Variable renewable energy

• 1. Introduction

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  Variable renewable energy sits at the heart of one of the most complex engineering challenges of the modern age. Wind turbines spin when the wind blows. Solar panels generate electricity when the sun shines. Neither can be switched on to order. That gap between what the grid needs and what nature provides is the defining problem the world must solve if it is to run on clean power. By 2021, wind and solar together supplied 11% of global electricity. In Denmark, Luxembourg, and Uruguay, that share had already crossed 40%. And in Britain, planners were targeting over 65% by 2030. The questions those numbers raise are the ones this documentary will explore: how does a grid stay stable when its main sources of power come and go with the weather, what tools exist to keep the lights on, and how much variable power can a modern grid actually absorb?

• 2. What Variability Means

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  Tidal power is the most predictable of all variable renewable energy sources. The tides reverse twice a day and, unlike wind or sun, they never disappear unexpectedly. That single fact highlights the wide spectrum inside the category called variable renewable energy. At one end sits tidal power, its rhythms locked to the orbit of the moon. At the other end sits solar, which produces nothing at night and very little in bad weather, and wind, whose output can shift by more than 10% within five hours.

  The industry uses a precise vocabulary to map this spectrum. Intermittency describes unpredictable fluctuation. Variability describes the predictable kind, such as the daily arc of the sun. Dispatchability is the ability to add output on demand. Penetration is the percentage of annual electricity consumption that a given source supplies. Capacity factor, perhaps the most useful of all these terms, is the ratio between electricity a plant actually produced and what it would have produced running at full nameplate capacity all year. Standard photovoltaic solar carries an annual average capacity factor of 10-20%. Panels that track the sun reach up to 30%. Thermal solar towers with storage can reach 73%.

  Wind sits in a different range. A typical wind farm runs at an annual capacity factor of 25-50%, with offshore installations outperforming onshore. That average, though, conceals extreme short-term variation. There is an 80% chance that wind output will change less than 10% in an hour. But over five hours, there is a 40% chance it will shift by more than 10%. The grid operator's job is to absorb those swings without the lights flickering.

• 3. The Grid's Built-In Tolerance

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  An equivalent-sized conventional power station can fail totally, instantaneously and without warning. A large wind farm, by contrast, is unlikely to have to shut down completely in less than half an hour even in the worst storms, and that shutdown is foreseeable through weather forecasting. This difference in failure mode is one reason grid operators view variability differently from fragility.

  Power grids have always carried uncertainty. Demand swings sharply at predictable moments, such as the end of a popular television broadcast, and unpredictably when something unexpected happens. A large conventional plant can trip offline in an instant. To handle these existing uncertainties, every managed grid already carries operational reserve and spinning reserve, which are partially loaded plants held ready to respond within 30 seconds to 30 minutes. Adding intermittent renewables does not automatically require 100% backup capacity, because reserves are calculated across the whole system, not plant by plant.

  At low levels of penetration, the capacity credit of wind, meaning the share of firm conventional capacity it can displace, roughly matches its capacity factor. As wind's share of the grid rises, that capacity credit percentage drops. The grid must work harder to stay balanced. Somewhere between 70% and 90% of combined wind and solar, without additional tools such as storage, interconnection, or demand management, technical limits begin to bite. With 12 hours of storage, that ceiling rises to around 94%.

• 4. Smoothing Supply

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  Quebec produces over 90% of its electricity from hydropower, and Hydro-Quebec is the largest hydropower producer in the world. Regions with that kind of hydro backing can absorb large swings in wind and solar output relatively easily, by ramping generation up or down to compensate. Norway, Brazil, Manitoba, and the U.S. Pacific Northwest share this advantage. For most of the world, the path to smooth supply requires assembling a toolkit rather than relying on a single solution.

  Geographic diversity is one of the cheapest tools available. A single wind turbine varies enormously from minute to minute. Link dozens of turbines in a wind farm, and the statistical variation drops because no two turbines face identical wind at the same moment. Spread farms across a wide region, and variation drops further still. The practical limit appears at roughly synoptic scale, around 1,000 km, beyond which a weather system covers the entire area and diversity stops helping.

