introduction to csp technologies

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1. Introduction to CSP Technologiesa. Why should we take interest in harvesting energy from solar?b. What is CSP?c. History of CSPd. CSP First Principlese. Parabolic Trough Systemf. Parabolic Dish Systemg. Central Receiver Systemh. Linear Fresneli. Hybrid System / ISCC2. Components of a CSP Power Planta. The Solar Field

i.

Concentrator structureii. Mirrors or reflectors

iii. Linear receiver or heat collection elementiv. Pump system for the HTFv. Collector balance of system

b. Thermal Energy Storagei. The two-tank direct system

ii. Two-tank indirect systemiii. Single-tank thermocline system

c. The Power Generation Systemi. Power cycles

1. Steam Rankine2. Organic Rankine3. Combined

ii. Fossil-fired (hybrid) backupiii. Wet and dry cooling

3. Summary

Introduction to CSP Technologies

Why should we take interest in harvesting energy from solar?

To understand the potential for harvesting energy from solar lets consider some quick statistics. In full

sun, about 100 watts of solar energy per square foot hits the earth. If you assume 12 hours of sun per day,


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this equates to 438,000 watt-hours per square foot per year. Based on 27,878,400 square feet per square

mile, sunlight bestows a whopping 12.2 trillion watt-hours per square mile per year.

With these assumptions, figuring out how much solar energy hits the entire planet is relatively simple.

12.2 trillion watt-hours converts to 12,211 gigawatt-hours, and based on 8,760 hours per year, and 197

million square miles of earths surface (including the oceans), the earth receives about 274 million

gigawatt-years of solar energy, which translates to an astonishing 8.2 million of quad Btu energy per year.

A quadBtu refers to one quadrillion British Thermal Units of energy, a common term used by energy

economists. The entire human race currently uses about 400 quads of energy (in all forms) per year. Put

another way, the solar energy hitting the earth exceeds the total energy consumed by humanity by a

factor of over 20,000 times.

Clearly there is enough solar energy available to fulfill all the human races energy requirements now,

and for all practical purposes, forever. The key is developing technologies that efficiently convert solar

power into usable energy in a cost-effective manner.

What is CSP?

Solar energy can be tapped to produce electricity in two distinct ways. One is using a photovoltaic

systemthat uses sunlight as it is and the other is by using solar concentrators. The basic idea is to

translate the sun's energy in the form of impending photons to usable electricity.

Solar Concentrators, such as that used in CSP, can be used to concentrate sunlight to be used either in a

Photovoltaic system or a solar thermal system. Though these two technologies essentially use focused

solar light to produce electricity, they differ in the way sunlight is converted to electricity. In the case of a

photo-voltaic device, the focused light is converted to electricity by exploiting the electronic properties of

semiconducting materials, whereas in the case of solar thermal, the focused light heats up a transport

fluid which is then used to generate steam which drives a turbine using the Rankine cycle generator.
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Whereas PV commonly uses the visible to UV part of the spectrum to generate electricity, solar thermal

uses the infrared energy to heat the HTF.

Concentrated Solar Power is used to produce electricity called solar thermoelectricity, usually generated

through steam. Basically CSP technology uses mirrors with tracking systems to focus a large area of

sunlight onto a small area. The concentrated light then causes a thermal storage material like oil, salt or

water to heat. This heat is then used as a heat source for a conventional power plant. The solar concentrators

used in CSP systems can often also be used to provide industrial process heating or cooling, such as in

solar air-conditioning.

To explore the energy storage that is possible using concentrated solar collector technology, it is necessary

to briefly visit the different solar harnessing technologies being used today. CSP is seen as a holistic

technology with other benefits as well involving the use of the waste heat from power generation.

History of CSP

According to legends, Archimedes used a "burning glass" to concentrate sunlight on the invading Romanfleet and repel them from Syracuse (Sicily). In 1973 a Greek scientist, Dr. Ioannis Sakkas, curious about

whether Archimedes could really have destroyed the Roman fleet in 212 BC, lined up nearly 60 Greek

sailors, each holding an oblong mirror tipped to catch the sun's rays and direct them at a tar-covered

plywood silhouette 160 feet away. The ship caught fire after a few minutes; however, historians continue

to doubt the Archimedes story.

In 1866, Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine. The first patent for a solar collector was obtained by the Italian Alessandro Battaglia in Genoa, Italy, in

1886. Over the following years, inventors such as John Ericsson and Frank Shuman developed

concentrating solar-powered devices for irrigation, refrigeration, and locomotion.

