Nellis Solar Power Plant in the United States,
one of the largest photovoltaicpower
plants in North
America.
Solar
energy,
radiant light and heat from the sun, has been harnessed by humans since ancient times using
a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered
resources such as wind and wave power, hydroelectricity and biomass, account for
most of the available renewable energy on
earth. Only a minuscule fraction of the available solar energy is
used.
Solar powered electrical
generation relies on heat
engines and photovoltaics. Solar
energy's uses are limited only by human ingenuity. A partial list of solar
applications includes space heating and cooling through solar
architecture, potable water via distillation and disinfection,daylighting, solar hot water, solar cooking, and
high temperature process heat for industrial purposes.To harvest the solar
energy, the most common way is to use solar panels.
Solar
technologies are broadly characterized as either passive solar or active solar depending
on the way they capture, convert and distribute solar energy. Active solar
techniques include the use of photovoltaic panels and solar
thermal collectors to harness the energy. Passive solar
techniques include orienting a building to the Sun, selecting materials with
favorable thermal
mass or light dispersing properties, and designing spaces
that naturally circulate air.
Energy from the Sun
Main
articles: Insolation and Solar radiation
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.[1] 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.[2]
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.[3] Sunlight
absorbed by the oceans and land masses keeps the surface at an average temperature
of 14 °C.[4] By photosynthesis green
plants convert solar energy into chemical energy,
which produces food, wood and the biomass from
which fossil fuels are derived.[5]
Yearly Solar fluxes & Human Energy Consumption
|
|
Solar
|
|
Wind
|
2,250 EJ[7]
|
Biomass
|
3,000 EJ[8]
|
Primary energy use
(2005)
|
487 EJ[9]
|
Electricity (2005)
|
56.7 EJ[10]
|
The
total solar energy absorbed by Earth's atmosphere, oceans and land masses is
approximately 3,850,000 exajoules (EJ)
per year.[6] In
2002, this was more energy in one hour than the world used in one year.[11][12] Photosynthesis
captures approximately 3,000 EJ per year in biomass.[8] 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 all of the
Earth's non-renewable resources of coal, oil, natural gas, and mined uranium
combined.[13]
Solar
energy can be harnessed in different levels around the world. Depending on a
geographical location the closer to the equator the more "potential"
solar energy is available.[14]
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.[15]
Architecture
and urban planning
Main
articles: Passive solar building design and Urban heat island
Darmstadt University of Technology inGermany won the
2007 Solar
Decathlon inWashington, D.C. with
this passive
housedesigned specifically for the humid and hot subtropical
climate.[16]
Sunlight
has influenced building design since the beginning of architectural history.[17] 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.[18]
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.[17] 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.[17] The
most recent approaches to solar design use computer modeling tying
together solar
lighting, heating and ventilation systems in an integratedsolar design package.[19] Active solar equipment
such as pumps, fans and switchable windows can complement passive design and
improve system performance.
Urban
heat islands (UHI) are metropolitan areas with higher temperatures than that of
the surrounding environment. The higher temperatures are a result of increased
absorption of the Solar light by urban materials such as asphalt and concrete,
which have lower albedos and
higherheat capacities than
those in 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.[20]
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.[21][22]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.[23] Applications
of solar energy in agriculture aside from growing crops include pumping water,
drying crops, brooding chicks and drying chicken manure.[24][25] More
recently the technology has been embraced by vinters, who use the energy
generated by solar panels to power grape presses.[26]
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.[27] The
first modern greenhouses were built in Europe in the 16th century to keep exotic
plants brought back from explorations abroad.[28] Greenhouses
remain an important part of horticulture today, and plastic transparent
materials have also been used to similar effect in polytunnels and row covers.
Solar
lighting
Daylighting features such
as this oculusat the top of
the Pantheon,
in Rome, Italy 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.[29][30] In
the 20th century artificial lighting became
the main source of interior illumination but daylighting techniques and hybrid
solar lighting solutions are ways to reduce energy consumption.
