7. Solar power

Figure 7.1 Solar race car designed and built by students at the University of Minnesota passes by a wind farm near Lake Benton, Minnesota. (DOE photo)

Key facts

• Solar is the fastest-growing energy source.

• It presently provides a tiny fraction of the world’s energy, considerably less than 1%, but if just 1% of Earth’s land area had photoelectric devices, all the world’s current energy needs would be met.

• And in about 20 years the solar energy generated on 1% of Earth’s surface would equal the amount of energy in all known fossil-fuel reserves.

• The same is true for the United States—1% of the land area would provide all of the nation’s energy needs, not just electricity.

• But at present, solar energy is the most expensive source of on-the-grid electricity, costing about twice the average price of electricity.

• However, solar energy can also be produced locally and in small amounts, and small solar-electric facilities are rapidly becoming more important in developing nations.

• In Kenya, some 10,000 women have been trained to use $10 solar cookers and show others how to use them. Hundreds of thousands of these solar cookers are also in use in India.

• In the 1990s, in Kenya alone, 120,000 photovoltaic units were sold to provide electricity for lighting, radio, television, and so on.

Crossing Australia at almost 60 miles an hour

Every other October since 1987, solar-powered cars have raced from Darwin to Adelaide, Australia, an 1,800-mile route that puts the latest alternative-energy technology to the test. The cars can run only on sunlight that their solar cells capture and convert to electricity. Electric motors that are at least 90% efficient are necessary. More than 50 teams—usually of college students, often backed by major aerospace and high-tech corporations—compete in the race, which takes a week.

In 2007, among the entrants was an all-women’s team from Annesley College in Adelaide; the team designed, built, and raced their car. Only 18 cars finished. The winning 2007 team, from the Netherlands, completed the race in 33 hours at an average speed of 57 mph. A car from the United States, designed, built, and raced by students from the University of Michigan, made it in just under 45 hours at an average speed of 42 mph. The only other U.S. entrants did not even finish the race: The Equinox from Stanford University reached 1,158 miles, more than halfway, and Houston’s Sundancer made it only 89 miles down the road from Darwin.¹ (Was this a sign that the United States might be losing its place as No. 1 in high-technology inventions, research, and development?)

As I write this in the summer of 2008, a website announces the next challenge in this race from the Swiss team, which promises more and more advances. Solar-powered car races aren’t all that frequent, but devices powered by the sun are becoming familiar, even ordinary. In the United States, if you look carefully you are likely to find solar-electric devices near home. A drive along a major highway reveals emergency telephones and emergency highway signs powered by solar-electric panels. Some wristwatches are solar-powered, and you can buy a little solar electrical generator to recharge your iPod and your Blackberry.

At a much larger industrial scale, solar energy parks, large facilities that generate electricity that goes onto an electric grid, are rapidly increasing in number, and at this point it’s hard to know at any time which is actually the world’s biggest working facility (Figures 7.2 A, B). At this writing, the prize goes to two locations in Spain, both with a 20-megawatt capacity—one in Jumilla, Murcia, and the other in Beneixama. Both are said to provide enough electricity for 20,000 houses, and both are where you would expect to find solar energy installations—in a warm sunny climate. (These sites average 300 sunny days a year.)

Figure 7.2 (A) Largest solar photovoltaic installation in the Western Hemisphere, near Orlando, Florida. (Photo courtesy of SunPower Corporation)

Figure 7.2 (B) Obama at that Western Hemisphere’s largest solar park. (Photo courtesy of SunPower Corporation)

The kinds of solar energy

Two kinds of solar energy are in use: active and passive. Passive solar energy is at work in buildings designed to absorb and reflect sunlight in ways that reduce the need to burn fuels for heating or cooling. No motors are used to move air or water or any material storing the energy. The way an automobile heats up when parked in the sun is an example of passive solar energy. Passive solar energy is often discussed as part of energy conservation, and we discuss it in Chapter 10, “Transporting Energy: The Grid, Hydrogen, Batteries, and More.” Right now we’ll talk about active solar energy.

