Published on EGEE 101: Energy and the Environment (https://www.e-education.psu.edu/egee101)

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Lesson 3: Renewables

4 small images: water in a dam, power lines, solar panels, and a windmill
Credit: Mike Fouque [1] / adobe.stock.com [2]

Overview

We have just examined electricity generation from fossil fuels and nuclear power. Now we expand into the renewable elecricity sources.

Lesson Objectives

  • demonstrate how electricity is generated by renewable approaches: wind, biomass, hydroelectric, geothermal, solar (photovoltaic and thermal concentrating), and tidal/wave
  • indicate the advantages and challenges with each
  • know how "electricity" is stored to meet later demand

Hydroelectric Power

Hydroelectric is one of the leading electricity sources for renewable energy, providing 7% of the US total electricity supply.

Renewable water power has been used for years to grind corn and grains into flour or to pump water. With hydropower, we convert stored potential energy into kinetic energy to do work or to generate electricity.

Picture of a historic water mill.
Wheels like this were used for centuries taking the flow of water from rivers and streams to provide power.
Credit: 수동 김 [3] / adobe.stock.com [4]

Potential Energy PE = mgh

M is mass
G is the acceleration due to gravity (~10 m/s2)
H is the height above a reference point

Imagine carrying a sack of potatoes up the stairs of a warehouse. Two things influence how much work you will have to do; the mass of the sack of potatoes, and the number of floors (the height) that you will have to carry the potatoes.

Our potato example would look like this in a formula;

To move a 60 kg sack 5 meters above the ground level becomes:

60 (kg) x 10 (meters per second per second) x 5 (meters)
= 3,000 kg m2s2
= 3,000 Joules (J) or
= 3 kJ

If you recall the energy laws, energy cannot be created or destroyed, and so, if we drop the sack of potatoes, we will obtain the same amount of energy back (but in various forms). Thus, potential energy is stored energy. Hydroelectric energy works in the same way. We use the stored energy in water to produce electricity by flowing the water through a turbine (to spin a generator as usual). The water flows because of the influence of gravity, and that it is above the reference height.

Hydroelectric energy is renewable because the water that flows through the turbine to create electricity is replaced naturally by the water cycle. The sun (solar energy) evaporates water from the seas and lakes; some of the water will form clouds high in the atmosphere that will drift over land, and the water will fall back to earth in some form of precipitation (hail, fog, snow, or rain). The water flows into rivers and streams and the kinetic energy can be used for useful work. In the case of the large hydroelectric operations, the rivers have been dammed to generate large lakes. The dam holds the water back and raises the level of the water above the reference level (the bottom of the dam). In the U.S. we have already used many of the primary sites for large-scale hydroelectric production and those that are left are unlikely to be dammed due to the damage it inflicts on the ecosystem. Hydroelectric power (large scale) was one of the cheapest of the renewable options and a source for a significant quantity of electricity in those regions where it could be employed at scale. For a long time, hydroelectricity was the leading renewable electricity source but has recently been surpassed by wind power in the U.S.

Annual electricity generation from wind and hydro2009 - 2019
Credit: International Energy Agency

China is the leading nation for electricity production and has been increasing its generation capacity with very large-scale additions such as the Three Gorges Dam [5] (almost twice the size of other large-scale sites). The U.S. hydroelectric production has however remained steady for decades.

Hydro electric power in various countries
Hydropower generation in terawatt-hours (TW) by country
Click for a text description.
  • China, 1,302 TW
  • Canada, 398 TW
  • Brazil, 386.95 TW
  • United States, 274 TW
  • Russia, 190.29 TW
  • India, 162.1 TW
  • Norway, 125.77 TW
  • Turkey, 87.09 TW
  • Japan, 86.67 TW
  • Venezuela, 72 TW
Credit: Statista

The energy we are trying to capture is kinetic energy:

Kinetic Energy = 1/2 mv2

Unfortunately, to change all the kinetic energy into electricity would require the water to have no velocity whilst exiting the turbine. This is impractical, as we do not transform all the kinetic energy into electrical energy. We can obtain more energy by increasing the flow rate of the water (increasing the mass of water) and by increasing the velocity of the water (increasing the height of the dam increases the water pressure and thus increases the velocity and the mass). PLEASE SEE THE HTML SOURCE BELOW

Additional types of hydroelectricity production

See the visualizations provided by the "Energy.gov" website, and their section on hydroelectric power [6]. Give it a look with the goal of understanding how hydroelectric power produces electricity. 

Advantages

  • It is able to easily follow the demand curve (change electricity production rapidly) that was covered in lesson 01
  • It is also a cheap large-scale electric source but quality locations are limited
  • One of the major advantages is that some sites can be used to store energy through pumped storage (particularly useful for the storage of renewable energy): The issue of energy storage is a major one. Listen to this audio file [7] for an explanation (transcript [8]).
  • Hydroelectric is a clean energy source producing no air pollution (once setup), and it is a renewable, domestic energy supply. Changes in the turbines might increase the capacity, and small scale additions may be added but with little impact on the overall contribution

Disadvantages

  • One of the environmental issues with hydroelectric power is the alteration of the species in the river. The fish that like the fast-moving cold waters of rivers do not do well in the warmer, deeper lakes. Thus, there is a reduction in the resident species when dams are built. Fish ladders and elevators can help move the fish around the physical barrier, but the impact on both the surrounding area and the ecology is devastating. There is a move now to dismantle some of the smaller hydroelectric facilities and return the rivers to their natural state. Run-of-river hydroelectric schemes might be used but they won't generate large quantities of electricity in a single site
  • Flooding of valleys and the destruction of that ecosystem
  • There is not much growth potential in the US as the likely spots are already used
  • Dams use a great deal of concrete that produces lots of CO2 (a greenhouse gas) for construction
  • With climate change and annual variations, there can be reduced snowfall in locations such as the Pacific Northwest. This has caused reduced hydroelectric electricity supply to California at times. This is one of the main variables related to the production of hydroelectric power — that it is very dependent on precipitation to replenish the water supply
  • Earthquakes or terrorist attacks can also destroy the dams, producing flooding; and a deadly, fast-moving flow of water. During the Iran-Iraq war, Iraq deliberately opened floodgates to flood (impact) a major oil-producing region in Iran. 

Three Gorges Dam

Photograph of the Hoover Dam showing water gushing from the outlets under high pressure
Hoover Dam
Credit: United States Bureau of Reclamation

The worlds' largest dam is the Three Gorges Dam. Producing 20,000 MW (for scale the Hoover Dam, pictured here, generates about 1,400 MW). The river navigation and flood control will also improve because of the dam (flooding has killed 300,000 last century). Hydroelectricity will provide clean electricity for the rapidly growing industries of eastern and central China. This is desirable because most of China's electricity comes from coal power plants close to the cities—lots of polluted air.  Of course, such a large project has rather large costs as well; one million-plus people will have to be relocated, fish migration will be stopped, and there may be increased threats to the survival of the Yangtze River dolphin, along with many other animal species. As is common with most issues of energy and the environment, however, there is plenty of disagreement over whether the dam is really the right thing for this country.

