Storing solar electricity in dams

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

12 December 2021

One of the greatest challenges to solar electricity is storage of the energy generated. The conventional solution is to use rechargeable batteries. However, there are other methods available. The most “low-tech” type is the pumped hydro-electric dam. In this case, the power from the solar cells is used to pumping water uphill into a dam. Then, when power is needed, the water is released to run a turbine connected to a generator.

Now in Kenya, we have many hydro-electric dams, the biggest being the Seven Forks on the river Tana. This comprises of five dams in series with a combined capacity of 599MW. One of the limitations here is inadequate water to ensure continuous electricity generation throughout the year.

If there was enough rain, the Seven Forks can potentially produce 599MW x 365 days x 24 hours = 5,247,240MWh annually; that is 5,247giga-watt-hours (GWh). However, in the year 2020/21, for example, only 2,631GWh (about half of peak capacity) was generated.

KenGen is building a 40MW solar power plant at Gitaru on the Seven Forks. The idea is to generate solar electricity during daylight and, consequently, reduce the generation using water. The question then is: what size of solar generators would be needed to close the gap between the real output (2,631GWh) and the peak potential (5,247GWh), that is, 2,616GWh?

Unlike other energy sources, the intensity of sunlight is not constant throughout the day. It starts off very low, peaks at around noon and then subsides to another very low level before sunset. As a result, the average power during a cloudless day is about half the peak rating. Thus, the 40MW station at Gitaru will average about 20MW on a very sunny day.

So, the maximum output of Gitaru Solar will be 20MW x 12h x 365 = 175,200MWh. But that assumes every day has 12h of cloudless sunshine. In reality, the output is about half of that figure, thus the annual output will no more than 85,000MWh, or 85GWh.

Therefore, to cover the 2,616GWh gap, we need about 30 solar power stations the size of the one at Gitaru; that is a 1,200MW installation. The Gitaru solar project sits on 80 hectares, so, we will need about 2,500ha for the expanded station. This is an area measuring 5km-by-5km.

Can it be done? Yes! After all, there are seven other solar farms in the world that are bigger that this. All that is needed is commitment and dedication.