Active solar energy uses electrical, electronic, or mechanical technologies to store, collect, and distribute solar energy for heating and cooling; to generate electricity; or (much more rarely) to do mechanical work directly. This chapter focuses on active solar energy technologies as sources of energy, primarily to generate electricity, but also, once the electricity is available, to use that energy to make chemical fuels.

Active solar energy, in turn, divides into two major technologies: those that convert sunlight directly into electricity, and those that use sunlight to heat (and boil) water, which is then used to run a conventional electrical generator or to provide hot water and space heating in a system that involves mechanical pumps and various control devices, including computers. (The pumps and controls distinguish active solar energy from passive solar heating of buildings.)

Solar thermal generators: sunlight to steam to electricity and big bright lights

The first large-scale test of using sunlight to heat water and using that to run an electric generator was Solar One, funded by the U.S. Department of Energy, built in 1981 by Southern California Edison and operated by that company along with the Los Angeles Department of Water & Power and the California Energy Commission. Here’s the way it worked: Sunlight was focused and concentrated onto the top of a tower by 1,818 large mirrors (each about 20 feet in diameter) that were mechanically linked to each other and tracked the sun. It is a system that would have delighted Archimedes, who is said to have used mirrors to focus sunlight on enemy ships and burn them, probably the first military use of solar energy.

Solar One became famous to those of us in Southern California interested in the environment. I was teaching at the time at the University of California, Santa Barbara, and I drove on I-40 to Barstow to see this remarkable new device. It was impressive, especially when viewed in late afternoon among the long shadows of the surrounding desert. The mirrors reflected so much sunlight onto the tower that its top seemed aglow, as if it were a miniature sun emitting its own, not reflected, light. In fact, it was so bright that you couldn’t look at it directly for long, as the accompanying photograph shows somewhat (Figure 7.3).

Figure 7.3 Solar One, the first major solar thermal tower, was built in the Mojave Desert near Barstow, California, several hundred miles east of Los Angeles. Originally it was an experiment funded by the Department of Energy and built and operated by Southern California Edison. (Photo by Daniel B. Botkin)

Solar One, still at the same location in the California desert, was improved in the mid-1990s and called Solar Two. Its mirrors and everything else necessary to run the system took up 126 acres—the mirrors alone had an area of 20 1/2 acres of reflecting surface!—and it operated with a capacity of 10 megawatts until it was shut down in 1999. Even today, it stands out in my memory as the most impressive view of the use of solar energy, almost magical in its brightness.

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 (Figure 7.4). This is a simpler system and has been considered cheaper and more reliable.

Figure 7.4 Researchers analyze the efficiency of using parabolic troughs for solar thermal systems. (DOE photo)

Solar electric generators: using very large, smooth surfaces to convert sunlight to electricity

The other major active solar energy technology uses solid-state devices that generate an electric current when exposed to light. If they weren’t so common today, they would seem quite magical, like the solar energy flashlight I bought recently—you just lay it on the windowsill, and whenever you need light to use in the dark, you have it. These devices are called photovoltaic cells, and the most common of them is a silicon wafer, which is made of crystals of silicon. These make up 60% of the devices sold.² This is the same kind of material used to make computer integrated circuits. In fact, one photovoltaic company, Astropower, now out of business, used rejected materials from the manufacturing of computer chips. Silicon, by the way, combined with oxygen makes up most of the sand on beaches and is a very common material.

More recently, photovoltaic thin films have been developed, and some are in commercial production. (These make up almost 40% of the devices sold.) These use very small amounts of certain rare metal compounds, including cadmium telluride (a compound of cadmium and tellurium), CIGS (a chemical compound of copper, indium, gallium, and selenium), and a noncrystalline form of silicon.

