Engineering has given a lot to the world.
It’s transformed the nature of work, improved
sanitation and helped create vital infrastructure.
The bad news is that to power the tools and
processes behind those developments, we’ve
relied on non-renewable fuels –
the kind that get produced at a much
slower rate than we use them.
As the name implies, non-renewables won’t
be around forever.
Resources like oil and natural gas might be
gone in just half a century.
And using them has been, frankly, pretty terrible
for the environment.
87% of the harmful carbon dioxide emitted by humans in the last 50 years has come from burning fuels such as coal, oil, and natural gas, known collectively as fossil fuels.
It’s been terrible for the atmosphere and oceans,
and is changing our climate in dangerous ways.
Whether we like it or not, we’re gonna have
to find new ways to power our world.
[Theme Music]
Despite their terrible effects on the environment
and limited supply, for now, non-renewables do
a really good job of meeting our energy needs.
In 2017, 80% of the power used in the United
States was supplied using fossil fuels.
And the need for energy doesn’t appear to
be shrinking any time soon.
Another 9% was delivered from nuclear fission,
the process of splitting atoms, which releases
far less CO2.
Unfortunately, fission produces radioactive
waste and also relies on non-renewable fuel
sources such as uranium and plutonium.
All of these methods operate on broadly the
same principle, essentially operating as a
heat engine.
A working fluid, often water, is heated by
the fuel to expand and do work, turning the
blades of a turbine.
The turbine is connected to an electrical generator
that converts the rotational motion of the blades into
electrical power, which is then fed into the grid.
So what about that remaining 11% of power?
That came from renewable energy sources – the
kind that are generated about as fast as we use them.
Some of the major renewable energy sources
come from processes that are naturally occurring
on Earth:
wind power; solar power; hydropower – which is
based on flowing water; and geothermal power,
which uses the heat of the Earth deep underground.
None of these sources are things we’ll run
out of – we have a good few billion years 
left of sunlight, for example.
And what’s more, renewable energy tends
to release fewer harmful byproducts, like
carbon dioxide, into the environment.
Take hydropower, for example, which converts
the kinetic energy from the motion of running
water into electrical power.
In a fast flowing river, a run-of-river power plant
diverts part of the river’s flow, sometimes through
a tunnel, to turn the turbinesof a generator.
That works well in some places, but the problem
with this approach is that it’s tricky to control the
generation of energy to meet demand.
You don’t want to put lots of power into the grid when
it won’t get used, and you want to be able to ramp up
the supply when the demand suddenly spikes.
Like during the halftime break when the English
football team – that’s soccer to you Americans –
played Colombia in the 2018 World Cup.
A huge number of people in the UK opened their
refrigerators to grab a drink or a snack, causing
the compressors inside them to turn on.
Then there were the people who’d already
had a bunch of drinks.
All those people simultaneously flushing their toilets during the break created an increased demand for power on the local pumping stations that maintain pressure in the water system.
The total increase in demand was measured
to be 1200 megawatts.
That’s an extra demand for power equivalent
to several power plants!
With fossil fuels, you can control the amount
of fuel being burned, and therefore the amount
of power being produced.
Run-of-river power plants struggle with this
because the amount of power they generate
depends on the flow of the river,
which in turn depends on things like the rainfall
during the time period and even the temperature –
both things we can’t control.
To get around this, the more common form of
hydropower is a hydroelectric dam.
In this case, you can install a dam that floods
an area and creates a huge reservoir of water.
The water then falls through the generator’s
turbines at the bottom of the dam, which turn
the water’s kinetic energy into power.
If you install an intake valve that opens
or shuts to control the water flow through it,
you can even manage the production of energy to
meet the changing demands of the electrical grid.
Unfortunately, flooding an area with water
isn’t consequence-free.
Changing the environment so suddenly and preventing
the natural flow of water downstream can have
devastating consequences for the local ecology.
There’s also the risk of the dam breaking
if it was built improperly.
Despite those challenges, hydropower has been
enormously helpful.
In recent years, it’s produced as much as
16% of the world’s energy and up to 70%
of all the world’s renewable energy.
The other renewable energy source that works
very similarly to hydropower is wind power,
which also uses turbines.
The main difference is that the fluid doing
work on the wind turbines is air instead of
water.
One of the biggest engineering challenges
here is designing the turbine blades to efficiently
extract energy from that air.
As we saw with fluid mechanics, predicting
the flow of a fluid around an object can get
seriously tricky!
Blades have to be engineered to withstand the stress
they’re subjected to while also allowing the wind to
efficiently rotate them to power the generator.
It’s as complicated as designing an airplane
wing.
Once again, you run into the problem of demand:
you can’t control the strength of the wind to increase
or decrease power generation as you need it.
Even if that were possible, you’d still have to
transport it from the sparsely populated, open
plains where the wind blows more easily,
to dense urban centers with low amounts of
wind but high demand for power.
Transporting that power becomes even trickier
over long distances because you lose some energy
as the electricity travels through the wires.
For that reason and others, engineering considerations
often play a big role in deciding where wind farms (as
a collection of turbines is known) should be built.
So wind power has only generated 4% of the
world’s total power supply in recent years.
Location also plays an important role in another
renewable energy source: geothermal power.
