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 For more info, see Micro hydro links.
¿Qué es un Sistema Micro-hidráulico? (en espanol)
Micro hydro technology is an appropriate, environmentally benign energy form, which can produce electricity at low cost in isolated communities internationally wherever there are mountains and streams. A micro hydro system uses the flowing water of a stream to turn a turbine-generator to produce electricity.
The two key components of any hydro system are head (or vertical drop of the water from its intake to where it turns the turbine) and flow (the volume of water available to turn the turbine).
A simple micro hydro system is shown in Figure 1.
FIGURE 1: MICRO-HYDRO SYSTEM
A. Intake B. Power canal/conduit C. Penstock D. Powerhouse E. Tailrace
F. Electrical Transmission Line G. Transformer House H. Village/Longhouse
The smallest applications up to 3 kW (3000 watts) may use a car alternator to directly produce DC (Direct Current) electricity or AC (Alternating Current) by adding extra conversion equipment. The larger applications in the 10 kW to 30 kW range will tend to use induction or synchronous generators and produce AC current. High Head, AC or DC Systems.
Water flow from a portion of a stream is diverted by a small wall of loose boulders or other appropriate intake to a submerged pipe of a size depending on power needs and cost limitations. This pipe, the penstock runs straight down, as steeply as possible, along the ground sometimes for hundreds of meters until it reaches the turbine-generator-battery site near the same stream or another stream. The turbine-generator-battery site is located near to where the electricity is to be used to minimize power transmission losses. The long fall of water through the pipe causes a small, high-speed stream of water to exit the pipe. This water stream strikes a small turbine (a water wheel about 4" diameter), causing it to turn at a very high speed.
DC System
As the turbine rotates, a generator on the same shaft rotates and so produces DC electricity. The generator can be a used car alternator. Wires carry this electricity to a couple of batteries and then from the turbine-generator-battery box to the site where the electricity will be used. After turning the turbine, the water drops in to a collection ditch that returns it to the river. Since the micro hydro system charges the batteries non-stop, the batteries will be fully charged by evening when most of the electricity is needed for lights and other DC appliances.
AC System
There are two possibilities for producing AC. The first which is more appropriate for smaller systems is to use a DC alternator to generate DC, which can then be converted using an inverter to AC power with voltage and frequency that matches the loads.
The other method for larger systems is to use a standard AC induction motor to directly produce AC power of the same voltage and frequency as the loads. For technical details on how this system works refer to the section on Induction motors used as stand-along induction generators below.
Low-Head, High-Flow System
Many situations occur where a high-head system is inappropriate. Reasons for this might be:
1. Generally flat terrain
2. Conflicts with existing irrigation systems
3. Prohibitive cost of long penstocks.
Nevertheless, a micro hydro system may work if there is sufficient flow and if a head of only 10 ft (or less in some situations) can be developed by building a canal (similar to an irrigation ditch) or a low dam or weir. In such cases, a crossflow, propeller or vertical axis turbine can be used to drive a generator, sometimes using pulleys and belts to achieve proper generator speeds when the turbine is large and rotation speeds are low.
Turbines
There are two basic families of turbines- reaction turbines which include propellers and Francis turbines, and impulse turbines which include Pelton, Turgo and crossflow turbines. They all use water pressure to set a mechanical wheel spinning. This spinning wheel in turn drives a generator which produces electricity.
The type of turbine best for your system depends most on three factors:
 H = head or pressure (m)
P = required power (k/W)
N =-working turbine speed (rev/min)
These are related by the equation:
Ns = NÖ P/H5/4
Where Ns = specific speed (rev/min)
The specific speeds for different turbine types are:
Type Ns
Pelton 12-30
Turgo 20-70
Crossflow 20-80
Francis 80-400
Propellor and Kaplan 340-1000
*Technical information in these sections is from micro hydropower Sourcebook A Practical Guide to Design and Implementation in Developing Countries, by Allen R Inversin, NRECA International Foundation, Arlington VA., USA
Induction motors used as stand-alone Induction Generators
Synchronous generators are almost exclusively used for large scale commercial power generation. The use of induction generators connected to a utility grid system where the utility maintains the voltage and frequency is less common but is still a familiar concept. Typically, in large-scale industrial applications, the induction generator is much cheaper and simpler than a synchronous generator and it is used only to supplement the main source of power. Typical applications are where there is a so-called "free" source of drive power such as a pressure let-down station The use of an induction generator in an isolated system with no other generation source is not as familiar. An induction generator is a simpler, more rugged, less expensive machine requiring less maintenance than a synchronous machine.
