How should we generate electricity?

Last post, we discussed various alternatives to fossil fuels. In this post, we will take the perspective of the grid. How do each of these power sources impact the functioning of the grid?

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Prerequisites: Part I: Overview of Alternative Power Sources.

Originally Written: March 2021.

Confidence Level: Numbers are approximate, but are the right order of magnitude. Finding all of the sources for this was as hard as writing it, so feel free to check them.

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Introduction

Our electrical grid is extremely impressive. It makes energy extraordinarily accessible, which we use to improve our lives in numerous ways.

The amount of electricity produced must always equal the amount of electricity used. If more electricity is used than produced, there will be blackouts. If more electricity is produced than used, we can store a little of it, but most of it will be wasted.

This is the main challenge from the perspective of the grid. How do you ensure that the amount of electricity produced always equals the amount of electricity used?

Electricity Demand

The amount of electricity used is not constant. More electricity is used during the day than at night. Electricity use also changes on the weekends and seasonally.

Grid engineers divide this demand into three parts. Baseline demand is the amount of electricity used all the time. Variable demand is the amount of electricity used which changes predictably. Surge demand is when there is unusually large amount of electricity used. It could be caused by holidays, extreme weather, or something else. While surge demand is important in determining how much excess capacity you need, I will mostly neglect it to make my model simpler (and more favorable to non-fossil fuels).

Types of Electricity Generation

Power sources can be divided into three categories based on how they produce electricity: steady, on demand, and intermittent.

On Demand Electricity Generation

Hydroelectric dams and natural gas power plants produce electricity on demand. These are also called ‘dispatchable’ power sources.

You can control how much electricity is produced. It takes at most a few minutes to turn them on or off.

These are the most useful power sources from the perspective of the grid, because you can use them to match rapid changes in demand.

Steady Electricity Generation

Nuclear power plants and coal power plants are both steady. It is prohibitively difficult to turn them up or down. Coal power plants take hours to fire up or to turn off. Nuclear power plants take weeks or months.

If there are variations in electricity demand that are slower than the time it takes to turn a steady electricity source up or down, then steady power sources act like on demand sources. Coal and nuclear can respond to the predictable change in seasonal demand, but not to fast fluctuations.

Intermittent Electricity Generation

Solar panels and wind turbines produce electricity intermittently. You do not control how much electricity is being produced right now. This is challenging from the perspective of the grid. The easiest thing to do is to have on demand backup that you can turn on whenever the intermittent power source goes off. You could also store energy from when there is more electricity produced to use later.

There are three important types of intermittency: daily, seasonal, and lulls.^[1]Not to be confused with LOLs.

Daily

The sun does not shine at night. The production of solar energy drops to zero. This is extremely predictable and only short-term. Demand is also lower at night and the wind is usually stronger. It is plausible (but difficult) that we could deal with this by storing the energy needed at night.

Seasonal

There is more sunlight in the summer than the winter. There is more wind in the winter than the summer.^[2]This may not be true at every location, but it is usually true. Wind is driven by temperature differences and there is a larger difference in temperature between the equator and the pole in winter … Continue reading Demand is higher in the summer in warm climates due to air conditioning and higher in the winter in cold climates due to heating. There is no way we could store months worth of energy.

If we want to use solar and wind for the majority of our power, we need to choose the ratio of solar to wind so that the combined seasonal fluctuations match the variations in demand. Studies from Europe suggest about 75% wind to 25% solar is best,^[3]Source. but this is different for different climates. Solar and wind should be treated as a single power source, if they are used in large enough quantities, because their ratio is determined by the seasonal variability.

Lulls

Lulls are caused by weather systems. Solar panels are much less productive when it’s cloudy. Wind turbines don’t produce any electricity when the wind is not blowing. The largest lulls last for about a week and have an area the size of small continents.^[4]The most common large lulls are due to major storms (hurricanes, nor’easters, etc.) or due to meteorological blocks. Lulls are also unpredictable.

To deal with intermittency due to lulls, you either need energy storage for several days or you need to be able to transport enough electricity a distance larger than the lull.

Intermittency due to lulls is the biggest technical challenge to using solar and wind to produce all of our electricity.

Capacity and Production

Since many power sources do not produce all the time, it is useful to distinguish electrical capacity from electrical production. Capacity is the maximum about of power that could be produced at ideal operating conditions. Production is the average amount of power produced. The ratio is called the capacity factor.

Steady power sources tend to have high capacity factors. Nuclear power plants have a capacity factor of about 90%. Production is 90% of capacity.

Intermittent power sources tend to have low capacity factors. Wind turbines have capacity factors of 30%-40%. Solar panels have capacity factors of 15%-30%.^[5]These are averages for the US. Source. This depends on the season and where the capacity is located. Solar has a much higher capacity factor in the Southwest US than in Northern Europe.

On demand power sources have widely varying capacity factors because they can be controlled.

Energy Storage

Energy storage looks like an attractive way to deal with intermittent power sources. We could store the extra electricity produced when the sun is shining and the wind is blowing for when it’s needed during the lulls.

Today, the US has 250 GW-hr of electric energy storage.^[6]Source. This is a Wikipedia link. The relevant citation [3] takes you to a dead link. Pulling up the link on the archive for July 7, 2020 gave a database of energy storage project, without aggregate … Continue reading Recall that the US uses 489 GW of electric power. We can currently store only about half an hour of electricity.

Pumped hydroelectricity accounts for 95% of the electricity storage in the US.^[7]Source. Note that they are more interested in maximum power supplied by the storage than in the amount of energy stored. These numbers are consistent: if it takes about 10 hours to discharge the … Continue reading Water is pumped up a hill, and then hydroelectricity is generated when it flows back down the hill. We cannot expand this very much, for the same reasons that we can’t expand hydroelectricity very much: most of the best sites are already used.

There are various other ways to store electrical power: compressed air, thermal storage, flywheels, and batteries. Each of these would have to be scaled up dramatically to be relevant at the scale of the grid.

I will focus on lithium ion batteries. While lithium ion batteries are not being used extensively for electricity storage, they are now mass produced for electric vehicles. We have some sense for how their economics scales.

The costs of lithium ion batteries has plummeted by 90% over the last decade. They currently cost $132 per kW-hr.^[8]Source. At this rate, it would cost $65 billion to buy enough lithium ion batteries to store one hour of electricity. This is about how many lithium ion batteries are currently being made in the world in a year and 10 times the battery production rate for the US.^[9]We would need 489 GW-hr of batteries for this project. In 2020, the world produced 500 GW-hr of lithium ion batteries, including about 50 GW-hr in the US. By 2030, this is planned to increase to … Continue reading

Lithium ion batteries last for about 5 years or 2,000 charging cycles. ^[10]Source.

Grid scale energy storage requires significant new developments in battery technology or production rates. This is not impossible, especially considering how much these and other costs have fallen in the last decade. Today, research in energy storage is more important than research in solar panels and wind turbines.

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The next post will put these estimates together to form a simple model to look at how we could transition a country’s electricity production away from fossil fuels.