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Solar energy

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The Sun provides approximately 1000 watts/meter² on Earth's surface.
US annual average solar energy received by a latitude tilt photovoltaic cell.

Solar power is the technology of obtaining usable energy from the light of the Sun. Solar energy has been used in many traditional technologies for centuries and has come into widespread use where other power supplies are absent, such as in remote locations and in space. Its use is spreading as the environmental costs and limited supply of other power sources such as fossil fuels are realized.

Solar energy can be harnessed in a number of manners for:

Energy from the Sun

Theoretical annual mean insolation, at the top of Earth's atmosphere (top) and at the surface on a horizontal square meter .
Global solar energy resources. The colors in the map show the actual local solar energy, averaged through the years of 1991-1993. The scale is in watts per square meter.
The land area required to supply the current global primary energy demand by solar energy using available technology is represented by the dark disks.

The rate at which solar radiation reaches a unit of area in space in the region of the Earth's orbit is1,366 W/m², as measured upon a surface normal (at a right angle) to the Sun. This number is referred to as the solar constant.[1] The atmosphere reflects 6% and absorbs 16% of incoming radiation resulting in a peak power at sea level of 1,020 W/m² . [2] [3] Average cloud cover reduces incoming radiation by 20% through reflection and 16% through absorption.[4] The image on the right shows the average solar power available on the surface in W/m² calculated from satellite cloud data averaged over three years from 1991 to 1993 (24 hours a day). For example, in North America the average power of the solar radiation lies somewhere between 125 and 375 W/m², between 3 and 9 kWh/m²/day. [5]

It should be noted that this is the maximum available power, and not the power delivered by solar power technology. For example, photovoltaic panels currently have an efficiency of about 15% and, hence, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45-1.35 kWh/m²/day. [6] The dark disks in the image on the right are an example for the land areas that, if covered with solar panels, would produce slightly more energy in the form of electricity than the total primary energy supply in 2003. [7] That is, solar cells with an assumed 8% efficiency installed in these areas would deliver a bit more energy in the form of electricity than what is currently available from oil, gas, hydropower, nuclear power, etc. combined.

It should also be noted that a recent concern is that of Global dimming, an effect of pollution that is allowing less and less sunlight to reach the Earth's surface. It is intricately linked with pollution particles and Global warming, and is mostly of concern for issues of Global climate change, but is also of concern to proponents of Solar Power due to the existing and potential future decreases in available Solar Energy. The order of magnitude is about 4% less solar energy available at sea level over the timeframe 1961–90, mostly due to more intense cloud reflections back into outer space. [8]

After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and Infrared radiations. Plants use solar energy to create chemical energy through photosynthesis. Humans regularly use this energy burning wood or fossil fuels, or when simply eating the plants.

Types of technologies

Many technologies have been developed to make use of solar radiation. Some of these technologies make direct use of the solar energy (e.g. to provide light, heat, etc.), while other technologies produce electricity.

Solar design in architecture

Main articles: Passive solar and Active solar

Solar design can be used to achieve comfortable temperature and light levels with little or no additional energy. This can be through passive solar, where maximising the entrance of sunlight in cold conditions and reducing it in hot weather; and active solar, using additional devices such as pumps and fans to direct warm and cool air or fluid.

Solar heating systems

Main article: Solar hot water

Solar hot water systems use sunlight to heat water. These systems may be used to heat domestic hot water or for space heating. These systems are basically composed of solar thermal collectors and a storage tank.[9] The three basic classifications of solar water heaters are:

  • Active systems which use pumps to circulate water or a heat transfer fluid.
  • Passive systems which circulate water or a heat transfer fluid by natural circulation. These are also called thermosiphon systems.
  • Batch systems using a tank directly heated by sunlight.

A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between glazed windows and a sun-facing wall. Sunlight heats the thermal mass during the day and drives natural circulation through vents at the top and bottom of the wall. During the evening the trombe wall radiates stored heat.[10]

A transpired collector is an active solar heating and ventilation system consisting of a perforated sun-facing wall which acts as a solar thermal collector. The collector pre-heats air as it is drawn into the building's ventilation system through the perforations. These systems are inexpensive and commercial models have achieved efficiencies above 70 percent. [11]

Solar cooking

Main article: Solar cooker
Solar Cookers use sunshine as an alternative to fire for cooking.