The induction generator is the same physical device as an induction motor, the only difference being the direction of real power flow. When connected to a utility grid, the induction generator derives its required excitation (referred to as reactive or magnetizing power) from the utility system in the same way that an induction motor does. The reactive power source can be from either rotating synchronous machines or static capacitors or a combination of the two. When the induction generator is mechanically driven at a speed higher than the synchronous speed of the motor (determined by the system frequency and the number of poles on the rotor of the motor), the generator supplies electrical power to the system. The deviation of speed from the synchronous speed is known as slip, and over the normal operating range of the generator, torque and power are roughly proportional to slip. This is similar to the operation of an induction motor except that the direction of power flow is reversed and the induction motor operates at a speed less than the synchronous speed of the motor. The induction generator is said to have negative slip when operating above synchronous speed.
For the applications under consideration, a standard off-the-shelf induction motor is operated as an induction generator. When used in an isolated electrical system though, the utility system is not available as a reference to maintain the frequency and voltage or to supply the required excitation. In order for an induction generator to produce approximately 60 Hertz as an output frequency, the generator must run at 3-4% above synchronous speed when delivering rated power.
On a system where there is no means available to control the water flow driving the turbine, the only way that the speed, and hence the output frequency, can be controlled is to match the generator output power to the input mechanical power. If the water flow were constant, fixed speed would be obtained for a particular value of fixed electrical load. If the flow varies on a daily or seasonal basis, there must be some means to vary the electrical load proportionately in order to maintain the output frequency reasonably close to 60 Hertz. Non-critical loads such as water heaters may provide the means by which the total system load can be automatically varied to match the input power.
Since there is no rotating source of excitation on an isolated electrical system, the excitation for the induction generator must be derived from static capacitors of the type used for power factor correction. The reactive power produced by the capacitors is a function of the generator output voltage so the initial start-up of the induction generator is dependent upon some residual magnetism in the rotor of the induction motor. Without this residual magnetism, the rotation of the generator would produce no voltage, and the connected capacitors in turn would provide no excitation current to the generator.
If the loads are large enough to warrant a three phase distribution system, a standard three phase induction motor can be used as an induction generator and a properly sized, balanced three phase capacitor bank connected to its terminals will supply the required excitation.
In smaller systems, where only single phase distribution to the loads is required, a three-phase induction motor can still be used as the generator, but now only two of the phases deliver current. This unbalance in currents causes circulating current and excessive heating in the generator rotor and thermally limits the output of the generator to less than its rating. In order to minimize the unbalanced currents, the static capacitors will only be connected to two of the phases. By judicious choice of the capacitor values and by taking note of the phase sequence of the generator phases, the amount of unbalanced current can be minimized. A wealth of experience is available from proven operating systems on the proper choice of motor, capacitors and control for any new application.
Direct Current Alternators (Car or Truck)
A number of mini-hydro schemes have utilized used car or truck alternators as the generation source. These systems are of necessity low-voltage, low-power schemes but may well be appropriate if 12 volt lighting, radios, TV's are the intended loads.
Connection of a DC alternator with a variable load to a turbine driven by a variable flow water source is similar to the application of the alternator in an automobile where both the engine speed and the alternator load can vary widely.
Connection of a DC alternator with a variable load to a turbine driven by a variable flow water source is similar to the application of the alternator in an automobile where both the engine speed and the alternator load can vary widely.
The output voltage of a alternator depends on three factors:
1) the number of turns of wire in the stator
2) the strength of the magnetic field of the rotor
3) the speed of rotation of the rotor
All loads operate successfully if voltage close to their rated supply voltage is available. Since the speed of the alternator in a mini-hydro scheme is dependent on the available water flow and this is usually not controllable, the only way to regulate the alternator output voltage is to vary the strength of the magnetic field of the rotor. The voltage regulator supplied with an alternator accomplishes this by automatically varying the amount of excitation current supplied to the rotor field windings.