Future Directions for the Hydroelectric Industry

Some consider the hydroelectric industry as "mature" and that the technical and operational aspects of the industry have not changed in the past 60 years. Research is currently underway that concentrates on new concepts for the industry, and one project is testing new turbine designs. This project will hopefully recommend a final blade configuration that will allow safe passage of more than 98% of the fish that are directed through the turbine (I wouldn't want to be part of the 2%.) The US Department of Energy has identified more than 30 million kilowatts of untapped hydroelectric capacity that could be constructed with minimal environmental effects at EXISTING dams that do not have generating facilities at the present time, and also at EXISTING dams that are underutilized, and at a number of sites where dams do not presently exist. This research and planning activity suggests that hydroelectric power could continue to be an important part of the US energy picture for some time to come. In-river hydro is also an option (kinetic energy from flowing water WITHOUT the dam). (See Ocean Energy Technologies [9] of D.O.E.) 

Here is a good overview video (3:50 min) by the U.S. Department of Energy. It is a few years old so it still (now incorrectly) claims that hydroelectricity is the leading renewable energy source for U.S. electricity.

Click here for a transcript.

PRESENTER: People have been capturing the energy and moving water for thousands of years. And today, it's still a powerful resource that can generate clean, renewable, and affordable electricity. You see, we harness energy from flowing water and convert it to electricity. That's what we call hydroelectric power, or hydropower. Water flows from a higher elevation to a lower elevation. And a hydropower facility uses turbines and generators to convert this motion into electricity.

America has been using hydropower to generate electricity for more than 100 years now. And today, about 7% of all our electricity is generated from hydropower, making it the largest source of renewable power.

[MUSIC PLAYING]

So what makes hydropower renewable? It's simple, water. Water evaporates into clouds and recycles back to Earth as precipitation. The water cycle is constantly recharging, and can be used to produce electricity along the way. How does it work? Basically, there are several ways hydropower technologies can generate electricity. You may recognize dams, like this one. This technology is called an impoundment. The impoundment stores water in a reservoir. When the water is released, it flows through and spins a turbine, turning a generator that produces electricity.

Here's another technology. This is a diversion. It channels a portion of a river through a canal or pipe into a turbine and generator system. What's cool about this method is that it uses the natural flow of the river, and usually doesn't require a large dam.

And have a look at this. This is called pumped storage hydropower. Basically, it works like a huge battery. To charge the battery, water is pumped back up into a reservoir during periods of low energy use, often during the night when people are using fewer appliances. Then when people need more power during the day, the water can be released to produce electricity.

[MUSIC PLAYING]

As long as we've been capturing energy from water, you may think there's nothing new in hydropower technology. Think again. The Department of Energy is helping to upgrade older facilities by increasing the efficiency of the turbines and generators. Operators of neighboring hydropower facilities are also working together to optimize energy production across whole river systems, instead of each dam working alone. And we can add generators or retrofit dams that were built without power, like dams used to water crops or prevent floods.

Today, there are about 80,000 dams in the US. But less than 3% of these dams produce power. That means there's a big opportunity to generate more clean renewable power at dams we've already built.

New technology is also making hydropower even more environmentally-friendly. For example, researchers are reducing adverse impacts on fish with fish-friendly turbines. And fish ladders like these let them swim around dams.

Hydropower is an essential, reliable, and renewable source of clean energy with a rich history, and it's meeting substantial energy demands today. With new technologies, it will be even more efficient and have greater production capacity, powering US homes and businesses for centuries to come.

Credit: The Department of Energy

Wind

Wind power generated 7% of the electricity in the U.S. and is now the leading renewable energy source for electricity in the U.S. While wind power is one of the oldest forms of energy it has rapidly increased its contribution in the last decade. The wind blows because of uneven heating of the earth's surface, producing high and low pressure, and thus results in airflow (wind). It is a renewable energy because solar energy (heating the earth's surface) is renewable. Early windmills were used to grind grain or to pump water. When we generate electricity we use wind turbines (not wind mills).

 Map of the United States and its potential for wind energy (concentrated in the mid-west).
United States Annual Average Wind Power map. This map of the US shows the areas where wind farms are more feasible. The velocity, duration, and probability (distribution) of the wind blowing are all factored in when deciding if a particular location is a good site for wind-powered electricity generation. Those locations that are darker in color are better. This is in the mountain regions, Midwest, and some of the coastlines. Other locations are also being used as you can match the wind turbine to the wind resource. Locations with more expensive electricity are also more desirable (can charge more for the electricity generated) so it is not just about the wind resource (renewable portfolio standards, tax breaks, electricity prices, and the availability of a market)  — are all influencing factors.
Credit: NREL

Wind power is one of the fastest-growing sources of electricity. It is now the leading renewable energy source for electricity in the U.S. (surpassing hydroelectric power in 2018). The reason we were historically not utilizing wind is simple: fossil fuels were cheaper alternatives (and more reliable) for electricity generation. As the cost of wind-generated electricity has drastically declined over the last decade, wind-generated power has increased dramatically here and in many other nations with the potential for more improvements. That said, solar energy generation is becoming more desirable and is expected to be much of the renewable electricity increase in capacity.

Graph showing the growth of wind starting around 2006 and surpassing hydroelectricty in 2018
U.S. electricity generation from wind and hydrio in million megawatt-hours (1990 - 2019)
Credit: EIA

So how does it work?

Photograph showing the scale of modern wind turbines with workers standing on the nacelle.
Wind turbines can be big! Here you can see the scale and the inner workings within the nacelle
Credit: NREL

Wind turbines come in different shapes and sizes. The vertical ones tend to be tall to take advantage of the higher wind velocities (more energy), the slower wind velocity at ground level is caused by friction or drag with the earth's surface. One of the tallest wind turbines constructed so far is in Hawaii; the blade is longer than a football field and is 20 stories tall. The blades are shaped so from the side, they are similar to an aircraft wing, with the other side of the blade (from the point of rotation) turned the other way. The blades "push" to produce the spinning rotation. This rotating motion turns gears that produce a much faster rotation within a small generator in the nacelle — producing electricity. The blades can pitch to capture more of the wind energy or to stop the blades when the wind velocities are too high.

We are transforming kinetic energy into electricity.

Recall that KE =1/2mv2

The mass of the wind can be increased by using a bigger blade to "catch" the wind (area of a circle is Pi times the radius squared, so doubling the radius quadruples the area), or having a higher velocity wind (because more air flows within the turbine diameter, but more importantly because the energy in wind is proportional to velocity cubed). Density changes in the air can also increase the kinetic energy of the wind and thus the mass will be greater (density = mass/volume). The density of air is lower than the density of water, so water with a density of one will have much more kinetic energy than the wind at the same conditions of flow! Also, our extraction efficiency is limited, because to transform all of the kinetic energy from the wind into electricity would require the wind to have zero velocity, which if it did the turbine would not turn! Thus, there is a maximum efficiency of 59% (Betz Law).

Watch the following 2:46 minute video about how a wind turbine generates electricity.

Click here for a transcript of the video.

[MUSIC PLAYING]

PRESENTER: How does a wind turbine work? You've probably seen a wind farm. But do you know how wind force is converted into electrical energy? We are going to show you how a wind turbine works.

Each wind turbine has a wind vane at the top that indicates the wind direction. This allows the turbine to rotate on the tower and face the wind. The blades also rotate on their axis for maximum resistance. Wind force, that is the kinetic energy contained in moving air currents, spins the blades.

These are designed to fully capture its energy. They can be as long as 60 meters each and are made of very light and resistant materials for ease of movement. This is why they can produce energy even with very light winds, starting from about 11 kilometers per hour. With very strong winds, above 90 kilometers per hour, the blades are placed in the feathered position, and the turbines stop spinning for safety reasons.

