Global Climate Change and Energy
Alternative Energy Sources: Solar Power

When we think of renewable energy sources, the Sun comes to mind first. Sunlight is warm, bright, and readily available. The problem: how to take the energy in sunlight and convert it to forms people can use, such as hot water, steam, or electricity.

We have known since approximately the 7th century b.c. that concentrating the Sun’s rays through a magnifying glass will cause a fire. The Greeks, Romans, and Chinese all used mirrors to concentrate and reflect sunlight to light torches for religious ceremonies. But creating power on a larger scale is much more complex.

Scientists and inventors have been working on it for over 200 years. But in the 1970s fossil-fuel prices began to climb. In conjunction there was greatly increased concern about the impact on climate of the pollution caused by burning fossil fuels. For a long time solar power was very inefficient. It was also expensive to install. Only since the 1980s has solar power come into more common use. Slowly, quietly, solar power has crept into all sorts of everyday uses. Solar units power everything from the main power grid in some regions to streetlights, pools, and calculators.

Solar power isn’t just one type of technology. In fact there are three methods in common use, each best suited for its own set of uses. Some of these technologies are based on concepts developed years ago, while others are newer. Let’s take a look at how these technologies developed and what is available now.

History

Scientists have been intrigued by the power potential of solar energy for ages. The first attempts to use technology to harness the sun’s energy came in the 1800s. In the early 1860s, Auguste Mouchout of France placed an iron cauldron of water under glass. Light from the Sun heated the cauldron and boiled the water. Mouchout found that adding a polished metal reflector concentrated the sunlight, and brought the water to the boiling point more quickly. This increased the amount of steam produced. With a few more minor changes to his system, he was able to make enough steam to power a small engine. This was the first step toward the modern concentrating solar collector.

During the late 1870s, William Adams, a British official in India, further developed Mouchout’s ideas. He replaced the polished metal reflector with a group of mirrors arranged in a semicircle around the boiler. This array of mirrors concentrated and focused the light, making the water boil more quickly. Adams’s design is still in use today in the form of the power tower. Adams also wrote the first book about solar power, called Solar Energy: A Substitute for Fuel in Tropical Countries.

American inventor Charles Fritts built the first solar cell, using selenium, in the early 1880s. This device converted less than 1% of light into electricity, not efficient or very useful.

In the late 1880s, France’s Charles Tellier built a solar collector very similar to the modern flat-plate collector. He set up ten plates, each consisting of two iron sheets riveted together. Tubes filled with ammonia connected the plates. Tellier chose ammonia because it boils faster than water. Heat from sunlight striking the plates converted the ammonia to ammonia steam. The steam powered an engine for a water pump. Later Tellier enclosed the top of the device in glass and insulated the bottom, to increase efficiency. However, he did not pursue this project any further, moving on instead to develop refrigeration technology.

Clarence Kemp of Baltimore, Maryland, patented the first commercial solar water heater in 1891. His water heater started with a device called the hot box—an insulated box painted black inside with a glass top. Knowing that metal containers heat what is inside, Kemp put a metal water tank inside the hot box. The combination of the metal, black paint, and enclosed box helped water in the tank retain the day’s heat for a longer time than would water in a bare tank.

After about ten years of experimenting with solar motors, Aubrey Eneas of Boston formed the Solar Motor Co. in 1900. In 1904 he demonstrated his device. The huge reflector, which spanned 10 m (33 ft) in diameter, contained 1,788 individual mirrors. The boiler held 378.5 l (100 gal) of water and produced 2.5 horsepower of steam. Eneas sold two of these machines. Unfortunately, his solar motors could not withstand storms: the first was destroyed in a windstorm, the second, in a hailstorm.

American entrepreneur Frank Shuman built the first solar power plant in 1912, to power an irrigation pump in the Egyptian desert outside Cairo. His Sun Power Co. erected rows of parabolic troughs that focused the Sun’s energy on glass-enclosed water-filled tubes. In a system amazingly similar to modern solar power plants, the water in the tubes turned to steam, which powered a water pump. Shuman’s plant succeeded during tests, but before it could begin operating for real, World War I started. The plant was destroyed during the battles in North Africa.

After World War I, interest in solar power declined. Fossil fuels were readily available and inexpensive. Commercial development of solar technologies halted. However, research on solar power continued. The systems now in use or under development build on the work of these solar-power pioneers.

Concentrating Solar Collectors

Concentrating solar collectors use mirrors to collect and concentrate sunlight to make large amounts of electricity. The collected sunlight heats water or some other fluid to make steam. That steam powers a generator to make electricity. The amount of energy produced by solar collector systems varies. Smaller systems can power an entire rural village. Larger systems can provide the electricity for the basic power grid of a region.

There are three types of solar collectors. Larger solar power plants use long parabolic troughs to collect and focus the sunlight and convert it to energy. The troughs align on a north-south axis to follow the Sun’s movement. At the center of the trough is a tube filled with a heat transfer fluid, most often oil. The reflective trough concentrates the Sun’s heat on the tube. The heated fluid then heats water to steam. This provides power to a steam generator. The troughs are arranged in parallel rows to form a collector field. Some plants even have storage capabilities to save thermal energy to be used during the night. Large trough systems generate up to 80 megawatts of power. This is enough to contribute to the basic power grid for a region!