In 1913 Shuman finished a 55 HP parabolic solar thermal energy station in Meadi, Egypt for irrigation. The first solar-power system using a mirror dish was built by Dr. R.H. Goddard, who was already well

known for his research on liquid-fueled rockets and wrote an article in 1929 in which he asserted that all

the previous obstacles had been addressed.

Professor Giovanni Francia (19111980) designed and built the first concentrated-solar plant. whichentered into operation in Sant'Ilario, near Genoa, Italy in 1968. This plant had the architecture of today's

concentrated-solar plants with a solar receiver in the center of a field of solar collectors. The plant was

able to produce 1 MW with superheated steam at 100 bar and 500 degrees Celsius.The 10 MW Solar One

power tower was developed in Southern California in 1981, but the parabolic-trough technology of the

nearby Solar Energy Generating Systems (SEGS), begun in 1984, was more workable. The 354 MW SEGS

is still the largest solar power plant in the world.

CSP First Principles


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Fig. This image shows the process by which a typical parabolic trough solar-to-electricity conversion takes

place in a solar thermal system. The entire system can be subdivided into three distinct sections:

Solar Field the solar field is composed of rows of parabolic-shaped mirrors. An array of tracking

parabolic dishes can be arranged in a so-called distributed system so that the working fluid from each

dish is piped to a central power conversion station. The mirrors follow the sun to catch as much energy as

possible. The curvature of these mirrors focuses the sunlight on a central receiver tube that runs the

length of the mirror. The central receiver tubes are hollow and filled with a heat transfer fluid (HTF). The

HTF, warmed by the sunlight to more than 400C, then flows to the power block or the thermal energy

storage system, depending on the mode of operation.

Thermal Energy Storage System Hot HTF is also transported to thermal storage tanks. The heat from

the HTF is transferred to the molten salts where it is stored for later use e.g., when clouds pass by, after

sunset, or before sunrise.

Power Block Hot HTF is transported to the power block where it is used to boil water to generate

steam for use in a conventional steam generator to produce electricity.

Solar thermal conversion process of solar energy is based on well-known phenomena of heat transfer. In

all thermal conversion processes, solar radiation is absorbed at the surface of a receiver, which contains or

is in contact with flow passages through which a working fluid passes. As the receiver heats up, heat is
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transferred to the working fluid which may be air, water, oil, or molten salt. The upper temperature that

can be achieved in solar thermal conversion depends on

the isolation The degree to which the sunlight is concentrated and the measures taken to reduce heat losses from the

working fluid.

The temperature level of the working fluid can be controlled by the velocity at which it is circulated. It is

possible to match solar energy to the load requirements according to the amount of heat and temperature

level. In this manner, it is possible to design conversion systems that are optimized according to both the

first and the second laws of thermodynamics. High temperature heat is needed by industry for process

heat and by utilities for electricity. The collection and conversion of the solar radiation to thermal energy

depends on the collector design and the relative amounts of direct beam and diffuse radiation absorbed

by the collector.

By incorporating tracking devices, the collectors can deliver intensities in the order of 50 suns and

temperatures about 450 deg C. Trackers can be single axis or dual axis based on the quality of tracking

required. An alternative approach, also using tracking parabolic dishes, is to locate a heat engine that can

generate electricity at the focal point of each dish and to transport electric current rather than a hot fluid.

The solar CSP technology can be broadly classified as follows:

The following are the types of collectors used for concentrated solar thermal:

Parabolic Trough System

Parabolic Trough Systems use a linear parabolic concentrator with a mirrored surface to focus solar

radiation on an absorber pipe running along the focal line of the parabola. The absorber pipe contains the

Heat Transfer Fluid or HTF which is heated and pumped to the steam generator. The steam generator is


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in-turn connected to a steam turbine. For change of the daily position of the sun perpendicular to the

receiver, the trough tilts east to west so that the direct radiation remains focused on the receiver.

However, seasonal changes in the in angle of sunlight parallel to the trough does not require adjustment

of the mirrors, since the light is simply concentrated elsewhere on the receiver. The receiver containing

the HTF can reach temperatures of 400 C and generates live steam to drive the steam turbine generator

of a conventional power block.

The receiver may be enclosed in a glass vacuum chamber. The vacuum significantly reduces convective

heat loss.

Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area ofland. Since 1985 a solar thermal system using this principle has been in full operation in California in the

United States. It is called the SEGS system). Other CSP designs lack this kind of long experience and

therefore it can currently be said that the parabolic trough design is the most thoroughly proven CSP

technology.

