Solar Energy

Botkin, Daniel B. Environmental Science: Earth as a Living Planet, 9th Edition. Wiley, 2013-12-23. VitalBook file.

16.1 Introduction to Alternative Energy Sources

Today fossil fuels supply approximately 90% of the en- ergy consumed by people. Fossil fuels and nuclear power are considered conventional energy sources. All other sources are considered alternative energy and are divided into renewable energy and nonrenewable energy. Nonre- newable alternative energy sources include deep-earth geothermal energy (the energy from the Earth’s geologi- cal processes). This kind of geothermal energy is consid- ered nonrenewable for the most part because heat can be extracted from Earth faster than it is naturally replen- ished—that is, output exceeds input (see Chapter 4).

The renewable energy sources are: direct solar (both active and passive); freshwater (hydro); wind; ocean; low-density, near-surface geothermal; and biofuels. Low- density, near-surface geothermal is simply solar energy stored by soil and rock near the surface. It is widespread and easily obtained and is renewed by the sun. The en- ergy of biofuels comes from photosynthesis—biologi- cal carbon fixation. Sources include trees, grasses, peat; agrifuels; organic waste, including landfill gases; and ethanol—alcohol produced directly by some algae or as a by-product of bacteria’s decomposition of organic wastes. These are renewable because they are regenerated by the energy from the sun (Figure 16.2).

16.2 Solar Energy

The total amount of solar energy reaching Earth’s surface is tremendous. For example, on a global scale, ten weeks of solar energy is roughly equal to the energy stored in all known reserves of coal, oil, and natural gas on Earth. Solar energy is absorbed at Earth’s surface at an average rate of 90,000 terawatts (1 TW equals 1012 W), which is about 7,000 times the total global demand for energy.3 In the United States, on aver- age, 13% of the sun’s original energy en- tering the atmosphere arrives at the surface (equivalent to approximately 177 W/m2, or about 16 W/ft2, on a continuous basis). The estimated year-round availability of so- lar energy in the United States is shown in Figure 16.3. However, solar energy is site- specific, and detailed observation of a po- tential site is necessary to evaluate the dailyand seasonal variability of its solar energy potential. It of- ten surprises people that some major solar energy installa- tions are in areas not considered especially sunny, such as northern Michigan and western Massachusetts, which have more rain and snow than many other parts of the coun- try, and less direct sunlight than desert areas like Arizona.4 Solar energy is captured through passive and active solar systems. Passive solar energy systems do not use mechani- cal pumps or other active technologies to move air or water.

Passive Solar Energy

Passive solar makes use of architectural designs and materials that enhance the absorption of solar energy (Figure 16.4). Prehistoric cave dwellers often chose limestone caves because limestone absorbs sunlight and stays warm. Since the rise of civilization, many have designed buildings to face south in the Northern Hemisphere, as in ancient Rome (see Chapter 22). Houses built by American Indians long ago, now in Mesa Verde National Park, lie below the surface of the mesa and therefore are protected from much of the area winds; they face south, to gather sunlight. The mesa overhangs the ouses, providing some shade during the summer when the sun is high overhead (Figure 16.5) The early settlers of the American West sometimes lived in dugouts, which were homes cut into the soil, because soil absorbs and stores solar energy and the structures are out of the wind, so that they do not readily lose heat by convection.

Today, thousands of buildings in the United States— not just in the sunny Southwest but in other parts of the country, such as New England—use passive solar systems. Passive solar energy promotes cooling in hot weather and retaining heat in cold weather. Methods include (1) add- ing overhangs on buildings to block summer (high-angle) sunlight but allow winter (low-angle) sunlight to penetrate and warm rooms; (2) building a wall that absorbs sunlight during the day and radiates heat that warms the room at night; and (3) planting deciduous trees on the sunny side of a building. In summer, these shade and cool the building; in the winter, with the leaves gone, they let the sunlight in.

Passive solar energy also provides natural lighting to buildings through windows and skylights. Modern

window glass can have a special glazing that transmits vis- ible light, blocks infrared, and provides insulation.

Active Solar Energy

Active solar energy systems require mechanical power, such as electric pumps, to circulate air, water, or other flu- ids from solar collectors to a location where the heat is stored and then pumped to where the energy is used.