At the time of this writing, some research is also exploring organic compounds that emit electricity when exposed to light, and there has been research over many years to try to take chlorophyll from green plants and use that in nonliving materials to generate electricity. But the majority of research and development of active solar energy is with inorganic photovoltaic systems.³

How much energy does solar provide now?

The amazing thing about commercially available photovoltaics is how efficient they are—how much of the energy they receive as sunlight goes out a wire as useful electricity. The record so far is 24% from crystalline silicon, and the thin films have reached 18% and 19%. Compare this to green plants that on average fix a miserly 3%.

By the end of 2007 there were many large installations, called solar energy parks. The 25 largest of these each had an electrical generating capacity of 5 megawatts or more from photovoltaic cells. Eleven of the largest installations were in Spain, ten in Germany, one each in Portugal and Japan, and only two in the United States.⁴ These 25 largest solar parks by themselves have a generating capacity of more than 225 megawatts, enough to provide electricity for more than 225,000 homes. Worldwide, 880 photovoltaic power plants, each with at least 200-kilowatt capacity, are in operation, with a total capacity of 955 megawatts. Most of these large-scale plants are in Germany (390), the United States (225), and Spain (130).⁵

Although these individual systems are impressive both visually and in their electrical generation, they are dwarfed by the largest solar thermal plants. In fact, the largest solar energy installation of any kind operating today is Solar Energy Generating Systems (SEGS) in the Mojave Desert, the same region where Solar One was built. This is actually nine individual solar thermal plants with a combined capacity of 310 megawatts. Its 400,000 mirrors occupy 1,000 acres (about 1.6 square miles).⁶ Operated by Florida Power & Light and Southern California Edison, its capacity is larger than the sum of the 25 largest photovoltaic installations in the world.

What contribution do these systems make to the total energy supply? In the United States, total energy generated from all sources in 2005 was 29,590 kilowatt-hours.⁷ (This is the standard value we are using throughout this book.) In 2008, the most recent year for which data are available, electricity generated in the U.S. totaled 4,119 billion kilowatt-hours. Solar energy provided just 0.02% (two-hundredths of a percent) of the electricity and 0.003% (three-thousandths) of the total energy from all sources. Wind provided much more: 1.34% of the electricity and 0.86% (almost 1%) of the total energy from all sources. So together, these two kinds of renewable energy provided a very small amount of the electricity and of the total energy. The situation is similar worldwide. At present, solar photovoltaic electricity provides less than 1% of the world’s electrical energy and only about 0.1% of the world’s energy.⁸ So the take-home lesson here is that today solar photovoltaic systems provide a very small percentage of the world’s electrical energy and therefore of the world’s total energy use.

How much energy could solar energy provide?

The short answer: a huge amount. I got interested in this several years ago when what was then the largest photovoltaic system in the world, the one in Bavaria, began operating. I made calculations, based on installed facilities—not theoretical possibilities—for a variety of sites established by PowerLight Corporation (now part of U.S. SunPower Corp.). Here I need to make a disclaimer. My son, Jonathan Botkin, is an engineer at this company and has been especially helpful in making sure that my calculations are correct. When I gave a talk about solar energy to some journalists a few years ago and mentioned this, they seemed to think I was therefore lobbying and advertising for that corporation. Although I do think this company is doing a good job, I’m not touting them, just turning to someone I trust implicitly to make sure my figures are correct.

Here are some of the calculations I made. Although the largest solar energy parks are in Spain, it wasn’t very long ago, in 2005, that the world’s largest solar energy facility started producing electricity in what would seem an unlikely place, the small city of Mühlhausen, in Germany’s Bavaria (Figure 7.5). Mühlhausen is famous for Johann Christian Bach’s life there as an organist, but although it’s a picturesque tourist destination, it probably wouldn’t go on your list of sunbathing resorts. Still, in 2005, one of the best answers to our energy crisis was right there, in farm fields where sheep grazed beneath an unusual crop: an array of rectangles mounted on long metal tubes that rotate slowly during the day, following the sun like mechanical sunflowers. Yes, this was, a few years ago, the world’s largest solar electric installation, generating 10 megawatts on 62 acres. Scaled up to just 3.5% of Germany’s land area, this kind of solar power could provide all the energy used in Germany—by cars, trucks, trains, manufacturing, everything!