[MUSIC PLAYING]

The blades are attached to the wind turbine through the HUB, which is coupled to the low-speed shaft. The low-speed shaft is given this name because it spins at the same speed of the blades, between 7 and 12 revolutions per minute. To produce electricity, it is necessary to increase the turning speed of the low-speed shaft. That is the mission of the gearbox, which raises the speed over 100 times and transfers it to the high-speed shaft.

The high-speed shaft, that rotates it up to 1,500 revolutions per minute, is connected to a generator. The generator converts the kinetic energy into electricity, a source of energy that is easier to transport and use. The electricity produced in the generator is conducted through the interior of the tower to the base. There, the transformer raises the voltage for transport inside the wind farm.

From each turbine, alternating current is sent to the substation to underground cables. Here, the voltage is increased again to feed it into the power grid and transport it to end consumers. This is how we use the wind to light cities, feed industries, schools, or hospitals, or operate our household appliances in a clean and sustainable way.

[MUSIC PLAYING]

Credit: ACCIONA [10]

Advantages

  • It is renewable
  • It produces no air pollution or greenhouse gases 

Disadvantages

  • Land Use
    It requires a large plot of land to house enough wind turbines to make the wind farm and produce enough electricity. The land below the turbines can still be used for grazing or for crops. Unfortunately, land near population centers is expensive and it is impracticable to have less populated areas like North Dakota produce lots of wind power and ship it to the more populated areas of the U.S.

  • Intermittency and Low Capacity Factors
    The wind does not blow all the time. Thus, you might be getting electricity at a time when it is not needed (we always try to use the cheapest electricity source). The capacity factor is a measure of how much electricity is generated in relation to the maximum quantity. For many wind turbines, this is ~34%.

  • Storage
    Electricity storage is a challenge. Batteries are traditionally expensive, heavy, and not a good storage option but advances in this area are producing near-term viable options but they're not quite there yet (more on this later). Pumped storage is a viable but limited option, large-scale battery operations are also starting to emerge.

  • Noise Pollution
    Wind farms are not the serene creaking and groaning of the old windmills. There is noise, which can be an issue if it is close to residences. Some might add that they are ugly too, but beauty is in the eye of the beholder.

  • Jonathan and a windmill

    Bat and Avian Issues
    Birds seem to have the tendency of flying into the blades, which kills them! For bats, it is getting close to the low-pressure region (they dodge the blades) but then get the "bends" in the decompression zone of low pressure (similar to a deep-sea diver coming to the surface too fast). In some bat sensitive locations, they do not use the turbines in select evenings when the conditions are conducive to bat activity. Many of the avian and noise issues have been solved by going to longer blades, and improved locations (avoiding migration routes). While visiting several windmill farms I could not find a single dead bird. Bats tend to like to swarm the tallest object around and the largest number of kills occur on a single summer night. Some wind turbines may have to stop running on certain evenings in the summer season if there is a lot of bat activity.

  • Supply and Demand
    In some cases, there is a mismatch between where the wind resource is located and where the electricity is needed (population centers). In locations such as Germany, the higher quality wind resource (higher wind speeds and higher capacity factors) is in the north while the population centers are in the south. Thus, new high voltage lines are needed to deliver the electricity, increasing the initial cost. Despite this, they are using more wind power 40% than many nations (however, they also have much higher electricity costs).

Look Further

Spend a few minutes learning more about Wind Power via the US Dept. of Energy's website [11] on wind energy.

Also, note that part of the reason for the growth in electricity generation from wind is the lower cost allowed by using larger turbine sizes (source EWEA).

Figure showing the increasing hub height, blade length, and generation capacity.
Wind turbines have been getting taller (access to higher velocity wind) and providing more power per turbine (so we need fewer larger turbines)
Credit: adapted from EWEA, 2009

Many of the problems can be alleviated and higher energy generation can be generated using off shore wind farms. These have started to emerge as viable options and will start moving to deeper water.

Photograph of an off shore wind warm, ~25 wind turbines off the coast of France
Offshore farm wind turbines near the Dutch coast
Credit: Kruwt [12] / adobe.stock.com [13]

Solar

Recall that we have already covered solar energy from the standpoint of home heating and water heating (passive and active solar heating). Solar energy is also responsible for nearly all the other renewable energy sources as well (solar energy drives the water cycle, creates wind, and is the energy source for biomass). Historically it has had a relatively low contribution to electricity generation in the U.S. That however is about to change. On this page we are concerned with solar energy being converted into electricity, whether directly via solar cells (photovoltaic) which is the majority of the contribution, or indirectly via heating some type of medium (thermal solar), which is then used to generate electricity from steam.

Solar Cells (Photovoltaic)

Solar cells generate 2% of the U.S. electricity supply. We can directly turn solar energy into electricity utilizing photovoltaic technology.  Solar cells are also used in remote locations such as space, and in remote locations where there is not a connection to the electric grid, on RT 322 across PA to power some of the temporary signs (otherwise someone would have to come out and either recharge or replace the battery), on satellite wings, and many other locations. There is a wide range of efficiencies but ~20% is common for solar cells. Cheaper solar cells are less efficient. A dozen or so solar cells can either be placed on the roof (or close to home) or in solar farms where 1,000's of larger cells can be present.

Watch

The following 2 minute video does a great job showing how Photovoltaic (PV) panels convert solar energy into renewable electricity.

Solar PV
Click here for a transcript of the Solar PV video.

PRESENTER: All right, we all know that the sun's energy creates heat and light. But it can also be converted to make electricity-- and lots of it. One technology is called Solar Photovoltaics, or PV for short. You've probably seen PV panels around for years but recent advancements have greatly improved their efficiency and electrical output. Enough energy from the sun hits the Earth every hour to power the planet for an entire year.

Here's how it works. You see, sunlight is made up of tiny packets of energy called photons. These photons radiate out from the sun and about 93 million miles later, they collide with a semiconductor on a solar panel here on earth. It all happens at the speed of light. Take a closer look and you can see the panel is made up of several individual cells, each with a positive and a negative layer, which create an electric field. It works something like a battery.

So, the photons strike the cell, and their energy frees some electrons in the semiconductor material. The electrons create an electric current, which is harnessed by wires connected to the positive and negative sides of the cell. The electricity created is multiplied by the number of cells in each panel and the number of panels in each solar array.

Combined, a solar array can make a lot of electricity for your home or business. This rooftop solar array powers this home. And the array on top of this warehouse creates enough electricity for about 1,000 homes.

OK, there are some obvious advantages to solar PV technology. It produces clean energy. It has no emissions, no moving parts. It doesn't make any noise, and it doesn't need water or fossil fuels to produce power. And it can be located right where the power is needed, in the middle of nowhere. Or it can be tied into the power grid. Solar PV is growing fast, and it can play a big role in America's clean energy economy, anywhere the sun shines.

Credit: DOE

Read

Another nice explanation of How Solar Photovoltaic Cells Work [14] can be found on the DOE's Office of Energy Efficiency & Renewable Energy website.