Daylighting systems
collect and distribute sunlight to provide interior illumination. This passive
technology directly offsets energy use by replacing artificial lighting, and
indirectly offsets non-solar energy use by reducing the need for air-conditioning.[31] Although
difficult to quantify, the use of natural
lighting also offers physiological and psychological benefits
compared to artificial
lighting.[31] Daylighting
design implies careful selection of window types, sizes and orientation;
exterior shading devices may be considered as well. Individual features include
sawtooth roofs, clerestory
windows, light shelves, skylights and light tubes. They
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%.[32]
Hybrid solar lighting is an active solar method
of providing interior illumination. HSL systems collect sunlight using focusing
mirrors that track
the Sun and use optical fibers to
transmit it inside the building to supplement conventional lighting. In
single-story applications these systems are able to transmit 50% of the direct
sunlight received.[33]
Solar
lights that charge during the day and light up at dusk are a common sight along
walkways.[34] Solar-charged
lanterns have become popular in developing countries where they provide a safer
and cheaper alternative to kerosene lamps.[35]
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.[36]
Solar
thermal
Main
article: Solar
thermal energy
Solar
thermal technologies can be used for water heating, space heating, space
cooling and process heat generation.[37]
Water
heating
Main
articles: Solar
hot water and Solar combisystem
Solar water heaters facing
the Sun to maximize
gain.
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.[38] 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.[39]
As
of 2007, the total installed capacity of solar hot water systems is
approximately 154 GW.[40] 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.[41] Israel
and Cyprus are the
per capita leaders in the use of solar hot water systems with over 90% of homes
using them.[42] 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.[15]
Heating,
cooling and ventilation
Main
articles: Solar
heating, Thermal
mass, Solar
chimney, and Solar air conditioning
Solar House #1 of Massachusetts Institute of Technology in
the United States, built in 1939, used seasonal thermal storagefor 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.[32][43] 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.[44]
A
solar chimney (or thermal chimney, in this context) 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[45] in a
way that mimics greenhouses.
Deciduous trees
and plants have been promoted as a means of controlling solar heating and
cooling. When planted on the southern side of a building, their leaves provide
shade during the summer, while the bare limbs allow light to pass during the
winter.[46] 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.[47] 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. They can, however, be used on the east and west sides to
provide a degree of summer shading without appreciably affecting winter solar
gain.[48]
Water
treatment
Small scale solar powered
sewerage treatment plant.
Solar
distillation can be used to make saline or brackish water potable.
The first recorded instance of this was by 16th century Arab alchemists.[49] A
large-scale solar distillation project was first constructed in 1872 in
the Chilean mining
town of Las Salinas.[50] 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.[50] Individual still designs include single-slope,
double-slope (or greenhouse type), vertical, conical, inverted absorber,
multi-wick, and multiple effect.[49] 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.[49]
Solar
water disinfection (SODIS)
involves exposing water-filled plastic polyethylene terephthalate (PET) bottles to
sunlight for several hours.[51]Exposure
times vary depending on weather and climate from a minimum of six hours to two
days during fully overcast conditions.[52] It
is recommended by the World Health Organization as a viable method for
household water treatment and safe storage.[53] Over
two million people in developing countries use this method for their daily
drinking water.[52]
Solar
energy may be used in a water stabilisation pond to treat waste water without
chemicals or electricity. A further environmental advantage is that algae grow in such ponds and
consume carbon
dioxide in photosynthesis, although algae may produce toxic
chemicals that make the water unusable.[54][55]
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. They
can be grouped into three broad categories: box cookers, panel cookers and
reflector cookers.[56] The
simplest solar cooker is the box cooker first built by Horace
de Saussure in 1767.[57] A
basic box cooker consists of an insulated container with a transparent lid. It
can be used effectively with partially overcast skies and will typically reach
temperatures of 90–150 °C.[58] 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.[59]
The solar bowl is a
concentrating technology employed by the Solar Kitchen in Auroville, Pondicherry, India, where 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.[60]
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 simplifies cooking.[61] The
world's largest Scheffler reflector system in Abu Road, Rajasthan, India is
capable of cooking up to 35,000 meals a day.[62] As
of 2008, over 2,000 large Scheffler cookers had been built worldwide.[63]
Process
heat
Main
articles: Solar
pond, Salt evaporation pond, and Solar furnace
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, USA 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.[64]
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.[65]
Clothes lines, clotheshorses, and
clothes racks dry clothes through evaporation by wind and sunlight without
consuming electricity or gas. In some states of the United States legislation
protects the "right to dry" clothes.[66]
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.[67] The
short payback period of transpired collectors (3 to 12 years) makes them a
more cost-effective alternative than glazed collection systems.[67] As
of 2003, over 80 systems with a combined collector area of 35,000 m2 had
been installed worldwide, including an 860 m2 collector
in Costa Rica used
for drying coffee beans and a 1,300 m2 collector in Coimbatore, India
used for drying marigolds.[25]
Solar
power
Main
article: Solar
power
The PS10 concentrates sunlight from a field of
heliostats on a central tower.