Figure 7.5 The Bavaria Solarpark, in 2005 the world’s largest solar energy installation, lies within a picturesque landscape famous in history but not especially sunny. (Courtesy of SunPower Corporation)

Here are some U.S. examples. Based on already installed photovoltaic systems in San Francisco’s Bay Area, one acre covered by a photovoltaic system provides enough electricity for 379 houses.⁹ San Francisco covers 46 square miles and in 1990 had a population of 723,959. Figuring about three people per house, this is equivalent to 241,320 houses, and just 1,910 acres of solar collectors would be enough to provide all the domestic electricity for the residents. Thus, solar photovoltaic devices would occupy only 6.5% of the city’s land area, and if the solar collectors were on house roofs, it is quite likely that the existing roofs would provide adequate area for domestic electricity needs.

Let’s use the same basic information for Arizona, even though it actually gets a lot more sunlight than San Francisco, but just to be on the conservative side. Arizona occupies 113,642 square miles of land, or 72,730,880 acres. At the installed energy yield I have been discussing—enough to provide electricity for 379 houses per acre—the entire state could provide electric power for 27 to 28 billion houses, and at an average of 3 people per house, that’s enough for 81 billion people, or 13 times the population of the Earth. If 1% of Arizona’s land area were used for photovoltaics, enough electricity could be produced for more than 275 million houses, which is considerably more houses than exist in the United States. At an average of three people per house, this area of photovoltaics would provide electricity for 837 million people, or about 28% of the world’s population.

Other people have made similar calculations—for example, Professor Nathan Lewis of Caltech, a expert on energy supply. He writes that in 1990 the total world energy demand was 10 billion kilowatts, and that using reasonable estimates of the increase in the human population and the increase in the demand for energy, by 2050 the demand could more than double, so that 28 billion kilowatts might be required to meet all energy demands.

Professor Lewis also points out that our planet receives 120,000 billion kilowatts of energy continuously, on average, from the sun. So current world energy use by people is just two-hundredths of the total energy our planet receives from the sun.

This means that all the energy used by the world’s people in 1990 could have been provided by covering just 0.1% of Earth’s area (and just 5.5% of the United States) with photovoltaics that were just 10% efficient, and that the estimated energy demand in the year 2050 could be met by the same photovoltaics covering 0.16% of Earth’s surface, or 8.8% of the United States.¹⁰ Indeed, the total energy demand in the United States alone could be met if photovoltaics occupied 1.7% of the land. In about 20 years of collecting solar energy in this way, we would have collected as much energy as is contained in all known fossil-fuel reserves—and that’s assuming a conservative efficiency of 10% in transferring solar energy to electrical output.

In short, the United States could become a net exporter of energy, either in the form of electricity or in the form of a fuel made with that electrical energy, such as hydrogen or a small hydrocarbon derived from hydrogen and carbon dioxide. And even if things did not work out exactly as suggested here, and required twice as much land area, we are still talking about a small portion of the land area of a nation and of the world.

Another approach: solar energy off the grid

There is an ongoing debate between proponents of on-the-grid and off-the-grid alternative energy. On-the-grid refers to solar energy whose electricity is put directly onto the electrical grid and becomes part of a major energy system for a region and for an entire nation, and in this way contributes to the world energy supply. Off-the-grid refers to solar energy whose use is local, ranging from providing power to a single house, to individual housing developments, to small villages, or small industries.

