Renewable energy sources and applications

Renewable Energy Sources Today: A Review

The emergence of modern-day developed economies depended heavily on the availability of cheap and abundant energy, but the planet's oil reserves, which supplies over 35% of the world's energy needs, are projected to be depleted within a hundred years (Balat 19). As this resource is depleted, demand will increase and drive prices to unheard of levels, thereby threatening the stability of economies in some of the most powerful nations the world. To avoid a catastrophic meltdown of what is now a global economic engine; governments worldwide have begun to invest in developing and promoting the use of renewable forms of energy. Although reducing fossil fuel use to lower green house gas emissions is a more laudable goal, it seems likely that economic incentives will be the primary driving force behind renewable energy development and implementation. This article reviews the various types of renewable energy in use today that will likely help reduce our dependence on fossil fuels.

Solar Energy

Sources of solar energy can be divided into direct or indirect. Direct solar energy converts photons produced by the sun directly into either electricity or heat. The primary indirect forms of solar energy (energy carriers) are created by the power of the sun and include biomass, wind, ocean currents, and hydropower.

Direct Solar Energy

Passive Solar Heating

Passive solar heating captures the energy contained in direct sunlight to heat water, cook food, dry clothing or agricultural products, heat the interiors of buildings, provide air-conditioning, power desalination and water purification projects, and generate electricity. Given that building structures consume 35.3% of energy worldwide (Chan, Riffat, and Zhu 781), the development and use of passive solar technology will become critical to relieving our dependence on fossil fuels.

Photovoltaic Cells

Photovoltaic cells are able to directly convert solar photons into electricity, because the photons are have enough energy to free electrons on one side of a light-absorbing material, thereby creating a charge differential. A single cell doesn't produce a significant amount of power, but when linked together in arrays they produce enough to energy to support the needs of whole households and neighborhoods. The primary limitation is the cost, which is typically more than five times higher than hydropower, geothermal, or wind (Balat 20). Concerns about cost, reliability, and performance explains why photovoltaic cells produced only 1.8 GWh (Balat 25) of the 3,157,000 GWh of all electricity produced by renewable energy sources in 2004 (Balat 19). In spite of the high cost, the annual growth rate in photovoltaic use between 2000 and 2004 was an unprecedented 60% (Balat 25).

Solar Energy Carriers

Hydropower

Solar energy drives weather patterns, which in turn carries precipitation to higher elevations. Solar energy is thereby stored in the form of mountain snow packs and watersheds, which can be harvested by hydroelectric power plants positioned on a river or at the base of a reservoir. Because of the low cost and reliability of hydropower, close to 89% of all electricity produced by renewable energy sources comes from hydroelectric power plants (Balat 21). The annual growth of hydroelectric power is low compared to wind and photovoltaic cells, averaging between 1-2% annually. Market saturation in developed countries, environmental concerns, and population displacement contribute to the slow growth of hydroelectric plants worldwide.

Wind Power

Wind turbines are second only to hydropower in terms of cost and reliability and represent one of the fastest growing renewable energy sectors. In 2004 alone, over 6000 wind turbines, with an average capacity of 1.25 MW, were put into use worldwide (Balat 23). On a global scale though, wind turbines contribute only 0.5% of all electricity consumed (Balat 21). Given the annual growth rate in wind turbine capacity, which was close to 29% between 1994 and 2004, the contribution from wind energy should steadily increase. Currently, the vast majority of wind turbine capacity is located in only a few countries, including Germany, Spain, Japan, Italy, and the United States.

Biomass

Almost without exception, all living matter derives most of its energy from the sun and stores it through carbon fixation. Traditional methods of converting solar energy into a useable form include the burning of firewood, straw, and animal dung for cooking and warmth, which still represents a major energy sourced in undeveloped countries. In developed countries the sources of biomass include agricultural and forestry residues, and municipal waste. Although still an important source of energy in developed economies, biomass provides a much smaller share of the energy consumed when compared to third world economies. The global energy contribution of biomass is around 9.5%, which represents 67.9% of the energy supplied by renewable energy sources (Balat 18). Biomass is converted to useable energy through combustion to generate heat, anaerobic digestion to produce methane, oil extraction for biofuels, or gasification to produce high grade fuels (Panwar, Kaushik, and Kothari). Unfortunately, the use of biomass generates greenhouse gases at almost the same rate as fossil fuels and therefore represents a threat to the environment.

Geothermal

The fourth biggest producer of renewable energy in the world is geothermal energy, which provides about 0.5% of the world's energy needs (Philips 2415). Geothermal energy exploits the heat stored within the earth, which was formed by the heat already present in the matter prior to the formation of earth, created as gravitational energy was spent as the matter combined to form the earth, and by ongoing radioisotope decay. From a human perspective, geothermal energy is essentially limitless.

Energy is produced by either tapping underground hot water reservoirs or pumping surface water deep into a borehole to heat it. If the temperature is hot enough to produce steam then electricity can be generated, otherwise it can be used as a heat source. There are several important limitations to using geothermal energy. For example, the steam derived from a geothermal reservoir can sometimes contain enough greenhouse gases to rival that produced by fossil fuels. The steam may also contain toxins, like mercury, boron, and radon. Other potential environmental hazards include water pollution, damage caused during the construction and operation of a geothermal power plant, soil erosion, and noise pollution. Still, with an annual rate of growth around 3.7% there is continued interest in exploiting this renewable energy source (Balat 21).