SOLAR ENERGY, HISTORICAL EVOLUTION OF THE USE OF

Every ninety-five minutes an object the size of a bus circles Earth. It represents an unparalleled achievement in space observation technology and is called the Hubble Space Telescope (HST). Since its deployment in 1990, the HST has brought images to the astronomy community that traditionally had been reserved for expensive long-range space missions by unmanned craft. One of the essential technological developments that make the HST such an attractive investment is its use of one "natural resource" found in space, the sun. The HST utilizes photovoltaic technology to capture the sun's rays and convert them directly into electricity. This procedure is commonly referred to active solar generation.

Active solar generation is nothing new to the space industry. Photovoltaics have been used to power satellites since the late 1950s and are an essential element to the new international space station. Closer to home, active solar technologies are used on houses and buildings to generate heat, hot water, and electricity. Spacecraft use of the sun is indeed a technological marvel. However, strategic use of the sun for heating and cooling is not a new discovery. In fact, 2,400 years ago the Greek philosopher Socrates observed:

Now in houses with a south aspect, the sun's rays penetrate into the porticos in winter, but in the summer, the path of the sun is right over our heads and above the roof, so that there is shade. If then this is the best arrangement, we should build the south side loftier to get the winter sun and the north side lower to keep out the winter winds. (Xenophon, 1994, chap. 2, sec. 8)

Socrates has described the strategic placement of a home to best utilize the rotation of Earth, to receive maximum sunlight during the winter and minimal sunlight in summer. The strategic utilization of the seasonal course of the sun in home design is called passive solar architecture. Passive solar use is best described as the greenhouse effect. This simple phenomenon is illustrated by the experience of returning to your car on a sunny, cool day and finding it heated. Hubble and Socrates represent opposite ends of the solar historical spectrum. However, solar technologies have historically been met with mixed results. While the technology has consistently proved itself to be useful and environmentally friendly, it has often been pushed into the shadows by cheaper, less environmentally friendly energy technologies.

Historically the most common problems associated with solar technologies are efficiency and economics compared to competing energy technologies. During the Industrial Revolution coal was relatively cheap and, for the most part, readily available in most of the world. These facts, which still hold true today, make coal a powerful competitor against alternative power technologies. However, solar technologies have been able to survive in niche markets long enough to increase efficiency and lower costs. Although sunlight is free and also readily available, solar technologies have experienced little success in large-scale applications. Solar technologies also have been able to take advantage of advances in silicon technology. An initial niche market for solar technology was the colonies of Europe.

EARLY APPLICATIONS: SOLAR THERMAL

One of the earliest forays in the use of solar technologies for power generation was in 1861. French engineer Auguste Mouchout utilized solar thermal technology by focusing the sun's rays to create steam, which in turn was used to power engines. Mouchout's design incorporated a water-filled cauldron surrounded by a polished metal dish that focused the sun's rays on the cauldron, creating steam. Mouchout was granted the first solar patent for an elementary one-half horsepower steam engine powered by a solar reflective dish. Several of Mouchout's solar motors were deployed in Algeria, then a French protectorate, where coal was expensive due to transportation costs. One-half horsepower may not seem like much, but it was a considerable advance in alternative power generating. At the time, the major source of energy was man and draft animals, the steam engine was in its infancy, and water power was not very portable. Mouchout was able to expand his design to a larger scale, but the project was abandoned due to new innovations in coal transportation that made coal a more efficient investment.

Solar thermal technology survived Mouchout and was further developed by Swiss-born engineer John Ericsson. In 1870 Ericsson created a variation of Mouchout's design that used a trough instead of a dish to reflect the Sun's rays. Ericsson's basic design would later become the standard for modern-day parabolic troughs. He allegedly refined his design to the point where it would be able to compete actively with conventional generation techniques of the time but he died without leaving any plans or blueprints.