 Grapic representation of the process used to turn solar energy into electricity.
At the heart of the solar cell is a sandwich of two semiconductors: the n-type and the p-type. The photons in light (if they have the right amount of energy) enter the positive silicon layer (p-type) and excite an electron to "jump" the bandgap into the negative (n-type) layer. The electron leaves behind a "hole" that allows other electrons in the p-type layer to hop in and out. The n-type has atoms that can hold an extra electron and so the electrons can flow (make a circuit). Any photon with too little or too much energy will not promote the electron into the n-type layer. The Silicon chips here are very similar to the chips in your computer. Both require highly purified and crystalline silicon. One of the reasons for the initial high cost of this technology was the silicon material however, the cost has fallen drastically and expectations are for a much greater contribution to electricity generation in the future.
Credit: DOE

Picture of three solar panels.
Left: Backwoods Solar Electric Systems, outside of Sandpoint, Idaho, is located 2 miles from utility lines and uses their off-grid system for home and business power. The system includes photovoltaics, wind power, and a backup generator. 3000 watts of power total are created in this system that was installed in 1978 and upgraded in 1990 and 1999. Right: Part of Littleton, Colorado's "10,000 Trees" project
Credit: LEft, Backwoods Solar Electric Systems,  Right: NREL

Advantages of Solar (PV) Technology

a field completely covered in solar arrays.
Credit: Vladimir Gerasimov [15] / adobe.stock.com [16]

The advantage of using solar technology is no fuel costs, it is a renewable energy that is clean during operation (no air pollution or greenhouse gases) but obviously, energy is used in the creation of the panels, etc. Solar derived electricity had a limited contribution to the US energy profile until recently when prices became more competitive. Where solar panels had initially been most beneficial was in remote locations, where it would be expensive or impossible to link power lines. Falling production costs mean that we can not have large-scale solar farms using photovoltaic approaches.  Tax credits also lower the overall cost of the systems.

Disadvantages of Solar (PV) Technology

The disadvantages of solar energy in general (for electricity generation) are the large plots of land required, and the inconvenience of those cloudy days (intermittency) and at night. Regional haze also reduces the amount of solar energy that reaches the surface so sunny locations like Florida become less economic because of the haze as other locations (we will see a map a little later on). Electricity storage is also an issue unless the photovoltaic can be hooked into the local electricity grid or large-scale battery storage. Some nations such as Germany, however, have managed to generate significant electricity with the photovoltaic approach often now being coupled to some storage solution such as large-scale batteries (or currently in Germany to coal-plants that can quickly generate electricity). Similar to wind turbines, solar panels also use rare earth elements — that are in limited supply.

Thermal Solar Plants

Thermal solar relies on concentrating the solar resource. There are two options: reflective mirrors on a tower and parabolic collectors (troughs). For the concentrating tower solar sites many in the U.S. are in California (high electricity prices, and a very good solar resource). Spain also has a large number of these types of solar thermal plants.

Watch

The following 2:16 minute video explains how CSP works to produce electricity.

Click here for a transcript of Concentrating Solar Power.

PRESENTER: OK. Take the natural heat from the sun. Reflect IT against a mirror. Focus all of that heat on one area. Send it through a power system. And you've got a renewable way of making electricity. It's called concentrating solar power OR CSP.

Now, there are many types of CSP technologies, towers, dishes, linear mirrors, and troughs. OK, have a look at this parabolic trough system. Parabolic troughs are large mirrors shaped like a giant U. These troughs are connected together in long lines and will track the sun throughout the day. When the sun's heat is reflected off the mirror, the curved shape sends most of that reflected heat onto a receiver.

The receiver tube is filled with the fluid, and it could be oil, molten salt, something that holds the heat well. Basically, this super hot liquid heats water in this thing called a heat exchanger. And the water turns to steam. Now the steam is sent off to a turbine, and from there it's business as usual inside a power plant.

A steam turbine spins a generator and the generator makes electricity. Once the fluid transfers its heat, it's recycled and used over and over. And the steam is also cooled, condensed, and recycled again and again. One big advantage of these trough systems is that the heated fluid can be stored and used later to keep making electricity when the sun isn't shining.

Sunny skies and hot temperatures make the southwest us an ideal place for these kinds of power plants. Many concentrated solar power plants could be built within the next several years. And a single plant can generate 250 megawatts or more, which is enough to power about 90,000 homes. That's a lot of electricity to meet America's power needs.

Credit: DOE

Read

Another nice explanation of Concentrating Solar-Thermal Power [17] can be found on the DOE's Office of Energy Efficiency & Renewable Energy website.

 A Solar panel field in Spain.
Left: This solar collector is in Spain. The heliostat field focuses on the top tower. The mirrors track the motion of the sun across the sky and reflect the solar energy onto the receiver. The receiver can be a simple boiler producing steam to generate electricity or it can be a molten salt boiler that is used to generate steam (the molten salt will stay hot longer permitting electricity generation later into the evening).
Right: A large solar plant with five Solar Electric Generating Stations (SEGS), with a combined capacity of 150 megawatts. The mirror is blue because of the reflection of a very blue sky.
Credit: Left: SANDIA National Lab, Right: DOE

The DOE says the following about the capacity for a large-scale plant. "At capacity, there is enough power for 150,000 homes. The facility covers more than 1000 acres, with over 1 million square meters of collector surface. The SEGS utilize parabolic trough collectors to focus the sun's energy on a pipe carrying a flow of heat transfer fluid (synthetic oil). The fluid flows to heat exchangers where the heat turns water into steam to drive conventional steam turbine generators, which produce electrical power."

 Closeup of a parabolic mirror.
Here you see a closer view of a smaller system with the pipe carrying oil at the focal point of the parabolic mirror (you own a couple of these, they are in the headlights of many cars). This one is actually using solar energy to kill bacteria in water. The mirror rotates to ensure maximum exposure to the sun.
Credit: DOE

Geothermal Electricity

In lesson 1, we discussed geothermal heat pumps as a method of producing home heating, cooling, and hot water by taking advantage of the solar energy captured by the first meter or so of the earth's surface. Here we are considering the energy that is contained within the earth, specifically the hot molten core for electricity generation (not home heating and cooling!!!!). The planet is comprised of a molten core surrounded by a crust. Unfortunately, the amount of energy coming to the surface of the earth from geothermal is small in most locations. Currently geothermal contributes 0.5% to the electricity generation in the U.S.

Watch

The following video 3:47 minute video. It provides an excellent overview of geothermal energy production.

Click here for a transcript of the Geothermal Energy video.

You may have relaxed in a natural hot springs pool or seen the Old Faithful geyser blasting hot water into the air at Yellowstone National Park. But have you ever thought of where all that heat comes from? Well, it comes from deep beneath the surface of the Earth. And it's called geothermal energy. And we can use it to generate clean, renewable electricity.

OK, here's how geothermal works. Heat from the Earth's crust warms water that has seeped into underground reservoirs. In some places, when water becomes hot enough, it can break through the Earth's surface as steam or hot water. This usually happens where the Earth's crust or plates meet and shift. In the past, taking advantage of geothermal energy was limited to areas where hot water flowed near the surface. But as geothermal technologies advance, we can leverage even more of these natural renewable energy sources.

Engineers have developed a few different ways to produce power from geothermal wells drilled into the ground. Have a look at this. It's a dry steam geothermal power plant. And it's the most common type of geothermal technology used today. Underground steam flows directly to a turbine to drive a generator that produces electricity. Pretty straightforward.

Another geothermal technology is called a flash steam power plant. A pump pushes hot fluid into a tank at the surface where it cools. As it cools, the fluid flashes or quickly turns into vapor. The vapor then drives a turbine and powers a generator.

A binary cycle plant works differently. It uses two types of fluid. Hot fluid from underground heats a second fluid called a heat transfer fluid in a giant heat exchanger. The second fluid has a much lower boiling point than the first fluid. And so it flashes into vapor at a lower temperature. When the second fluid flashes, it spins a turbine that drives a generator.

