A wind turbine that cannot spin because the air is still, or a solar panel shrouded by cloud, does not simply produce less electricity — for the grid operator balancing supply against demand every few seconds, it produces a scheduling problem that fossil-fuel plants, which can be throttled up or down more or less on command, rarely posed at the same scale. As renewable sources have grown from a marginal contributor to a substantial share of many national grids, the question of how to store surplus electricity generated when the wind blows or the sun shines, for release when it does neither, has shifted from an engineering curiosity to a central constraint on how fast decarbonisation can proceed.
Lithium-ion batteries, now familiar from electric vehicles, dominate short-duration grid storage — smoothing supply over minutes to a few hours — because their cost has fallen sharply and their response time is nearly instantaneous. But batteries of this kind remain a poor fit for the multi-day lulls that wind and solar generation periodically suffer, since scaling battery capacity to cover several consecutive cloudy, windless days would require quantities of lithium, cobalt and manufacturing capacity that remain prohibitively expensive at grid scale. For that longer duration, other technologies compete: pumped hydro storage, which uses surplus electricity to pump water uphill and releases it through turbines when needed, remains the most widely deployed form of large-scale storage worldwide, but is geographically constrained to locations with suitable elevation and water access; newer entrants, including compressed-air storage and various forms of thermal storage, aim to fill the gap in regions without such terrain, though none has yet achieved pumped hydro's combination of scale and cost.
None of these solutions, on their own, resolves the underlying mismatch between an intermittent supply and a demand that must be met continuously; grid planners increasingly speak instead of a "storage portfolio" — layering technologies with different response times and durations, alongside transmission links that move surplus power between regions with complementary weather, and demand-side flexibility that shifts some consumption to match supply rather than the reverse. Whether this layered approach can scale quickly enough to keep pace with renewable deployment remains, by most technical accounts, the more binding constraint on decarbonisation timelines than the cost of generation itself.
Lithium-ion batteries, now familiar from electric vehicles, dominate short-duration grid storage — smoothing supply over minutes to a few hours — because their cost has fallen sharply and their response time is nearly instantaneous. But batteries of this kind remain a poor fit for the multi-day lulls that wind and solar generation periodically suffer, since scaling battery capacity to cover several consecutive cloudy, windless days would require quantities of lithium, cobalt and manufacturing capacity that remain prohibitively expensive at grid scale. For that longer duration, other technologies compete: pumped hydro storage, which uses surplus electricity to pump water uphill and releases it through turbines when needed, remains the most widely deployed form of large-scale storage worldwide, but is geographically constrained to locations with suitable elevation and water access; newer entrants, including compressed-air storage and various forms of thermal storage, aim to fill the gap in regions without such terrain, though none has yet achieved pumped hydro's combination of scale and cost.
None of these solutions, on their own, resolves the underlying mismatch between an intermittent supply and a demand that must be met continuously; grid planners increasingly speak instead of a "storage portfolio" — layering technologies with different response times and durations, alongside transmission links that move surplus power between regions with complementary weather, and demand-side flexibility that shifts some consumption to match supply rather than the reverse. Whether this layered approach can scale quickly enough to keep pace with renewable deployment remains, by most technical accounts, the more binding constraint on decarbonisation timelines than the cost of generation itself.
Question 1
Single correct option
According to the passage, the central challenge posed by renewable energy sources for grid operators is primarily one of:
- A. insufficient total generation capacity compared to fossil fuels.
- B. matching an intermittent supply to a continuous demand, given limited ability to throttle output on command.
- C. the high cost of building new wind and solar installations.
- D. public resistance to renewable energy infrastructure.
Show answer and solution
Answer: (B) matching an intermittent supply to a continuous demand, given limited ability to throttle output on command.
Paragraph 1 frames the issue as a 'scheduling problem' because renewables cannot be throttled 'on command' the way fossil plants can, and the final paragraph reiterates 'the underlying mismatch between an intermittent supply and a demand that must be met continuously' — matching B, not A, C or D, none of which the passage discusses as the central issue.
Question 2
Single correct option
The passage identifies lithium-ion batteries' main limitation, for grid-scale use, as their unsuitability for:
- A. responding quickly to sudden changes in demand.
- B. covering multi-day lulls in wind and solar generation due to prohibitive scale-up costs and material requirements.
- C. use in electric vehicles.
- D. any form of short-duration storage.
Show answer and solution
Answer: (B) covering multi-day lulls in wind and solar generation due to prohibitive scale-up costs and material requirements.
Paragraph 2 states batteries dominate short-duration storage due to fast response, but are 'a poor fit for the multi-day lulls' because scaling to cover them 'would require quantities of lithium, cobalt and manufacturing capacity that remain prohibitively expensive' — matching B and contradicting A and D.
Question 3
Single correct option
Pumped hydro storage is described in the passage as having which combination of characteristics?
- A. Widely deployed and geographically unconstrained.
- B. The most widely deployed large-scale storage method, but limited to locations with suitable terrain and water access.
- C. A newer technology still competing to prove itself at scale.
- D. Cheaper than batteries but rarely used in practice.
Show answer and solution
Answer: (B) The most widely deployed large-scale storage method, but limited to locations with suitable terrain and water access.
The passage calls pumped hydro 'the most widely deployed form of large-scale storage worldwide, but... geographically constrained to locations with suitable elevation and water access' — precisely option B.
Question 4
Single correct option
The passage's closing claim — that the pace of scaling a 'storage portfolio' is a more binding constraint on decarbonisation than generation cost — functions mainly to:
- A. contradict the passage's earlier emphasis on storage challenges.
- B. reinforce the passage's argument by identifying storage, not generation cost, as the key bottleneck going forward.
- C. suggest that renewable generation costs are currently too high to be practical.
- D. argue that fossil fuel plants should be retained indefinitely.
Show answer and solution
Answer: (B) reinforce the passage's argument by identifying storage, not generation cost, as the key bottleneck going forward.
This final claim is consistent with, and reinforces, the passage's throughline that storage (not generation) is the harder problem — matching option B. It does not contradict earlier points (ruling out A), nor does it comment on generation cost being 'too high' (ruling out C) or advocate retaining fossil plants (ruling out D).
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