Solar Energy Perspectives - IEA

Solar Energy Perspectives: Testing the limits
energy, including the electricity, will presumably not differ much from the cost of gasoline or
diesel fuel in ten years from now. It remains to be seen to what extent V2G is to be preferred
over large-scale electricity storage options, or whether it should be seen instead as an ultimate
resource to avoid black-outs in very rare occurrences. 2 One can assume, though, that the
required storage capacities are driven much more by the constraints of responding to peak
demand than by the constraints of avoiding curtailment of renewable variables.
Large-scale electricity storage
Technology options for electricity storage are examined in Chapter 3. The primary candidate
for very large-scale electricity storage is pumped-hydro. The power capacity of some existing
plants might be increased through more frequent use of their existing reservoir capabilities.
But many new stations would be necessary to fulfil the storage requirement of a high
penetration of variable renewables.
The overall potential offered by natural relief is not precisely known, and not all countries
have mountains, but it is probably much larger than the current capacities. Many existing
hydropower stations, sometimes already consisting of several successive basins, could be
turned into pumped hydro stations. In North America, for example, with 100 metres altitude
difference, the lakes Erie and Ontario could provide for 10 GW peaking capacities and very
large storage capacities due to their large surface areas. They could offer one of the few
affordable seasonal storage options, providing the investment in waterways and pumps/
turbines is paid for by daily operations.
New options are also emerging that would allow pumped-hydro stations using the sea as the
lower reservoir. One such pilot plant is already in service in the Japanese island of Okinawa
(Photo 11.1). Seawater pumped-hydro facilities could be built in many places. Ideally
situated sites would allow for a water head of several dozen metres difference between a cliff
top basin and sea level.
If mountainous and coastal natural-lift pumped-hydro plants were not sufficient, it is possible
to build new plants on the sea, entirely offshore or, more likely, coastal, as this would limit
the length of the necessary dykes. The idea is to use dykes to create a basin (Figure 11.7) that
is either higher or lower than sea level. The necessarily low water head in such cases,
however, would require large water flows. In total the costs for very large seawater plants
might be 50% higher than in the cases described earlier, and even higher for smaller plants.
Economies of scale are important here, as the storage capacity increases with the square of
the dyke lengths, which account for a large share of the costs. Although no such plant exists
yet, the concept involves no more than a combination of existing marine technologies.
Resistance to corrosion from salty waters, in particular, has been proven for half a century
with tidal plants such as La Rance in France.
Because use of pumped-hydro storage to manage variable renewable sources is based on
daily operations, it offers a much smaller footprint than ordinary hydro power of similar
electric capacities. A global capacity of 2 500 GW pumped-hydro storage for 50 hours would
require (assuming standard depths for the basins) less than 40 000 km 2 of surface area,
compared to 300 000 km 2 for existing hydropower plants.
2. See Jacobson and Delucchi, 2011b for an extensive discussion of the costs of V2G.
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