From Wikipedia, the free encyclopedia
At the equator, the Sun provides approximately 1000
watts per square meter on Earth's surface.
Solar power is the technology of obtaining usable
energy from the
light
of the
Sun. Solar energy has been used in many traditional
technologies for centuries and has come into widespread use
where other power supplies are absent, such as in remote
locations and in
space.
Solar energy is currently used in a number of applications:
Energy from the Sun
Map of global solar energy resources. The colors show the
average available solar energy on the surface during 1991
to 1993. The scale is in
watts per
square meter. For comparison, the dark disks represent
the land area required to supply the primary energy demand
in the year 2010, using currently available technology.
Solar radiation reaches the Earth's upper
atmosphere at a rate of
1366
watts
per
square meter (W/m2).[1]
The first map shows how the solar energy varies in different
latitudes.
While traveling through the atmosphere 6% of the incoming
solar radiation (insolation)
is
reflected and 16% is
absorbed resulting in a peak
irradiance at the
equator of 1,020 W/m².[2]
Average atmospheric conditions (clouds,
dust,
pollutants) further reduce insolation by 20% through
reflection and 3% through absorption.[3]
Atmospheric conditions not only reduce the quantity of
insolation reaching the earth's surface but also affect the
quality of insolation by
diffusing incoming light and altering its
spectrum.
The second map shows the average global irradiance calculated
from satellite data collected from 1991 to 1993. For example, in
North America the average insolation at ground level over an
entire year (including nights and periods of cloudy weather)
lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).[4]
This represents the available power, and not the delivered
power. At present,
photovoltaic panels typically convert about 15% of incident
sunlight into electricity; therefore, a solar panel in the
contiguous United States on average delivers 19 to 56 W/m² or
0.45 - 1.35 (kW·h/m²)/day.[5]
The dark disks in the third map on the right are an example
of the land areas that, if covered with 8% efficient solar
panels, would produce slightly more energy in the form of
electricity than the total world primary energy supply in 2003.[6]
While average insolation and power offer insight into solar
power's potential on a regional scale, locally relevant
conditions are also important to the potential of a specific
site.
After passing through the Earth's atmosphere, most of the
sun's energy is in the form of
visible and
Infrared radiations. Plants use solar energy to create
chemical energy through
photosynthesis. Humans regularly use this energy burning
wood or
fossil fuels, or when simply
eating the plants.
A recent concern is
global dimming, an effect of pollution that is allowing less
sunlight to reach the Earth's surface. It is intricately linked
with pollution particles and
global warming, and it is mostly of concern for issues of
global climate change, but is also of concern to proponents
of solar power because of the existing and potential future
decreases in available solar energy. The order of magnitude is
about 4% less solar energy available at sea level over the
timeframe 1961–90, mostly from increased reflection from clouds
back into outer space.[7]
Types of technologies
Many technologies have been developed to make use of solar
radiation. Some of these technologies make direct use of the
solar energy (e.g. to provide light, heat, etc.), while others
produce electricity.
Solar design in architecture
-
Solar design in architecture involves the use of appropriate
solar technologies to maintain a building’s environment at a
comfortable temperature through the sun's daily and annual
cycles. It may do this by storing solar energy as heat in the
walls of a building, which then acts to heat the building at
night. Another approach is to keep the interior cool during a
hot day by designing in natural convection through the
building’s interior.
Solar heating systems
-
Solar hot water systems use sunlight to heat water. They may
be used to heat domestic hot water or for space heating. These
systems are basically composed of solar thermal collectors and a
storage tank.[8]
The three basic classifications of solar water heaters are:
- Active systems which use pumps to circulate water or a
heat transfer fluid.
- Passive systems which circulate water or a heat transfer
fluid by
natural circulation. These are also called
thermosiphon systems.
- Batch systems using a tank directly heated by sunlight.
A
Trombe wall is a passive solar heating and ventilation
system consisting of an air channel sandwiched between a window
and a sun-facing wall. Sunlight heats the air space during the
day causing
natural circulation through vents at the top and bottom of
the wall and storing heat in the
thermal mass. During the evening the Trombe wall radiates
stored heat.[9]
A transpired collector is an active solar heating and
ventilation system consisting of a perforated sun-facing wall
which acts as a
solar thermal collector. The collector pre-heats air as it
is drawn into the building's ventilation system through the
perforations. These systems are inexpensive and commercial
models have achieved efficiencies above 70%. Most systems pay
for themselves within 4-8 years.[10]
Solar cooking
-
Solar Cookers use sunshine as an alternative to fire for
cooking.
