Concentrated solar power
(CSP) systems use
lenses or mirrors to focus a large area of sunlight onto a small area.
Electrical power is produced when the concentrated light is directed onto
photovoltaic surfaces or used to heat a transfer fluid for a conventional power
plant.
Concentrated solar power systems are divided
into
● concentrated solar thermal (CST)
● concentrated photovoltaics (CPV)
● concentrating photovoltaics and thermal
(CPT)
Concentrated solar thermal (CST) is used to produce renewable heat
or electricity (generally, in the latter case, through steam). Concentrated
solar thermal (CST) systems use lenses or mirrors and tracking systems
to focus a large area of sunlight onto a small area. The concentrated light is
then used as heat or as a heat source for a conventional power plant (solar
thermoelectricity).
A wide range of concentrating technologies exist, including
the parabolic trough, Dish Stirling, Concentrating Linear Fresnel Reflector,
Solar chimney and solar power tower. Each concentration method is
capable of producing high temperatures and correspondingly high thermodynamic
efficiencies, but they vary in the way that they track the Sun and focus light.
Due to new innovations in the technology, concentrating solar thermal is
becoming more and more cost-effective.
A parabolic trough consists of a linear parabolic reflector
that concentrates light onto a receiver positioned along the reflector's focal
line. The receiver is a tube positioned directly above the middle of the
parabolic mirror and is filled with a working fluid. The reflector follows the
Sun during the daylight hours by tracking along a single axis. A working fluid
is heated to 150–350 °C (423–623 K (302–662 °F)) as it flows through the receiver
and is then used as a heat source for a power generation system. Trough systems
are the most developed CSP technology. The Solar Energy Generating Systems
(SEGS) plants in California,Acciona's Nevada
Solar One near Boulder City, Nevada, andPlataforma Solar de Almería's SSPS-DCS plant in Spain are representative of this
technology.
Concentrating Linear Fresnel Reflectors are CSP-plants which
use many thin mirror strips instead of parabolic mirrors to concentrate
sunlight onto two tubes with working fluid. This has the advantage that flat
mirrors can be used which are much cheaper than parabolic mirrors, and that
more reflectors can be placed in the same amount of space, allowing more of the
available sunlight to be used. Concentrating Linear Fresnel reflector can come
in large plants or more compact plants.
A Dish Stirling or dish engine 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. The working fluid in the receiver is heated to 250–700 °C (523–973 K
(482–1,292 °F)) and then used by a Stirling engine to generate power.Parabolic dish systems provide the highest
solar-to-electric efficiency among CSP technologies, and their modular nature
provides scalability. The Stirling Energy Systems (SES) and Science
Applications International Corporation (SAIC) dishes at UNLV, and the Big Dish
in Canberra, Australia are representative of this technology.
A Solar chimney consists of a transparent large room (usually
completely in glass) which is sloped gently up to a central hollow tower or
chimney. The sun heats the air in this greenhouse-type structure which then
rises up the chimney, hereby driving an air turbine as it rises. This air turbine
hereby creates electricity. Solar chimneys are very simple in design and could
therefore be a viable option for projects in the developing world.
A solar power tower consists of an array of
dual-axis tracking reflectors (heliostats) that concentrate light on a central
receiver atop a tower; the receiver contains a fluid deposit, which can consist
of sea water. The working fluid in the receiver is heated to 500–1000 °C
(773–1,273 K (932–1,832 °F)) and then used as a heat source for a power
generation or energy storage system. Power tower development is less advanced
than trough systems, but they offer higher efficiency and better energy storage
capability. The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representative
of this technology. eSolar's 5 MW
Sierra SunTower located in Lancaster, California
and is the only CSP tower facility operating in North America.
Concentrated Solar Thermal Power (CSP) is the main technology proposed
for a cooperation to produce electricity and desalinated water in the arid
regions of North Africa and Southern Europe by the Trans-Mediterranean
Renewable Energy Cooperation DESERTEC.
