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Solar energy



Main articles: Solar vehicle, Electric boat, and Solar balloon
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.
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 center of this development is the World Solar Challenge, a biannual solar-powered car race in which 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).[104] The 2007 race included a new challenge class using cars with an upright seating position and which, with little modification, could be a practical proposition for sustainable transport. The winning car averaged 90.87 kilometres per hour (56.46 mph). The North American Solar Challenge (formerly Sunrayce USA) and the planned South African Solar Challenge are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles.

In 1975, the first practical solar boat was constructed in England.[105] By 1995, passenger boats incorporating PV panels began appearing and are now used extensively.[106] In 1996, Kenichi Horie made the first solar powered crossing of the Pacific Ocean, and the sun21 catamaran made the first solar powered crossing of the Atlantic Ocean in the winter of 2006/2007.[107] Plans to circumnavigate the globe in 2009 are indicative of the progress solar boats have made.

Helios UAV in solar powered flight
Helios UAV in solar powered flight

In 1974, the unmanned Sunrise II inaugurated the era of solar flight. In 1980, the Gossamer Penguin made the first piloted flights powered solely by photovoltaics. This was quickly followed by the Solar Challenger which demonstrated a more airworthy design with its crossing of the English Channel in July, 1981. Developments then turned back to unmanned aerial vehicles with the Pathfinder (1997), Pathfinder Plus (1998) and Centurion (1998) each building on one another.[108] These designs culminated in the Helios which set the altitude record for a non-rocket-propelled aircraft of 29,524 metres (96,860 ft) in 2001. The Zephyr, developed by BAE Systems, is the latest in a line of record breaking solar aircraft. This aircraft made a record setting 54 hours duration flight in 2007, and month long duration flights are envisioned by 2010.[109]

A solar balloon is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands, causing an upward buoyancy force, much like an artificially-heated hot air balloon. Some solar balloons are large enough for human flight, but usage is limited to the toy market as the surface-area to payload-weight ratio is rather high.

Solar sails are a proposed form of spacecraft propulsion using large membrane mirrors. Radiation pressure is small and decreases by the square of the distance from the Sun, but unlike rockets, solar sails require no fuel. Although the thrust is small compared to rockets, it continues as long as the Sun shines and the sail is deployed and in the frictionless vacuum of space significant speeds can eventually be achieved.[110]

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Thermal and electrical storage

Main articles: Thermal mass, Thermal energy storage, Phase change material, and Grid energy storage
Solar Two's thermal storage system allowed it to generate electricity during cloudy weather and at night.
Solar Two's thermal storage system allowed it to generate electricity during cloudy weather and at night.

Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. Solar energy is not available at night, and the performance of solar power systems is affected by unpredictable weather patterns; therefore, a storage medium or back-up power systems must be used.

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.

Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are non-flammable, nontoxic, low-cost, have a high specific heat capacity, and can deliver heat at temperatures compatible with conventional power systems. A molten salt storage system consists of a salt loop connected to an insulated storage tank. During the heating cycle, the salt mixture is heated from an initial temperature of 290 °C up to 565 °C. During the power cycle, the salt is used to make steam for a thermal power station. The Solar Two used this method of energy storage, allowing it to store 1.44 TJ in its 68 m³ storage tank with an annual storage efficiency of about 99%.[111]

A Paraffin wax thermal storage system consists of a solar hot water loop connected to a paraffin wax tank. During the storage cycle, hot water flows through the storage tank melting the paraffin. The enthalpy of fusion for paraffin is 210-230 kJ/kg. During the heating cycle, stored heat is extracted from the tank as the wax resolidifies. These systems heat air and water to 64 °C and can reduce conventional energy use by 50 to 70%.[112][113]

Eutectic salts such as Glauber's salt also can be employed in thermal storage systems. Glauber's salt is inexpensive and readily available. It can store 347 kJ/kg and deliver heat at 64 °C. The "Dover House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating system in 1948.[114]

Rechargeable batteries can be used to store excess electricity from a photovoltaic system. Lead acid batteries are the most common type of battery associated with photovoltaic systems because they are cheap and available. Batteries used in off-grid applications should be sized for three to five days of capacity.[115]

Excess electricity can also be fed into the transmission grid. Net metering programs give photovoltaic system owners a credit for the electricity they deliver to the grid. This credit is used to offset electricity provided from the grid when the photovoltaic system cannot meet demand, effectively acting as a giant battery. For large scale use of renewable energy the most practical storage is hydro-storage, although V2G (Vehicle to Grid) is also being developed, which will become viable when more plug-in hybrids and electric cars are in use.

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Development, deployment and economics

Main article: Deployment of solar power to energy grids
11 MW Serpa solar power plant in Portugal
11 MW Serpa solar power plant in Portugal

Beginning with the surge in coal use which accompanied the Industrial Revolution in the late 18th century, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The first oil well in 1859 accelerated the energy transition so that by the mid-1880s the U.S. consumption of fossil fuels surpassed the consumption of wood which had traditionally been the main energy resource.[116] The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce but solar development stagnated in the early 20th century in the face of the increasing availability, economy, and utility of fossil fuels such as coal and petroleum.[117]

The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies. Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the USA (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE).[118][119]

Between 1970 and 1983, photovoltaic installations grew rapidly, but dropping oil prices in the early 1980s moderated the growth of PV from 1984 through 1996. Since 1997, PV development has accelerated due to supply issues with oil and natural gas, global warming concerns (see Kyoto Protocol), and the improving economic position of PV relative to other energy technologies. Incentives first in Japan and then Germany have resulted in increased production and has reduced prices. Photovoltaic electricity is now competitive with conventional electricity rates in many locations. Particularly due to sale of renewable energy credits and other incentives, Nellis Air Force Base is obtaining photovoltaic power for about 2.2 ¢/kWh and grid power for 9 ¢/kWh.[120] Typical payback periods for installing photovoltaics are 15 to 25 years.[121] Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6 GW at the end of 2007.[43] By 2006 more polysilicon was used for photovoltaics than for computer chips.

Commercial solar water heaters began appearing in the United States in the 1890s.[122] These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels.[123] As with photovoltaics, solar water heating attracted renewed attention as a result of the oil crises in the 1970s but interest subsided in the 1980s due to falling petroleum prices. Development in the solar water heating sector progressed steadily throughout the 1990s and growth rates have averaged 20% per year since 1999.[46] Estimated payback time for solar water heaters is about 9 years.[124] Although generally underestimated, solar water heating is by far the most widely deployed solar technology with an estimated capacity of 154 GW as of 2007.[46]

Commercial concentrating solar power (CSP) plants were first developed in the 1980s. CSP plants such as SEGS project in the United States have a levelized cost of energy (LCOE) of 12-14 cents/kWh.[125] The 11 MW PS10 power tower in Spain, completed in late 2005, is Europe's first commercial CSP system and a total capacity of 300 MW is expected to be installed in the same area by 2013.[126]

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See also

Sustainable development Portal
Energy Portal

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Notes

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References

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External links

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