The Solar Energy Generating System (SEGS) is a collection of nine plants with a total capacity of 350MW.It is currently the largest operational solar system (both thermal and non-thermal). A newer plant is the

Nevada Solar One plant with a capacity of 64MW. Under construction are Andasol 1 and Andasol 2 in

Spain with each site having a capacity of 50MW. Note however, that those plants have heat storage which

requires a larger field of solar collectors relative to the size of the steam turbine-generator to store heat

and send heat to the steam turbine at the same time. Heat storage enables better utilization of the steam

turbine. With day and some nighttime operation of the steam-turbine Andasol 1 at 50MW peak capacity

produces more energy than Nevada Solar One at 64 MW peak capacities, due to the former plant'sthermal energy storage system and larger solar field.


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Technology Constraints:

The upper process temperature is currently limited by the heat transfer thermal oil to 400C. The heat transfer thermal oil adds extra costs of investment and of operating and maintenance. Depending on national regulations, environmental constraints from ground pollution by spillage of

thermal oil could occur.

High winds may break mirror reflectors at field corners. Low-cost and efficient energy storage systems have not been demonstrated up to now. Heat losses between the receivers and the central conversion unit are high Complexity of flexible connections necessary between moving receivers and stationary piping reduces

the reliability of such distributed systems.

Parabolic Dish System

A Parabolic Dish System or PDS uses a dish shaped mirror to focus sunlight onto receivers located at the

focal point of the dish. The focused sunlight is converted to thermal energy which heats the HTF. The

heated HFT can be transported to a central generator for conversion to electricity or it can be done at the

receiver itself. The heliostats allow a two axis rotation in the X and Y directions. This affords greatest

conversion efficiencies among solar collector systems and results in receiver temperatures reaching as

high as 1500 deg C or more. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System,


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but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to

electricity using an electric generator.

The advantage of a dish system is that it can achieve much higher temperatures due to the higher

concentration of light (as in tower designs). Higher temperatures lead to better conversion to electricity

and the dish system is very efficient in doing this.

The main challenges facing PDSs is developing efficient thermal conversion units which have low capital

and maintenance costs, long life and the ability to operate independently. Several different engines such

as reciprocating steam engines, organic Rankine engines, gas turbine engines and Stirling engines have

been explored.

In 2005 Southern California Edison announced an agreement to purchase solar powered Stirling enginesfrom Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to

generate 500 megawatts of electricity. In January 2010, Stirling Energy Systems and Tessera Solar

commissioned the first demonstration 1.5-megawatt power plant ("Maricopa Solar") using Stirling

technology in Peoria, Arizona. At the beginning of 2011 Stirling Energy's development arm, Tessera Solar,

sold off its two large projects, the 709 MW Imperial project and the 850 MW Calico project to AES Solar

and K.Road, respectively, and in the fall of 2011 Stirling Energy Systems applied for Chapter 7

bankruptcy due to competition from low cost photovoltaics.

In July 2011, Iran inaugurated Iran's biggest solar power plant in Mashhad which produces 72,000kilowatt-hour electricity per year.

Fig. This is a six dish stirling system developed by Schlaich Bergermann und Partner of Stuttgart,

Germany, in operation at the Plataforma Solar de Almeria in Spain. The systems produce 10 kW of power

from a Solo Kleinmotoren engine. Wikipedia


The electricity output of single dish/Stirling unit is limited to small ratings due to geometric and physicalreasons.


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Large-scale deployment has not yet occurred because of various reasons such as production,transportation and installation difficulties.

Projections of capital costs, operation and maintenance costs, electricity costs, system performance and ofthe annual plant availability over the long run are not available or verified.

No adequate energy storage system is applicable or available. Heat to electricity conversion requires moving parts and that results in maintenance. In general, a centralized approach for this conversion is better than the decentralized concept in the dish

design.

The (heavy) engine is part of the moving structure, which requires a rigid frame and strong trackingsystem.

Furthermore, parabolic mirrors are used instead of flat mirrors and tracking must be dual-axis.Central Receiver System

In a Central Receiver or Tower systems the sunlight is concentrated to a central tower mounted receiverwhere the thermal energy is transferred to the HTF. This energy is then passed on to a thermal storage

system or for immediate power conversion. The major components in such a system include the heliostat

controllers, the receiver, the storage system, the heat exchanger and the thermal engine. Newer tower

systems with storage today use a HTF of thermal fluid and a thermal salt storage in molten state to

provide electricity when the sun is down. The receiver temperatures in such as system can reach as high

as 1500 deg C. Thermal storage reduces the load on backup fuel resources.

