Different ways of increasing generating capacity of a hydroelectric dam

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

17 March 2024

A reader of this column engaged me on the Internet social media about electricity production in Kenya. Like many people, he was surprised to learn that, out of the five dams on the river Tana (the so-called Seven Forks dams), Masinga has the smallest generators – totalling only 40 megawatts (MW).

This is despite the fact that Masinga has the largest water reservoir in the series: it stretches for over 40km from the dam head to the farthest tip. With so much water, why not add more generators to boost the electricity output?

It comes down to the age-old golden law: energy cannot be created (or destroyed), it can only be transformed from one form to another. In this case, the water at Masinga is dropped down a 60m-tall pipe to strike a turbine at the bottom. The force from this fall spins the turbine, which, in turn, spins the electricity generator.

There are only two ways of getting more electrical energy: either increase the height through which the water falls or increase the quantity of the water falling. There are two ways of implementing the first option. The height of the dam wall can be raised or the location of the turbine can be lowered far below ground level.

The second method is what was done at Kamburu dam: the turbines and generators are 30m below the ground. From the turbines, the water flows downstream in a 3km underground tunnel only to emerge at the inlet into Gitaru dam. The river actually disappears from view over that distance.

Increasing the volume of water flowing out of the Masinga dam can be achieved by adding another 60m-pipe to turn an additional turbine and generator. However, doing that would empty the dam faster than normal. This would fill up the four downstream dams quicker than usual and force the opening of over-flow spillways. This not only wastes water but can also cause flooding.

Clearly then, increasing the electricity generating capacity of Masinga dam (or any of the other dams in the Seven Forks) is not as easy as simply adding more powerful generators.

In closing, let me correct a common misconception. The dams on the river Tana are not called Seven Forks because there were seven of them planned! The name arises from the behaviour of the river as it enters Kindaruma dam. It breaks into seven parts which then re-combine in Kindaruma. Kindaruma was actually called the Seven Forks dam in its early days.