Englishman William Adams designed another interesting variation of Mouchout's work in 1878. Adams's design used mirrors that could be moved to track sunlight and focus the rays on a stationary cauldron. In theory, a larger number of mirrors would generate increased heat levels and greater steam output for larger engines. Adams was able to power a twoand-a-half-horsepower engine during daylight hours and viewed his invention as an alternative fuel source in tropical countries. Although Adams was more interested in proving that it could be done than in developing the technology, his basic design would become the prototype for contemporary large-scale centralized solar plants called "power towers" that are currently in operation in the United States and other countries.

CONTEMPORARY USES: SOLAR THERMAL

Solar thermal technology varied little in design from 1870 to 1970. Advances were made in the materials, such as lower-boiling-point fluids and higher-efficiency motors, but the basic design of using mirrors to focus sunlight remained the same. The aforementioned introductory ventures in the use of solar technology provided the intellectual foundation for growth and extensive technological development of solar use in the twentieth century. These pioneers helped to establish solar as a viable alternative to conventional power generation. However, many of the same barriers resurfaced, preventing widespread implementation. Many of the previously mentioned designs survived by gaining support within environmental communities as environmentally friendly alternatives to fossil fuels, as well as a safer alternative to nuclear power. In addition, solar technologies were able to establish themselves within new niche markets and to gain further interest during the oil crisis of the 1970s.

As previously mentioned, both Ericsson's and Adams's original designs were upgraded and implemented as modern solar power generating facilities. Contemporary versions of the solar trough and power tower were constructed during the 1980s in the Mojave Desert of California by LUZ International Ltd. Currently the corporate successor of LUZ, which went bankrupt in 1991, partly due to dependence on a federal subsidy (tax credits) not renewed in 1991, has nine solar thermal plants in California and has generated more energy from solar thermal than any other company in the world.

EARLY APPLICATIONS: PHOTOVOLTAICS

Use of solar panels or photovoltaics (PVs) is another popular way to generate solar electricity. The space program is perhaps the most recognized user of PVs and is responsible for most of the advancements in PVs. Many people are familiar with PVs through small applications such as calculators and perhaps solar water heaters, but early forays in PV experimentation were little more than noted side observations in non-PV experiments.

In 1839 Alexandre-Edmund Becquerel, a French experimental physicist, did the earliest recorded experiments with the photovoltaic effect. Becquerel discovered the photovoltaic effect while experimenting with an electrolytic cell made up of two metal electrodes placed in an electricity-conducting solution. When exposed to sunlight, the generation increased. Between 1873 and 1883 several scientists observed the photoconductivity of selenium. American inventor Charles Fritts described the first experimental selenium solar cells in 1883. Notable scientists such Albert Einstein and Wilhelm Hallwachs conducted many experiments with various elements to explore it further. In 1921 Einstein won the Nobel Prize in Physics for his theories explaining the photoelectric effect.

Many elements were found to experience the photoelectric effect. Germanium, copper, selenium, and cuprous oxide comprised many of the early experimental cells. In 1953 Bell Laboratories scientists Calvin Fuller and Gerald Pearson were conducting experiments that brought the silicon transistor from theory to working device. When they exposed their transistors to lamplight, they produced a significant amount of electricity. Their colleague Darryl Chaplin was looking for better ways to provide power for electrical equipment in remote locations. After a year of refinements Bell Laboratories presented the first working silicon solar cell, which would later become the core of contemporary PV applications.

CONTEMPORARY USE: PHOTOVOLTAICS

The utility industry has experimented with large-scale central station PV plants but concluded that in most cases they are not cost-effective due to their perceived low efficiency and high capital costs. However, PV has proven useful in niche markets within the utility system as remote power stations, peak generation applications, and in a distributed generation scheme. Distributed generation is the inverse of central generation. Instead of constructing large generating facilities energy has to be sent to consumers, energy is generated where it is used. Highway signs are a good example of distributed generation. They use solar panels and batteries on the signs to collect and store the power used to illuminate the signs at night. Because very energy-efficient LED lighting replaced incandescent bulbs, these signs could be powered much more economically by PVs than by traditional gasoline-powered generators. Perhaps the most prolific use of PVs is in the space program. The sun, the only natural energy source available in space, is the logical choice for powering space vehicles. Photovolttaics are safer and longer-lasting energy sources than nuclear power is.