One can make the case that the on-the-grid/off-the-grid debate goes back to the very origins of the development of electrical energy during Thomas Edison’s time. After inventing the lightbulb and helping with the invention and development of electric motors and generators, Edison promoted a direct-current system. But the problem was, direct current could not be transmitted efficiently over a long distance; there was just too much power loss. Edison lost out in the advancement of electricity when the first large hydroelectric plants were installed at Niagara Falls and the electricity was transmitted long distances as alternating current.

Later, the establishment of the Bonneville Power Administration and the Tennessee Valley Authority in the 1930s to build large hydroelectric dams enhanced and increased a regional, centralized electrical production and distribution system. It was viewed as government providing a service to all the people, but was also consistent with a large-scale, big-industrial approach to providing energy.

A different political philosophy with a different technological approach, could have taken the same money and promoted local production of electricity. But the technology wasn’t really ready for that. Only now, with the development of late-20th-century and early-21st-century wind and solar energy technologies, can this kind of intense off-the-grid electrical generation be seen as a competitive approach to energy production.

Off-the-grid solar energy for rural areas, for the poor, for single-family homeowners, and for less-developed nations

Like wind power, solar energy is useful for those who lack easy access to other forms of energy or simply prefer to be energy-independent. The potential for off-the-grid, locally generated solar energy to help people in developing nations led to the development of solar cookers, but the earliest versions did not become widely used. These early solar stoves concentrated sunlight enough to cook food, but they were unfamiliar devices, out of context for the cultures and rural societies they were supposed to help, and have been referred to as solutions in search of a problem. It would probably be more accurate to say they were a Western male engineer’s concept of a device to be used in non-Western civilizations by rural women used to cooking in traditional ways.

All that has changed rapidly. In recent years, the most famous proponent of solar cooking for developing nations is Margaret Owino, director of Solar Cookers International, East Africa, who has successfully promoted the use of these cookers in Southern Africa. More than 10,000 women in Kenya have learned how to use and promote them, and there are many testimonials about how they are helping. For example, Elizabeth Leshom, who lives in Kajiado about 50 miles south of Nairobi, has found that the solar cooker has cut her family’s use of charcoal in half and considerably reduced her use of firewood.

Solar cookers come in two major types, simple hot boxes (Figure 7.6, top) and parabolic mirrors that concentrate sunlight onto a point (Figure 7.6, bottom). Hot boxes simply heat up enough to cook food. Their advantages are that a number of pots can fit into one box and they are cheap. The least expensive of these are made of waxed cardboard cartons with foil surfaces and can heat up to 275°F. In Kenya, these have sold for the equivalent of $5.60 to $7.90. Worldwide, these are the most widely used individual solar cookers, with several hundred thousand in use in India alone.¹¹

Figure 7.6 (Top) A hot-box solar cooker (Solar Cookers International) and (Bottom) a parabolic solar cooker, whose mirror concentrates the heat at a point. (Maarten Olthof/Vajra Foundation).

Opportunities for entrepreneurs

According to an article in the Wall Street Journal, some companies are beginning to see a market in small and inexpensive solar energy devices for the Third World, including lighting. One company, Cosmos Ignite, sells solar-powered MightyLights for US$40, about the cost of a few months’ supply of kerosene. This company came about as a project in a class at Stanford Business School, where the students were asked to develop a cheap alternative to artificial lighting for developing countries. One of the students, Matt Scott, developed a light and founded this company. In India alone, 10,000 of these have been sold.¹²

To give you a clear picture of the potential, here are some basic facts. In full sunlight, a square meter (think a little bigger than a square yard) receives 1,000 watts of sunlight, enough energy to light ten 100-watt lightbulbs. A silicon chip 4 inches square generates 1 watt of electricity in full sunlight at ordinary outdoor temperatures. A small photovoltaic system that generates 50 watts in full sun provides enough energy “for four or five small fluorescent bulbs, a radio, and a 15-inch black-and-white television set for up to 5 hours a day,” according to Robert Foster of the Southwest Technology Development Institute at New Mexico State University.¹³