Just as is done in a car, batteries can be supplied in the DC scheme to permit storage of energy when loads are light. This energy can be released when loads exceed the power output capability of the alternator. This may be due either to the occurrence of short-term peak electrical loads or to fluctuations of the water flow driving the turbine. The storage capability of the batteries ( measured in Ampere-hours) depends on the number and type of batteries used.
Distribution Systems
In designing the nano-hydro systems, it is of course essential to ensure a good match between the turbine and the generator as well as between the generator and the connected load. Equally critical in the success of the project is the design of the electrical distribution system between the generation source and the connected loads.
Issues that need to be considered:
Voltage drop between source and load
Energy loss in the distribution system
Personnel safety
Protection of equipment
Switching and isolation of parts of the system
Relative criticality of the loads
The following is a preliminary list of information that should be provided prior to considering how the power will be distributed throughout the site:
1) A physical layout of the site showing the magnitude and location of at least the major loads should be provided. In order to supply a voltage reasonably close to the voltage ratings of the connected equipment, the designer must be able to calculate the voltage drop between the source and the loads. ( Lamps operating at less than rated voltage produce less than rated output, and if operated above rated voltage burn out much sooner. Motors operating at less than rated voltage draw excessive current and the subsequent overheating shortens the motor life.) Various design techniques such as over-sizing conductors, correcting low power factor, using higher voltage levels for distribution of large blocks of power over long distances, or adding voltage boosting transformers can be used to minimize excessive voltage drop and energy loss in the distribution system.
2) Thought must be given to minimizing the exposure of personnel to live conductors or equipment. Even the low voltages anticipated to be in use in these small projects have the potential for fatal accidents. Careful routing of conductors and physical isolation of components can contribute a great deal to the safety of personnel.
3) The user of the power should define the relative criticality of the loads on the system. Does loss of power to certain loads merely represent an inconvenience or is there the possibility of loss of revenue due to curtailed production? Is there any possibility of equipment damage if power is lost?
4) The above considerations will affect the amount and arrangement of the protection and switching devices that are installed in the system. For example, the simplest arrangement would be to have a fused switch located at the generator. A short circuit anywhere in the system would blow this fuse and cause the whole system to be shut down. If equipment maintenance were required anywhere in the system, the main switch would have to be opened to shut down the whole system to enable the work to be carried out safely. It may be more convenient to provide more fused switches out in the distribution system to allow isolating part of the system without disturbing other parts.
This extra complexity entails additional cost so each project should be carefully tailored to meet the needs of the users.
Often overlooked in the design of a distribution system is the consideration of the future addition of generation and loads. Although limited funds and resources may constrain the additional installation, some consideration should be given to the possibility of future additional load. If the probability of these future additions is relatively high, it may be prudent to spend additional funds in the initial stages to avoid bottlenecks later. In some cases, it can be possible to allow for future expansions without a large outlay of capital. It may be as simple as considering where additional equipment would be installed if and when required.
Electrical Calculations
You can use this formula to determine potential power.
The amount of electrical power that can be generated at any particular site can be calculated from the following equation that uses the water flow quantities measured on site.
P = .0098 Q Hg
P = power (kW)
Q= flow (l/sec)
Hg = gross head or elevation drop (m)
To begin, you can assume that your overall system efficiency is 50%. Then you can use: P= 5QHg to find how much power you might generate.
Thus, the head you will need is:
Hg = P/5Q
To measure the head at your site click here to read Instructions on Measuring Head and Flow.
This drop or head can occur suddenly as in a waterfall or more gradually. The actual power generated will depend on how efficiently the water is carried from the top to the bottom (length, size, and type of pipe used) and then how the energy is converted to electricity. The electrical power must also be transmitted from the generator to the point of use- homes, machinery, etc. This also involves some energy losses. Generally, more efficiency involves higher system cost.
If you find that you have sufficient flow and head to produce the power you need, then you can move forward to other considerations such as, environmental effects, available materials, cost, labor involved. These all vary widely for different situations.
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