The Handy Chemistry Answer Book (2014)

EMERGING SOURCES OF ENERGY

How do solar cells work?

The Sun gives off 1,000 watts of energy per square meter of the Earth’s surface, which, if we could harness all of it, is more than enough to fulfill all of the world’s current electricity needs. It’s not easy to do this, however, which is why we still rely primarily on other energy sources. A solar cell, or photovoltaic cell, harnesses energy from photons of sunlight by having them excite electrons in a material. An electric field is established within the solar cell using a process called doping, and this makes it so that the excited electrons can only flow in one direction. Thus, when sunlight strikes the solar cell, a current flows, and this is the basic principle on which solar cells operate to capture energy. The harnessed energy can either be used immediately or it can be stored for later use. Designing more efficient solar cells is a very active area of research, and you can expect their efficiency to continue to increase in coming years.

How do wind turbines generate energy?

Wind can be converted into energy by a wind turbine, which typically consists of a wheel with large blades connected to a series of gears. As wind causes the blades of the turbine to spin the turbine collects mechanical/kinetic energy, which can either perform mechanical processes or be converted into electrical/potential energy. Some mechanical processes that make use of wind turbines are pumping water and grinding grain. To store electricity the turbine must be connected to a generator, which converts the mechanical energy from the blades into stored electrical energy. Typically, wind speeds of 7–10 mph are needed to generate energy using a wind turbine. The orientation of a wind turbine can be controlled by a computer to optimize the amount of energy it collects.

Photovoltaic cells, or solar cells, work by containing materials such as mono- or polycrystaline silicon, cadmium telluride, or copper indium selenide whose electrons are easily excited by photons from the sun, creating electricity.

What are carbon offsets?

A carbon offset involves a party committed to reducing emissions of greenhouse gases in order to compensate for emissions made elsewhere. The gases involved include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), sulfur hexafluoride (SF₆), perfluorocarbons, and hydrofluorocarbons. Conversion factors are used to attempt to equate the negative effects of the different gases on the atmosphere. Carbon offsets are typically purchased by either companies or governments under regulations with regard to greenhouse gas emissions or by personal consumers who wish to offset their own contributions to greenhouse gas emissions.

What is biodiesel?

Biodiesel is a type of fuel made from vegetable oil and/or animal fat which consists of long alkyl esters (see “Organic Chemistry” to review functional groups). Biodiesel fuel is commonly distributed for sale as a mixture with petrodiesel and is labeled with a “B factor” describing the fraction of biodiesel present. B100 is pure biodiesel, while B20 would represent a mixture of 20% biodiesel with 80% petrodiesel.

How is biodiesel produced?

The production of biodiesel fuel involves a chemical reaction called transesterification of lipids (those from the vegetable oil or fat) with alcohols to produce alkyl esters. Methanol is most commonly used as the alcohol in these reactions, which leads to methyl esters, though other alcohols have also been used.

Glycerol is a byproduct of the transesterification reaction, and this compound is actually formed in fairly substantial quantities (ca. 10% by mass). This has given rise to research directed toward finding ways to carry out chemical reactions beginning with, or involving, glycerol, such that the cost efficiency of the overall process of biodiesel production might be improved.

How is ethanol produced for use as a fuel?

Ethanol is produced by fermenting sugars from plants like corn, soybeans, and sugarcane. The sugars in the plant material are first broken down and then “fed” to yeast, which ferments them to produce ethanol as a byproduct.

How does ethanol work as a fuel?

Ethanol works as a fuel in much the same way as gasoline; it is burned in a combustion reaction to release energy. In automobiles, ethanol is usually mixed with gasoline. Most cars can run on an ethanol-gasoline mixture with 10% ethanol (E10)—but to use an 85% ethanol mixture (E85) requires a specially designed system. Ethanol burns much cleaner than gasoline, making it less hazardous to the environment. It is also a renewable resource that can be produced by growing crops, so it has the potential to reduce dependence on foreign oil sources.

How could algae potentially be used as a source of energy?

Algae are potentially a very useful source for biofuels, but currently the cost of using them as a source of biofuels is too high to make them practically useful. One current area of research involves using the algae biofuel production process to produce a by-product that is rich in protein and that could be used to feed farm animals. This would help to offset the cost associated with algae biofuel production. Since corn is currently used to both feed animals and to produce ethanol for fuel, the protein-rich by-product could also help by reducing the amount of corn that needs to be grown.