An instance where large-scale grid-connected PV generation has occurred is the Sacramento Municipal Utility District (SMUD). SMUD built several dozen "solar buildings" and placed PV systems directly on their clients' homes. SMUD purchased one megawatt of distributed PV systems each year until the year 2000.

SMUD's approach introduced another distributed application concept for PV, called building integrated photovoltaics or BIPV. Currently Japan has one of the largest BIPV programs, started in 1996. The simplest application of BIPV is the installation of solar panels directly on a building. While this method is perhaps the easiest, it does not reflect the best use of PV technology. The best potential for the growth of PV is for PV to double as building materials. Examples include exterior insulation cover, roofing shingles, windows, and skylights that incorporate solar cells. Incorporating PV into construction materials is very complex and requires joint cooperation among traditionally separate entities of the building sector. Although no actual products exist, many of the major manufacturers began experimenting with prototypes in the late 1990s and hope to introduce cost-effective products on the market by 2005. Additional uses for PV include desalinization of seawater for fresh water, solar water heaters, and solar ovens.

IMPLEMENTATION AND BARRIERS

Solar energy is used worldwide in many applications, from niche markets in developed countries to primary village power in rural and developing communities. Its attractiveness can be attributed to several factors:

• Solar power works well in small facilities, and it may be comparatively cost-effective when all factors (transmission, fuel transportation costs, service life, etc.) are taken into consideration. In many instances solar energy is the only viable energy option in the region.
• Solar distributed energy systems provide security and flexibility during peak use and outages associated with environmental disasters. In addition, programs that allow consumers to sell surplus energy they generate back to the utility, called "net metering," will make residential solar technologies more attractive.
• Solar power is environmentally friendly and will help mitigate global environmental concerns associated with climate change.

These claims have been the primary call to arms for many solar advocates and industries since the 1970s, but solar technologies have not enjoyed widespread use except within niche markets, especially among developed countries. According to a U.S. Department of Energy report by the Energy Information Administration, solar technologies have only experienced modest sector growth domestically because domestic solar thermal technologies face stiff competition from fossil-fueled energy generation. PV growth faces similar challenges, since deregulation is likely to lower the cost of electric power from natural gas and coal.

Because of the wide range of ideological leanings of various administrations, direct subsidies and federally funded research and development have fluctuated wildly since the late 1970s. In 1980 the United States spent $260 million on solar research and development, but then the amount fell to $38 million by 1990. Between 1990 and 1994 support doubled, reaching $78 million.

Because of the problems associated with nuclear power and burning fossil fuels, solar technologies may again be viewed as a viable alternative. However, solar technology faces several challenges

First. the energy expended in the production of PV panels is considerable. Second, solar cells become less efficient over time (degradation is slow, but eventually they lose most of their conductivity). Third, contemporary solar cells use mercury in their construction and present toxic waste disposal problems. Finally, solar technologies are part-time power sources. Solar thermal technology requires full, direct sunlight, and PVs do not work at night (although batteries and supplementary generating applications such as wind power can be combined to provide full-time generation).

The need for long-term clean power will be a controversial but necessary issue in the twenty-first century. The debate will focus on the deregulation of the power industry and the needs of consumers, coupled with what is best for the environment. For certain applications, solar technology has proven to be efficient and reliable. The question remains whether solar will be wholly embraced as energy options for a sustainable future in a climate of cheap fossil fuels and less than enthusiastic public support.

J. Bernard Moore

See also: Batteries; Becquerel, Alexandre-Edmund; Einstein, Albert; Electric Power, Generation of; Fuel Cells; Heat and Heating; Heat Transfer; Power; Renewable Energy; Spacecraft Energy Systems; Solar Energy; Thermal Energy; Water Heating.

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