There are many options theoretically feasible for “firming” solar & wind energy from Africa, including dispatchable hydro, gas turbines, and storing off-peak energy. Energy storage options include compressed air energy storage, flywheels and various kinds of batteries, for example, but by far the most widely deployed energy storage option is pumped storage. Modern pumped storage has about an 83% round trip efficiency for energy storage (provided the two reservoirs are close together), and the facilities have a very long life. With variable frequency controls, very fast regulation functions are now possible for pumped storage facilities, similar to the function of flywheels or batteries for voltage support and frequency regulation, but at lower cost. There are several conventional pumped storage facilities in Africa at present, and no doubt more would be constructed if the continental scale grid of Figure 3 in this document:
were ever built, according to market needs. There are also three particular sites in Africa that allow for the possibility of high capacity pumped storage operating between the ocean and a depression that is below sea level: the Qattara depression in Egypt, the Danakil depression in Eritrea, Ethiopia and at Lake Assal and the surrounding Afal depression area in Djibouti.
The lowest cost of any of these sea water based pumped storage projects to develop would be the Danakil land depression, which covers some 9700 square km along the Red Sea coast of Eritrea and stretches into Ethiopia; only about a 20-km access canal from the Red Sea would be needed to access this region for pumped storage (longer canals and/or tunnels would be required to access Qattara). It is far easier to build access canals to Danakil than is the case at Qattara because there are mountains in the way at Quattara, and a Danikil pumped storage project would not be disrupting one of the wonders of the world, as would be the case in flooding Lake Assal or the Dead Sea, for example. The land depression at Danakil is as much as 125 meters below sea level. I have arbitrarily assumed for the sake of calculation that the lower reservoir (the “Danakil Sea”) would have a surface area that is 50% of the total area of the depression at 50 meters below sea level for the calculations below.
There is potential to partially flood the Danakil land depression and operate it as a pumped hydraulic energy storage system. Seawater would flow in during periods of high demand, and be pumped back into the red sea in periods of low demand, as in any pumped storage facility. The enormous size of the lower reservoir (the “Danakil Sea”) opens up the possibility that a seasonal cycle could be superimposed upon the normal daily/weekly energy storage cycle.
The enormous potential energy storage capacity of Danakil could be as high as 6000 GW-hours, by allowing a large 10-meter swing in depth between fully charged & fully discharged states, as is typical in dedicated pumped storage reservoirs today. (This assumes an average depth below sea level of the Danakil Sea of 50 meters, and an average surface area of the lower reservoir that is half the total area of the depression.) If a maximum swing in the level of the Danikil Sea of about one meter per day is allowed, corresponding to the typical swing of natural tides, this would imply a daily cycle round-trip capacity of about 600 GW-hours. For 12 hours charging, 12 hours generating, this would mean 46 GW maximum output, 57 GW maximum input, which would be by far the largest pumped storage project in the world. The multi-week up to seasonal storage capacity of Danakil would be even greater than the 1300 GW-hour potential of the Lake Erie-Lake Ontario system in North America[i]
if a 2-meter swing in level of the Danakil Sea was allowed seasonally. Maximum swings greater than 3 meters in the Danakil Sea would not be consistent with healthy coral reefs, so a practical maximum energy storage capacity at Danakil would be around 1800 GW-hours maximum while still managing the Danakil Sea as a healthy marine environment (to be refined with better data later).
[i] Faulkner, Roger; Mumby, Ken; “Evaluation of a Pumped Storage Facility Operating Between Lake Erie and Lake Ontario,” presented at the Energy Storage Association meeting, May 22, 2009
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