The environmental benefits of this clean, round-the-clock, renewable energy source are substantial. Low emissions, small physical footprint, and minimal environmental impact. The few byproducts that can come up are often reinjected underground.

Geothermal energy can also help recycle wastewater. In California, wastewater from the city of Santa Rosa is injected into the ground to generate more geothermal energy. Some plants do produce solid waste. But that solid waste may contain minerals that we can remove and sell, which lowers the cost of this energy source.

The US Geological Survey estimates that untapped geothermal resources in the United States if developed could supply the equivalent of 10% of today's energy needs and cut our dependence on fossil fuels. In fact, electricity generated by geothermal energy already provides about 60% of the power along the Northern California coast. From The Golden Gate Bridge to the Oregon State line, geothermal energy-- helping to push America toward energy independence and a clean, renewable way to meet our growing energy demands.

Credit: U.S. Department of Energy

Read

Another nice explanation about Geothermal Basics [18] can be found on the DOE's Office of Energy Efficiency & Renewable Energy website.

Averaged over the earth's surface, the heat energy flow is 0.06 Watts per square meter (500 times less than the incoming solar energy flux). This is much smaller than incoming solar energy and so for most locations extracting geothermal energy (other than surface heat pump applications) will be too costly to drill down deep towards the core of the planet. In some locations, however, geothermal energy is far more concentrated and accessible. Locations such as Jellystone (oops, Yogi Bear slip), Yellowstone National Park's "Old Faithful" are recognizable and spectacular examples of geothermal activity.

As can be seen in the map below, certain locations have easy access to geothermal energy. Many of the island chains owe their existence to volcanic activity. Iceland, New Zealand and Hawaii have ample geothermal energy and use this renewable energy for the generation of electricity, home heating, hot water, etc. In the US, geothermal accounts for only about 0.4% of our electricity.

Graphic of a world map outlining the tectonic plates.
The tectonic plates sit on top of the magma and are constantly moving.  At the junctions of the plates, the magma has an easier access to the surface, spectacularly via volcanoes (red dots). The ring of fire goes up the West coast of Mexico, the US and across and down along Asia. This is where there is a higher risk of earthquakes and volcanic eruptions.
Credit:  Living Earth and Mark Wherley

How is geothermal energy turned into electricity?

How the electricity is generated from geothermal follows the same principles as the techniques already covered in this course. In an open-loop system, water and steam are separated. The high temperature, high-pressure steam turns a turbine, that spins a generator, that produces electricity. The steam is cooled and the water injected back into the ground to ensure that the system is renewable. In closed-loop systems, water is injected into the ground in a pipe where heat exchange warms it up and returns it as steam or hot water. In a binary system, ammonia is used in place of water as the working fluid. Ammonia will be a liquid at normal conditions but can easily be converted into ammonia gas (it has a low boiling point). The ammonia is used to turn the turbine (this technique is also used in Ocean Thermal Energy Conversion (OTEC), more on that later). The advantage of this method is that it can be used when the thermal gradient is not as great. Most of the geothermal plants use the open-loop system.

Geysers Geothermal Field

Here is a domestic geothermal power plant located in California (one of the 18 plants operating within the Geysers geothermal field). Multiple wells were drilled to supply the steam to the power plants (some as deep as 3 km to reach the higher temperature water). This is a mature site with over 60-years of operation. It was approaching the end of life as the water resource had been depleted (and water is expensive in California) but has been extended by injecting wastewater. There are geothermal pools in the region but interestingly no actual geysers!

 Image of a geothermal site on a hill.
The Geysers geothermal field in California producing electricity. The cooling towers are an obvious sign of electricity generation via steam.
Credit: DOE Office of Energy Efficiency & Renewable Energy [19]

Advantages of Geothermal Energy

  • Geothermal is a renewable energy source and can operate 24 hours a day
  • Much less air pollution is generated than electricity generated from fossil fuels although they do emit some pollutants
  • Small footprint - it doesn't require a great deal of land
  • Enhances national security and trade deficit reduction
  • It's a cheap electricity source. If interested you can also look at this site: 5 things to know about geothermal power. [20]

Disadvantages of Geothermal Energy

  • There are limited locations where it is a viable option. The western United States is the most likely area for generating geothermal energy on the US mainland.
  • It emits sulfur dioxide (sulfur is the yellow-colored material in some of the geyser basins), and NOx as well as greenhouse gases such as CO2

Biomass

Biomass does not contribute very much to our electricity supply in the U.S. at only about 2%. However, agricultural and timber wastes are used to generate steam and heat for industries. As a source of energy, biomass offers a host of positive qualities; It is fairly plentiful, relatively inexpensive to use, and helps reduce agricultural waste problems. Most importantly, however, biomass is a renewable energy source, thanks to the carbon cycle and solar energy.

Solar energy is used by plants to generate energy in chemical form (glucose).

6CO2+6H2O + solar energy → C6H12O6+6O2

When plants die, this process simply works in reverse.

C6H12O6+6O2  → 6CO2+6H2O + energy

Walking through almost any forest is the best way to witness the decaying process in action. The ground is generally strewn with dead and decaying leaves, limbs, branches, and sometimes entire trees. If they did not decay, we would be faced with a serious dead-tree problem in our forests. Then, imagine the scope of this problem over millions of years. This helps illustrate the nature and value of the carbon cycle.

However, in Pennsylvania, about 320 million years ago (and even today), the forests did not decay. Instead, the trees fell into swamps (bogs) and were protected from the decay process. Eventually, these trees formed coal, and in the oceans, plankton and algae went through similar processes. There, the stored solar energy eventually formed oil (protection from oxygen at the bottom of the ocean, with sediment burial). Now as we use the fossil fuels (combustion), we release CO2 back into the atmosphere.

So, instead of allowing the biomass to rot naturally as explained above, we can harvest it and combust the biomass for home heating, industrial use, or electricity generation.

Let Dr. Mathews tell you the full story in the (:46) video below:

Click here for a transcript of Part 1 - Energy and Biomass video.

[Video opens with Dr. Mathews standing in front of a display of biomasses.] Dr. Mathews: Biomass, if it is an agricultural waste or even a plant deliberately grown for use in biomass, then we have an excellent source of free energy. We just have to harvest it; it is free solar energy being stored in the plant. The plant grows, and we can come and harvest it. And one of the other nice things about it is it's CO2 neutral. Even though when we combust it we produce carbon dioxide, when you grow the tree again, or we grow the plant again, or even livestock, that CO2 is stored back in that animal or that plant. And, really, there are 4 classifications behind me which we are going to look at. There are the grasses, there are the woods, there are the proteins, and there are the fats and lards. We are going to have a look at each one of those. [Video ends.]

Credit: John A. Dutton e-Education Institute

Now that you've been introduced to the idea, you're ready for the details.

Weeds and Grasses (:32) Video

Click here for a transcript of the Part 2 - Weeds and Grasses video.

[Video opens with the caption: Biomass - Weeds and Grasses.] Dr. Mathews: This is switchgrass. [Dr. Mathews holds up a small bottle.] Dr. Mathews: Things like this are wonderful. They are more or fewer weeds but they are going to grow very quickly. And that is also desirable. You wouldn't want to have to plant an oak tree and wait 30 years before you could harvest it. But things like switchgrass are harvestable within 6 months. So you can get a couple of crops a year. And so anything like this that will grow quickly is possibly something that we use. [Video ends]

Credit: John A. Dutton e-Education Institute

Wood Products (1:00) Video

Click here for a transcript of the Wood Products video.

