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Micro/Mini Hydro Systems

 

Small HydroPower generation projects can be defined as a generating capacity of up to 10 Megawatts (mw) which further sub-divided into mini Hydro which is typically defined as up to 1 mw (1000 Kilowatts-kw) and a smaller micro Hydro up to 100kw.  As a practical example micro hydro projects will typically supply enough power for from 5-100 homes, mini Hydro up to 1000 homes and a small hydro for a community of up to 10,000 homes.

 

The key to the amount of power available at a particular site is based on a dependable flow of water and a reasonable height of fall of water, called the head.  In a typical installation, water is fed from a reservoir through a channel or pipe into a turbine..  The pressure of the flowing water on the turbine blades causes the shaft to rotate.  The rotating shaft is connected to an electrical generator which converts the motion of the shaft into electrical energy.

 

 Small hydro plants may be connected to conventional electrical distribution networks as a source of low-cost renewable energy.  Alternatively, small Hydro projects may be built in isolated areas wher there is no electrical distribution network.  Since small Hydro projects usually have minimal reservoirs and civil construction work, they are seen as having a relatively low environmental impact.

this decreased environmental impact depends srongly on the balance between stream flow and power production.  one tool that helps evaluate this issue is the Flow Duration Curve of FDC which Charts a streams daily flow rate vs. Frequency.  The Hydro systems designer and site developer must strike a balance to maintain both the health of the stream and the systems economic fesibility.

 Rickly Hydro offers standardized turbine packages that range in size from 1 to 10 MW. These "water to wire" packages simplify the planning and development of the site since thisbecomes a one stop shop for most of the equipment supply.  Non-recurring engineering costs are minimized and Rickly Hydro takes responsibility for all aspects of the mechanical and electrical system package.

 

To calculate the maximum power available or the amount of energy in watts, the head and the flow have to be calculated.  Several equations can be used.  

 

For a typical micro systems calculation that takes typical efficencies into account would be:

  • Flow in gallons per minute(GPM) X Head, in feet of elevation drop divided by 10 = watts or

Watts = (GPM x HEAD,FT)/10

For a typical micro systems calculation in metric units a common equation is:

Watts = Flow Liters/sec x 9.8 m/sec(2) x Head, m x 0.9

 

TURBINE TYPES

Another consideration is high speed versus low head systems and the particular turbine sesign best used for each.

High Head systems use Turgo or Pelton turbines which would typically discharge a small percentage of the flow under high pressure and because high-head sites tend to be at the heads of rivers in mountainous areas, so the ground slopes steeply enabling high heads to be created, but the rainfall catchment of the watershed is relatively small, so the flow rate is small.

For Low-Head installations, Francis, Kaplan or Axial Flow turbines are used.  very low head installations of only of a few meters may use Axial flow turbines in a pit.  In low-head installations, maintenance costs often become important.  A Low-Head ssystem moves larger amounts of water, and is more likely to encounter surface debris.

REGULATION & OPERATION

An automatic controller operates the turbine inlet valve to maintain constant speed (and frequency) when the load changes on the generator.  In a sydtem connected to a grid with multiple sources, the turbine control ensures that the power always flos out from the generator to the system.  the frequency of the alternating current generated needs to match the local standard utility frequency.  In some systems, if the useful load on the generator is not high enough, a load bank may be automatically connected to the generator to dissipate energy not required by the load: while this wastes energy, it may be required if its not possible to stop the water flow through the turbine.

An induction generator always operates at the grid frequency irrespective of its rotation speed: all that is necessary is to ensure that it isdriven by the turbine faster than the synchronous speed so that it generates power rather than consuming it.  Other types of generators require a speed control systems for frequency matching.

Most Micro/Mini Hydro Systems operate the generator at an arbitrary frequency and feeds its output through an inverter which produces output at grid frequency.  Power electronics now allow the use of permanent magnet alternators that produce wild AC to be stabilised.  This approach allows low speed/low head water turbines to be competitive: they can run at the best speed for extraction of energy, and the power frequency is controlled by the electronis instead of the generator.  High Speed/High Speed Turbines also use this arbitrary frequency output and inverter.

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