In Kenya, more than 200,000 small solar-electric systems have been sold. These include basic array of photovoltaic units, a storage battery, and whatever wiring and electronic devices were necessary to make a system work.¹⁴ These have provided home lighting and power for radios, televisions, computers, and so on at a cost of a few hundred dollars for the smallest system (less than 16 watts output) to more than $800 for larger systems (more than 45 watts).¹⁵ The cost to install these systems worked out to be $15 to $18 per watt, and of course there were no monthly fuel costs. By comparison, a gasoline electrical generator cost at least $500 to install and required $64 worth of fuel per month to run. Rural Kenyans without electrical generating systems buy kerosene for lighting and dry-cell batteries to operate radios, at a cost of $5 to $10 a month. Thus, a small photovoltaic system pays for itself in two years or so. Many small companies have started up in Kenya to sell, install, and maintain these systems.

By the end of the 20th century, only 62,000 Kenyan households—less than 1% of the total in that nation—were on an electrical grid.¹⁶ A good argument can be made that in nations with economic situations similar to Kenya’s, an electrical grid is not cost-effective and is unlikely to be developed; therefore, local, off-the-grid small systems will be the major way for their citizens to have access to computers, radio, television, and other modern technology. Small solar energy systems like those described above are helping.

For example, on the Philippine Island of Mindanao, U.S. AID (Agency for International Development) has funded the installation of solar cells (and small hydropower systems) to provide electricity where it was difficult to establish an electrical grid. I was a Peace Corps volunteer on that island in the 1960s. We lived in Marawi City, the capital of Mindanao Province, on the shores of beautiful Lake Lanao. About 30 miles down the river that flowed out from that lake, at the city of Iligan, was a hydroelectric power plant at Maria Christina Falls. The city of Iligan, on the coast and right near the falls, had a good electrical supply, but Marawi City had no grid system and no electricity except for individual gasoline and diesel generators. At the university where I was teaching, there were three engines: an electrical generator, a water pump, and a refrigerator. One day all three broke down, reminding all of us of the benefits of an electrical power system with some redundancies.

There were telephone lines that had been constructed when the Philippines was a U.S. territory, but with the renewal of fighting between people of Mindanao and the central Philippine government, the system had fallen into disrepair, and we were told that bandits had stolen the telephone wires to sell the copper. More recently, fighting between the Mindanao Muslims and the central government also made it impossible to develop an electrical grid. As a result, the Alliance for Mindanao Off-grid Renewable Energy (AMORE) began a new program that has electrified more than 500 villages with these small local systems.¹⁷

Other definitely off-the-grid solar technologies

Solar-powered vehicles. Paul MacCready, already famous for creating a human-powered aircraft that flew across the English Channel, designed and built Solar Challenger, which in 1981 flew from Paris to Canterbury, England, across the English Channel, flying a total of 163 miles and reaching an altitude of 11,000 feet. NASA developed solar-powered aircraft that have greatly exceeded that record.¹⁸

Space travel and solar energy. If you’re planning a trip to outer space, perhaps to the moon or to Mars, you have two sources of energy for the long term, nuclear and solar, and space vehicles make use of both. The space station relies on solar energy, and so do the cute little Mars rovers that captured public attention when they began to rove slowly over the Martian landscape doing the bidding of Earthbound planetary geologists.

Downsides

Why aren’t nations rushing even faster to install solar power facilities? And especially, why isn’t the United States—the world’s largest energy user—rushing to become a world leader in solar energy production? We can ask the same question about China and India, the two most populous nations and the two that have had the most rapid recent increase in energy use.

