Run-of-River Hydro » How it works

Scheme Constituents

The main constituents of a run-of-river hydro scheme are:

  • Intake(s)
  • Pipeline
  • Powerhouse
  • Outfall
  • Grid Connection

Intake

At an intake water is abstracted from the river according to the guidelines agreed with the relevant statutory authority. Run-of-river schemes are not environmentally sensitive, since a sensible proportion of the river flow is always allowed to continue downstream. Figure 1 shows a typical intake arrangement. A small weir is placed across the river. Water flows through a brass V-shaped plate embedded in the river weir, allowing accurate calculation of the flow progressing downstream for a given water level. A second weir (intake weir) is constructed on one of the banks immediately upstream of the river weir, behind which is the pipeline inlet. Water flows over the intake weir once the water level is sufficient for an acceptable amount of water to continue downstream.

Pipeline

The pipeline carries the abstracted water from the intake(s) to the powerhouse. Total pipe length usually varies from 500m up to several km. The pipeline usually comprises a mixture of plastic and ductile iron / steel pipe, the balance depending upon pressure profile. Pipe diameters are chosen to suit the maximum abstracted flow. The greater the water flow, the more the loss of power through friction in a pipeline of a specific size. Friction is smaller in a larger diameter pipeline due to the greater ratio of cross-sectional area to circumference. However the gain in power due to reduced friction must be weighed-up against the increased capital cost of buying and laying larger pipes.

Powerhouse

Water enters the powerhouse under high pressure, proportional to the difference in elevation between the intake(s) and powerhouse. The equipment within the powerhouse manages the process of converting this energy into a form compatible with the national electricity network (see Equipment for more information). Powerhouses typically range in plan size from about 50m2 up to 100m2 depending on scheme output and are usually buried to minimise visual impact (Figure 2), however over-ground designs sometimes provide the best solution. In some cases a further building or room is required to house equipment belonging to the electricity distribution network, this varying in size from 10-20m2.

Outfall

Water leaving the powerhouse enters an outfall duct on its way back to the river. Before entering the river, the water flows into a small outfall structure designed to reduce water velocity so that modifications to the watercourse do not occur close to the point of return. Figure 3 depicts a typical outfall structure.