Concentrated photovoltaics (CPV) systems employ sunlight
concentrated onto photovoltaic surfaces for the purpose of electrical power
production. Solar concentrators of all varieties may be used, and these are
often mounted on a solar tracker in order to keep the focal point upon the cell
as the Sun moves across the sky.
Serious research and development work on concentrator PV
systems has been conducted since the 1970s. For example, a linear-trough
concentrator system was tested and installed at Sandia National Laboratories,
and the first modern point focus photovoltaic concentrating system was
developed in the Sandia, both late in that decade. The latter system used a
point focus acrylic Fresnel lens focusing on water-cooled silicon cells and two
axis tracking. A similar concept was used in other prototypes. Ramón Areces' system, developed in the late 1970’s,
used hybrid silicone-glass Fresnel lenses, while cooling of silicon cells was
achieved with a passive heat sink.
Luminescent solar concentrators (when combined with a
PV-solar cell) can also be regarded as a Concentrating photovoltaics (CPV)
system. Luminescent solar concentrators are useful as they can improve
performance of PV-solar panels drastically.
Semiconductor properties allow solar cells to operate more
efficiently in concentrated light, as long as the cell junction temperature is
kept cool by suitable heat sinks. CPV operates most effectively in sunny
weather since clouds and overcast conditions create diffuse light, which
essentially cannot be concentrated.
Expected future efficiencies are nearly 50%.
Compared to conventional flat panel solar cells,
CPV is advantageous because the solar collector is less expensive than an
equivalent area of solar cells. CPV hardware (solar collector and tracker) is
targeted to be priced well under 3 USD/Watt, whereas silicon flat panels that
are commonly sold are 3 to 5 USD/Watt (not including any associated power
systems or installation charges).
CPV could reach grid parity in 2011.
Low concentration CPV are systems with a solar concentration
of 2-100 suns. For economic reasons, conventional or modified silicon solar
cells are typically used, and, at these concentrations, the heat flux is low
enough that the cells do not need to be actively cooled. The laws of optics
dictate that a solar collector with a low concentration ratio can have a high acceptance
angle and thus in some instances does not require active solar tracking.
From concentrations of 100 to 300 suns, the CPV systems
require two-axes solar tracking and cooling
(whether passive or active), which makes them more complex.
High concentration photovoltaics (HCPV) systems employ concentrating optics
consisting of dish reflectors or fresnel lenses
that concentrate sunlight to intensities of 300 suns or more. The solar cells
require high-capacity heat sinks to prevent thermal destruction and to manage
temperature related performance losses. Multijunction solar cells are currently favored over silicon as they are more
efficient. The efficiency of both cell types rises with increased
concentration; multijunction efficiency also rises
faster. Multijunction solar cells, originally designed
for non-concentrating space-based satellites, have been re-designed due to the
high-current density encountered with CPV (typically 8 A/cm2 at
500 suns). Though the cost of multijunction solar cells is roughly 100 times that of comparable
silicon cells, the cell cost remains a small fraction of the cost of the
overall concentrating PV system, so the system economics might still favor themultijunction cells.
Much of the original research into multijunction photovoltaics was sponsored by
governments and the astronautics industry. More recently, the technical
research and product development of CPV systems has grown due to investment in
terrestrial electric generating systems. Recent technological advances in
triple-junction solar cells by Fraunhofer Institute ISE have yielded 41.1% conversion efficiency.
In May 2008, IBM demonstrated a prototype CPV using computer
chip cooling techniques to achieve an energy density of 2300 suns.
Recently, Concentrix (Germany) and Amonix (USA) have announced operating AC
efficiencies of 23% and 25%,respectively. These
numbers point to significantly higher annual energy generation per receiver
area unit with HCPV than with competing technologies.
In November 2009, NREL released a technical report presenting
the opportunities and challenges of CPV technology, from a state of the art
review.
In March 2010, OPEL Solar 330 kilowatts (kW) utility-grade
solar photovoltaic power plant in Spain is one of the first HCPV installations
to be recognized with an feed-in tariff
(FIT) and guaranteed investment rate of return. The installation features
dual-axis tracker-mounted Opel Mk-I HCPV panels, which can focus more than 500
suns onto high-efficiency multijunction GaAs solar cells. and has
conversion efficiency up to twice that of silicon solar panels and more than
three times that of thin-film solar panels.
