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.
You will discover in the Energy and Action section ahead that many people around the world are working on energy solutions through solar projects.
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).
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.
Find out more about how solar panels capture sunlight and convert it to electricity.
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. Hydrogen can also be used to store energy as backup.
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).
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.
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).
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 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 4.5-10oC (40-50oF) 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
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 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.
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.
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
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.
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.
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.