[Video opens with a caption: Biomass - Wood Products.] Dr. Mathews: Wood again is another one. [Dr. Mathews picks up a small bottle.] Dr. Mathews: This is a tree of heaven. It is again a very quickly growing small tree, but like a bush. It is somewhat of a problematic plant. It grows on the sides of the roads and some parts of the south they have a major weed status. But again, quickly growing, we can crush it up and fire it either by itself or we can add this, to say, a pulverized coal facility and burn perhaps up to 10 percent the material being this biomass, this wood. Doesn't have as much energy content as coal because it is not concentrated, it has a lot of oxygen. But it is still very useful. But remember one of the main advantages of the biomass is that it is CO2 neutral. And so you plant the seed, you grow the seed, you cut it down, and you burn it. Provided you go and replant the seed, you are going to keep that CO2 locked away. [Video ends showing 4 bottles of this woody biomass.]

Credit: John A. Dutton e-Education Institute

Oils, Grease, Animal Fat & Lard (2:10) Video

Click here for a transcript of the Oils, Grease, Animal Fat & Lard video.

[Video opens with a caption saying: Biomass - Oil, Grease, Animal Fat, and Lard.] Dr. Mathews: McDonald's grease, something else that you can use. It is a waste product; it was already been used to fry french fries or various other components in it. You are not going to use it anymore to fry food in, but it still has calorific value. Remember, the difference between vegetable oils for cooking and oils for combustion is very little. Is just about length, but we can still burn these things. [The camera pans past several bottles of oil.] Dr. Mathews: This is choice white grease, we have coconut, you can crush coconuts and extract their oil. That again is something you can use for cooking purposes and once you have fried things in it you can go ahead and use it for other things - combustion. [Dr. Mathews holds up a small bottle.] This is poultry fat. We kill an amazing number of chickens every year. Not much is wasted. This fat is something else that you can think of as a waste product; something else you would need to dispose of, or you can use it as a fuel. [Dr. Mathews puts that bottle down and picks up another one.] Dr. Mathews: Ah, pork. There is a saying that you use everything from the pig, including the squeal. [Sound of a pig plays in the background.] Dr. Mathews: Well that is pretty much true. In the old days you could use the pork fat to make candle. You could use the liquid fuel for lighting prior to kerosene. Now pork oils have a value. Lard has a value. But if, for say, we were looking at having lower cholesterol, and going with a non-fatty, non-high cholesterol fat, then the pork market might go away for lard. In which case it would have to be disposed of. It is a beautiful boiler fuel. We heat it up to melt it and turn it into a liquid, and inject it into our boilers. It has very low sulfur content. And so pollution really isn’t an issue and it burns very nicely. It isn't quite as nice as, or have as much energy content as fuel oil, but you just have to burn a little bit more. Again, if it ever came to a problem that we had to dispose of it, combustion is one way we could dispose of it very cleanly and get a useful product at the same time. [Video ends showing several bottles of the fats, greases, and lards.]

Credit: John A. Dutton e-Education Institute

Animal Proteins (1:15) Video

Click here for a transcript of the Animal Proteins video.

[Video opens with a caption saying: Biomass - Animal By-Products.] Dr. Mathews: Here are some other rather strange biomasses. [Dr. Mathews picks up 2 containers.] Dr. Mathews: This is feather meal. Again the chickens, we have no use for the feathers. If you grind them and extract them down this can be fed to other animals. Although with the Bovine Spongiform Encephalopathy scare, mad cow disease to you and I, we have gone away from feeding some of these other animal products to animals. And because I ate beef in England for a couple of years before coming here, I can't even give blood anymore because they are frightened that I have mad cow disease. [A caption reads: ...mad professor maybe...] Dr. Mathews: They might be right. Again, pork meal. It is essentially animal proteins that we can extract. There is still a lot of energy in these so we can go ahead and burn them. Manure would be something else we could use. And we have certainly looked into chicken litter (which is really chicken poop) and other manure. Obviously, in Africa they use certain manure for biomass applications and so we can certainly do the same things here if we had the desire to. [Video ends]

Credit: John A. Dutton e-Education Institute

Biomass does not contribute as much to electricity as it does to industrial heat generation where industries that produce a combustible biomass waste product and require heat use the waste. Paper mills are a good example of this. The whole tree does not go into the paper manufacturing; bark, leaves, and small branches are combusted to generate the heat to drive off the water from the water/cellulose slurry.

When we discussed lighting, sperm whale oil was the fuel of choice for some of our ancestors. Candles would have also been produced from animal fat (pig fat worked well), and if the need arose, we could combust fats (currently pig fat is too valuable to burn; it is used in frying potato chips.)

Advantages of Biomass

  • It is renewable
  • There is no net carbon dioxide released into the atmosphere (greenhouse gas)
  • Stimulate American jobs in agricultural areas
  • Can be co-fired with coal to reduce the greenhouse gas emissions

Disadvantages of Biomass

  • Biomass requires large plots of land, increased fertilizer (produced mostly from natural gas!), and more insecticide use (perhaps)
  • In a world where starvation occurs, using food as fuel (such as corn) is an abuse
  • Biomass is heavy (high water content), so transportation costs can be high, influencing the economic feasibility of biomass use
  • Energy demand is constant, but only a percentage of biomass crops can yield harvests year-round, requiring the need for storage. Harvests will also be weather and disease dependent

The bottom line: Expect to see biomass being integrated into existing utilities that burn other fuels, rather than the creation of large biomass-only utilities. It is already a requirement in many states that renewable energy contributes to electricity generation. (This is known as the renewable portfolio standards.) In Pennsylvania, this can include photovoltaic energy, solar thermal energy, wind, low-impact hydro, geothermal, biomass, biologically-derived methane gas, coal-mine methane, coal-waste (those culm/gob piles), and from using fuel cells.

Tidal and Wave Power

The water in the ocean is constantly moving due to waves (from the wind) and tides (mostly from the moon). This movement can be turned into energy in a number of different ways which we will discuss here. However, there are only a few locations where the electricity-generated is currently significant.

Tidal Power

The tides are associated with the gravitational pull from the moon (and to a lesser extent the sun). The moon rotates around the earth on a lunar cycle that is close to a calendar month. The waxing and the waning of the moon are associated with the sunlight being reflected from the surface of the moon (the moon does not generate light so the "Dark Side of the Moon" great Pink Floyd song is somewhat misleading). As the moon rotates around the earth, it exerts a gravitational pull on the oceans such that during a 24-hour cycle there will be at least one high and low tide. Hence, tidal energy is renewable. Unfortunately, the tides do not always coincide with the peak demand times but the production of electricity output is predictable.

The technology used for the conversion of tidal energy into electricity is similar to the technology used in hydroelectric power plants. There are various systems that are used to harness the potential energy supply by turning a turbine, to turn a generator, to electricity. The approaches for tidal power are either a barrage (similar to a dam),  to create a catchment zone, or to directly ??? from the flow. With a barrage, you close off the flow — allow the tide to come in when it is much higher than the water height in the catchment area: the flow is opened and water rushes in. Before the tide goes out the high tide water is captured, the flow stopped and the flow is started again when the tide is low so the water flows out of the catchment area. The process then repeats.

Watch

Click here for a transcript of the Tidal Power 101 video.

Tidal power. Tidal power is a form of hydro power that converts the energy from the natural rise and fall of the tides into electricity. Tides are caused by the combined effects of gravitational forces exerted by the moon, the sun, and the rotation of the Earth.

