13-12-2012, 04:14 PM
Small Pumped Hydro Storage
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Overview:
Flowing and falling water have potential energy. Hydropower comes from converting energy in flowing water by means of a water wheel or through a turbine into useful mechanical power. This power is converted into electricity using an electric generator or is used directly to run milling machines. Most people in North America understand hydropower as involving big dams and large-scale generating facilities. Small-scale hydropower systems, however, are receiving a great deal of public interest as a promising, renewable source of electrical power for homes, parks and remote communities.
Hydropower systems are classified as large, medium, small, mini and micro according to their installed power generation capacity. Electrical power is measured in watts (W), kilowatts (kW) or megawatts (MW). A micro-hydropower system is generally classified as having a generating capacity of less than 100 kW. Systems that have an installation capacity of between 100 kW and 1000 kW (1.0 MW) are referred to as mini-hydro. Small hydro is defined as having a capacity of more than 1.0 MW and up to 10 MW,
Small and micro-hydro systems have the following components:
• A water turbine that converts the energy of flowing or falling water into mechanical energy that drives a generator, which generates electrical power – this is the heart of a micro-hydropower system
• A control mechanism to provide stable electrical power
• Electrical transmission lines to deliver the power to its destination.
How to Measure Potential Power and Energy
The first step is to determine the hydro potential of water flowing from the river or stream. You will need to know the flow rate of the water and the head through which the water can fall, as defined in the following:
• The flow rate is the quantity of water flowing past a point at a given time. Typical units used for flow rate are cubic metres per second (m3/s), litres per second (lps), gallons per minute (gpm) and cubic feet per minute (cfm).
• The head is the vertical height in metres (m) or feet (ft.) from the level where the water enters the intake pipe (penstock) to the level where the water leaves the turbine housing (see Figure 1).
Pump-as-Turbine
For a number of years there has been wide interest in reverse-engineered conventional pumps that can be used as hydraulic turbines. The action of a centrifugal pump operates like a water turbine when it is run in reverse. Because the pumps are mass-produced, they are more readily available and less expensive than turbines. It is estimated that the cost of a pump-as-turbine (PAT) is at least 50 percent less or even lower than that of a comparable turbine. However, for adequate performance, a micro-
hydropower site must have a fairly constant head and flow because PATs have very poor partial-flow efficiency. It is possible to obtain full efficiency from PATs by installing multiple units, where they can be turned on or off depending on the availability of water in the stream. PATs are most efficient in the range of 13 to 75 m (40 to 250 ft.) of gross head. The higher the head, the less expensive the cost per kilowatt; this is generally the case with all turbines.
Generators
Generators convert the mechanical (rotational) energy produced by the turbine to electrical energy; this is the heart of any hydro-electrical power system. The principle of generator operation is quite simple: when a coil of wire is moved past a magnetic field, a voltage is induced in the wire.
Alternating current (AC) generators are also referred to as alternators. They generate varying voltages, which alternate above and below the zero voltage point. It is this process that produces AC electricity. This same principle is used in all electric generators, from large hydro and nuclear plants to the alternator in your car, although the speed will vary depending on the type of generator used. There are two types of generators: synchronous and asynchronous. Synchronous generators are standard in electrical power generation and are used in most power plants. Asynchronous generators are more commonly known as induction generators. Both of these generators are available in three-phase or single-phase systems. System capacity, type of load and length of the transmission/distribution network dictate whether a single- or three-phase generator should be used.