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INTRODUCTION
After having been blocked for a long time in its
development, the concentrating solar power (CSP) sector is undergoing a
full revival today due to falling costs, more effective technologies and
policies sensitive to environmental questions.
Technically, concentrating solar power consists in
focusing sunlight to heat a fluid to a sufficiently high temperature to
produce electricity. Exploitation of solar energy in this form requires
sunlight conditions that are particular to certain regions of the world
only. The best zones are the Sahara and the Australian and Californian
deserts, as well as Mediterranean areas like Spain, the South of France,
Italy and North Africa.
In terms of technology, this sector exists in three
distinct techniques. The first consists of parabolic collectors where
the sun's rays converge towards a single point, the focal point of a
parabolic collector. The second technique is implemented in solar tower
power plants where hundreds, or even thousands, of mirrors follow the
sun's path and concentrate its rays on a central receiver positioned at
the top of a tower. The last technique is based on cylindrical-parabolic
collectors: parabolic-section mirrors concentrate the sun's rays towards
a focal line.
The USA, the birthplace of this sector, today
concentrates practically all of the installed CSP capacity in the world
with 355 MW. Four American States are particularly involved in
developing new projects: Nevada with 64 MWe that is to be commissioned
in 2007 (the Acciona and Solargenix companies), California where two
contracts have been signed to develop 800 MWe between 2008 and 2011,
Arizona and New Mexico.
But the sector’s rebirth is now being carried onward
by Spain, which set an objective of 500 MWe for 2010. Wiser from the
experience obtained from its first three CSP sites located in Almeria,
Spain inaugurated it’s first commercial power plant in Seville in 2006
(PS10 with 11 MWe), with the next one expected for 2008 in Grenada (Andasol
1 with 50 MWe). In the end, more than 700 MWe are on the drawingboards.
In Germany, several industrialists and consulting firms are working on
techniques intended to be developed in the
countries of the South. Schott has opened a production plant to equip
solar power plants under construction, Schlaich Bergermann und Partner (SBP)
are taking part in developing the parabolic collector “Eurodish”, seven
10 kW capacity models of which are currently operating in Europe and in
India. A dozen heliothermodynamic power plant projects are being studied
throughout the world (in Iran, South Africa, Egypt, Israel, Morocco,
etc.).
In France, the sector
continues to mobilise actors even though all of the sites that were
developed in the early 1980s have now been converted into research
centres. In this way, the CNRS (National Centre of Scientific Research)
has launched the Pégase Project: a new installation coupled with a gas
turbine is to be erected on the historical site of the Thémis solar
tower power plant. Experimentation should begin in the year 2009, if
financing follows. A second project, named “ThemDish”, provides for the
installation of five Eurodish parabolas for the end of 2007 or the
beginning of 2008.
TECHNOLOGY
Concentrating Solar Power systems use concentrated solar
radiation as a high temperature energy source to produce electrical power.
These clean energy technologies are appropriate for applications where direct solar radiation
is high. The first commercial plants have been in operation in California since the mid-1980s,
providing 354 MW of solar power.The many types of systems under development (including
parabolic troughs, power towers,and dish/engine systems) for different markets vary
according to the concentration devices,energy conversion methods, storage options and other
design variables. Solar radiation can also drive chemical reactions for the production of fuels
and chemicals. Additional uses include environmentally benign technologies in fields such
as detoxification of chemical wastes and energy storage which are aimed at the medium to
long term.
1. Parabolic troughs solar collector system
A parabolic trough solar collector is part of a solar
collector field, designed to collect heat from the sun. Solar radiation is concentrated via
parabolic curved solar reflectors to a heat collecting element (HCE) located in the optical focal line
of the collector. The solar collectors are continuously tracked to direct to the sun. A heat
transfer fluid is circulated inside the HCE tubes and transports the absorbed energy to a conventional
power block where electricity is generated (see figure 1). The smallest subunits of the
collector field, the so-called solar collector ele ments (SCE) are shown in figure 2. They are
made up of a steel space frame structure of approx. 12 m length. 28 curved solar
reflectors are attached to the steel structure in 4 rows, forming the parabolic collector with approx. 12
m length and 5.76 m width. The absorber or HCE is fixed to the steel structure by means
of steel supports, one at every 4 m.Each SCE is supported at its end by pylons. They are
provided with plain bearings, allowing for rotation along the collector longitudinal axis. 12
SCES form the solar collector assembly (SCA) with a length of approx. 150 m. All collectors of
the SCA are connected by means of torque transfer units to permit joint movement. Therefore
the pylon in the middle of the 12 SCES is equipped with a hydraulic drive unit. 4 SCAs are
combined to a so-called solar collector loop. Within the loop all absorbers (HCEs) are
connected, the heat transfer fluid flows through all of them, heating up in every collector.
2. Solar power tower systems
A short description of the major Solar
Power Tower (SPT) System Elements follows. In figure 3, an image of a SPT
plant and figure 4 its main components and their relationship are shown.
2.1. Collector System (CS)
The CS contains the collector field and heliostats that redirect and
focuses sunlight on the receiver. The major system elements are two-axis
tracking mirrors (heliostats), heliostat controllers (HCs), heliostat
array controller (HAC), and communications link between the HCs and the
HAC. The number of heliostats will vary for a particular receiver thermal
duty and a specific heliostat design.
2.2. Receiver System (RS)
The RS converts the redirected solar flux into thermal energy. The
receiver is a cylindrical tube wall heat exchanger that heats molten
nitrate salt from 290°C (550°F) to 565°C (1050°F) and includes the
associated piping, valves and controls and unique RS control system
2.3. Steam Generation System (SGS)
The SGS uses thermal energy from the hot nitrate salt to produce
superheated steam at the conditions required by the turbine-generator and
auxiliary steam systems.
2.4 Thermal Storage System (TSS)
The TSS stores high temperature nitrate salt 565°C (1050°F) from the
receiver for use by the steam generator, and stores low temperature
nitrate salt 290°C (550°F) from the steam generator for use by the
receiver. The TSS system components are the: cold nitrate salt tank;hot
nitrate salt tank; pressure relief valves (over- and under- pressure
relief); tank foundations;nitrate salt inventory; tank immersion heaters
and tank insulation system.
2.5 Master Control System (MCS)
The MCS controls and monitors all SPT process functions for all system
equipment through all states and transitions in response to operator
commands. The MCS is comprised of the following major subsystems: a
Distributed Control System (DCS), Heliostat Array Controller (HAC) and
Administrative and Data Analysis System.
2.6 Electric Heat Tracing System (EHTS)
The EHTS provides nitrate salt freeze protection to all process equipment
and components;thermal conditioning of all process equipment and
components for plant startup, and protects equipment from extreme thermal
gradients and excessive thermal stresses
2.7 Electric Power Generation System (EPGS)*
The electric power generation system converts the energy in the main steam
into electric power for delivery to the electric grid. The EPGS consists
of the turbine-generator and asociated equipment.
2.8. Balance of Plant (BOP)
The BOP supports all other plant systems and includes, among other: Switch
yard / main power distribution system, emergency and uninterruptible power
supply system, RS tower cranes. |