Tidal plants can only be installed along coastlines. Coastlines often experience two high tides and two low tides on a daily basis. The difference in water levels must be at least five meters high to produce electricity. Tidal electricity can be created from several technologies, the main ones being tidal barrages, tidal fences, and tidal turbines.

Tidal barrages are the most efficient tidal energy sources. A tidal barrage is a dam that utilizes the potential energy generated by the change in height between high and low tides. This energy turns a turbine or compresses air, which in turn creates electricity. Tidal fences are turbines that operate like giant turnstiles, whereas tidal turbines are similar to wind turbines only underwater. In both cases, electricity is generated when the mechanical energy of tidal currents turns turbines connected to a generator. The generator produces electricity.

Ocean currents generate relatively more energy than air currents because ocean water is 832 times more dense than air and therefore applies greater force on the turbines. Tidal power is easy to install and renewable, having no direct greenhouse gas emissions and a low environmental impact. Because the ocean's tidal patterns are well understood, tidal energy is a very predictable energy source, making it highly attractive for electrical grid management. This sets it apart from other renewables that can be more unpredictable.

However, adoption of tidal technologies has been slow, and so far the amount of power generated using tidal power plants is very small. This is due largely to the very specific site requirements necessary to produce tidal electricity. Additionally, tide cycles do not always match the daily consumption patterns of electricity and therefore do not provide sufficient capacity to satisfy demand. That's tidal power.

Credit: Student Energy [21]

Read 

The U.S. Energy Information Administration Tidal Power webpage [22] provides additional information on the three different tidal technologies: tidal barrages (currently in use), tidal turbines, and tidal fences are both emerging technologies.

Advantages of Tidal Power (barrage generated)

  • no air pollution
  • no fuel needed, no waste produced
  • relatively inexpensive to operate and maintain
  • can produce a great deal of energy
  • predictable energy output – tides are predictable and mostly constant
  • as much of the coastline is populated, there is often a site at which the product can be linked to the electric grid

Disadvantages of Tidal Power (barrage generated)

  • there is the possibility that some of the undesirable results of hydroelectric electric power generation can be carried over to tidal power. Consideration of fish movement, sediment flow, navigation, wetland areas, and other environmental issues must be made and environmental impact studies are required
  • tides do not always coincide with the peak demand times
  • must be able to withstand very rough weather
  • it is currently expensive to construct tidal power plants as they require high capital investments
  • environmental issues such as habitat change, particularly with tidal barrages

Wave Power

While tidal power is a proven technology, wave power is an emerging one. There are a few different ideas ranging from using wave power to move air to make electricity, underwater turbines similar to wind turbines (but much smaller), or devices that articulate.

Watch

Here is a 2:35 minute video about an emerging technology for using wave power at the coast. The wave is used to create a column of water, that works like a piston to compress air — moving it up though a Wells turbine on the upstroke, and moving the air in the other direction as the water column falls.
Click here for a transcript of the Harvesting the Waves video.

One concept for harnessing the wave energy is the oscillating water column principle. The take off for this technology is typically an air turbine. The most commonly used air turbine for this application is the Well turbine, which we will explain in this episode.

The water level in the chamber rises and falls with the rhythm of the waves and act as a piston. The air is forced forwards and backwards through the turbine and causes the rotation of the turbine. This generates mechanical energy that is converted into electricity by a generator.

The Wells turbine has a special feature. It always rotates in the same direction, regardless of which direction the airflow comes from. How is this possible? This is feasible because of the symmetrical shape of the rotor blades. As the air hits the rotor blade, most of the flow is deflected in one direction and pushes the blade in the opposite direction. Due to the symmetrical shape of the rotor blades, the same effect happens when the airflow comes from the other direction. Therefore, this Wells turbine always rotates in one direction, regardless of the air flow direction and guarantees the continuous rotation of the turbine.

The Wells turbine must be turned on initially. The airflow alone does not get it to rotate. This turbine is one of the simplest turbines for wave energy conversion. It has very few moving parts. None of them are in the water, need no gearboxes, is easy to maintain, and achieves an efficiency between 40% and 70%.

This turbine was tested in several research plants under real conditions. Limpert, the power plant on the Isle of Islay was the first commercial plant in operation between the year 2000 and 2018. It generated 500 kilowatts with two Wells turbines.

The Mutriku Breakwater Wave Plant in Bay of Biscay on the coast of the Basque country in Spain started in 2011. It has 16 Wells turbines and supplies 250 households with energy. The Wells turbine has the most operational experience and running hours of all air turbines for oscillating water column concept of harnessing the wave energy, and it makes a small contribution to the generation of sustainable energy.

Credit: Ideas for the Green Planet [23]

Read

The U.S. Energy Information Administration's Wave Power webpage [24] provides additional information on ways to capture the energy in the waves.

Advantages of Wave Power

  • Low cost
  • Predictable and reliable
  • Renewable
  • Produces no emissions

Disadvantages of Wave Power

  • High cost to build the initial equipment and machinery. Therefore, these devices must be placed in areas with the strongest waves
  • Might cause a negative effect on the marine system
  • Waves strengths can be variable (thus the electricity produced is variable

Electricity Storage

Challenges of Electricity Storage

We have already examined how the electricity demand cycle changes according to the time of day, day of the week, season, and needs to accommodate temperature changes (due to electricity use in heating and cooling). Here are examples of the weekly electricity demand cycle for various months. Note that day and time have an impact on demand. Also, these are averages so the variance due to weather (especially cold/hot days and holidays are muted). When you use electricity it has a very significant impact on the cost to generate that supply (the peak will be more expensive than the electricity generated for the trough).

Electricity demand cycle changes for various months
Average hourly U.S. electricity load during a typical week (selected months)
What else influences the non-averaged electricity demand? Would California differ from Vermont? Does season or weather impact usage? What about the economy? Our electricity management in PA is controlled by the PJM which is a regional transmission organization for the movement of wholesale electricity. It covers PA and parts or all of 11 other states.
Credit: Energy Information Agency

Thus, to meet this demand we need to have either extra generation capacity or electricity storage. The traditional approach was to have additional generation capacity, often smaller natural gas-fired "peaking" units that were activated when the demand was high. As these were not used for extended periods (and they were small) they were/are expensive to operate. The owners also need to be paid (a financial incentive) to keep this electricity generation in reserve for when it is needed (the hotter afternoons in the summer, colder winter mornings/afternoons). A few locations used pumped-storage: using coal-fired and nuclear-derived electricity at night (when the demand was lower) to effectively store the electricity as potential energy in water that could be returned to electricity when demand was again high via the hydroelectricity approach. In some cases, this was a cheaper option than the addition of new capacity that was needed for the then growing electricity demand (currently electricity demand growth is very low). Our current situation however is very different. There is now an ongoing transition towards increasing the renewable energy contribution from solar and wind. Both suffer from an intermittency challenge: so we need either many more wind turbines and solar farms (well-dispersed) to accommodate the expectation for always available electricity or we need electricity storage options. The closure of coal-fired plants and the impending closure of nuclear plants will make this challenge much greater (electricity was available with a high probability unless maintenance or unscheduled off-grid issues). 

