From the perspective of ongoing sustainable use of energy resources, we have classified energy sources into two categories: renewable energy and nonrenewable energy. Renewable energy is energy derived from sources that replenished faster than we use them. Most renewable energy sources are also clean in that they do not require combustion, which releases pollutants and greenhouse gases into the environment. Nonrenewable energy comes from sources that are used faster than they replenish (they may be finite or not). Biomass and biofuels are an interesting case: if they are used faster than their natural growth rate, or in an unsustainable way, they can also become nonrenewable. About 90% of global energy consumption is supplied by nonrenewable sources. This is extremely problematic because nonrenewable resources will soon be exhausted, and most of these sources are also major greenhouse gas emitters. On the other hand, renewable energy sources (including biomass, biofuels, hydropower, geothermal, solar, and wind) accounted for only 11% of the global energy consumption in 2010, and are projected to account for only 15% by 2040 (from U.S. Energy Information Administration, International Energy Outlook 2013). Of these, solar, wind, geothermal, and hydropower are the cleanest energy sources as they do not require combustion and therefore have no direct greenhouse gas or air pollutant emissions, as compared to the burning of biomass and biofuels which emit greenhouse gases and particles which pollute the air and cause climate change. Nuclear power, like fossil fuels, is a non-renewable source because there is a finite amount of uranium available in the Earth’s crust. However, nuclear power generation does not emit greenhouse gases or other air pollutants. Let’s take a more in depth look at nonrenewable and renewable energy sources.

Closer Look


Read more about fossil fuels.

Nonrenewable Energy Sources

Fossil fuels comprise the world’s primary energy sources, which are projected to continue to account for nearly 80% of the world’s energy consumption through 2040 (US EIA International Energy Outlook 2013). While fossil fuel formation is an ongoing process on Earth, new fossil fuel formation takes hundreds of millions of years. The rate at which fossil fuels are being extracted far exceeds the rate at which they are produced; hence they are classified as nonrenewable.

Figure 9: The Susquehanna Nuclear Power Plant is located in Pennsylvania, U.S.A. and generates 63 million kilowatt hours of electricity per day. Note the two large cooling towers emitting steam. 1

Nuclear power is the energy that comes from reactions which change the nucleus of an atom. Nuclear fission is an atomic reaction whereby one atom’s nucleus is split into two smaller nuclei, and energy is released. Nuclear fusion is an atomic reaction where two smaller nuclei fuse to form one larger one, energy is released in this reaction as well. The energy released is typically in the form of high energy radiation such as gamma rays and x-rays, which can be highly dangerous to living organisms when exposure is acute.

Nuclear Energy makes up about 8% of the world’s total energy. Nuclear energy is very efficient because there is an enormous amount of energy yielded from just a tiny amount of uranium- 235 in the initial reaction. Although nuclear energy is valuable due to its high yield, efficiency, and lack of greenhouse gas output, the technology required to produce safe nuclear energy is complex and controversial (Figure 9).  

Closer Look


Learn how nuclear energy is generated from uranium.

The drawbacks to nuclear power production include the barium and krypton waste produced by the reaction (fission products) which is highly toxic to the environment as it continues to emit radiation. Of course, many things emit radiation, including rocks, soil, food, water, and air. However, not all types of radiation that we are exposed to are damaging.

Radiation can be classified by the Electromagnetic Spectrum (Figure 10), which defines different types of radiation according to its wavelength or energy. Wavelength is defined as the distance between successive crests of a wave. The longer the wavelength, the less energy is contained in the radiation. Thus, radio waves transmit very little energy, while gamma rays are powerfully energetic.

Closer Look


Read more about electromagnetic waves from the Khan Academy.

