02-07-2013, 03:53 PM
THE ULTIMATE RENEWABLE ENERGY SOURCE
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INTRODUCTION
Renewable energy sources are often considered alternative sources because,in general, most industrialized countries do not rely on them as their main energysource. Instead, they tend to rely on non-renewable sources such as fossil fuelsor nuclear power. Because the energy crisis in the United States during thepower, usage of renewable energy sources such as solar energy, hydroelectric,wind, biomass, and geothermal has grown.Renewable energy comes from the sun (considered an "unlimited" supply) orother sources that can theoretically be renewed at least as quickly as they areconsumed. If used at a sustainable rate, these sources will be available forconsumption for thousands of years or longer. Unfortunately, some potentiallyrenewable energy sources, such as biomass and geothermal, are actually beingdepleted in some areas because the usage rate exceeds the renewal rate.
SOLAR ENERGY
Solar energy is the ultimate energy source driving the earth. Though only onebillionth of the energy that leaves the sun actually reaches the earth's surface,this is more than enough to meet the world's energy requirements. In fact, allother sources of energy, renewable and non-renewable, are actually storedforms of solar energy. The process of directly converting solar energy to heat orelectricity is considered a renewable energy source. Solar energy represents anessentially unlimited supply of energy as the sun will long outlast humancivilization on earth. The difficulties lie in harnessing the energy. Solar energyhas been used for centuries to heat homes and water, and modern technology(photovoltaic cells)has provided a way to produce electricity from sunlight.There are two basic forms of radiant solar energy use: passive and active.Passive solar energy systems are static, and do not require the input of energyin the form of moving parts or pumping fluids to utilize the sun's energy. Buildingscan be designed to capture and collect the sun's energy directly. Materials areselected for their special characteristics: glass allows the sun to enter thebuilding to provide light and heat; water and stone materials have high heatcapacities. They can absorb large amounts of solar energy during the day, whichcan then be used during the night. A southern exposure greenhouse with glasswindows and a concrete floor is an example of a passive solar heating system.Active solar energy systems require the input of some energy to drivemechanical devices (e.g., solar panels), which collect the energy and pump fluidsused to store and distribute the energy.
Energy from the Sun
About half the incoming solar energy reaches the Earth's surface.The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere.[3] Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet.[4]
Earth's land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the Earth's surface, completing the water cycle. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind, cyclones and anti-cyclones.[5] Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C.[6] By photosynthesis green plants convert solar energy into chemical energy, which produces food, wood and the biomass from which fossil fuels are derived.
Applications of solar technology
Average insolation showing land area (small black dots) required to replace the world primary energy supply with solar electricity. 18 TW is 568 Exajoule (EJ) per year. Insolation for most people is from 150 to 300 W/m2 or 3.5 to 7.0 kWh/m2/day.
Solar energy refers primarily to the use of solar radiation for practical ends. However, all renewable energies, other than geothermal and tidal, derive their energy from the sun.
Solar technologies are broadly characterized as either passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques use photovoltaic panels, pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. Active solar technologies increase the supply of energy and are considered supply side technologies, while passive solar technologies reduce the need for alternate resources and are generally considered demand side technologies.[18]
Architecture and urban planning
Darmstadt University of Technology in Germany won the 2007 Solar Decathlon in Washington, D.C. with this passive house designed specifically for the humid and hot subtropical climate.[19]
Sunlight has influenced building design since the beginning of architectural history.[20] Advanced solar architecture and urban planning methods were first employed by the Greeks and Chinese, who oriented their buildings toward the south to provide light and warmth.[21]
The common features of passive solar architecture are orientation relative to the Sun, compact proportion (a low surface area to volume ratio), selective shading (overhangs) and thermal mass.[20] When these features are tailored to the local climate and environment they can produce well-lit spaces that stay in a comfortable temperature range. Socrates' Megaron House is a classic example of passive solar design.[20] The most recent approaches to solar design use computer modeling tying together solar lighting, heating and ventilation systems in an integrated solar design package.[22] Active solar equipment such as pumps, fans and switchable windows can complement passive design and improve system performance.
Agriculture and horticulture
Greenhouses like these in the Westland municipality of the Netherlands grow vegetables, fruits and flowers.