A
solar box cooker traps the sun's energy in an insulated box;
such boxes have been successfully used for cooking,
pasteurization and fruit canning. Solar cooking is helping
many developing countries, both reducing the demands for local
firewood and maintaining a cleaner breathing environment for the
cooks.
The first known western solar oven is attributed to
Horace de Saussure in 1767, which impressed Sir John
Herschel enough to build one for cooking meals on his
astronomical expedition to the Cape of Good Hope in Africa in
1830.
[11] Today, there are many
different designs in use around the world.[12]
Solar lighting
-
Solar lighting or
daylighting is the use of natural light to provide
illumination. Daylighting offsets energy use in electric
lighting systems and reduces the cooling load on
HVAC
systems (this assumes that daylighting is replacing
incandescent lighting, which produces more heat than light).
The use of natural light also offers physiological and
psychological benefits, although this is difficult to quantify.
Daylighting features include building orientation,
window orientation, exterior shading, sawtooth roofs,
clerestory windows, light shelves,
skylights and
light tubes.[13]
These features may be incorporated in existing structures but
are most effective when integrated in a
solar design package which accounts for factors such as
glare, heat gain, heat loss and time-of-use. Architectural
trends increasingly recognize daylighting as a cornerstone of
sustainable design.
Daylight saving time (DST) can be seen as a method of
utilising solar energy by matching available sunlight to the
hours of the day in which it is most useful. DST energy savings
have been estimated to reduce total electricity use in
California by 0.5% (3400 MW·h) and peak electricity use by
3% (1000 MW).[14]
Photovoltaics
-
The solar panels (photovoltaic arrays) on this small yacht
at sea can charge the 12 V batteries at up to 9 A in full,
direct sunlight
Solar cells, also referred to as photovoltaic cells, are
devices or banks of devices that use the
photovoltaic effect of
semiconductors to generate electricity directly from
sunlight. Until recently, their use has been limited because of
high manufacturing costs. One cost effective use has been in
very low-power devices such as
calculators with
LCDs. Another use has been in remote applications such as
roadside emergency telephones, remote sensing,
cathodic protection of pipe lines, and limited "off grid"
home power applications. A third use has been in powering
orbiting
satellites and
spacecraft.
Total peak power of installed PV is around 1,700 MW as of the
end of 2005.[15]
This is only one part of solar-generated electric power.
Declining manufacturing costs (dropping at 3 to 5% a year in
recent years) are expanding the range of cost-effective uses.
The average lowest retail cost of a large
photovoltaic array declined from $7.50 to $4 per watt
between 1990 and
2005.[16]
With many jurisdictions now giving tax and rebate incentives,
solar electric power can now pay for itself in five to ten years
in many places. "Grid-connected" systems - those systems that
use an
inverter to connect to the
utility grid instead of relying on batteries - now make up
the largest part of the market.
In 2003, worldwide production of solar cells increased by
32%.[17]
Between 2000 and 2004, the increase in worldwide solar energy
capacity was an annualized 60%.[18]
2005 was expected to see large growth again, but shortages of
refined
silicon have been hampering production worldwide since late
2004.[19]
Analysts have predicted similar supply problems for 2006 and
2007.[20]
Solar thermal electric power
plants
Solar Two, a concentrating solar
power tower (an example of
solar thermal energy
applied to electrical power production).
-
Solar thermal energy can be focused on a
heat exchanger, and converted in a
heat engine to produce
electric power or applied to other industrial processes.
Power towers
-
Power towers use an array of flat, movable mirrors (called
heliostats) to focus the sun's rays upon a collector tower (the
target). The high energy at this point of concentrated sunlight
is transferred to a working fluid for conversion to electrical
energy in a heat engine, or in some instances, stored for
nighttime usage, in order to provide a more continuous output.
Parabolic troughs
-
A long row of parabolic mirrors concentrates sunlight on a
tube filled with a
heat transfer fluid (usually oil). As with the power tower,
this heated oil is used to power a conventional steam turbine,
or stored for nighttime use. The largest operating solar power
plant, as of 2007, is one of the
SEGS
parabolic trough systems in the
Mojave Desert in California, USA.
Concentrating collector with steam
engine
Solar energy converted to heat in a concentrating collector
can be used to boil water into steam (as is done in nuclear and
coal power plants) to drive a
steam engine or
steam turbine. The concentrating collector can be a trough
collector, parabolic collector, or power tower.
