### Solar Energy

The Sun is critically important to life on Earth, and sunlight has shaped the terrestrial environment since the Earth was formed. It is difficult to overstate the importance of the Sun, and one way of looking at this is to consider what types of life would be possible without the Sun.

What types of terrestrial life can survive without sunlight?

# The Essential Energy of Sunlight

Select one or more

With the exception of bacteria deep in the ocean that use the energy from hydro-thermal vents and organisms that eat those bacteria, every living thing on Earth depends on sunlight to survive. Plants rely on sunlight to grow, and would therefore die without sunlight. This means that any plant-eating animal also depends on sunlight for survival, as does any animal that eats a plant-eating animal, and so on up the food chain.

# The Essential Energy of Sunlight

You may think that humans required sunlight for survival in the past, but with modern innovations and technology we could live comfortably even without sunlight. A challenge to this idea is that most modern technology requires electricity, and almost all of the electricity we generate comes from sources that wouldn't exist without the Sun.

Which electricity sources ultimately come from sunlight?

# The Essential Energy of Sunlight

Select one or more

Most of the energy sources humans use to generate electricity come, ultimately, from the Sun (the exceptions are tidal, geothermal, and nuclear power). Windmills depend on the Sun, as wind fundamentally results from uneven heating of the Earth's surface due to sunlight. Hydro power depends on the Sun because rivers would not flow without the Sun powering the water cycle, evaporating water, and precipitating it at higher elevations.

Even fossil fuels depend on the Sun, as they originally came from plants, with thanks to photosynthesis and the Sun. Least surprising of all, solar power depends on the Sun for its power. Without the Sun, our ability to generate electricity would be severely handicapped.

# The Essential Energy of Sunlight

While there are a wide variety of ways to generate electricity, most electricity is generated by burning fossil fuels like coal, natural gas, and oil. Fossil fuels form after organic matter (primarily from plants) is exposed to heat and pressure in the Earth’s crust. The process of converting organic matter into fossil fuels can take millions of years.

By some estimates, the total supply of energy in fossil fuels in the Earth's crust was around $10^{23} \si{\joule}$ before humans started extracting and burning these fuels. If it took 500 million years to accumulate those fossil fuels, what is a reasonable estimate for a sustainable annual usage rate of fossil fuel energy?

# The Essential Energy of Sunlight

Humans currently use about $10^{20} \si{\joule / year}$ of electricity globally, which is much higher than the sustainable utilization rate for fossil fuels.

Adding to the challenge of dwindling fossil fuel reserves are the facts that:

1. The energy in fossil fuels can't be converted to electricity at 100% efficiency. This means generating $10^{20} \si{\joule / year}$ of electricity requires burning more than $10^{20} \si{\joule / year}$ of fossil fuels.

2. Fossil fuels are burned for more than just generating electricity - we also burn them to power most cars, for example.

At current usage rates we will run out of fossil fuels in roughly 100 years. If we want to continue our high energy consumption habits for the foreseeable future, we need to transition from fossil fuels to other electricity generation technologies.

# The Essential Energy of Sunlight

Since humans can use up fossil fuels much faster than they can be replaced, fossil fuels are known as a non-renewable energy source. This is in contrast to renewable sources, which don't get depleted by human behavior. For example, building windmills allows us to extract power from the wind, but if those windmills are taken down, the wind will blow just as strong as it did before the windmills were constructed. Dismantling a coal power plant, on the other hand, doesn't replenish the reserves of coal in the Earth's crust that were burned in that power plant. Therefore wind is a renewable energy source and coal is not.

Renewable energy sources don't get depleted like non-renewable sources do, but there is a limit to how much power they can provide. For example, if there are $\SI{100}{\tera\watt}$ of wind on Earth, it would be impossible to generate more than $\SI{100}{\tera\watt}$ of electricity from wind power, no matter how many windmills we build.

Which renewable energy source is the most abundant on Earth? That is, which renewable energy source has the potential to provide the most electrical power?

Hint: you can estimate the abundance of an energy source by predicting how much the climate/temperature would change if that energy source was no longer available. Removing a more abundant energy source will have a greater impact on the climate.

# The Essential Energy of Sunlight

Solar energy is by far the most abundant renewable energy source, and the amount of solar power that reaches the Earth's surface is orders of magnitude greater than current global energy demand (as well as the current reserves of fossil fuels). Therefore, with effective and efficient technologies for collecting solar energy, we could conceivably meet the entire global energy demand with solar power.

Relative availability of various energy sources: Note that renewable resources are reported in terms of their annual replenishment (in $\text{TWy/y}$), while finite resources (like coal) are reported in terms of the total amount remaining on Earth (in $\text{TWy}$).

While solar power has the advantage of sunlight being abundant, it faces other challenges. In fact, despite the abundance of solar energy on Earth, solar power currently provides a very small portion of the world's electricity, even compared to some other renewable sources. In this course, we will explore the promises and challenges of harnessing solar energy.

# The Essential Energy of Sunlight

When most people hear "solar power," they probably think of a technology called photovoltaics, or PV for short. PV is the most common solar electricity technology, and in some cities it is common to see PV panels of the roofs of homes and businesses. In photovoltaics, sunlight incident on a special material called a semiconductor excites electrons, and these excited electrons are collected as electrical current. PV provides a very direct method for converting sunlight to electricity.

PV panels on the roof of a home. Photo by Gray Watson

# The Essential Energy of Sunlight

The second most common solar technology used for electricity generation is called solar thermal energy. In solar thermal energy systems, mirrors concentrate sunlight onto absorbers, where that sunlight is absorbed as high temperature thermal energy. That high temperature thermal energy is then used to turn a turbine (for example, by boiling water into steam and directing that expanding steam through the turbine blades) which turns a generator to generate electricity. A solar thermal plant is like a conventional fossil fuel fired power plant, but rather than burning fossil fuels to provide heat input, sunlight is used instead.

# The Essential Energy of Sunlight

Solar energy is a promising renewable energy source, but understanding the technologies behind solar energy conversion requires deeper knowledge about sunlight, radiation, and physics than most of us pick up in day to day life. The rest of this chapter will introduce the physics behind sunlight and the next chapter will examine the availability of the solar resource on Earth. These first two chapters will provide the foundation required to understand specific solar energy technologies, which are explored over the rest of the course.

# The Essential Energy of Sunlight

×