Solar
power is the conversion of sunlight into electricity, either
directly using photovoltaics (PV),
or indirectly using concentrated solar power(CSP). CSP systems use lenses
or mirrors and tracking systems to focus a large area of sunlight into a small
beam. PV converts light into electric current using the photoelectric
effect.
Commercial
CSP plants were first developed in the 1980s, and the 354 MW SEGS CSP installation is the largest
solar power plant in the world and is located in the Mojave Desert of
California. Other large CSP plants include the Solnova Solar Power Station (150 MW) and
the Andasol solar power station (100 MW), both in
Spain. The 97 MW Sarnia Photovoltaic Power Plant in Canada, is the world’s largest photovoltaic
plant.
Concentrated
solar power
See
also: Concentrated solar power
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 heat is then used as
a heat source for a conventional power plant. A wide range of concentrating
technologies exists; the most developed are the parabolic trough, the
concentrating linear fresnel reflector, the Stirling dish and the solar power
tower. Various techniques are used to track the Sun and focus light. In all of
these systems a working
fluid is heated by the concentrated sunlight, and is then used
for power generation or energy storage.[68]
Photovoltaics
Main
article: Photovoltaics
The 71.8 MW Lieberose Photovoltaic Parkin Germany.
A solar cell, or
photovoltaic cell (PV), is a device that converts light into electric current
using the photoelectric
effect. The first solar cell was constructed by Charles Fritts in
the 1880s.[69] In
1931 a German engineer, Dr Bruno Lange, developed a photo cell using silver
selenide in place of copper oxide.[70] 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.[71] Following
the work of Russell
Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and
Daryl Chapin created the silicon solar
cell in 1954.[72] These
early solar cells cost 286 USD/watt and reached efficiencies of 4.5–6%.[73]
Solar
chemical
Main
article: Solar
chemical
Solar
chemical processes use solar energy to drive chemical reactions. These
processes offset energy that would otherwise come from a fossil fuel source and
can also convert solar energy into storable and transportable fuels. Solar
induced chemical reactions can be divided into thermochemical or photochemical.[74] A
variety of fuels can be produced by artificial photosynthesis.[75] The
multielectron catalytic chemistry involved in making carbon-based fuels (such
as methanol) from
reduction of carbon
dioxide is challenging; a feasible alternative is hydrogen production
from protons, though use of water as the source of electrons (as plants do)
requires mastering the multielectron oxidation of two water molecules to molecular
oxygen.[76] Some
have envisaged working solar fuel plants in coastal metropolitan areas by 2050-
the splitting of sea water providing hydrogen to be run through adjacent fuel-cell
electric power plants and the pure water by-product going directly into the
municipal water system.[77]
Hydrogen
production technologies been a significant area of solar
chemical 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 into oxygen and
hydrogen at high temperatures (2300-2600 °C).[78] Another
approach uses the heat from solar concentrators to drive the steam reformation of
natural gas thereby increasing the overall hydrogen yield compared to
conventional reforming methods.[79] 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.[80]
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 carbon monoxide can then be used to synthesize conventional fuels such as
methanol, gasoline and jet fuel.[81]
A
photogalvanic device is a type of battery in which the cell solution (or
equivalent) forms energy-rich chemical intermediates when illuminated. These
energy-rich intermediates can potentially be stored and subsequently reacted at
the electrodes to produce an electric potential. The ferric-thionine chemical
cell is an example of this technology.[82]
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.[83]
A
combination thermal/photochemical cell has also been proposed. The Stanford
PETE process uses solar thermal energy to raise the temperature of a thermionic
metal to about 800C to increase the rate of production of electricity to
electrolyse atmospheric CO2 down to carbon or carbon monoxide which can then be
used for fuel production, and the waste heat can be used as well.[84]
Solar
vehicles
Main
articles: Solar