Visible radiation (or visible light, the solar radiation with wavelengths ranging between 400–800 nm on the electromagnetic spectrum) is the solar radiation absorbed by plants to stimulate photosynthesis. This radiation is typically not harmful to any life forms, including humans. Alternatively, gamma and X-rays, which are the most energetic, can be extremely harmful to the health of humans, plants, and animals. This type of radiation is harmful because it destroys the integrity of DNA molecules in living cells. Similarly, too much exposure to the sun’s ultra violet radiation can cause sunburn and skin cancer.

electromagnetic spectrum
Figure 10: The Electromagnetic Spectrum. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. 2

Inspired People

Amory Lovins

Amory Bloch Lovins is a physicist, environmental scientist, and co- founder of the Rocky Mountain Institute, a non-profit organization dedicated to energy efficiency and transition to renewable energy. In 2007 he has received the Blue Planet Prize for outstanding efforts at solving global environmental problems. 3

Because radioactive waste contains gamma and X-rays, it poses a problem both today and for future civilizations. Currently, most nuclear waste is disposed of in large cement containment vats at the facilities where it is created. Continuing to store this waste above ground poses security risks, such as damage from earthquakes, storms, or vandalism. It also requires future societies to maintain these storage facilities. Human error is also a concern when it comes to waste management of nuclear matter. An alternate option for the disposal of nuclear waste is to reuse the waste as fuel. This beneficial use of nuclear waste is currently being practiced with the use of breeder reactors in nations such as France, the United Kingdom, and Germany. However, breeder reactors also facilitate the production of nuclear weapons, so they also increase the risk of nuclear proliferation.

Another downside of nonrenewable energy sources is that they cause severe environmental problems, including climate change and air pollution, which pose a threat to humans, plants, microorganisms, and animals. Furthermore, nonrenewable energy sources will eventually run out if used at current rates (see Table 1). Therefore, for the protection of the environment and for ensuring the energy security and health of human societies, we must move away from our reliance on non-renewable energy sources and increase the use of renewable energy sources.

table of energy sources
Table 1: Comparisons of abundance among energy sources.

Renewable Energy

Supply of renewable energy sources worldwide far exceeds the global energy demand by several orders of magnitude. For example, it is projected that in 2030 the world will require 16.9 terawatts of power, and wind power alone can provide 40 to 85 terawatts. However, we only generate 0.02 terawatts of wind power worldwide today, only a tiny percent of what is available (see Table 1).

In the past few decades, renewable forms of energy (especially solar and wind) have become more widely used in response to the need to find energy sources that can replenish themselves without producing harmful climate-changing by-products.

Looking Ahead


You will discover in the Energy and Action section ahead that many people around the world are working on energy solutions through solar projects.

Solar Energy

Harvesting energy from the sun is one of the most straightforward methods to power our activities. In fact, each day Earth receives enough energy from the sun to power the planet’s current energy demands for a quarter of a century. Solar energy can be captured “actively” or “passively.”

Active solar energy uses special technology to capture the energy within the sun’s rays (much like a plant’s photosynthetic apparatus) and convert it to electricity. The two main types of equipment used in this process are photovoltaic cells and mirrors. Photovoltaic cells (also called PV cells or solar panels) absorb sunlight and convert it directly into electricity through the Photoelectric Effect and the Photovoltaic Effect (Figure 11).

solar power station in Spain
Figure 11: The Andasol Solar Power Station of Andalusia, Spain is the first commercial parabolic trough (curved metal mirror) solar thermal power plant in Europe. The heat collected by the concave mirrors turns a turbine which generates electricity. Some of the heat produced during the day is stored in molten salt (3:2 ratio of sodium nitrate: potassium nitrate), which is used to produce electricity at night and during overcast days. 4

Closer Look


Find out more about how solar panels capture sunlight and convert it to electricity.

Mirrors can be used to focus large amounts of sunlight onto a single point. Enough light focused on a single point can create a large amount of heat energy. For example, large dish-shaped mirrors can collect sunlight and focus it onto a kettle for cooking, or a tub for heating water, or a Stirling engine. Other mirrors are used in parabolic trough or central tower solar thermoelectric plants: the heat is used to generate steam that is then converted into electricity with a steam turbine. In these ways, active solar energy technologies directly generate electricity from sunlight that can be used to power electronic devices. See, for example, the solar powered bench or try making your own solar powered backpack.

Figure 12: Simple model of rooftop solar thermal hot water heaters which are used in the vast majority of homes in Israel. Water from the home system runs through coils in the solar collector panel, which heats the water. Heated water is stored in the tank for distribution back into the home system. 5

In comparison to active solar energy, passive solar energy does not require any conversion equipment. Instead, passive solar energy maximizes the use of solar radiation for heating and lighting a building through intelligent design of the structure. For example, a house can be built so that its windows face the path of the rising and setting sun; this lets in more light and heat and reduces the need for electrical lighting and heating. Another example of a passive solar technology is a rooftop solar water heater where sunlight is absorbed by a large dark-colored water tank which reduces the need for electricity or gas to heat household or commercial water (Figure 12).