Concentrating collector with
Stirling engine
A parabolic solar collector concentrating the sun's rays
on the heating element of a
Stirling engine. The entire unit acts as a
solar tracker.
Solar energy converted to heat in a concentrating (dish or
trough parabolic) collector can be used to drive a
Stirling engine, a type of heat engine which uses a sealed
working gas (i.e. a closed cycle) and does not require a water
supply.
Until recently, a solar Stirling system held the record for
converting solar energy into electricity (30% at 1,000 watts per
square meter).[21]
Such concentrating systems produce little or no power in
overcast conditions and incorporate a
solar tracker to point the device directly at the sun. That
record has been broken by a so-called concentrator solar cell
produced by Boeing-Spectrolab which claims a conversion
efficiency of 40.7 percent.[22]
Solar updraft tower
-
A
solar updraft tower (also known as a solar chimney, but this
term is avoided by many proponents due to its association with
fossil fuels) is a relatively low-tech solar thermal power plant
where air passes under a very large agricultural glass house
(between 2 and 8 km in diameter), is heated by the sun and
channeled upwards towards a convection tower. It then rises
naturally and is used to drive turbines, which generate
electricity.
Energy tower
An
energy tower is an alternative proposal to the solar updraft
tower. It is driven by spraying water at the top of the tower,
evaporation of water causes a downdraft by cooling the air
thereby increasing its density, driving wind turbines at the
bottom of the tower. It requires a hot arid climate and large
quantities of water (seawater may be used) but does not require
the large glass house of the solar updraft tower.
Solar pond
A
solar pond is simply a pool of water which collects and
stores solar energy. It contains layers of salt solutions with
increasing concentration (and therefore density) to a certain
depth, below which the solution has a uniform high salt
concentration. It is a relatively low-tech, low-cost approach to
harvesting solar energy. The principle is to fill a pond with 3
layers of water:
- A top layer with a low salt content.
- An intermediate insulating layer with a salt gradient,
which sets up a
density gradient that prevents heat exchange by natural
convection in the water.
- A bottom layer with a
high salt content which reaches a temperature approaching
90 degrees Celsius.
The layers have different densities due to their different
salt content, and this prevents the development of
convection currents which would otherwise transfer the heat
to the surface and then to the air above. The heat trapped in
the salty bottom layer can be used for heating of buildings,
industrial processes, generating electricity or other purposes.
One such system is in use at
Bhuj,
Gujarat,
India[23]
and another at the
University of Texas El Paso.[24]
Solar chemical
Solar chemical is any process that harnesses solar energy by
absorbing sunlight in a
chemical reaction in a way similar to
photosynthesis in plants but without using living organisms.
No practical process has yet emerged.
A promising approach is to use focused sunlight to provide
the energy needed to split water into its constituent
hydrogen and
oxygen in the presence of a metallic catalyst such as
zinc.[25]
While metals, such as zinc, have been shown to drive
photoelectrolysis of water, more research has focused on
semiconductors. Further research has examined
transition metal compounds, in particular
titanium,
niobium and
tantalum oxides.[26]
Unfortunately, these materials exhibit very low efficiencies,
because they require
ultraviolet light to drive the
photoelectrolysis of water. Current materials also require
an electrical voltage bias for the hydrogen and oxygen gas to
evolve from the surface, another disadvantage. Current research
is focusing on the development of materials capable of the same
water splitting reaction using lower energy visible light.
It is also possible to use solar energy to drive industrial
chemical processes without a requirement for fossil fuel.
Biofuels
-
The oil in plant
seeds,
in chemical terms, very closely resembles that of petroleum.
Many, since the invention of the Diesel engine, have been using
this form of captured solar energy as a fuel comparable to
petrodiesel—for functional use in any diesel engine or generator
and known as
biodiesel.
A 1998 joint study by the U.S. Department of Energy (DOE) and
the U.S. Department of Agriculture (USDA) traced many of the
various costs involved in the production of biodiesel and found
that overall, it yields 3.2 units of fuel product energy for
every unit of fossil fuel energy consumed.[27]
Other biofuels include
ethanol, wood for stoves, ovens and furnaces, and methane
gas produced from biofuels through chemical processes.
Classifications of solar power
technology
Solar power technologies can be classified in a number of
ways.
