Sunlight can be converted into electricity using photovoltaics (PV), concentrating solar power (CSP), and various experimental technologies. PV has mainly been used to power small and medium-sized applications, from the
calculator powered by a single solar cell to off-grid homes powered by a
photovoltaic array. For large-scale generation, CSP plants like
SEGS have been the norm but recently multi-megawatt PV plants are becoming common. Completed in 2007, the 14 MW power station in
Clark County,
Nevada and the 20 MW site in Beneixama, Spain are characteristic of the trend toward larger
photovoltaic power stations in the US and Europe.
Photovoltaics
A
solar cell, or photovoltaic cell (PV), is a device that converts light into
direct current using the
photoelectric effect. The first solar cell was constructed by
Charles Fritts in the 1880s.
[62] Although the prototype
selenium cells converted less than 1% of incident light into electricity, both
Ernst Werner von Siemens and
James Clerk Maxwell recognized the importance of this discovery.
[63] Following the work of
Russell Ohl in the 1940s, researchers Gerald Pearson,
Calvin Fuller and Daryl Chapin created the
silicon solar cell in 1954.
[64] These early solar cells cost 286 USD/watt and reached efficiencies of 4.5–6%.
[65]The earliest significant application of solar cells was as a back-up power source to the
Vanguard I satellite, which allowed it to continue transmitting for over a year after its chemical battery was exhausted.
[66] The successful operation of solar cells on this mission was duplicated in many other
Soviet and
American satellites, and by the late 1960s, PV had become the established source of power for them.
[67] Photovoltaics went on to play an essential part in the success of early commercial satellites such as
Telstar, and they remain vital to the telecommunications infrastructure today.
[68]The high cost of solar cells limited terrestrial uses throughout the 1960s. This changed in the early 1970s when prices reached levels that made PV generation competitive in remote areas without
grid access. Early terrestrial uses included powering telecommunication stations, off-shore
oil rigs,
navigational buoys and railroad crossings.
[69] These
off-grid applications have proven very successful and accounted for over half of worldwide installed capacity until 2004.
[37]Building-integrated photovoltaics cover the roofs of the increasing number of homes.
The
1973 oil crisis stimulated a rapid rise in the production of PV during the 1970s and early 1980s.
[70] Economies of scale which resulted from increasing production along with improvements in system performance brought the price of PV down from 100 USD/watt in 1971 to 7 USD/watt in 1985.
[71] Steadily
falling oil prices during the early 1980s led to a reduction in funding for photovoltaic R&D and a discontinuation of the tax credits associated with the
Energy Tax Act of 1978. These factors moderated growth to approximately 15% per year from 1984 through 1996.
[72]Since the mid-1990s, leadership in the PV sector has shifted from the US to
Japan and
Germany. Between 1992 and 1994 Japan increased R&D funding, established
net metering guidelines, and introduced a subsidy program to encourage the installation of residential PV systems.
[73] As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999,
[74] and worldwide production growth increased to 30% in the late 1990s.
[75]Germany has become the leading PV market worldwide since revising its
Feed-in tariff system as part of the Renewable Energy Sources Act. Installed PV capacity has risen from 100 MW in 2000 to approximately 4,150 MW at the end of 2007.
[76][77] Spain has become the third largest PV market after adopting a similar feed-in tariff structure in 2004, while France, Italy, South Korea and the US have seen rapid growth recently due to various incentive programs and local market conditions.
Concentrating solar power
Concentrated sunlight has been used to perform useful tasks since the time of
ancient China. A legend claims that
Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from
Syracuse.
[79] Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine in 1866, and subsequent developments led to the use of concentrating solar-powered devices for irrigation, refrigeration and locomotion.
[80]Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exist; the most developed are the solar trough, parabolic dish and solar power tower. These methods vary in the way they track the Sun and focus light. In all these systems a
working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.
[81]The
PS10 concentrates sunlight from a field of heliostats on a central tower.
A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the Sun during the daylight hours by tracking along a single axis. Trough systems provide the best land-use factor of any solar technology.
[82] The
SEGS plants in California and Acciona's
Nevada Solar One near
Boulder City, Nevada are representatives of this technology.
[83][84]A parabolic dish system consists of a stand-alone
parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the Sun along two axes. Parabolic dish systems give the highest efficiency among CSP technologies.
[85] The 50 kW Big Dish in
Canberra, Australia is an example of this technology.
[83]A solar power tower uses an array of tracking reflectors (
heliostats) to concentrate light on a central receiver atop a tower. Power towers are less advanced than trough systems but offer higher efficiency and better energy storage capability.
[83] The
Solar Two in Barstow, California and the
Planta Solar 10 in
Sanlucar la Mayor, Spain are representatives of this technology.
Experimental solar power
A solar updraft tower (also known as a solar chimney or solar tower) consists of a large greenhouse that funnels into a central tower. As sunlight shines on the greenhouse, the air inside is heated, and expands. The expanding air flows toward the central tower, where a turbine converts the air flow into electricity. A 50 kW prototype was constructed in
Ciudad Real, Spain and operated for eight years before decommissioning in 1989.
[87]A
solar pond is a pool of salt water (usually 1–2
m deep) that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after he came across reports of a lake in
Hungary in which the temperature increased with depth. This effect was due to salts in the lake's water, which created a "density gradient" that prevented
convection currents. A prototype was constructed in 1958 on the shores of the Dead Sea near
Jerusalem.
[88] The pond consisted of layers of water that successively increased from a weak salt solution at the top to a
high salt solution at the bottom. This solar pond was capable of producing temperatures of 90 °C in its bottom layer and had an estimated solar-to-electric efficiency of two percent.
Thermoelectric, or "thermovoltaic" devices convert a temperature difference between dissimilar materials into an electric current. First proposed as a method to store solar energy by solar pioneer Mouchout in the 1800s,
[89] thermoelectrics reemerged in the Soviet Union during the 1930s. Under the direction of Soviet scientist
Abram Ioffe a concentrating system was used to thermoelectrically generate power for a 1
hp engine.
[90] Thermogenerators were later used in the US space program as an energy conversion technology for powering deep space missions such as
Cassini,
Galileo and
Viking. Research in this area is focused on raising the efficiency of these devices from 7–8% to 15–20%.
[91]Space solar power systems would use a large solar array in
geosynchronous orbit to collect sunlight and beam this energy in the form of microwave radiation to receivers (
rectennas) on Earth for distribution. This concept was first proposed by
Dr. Peter Glaser in 1968 and since then a wide variety of systems have been studied with both photovoltaic and concentrating solar thermal technologies being proposed. Although still in the concept stage, these systems offer the possibility of delivering power approximately 96% of the time.
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).[100] 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.[101][102]
Helios UAV in solar powered flight
