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Hydropower – Systems Overview (2)

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Hydropower - Systems Overview

Hydropower - Systems Overview

Continued from first part of article:
Hydropower – Systems Overview (1)

Generator

The generator converts the rotational energy from the turbine shaft into electricity.

Efficiency is important at this stage too, but most modern, well-built generators deliver good efficiency.

Direct current (DC) generators, or alternators with rectifiers, are typically used with small household systems, and are usually augmented with batteries for reserve capacity, as well as inverters for converting the electricity into the AC required by most appliances. DC generators are available in a variety of voltages and power outputs.

AC generators are typically used with systems producing about 3 KW or more. AC voltage is also easily changed using transformers, which can improve efficiency with long transmission lines.

A view into a turbine shows a relatively large (2 feet in diameter) Pelton wheel. Peltons vary in size from 3 inches to 13 feet or more, depending on head and flow.

A view into a turbine shows a relatively large (2 feet in diameter) Pelton wheel. Peltons vary in size from 3 inches to 13 feet or more, depending on head and flow.

Depending on your requirements, you can choose either single-phase or three-phase AC generators in a variety of voltages. One critical aspect of AC is frequency, typically measured as cycles per second (cps) or Hertz (Hz).

Most household appliances and motors run on either 50 Hz or 60 Hz (depending on where you are in the world), as do the major grids that interconnect large generating stations. Frequency is determined by the rotational speed of the generator shaft; faster rotation generates a higher frequency.

In battery-based hydro systems, the inverter produces an AC waveform at a fixed frequency. In batteryless hydro systems, the turbine controller regulates the frequency.

AC Controls
At the bottom of the penstock, a manifold routes water to the four nozzles of a Harris Pelton turbine that drives a permanent magnet alternator.

At the bottom of the penstock, a manifold routes water to the four nozzles of a Harris Pelton turbine that drives a permanent magnet alternator.

Pure AC hydro systems have no batteries or inverter. AC is used by loads directly from the generator, and surplus electricity is burned off in dump loads—usually resistance heaters.

Governors and other controls help ensure that an AC generator constantly spins at its correct speed. The most common types of governors for small hydro systems accomplish this by managing the load on the generator. With no load, the generator would “freewheel,” and run at a very high rpm. By adding progressively higher loads, you can eventually slow the generator until it reaches the exact rpm for proper AC voltage and frequency.

As long as you maintain this “perfect” load, known as the design load, electrical output will be correct. You might be able to maintain the correct load yourself by manually switching devices on and off, but a governor can do a better job— automatically.

By connecting your hydro system to the utility grid, you can draw energy from the grid during peak usage times when your hydro system can’t keep up, and feed excess electricity back into the grid when your usage is low. In effect, the grid acts as a large battery with infinite capacity.
If you choose to connect to the grid, however, keep in mind that significant synchronization and safeguards must be in place. Grid interconnection controls do both. They will monitor the grid and ensure that your system is generating compatible voltage, frequency, and phase. They will also instantly disconnect from the grid if major fluctuations occur on either end. Automatic disconnection is critical to the safety of all parties. At the same time, emergency shutdown systems interrupt the water flow to the turbine, causing the system to coast to a stop, and protecting the turbine from overspeed.

DC Controls

A DC hydro system works very differently from an AC system. The alternator or generator output charges batteries. A diversion controller shunts excess energy to a dump load. An inverter converts DC electricity to AC electricity for home use. DC systems make sense for smaller streams with potential of less than 3 KW.
AC systems are limited to a peak load that is equivalent to the output of the generator. With a battery bank and large inverter, DC systems can supply a high peak load from the batteries even though the generating capacity is lower.
Series charge controllers, like those used with solar- electric systems, are not used with hydro systems since the generators cannot run without a load (open circuit). This can potentially damage the alternator windings and bearings from overspeeding. Instead, a diversion (or shunt) controller must be used. These normally divert energy from the battery to a resistance heater (air or water), to keep the battery voltage at the desired level while maintaining a constant load on the generator.

The inverter and battery bank in a DC hydro system are exactly the same as those used in battery-based, solar-electric or wind-electric systems. No other special equipment is needed. Charge controller settings may be lower than used in typical PV and wind systems, since hydro systems are constant and tend to run with full batteries much of the time.

Head, Flow, & Efficiency

If you expect to sell electricity back to the utility, pay extra attention to the efficiency of your hydro system because higher output and a lower cost-per-watt will go straight to your bottom line. Your turbine manufacturer can give you guidance on the most efficient design, as well as grid interconnection controls and safeguards. If you’re off- grid, and your site doesn’t have lots of head and flow, high efficiency can make the difference between ample electricity for your needs and having to use a backup, gasoline- powered generator.

Click on turbine images to see enlarged:
A Canadian-made Energy Systems and Design turbine uses a permanent magnet alternator and a turgo runner.

A Canadian-made Energy Systems and Design turbine uses a permanent magnet alternator and a turgo runner.

A Power Pal turbine with a Francis runner direct-coupled to the alternator

A Power Pal turbine with a Francis runner direct-coupled to the alternator

The 4-inch (10 cm) turgo runner in an Australian-made Platypus turbine.

The 4-inch (10 cm) turgo runner in an Australian-made Platypus turbine.

The underside of a low-head, high-flow Nautilus turbine showing the Francis runner, and above it, the innovative nautilus-shaped headrace.

The underside of a low-head, high-flow Nautilus turbine showing the Francis runner, and above it, the innovative nautilus-shaped headrace.

Whether a hydro system generates a few watts or hundreds of megawatts, the fundamentals are the same. Head and flow determine how much raw water power is available, and the system efficiency affects how much electricity will come out the other end. Each component of a hydro system affects efficiency, so it’s worthwhile to optimize your design every step of the way.

More Hydro Terms
Pipe LossPressureReaction TurbineRunner
Frictional Head Loss: Refers to the quantity of water supplied from a water source or exiting a nozzle per unit of time. Commonly measured in gallons per minute (gpm).A type of reaction hydro-turbine used in low to medium heads. It consists of fixed vanes on a shaft. Water flows down through the vanes, driving the shaft.Lost energy due to pipe friction. In hydro systems, pipe sized too small can lead to serious friction losses.The difference in elevation between a source of water and the location at which the water from that source may be used (synonym: vertical drop). Expressed in vertical distance or pressure.
TailraceTrash RackTurgo
A flume or channel that feeds water into a hydro turbine.Any electricity that is generated by the flow of water.Turbines with runners that operate in air, driven by one or more high-velocity jets of water from nozzles. Typically used with moderate- to high- head systems. Examples include Pelton and turgo.

SOURCE: Dan New, homepower.com

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    1. linke.rs says:

      Hydropower – Systems Overview (2) | CsanyiGroup…

      The generator converts the rotational energy from the turbine shaft into electricity. Efficiency is important at this stage too, but most modern, well-built generators deliver good efficiency. Direct current (DC) generators, or alternators with rectifi…

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      This post was mentioned on Twitter by ecsanyi: Hydropower – Systems Overview (2) – http://www.csanyigroup.com/hydropower-systems-overview-2...