Green Energy

Renewable Energy: Solar

The sun gives us energy in two forms: light and heat. Technologies, such as photovoltaics (PV) and solar thermal systems, harness energy from the sun providing power in two forms: light and heat. Solar energy is a plentiful and renewable resource that releases no greenhouse emissions. Enough sunlight reaches the earth's surface each year to produce approximately 1,000 times the amount of energy produced by burning all fossil fuels mined and extracted during the same time period. With solar energy we can both heat our water and homes and provide electricity to power our lights, stoves, refrigerators and other appliances. For many years, people have been using the sun's energy to make their homes brighter and warmer.

Although a few attempts were made in the 1950s to use silicon cells in commercial applications such as rural telecommunications, it was the new space program that gave the technology its first major application. In 1958, the U.S. Vanguard space satellite carried a small array of PV cells to power its radio. The cells worked so well that PV technology has been part of the space program ever since. Today, solar cells power virtually all satellites, including those used for communications, defense, and scientific research.

Despite this, PV cells were still too expensive for most "down-to-Earth" applications.  Rising energy prices in the mid-70s, sparked by a world oil crisis, renewed interest in PV technology. In the 1970s and '80s several projects were initiated all over the world and worldwide PV production exceeded 21.3 MW, and sales exceeded $250 million. Since then, several national governments, and bilateral and multilateral agencies have invested billions of dollars in research, development, and production.

The computer industry, especially transistor semiconductor technology, also contributed to the development of PV cells. Transistors and PV cells are made from similar materials and operate on the basis of similar physical mechanisms. As a result, advances in transistor research provided a steady flow of new information about PV cell technology. Today, however, this technology transfer process often works in reverse, as advances in PV research and development are sometimes adopted by the semiconductor industry.

Today's commercial PV systems can convert from 7 to 17% of sunlight into electricity. They are highly reliable and last 20 years or longer. The cost of PV-generated electricity has dropped 15- to 20-fold, and PV modules now cost around $6 per watt and produce electricity for as little as 25 to 50 cents per kilowatt-hour.

Photovoltaic systems use semiconductor technology to convert sunlight directly into electricity. These systems have numerous advantages as an energy source with minimal impact on the environment. Sunlight does not have to be explored, extracted, transported, combusted, transmitted or imported. Solar energy also produces negligible air and water pollution emissions. Not only environmentally sound, these systems run silently because there are no moving parts and are relatively simple in construction. They can be constructed in an infinite number of sizes ranging from a single solar cell in a calculator to an industrial scale sized module system. Consequently, PV systems are highly reliable and amount to low operating costs as they can run for long periods of time with no maintenance. 

For many regions in the world, construction costs for PV systems are lower when compared to expanding utility grid lines to remote villages and locations. Developing countries, in particular, find that solar energy is much more cost effective because there is less wiring and no need for step down transformers from the utility grid line.

Solar energy does have its disadvantages. Without energy storage, photovoltaic systems cannot provide continuous power. However, solar energy is abundant when energy demands are at its highest during the day. PV systems also carry high initial capital costs. Unless the location is off the grid or remote, conventional electricity from utilities is cheaper in cost.

In contrast to photovoltaic systems, solar thermal systems produce heat which can be used directly as heat energy or converted into electricity. Typically, sunlight is reflected by mirrors and concentrated onto a receiver. The high temperature energy produces heat which boils water to make steam. The steam's pressure flows through a turbine, turning the shaft that is connected to a generator in which electricity is produced. There are three solar electric thermal technologies being developed: parabolic troughs, central receivers, and parabolic dishes. All of these technologies use tracking mirrors to reflect and concentrate sunlight and can operate independently or as part of a hybrid system.

Parabolic troughs are constructed by long rows of concentrators that are curved in only one dimension forming troughs. This type of solar thermal technology requires a supplemental fuel source and considerable amounts of coolant water. Parabolic troughs have reached an advanced stage of commercialization compared to other technologies.

Central receivers, commonly called power towers, consist of a fixed receiver mounted on a tower surrounded by a large array of mirrors called heliostats. Power towers provide a centralized power supply with the ability to store energy. This results in a highly dispatchable solar power source. Other advantages include lower capital and operating costs because less piping and plumbing are needed when compared to the parabolic trough. Currently, the Solar Two power tower in Dagget, California is the world's largest central receiver system at 10 MW.

Parabolic dishes consist of parabolic-shaped point focus concentrators that reflect solar energy onto a receiver mounted at the focal point. Along with central receivers, parabolic dishes typically achieve higher conversion efficiencies than parabolic troughs. Advantages to this type of solar thermal technology include modularity, short installation times, siting flexibility, minimal water requirements, and high conversion efficiencies. Despite these advantages, however, parabolic dish commercialization is hindered by a lack of commercial experience, concern for excessive operation and maintenance costs, and a lack of storage capacity.

Solar thermal technologies have a promising future, and once commercialized, could become an economically viable energy source. Like electricity from PV systems, solar thermal energy has its advantages and disadvantages. Sunlight is a free and renewable source of fuel. Solar thermal technologies minimally impact the environment and expel few greenhouse gas emissions. The sun's intermittent nature and the fact that solar thermal technologies are not fully commercialized tend to be a disadvantage. Constructing systems as hybrids fueled by other energy sources tend to resolve part of this problem and a lot is being done to increase the commercialization of solar thermal technologies.