There are some challenges associated with solar power that scientists and engineers are currently addressing. The intensity of incoming solar radiation varies for different parts of the world based on geographic location, time of year, and time of day. A back-up source of energy may be required during the nighttime in some locations, but this can be solved by charging a storage system (such as a battery) during the day to store energy for use at night. Scientists are developing better batteries for the power grid.

Switching a single fossil fuel-powered home to one that runs on solar energy can prevent over 3.5 tons of CO2 emissions per year from being displaced into the atmosphere. Moreover, at today’s level of global energy consumption — 18,000 terawatt-hours per year — we would need to cover just 3/1000th of Earth’s desert regions with solar panels, about 90,000 square kilometers, to meet the total world demand (see Figure 13).

solar thermal farm
Figure 13: The largest solar thermal farm in the world. The project relies on a technology known as solar thermal energy. 350,000 mirrors reflect light onto boilers. When the water boils, the steam turns a turbine, creating electricity. The project provides clean, renewable energy for 140,000 homes and is supported by a U.S. Energy Department loan guarantee. 6

Wind Power

Figure 14: A wind farm in the U.S.A. generates electricity from wind turbines that stand 125m tall. 7
Wind energy, energy derived from the movement of air, is actually an indirect form of solar energy. Due to the spherical shape of the Earth, solar radiation warms different regions of the atmosphere at different rates; more concentrated solar rays hit the equator while less intense solar radiation reaches the poles. Air tends to flow from the warmer regions to cooler regions through the process of convection, causing winds to occur in those areas of transition. Wind patterns are then set by the variation in air temperature coupled with the rotation of the Earth (this process is described more fully in the Water Chapter’s discussion of the Hadley Cells). Wind turbines can be used to harvest this wind energy.

Looking Ahead


You will learn in the Energy and Action section ahead that people are working on ways to protect bird populations from destruction by wind turbines.

Similar to a windmill, the wind turbine is usually a large wheel with a set of three thin blades mounted on top of a large tower (about 39 to 105 meters high). Wind turbines then produce electricity when wind sweeping across the turbine causes the blades to rotate and subsequently turn a gear that is attached to the blade (Figure 14). This gear is connected to a metal coil that rotates inside a magnet. Similar to a generator at a nuclear or coal powered plant, the laws of physics converts this mechanical motion into the motion of electrons on a wire, producing electricity.

Groups of these wind turbines are called “wind farms” and are typically located near farmlands, in open mountain passes, and even out in the ocean. Wind farms can be created wherever there is a long, uninterrupted flat landscape where wind can gain speed and remain laminar (non-turbulent).

Closer Look


Read more about radioactive decay.

As with solar power, there are some minor drawbacks to wind energy production systems technology at its current stage of development. For example, wind speeds can vary based on time of day, weather conditions, and geographic location. Good wind resources are often not located near a population’s center, requiring an electricity transmission network to be developed for the electricity to be transported to population centers. To further explore some of the practical challenges of providing energy to whole communities through wind power, read more about the El Hierro island community introduced in this chapter’s case study.

As with solar energy, wind energy capture and conversion to electricity produces no carbon emissions. However, wind farms need to be sited carefully to avoid potential impacts to bird or bat populations.

Wind energy capture and conversion to electricity has the potential of preventing the release of about 1500 tons of carbon dioxide per year into the atmosphere (equivalent to the emissions released from about 6.5 million cars per day). Wind farms also use less water than conventional power plants, produce a substantial amount of energy when optimized, and create jobs and revenue.

Geothermal Energy

Figure 15: The eruption of Castle geyser in Yellowstone National Park, U.S.A. is an example of the heat in the Earth’s core escaping from a vent at the Earth’s surface. 8

Geothermal energy originates from the natural radioactive decay of elements under extremely high pressures that are located deep in the interior of the Earth. As a result of this process, the heat of the Earth’s interior core rises to Earth’s surface through cracks and fissures. Geothermal energy is powerful enough to melt underground rocks into magma and cause lava to flow from volcanoes. It also creates the hot water that sprays out of a geyser or hydrothermal vents (Figure 15).

