Solar energy is the utilization of the
radiant energy from the
Sun. Solar power is
often used interchangeably with solar energy but refers more specifically to
the conversion of sunlight into
electricity, either by
photovoltaics and concentrating solar thermal devices, or by one of
several experimental technologies such as thermoelectric converters, solar
chimneys or solar ponds.
Solar energy and shading are important considerations in building
design.
Thermal mass is used to conserve the heat that sunshine delivers to all
buildings.
Daylighting techniques optimize the use of light in buildings.
Solar water heaters heat swimming pools and provide domestic hot water. In
agriculture,
greenhouses expand growing seasons and pumps powered by
solar cells (also known as photovoltaics) provide water for grazing
animals.
Evaporation ponds are used to harvest salt and clean waste streams of
contaminants.
Solar
distillation and
disinfection techniques produce
potable water for millions of people worldwide. Simple applications
include
clotheslines and
solar cookers which concentrate sunlight for cooking, drying and
pasteurization. More sophisticated concentrating technologies magnify the
rays of the Sun for high-temperature material testing, metal
smelting
and industrial
chemical production. A range of
prototype
solar vehicles provide ground, air and sea transportation.
Solar energy
-
About half the incoming solar energy reaches the earth's surface.
Incident solar energy (left) compared to global energy consumption
(right)[1]
The Earth receives 174 petawatts
(PW) of incoming solar radiation (insolation)
at the upper
atmosphere.[2]
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.[3]
The absorbed solar light heats the land surface, oceans and atmosphere. The
warm air containing evaporated water from the oceans rises, driving
atmospheric circulation or
convection. When this 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
cyclones
and
anti-cyclones.
Wind is a manifestation of the atmospheric circulation driven by solar
energy.[4]
Sunlight absorbed by the oceans and land masses keeps the surface at an
average temperature of 14 °C.[5]
The conversion of solar energy into chemical energy via
photosynthesis produces food, wood and the
biomass
from which fossil fuels are derived.[6]
Solar radiation along with secondary solar resources such as
wind
and wave
power,
hydroelectricity and
biomass
account for over
99.9% of the available flow of
renewable energy on Earth.[7][8]
Flows and stores of solar energy in the environment are vast. The total solar
energy absorbed by Earth's atmosphere, oceans and land masses is approximately
3,850 zettajoules
(ZJ) per year.[9]
In 2002, this was more energy in one hour than the world used in one year.[10][11]
Photosynthesis captures approximately 3 ZJ per year in biomass.[12]
The amount of solar energy reaching the surface of the planet is so vast that
in one year it is about twice as much as will ever be obtained from the all of
earth's non-renewable resources of coal, oil, natural gas, and mined uranium
combined.[13]
Applications of solar energy technology
Average
insolation showing land area (small black dots) required to replace
the total world energy supply with solar electricity
Solar energy technologies use
solar radiation for practical ends. Technologies that use secondary solar
resources such as biomass, wind, waves and ocean thermal gradients can be
included in a broader description of solar energy but only primary resource
applications are discussed here. Because the performance of solar technologies
varies widely between regions, solar technologies should be deployed in a way
that carefully considers these variations.
Solar technologies such as photovoltaics and water heaters increase the
supply of energy and may be characterized as
supply side technologies.[citation
needed] Technologies such as passive design and shading
devices reduce the need for alternate resources and may be characterized as
demand side. Optimizing the performance of solar technologies is often a
matter of controlling the resource rather than simply maximizing its
collection.[citation
needed]
Architecture and urban planning
-
Sunlight has influenced building design since the beginning of
architectural history.[15]
Fully developed 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.[16]
The elemental features of
passive solar architecture are Sun orientation, compact proportion (a low
surface area to volume ratio), selective shading (overhangs) and
thermal mass.[15]
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.[15]
The most recent approaches to solar design use computer modeling to tie
together
solar
lighting,
heating and
ventilation systems in an integrated
solar design package.[17]
Active solar equipment such as pumps, fans and switchable windows can also
complement passive design and improve system performance.
Urban heat islands (UHI) are metropolitan areas with higher temperatures
than the surrounding environment. These higher temperatures are the result of
urban materials such as asphalt and concrete that have lower
albedos and
higher
heat capacities than the natural environment. A straightforward method of
counteracting the UHI effect is to paint buildings and roads white and plant
trees. Using these methods, a hypothetical "cool communities" program in
Los Angeles has projected that urban temperatures could be reduced by
approximately 3 °C at an estimated cost of US$1 billion, giving estimated
total annual benefits of US$530 million from reduced air-conditioning costs
and healthcare savings.[18]
Agriculture and horticulture
-
Greenhouses like these in the Netherland's Westland municipality grow
a wide variety of vegetables, fruits and flowers.
