Developments in the 1800s saw the waterwheel being superseeded by new turbine developments which were then combined with generators to allow for the electricity production. The late 1800s and early 1900s saw installations of hydro electricity grow at a significant rate. In many cases electricity from hydro was the first electricity to be provided in towns/regions.

Hydro-power system components

A typical hydro system will have the following key components.
a) Water source, from a river, an intake weir or a water reservoir to a
b) Channel, to deliver water to the
c) Forebay Tank, a limited capacity tank to guarantee a continuous steady flow of water into the
d) Penstock, through which water is channelled   or piped under pressure (depending on the head) directly into the
e) Power House containing the
f) Turbine, the
g) Gearbox (this may be required as the speed of rotation of the turbine
can be lower than the speed at which the generator will operate), and the
h) Generator, to produce electricity.
It is equally important that the water leaves the turbine as smoothly as possible and this requires a suitably designed
i) Draft Tube or Tailrace, to take the water away from the turbine.

Source: World Commission on Dams (link)

Hydro-power system types

Hydro electricity installations are typically described in a number of ways:

a) By the head of water available
Systems can generally be defined as low, medium or high head sites. Low head is typically below 10m and high head above 100m but these are not definitive boundaries. In Ireland the majority of sites with remaining potential for development would be classified as 'run of the river' systems. This is where a turbine site would be placed in a part of the river to capture the energy available. These would mostly be systems in the mini or micro scale and would be low to medium head systems.

b) By the capacity of the plant, expressedin kilowatts.

c) By the type of turbine used.

d) By the location and type of dam or reservoir.

Further definition of systems types can be taken from the European Commission (2000):
Run-of-river hydro plants use the river flow as it occurs, the filling period of its reservoir being practically negligible. The majority of small hydropower plants are run-of-river plants because of the high construction cost of a reservoir.
Pondage hydro plants are plants in which the reservoir permits the storage of water over a period of a few weeks at most. In particular, a pondage hydro plant permits water to be stored during periods of low load to enable the turbine to operate during periods of high load on the same or following days. Some small hydropower plants fall into this type, especially high head ones with high installed capacities (> 1.000 kW).
Reservoir hydro plants are plants in which the filling period of the reservoir is longer than several weeks. It generally permits water to be stored during high water periods to enable the turbine to operate during later high load periods. As the operation of these plants requires the construction of very large basins, practically no small or micro hydropower plant is of this type."

Hydro-power system scale

Using water to produce electricity can be done at a wide range of scales. Typically, these are classified into three levels or scales of development (ESHA, 1998)
•  Large scale or full scale hydro projects are typically multi-megawatt (>10MW) size developments. The development on the River Shannon in Ireland (Ardnacrusha) is an example with a capacity of 96MW.   At the upper end of this scale is the Three Gorges project in China which has a capacity of 18.2 GW at a cost of $1,200bn. These systems always supply electricity to the electricity grid.
•  Mini-Hydro is generally taken to be between 300kW to 10MW.  These systems are also generally connected to the electricity grid.
•  Micro-Hydro systems are generally taken as being less that 300kW and may or may not supply electricity to the electricity grid. In many cases, the smaller micro-hydro systems will be independent of the grid and supply electricity to residential or commercial units.

N.B. For basic information please refer to Chapter 8 - Small Scale Hydro of the ELREN Training Manual available for download at this website homepage (leftside) upon registration (rightside).