Costs

Today, primarily because of the cost of manufacturing photovoltaic devices, electricity from solar energy is more expensive than from most other sources, including fossil fuels. In the United States, according to the Department of Energy, electricity from solar energy costs 21¢ per kilowatt-hour for industrial production and 38¢ per kilowatt-hour for residential production,¹⁹ while the national average price to consumers is 13¢ for industrial users and 16¢ for residential users.²⁰ Solar thermal towers and systems with parabolic reflecting mirrors have been cheaper to operate, providing electricity at about 12¢ per kilowatt-hour.²¹

Are the costs prohibitive? In 2002 Con Edison built New York City’s largest commercial rooftop solar energy system for $900,000, providing energy for 100 houses. Assuming an average of three people per home, the installed cost is $3,000 per person. For all 300 million U.S. residents, the installation cost would be $900 billion.

The U.S. balance of trade is in the red by about $60 billion a month, or $720 billion a year, and much of this trade imbalance is due to the cost of foreign oil. So, for the equivalent of one year’s trade imbalance, the United States could pay at least 80% of the cost of installing solar energy facilities for all domestic electrical consumption. The war in Iraq—justified, many say, in part to protect our petroleum sources—has cost an official federal allocation of more than $600 billion. And the Pentagon’s acquisition budget reached $1.6 trillion in 2007.²² In March 2008, a report by Nobel Prize-winning economist Joseph Stiglitz estimated that the true direct costs of the Iraq war will be $1.5 trillion or more, and the total costs, including the costs of health care and rehabilitation of veterans, will be more than $3 trillion.²³

For the cost of the Iraq war, or perhaps just one-half or one-quarter of those costs, solar energy systems could have been installed to provide domestic electricity for all the people in America—energy forever!

As we saw earlier, the numbers become even more amazing for the dry, sunny climate of Arizona, where covering just 1% of the land with solar collectors would produce electricity for more houses than exist in the entire United States.

New solar cell technologies may lower costs. Although solar electrical devices are amazingly efficient, this technology is developing rapidly and costs could go down. Right now the best candidates for new kinds of solar collectors are thin film (a variation on the silicon cells that have been well established) and organic compounds. Crystalline silicon oxide, the material from which photovoltaic chips are presently made, is more expensive to make but more efficient than others. Amorphous silicon is used in “thin film” and is cheaper to manufacturer but less efficient. The engineering question is whether it is economically advantageous to pay more for higher efficiency of the fundamental receptor or to go with the cheaper basic unit. The latter will be the best choice only if the total cost is due largely to the cost of the primary photovoltaic cells.

This is not generally the case at present, but could change. Right now, photovoltaic cells represent about half the cost of a solar-electric installation,²⁴ suggesting that perhaps more costly, more efficient units are a better economic approach than cheaper, less efficient photovoltaic cells. But this debate is ongoing and will be resolved only by more research and development. It is beneficial at present to have both approaches taken, as is happening now because some corporations are producing the crystalline product and others are producing thin films.

Although silicon is the basis of most of today’s photovoltaic cells, other chemical elements also produce a photoelectric effect, in particular cadmium and gallium-arsenide, toxic elements whose use should be either restricted or carefully monitored and controlled for health and safety.

Manufacturing limits

One of the major downsides, perhaps the major one, is that the manufacturing capacity to produce photovoltaics and solar thermal systems is presently inadequate to meet growing U.S. and global energy needs.²⁵ But the good news is that the number of photovoltaic cells manufactured in the United States is growing about 40% per year. If this rate of increase continues, solar photovoltaics could provide as much as one-third of the total energy the United States will need in 2050, as I discuss in Chapter 13, “Solutions.” Whether this can happen without large-scale government investment that looks beyond the immediate market is unclear. Professor Nathan Lewis of Caltech writes: “Researching, developing, and commercializing carbon-free primary power technologies capable of 10–30 TW by the mid-21st century could require efforts, perhaps international, pursued with the urgency of the Manhattan Project or the Apollo Space Program.”

Energy storage

A downside that is always pointed out is how to store the energy from sunlight. (This is true of wind energy as well.) The problem is perhaps most spectacularly illustrated by attempts to make solar-powered airplanes.