Underground geothermal heat can be accessed if the heat source is close enough to the surface. One specific way of accessing geothermal energy is with “geothermal heat pumps” also known as ground source heat pumps (GSHP). In these systems, pipes filled with water loop between a deep underground heat source and an above ground reservoir for use in heating buildings and houses. Areas in colder climates, such as Iceland heat most of their homes using GSHP systems. In some locations, water is pumped underground through pipe-loops placed close to geothermal heat sources. In this case, the water in the pipes is turned into steam that can then be converted into electrical energy through the use of heat engines.

There are many benefits to geothermal heating. Compared to typical electrical heating elements, GSHPs can reduce carbon emissions by up to 70%. However, one drawback to geothermal heating is that there are only a few locations in the world where we can access active sites for geothermal heat production, and these sites can shift over time.

In temperate zones of the world where there are four distinct seasons per year, with summers hot enough to require cooling of buildings and winters that require warming, simple closed-loop geothermal systems can provide both cooling and heating from the earth. Unlike GSHP systems, these simple geothermal systems do not utilize heat from the radioactive decay of elements. Instead, they utilize the earth’s constant temperature ranging between 4.5-10°C (40-50°F) year-round at relatively shallow depth of 15-150m (50-500ft) below the surface. Loops of pipes are placed either vertically or horizontally underground and water mixed with antifreeze is pumped through the closed-loop pipe system to either gain heat for the building in winter, or lose heat from the building in summer. This is a low-energy cost mechanism for providing all or nearly all of the heating and cooling needed by buildings in these climates.

Hydroelectric Power Technology

Figure 16: Inside a hydroelectric power plant. 9

Hydroelectric energy is produced by capturing the energy of flowing water of streams and rivers caused by gravity (i.e. water flowing downhill). This is one of the oldest and most widely used forms of renewable energy. It currently provides almost one-fifth of the world’s electricity. Most hydroelectric power plants are located on large human-made dams, which control and block the flow of a river to create an artificial lake, or reservoir. A controlled amount of water from the reservoir flows through tunnels in the dam by gravity. As water flows through the tunnels, it turns huge turbines and generates electricity (Figure 16). Although hydroelectricity from large dams produces no direct air pollution, its construction can be detrimental to the flow of water, aquatic life in rivers, ecosystems, and nearby terrestrial wildlife. Also, in some cases, methane (a very powerful greenhouse gas) is also released by the dammed water, from the decomposition of the organic matter below.

Run-of-the-river hydroelectric does not rely on dams, and hence creates much lower environmental impact, although it also generates less electricity because potential energy is lower.

Biomass Energy

Biomass refers to organic materials that have been derived from living or recently living organisms that contain chemical energy which was originally harnessed from the sun. Energy obtained from biomass resources is referred to as biomass energy. Sources of biomass energy include: plant matter, wood, peat, animal waste products such as manure, or even sewage. In the developing world, many people use wood and animal dung as the primary source of energy for cooking, heating, and lighting (using biomass accounts for 35-90% of the energy sources in these nations). Burning this biomass also creates huge health impacts on these populations, particularly in women and children.

In industrialized countries, new sources of biomass energy are being innovated. Depending on how the biomass is used, these new innovations are referred to as either biofuels or biopower. Biofuels are specifically used to power vehicles. Biopower is used to generate heat and electricity.

Biopower, or biomass energy, primarily takes advantage of unused plant material from crops (i.e. cornstalks), unused tree-top wood waste from logging efforts, and solid waste produced by livestock to burn and generate electricity. New research has led to the discovery of bioenergy crops, where current fast growing grasses such as bamboo, fescue, and switchgrass, as well as trees such as willows and poplar, are grown to produce biomass for biopower and biofuel.

These sources of energy can replace fossil fuels in coal-fired plants. In situations where 100% replacement with biomass is not possible, co-firing can be used, allowing for biomass to replace some of the coal (but not all). Another advantage of biopower is the fact that it uses biomass which is currently seen as waste. Additionally, biopower reduces the amount of sulfur dioxide being emitted into the atmosphere (as with coal), and reduces carbon emissions by using active carbon sinks as opposed to stored carbon sinks. However, extracting plant matter for the sole purpose of generating energy depletes the soil and can displace natural ecosystems and/or cropland. Furthermore, this type of energy may not be sustainable if we harvest biomass faster than it can be replenished.