Figure shows two pie charts, the first showing pumped storage being ~95% of the U.S. capacity. The other 5% is mostly thermal storage and battery storage with limited contribution from compressed air and flywheel technologies.
Pumped storage is the leading contributor to our electricity storage but large-scale battery farms are now being added.
Credit: Department of Energy

Pumped Storage

This was covered earlier in the lesson. It is one of the few electricity "storage" options that have been available for decades and at scale. Here the electricity is converted to the potential energy in the water that is pumped to a higher elevation. This potential energy is converted back into electricity when needed (peak demand or to fill in the gap if renewable electricity is not meeting the need). This supplies 95% of the electricity "storage" in the U.S. There are some other emerging options, however.

An illustration showing the pumped-storage concept of an upper and lower reservoir with a pump house / generator
Pumped storage illustration (not to scale).
Credit: energy.gov

Large-Scale Battery Storage

A large-scale battery storage
location in Germany
Credit: BMW

We have used rechargeable batteries for decades, such as the lead-acid battery in gasoline and diesel vehicles (used to start the engine). However, the cost of alternative batteries has significantly declined and large-scale electricity storage plants are now possible using advanced Lithium-ion batteries. These plants make money by buying electricity when it is cheaper (when the demand is low) and selling it when the demand is higher (and the electricity cost is higher). Alternatively, they are now being paired with wind and solar farms to permit a more consistent electricity supply. The cheaper batteries are also "driving" the transportation transitions to electric vehicles (the pun was intentional — sorry) such as Tesla vehicles. If we electrify the transportation market, we will need much more electricity production. Some experts also predict we may have electricity storage at home in the future using some of these batteries (not just for rooftop solar cells). Again the system would store cheaper electricity and would provide electricity to the home when electricity costs are higher — as well as providing backup power for grid disruptions (blackouts).

Thermal Storage

Thermal storage was discussed with concentrated solar plants. By using a molten metal salt, there is a heat source (heat storage) that can be used when the sun goes down or on cloudy days. This could be heated by other power-plants.

Concentration solar thermal plant
A concentrated solar power plant. Notice the addition of the thermal energy storage tanks to increase the mass of the molten (very hot) metal salt from the receiving tower. These units extend the solar electricity generation period into the evening hours.
Credit: Clean Leap

Electricity Generation by the Numbers

In 2010 the electricity generation in the US was dominated by a single fossil fuel: coal at 45%. By 2019  coal had declined to ~25% (the change is due to the availability of cheap natural gas with some wind, and limited solar). I expect coal will stay around this number (even with the closure of older plants) as their capacity factor will increase (produce more electricity by either being 'on' more often or generating more electricity (they often run well below maximum capacity). Natural gas is the leading source at 38%. Nuclear was the third (~20%) although the expectation is that this will decrease as units are dismantled than are constructed and as the total electricity production grows. Hydroelectric power was 7%, with the wind also contributing 7%. Note that petroleum (in the form of fuel oil and coke) did not contribute very much to electricity generation (~1%). We are still highly reliant on fossil fuels, and nuclear for electricity generation!

Figure showing the leading source of fuel for electricity generation in 2019 for the U.S. Described above
Sources of U.S. electricity generation, 2019
Click for a text description.
Sources of U.S. electricity generation, 2019
Total = 4.12 trillion kilowatthours
  • Natural gas = 38%
  • coal = 23%
  • nuclear = 20%
  • renewables = 17%
    • wind = 7.3%
    • hydro = 6.6%
    • solar = 1.8%
    • biomass = 1.4%
    • geothermal = 0.4%

Note: Electricity generation from utility-scale facilities. Sum of percentages may not equal 100% because of independent rounding. 

Credit: U.S. Energy Information Administration, Electric Power Monthly [25], February 2020, preliminary data

Changing Energy Economics Change the Renewables New Capacity Mix

As we look forward the predictions are that coal and nuclear will decrease their contribution and stabilize while natural gas and renewables will increase.

Plot of the electricity source for the U.S with projections.
Net electric generation from select fuels
Click for a text description.
The is a line graph that shows historic (1990 - 2017) and projected (2017 - 2050) electric generation by source.
Credit: EIA

Here is another chart showing the contribution to renewables. Note the increasing role of wind and solar.

electricity generation. See text description in caption
U.S. electricity generation, 2010 - 2050
Click for a text description.
Add text description here
Credit: EIA

The last few lessons have had an electricity focus. However, it is important for us to keep in mind that we also have primary energy: transportation, electricity, heating (natural gas and other fuels), and fuel for industrial operations. Note that we use a great deal of petroleum (we will see in Unit 2 that this is used in transportation fuels), and natural gas! Also, note the higher contribution of biomass within renewables. It is used as industrial fuel for steam etc.

US primary energy consumption. See text description
U.S. primary energy consumption by energy source, 2019
Click for a text description.
Add text description here
Credit: U.S. Energy Information Administration, Monthly Energy Review [26], April 2020

Lesson 3 Coverage Map

Like the coverage map in lesson 1, this map represents a summary of lesson 3, providing you with a way to quickly refresh yourself on the big ideas and connections in this lesson. Hover over the text boxes for more information.

Accessible Version (word document) [27]

Deliverable

After looking at this map, please take the L03 quiz.


Source URL: https://www.e-education.psu.edu/egee101/node/772

Links
[1] https://stock.adobe.com/contributor/205494801/mike-fouque?load_type=author&prev_url=detail
[2] https://stock.adobe.com/346458341?as_campaign=TinEye&as_content=tineye_match&epi1=346458341&tduid=162c3721f33a81b2992acf0c8bd83f41&as_channel=affiliate&as_campclass=redirect&as_source=arvato
[3] https://stock.adobe.com/contributor/206691374/%EC%88%98%EB%8F%99-%EA%B9%80?load_type=author&prev_url=detail
[4] https://stock.adobe.com/images/water-wheel-in-korea/222319660
[5] https://en.wikipedia.org/wiki/Three_Gorges_Dam
[6] https://energy.gov/eere/water/how-hydropower-works
[7] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson02/L02_pumped_storage.mp3
[8] https://www.e-education.psu.edu/egee101/node/823
[9] http://www.oceanenergycouncil.com/
[10] https://www.youtube.com/c/Acciona_Corp/about
[11] http://energy.gov/eere/renewables/wind
[12] https://stock.adobe.com/contributor/201003291/kruwt?load_type=author&prev_url=detail
[13] https://stock.adobe.com/images/offshore-farm-windturbines-near-dutch-coast/143721774?prev_url=detail
[14] https://www.energy.gov/eere/solar/solar-photovoltaic-technology-basics
[15] https://stock.adobe.com/contributor/200994435/vladimir-gerasimov?load_type=author&prev_url=detail
[16] https://stock.adobe.com/images/aerial-top-view-of-a-solar-pannels-power-plant/284063002?prev_url=detail
[17] https://www.energy.gov/eere/solar/concentrating-solar-thermal-power
[18] https://www.energy.gov/eere/geothermal/geothermal-basics
[19] https://www.energy.gov/eere/geothermal/hydrothermal-resources
[20] https://www.energy.gov/eere/articles/5-things-know-about-geothermal-power
[21] https://www.youtube.com/channel/UCbVKIQMvSWEDLnQIAWE9IgA
[22] https://www.eia.gov/energyexplained/hydropower/tidal-power.php
[23] https://www.youtube.com/channel/UCr2F-Kyi8vFXlRgEUozAD5w
[24] https://www.eia.gov/energyexplained/hydropower/wave-power.php
[25] https://www.eia.gov/electricity/monthly/
[26] https://www.eia.gov/totalenergy/data/monthly/previous.php
[27] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson02/Lesson%202%20and%203%20Coverage%20Map.docx