Solar Collectors

Solar collectors that provide space heating or hot wa- ter are usually flat, glass-covered plates over a black back- ground where a heat-absorbing fluid (water or some other liquid) is circulated through tubes (Figure 16.6). Solar ra- diation enters the glass and is absorbed by the black back- ground. Heat is emitted from the black material, heating the fluid in the tubes.

A second type is the evacuated tube collector, which is similar to the flat-plate collector, except that each tube, along with its absorbing fluid, passes through a larger tube that helps reduce heat loss. The use of solar collectors is expanding very rapidly; the global market grew about 50% from 2001 to 2004. In the United States, solar wa- ter-heating systems generally pay for themselves in only four to eight years.4

Photovoltaics

Photovoltaics convert sunlight directly into electricity (Figure 16.7). The systems use solar cells, also called photo- voltaic cells, made of thin layers of semiconductors (silicon or other materials) and solid-state electronic components with few or no moving parts. The cells are constructed in standardized modules and encapsulated in plastic or glass, which can be combined to produce systems of various sizes so that power output can be matched to the intended use. Electricity is produced when sunlight strikes the cell. The different electronic properties of the layers cause electrons to flow out of the cell through electrical wires.

Large photovoltaic installations may be connected to an electrical grid. Off-the-grid applications can be large or small and include powering satellites and space vehicles, and powering electric equipment, such as water-level sen- sors, meteorological stations, and emergency telephones in remote areas (Figure 16.8).

Off-the-grid photovoltaics are emerging as a major contributor to developing countries that can’t afford to build electrical grids or large central power plants that burn fossil fuels. One company in the United States is manufacturing photovoltaic systems that power lights and televisions at an installed cost of less than $400 per house- hold.5 About half a million homes, mostly in villages not linked to a countrywide electrical grid, now receive their electricity from photovoltaic cells.6 Photovoltaics are the world’s fastest growing source of energy. In the United States, on-the-grid utility-scale solar energy capacity grew from 70 megawatts (MW) in 2008 to 1,052 MW in 2011 and doubled in 2011 alone. Total solar electric installa- tions, including residential, grew from less 7 than 2,000 MW in 2008 to almost 3,000 MW by 2011.

Solar cell technology is advancing rapidly. While a few decades ago solar cells converted only about 1 or 2% of sunlight into electricity, today they convert 20% or more.

And the costs are dropping fast as well. As recently as 2008, installation of solar cells cost an average of $6.71/watt, but by 2013 had dropped to about $4.00/watt, a decrease of 40% and in some situations even more, depending on the complexity of the site.8,4 And some forecast that the manu- facturing costs1of solar cells might drop to less than $0.50/ watt by 2015. (This cost is for the cells only and doesn’t include the cost of installation, inverters, etc.)

Solar Thermal Generators

Solar thermal generators focus sunlight onto water- holding containers. The water boils and is used to run such machines as conventional steam-driven electrical generators. These include solar power towers, shown in Figure 16.9. The first large-scale test of using sunlight to boil water and using the steam to run an electric genera- tor was “Solar One,” funded by the U.S. Department of Energy. It was built in 1981 by Southern Califor- nia Edison and operated by that company along with the Los Angeles Department of Water & Power and the California Energy Commission. Sunlight was concen- trated onto the top of the tower by 1,818 large mirrors (each about 20 ft in diameter) that were mechanically linked to each other and tracked the sun. At the end of 1999, this power tower was shut down, in part because the plant was not economically competitive with other sources of electricity. New solar thermal generators are being built with very large output (Figure 16.10).

More recently, solar devices that heat a liquid and produce electricity from steam have used many mirrors without a tower, each mirror concentrating sunlight onto a pipe containing the liquid (as shown in Figures 16.9 and 16.10). This is a simpler system and has been considered cheaper and more reliable.

Solar Energy and the Environment

The use of solar energy generally has a relatively low impact on the environment, but there are some environmental concerns nonetheless. As we saw in the chapter’s opening case study, there is concern about the effects of installing solar energy near national parks and on public lands that have other important environmental uses and benefits. This is likely to be the primary environmental conflict among environmentalists concerning solar energy. Another con- cern is the large variety of metals, glass, plastics, and flu- ids used in the manufacture and use of solar equipment. Some of these substances may cause environmental prob- lems through production and by accidental release of toxic materials.