An experimental fuel cell car is shown here. Fuel cell technology has been deployed commercially in recent years, but is more often seen in public vehicles such as buses than in private cars.

How is hydrogen produced for use as a fuel?

Most of the current hydrogen production in the U.S. is carried out by steam reforming natural gas (methane), which involves reacting steam with methane to generate H₂. To make hydrogen a viable fuel source, more efficient ways of producing hydrogen on the large scale will be needed. Many scientists are currently investigating chemical and biological catalysts capable of carrying out a process called water splitting, which is the production of H₂ and O₂from water (H₂O). Water splitting may have the potential to make hydrogen into a viable fuel source for vehicles.

How do hydrogen-powered cars get their energy?

Hydrogen-powered cars are based on fuel cells that store hydrogen, or H₂ gas, inside a material called a polymer exchange membrane. The fuel cell contains two electrodes: an anode (negative side) and a cathode (positive side). At the anode, the H₂ molecules are split into protons and electrons. The protons pass through a polymer exchange membrane, while the electrons are unable to pass through this membrane and thus have to flow in a different direction. This creates a current of electricity by which the car is powered.

What wavelengths of light from the Sun reach Earth’s surface?

The light from the Sun reaching the Earth’s surface spans a range of wavelengths between roughly 300 to 2,500 nanometers with a few gaps in between where atmospheric water and carbon dioxide absorb radiation. The range of wavelengths spanned by light in the Earth’s upper atmosphere is slightly broader, demonstrating that gases present in the atmosphere absorb a notable amount of light before they reach the Earth’s surface.

How much energy in the United States comes from hydroelectric power sources?

About ten percent of the electricity in the United States comes from hydroelectric power sources. The state of Washington leads the United States in hydroelectric energy production, with roughly 87% of this state’s electricity coming from hydroelectric power!

As sunlight reaches the earth, it must penetrate the atmosphere, which absorbs or reflects much of the radiation before it reaches the surface of the planet.

What is natural gas, and where does it come from?

Natural gas is primarily composed of methane (CH₄), and like other hydrocarbons, methane provides energy via combustion reactions. Natural gas is typically found underground, near sources of petroleum, and can be pumped up via pipelines. Actually, natural gas does not have a smell, so it is mixed with a small amount of a strong-smelling thiol compound so that you can tell if you have a gas leak in your home.

What is fusion and how might it be used as an energy source?

We looked at fusion in “Nuclear Chemistry.” To recap, fusion involves the combination of two nuclei to form a single nucleus. For lightweight nuclei, this process typically involves the release of energy. Unfortunately, it’s currently very difficult to make fusion happen under practical conditions; nuclei typically have to be accelerated to very, very high speeds to make fusion take place.

What is a renewable energy source?

A renewable energy source is one that is provided by resources that are continuously replenished or which will always be available during the foreseeable future of the planet.

So if the radiation coming from the sun more or less covers all wavelengths of visible light, why aren’t plants black?

Let’s start with explaining the question a bit more. You can see from the graph on the preceding page that the sun provides light in almost every wavelength. The vast majority of plants, however, appear green, which means they are absorbing blue and red light and reflecting the green light back into your eyeballs. If plants were taking in all the energy the sun is providing, they would appear black, as no light would be reflected. So, to restate the question, why do plants reflect any light at all?

The easiest answer here is that absorbing red and blue energy obviously works well enough, so evolution stuck with it. Evolution, after all, doesn’t provide the best solution, but rather a solution. Nonetheless, let’s make some guesses as to why plants are green. Chlorophyll isn’t just after energy in general, but needs very specific wavelengths to pass along energy to the reactive centers of Photosystems I and II. The second reasonable guess is that too much energy is not a good thing. Absorption of light creates high-energy species, and if they aren’t used in some productive way the energy will find something else to do. These other reactions could be destructive to cells. In sum, chlorophyll only takes what it needs or what it can use.

Which energy sources are renewable?

Renewable energy sources include wind, hydroelectric, solar, biomass/biodiesel, and geothermally derived power.

What fraction of power globally comes from renewable energy sources?

It is estimated that about 16% of global energy consumption currently comes from renewable resources. Hopefully this number will increase significantly in the near future.