Variable renewable energy

The sun and wind make power.

But they do not work all the time. The wind may stop blowing. The sun goes away at night.

This can make it hard to have light. We can use big batteries to save the power. Some plants use heat to keep power for later.

This helps us have power when we need it. It is good to use many kinds of energy.

Sun and wind make clean power. But they are variable. This means they change all the time.

Solar power needs the sun. It does not work at night. Clouds or dust can also slow it down.

Wind power depends on the wind. If the wind is too low, turbines do not spin. If it is too high, they must stop.

This can be a problem for the power grid. A grid must match supply with demand. This means power must be ready when people use it. We can fix this in many ways. One way is to use storage. We can use batteries to save power.

Some solar plants use heat. They store heat in tanks of salt. This lets them make power even after sunset.

We can also use tidal power. Tides move in a predictable way. This makes them very reliable. Using many different sources helps keep the lights on.

Some energy sources are hard to control. We call these variable renewable energy sources. This means the power comes from things like the sun and the wind. These sources are not dispatchable. Dispatchable means you can turn them up or down whenever you want. Wind and solar change because the weather changes. This is different from geothermal power or dammed water. Those sources are more constant and easier to manage.

How does this work in our world? The sun shines and the wind blows, but they do not stay the same. Solar power depends on sunlight at a specific place and time. It can be slowed by dust, fog, or clouds. It also stops working at night. Wind power depends on how fast the air moves. If the wind is too low, turbines cannot make electricity. If the wind is too high, turbines must shut down to stay safe.

Scientists and workers have studied this for a long time. In 2021, wind and solar made a small part of global power. Wind made 7% and solar made 4%. However, some places use much more. Denmark, Luxembourg, and Uruguay made over 40% of their electricity from wind and solar in 2021.

We use weather forecasts to help plan for this. These forecasts help grid operators know how much wind or sun to expect.

There are many real numbers to know about these sources. Solar panels have a capacity factor of 10% to 20%. If the panels move to track the sun, they reach 30%. Some solar plants use heat to reach 73%. Wind power usually has a capacity factor of 25% to 50%. Offshore wind often does better than wind on land.

Tidal power is special because it is very predictable. The tides change twice a day because of the moon.

We can solve the problem of changing power with clever tools. One way is to use energy storage. We can turn sunlight into stored energy in batteries. This energy can be used later when the sun is gone.

Some solar plants use tanks of molten salt to store heat. This lets them make power even after the sun sets.

Using many different sources helps keep the power grid steady and strong.

Variable renewable energy (VRE) refers to energy sources that fluctuate based on natural conditions. These sources are often called intermittent renewable energy sources (IRES). Unlike controllable sources, VRE is not dispatchable. Dispatchability is the ability of a power source to increase or decrease its output on demand. This makes managing the electrical grid a complex task for operators. They must constantly match the supply of electricity to the actual demand from consumers.

The mechanism of variability involves both predictable and unpredictable changes. Predictable variability follows regular patterns, such as the day-night cycle for solar energy. Unpredictable variability comes from local weather changes that are hard to forecast perfectly. For wind power, output depends on wind speed, air density, and specific turbine characteristics. If wind speeds are too low, turbines cannot generate electricity. If speeds are too high, turbines must shut down to prevent damage.

There are several distinct types of variable renewable energy. Solar power depends on sunlight and can be affected by dust, fog, or clouds. It naturally stops producing power at night. Wind power varies across different geographical scales. While a single turbine might change rapidly, connecting many turbines over large areas can smooth out the average output. Tidal power is a unique type because it is highly predictable. Because tides are driven by the moon, they are reliable and reverse twice a day.

Understanding the efficiency of these sources requires looking at the capacity factor. The capacity factor is the ratio of actual electricity produced over a period to the amount that would have been produced if the plant ran at its nameplate capacity. Nameplate capacity is the theoretical maximum output under ideal conditions. For standard solar panels, the annual capacity factor is usually between 10% and 20%. If the panels move to track the sun, this can rise to 30%. Wind power typically has an annual capacity factor between 25% and 50%, with offshore wind often performing better than onshore wind.

In 2021, the global penetration of these technologies was relatively low. Wind accounted for 7% of global electricity, and solar accounted for 4%. However, some nations have much higher levels of penetration. In 2021, Denmark, Luxembourg, and Uruguay each generated over 40% of their electricity from wind and solar. As the concentration of wind power on a grid rises, its capacity credit—the amount of conventional power that can be removed while maintaining reliability—tends to drop.

Engineers use several solutions to integrate large amounts of VRE into the grid. One method is energy storage, which converts sunlight or wind into potential energy in batteries. This energy is released when the original source is unavailable. Some solar thermal plants use molten salt tanks to store heat. This allows them to continue generating electricity even after the sun sets.

Another solution is sector coupling, which connects the energy sector to buildings, transport, and industry.

These systems connect to broader goals of grid stability and decarbonization. While small amounts of intermittent power have little effect, large amounts may require a total redesign of grid infrastructure. Operators can also use overcapacity to ensure enough energy is produced during poor weather. They can also use dispatchable sources, like hydroelectricity or biomass, to balance the fluctuations. By combining many different sources and storage methods, the grid can remain reliable even as we move toward more renewable energy.