The heliostats capture and focus the sun's thermal energy with thousands of tracking mirrors (called

heliostats) in roughly a two square mile field. A tower resides in the center of the heliostat field. Theheliostats focus concentrated sunlight on a receiver which sits on top of the tower. Within the receiver the

concentrated sunlight heats molten salt to over 1,000 F(538 C). The heated molten salt then flows into a

thermal storage tank where it is stored, maintaining 98% thermal efficiency, and eventually pumped to a

steam generator. The steam drives a standard turbine to generate electricity. This process, also known as

the "Rankine cycle" is similar to a standard coal-fired power plant, except it is fueled by clean and free

solar energy.

The advantage of this design above the parabolic trough design is the higher temperature just as the dish

system. Thermal energy at higher temperatures can be converted to electricity more efficiently and can be

more cheaply stored for later use. Furthermore, there is less need to flatten the ground area. In principle a

power tower can be built on a hillside. Mirrors can be flat and plumbing is concentrated in the tower.

In June 2008, eSolar, a Pasadena, CA-based company founded by Idealab CEO Bill Gross with fundingfrom Google, announced a power purchase agreement (PPA) with the utility Southern California Edison

to produce 245 megawatts of power. Also, in February 2009, eSolar announced it had licensed its


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technology to two development partners, the Princeton, N.J.-based NRG Energy, Inc., and the India-

based ACME Group. In the deal with NRG, the companies announced plans to jointly build 500

megawatts of concentrating solar thermal plants throughout the United States. The target goal for the

ACME Group was nearly double; ACME plans to start construction on its first eSolar power plant this

year, and will build a total of 1 giga watt over the next 10 years.

eSolar's proprietary sun-tracking software coordinates the movement of 24,000 1 meter-square mirrorsper 1 tower using optical sensors to adjust and calibrate the mirrors in real time. This allows for a high

density of reflective material which enables the development of modular concentrating solar thermal

(CSP) power plants in 46 megawatt (MW) units on approximately square mile parcels of land, resulting

in a land-to-power ratio of 4 acres (16,000 m2) per 1 megawatt.

Bright Source Energy entered into a series of power purchase agreements with Pacific Gas and ElectricCompany in March 2008 for up to 900MW of electricity, the largest solar power commitment ever made

by a utility. Bright Source is currently developing a number of solar power plants in Southern California,with construction of the first plant planned to start in 2009.

In June 2008, Bright Source Energy dedicated its 4-6 MW Solar Energy Development Center (SEDC) inIsrael's Negev Desert. The site, located in the Rotem Industrial Park, features more than 1,600 heliostats

that track the sun and reflect light onto a 60 meter-high tower. The concentrated energy is then used to

heat a boiler atop the tower to 550 degrees Celsius, generating superheated steam.

A working tower power plant is PS10 in Spain with a capacity of 11MW. The 15MW Solar Trees plant with heat storage is under construction in Spain. In South Africa, a 100MW

solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m. A

10MW power plant in Cloncurry, Australia (with purified graphite as heat storage located on the tower

directly by the receiver.

Morocco is building five solar thermal power plants around Ouasarzate. The sites will produce about2000 MW by 2012. Over ten thousand hectors of land will be needed to sustain all of the sites.

Out of commission are the 10MW Solar One (later redeveloped and made into Solar Two) and the 2MWThemis plants.

A cost/performance comparison between power tower and parabolic trough concentrators was made bythe NREL which estimated that by 2020 electricity could be produced from power towers for 5.47 /kWh

and for 6.21

/kWh from parabolic troughs. The capacity factor for power towers was estimated to be72.9% and 56.2% for parabolic troughs. There is some hope that the development of cheap, durable, mass

producible heliostat power plant components could bring this cost down.

Technology advances in tower systems are exploring the use of stretched-membrane heliostats andpolymer reflectors.


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Fig. A power tower system in Barstow, California. Source: http://www.solarpaces.org


Currently promising energy storage system such as molten salt and volumetric air receiver technology

are limited by their proven generation capabilities.

The industrial demonstration of volume production of heliostat components is still missing.

Each mirror must have its own dual-axis control, while in the parabolic trough design one axis can be

shared for a large array of mirrors.