Agriculture and horticulture seek to optimize the capture of solar energy in order to optimize the productivity of plants. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows and the mixing of plant varieties can improve crop yields.[24][25] While sunlight is generally considered a plentiful resource, the exceptions highlight the importance of solar energy to agriculture. During the short growing seasons of the Little Ice Age, French and English farmers employed fruit walls to maximize the collection of solar energy. These walls acted as thermal masses and accelerated ripening by keeping plants warm. Early fruit walls were built perpendicular to the ground and facing south, but over time, sloping walls were developed to make better use of sunlight. In 1699, Nicolas Fatio de Duillier even suggested using a tracking mechanism which could pivot to follow the Sun.[26] Applications of solar energy in agriculture aside from growing crops include pumping water, drying crops, brooding chicks and drying chicken manure.[27][28] More recently the technology has been embraced by vinters, who use the energy generated by solar panels to power grape presses.[29]
Transport and reconnaissance
Australia hosts the World Solar Challenge where solar cars like the Nuna3 race through a 3,021 km (1,877 mi) course from Darwin to Adelaide.
Development of a solar powered car has been an engineering goal since the 1980s. The World Solar Challenge is a biannual solar-powered car race, where teams from universities and enterprises compete over 3,021 kilometres (1,877 mi) across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 kilometres per hour (42 mph) and by 2007 the winner's average speed had improved to 90.87 kilometres per hour (56.46 mph).[32] The North American Solar Challenge and the planned South African Solar Challenge are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles.[33][34]
Solar thermal
Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation.Solar hot water systems use sunlight to heat water. In low geographical latitudes (below 40 degrees) from 60 to 70% of the domestic hot water use with temperatures up to 60 °C can be provided by solar heating systems.[55] The most common types of solar water heaters are evacuated tube collectors (44%) and glazed flat plate collectors (34%) generally used for domestic hot water; and unglazed plastic collectors (21%) used mainly to heat swimming pools.[56]
As of 2007, the total installed capacity of solar hot water systems is approximately 154 GW.[57] China is the world leader in their deployment with 70 GW installed as of 2006 and a long term goal of 210 GW by 2020.[58] Israel and Cyprus are the per capita leaders in the use of solar hot water systems with over 90% of homes using them.[59] In the United States, Canada and Australia heating swimming pools is the dominant application of solar hot water with an installed capacity of 18 GW as of 2005.[18]
Heating, cooling and ventilation
Solar House #1 of Massachusetts Institute of Technology in the United States, built in 1939, used Seasonal thermal energy storage (STES) for year-round heating.
In the United States, heating, ventilation and air conditioning (HVAC) systems account for 30% (4.65 EJ) of the energy used in commercial buildings and nearly 50% (10.1 EJ) of the energy used in residential buildings.[49][60] Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy.
Thermal mass is any material that can be used to store heat—heat from the Sun in the case of solar energy. Common thermal mass materials include stone, cement and water. Historically they have been used in arid climates or warm temperate regions to keep buildings cool by absorbing solar energy during the day and radiating stored heat to the cooler atmosphere at night. However they can be used in cold temperate areas to maintain warmth as well. The size and placement of thermal mass depend on several factors such as climate, daylighting and shading conditions. When properly incorporated, thermal mass maintains space temperatures in a comfortable range and reduces the need for auxiliary heating and cooling equipment.[61]
Water treatment
Solar distillation can be used to make saline or brackish water potable. The first recorded instance of this was by 16th century Arab alchemists.[66] A large-scale solar distillation project was first constructed in 1872 in the Chilean mining town of Las Salinas.[67] The plant, which had solar collection area of 4,700 m2, could produce up to 22,700 L per day and operated for 40 years.[67] Individual still designs include single-slope, double-slope (or greenhouse type), vertical, conical, inverted absorber, multi-wick, and multiple effect.[66] These stills can operate in passive, active, or hybrid modes. Double-slope stills are the most economical for decentralized domestic purposes, while active multiple effect units are more suitable for large-scale applications.[66]
Energy storage methods
The 150 MW Andasol solar power station is a commercial parabolic trough solar thermal power plant, located in Spain. The Andasol plant uses tanks of molten salt to store solar energy so that it can continue generating electricity even when the sun isn't shining.[102]
Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or seasonal durations. Thermal storage systems generally use readily available materials with high specific heat capacities such as water, earth and stone. Well-designed systems can lower peak demand, shift time-of-use to off-peak hours and reduce overall heating and cooling requirements.[103][104]
Phase change materials such as paraffin wax and Glauber's salt are another thermal storage media. These materials are inexpensive, readily available, and can deliver domestically useful temperatures (approximately 64 °C). The "Dover House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating system, in 1948.[105]