In China Suntrix design and produce HCPV module.They have 500x and 1000x products.
Concentrating Photovoltaics and Thermal (CPVT) technologyproduces both electricity and thermal
heat in the same module. Thermal heat that can be employed for hot tap water,
heating and heat-powered air conditioning (solar cooling), desalination or
solar process heat.
CPVT systems can be used in private homes and increase total
energy output to 40-50%, as compared with normal PV panels with 10-20%
efficiency, and they produce more thermal heat in wintertime compared with
normal thermal collectors. Also, thermal systems do not overheat.
Australian, American, and Chinese researchers are exploring
the potential for Combined Heat and Power Solar (CHAPS), while Europeans are
now producing CHAPS systems.
As at September 9, 2009; 8 months ago, the cost of building a
CSP station was typically about $2.5 to $4 per watt, while the fuel (the sun's
radiation) is free. Therefore a 250MW CSP station would have cost $600–1000
million to build. That works out to 12 to 18 cents per kilowatt-hour.
A study done by Greenpeace International, the European Solar
Thermal Electricity Association, and the International Energy Agency's SolarPACES group investigated the potential
and future of concentrated solar power. The study found that concentrated solar
power could account for up to 25% of the world's energy needs by 2050. Also,
with this expansion of concentrated solar power, thousands of new jobs would be
created and millions of tonnes of carbon dioxide would be prevented from being
released. The increase in investment would be from 2 billion euros worldwide to
92.5 billion euros in that time period. Spain is the leader in concentrated
solar power technology, with more than 50 projects approved by the government
in the works. Also, it exports its technology, further increasing the
technology's stake in energy worldwide. Because of the nature of the technology
needing a desert like area, experts predicted the biggest growth in places like
Africa, Mexico, the southwest United States.
The study examined three different outcomes for this technology: no increases
in CSP technology, investment continuing as it has been in Spain and the US,
and finally the true potential of CSP without any barriers on its growth. The
findings of the third part are shown in the table below:
|
TIME |
INVESTMENT |
CAPACITY |
|
2015 |
21 billion euros a year |
420 megawatts |
|
2050 |
174 billion euros a year |
1500 gigawatts |
Finally, the study acknowledged how technology for CSP was
improving and how this would result in a drastic price decrease by 2050. It
predicted a drop from the current range of .23 to .15 euros per kilowatt, down
to .14 to .10 euros a kilowatt. Recently the EU has begun to look into developing
a €400 billion ($774 billion) solar power plant based in the Sahara region
using CSP technology known as Desertec. It is part of a wider plan to create "a new
carbon-free network linking Europe, the Middle East and North Africa". The
plan is backed mainly by German industrialists and predicts production of 15%
of Europe's power by 2050. Morocco is a major partner in Desertec and as it has barely 1% of the
electricity consumption of the EU, it will produce more than enough energy for
the entire country with a large energy surplus to deliver to Europe.
Other organizations expect CSP to cost $0.06(US)/kWh by 2015
due to efficiency improvements and mass production of equipment. That would
make CSP as cheap as conventional power. Investors such as venture capitalist
Vinod Khosla expect CSP to continuously reduce costs and actually be cheaper
than coal power after 2015.
On September 9, 2009; 8 months ago, Bill Weihl, Google.org'sgreen energy czar said that the firm was conducting research
on the heliostat mirrors and gas turbine technology, which he expects will drop
the cost of solar thermal electric power to less than $0.05/kWh in 2 or 3 years.
In 2009, scientists at the
National Renewable Energy Laboratory (NREL) and SkyFuel teamed to develop large curved sheets of metal that
have the potential to be 30% less expensive than today's best collectors of
concentrated solar power by replacing glass-based models with a silver polymer
sheet that has the same performance as the heavy glass mirrors, but at a much lower
cost and much lower weight. It also is much easier to deploy and install. The
glossy film uses several layers of polymers, with an inner layer of pure silver.