After Paul MacCready’s successful design of the Solar Challenger, the solar-powered airplane that crossed the English Channel in the 1980s, NASA experimented with a remote-controlled solar-powered light aircraft called the Pathfinder. The best of these, Pathfinder Plus, flew to an altitude of 80,201 feet on August 6, 1998 (Figure 7.7). However impressive this was, the Pathfinder Plus had a limitation: It could carry only enough batteries for a few hours of flight after dark. As a kind of diurnal creature that had to land not too long after dark, it was not really a practical airplane.

Figure 7.7 NASA’s Pathfinder Plus solar-powered airplane.²⁶ (Nick Galante/NASA Dryden Historical Aircraft Photo Collection)

There’s been a lot of headshaking about the problem of storing the energy from sunlight and wind, as if this problem were unique to these two energy sources. But energy storage is also a problem for nuclear power plants, because they are most efficient when they run at the maximum electrical output all the time. In some cases, nuclear power plants have been linked to reservoirs, pumping water up into the reservoir at night when the demand for electricity was low, and generating electricity during peak demand from both nuclear reactions and waterpower. During a drought or a rainy season, hydroelectric dams and reservoirs, too, have a storage problem.

We talk about solutions to energy storage in Chapter 10. In brief, the problem can be partially overcome by (1) connecting solar generators to the grid, (2) using solar energy to heat water, and (3) using the electricity to make gaseous and liquid fuels (starting with hydrogen taken from water). Of course, (4) storing electricity in batteries is always an option, but as NASA’s Pathfinder Plus demonstrated, this has its limitations. We can also use the energy to do tasks for us whose timing is not very important, such as pumping water up into water towers for distribution later and desalinating water (processes that can be done whenever the energy is available).

Other means of storage have been proposed, and some tested. One of them is to store the energy mechanically in a flywheel and use that energy as needed by having the spinning wheel connected to an electric generator or directly to the wheels of a land vehicle.

Storage is not a simple problem with a single simple solution, but it is solvable, as I explain later.

Environmental effects: landscape beauty and competition for space

It would be naive to think that any source of energy had absolutely no undesirable effects, especially environmental effects. As Barry Commoner told us a long time ago, there is no such thing as a free lunch in nature. So probably some environmental problems will arise even from the use of solar energy. One that comes to mind is landscape beauty. Although solar collectors usually lie horizontally and thus have much less effect on scenery than do wind turbines, it will not be surprising if in some locations many acres covered by the black surfaces of photovoltaics are considered a blight on the landscape. As with wind power, solar facilities should be situated with the help of professional landscape architects and planners and experts in ecology to minimize potentially negative effects.

Solar parks will in some cases be seen as competing with other uses for land, but one advantage of some solar park designs is that the land can be open to multiple uses. For example, many solar installations are on rooftops. The most likely environmental negative of solar energy is with the mining, manufacturing, and recycling of materials, especially once solar becomes one of the world’s major energy sources. Right now, recycling of batteries is not done efficiently. And while silicon forms some of the most common earth materials and is thus readily available, its mining creates fine dust that can be a local health and environmental problem and cause regional, even global, pollution if emitted high into the atmosphere. Surface mining for the large-scale manufacture of photovoltaic cells will damage landscapes and ecosystems in ways similar to surface mining, except that there will be less likelihood of acid drainage.

The bottom line

• The sun offers the greatest amount of energy, and could by itself, using a small percentage of Earth’s surface area, provide the equivalent of all the energy used in the world by people.

• Solar energy has great potential and is benefiting from rapid increases in research and development, which will lower its costs.

• European nations are taking the lead in the installation of large solar facilities, with Germany and Spain outstanding users of this energy source.

• Solar energy is providing electricity and heat for cooking in many developing nations where a large-scale electrical grid does not exist and may never be practical.

• For many of the world’s people, solar energy offers the only way to participate in modern, high-technology activities.

• Solar energy is bound to be a major player in the supply of energy in the future.