Like conventional power plants, geothermal
power relies on steam as the working fluid
on the turbines connected to the generator.
But in this case, you don’t need fuel to
generate the steam.
You can drill into underground deposits of hot, volcanic rocks, normally near the Earth’s tectonic plate boundaries, to use them as a heat source for a power plant.
Then, all you need is to pump water to that
location and create another channel for steam
to rise through to do work on the turbines.
The biggest problem comes with setting up
a geothermal power plant in the first place.
It can be expensive to drill and explore for
underground conditions that are exactly right,
and is only really possible in certain parts
of the world, like Iceland and Italy.
But there’s one source of renewable energy
that’s so abundant and easily accessible
you only have to step outside on a bright
sunny day to see it: solar energy!
In fact, the amount of sunlight the Earth
receives in just a single year
is twice the total amount of energy that will ever
be extracted from fossil fuels and the uranium used
in nuclear fission, combined.
The challenge is finding efficient ways to
harness that energy, because turning sunlight 
into electricity isn’t simple.
The most promising technology we have is called
the photovoltaic, or simply, PV cell.
Most people know them by the name given to
many cells arranged together: solar panels!
Unlike everything else we’ve looked at,
there’s no trace of a turbine here.
Instead, as we saw when looking at semiconductors,
solar panels use two different semiconducting pieces to set up an electrical field that biases the movement of free electrons inside the material in a particular direction.
In short, the materials encourage an electrical
current to flow when they receive energy,
which then travels through the circuit delivering
power to whatever’s connected to the PV cell.
That means solar panels can deliver power
directly to the grid.
Between that and the abundance of sunlight,
it seems like there shouldn’t be an energy
shortage problem at all.
But, as we’ve seen for the other energy
sources, costs, fluctuating demand, location,
and transmission all factor in here.
For starters, solar cells aren’t all that
efficient.
The very best solar cells can convert 40%
of the energy they absorb into electrical power,
but they’re expensive to produce because of
the high quality of silicon needed in manufacturing,
among other reasons.
On average, industrial PV cells are about
17% efficient.
Once you factor in the cost of making the cells and energy storage, solar power ends up being anywhere between 3-6 times as expensive to produce as that from fossil fuels.
Increasing solar panels’ efficiency would
bring this down dramatically.
Another big challenge with solar power is that, like with the hydroelectric dam, you need a way to store energy to control the production in line with power demand.
You won’t generate much solar power on a
cloudy day, whereas you might have a surplus
on sunny days.
But you can’t store sunlight directly!
Instead, engineers are working on ways to
temporarily store that extra solar power.
These include solutions like batteries, or even
pumping water up a column to later give up its energy
as hydropower during periods of high demand.
Once again though, efficiency plays a big
role in making both of these methods a suitable
form of energy storage.
Despite the efficiency and storage problems,
there is one major advantage to solar panels:
they can be deployed pretty much anywhere.
Rather than having to transmit power across
long distances, solar panels can simply be installed
on smaller scales close to areas of demand –
even on the roof of an individual home.
But manufacturing the panels themselves brings
its own set of issues.
One of the raw materials used to currently make solar
panels is quartz, which has to be processed to produce
the high quality silicon needed for making PV cells.
This itself is an energy intensive process,
which offsets some of the total energy production
of solar panels across their lifetime of usage.
Even worse, processing quartz can often produce toxic byproducts like tetrachloride, which can end up spilling into the environment and causing damage to soil.
That all sounds a little bleak, but the most
difficult challenges in engineering are often
the most important ones.
In fact, the National Academy of Engineering
in the US has identified making solar energy more economical as one of the Grand Challenges that engineers in the 21st Century need to solve.
Future engineers have lots of ways to contribute
towards making solar more feasible.
Currently, researchers are looking at new
storage systems, such as using solar power
to drive hydrogen fuel production,
which can be burned later on with no
carbon dioxide emissions.
More on that next time.
Engineers are also introducing new materials into the production of solar panels, and improving the ways in which PV cells themselves are linked and arranged on the panels.
There are even experimental methods being
developed that use new structures on a molecular
level, called nanocrystals.
These increase the amount of energy given to the
electrons in the material when ligh is absorbed
instead of losing the energy as heat.
That could drive the efficiency high enough to make it
economically competitive with current power sources
and increase the adoption of solar worldwide.
So there are lots of challenges ahead in bringing
renewable energy sources to the forefront
of electrical power production.
But that’s all the more space for future
engineers to have an impact and create new
solutions to the world’s energy needs!
In this episode we looked at renewable energy
sources and why we need them.
We looked at how hydropower, wind, geothermal,
and solar power are used to produce electricity,
some of the challenges faced in doing so,
and the areas engineers are working on to
make their use more widespread.
In our next episode, we’ll see how engineers
have moved beyond natural processes, to invent
entirely new ways of generating power.
Crash Course Engineering is produced in association
with PBS Digital Studios, which also produces Eons,
a series that journeys through the history of life on Earth.
With paleontology and natural history, Eons
takes you from the dawn of life, through the
so-called “Age of Dinosaurs”, and right
up to the end of the most recent Ice Age.
Crash Course is a Complexly production and
this episode was filmed in the Doctor Cheryl C.
Kinney Studio with the help of these wonderful
people.
And our amazing graphics team is Thought Cafe.