16.3 Converting electricity from renewable energy into a Fuel for Vehicles

An obvious question about solar energy, as well as energy from wind, oceans, and freshwater, is how to convert this energy into a form that we can easily transport and can use to power our motor vehicles. Basically, there are three choices: Use the electricity directly; convert the energy to mechanical motion, such as a spinning cylin- der; or convert the energy to another form, a liquid or gaseous fuel. Direct use includes storing the electricity in batteries.

Producing Liquid and Gaseous Fuels from Electricity

An electrical current can be used to separate water into hydrogen and oxygen. The hydrogen can be used directly—burned within an engine—or it can power fuel cells (see A Closer Look 16.1). In a fuel cell the hydrogen is combined again with oxygen, and the energy is released as electricity

In theory, another option is to combine the hydrogen with carbon to form methane (CH4), the major constitu- ent of natural gas. Or the methane can be combined with oxygen to form ethanol (C2H6O)—the alcohol already used as part of standard gasoline in the United States. This would be a kind of reverse refinery, which is not yet in use.

Hydrogen, like natural gas, can be transported in pipelines and stored in tanks. Furthermore, it is a clean fuel; the combustion of hydrogen produces water, so it does not contribute to global warming, air pollution, or acid rain.

uel Cells—An Attractive Alternative

Even if we weren’t going to run out of fossil fuels, we would still have to deal with the fact that burning fossil fuels, par- ticularly coal and fuels used in internal combustion engines (cars, trucks, ships, and locomotives), causes serious envi- ronmental problems. This is why we are searching not only for alternative energy sources but for those that are envi- ronmentally benign ways to convert a fuel to usable ways to generate power.a One promising technology uses fuel cells, which produce fewer pollutants, are relatively inexpensive, and have the potential to store and produce high-quality energy.

Fuel cells are highly efficient power-generating systems that produce electricity by combining fuel and oxygen in an electrochemical reaction. (They are not sources of energy but

a way to convert energy to a useful form.) Hydrogen is the most common fuel type, but fuel cells that run on methanol, ethanol, and natural gas are also available. Traditional gen- erating technologies require combustion of fuel to generate heat, then conversion of that heat into mechanical energy (to drive pistons or turbines), and conversion of the mechan- ical energy into electricity. With fuel cells, however, chemical energy is converted directly into electricity, thus increasing second-law efficiency (see Chapter 17) while reducing harm- ful emissions.

The basic components of a hydrogen-burning fuel cell are shown in Figure 16.11. Both hydrogen and oxygen are added to the fuel cell in an electrolyte solution. The hydrogen and oxygen remain separated from one another, and a platinum membrane prevents electrons from flowing directly to the positive side of the fuel cell. Instead, they are routed through an external circuit, and along the way from the negative to the positive electrode, they are diverted into an electrical motor, supplying current to keep the motor running.a, b To maintain this reaction, hydrogen and oxygen are added as needed. When hydrogen is used in a fuel cell, the only waste product is water.

Fuel cells are efficient and clean, and they can be arranged in a series to produce the appropriate amount of energy for a particular task. In addition, the efficiency of a fuel cell is large- ly independent of its size and energy output. They can also be used to store energy to be used as needed. Fuel cells are used in many locations. For example, they power buses at Los Angeles International Airport and in Vancouver. They also power a few buses in east San Francisco Bayc and provide heat and power at Vandenberg Air Force Base in California.d But this experimen- tal technology is expensive, with buses estimated to cost more than $1 million each.e

Technological improvements in producing hydrogen are certain, and the fuel price of hydrogen may be substantially lower in the future.

16.8 geothermal energy

There are two kinds of geothermal energy: deep-earth, high-density; and shallow-earth, low-density. The first makes use of energy within the Earth. The second is a form of solar energy: When the sun warms the surface soils, water, and rocks, some of this heat energy is gradu- ally transmitted down into the ground.