Agriculture inherently seeks to optimize the capture of solar energy, and
thereby plant productivity. Techniques such as timed planting cycles, tailored
row orientation, staggered heights between rows and the mixing of plant
varieties can improve crop yields.[19][20]
While sunlight is generally considered a plentiful resource, 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
with a south facing orientation 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.[21]
Solar energy applications in agriculture, aside from growing crops, include
pumping water, drying crops, brooding chicks and drying chicken manure.[22][23]
Greenhouses convert solar light to heat enabling year-round production and
the growth (in enclosed environments) of specialty crops and other plants not
naturally suited to the local climate. Primitive greenhouses were first used
during Roman times to produce cucumbers year-round for the Roman emperor
Tiberius.[24]
The first modern greenhouses were built in
Europe in the
16th century to keep exotic plants brought back from explorations abroad.[25]
Greenhouses remain an important part of horticulture today, while plastic
transparent materials have also been used to similar effect in
polytunnels and
row covers.
Solar lighting
Daylighting features such as this
oculus
at the top of the Pantheon in Rome have been in use since antiquity.
The history of lighting is dominated by the use of natural light. The
Romans recognized a
right to light as early as the
6th century and English law echoed these judgments with the Prescription
Act of 1832.[26][27]
In the 20th century artificial
lighting
became the main source of interior illumination.
Daylighting systems collect and distribute sunlight to provide interior
illumination; they are passive systems. These systems directly offset energy
use by replacing artificial lighting, and indirectly offset non-solar energy
use by reducing the need for
air-conditioning.[28]
The use of
natural lighting also offers physiological and psychological benefits
compared to
artificial lighting, albeit difficult to quantify.[28]
Daylighting design implies careful selection of window types, sizes and
orientation; exterior shading devices may also be considered. Individual
features include sawtooth roofs,
clerestory windows, light shelves,
skylights
and light
tubes. These features may be incorporated into existing structures, but
are most effective when integrated into a
solar design package that accounts for factors such as
glare, heat flux and
time-of-use. When daylighting features are properly implemented they can
reduce lighting-related energy requirements by 25%.[29]
An important
active solar lighting method is the hybrid solar lighting (HSL). HSL
systems collect sunlight using focusing mirrors that
track the Sun and use
optical fibers to transmit the light into a building's interior to
supplement conventional lighting. In single-story applications, these systems
are able to transmit 50% of the direct sunlight received.[30]
Although
daylight saving time is promoted as a way to use sunlight to save energy,
recent research has been limited and reports contradictory results: several
studies report savings, but just as many suggest no effect or even a net loss,
particularly when
gasoline
consumption is taken into account. Electricity use is greatly affected by
geography, climate and economics, making it hard to generalize from single
studies.[31]
Solar thermal
-
Solar thermal technologies can be used for water heating, space heating,
space cooling and process heat generation.[32]
Water heating
-
Solar water heaters must face the Sun to maximize gain.
Solar hot water systems use sunlight to heat water. In low geographical
latitudes (below 40 degrees) solar heating system can provide from 60 to 70%
of domestic hot water use with temperatures up to 60 °C.[33]
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.[34]
As of 2007, the total installed capacity of solar hot water systems is
approximately 154 GW.[35]
China is the
world leader in the deployment of solar hot water with 70 GW installed as of
2006 and a long term goal of 210 GW by 2020.[36]
Israel is the per capita leader in the use of solar hot water with 90% of
homes using this technology.[37]
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.[38]
Heating, cooling and ventilation
-
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.[39][29]
Solar heating, cooling and ventilation technologies can be used to offset a
portion of this energy.
Thermal mass, in the most general sense, is any material that has the
capacity to store heat. In the context of solar energy, thermal mass materials
are used to store heat from the Sun. Common thermal mass materials include
stone, cement and water. These materials have historically 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, but they can also be used in cold temperate areas to maintain warmth.
The size and placement of thermal mass should consider 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.[40]
A solar chimney (or thermal chimney) is a passive solar ventilation system
composed of a vertical shaft connecting the interior and exterior of a
building. As the chimney warms, the air inside is heated causing an
updraft that pulls air through the building. Performance can be improved
by using glazing and thermal mass materials in a way that mimics greenhouses.