vehicle, Solar-charged vehicle, Electric boat,
and Solar
balloon
Australia hosts the World Solar Challengewhere 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 toAdelaide. 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).[85] 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.[86][87]
Some
vehicles use solar panels for auxiliary power, such as for air conditioning, to
keep the interior cool, thus reducing fuel consumption.[88][89]
In
1975, the first practical solar boat was constructed in England.[90] By
1995, passenger boats incorporating PV panels began appearing and are now used
extensively.[91] 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.[92] There
are plans to circumnavigate the globe in 2010.[93]
Helios UAV in solar powered
flight.
In
1974, the unmanned AstroFlight
Sunrise plane made the first solar flight. On 29 April 1979,
the Solar
Riser made the first flight in a solar powered, fully
controlled, man carrying flying machine, reaching an altitude of 40 feet
(12 m). In 1980, the Gossamer
Penguin made the first piloted flights powered solely by
photovoltaics. This was quickly followed by the Solar Challenger which
crossed the English Channel in July 1981. In 1990 Eric
Scott Raymond in 21 hops flew from California to North Carolina
using solar power.[94] 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
(96,864 ft) in 2001.[95] 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.[96]
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 generally
limited to the toy market as the surface-area to payload-weight ratio is
relatively high.[97]
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 vacuum of space significant
speeds can eventually be achieved.[98]
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 DefenseMissile
Defense Agency has contracted Lockheed Martin to
construct it to enhance the Ballistic Missile Defense System (BMDS).[99] 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
Main
articles: Thermal
mass, Thermal energy storage, Phase change material, Grid
energy storage, and V2G
Solar Two's thermal storage
system generated electricity during cloudy weather and at night.
Solar
energy is not available at night, and energy storage is an important issue
because modern energy systems usually assume continuous availability of energy.[100]
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.[101][102]
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.[103]
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 m3 storage
tank with an annual storage efficiency of about 99%.[104]
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, while standard grid electricity can be used to
meet shortfalls. Net
metering programs give household systems a credit for any
electricity they deliver to the grid. This is often legally handled by 'rolling
back' the meter whenever the home produces more electricity than it consumes.
If the net electricity use is below zero, the utility is required to pay for
the extra at the same rate as they charge consumers.[105] Other
legal approaches involve the use of two meters, to measure electricity consumed
vs. electricity produced. This is less common due to the increased installation
cost of the second meter.
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.[106]
Development, deployment and economics
Main
article: Deployment of solar power to energy grids
See
also: Cost of electricity by source
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. However development of solar
technologies stagnated in the early 20th century in the face of the
increasing availability, economy, and utility of coal and petroleum.[107]
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.[108][109]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).[110]
Commercial
solar water heaters began appearing in the United States in the 1890s.[111] These
systems saw increasing use until the 1920s but were gradually replaced by
cheaper and more reliable heating fuels.[112] 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.[40] Although
generally underestimated, solar water heating and cooling is by far the most
widely deployed solar technology with an estimated capacity of 154 GW as
of 2007.[40]
ISO Standards
The International Organization for Standardization has
established a number of standards relating to solar energy equipment. For
example, ISO 9050 relates to glass in building while ISO 10217 relates to the
materials used in solar water heaters.
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