The compact dish-engine system uses a group of mirrors arranged in the shape of a dish. This dish moves with the Sun, collecting and concentrating the Sun’s energy. The mirrors focus that energy onto a receiver. Located at the focal point of the dish, the receiver includes a heat transfer medium—either tubes filled with hydrogen or helium gas or a fluid that boils to gas and then condenses—to transfer the heat to a small engine, often the Stirling engine. Also mounted at the focal point, the engine then creates mechani cal power to drive a generator and produce electricity. The dish-engine system operates at about 30% efficiency, the most efficient of the solar collectors.

The tall solar power tower sits amid an arrangement of Sun-tracking mirrors that focus sunlight on a receiver at the top of the tower. The mirrors, called heliostats, form a loosely circular arrangement around the tower. A heat transfer fluid inside the receiver generates steam to power a generator. In earlier power towers, the heat transfer fluid was steam. Molten nitrate salt has replaced steam, because it is better at transferring the heat and at storing it for use at a later time. Power tower plants can produce between 50 and 200 megawatts of power. Use of the power tower is being explored in South Africa, as well as in other parts of the world.

Photovoltaic Solar Systems

Photovoltaic (PV) solar energy systems use semiconductors—the same materials as in computer chips—to create electricity from sunlight. At the heart of a PV system is the PV cell. A cell consists of two semiconductor wafers containing the chemicals needed to create an electrical field. When sunlight hits the surface of the PV cell, the electrical field moves electrons in a specific direction. This creates an electrical current. Each cell produces only one or two watts. But cells can be put together in modules for more power; for even bigger uses, modules can be connected into arrays. An array can include one or more modules, depending on the amount of power needed. PV systems operate at a 10% efficiency, with research ongoing to raise efficiency to 20%.

The PV cell was developed in 1953. After five years of research and development, radiation-proof silicon PV cells were used on satellites. The American satellite Vanguard I, launched on March 17, 1958, operated on PV power. PV cells now provide most of the electricity in space. On the Earth, PV cells power solar calculators, streetlights, and road signs most commonly. But PV systems can be of any size, depending on the amount of electricity needed. Products under development include PV roofing, used to supplement traditional power. Many PV systems work with the local utility or a battery system, to guarantee backup power during the night or on cloudy days.

Solar Thermal Systems

Solar thermal systems heat water for swimming pools, homes, or offices. There are two kinds of collectors: flat-plate collectors and evacuated-tube collectors.

Flat-plate collectors work best for smaller residential uses, such as water heating or space heating. The device consists of an insulated metal box that contains a dark-colored copper or aluminum absorber plate. The dark color on the absorber plate comes from special coatings that absorb and retain heat better than bare metal or regular black paint. Glass or plastic glazing covers the box. Sunlight hits the flat-plate collector and heats the absorber plate.

The medium of choice that is used between the glazing and the absorber plate depends on whether the collector is a liquid flat-plate or air flat-plate collector. The liquid flat-plate collector, used to heat water, contains a rack of tubes that sit over the absorber plate. The tubes contain water heated by the absorber plate. The heated water can then be used in a house or pool. Flat-plate collectors used for outdoor pools are usually unglazed, to keep costs down. This is because the pool water needs to be only slightly warmer than the surrounding air temperature; indoor pools and spas use more expensive, glazed collectors. Air flat-plate collectors contain layers of metal sheets or mesh over the absorber plates, to heat up the air in the collector. These are used for space heating, and are generally less efficient than liquid flat-plate collectors. Flat-plate collectors heat liquid or air to temperatures less than 180°F (82.2°C).  These collectors are set up in panels, so the size of the system depends on the amount of hot water needed, whether for a pool, a house, or an office building.

Evacuated-tube collectors heat to temperatures in the range of 77° to 177°C (170° to 350°F). This means they make more power than the standard flat-plate collector. Evacuated-tube systems consist of parallel rows of glass tubes. Each glass tube contains another tube inside it. The inside tube is the absorber tube, made of metal with a heat-absorbent coating. The sunlight converts to thermal energy, which is transferred either directly to water that is stored or to a fluid that is used to heat water. Evacuated-tube collectors find more use for powering cooling applications as well as industrial or commercial applications.

A unique feature of the evacuated-tube system is that the space between the two tubes is a vacuum. The vacuum provides insulation, retaining the collected solar heat inside the tubes for a longer time. That means that the tubes lose only a minimal amount of heat to the environment. This kind of system can be used in colder climates or regions with cloudy weather.

Like the PV systems, oil or natural-gas heat provides a backup power source for solar thermal systems, to maintain the needed level of hot water.

Issues

The biggest hurdle for solar energy has for years been the price of installation; this remains true. Solar equipment costs significantly more than traditional energy equipment. It takes many years of use to see that investment pay off. For example, the evacuated-tube system is about twice the price of a flat-plate collector. In addition, the systems have about a 20-year life span. In the United States, each state may offer rebate plans to people who switch to some sort of solar heating product.

Down the Road

Still under development is hybrid solar lighting. This technology uses collectors mounted to the roof to transmit the energy directly to fiber-optic cables, which are connected to special light fixtures mounted in the room. The fixtures then give off light. The system is connected to an electrical system for cloudy days. This saves electricity, especially during times of high power use.