Deciduous trees and plants have often been promoted as a means of
controlling solar heating and cooling. When planted on the southern side of a
building, the leaves provide shade during the summer while the bare limbs
allow light and warmth to pass during the winter.[41]
Since bare, leafless trees shade 1/3 to 1/2 of incident solar radiation, there
is a balance between the benefits of summer shading and the corresponding loss
of winter heating.[42]
In climates with significant heating loads, deciduous trees should not be
planted on the southern side of a building because they will interfere with
winter solar availability but they can be used on the east and west sides to
provide a degree of summer shading without appreciably affecting winter solar
gain.[43]
Desalination and disinfection
-
A SODIS application in Indonesia demonstrates the simplicity of this
approach to water disinfection.
Solar distillation can be used to produce potable water from
saline or
brackish water. The first recorded use was by 16th century Arab
alchemists.[44]
The first large-scale solar distillation project was constructed in 1872 in
the Chilean
mining town of Las Salinas.[45]
This plant, which had solar collection area of 4,700 m˛, could produce up to
22,700 L
per day and operated for 40 years.[45]
Individual still
designs include single-slope, double-slope (or greenhouse type), vertical,
conical, inverted absorber, multi-wick and multiple effect.[44]
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.[44]
Solar water disinfection (SODIS) is a method of
disinfecting water by exposing water-filled plastic
polyethylene terephthalate (PET) bottles to several hours of sunlight.[46]
Exposure times vary depending on weather and climate from a minimum of six
hours to two days during fully overcast conditions.[47]
SODIS is recommended by the
World Health Organization as a viable method for household water treatment
and safe storage.[48]
Over two million people in developing countries use SODIS for their daily
drinking water needs.[47]
Cooking
-
Main article:
Solar cooker
The Solar Bowl in Auroville, India, concentrates sunlight on a
movable receiver to produce steam for cooking.
Solar cookers use sunlight for cooking, drying and
pasteurization. These devices can be grouped into three broad categories:
box cookers, panel cookers and reflector cookers.[49]
The simplest type of solar cooker is the box cooker first built by
Horace de Saussure in 1767.[50]
A basic box cooker consists of an insulated container with a transparent lid.
These cookers can be used effectively with partially overcast skies and will
typically reach temperatures of 90–150 °C.[51]
Panel cookers use a reflective panel to direct sunlight onto an insulated
container and reach temperatures comparable to box cookers. Reflector cookers
use various concentrating geometries (dish, trough, Fresnel mirrors) to focus
light on a cooking container. These cookers reach temperatures of 315 °C and
above but require direct light to function properly and must be repositioned
to track the Sun.[52]
The solar bowl is a concentrating technology employed by the Solar Kitchen
in
Auroville, India, in which a stationary spherical reflector focuses light
along a line perpendicular to the sphere's interior surface and a computer
control system moves the receiver to intersect this line. Steam is produced in
the receiver at temperatures reaching 150 °C and then used for process heat in
the kitchen.[53]
A reflector developed by
Wolfgang Scheffler in 1986 is used in many solar kitchens. Scheffler
reflectors are flexible parabolic dishes that combine aspects of trough and
power tower concentrators.
Polar tracking is used to follow the Sun's daily course and the curvature
of the reflector is adjusted for seasonal variations in the incident angle of
sunlight. These reflectors can reach temperatures of 450–650 °C and have a
fixed focal point which improves the ease of cooking.[54]
The world's largest Scheffler reflector system in Abu Road,
Rajasthan,
India is capable of cooking up to 35,000 meals a day.[55]
As of 2008, over 2,000 large Scheffler cookers had been built worldwide.[56]
Process heat
-
.png/200px-7_Meter_Sheet_Metal_Dishes_(Flipped).png)
STEP parabolic dishes used for steam production and electrical
generation
Solar concentrating technologies such as parabolic dish, trough and
Scheffler reflectors can provide process heat for commercial and industrial
applications. The first commercial system was the Solar Total Energy Project
(STEP) in Shenandoah, Georgia where a field of 114 parabolic dishes provided
50% of the process heating, air conditioning and electrical requirements for a
clothing factory. This grid-connected cogeneration system provided 400 kW of
electricity plus thermal energy in the form of 401 kW steam and 468 kW chilled
water, and had a one hour peak load thermal storage.[57]
Evaporation ponds are shallow pools that concentrate dissolved solids
through
evaporation. The use of evaporation ponds to obtain salt from sea water is
one of the oldest applications of solar energy. Modern uses include
concentrating brine solutions used in leach mining and removing dissolved
solids from waste streams.[58]
Clothes lines,
clotheshorses, and clothes racks dry clothes through evaporation. These
devices use wind and sunlight instead of electricity or natural gas.
