will convert the mechanical energy of the turbine into electrical energy. This conversion into electricity facilitates the provision on places that can be removed from the turbine. Today, the generator is an alternator that provides alternating current, a type of electricity that is more widespread than direct current. To provide an alternating current conforming to the needs of network users, the alternator must turn in a speed range set by the manufacturer. Between the generator and the turbine, which rotates much more slowly, a gearbox is installed to make the operation of the two machines compatible. The generator is set synchronous or asynchronous, as determined or set on the frequency of the cycles of current of the network, being 50 cycles per second except on the American continent where it is 60 cycles. If the plant is the only or main power source of the network, install a synchronous alternator.
# Electrical cabinet for control and distribution. For the safety of the facilities and also the people, one should distribute the current on the network via a switchboard where the devices (fuses, disconnectors) allow the controlling of the distribution. At the other end of the network similar equipment should be installed to control the distribution with the client.
# The electrical distribution network includes wiring that allows the transport of the power to the users. This network is either low voltage, ie operating voltage of client devices, or high voltage. This is necessary when the transport distance is over 500m. Indeed, to carry 20 kW at a voltage of 220V, one requires a 90 amp electrical cable of at least 10mm2 section. If the voltage is 5000V, a section of 1mm2 is sufficient. The cost of this cable will probably be cheaper than the other one.
# The leakage channel is the route of escape of the water that has gone through the turbine. This channel is connected to the stream that resupplies the watervolume that is diverted to the water intake. It is important that this channel is not flooded in case of riverflood. Indeed, the rise of water level can cause flooding of the engine room (generator and turbine) with a risk of damage.
Other equipment to provide is an sand trap on the feeder canal. The grains of sand that accompany the water on the turbine have an abrasive effect that reduces the life of the machine. The biggest materials (stones, bricks) have an even more destructive effect and gratings should be installed at the entrance to the intake line but also at the penstock to prevent such incidents.
===Key criteria for evaluating the hydropower potential===