A solar box cooker traps the Sun's energy in an insulated box; such boxes have been successfully used for cooking, pasteurization and fruit canning. Solar cooking is helping many developing countries, both reducing the demands for local firewood and maintaining a cleaner environment for the cooks. The first known western solar oven is attributed to Horace de Saussure in 1767, which impressed Sir John Herschel enough to build one for cooking meals on his astronomical expedition to the Cape of Good Hope in Africa in 1830. [12] Today, there are many different designs in use around the world. [13]

Solar lighting

Main articles: Daylighting and Light tube

The interior of a building can be lit during daylight hours using light tubes.

For instance, fiber optic light pipes can be connected to a parabolic collector mounted on the roof. The manufacturer claims this gives a more natural interior light and can be used to reduce the energy demands of electric lighting. [14]

Photovoltaics

Main article: Photovoltaics
The solar panels (photovoltaic arrays) on this small yacht at sea can charge the 12 V batteries at up to 9 Amps in full, direct sunlight

Solar cells, also referred to as photovoltaic cells, are devices or banks of devices that use the photovoltaic effect of semiconductors to generate electricity directly from sunlight. Until recently, their use has been limited due to high manufacturing costs. One cost effective use has been in very low-power devices such as calculators with LCDs. Another use has been in remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third use has been in powering orbiting satellites and other spacecraft.

Total peak power of installed PV is around 5,300 MW as of the end of 2005.[citation needed] This is only one part of solar-generated electric power. For solar reflector plants see below.

Declining manufacturing costs (dropping at 3 to 5% a year in recent years) are expanding the range of cost-effective uses. The average lowest retail cost of a large photovoltaic array declined from $7.50 to $4 per watt between 1990 and 2005[citation needed]. With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - that is, systems with no battery that connect to the utility grid through a special inverter - now make up the largest part of the market. In 2003 worldwide production of solar cells increased by 32%.[15] Between 2000 and 2004 the increase in worldwide solar energy capacity was an annual 60%.[16] 2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.[17] Analysts have predicted the similar supply problems during 2006 and 2007.[18]

Solar thermal electric power plants

Solar Two, a concentrating solar power plant (an example of solar thermal energy).
Main article: Solar thermal energy

Solar thermal energy can be used to heat a fluid to high temperatures and use it to produce electric power.

Solar updraft tower

Main article: Solar updraft tower

A Solar updraft tower is a relatively low tech solar thermal power plant where air passes under a very large agricultural glass house (between 2 and 8 km in diameter), is heated by the sun and channeled upwards towards a convection tower. It then rises naturally and is used to drive turbines, which generate electricity.

Energy Tower

An Energy tower is an alternative proposal for the Solar updraft tower. The "Energy Tower" is driven by spraying water at the top of the tower; evaporation of water causes a downdraft by cooling the air thereby increasing its density, driving windturbines at the bottom of the tower. It requires a hot arid climate and large quantities of water (seawater may be used for this purpose) but it does not require the large glass house of the Solar updraft tower.

Steam engine

Solar energy converted to heat in a concentrating collector can be used to boil water into steam to drive a Steam engine.

Stirling engine

A parabolic solar collector concentrating the sun's rays on the heating element of a Stirling engine. The entire unit acts as a solar tracker.

Solar energy converted to heat in a concentrating (dish or trough parabolic) collector can be used to drive a Stirling engine. A solar sterling system holds the record for converting solar energy into electricity (30 percent at 1,000 watts per square meter). [19] Such concentrating systems produce little or no power when the sun is not directly visible even in bright conditions (as in thin fog, stratus, or certain high clouds) and must also incorporate a solar tracker to point the device directly at the sun.

Solar pond

A solar pond is a relatively low-tech, low cost approach to harvesting solar energy. The principle is to fill a pond with 3 layers of water:

  1. A top layer with a low salt content
  2. An intermediate insulating layer with a salt gradient, which sets up a density gradient that prevents heat exchange by natural convection in the water.
  3. A bottom layer has with a high salt content which reaches a temperature approaching 90 degrees Celsius.