Florida
legislation specifically protects the 'right to dry' and similar solar rights
legislation has been passed in
Utah and
Hawaii.[59]
Unglazed transpired collectors (UTC) are perforated sun-facing walls used
for preheating ventilation air. UTCs can raise the incoming air temperature up
to 22 °C and deliver outlet temperatures of 45–60 °C.[60]
The short payback period of transpired collectors (3 to 12 years) makes them a
more cost-effective alternative than glazed collection systems.[60]
As of 2003, over 80 systems with a combined collector area of 35,000 m˛
had been installed worldwide, including an 860 m˛ collector in
Costa
Rica used for drying coffee beans and a 1,300 m˛ collector in
Coimbatore,
India used for drying marigolds.[23]
Solar electricity
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.[61]
Photovoltaics
-
Main article:
Photovoltaics
11 MW Serpa solar power plant in Portugal
A
solar cell (or photovoltaic cell) 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 fundamental 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 the satellite 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 satellites.[67]
Photovoltaics went on to play an essential part in the success of early
commercial satellites such as
Telstar and
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 and other
off-grid applications have proven very successful and accounted for over
half of worldwide installed capacity until 2004.[36]
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
also seen rapid growth recently due to various incentive programs and local
market conditions.[78]
Concentrating solar power
-
Solar troughs are the most widely deployed and cost-effective CSP
technology.
Concentrated sunlight has been used to perform useful tasks since the time
of
ancient China. A legend claims
Archimedes used polished shields to concentrate sunlight on the invading
Roman fleet and repel them from
Syracuse.[79]
In 1866, Auguste Mouchout used a parabolic trough to produce steam for the
first solar steam engine, 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 are the most mature CSP technology.[82]
The
SEGS plants in California and Acciona's
Nevada Solar One near
Boulder City, Nevada are representatives of this technology.[82][83]
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.[84]
The 50 kW Big Dish in
Canberra,
Australia is an example of this technology.[82]
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.[82]
The
Solar Two in Barstow, California and the
Planta Solar 10 in
Sanlucar la Mayor, Spain are representatives of this technology.[82][85]
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.[86]
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.[87]
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,[88]
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.[89]
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%.[90]
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.[91]
Solar chemical
-
Main article:
Solar chemical
Solar radiation stimulated chemical processes use solar energy to drive
chemical reactions. These processes offset energy that would otherwise be
required from an alternate source and can convert solar energy into a storable
and transportable fuel. Solar induced chemical reactions are diverse, but can
be divided into thermochemical or
photochemical.
Hydrogen production technologies involving the use of solar light have
been a significant area of research since the 1970s. Aside from electrolysis
driven by photovoltaic or photochemical cells, several thermochemical
processes have also been explored. One such route uses concentrators to split
water at high temperatures (2300-2600 °C), but this process has been limited
by complexity and low solar-to-hydrogen efficiency (1–2%).[92]
Another approach uses the heat from solar concentrators to drive the
steam reformation of natural gas thereby increasing the overall hydrogen
yield.
Thermochemical cycles characterized by the decomposition and regeneration
of reactants present another avenue for hydrogen production. The Solzinc
process under development at the
Weizmann Institute uses a 1 MW solar furnace to decompose
zinc
oxide (ZnO) at temperatures above 1200 °C. This initial reaction produces
pure zinc, which can subsequently be reacted with water to produce hydrogen.[93]
Sandia's Sunshine to Petrol (S2P) technology uses the high temperatures
generated by concentrating sunlight along with a
zirconia/ferrite
catalyst to break down atmospheric carbon dioxide into oxygen and
carbon monoxide (CO). The CO may then be used to synthesize methanol,
gasoline and jet fuel.[94]
Photoelectrochemical cells or PECs consist of a semiconductor, typically
titanium dioxide or related titanates, immersed in an electrolyte. When the
semiconductor is illuminated an electrical potential develops. There are two
types of photoelectrochemical cells: photoelectric cells that convert light
into electricity and photochemical cells that use light to drive chemical
reactions such as
electrolysis.[95]
A photogalvanic device is a type of battery in which the cell solution (or
equivalent) forms energy-rich chemical intermediates when illuminated. These
chemical intermediates then react at the electrodes to produce an electric
potential. The ferric-thionine chemical cell is an example of this technology.[96]
Solar vehicles
-
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, 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)
and by 2007 the winner's average speed had improved to 90.87 kilometres per
hour (56.46 mph).[97]
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.[98][99]
In 1975, the first practical solar boat was constructed in
England.[100]
By 1995, passenger boats incorporating PV panels began appearing and are now
used extensively.[101]
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.[102]
Plans to circumnavigate the globe in 2009 are indicative of the progress solar
boats have made.