Another way of using “waste” biomass is to produce biogas, which is the gaseous product of anaerobic digestion of organic matter (Figure 17). Biogas is primarily methane (CH4), which can be used for cooking, heating, and electricity generation.

Figure 17: Different types of biodigesters use plant and animal waste to feed methanogenic bacteria. The bacteria decompose the waste material producing methane (natural gas) which can be used for fuel, and a sludge that is nutrient rich and can be used as fertilizer. On the top, the Lübeck Waste Treatment Facility in Germany uses anaerobic digestion and air compression to perform mechanical biological treatment. Industrial scale plants such as this one process household and commercial wastes. On the bottom, a simple schematic of a handmade biodigester used in agriculture. 10


Looking Back


To review the environmental costs of deforestation, see Chapter 2.

Vegetable oil extracted from palm, canola, soy, and algae can be mixed with methanol and lye to produce biodiesel – a plant-derived fuel that can be used in regular petroleum-diesel engines. Similarly, corn starches and sugars can be fermented to produce liquid ethanol, also used to power automobiles. However, growing corn for the primary purpose of developing ethanol has raised the price of corn, which impacts the ability of less-developed nations to afford food. It has also caused destruction of precious cropland that could have been used to grow food. For example, in the Philippines the rising demand for biodiesel has driven deforestation of tropical rainforests in order to grow huge monocultures of palm for its oil.

Loyala University Chicago biodiesel reactor
Figure 18: Loyola University Chicago Biodiesel Reactor. Using this equipment, students make biodiesel out of the waste vegetable oil used to fry foods in the campus cafeterias. The biodiesel is used to fuel the shuttle buses that move students between two campuses in Chicago. Unlike growing soy and palm for use in biodiesel production, converting waste oil into biodiesel is efficient and sustainable. 11

Biodiesel and ethanol production also require substantial amounts of energy, water, fertilizer, and pesticide to produce, making the inputs more costly than the benefits derived from the product. For these reasons, experts feel that biodiesel and ethanol do not constitute sustainable fuel options. An exception is to produce biodiesel from used vegetable oil (e.g., cooking-oil waste). This converts a waste product into a non-petroleum based fuel and does not depend on converting land for the growth of oil-producing crops (Figure 18).

Energy Efficiency and Energy Transition

German energy transition chart
Figure 19 12

While renewable energy technologies are critical for reducing greenhouse gas emissions, reducing demand for energy imports, and lowering energy costs on a household and economy-wide level, improved ‘energy efficiency’ also makes an important contribution to these reductions. Energy efficiency simply means using less energy to perform the same task; that is, eliminating energy waste. Energy efficiency is the cheapest, and often most immediate, way to reduce the use of fossil fuels. While energy efficiency is a matter of changing energy use in an individual energy technology or fuel source, ‘energy transition’ is a matter of long-term structural change of entire energy systems. Such changes have occurred in the past, as in the transition from a pre-industrial system relying on traditional biomass and other renewable power sources (wind, water, and muscle power) to an industrial system characterized by pervasive mechanization (steam power) and the use of coal. Today, the global challenge is to transition from non-renewable energy systems to sustainable, renewable energy systems.

Looking Ahead


In the Energy and Ethics section you will explore the moral issue of energy availability to the poor and to future generations.

An example of transition toward sustainable energy, is the shift by Germany to decentralised renewable energy. Though the immediate shift has focused replacing nuclear energy, the next step is the abolishment of coal use. The overall goal for Germany is an entire energy system based on 60% renewable energy by 2050 (Fig 19).

The current state of the planet’s declining health and the knowledge we have gained from using fossil fuels has revealed that the negative impact which fossil fuel extraction and combustion has on the environment enormously outweighs its value in convenience and revenue. Alternative forms of clean and renewable energy offer a new approach to fueling the planet, but it is crucial that we continue to conscientiously conserve energy and reduce consumption while making headway on the new renewable energy technologies.

Questions to Consider

Return to the Energy Sources chart at the beginning of this Science section.

  • How many of the energy sources listed have you actually seen, felt, smelled, or touched?
  • Which of these sources provide most of the energy you use everyday?
  • Have you or anyone you have known ever been bodily harmed by any of the energy sources listed? Explain.