The different densities in the layers due to their salt content prevent convection currents developing which would normally transfer the heat to the surface and then to the air above. The heat trapped in the salty bottom layer can be used for different purposes, such as heating of buildings, industrial processes, or generating electricity. There is one in use at Bhuj, Gujarat, India [20] and another at the University of Texas El Paso [21].

Solar chemical

Solar chemical refers to a number of possible processes that harness solar energy by absorbing sunlight in a chemical reaction in a way similar to photosynthesis in plants but without using living organisms. No practical process has yet emerged.
A promising approach is to use focused sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc.[22]

While metals, such as zinc, have been shown to drive photoelectrolysis of water, more research has focused on semiconductors. Further research has examined transition metal compounds, in particular titania, titanates, niobates, tantalates. [citation needed]Unfortunately, these materials exhibit very low efficiencies, because they require ultraviolet light to drive the photoelectrolysis of water. Current materials also require an electrical voltage bias for the hydrogen and oxygen gas to evolve from the surface, another disadvantage. Current research is focusing on the development of materials capable of the same water splitting reaction using lower energy visible light.

It is also possible to use solar energy to drive industrial chemical processes without a requirement for fossil fuel.

Biofuels

Main article: Biofuel

The oil in plant seeds, in chemical terms, very closely resembles that of petroleum. Many, since the invention of the Diesel engine, have been using this form of captured solar energy as a fuel comparable to petrodiesel - for functional use in any diesel engine or generator and known as Biodiesel. A 1998 joint study by the U.S. Department of Energy (DOE) and the U.S. Department of Agriculture (USDA) traced many of the various costs involved in the production of biodiesel and found that overall, it yields 3.2 units of fuel product energy for every unit of fossil fuel energy consumed. [23] Other Biofuels include ethanol, wood for stoves, ovens and furnaces, and methane gas produced from biofuels through chemical processes.

Classifications of solar power technology

Solar power technologies can be classified in a number of ways.

Direct or Indirect

A photovoltaic cell produces electricity directly from solar energy

Direct solar power involves a single transformation of sunlight which results in a useable form of energy.

  • Sunlight hits a photovoltaic cell creating electricity.
  • Sunlight heats an absorber plate of a solar thermal collector.[24]
  • Sunlight strikes a solar sail on a space craft and is converted directly into a force on the sail which causes motion of the craft.
  • Sunlight is collected using focusing mirrors and transmitted via optical fibers into a building's interior to supplement lighting.[25]
  • Sunlight strikes a light mill and causes the vanes to rotate as mechanical energy, little practical application has yet been found for this effect.
File:Itaipu2.jpg
Hydroelectric power stations produce indirect solar power. The Itaipu Dam, Brazil / Paraguay

Indirect solar power involves multiple transformations of sunlight which result in a useable form of energy.

Passive or Active

Passive solar systems use non-mechanical techniques of capturing, converting and distributing sunlight into useable forms of energy such as heating, lighting or ventillation. These 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 systems use mechanical components such as pumps and fans to process sunlight into useable forms of energy.

Concentrating or Non-concentrating

A large parabolic reflector solar furnace is located in the Pyrenees at Odeillo, French Cerdagne. It is used for various research purposes.[27]

Concentrating solar power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam capable of producing high temperatures and correspondingly high thermodynamic efficiencies. Concentrating solar is generally associated with solar thermal applications but concentrating photovoltaic (CPV) applications exist as well and these technologies also exhibit improved efficiencies. CSP systems require direct sunlight to operate properly.[28]

Concentrating solar power systems are sub-classified by focus and tracking type.