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 (UAV)
with the
Pathfinder (1997) and subsequent designs, culminating in the
Helios which set the altitude record for a non-rocket-propelled
aircraft at 29,524 metres (96,860 ft) in 2001.[103]
The
Zephyr, developed by
BAE
Systems, is the latest in a line of record-breaking solar aircraft, making
a 54-hour flight in 2007, and month-long flights are envisioned by 2010.[104]
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 relatively high.[citation
needed]
Solar
sails are a proposed form of spacecraft propulsion using large membrane
mirrors to exploit radiation pressure from the sun. Unlike rockets, solar
sails require no fuel. Although the thrust is small compared to rockets, it
continues as long as the Sun shines onto the deployed sail and in the
frictionless vacuum of space significant speeds can eventually be achieved.[105]
The
High-altitude airship (HAA) is an unmanned, long-duration,
lighter-than-air vehicle using
helium gas
for lift, and thin-film solar cells for power. The
United States Department of Defense Missile Defense Agency has contracted
Lockheed Martin to construct it to enhance its
Ballistic Missile Defense System (BMDS).[106]
Airships have some advantages for solar-powered flight: they do not require
power to remain aloft, and an airship's envelope presents a large area to the
Sun.
Energy storage methods
-
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.[107]
Solar energy is not available at night, and the performance of solar power
systems is affected by unpredictable weather patterns; therefore, storage
media 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[108][109]
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.[110]
Solar energy can be stored at high temperatures using molten salts. Salts
are an effective storage medium because they are low-cost, have a high
specific heat capacity and can deliver heat at temperatures compatible with
conventional power systems. The
Solar Two used this method of energy storage, allowing it to store 1.44 TJ
in its 68 mł
storage tank with an annual storage efficiency of about 99%.[111]
Off-grid PV systems have traditionally used
rechargeable batteries to store excess electricity. With grid-tied
systems, excess electricity can be sent to the transmission
grid.
Net
metering programs give these systems a credit for the electricity they
deliver to the grid. This credit offsets electricity provided from the grid
when the system cannot meet demand, effectively using the grid as a storage
mechanism.[112]
Pumped-storage hydroelectricity stores energy in the form of water pumped
when energy is available from a lower elevation reservoir to a higher
elevation one. The energy is recovered when demand is high by releasing the
water to run through a hydroelectric power generator.[113]
Development, deployment and economics
-
Beginning with the surge in
coal use which
accompanied the
Industrial Revolution, energy consumption has steadily transitioned from
wood and biomass to
fossil
fuels. 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.[114]
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.[citation
needed] 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 US (SERI, now
NREL), Japan (NEDO),
and
Germany (Fraunhofer
Institute for Solar Energy Systems ISE).[115]
Between 1970 and 1983, photovoltaic installations grew rapidly, but falling
oil prices in the early 1980s moderated the growth of PV from 1984 to 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. Photovoltaic production growth has averaged 40% per
year since 2000 and installed capacity reached 10.6 GW at the end of 2007.[36]
Since 2006 it has been economical for investors to install photovoltaics for
free in return for a long term
power purchase agreement. 50% of commercial systems were installed in this
manner in 2007 and it is expected that 90% will by 2009.[116]
Nellis Air Force Base is receiving photoelectric power for about 2.2 ˘/kWh
and grid power for 9 ˘/kWh.[117][118]
Commercial solar water heaters began appearing in the United States in the
1890s.[119]
These systems saw increasing use until the 1920s but were gradually replaced
by cheaper and more reliable heating fuels.[120]
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.[35]
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.[35]
Commercial concentrating solar power (CSP) plants were first developed in
the 1980s. CSP plants such as
SEGS project in the United States have a LEC of 12–14 ˘/kWh.[121]
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.[122]
.png)
)
Balance
of system components
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