Linear Fresnel

Linear Fresnel reflectors differ from other concentrated solar power (CSP) technologies in that their long,

low mirrors reflect sunlight onto a single, horizontal tubular receiver, where as other CSPs require

multiple receivers. Linear Fresnel reflectors also require fewer acres because more mirrors can be

squeezed onto a smaller parcel of land. And significantly, they can produce high temperatures, which

lead to a more efficient conversion of sunlight into electricity.

These systems aim to offer lower overall costs by sharing a receiver between several mirrors (as

compared with trough and dish concepts), while still using the simple line-focus geometry with one axis

for tracking. This is similar to the trough design (and different from central towers and dishes with dual-

axis). The receiver is stationary and so fluid couplings are not required (as in troughs and dishes). The

mirrors also do not need to support the receiver, so they are structurally simpler. When suitable aiming

strategies are used (mirrors aimed at different receivers at different times of day), this can allow a denser

packing of mirrors on available land area.

Recent prototypes of these types of systems have been built in Australia (CLFR) and by Solar undo in

Belgium.

The Solarmundo research and development project, with its pilot plant at Lige, was closed down after

successful proof of concept of the Linear Fresnel technology. Subsequently, Solar Power Group GmbH


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(SPG), based in Munich, Germany, was founded by some Solarmundo team members. A Fresnel-based

prototype with direct steam generation was built by SPG in conjunction with the German Aerospace

Center (DLR).

Based on the Australian prototype, a 177MW plant had been proposed near San Luis Obispo in California

and would be built by Ausra.But Ausra sold its planned California solar farm to First Solar. First Solar (a

manufacturer of thin-film photovoltaic solar cells) will not build the Carrizo project, and the deal has

resulted in the cancellation of Ausras contract to provide 177 megawatts to P.G.& E.Small capacity plants

are an enormous economical challenge with conventional parabolic trough and drive design few

companies build such small projects. There are plans for SHP Europe, former Ausra subsidiary, to build a

6.5 MW combined cycle plant in Portugal. The German company SK Energy GmbH (company)|SK

Energy]) has plans to build several small 1-3 MW plants in Southern Europe (esp. in Spain) using Fresnel

mirror and steam drive technology.

In May 2008, the German Solar Power Group GmbH and the Spanish Laer S.L. agreed the joint execution

of a solar thermal power plant in central Spain. This will be the first commercial solar thermal power

plant in Spain based on the Fresnel collector technology of the Solar Power Group. The planned size of

the power plant will be 10 MW a solar thermal collector field with a fossil co-firing unit as backup system.

The start of constructions is planned for 2009. The project is located in Gotarrendura, a small renewable

energy pioneering village, about 100 km northwest of Madrid, Spain.

A Multi-Tower Solar Array (MTSA) concept, that uses a point-focus Fresnel reflector idea, has also been

developed, but has not yet been prototyped.

Since March 2009, the Fresnel solar power plant PE 1 of the German company Novatec Biosol is in

commercial operation in southern Spain. The solar thermal power plant is based on linear Fresnel

collector technology and has an electrical capacity of 1.4 MW. Beside a conventional power block, PE 1

comprises a solar boiler with mirror surface of around 18,000m. The steam is generated by concentrating

direct solar irradiation onto a linear receiver which is 7.40m above the ground. An absorber tube is

positioned in the focal line of the mirror field in which water is evaporated directly into saturated steam

at 270 C and at a pressure of 55 bar by the concentrated solar energy.

Hybrid Systems / ISCC

Hybrid systems combine power towers with natural gas power generators currently used at many power

plants, creating a system that can continuously generate electricity, even when the sun isn't shining.

Hybrid systems are not usually considered a separate category in concentrating solar thermal.

Components of a CSP Power Plant

The three main parts of CSP Power Plant are:


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The Solar Field

Thermal Energy Storage

The Power Generation System

On this site we will be learning mainly about the trough system, as this is the most prominent and

commercially proven technology.

The solar field

A parabolic trough power plant's solar field consists of a large, modular array of single-axis-tracking

parabolic trough solar collectors. Many parallel rows of these solar collectors span across the solar field,

usually aligned on a north-south horizontal axis. The basic component of a parabolic trough solar field is

the solar collector assembly or SCA. A solar field consists of hundreds or potentially thousands of solar

collector assemblies.

Each solar collector assembly is an independently tracking, parabolic trough solar collector composed of

the following key subsystems:

Concentrator structure

Mirrors or reflectors

Linear receiver or heat collection element

Pump system for the HTF

Collector balance of system

Also, each parabolic trough solar collector assembly consists of multiple, torque-tube or truss assemblies

(often referred to as solar collector elements or modules).