  Combining wind and solar directly addresses a seasonal mismatch. Wind tends to peak in winter and at night. Solar peaks in summer and midday. In many countries, the two sources naturally counterbalance each other. Nevada Solar One, a thermal solar plant, is somewhat matched to summer peak cooling loads in the southwestern United States. The small Spanish Gemasolar Thermosolar Plant uses thermal storage to extend generation beyond daylight hours, improving the match between supply and local consumption. The trade-off is that thermal storage with its longer generation window reduces maximum peak output while spreading power across more hours.

  International interconnection extends the same logic across borders. Denmark carries a high share of variable renewables within its own borders. Viewed as part of the wider German, Dutch, and Scandinavian interconnected grids, that share is considerably smaller as a proportion of the total system. Europe and the U.S.-Canada corridor already practise routine cross-border energy trading to balance supply and demand.

• 5. Storage and Demand Response

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  Pumped storage hydropower is the most prevalent storage technology in use today, and it carries a typical round-trip efficiency of around 80%. Water is pumped uphill when generation exceeds demand and released through turbines when demand exceeds supply. Its availability varies by geography. Not every grid has suitable terrain.

  Flywheel energy storage systems offer a different profile. They can go from full discharge to full charge within a few seconds, a response rate chemical batteries cannot match. They can be cycled frequently without significant life reduction, and they can be built from non-toxic, recyclable materials. Grid-scale battery plants, by contrast, provide immediately available power for roughly an hour, buying time for slower generators to start up and substantially reducing the spinning reserve a grid must carry at all times.

  Demand response flips the equation. Rather than adjusting supply to match demand, it adjusts demand to match supply. France runs a formal version of this system: large users including CERN cut power consumption as required by the system operator, EDF, under the terms of the EJP tariff. The American and British systems have created similar incentive structures, offering favorable rates or capital cost assistance to consumers who agree to reduce load during shortages or increase it during surpluses.

  Certain loads are particularly well suited to this flexibility. Desalination plants, electric boilers, and industrial refrigeration units can store their output, whether water or cold, and shift their electricity consumption to periods when power is cheap and abundant. The International Energy Agency identifies sector coupling, connecting electricity markets to mobility, heat, and industrial gas, as a necessary tool for handling the seasonal mismatch between renewable supply and demand. Electric vehicles represent one of the largest emerging sources of storage capacity within that framework, able to charge during low-demand periods and, in some grids, return power via vehicle-to-grid connections.

• 6. Economics of Integration

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  Transmission capacity for wind and solar tends to cost more per unit of energy delivered than for nuclear or coal, because lines are sized for peak output while average output is significantly lower. The cost per unit of energy actually transmitted is therefore higher. Transmission costs, though, remain a low fraction of total energy costs.

  At low penetration levels, the additional costs of operating reserve and grid balancing are widely considered insignificant. As penetration rises, those costs grow. The magnitude is contested and varies sharply by location. Fossil fuel generation carries its own external costs, including greenhouse gas emissions and habitat destruction, that are generally not directly charged to producers, which complicates direct comparisons.

  In many countries, governments manage investment risk by inviting companies to tender sealed bids to build a fixed capacity of solar connected to specified substations. The government accepts the lowest bid and commits to buy electricity at that price per kilowatt-hour for a fixed number of years, or up to a set total volume. This removes price uncertainty for investors. Currency risk remains for projects financed in foreign currency.

  Some forecasters expect that combinations of batteries with solar and wind power, which they call near-firm renewables, will undercut existing nuclear plant on cost before the end of the 2020s. Britain's National Grid Electricity System Operator has stated publicly that the system will be capable of operating zero-carbon by 2025 whenever sufficient renewable generation is available, and may reach carbon-negative status by 2033. Whether fossil fuel plants that can no longer compete on cost will become stranded assets before their planned service lives end is a question that will shape investment decisions for the next decade.