The first kind of geothermal energy—deep-earth, high-density—is natural heat from the interior of the Earth. It is mined and then used to heat buildings and generate elec- tricity. The idea of harnessing Earth’s internal heat goes back more than a century. As early as 1904, geothermal power was used in Italy. Today, Earth’s natural internal heat is being used to generate electricity in 21 countries, including Rus- sia, Japan, New Zealand, Iceland, Mexico, Ethiopia, Gua- temala, El Salvador, the Philippines, and the United States. Total worldwide production is approaching 9,000 MW (equivalent to nine large, modern coal-burning or nuclear power plants)—double the amount in 1980. Some 40 mil- lion people today receive their electricity from geothermal energy at a cost competitive with that of other energy sourc- es. In El Salvador, geothermal energy is supplying 25% of the total electric energy used. However, at the global level, geothermal energy accounts for less than 0.15% of the total energy supply.14 In 2011 geothermal energy provided only 0.4% of the electricity in the United States.

This kind of geothermal energy may be considered a nonrenewable energy source when rates of extraction are greater than rates of natural replenishment. However, geo- thermal energy has its origin in the natural heat produc- tion within Earth, and only a small fraction of the vast total resource base is being used today. Although most geothermal energy production involves tapping high-heat sources, people are also using the low-temperature geo- thermal energy of groundwater in some applications.

16.8 Geothermal Energy 381 Geothermal Systems

The average heat flow from the interior of the Earth is very low, about 0.06 watts per square meter (W/m2). This amount is trivial compared with the 177 W/m2 from sun- light at the surface, but in some areas the heat flow is high enough to be useful.15 For the most part, high heat flow oc- curs mostly at plate tectonic boundaries (see Chapter 4, in- cluding oceanic ridge systems—divergent plate boundaries) and areas where mountains are being uplifted and volcanic islands are forming (convergent plate boundaries). You can see the effects of such natural heat flow at Yellowstone Na- tional Park. On the basis of geologic criteria, several types of hot geothermal systems (with temperatures greater than about 80°C, or 176°F) have been defined (Figure 16.19).

Some communities in the United States, includ- ing Boise, Idaho, and Klamath Falls, Oregon, are using deep-earth, high-density geothermal heating systems. A common type of geothermal system uses hydrothermal convection, where the circulation of steam and/or hot water transfers heat from the depths to the surface. An example is the Geysers Geothermal Field, 145 km (90 mi) north of San Francisco. It is the largest geothermal power operation in the world (Figure 16.20), producing about 1,000 MW of electrical energy. At the Geysers, the hot water is maintained in part by injecting treated wastewa- ter from urban areas into hot rocks.

The second kind of geothermal energy—shallow- earth, low-density—is at much lower temperatures than geothermal sources and is used not to produce electric- ity but for heating buildings and swimming pools, and for heating soil to boost crop production in greenhous- es. The potential for this kind of energy is huge, and it is cheap to obtain.8 Such systems are extensively used in Iceland.

It may come as a surprise to learn that most ground- water can be considered a source of shallow-earth, low- density geothermal energy. It is geothermal because the normal heat flow from the sun to the Earth keeps the temperature of groundwater, at a depth of 100 m (320 ft), at about 13°C (55°F). This is cold for a shower but warm- er than winter temperatures in much of the United States, where it can help heat a house. In warmer regions, with summer temperatures of 30–35°C (86–95°F), groundwa- ter at 13°C (55°F) is cool and thus can be used to provide air conditioning, as it does today in coastal Florida and elsewhere. In summer, heat can be transferred from the warm air in a building to the cool groundwater. In win- ter, when the outdoor temperature is below about 4°C (40°F), heat can be transferred from the groundwater to the air in the building, reducing the need for heating from other sources (Figure 16.21). “Heat pumps” for this kind of heat transfer are used in warm locations such as Florida and as far north as Juneau, Alaska, but are limited by extreme temperatures and can’t function in the cold winters of northern New Hampshire or interior Alaska, such as in Fairbanks. (Juneau, on the coast, has a much more moderate climate because of the influence of the ocean waters.)