  • Line focus
    • A solar trough consists of an elongated parabolic reflector aligned on a north-south axis that uses single-axis tracking to follow the sun from east to west and concentrate light along a line formed at the parabola's focus.[29][30] The SEGS systems in California are an example of this type of system.
  • Point focus
    • A power tower consists of an array of flat reflectors that use dual-axis tracking to follow the sun and concentrate light at a single point on the tower where a thermal receiver is located.[31][32]
    • A parabolic dish or dish/engine system consists of a stand-alone unit that uses dual-axis tracking to follow the sun and focuses light at a single point where photovoltaic cells or a thermal receiver is located.[33][34]

Non-concentrating photovoltaic and solar thermal systems do not concentrate sunlight. While the maximum attainable temperatures (200 °C) and thermodynamic efficiencies are lower, these systems offer simplicity of design a have the ability to effectively utilize diffuse solar radiation.[35]

Advantages and disadvantages of Solar power

Advantages

  • The 122 PW of sunlight reaching the earth's surface is plentiful compared to the 13 TW of total energy consumed by humans.[36]
  • Solar power is pollution free during use. Production end wastes and emissions are manageable using existing pollution controls. Decommisioning end recycling technologies are under development. [37]
  • Facilities can operate with little maintenance or intervention after initial setup.
  • Solar electric generation is economically competitive where grid connection or fuel transport is difficult, costly or impossible. Examples include satellites, island communities, remote locations and ocean vessels.
  • When grid connected, solar electric generation can displace the highest cost electricity during times of peak demand (in most climatic regions), can reduce grid loading, and can eliminate the need for local battery power for use in times of darkness and high local demand; such application is encouraged by net metering. Time-of-use net metering can be highly favorable to small photovoltaic systems.
  • Grid connected solar electricity can be used locally thus minimizing transmission/distribution losses (approximately 7.2%).[38]

Disadvantages

  • Limited areal power density: For electrical generation with photovoltaics, the average irradiation power density is approximately 1 kW/m2 usable by 8-15% efficient solar panels.[citation needed]
  • Intermittency: It is not available at night and is reduced when there is cloud cover, decreasing the reliability of peak output performance or requiring a means of energy storage. For power grids to stay functional at all times, the addition of substantial amounts of solar generated electricity would require the expansion of energy storage facilities, other renewable energy sources, or the use of backup conventional powerplants. There is an energy cost to keep coal-burning power plants 'hot', which includes the burning of coal to keep boilers at temperature. However, natural gas power plants can quickly come up to full load without requiring significant standby idling [39].
  • Locations at high latitudes or with frequent substantial cloud cover offer reduced potential for solar power use.
  • It can only realistically be used to power transport vehicles by converting light energy into another form of energy (e.g. battery stored electricity or by electrolysing water to produce hydrogen) suitable for transport, incurring an energy penalty similar to coal or nuclear electricity generation. While the burning of gasoline in an internal combustion engine is only about 20%-25% efficient [40], depending on driving mode, the use of battery electric technology can match or exceed that efficiency when various external factors are included, such as the loss of energy in the production of gasoline and the energy cost of battery manufacture and recycling.
  • Solar cells produce DC which must be converted to AC when used in currently existing distribution grids. This incurs an energy penalty of 4-12%.[41]

Energy storage

Main article: Grid energy storage

For a stand-alone system, some means must be employed to store the collected energy for use during hours of darkness or cloud cover. The following list includes both mature and immature techniques:

Storage always has an extra stage of energy conversion, with consequent energy losses, greatly increasing capital costs. One way around this is to export excess power to the power grid, drawing it back when needed. This appears to use the power grid as a battery but in fact is relying on conventional energy production through the grid during the night. However, since the grid always has a positive outflow, the result is exactly the same.

Electric power costs are highly dependant on the consumption per time of day, since plants must be built for peak power (not average power). Expensive gas-fired "peaking generators" must be used when base capacity is insufficient. Fortunately for solar, solar capacity parallels energy demand -since much of the electricity is for removing heat produced by too much solar energy (air conditioners)! This is less true in the winter. Wind power complements solar power since it can produce energy when there is no sunlight.

Development & deployment of solar power

The solar powered car The Nuna 3 built by the Dutch Nuna team
See main article Deployment of solar power to energy grids

Deployment of solar power depends largely upon local conditions and requirements. All industrialised nations share a need for electricity and it is clear that solar power will increasingly be used as an option for electricity supply.