Photovoltaic cells produce electricity directly from
sunlight
Direct or Indirect
In general, direct solar power involves a single
transformation of sunlight which results in a usable form of
energy.
- Sunlight hits a
photovoltaic cell creating electricity.
- Sunlight warms a
thermal mass.
- Sunlight strikes a
solar sail on a space craft and is converted directly into
a force on the sail which causes motion of the craft.
- Sunlight strikes a
light mill and causes the vanes to rotate as mechanical
energy, little practical application has yet been found for
this effect.
- In a direct solar water heater the water heated in the
collector is used in the domestic water system.
In general, indirect solar power involves multiple
transformations of sunlight which result in a usable form of
energy.
-
Vegetation uses
photosynthesis to convert solar energy to
chemical energy. The resulting
biomass may be burned directly to produce heat and
electricity or processed into
ethanol,
methane,
hydrogen and other biofuels.
-
Hydroelectric dams and
wind turbines are powered by solar energy through its
interaction with the Earth's atmosphere and the resulting
weather phenomena.
-
Ocean thermal energy production uses the thermal gradients
present across ocean depths to generate power. These
temperature differences are produced by sunlight.[28]
-
Fossil fuels are ultimately derived from solar energy
captured by vegetation in the
geological past.
- In an indirect solar water heater the fluid heated in the
collector transfers its heat through a heat exchanger to a
separate domestic water system.
Passive or active
This distinction is made in the context of building
construction and building services engineering.
Passive solar systems use non-mechanical techniques of
capturing, converting and distributing sunlight into usable
outputs such as heating, lighting or ventilation. These
techniques include selecting materials with favorable thermal
properties, designing spaces that naturally circulate air and
referencing the position of a building to the sun.
Active solar systems use electrical and mechanical
components such as photovoltaic panels, pumps and fans to
process sunlight into usable outputs.
Concentrating or non-concentrating
Concentrating solar power (CSP) systems use lenses or mirrors
and
tracking systems to focus a large area of sunlight into a
small beam capable of producing high temperatures and
correspondingly high
thermodynamic efficiencies. Concentrating solar is generally
associated with
solar thermal applications but
concentrating photovoltaic (CPV) applications exist as well
and these technologies also exhibit improved efficiencies. CSP
systems require
direct insolation to operate properly.[30]
Concentrating solar power systems vary in the way they track
the sun and focus light.
- Line focus/Single-axis
- A
solar trough consists of a linear parabolic reflector
which concentrates light on a receiver positioned along the
reflector's focal line. These systems use single-axis
tracking to follow the sun. A working fluid (oil, water)
flows through the receiver and is heated up to 400 °C before
transferring its heat to a distillation or power generation
system.[31]
[32] Trough systems are
the most developed CSP technology. The Solar Electric
Generating System (SEGS)
plants in California and Plataforma Solar de Almería's
SSPS-DCS plant in Spain are representatives of this
technology.[33]
- Point focus/Dual-axis
- A
power tower consists of an array of flat reflectors (heliostats)
which concentrate light on a central receiver located on a
tower. These systems use dual-axis tracking to follow the
sun. A working fluid (air, water, molten salt) flows through
the receiver where it is heated up to 1000 °C before
transferring its heat to a power generation or energy
storage system. Power towers are less advanced than trough
systems but they offer higher efficiency and energy storage
capability.[34]
The
Solar Two in Daggett, California and the Planta Solar 10
(PS10) in Sanlucar la Mayor, Spain are representatives of
this technology.
- A
parabolic dish or dish/engine system consists of a
stand-alone parabolic reflector which concentrates light on
a receiver positioned at the reflector's focal point. These
systems use dual-axis tracking to follow the sun. A working
fluid (hydrogen, helium, air, water) flows through the
receiver where it is heated up to 1500 °C before
transferring its heat to a sterling engine for power
generation.[35][34]
Parabolic dish systems display the highest solar-to-electric
efficiency among CSP technologies and their modular nature
offers scalability. The Stirling Energy Systems (SES) and
Science Applications International Corporation (SAIC) dishes
at
UNLV and the Big Dish in Canberra, Australia are
representatives of this technology.
Non-concentrating photovoltaic and solar thermal systems do
not concentrate sunlight. While the maximum attainable
temperatures (200 °C) and thermodynamic efficiencies are lower,
these systems offer simplicity of design a have the ability to
effectively utilize
diffuse insolation.[34]
Flat-plate thermal and photovoltaic panels are representatives
of this technology.