Geothermal Energy and the Environment

Deep-earth, high-density, geothermal energy develop- ment produces considerable thermal pollution from its hot wastewaters, which may be saline and highly corrosive. Other environmental problems associated with this kind of geothermal energy use include on-site noise, emissions of gas, and disturbance of the land at drilling sites, disposal sites, roads and pipelines, and power plants. The good news is that the use of deep- earth, high-density geothermal energy releases almost 90% less carbon dioxide and sulfur dioxide than burn- ing coal releases to produce the same amount of elec- tricity.16 Furthermore, development of geothermal energy does not require large-scale transportation of raw materials or refining of chemicals, as development of fossil fuels does. Nor does geothermal energy pro- duce the atmospheric pollutants associated with burn- ing fossil fuels or the radioactive waste associated with nuclear energy.

Even so, deep-earth, high-density geothermal pow- er is not always popular. For instance, on the island of Hawaii, where active volcanoes provide abundant near-surface heat, some argue that the exploration and development of geothermal energy degrade the tropi- cal forest as developers construct roads, build facili- ties, and drill wells. In Hawaii, geothermal energy also raises religious and cultural issues. Some people, for instance, are offended by the use of the “breath and water of Pele,” the volcano goddess, to make electricity. This points out the importance of being sensitive to the values and cultures of people where development is planned.

The Future of Geothermal Energy

By 2011, the United States produced only 10,898 MW of geothermal energy.8 Globally, the likelihood is that high- density, deep-earth geothermal can be only a minor con- tributor to world energy demand, but that low-density, shallow-earth geothermal can be a major source of alternative and renewable energy.

SUMMARY

• The use of renewable alternative energy sources, such as wind and solar energy, is growing rapidly. These en- ergy sources do not cause air pollution, health prob- lems, or climate changes. They offer our best chance to replace fossil fuels and develop a sustainable energy policy.

• Passive solar energy systems often involve architectural designs that enhance absorption of solar energy without requiring mechanical power or moving parts.

• Some active solar energy systems use solar collectors to heat water for homes.

• Systems to produce heat or electricity include power towers and solar farms.

• Photovoltaics convert sunlight directly into electricity. • Hydrogen gas may be an important fuel of the future,

especially when used in fuel cells.

• Water power today provides about 10% of the total electricity produced in the United States. Except in some developing nations, good sites for large dams are already in use. Water power is clean, but there is an environmental price to pay in terms of disturbance of ecosystems, sediment trapped in reservoirs, loss of wild rivers, and loss of productive land.

• Wind power has tremendous potential as a source of electrical energy in many parts of the world. Many util- ity companies are using wind power as part of energy production or as part of long-term energy planning. Environmental impacts of wind installations include loss of land, killing of birds, and degradation of scenery.

• Biofuels are of three kinds: firewood, wastes, and crops grown to produce fuels. Firewood has been important historically and will remain so in many developing na- tions and many rural parts of developed nations. Burn- ing wastes is a good way to dispose of them, and their energy is a useful by-product. Crops grown solely as biofuels appear to be a net energy sink or of only mar- ginal benefit, at considerable environmental costs, in- cluding competition for land, water, and fertilizers, and the use of artificial pesticides. At present they are not a good option.

• Geothermal energy, natural heat from Earth’s interior, can be used as an energy source. The environmental ef- fects of developing geothermal energy relate to specific site conditions and the type of heat—steam, hot water, or warm water. Environmental impacts may involve on-site noise, industrial scars, emission of gas, and disposal of saline or corrosive waters.

Botkin, Daniel B. Environmental Science: Earth as a Living Planet, 9th Edition. Wiley, 2013-12-23. VitalBook file.

Multimedia

1. energynownews. (2011, August 23). Energy 101: Solar power [Video clip]. Retrieved from http://www.youtube.com/watch?v=NDZzAIcCQLQ

· Transcript

2. U.S. Department of Energy. (2011, January 4). Energy 101: Geothermal heat pumps [Video clip]. Retrieved from http://www.youtube.com/watch?v=y_ZGBhy48YI

· Captions are available by clicking the “CC” icon in the video window.

3. U.S. Department of Energy. (2011, February 8). Energy 101: Solar PV [Video clip]. Retrieved from http://www.youtube.com/watch?v=0elhIcPVtKE

· Captions are available by clicking the “CC” icon in the video window.

4. U.S. Department of Energy. (2011, July 8). Energy 101: Concentrating solar power [Video clip]. Retrieved from http://www.youtube.com/watch?v=rO5rUqeCFY4

· Captions are available by clicking the “CC” icon in the video window.