Development of a practical solar powered car has been an engineering goal for twenty years. The center of this development is the World Solar Challenge, a biannual solar powered car race over 3021 km through central Australia from Darwin to Adelaide. The race's stated objective is to promote research into solar-powered cars. Teams from universities and enterprises participate. In 1987 when it was founded the winner's average speed was 67 km/h.[43] By the 2005 race this had increased to an average speed of greater than 100 km/h, even though the cars were faced with the 110 km/h South Australia speed limit.[44]

See also

References

  1. ^ Solar Spectra: Standard Air Mass Zero
  2. ^ Earth Radiation Budget
  3. ^ SRRL: An overview of the Solar Radiation Research Laboratory
  4. ^ Earth Radiation Budget
  5. ^ NREL: Dynamic Maps, GIS Data, and Analysis Tools - Solar Maps
  6. ^ "us_pv_annual_may2004.jpg". National Renewable Energy Laboratory, US. Retrieved 2006-09-04.
  7. ^ International Energy Agency - Homepage
  8. ^ Liepert, B. G. (2002-05-02). "Observed Reductions in Surface Solar Radiation in the United States and Worldwide from 1961 to 1990" (PDF). GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 10, 1421. Retrieved 2006-09-04.
  9. ^ NREL - Solar Hot Water
  10. ^ EERE - Indirect Gain (Trombe Walls)
  11. ^ NREL - Transpired Air Collectors (Ventilation Preheating)
  12. ^ "Horace de Saussure and his Hot Boxes of the 1700's". Retrieved 2006-09-04.
  13. ^ "Solar Cooking Plans". Retrieved 2006-09-04.
  14. ^ Sunlight Direct Products
  15. ^ World Sales of Solar Cells Jump 32 PercentViviana Jiménez, 2004 Earth Policy Institute. Retrieved 4 September 2006.
  16. ^ Sun King Russell Flannery 27 March 2006. Retrieved 4 September 2006.
  17. ^ Silicon Shortage Stalls Solar John Gartner, Wired News, 28 March 2005. Retrieved 4 September 2006.
  18. ^ 2005 Solar Year-end Review & 2006 Solar Industry Forecast Jesse W. Pichel and Ming Yang, Research Analysts, Piper Jaffray, 11 January 2006. Retrieved 4 September 2006.
  19. ^ "Solar Stirling system ready for production". Retrieved 2006-09-06.
  20. ^ Solar pond in Gujarat
  21. ^ Solar pond at University of Texas El Paso
  22. ^ IsraCast: ZINC POWDER WILL DRIVE YOUR HYDROGEN CAR, Wired News: Sunlight to Fuel Hydrogen Future and Solar Technology Laboratory: SynMet
  23. ^ Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus
  24. ^ Schatz Energy Research Center - Solar Thermal Technology
  25. ^ ORNL - Solar Technologies Program
  26. ^ NREL - Ocen Energy Basics
  27. ^ Les Fours solaires
  28. ^ DOE - Solar Basics
  29. ^ Sandia - Concentrating Solar Power Overview
  30. ^ Volker Quaschning - Solar Thermal Power Plants
  31. ^ Sandia - Concentrating Solar Power Overview
  32. ^ Volker Quaschning - Solar Thermal Power Plants
  33. ^ Sandia - Concentrating Solar Power Overview
  34. ^ Volker Quaschning - Solar Thermal Power Plants
  35. ^ Volker-Quaschning - Solar Thermal Basics
  36. ^ Vaclav Smil - Energy at the Crossroads
  37. ^ Environmental Aspects of PV Power Systems
  38. ^ U.S. Climate Change Technology Program - Transmission and Distribution Technologies
  39. ^ Pratt & Whitney's Next Generation Turbine Program
  40. ^ Engine Efficiency: Good or Bad? - Efficiency Loss
  41. ^ Renewable Resource Data Center - PV Correction Factors
  42. ^ Solar Tres Project
  43. ^ History of World Solar Challenge The World Solar Challenge. Retrieved 4 September 2006.
  44. ^ Panasonic World Solar Challenge 21-28 October 2007 The World Solar Challenge. Retrieved 4 September 2006.

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