Advantages and disadvantages of
solar power
US annual average solar energy received by a latitude tilt
photovoltaic cell.
Advantages
- The 89
petawatts of sunlight reaching the earth's surface is
plentiful compared to the 15
terawatts of average power consumed by humans.[36]
Additionally, solar electric generation has the highest power
density (global mean of 170 W/m2) among renewable
energies.[36]
- Solar power is pollution free during use. Production end
wastes and emissions are manageable using existing pollution
controls. End-of-use recycling technologies are under
development.[37]
- Facilities can operate with little maintenance or
intervention after initial setup.[citation
needed]
- Solar electric generation is economically competitive
where grid connection or fuel transport is difficult, costly
or impossible. Examples include satellites, island
communities, remote locations and ocean vessels.
- When grid connected, solar electric generation can
displace the highest cost electricity during times of peak
demand (in most climatic regions), can reduce grid loading,
and can eliminate the need for local battery power for use in
times of darkness and high local demand; such application is
encouraged by
net metering. Time-of-use net metering can be highly
favorable to small photovoltaic systems.
- Grid connected solar electricity can be used locally thus
minimizing transmission/distribution losses (approximately
7.2%).[38]
Disadvantages
- Polysilicon Solar cells are costly, requiring a large
initial capital investment, and silicon shortages raise
prices. Costs are expected to come down, however, due to
increased manufacturing, economies of scale and Balance of
System planning. Thin film technology uses less silicon; and
Lease/Rental options* are currently being introduced.
- Limited power density: Average daily insolation in the
contiguous U.S. is 3-9 kW·h/m2 usable by 7-17.7%
efficient solar panels.[40][41]
[42]
- To get enough energy for larger applications, a large
number of photovoltaic cells is needed. This increases the
cost of the technology and requires a large plot of land.
- Like electricity from nuclear or fossil fuel plants, it
can only realistically be used to power transport vehicles by
converting light energy into another form of stored energy
(e.g. battery stored electricity or by electrolysing water to
produce hydrogen) suitable for transport.
- Solar cells produce
DC which must be converted to
AC when used in currently existing distribution grids.
This incurs an energy loss of 4-12%.[43]
Availability of solar energy
There is no shortage of solar-derived energy on Earth. Indeed
the storages and flows of energy on the planet are very large
relative to human needs.
- The amount of solar energy intercepted by the Earth every
minute is greater than the amount of energy the world uses in
fossil fuels each year.
- Tropical
oceans absorb 560 trillion
gigajoules (GJ) of solar energy each year, equivalent to
1,600 times the world’s annual energy use.
- The energy in the
winds
that blow across the United States each year could produce
more than 16 billion
GJ of
electricity—more than one and one-half times the
electricity consumed in the United States in 2000.
- Annual
photosynthesis by the vegetation in the United States is
50 billion GJ, equivalent to nearly 60% of the nation’s annual
fossil fuel use.
Plants, on average, capture 0.1% of the solar energy reaching
the Earth. The land area of the lower 48 United States
intercepts 50 trillion GJ per year, equivalent to 500 times of
the nation’s annual energy use.[44]
This energy is spread over 8 million square kilometers of land
area, so that the energy absorbed per unit area is 6.1 GJ per
square meter per year. This results in potential
biomass production of 6,100
GJ per square kilometer per year. Compared to the 0.1%
efficiency of vegetation, roof installable amorphous silicon
solar panels capture 8%-14% of the solar energy, while more
expensive crystalline panels capture 14%-20%, and large scale
desert mirror-concentrator heat engine based setups may capture
up to 30-50%.
Energy storage
-
For a stand-alone system, some means must be employed to
store the collected energy for use during hours of darkness or
cloud cover. The following list includes both mature and
immature techniques:
Storage always has an extra stage of energy conversion, with
consequent energy losses, increasing the total capital costs.
One way around this is to export excess power to the power grid,
drawing it back when needed. This appears to use the power grid
as a battery but in fact is relying on conventional energy
production through the grid during the night. However, since the
grid always has a positive outflow, the result is exactly the
same.
Electric power costs are highly dependent on the consumption
per time of day, since plants must be built for peak power (not
average power). Expensive gas-fired "peaking generators" must be
used when base capacity is insufficient. Fortunately for solar,
solar capacity parallels energy demand -since much of the
electricity is for removing heat produced by too much solar
energy (ie, air conditioners). This is less true in the winter.
Wind power complements solar power since it can produce
energy when there is no sunlight.