Concentrator Structure

The structural skeleton of the parabolic trough solar collector is the concentrator structure. The

concentrator structure:

Supports the mirrors and receivers, maintaining them in optical alignment

Withstands external forces, such as wind

Allows the collector to rotate, so the mirrors and receiver can track the sun.

The suppliers of the collector structure are more difficult to characterize. The only collector with a new

type of construction is the EuroTrough, which was designed and manufactured in many different

European countries.

Mirrors or Reflectors

The most obvious features of the parabolic trough solar collector are its parabolic-shaped mirrors or

reflectors.


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The German Flabeg GmbH & Co. KG (formerly part of the Pilkington Group), is one of the major

companies providing special glassware for technical applications and has the best bending technology for

mirrors. At present, the Flabeg Group is the only provider worldwide for the high precision solar

reflectors necessary for parabolic trough collector systems.

A number of alternative mirror concepts have been under development to reduce cost, improve

reliability, or increase performance.

Linear Receiver or Heat Collection Element

The parabolic trough linear receiver also called a heat collection element (HCE) is one of the primary

reasons for the high efficiency of the trough system. The receiver heats a special heat transfer fluid as it

circulates through the receiver tube. It is made up of stainless steel, special solar-selective absorber

surface surrounded by an anti-reflective glass tube.

Solel is one of the key technology suppliers of vacuum absorber tube. For the production of the absorber,

Sole purchases specific glass tubes at Schott Rohrglas GmbH in Germany, a company specialized in

sophisticated glass products for various applications. Schott Rohrglas currently develops a new type of

absorber tube on its own initiative, i.e. to become a competitor for Solel in this field.

Newer designs are being developed that can substantially improve:

Receiver reliability

Optical and thermal performance

The lifetime of receivers

Pump system for the HTF

Pumps for special technical applications are important for CSP, because it is a technological challenge to

manufacture pumps that are able to pump hot fluids, such as the heat transfer medium, of about 500

degree Celsius.

Collector Balance of System

A number of other key components make up the balance of system in the parabolic trough solar field,

including:

Pylons and foundations - The pylons support the collector structure. They allow the collector to rotate

and track the sun.

Drive - Each solar collector assembly includes one drive. The drive positions the collector to track the sun

during the day. It can be either a standard motor and gear box configuration or can use a hydraulic drive

system.


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Controls - Each solar collector assembly has its own local controller (LOC) that controls the tracking of

the collector. It communicates with a supervisory computer in the power plant control building to know

when to start tracking the sun or when to stop tracking at the end of the day.

Collector-interconnect It is used for connecting the receiver to header piping and between two adjacent

collectors. Earlier insulated flexible hoses were used but now new ball joint assembly is developed to

replace the flex hose.

Materials, sub-components and components required to make Solar Collector Assembly (SCA) are

Thermal Energy Storage

The availability of efficient and low-cost thermal storage is important for the long-term cost reduction of

trough technology and significantly increases potential market opportunities. Several thermal energy

storage (TES) technologies have been tested and implemented since 1985. These include:

The two-tank direct system,

Two-tank indirect system, and

Single-tank thermo cline system.

Two-Tank Direct

This system was used in early parabolic trough power plants. The energy generated is stored in the same

fluid used to collect it. The fluid is stored in two tanksone at high temperature and the other at low

temperature. Fluid from the low-temperature tank flows through the solar collector or receiver, where

solar energy heats it to a high temperature and it then flows to the high-temperature tank for storage. The

trough plants used mineral oil as the heat-transfer and storage fluid.

Two-Tank Indirect


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Two-tank indirect systems function in the same way as two-tank direct systems, except different fluids

are used as the heat-transfer and storage fluids. The plants will use organic oil as the heat-transfer fluid

and molten salt as the storage fluid. The indirect system requires an extra heat exchanger, which adds

cost to the system.

Single-Tank Thermo cline

Single-tank thermo cline systems store thermal energy in a solid mediummost commonly, silica sand

located in a single tank. At any time during operation, a portion of the medium is at high temperature,

and a portion is at low temperature. The hot- and cold-temperature regions are separated by a

temperature gradient or thermo cline. High-temperature heat-transfer fluid flows into the top of the

thermo cline and exits the bottom at low temperature. This process moves the thermo cline downward

and adds thermal energy to the system for storage. Reversing the flow moves the thermo cline upward

and removes thermal energy from the system to generate steam and electricity. Using a solid storage

medium and only needing one tank reduces the cost of this system relative to two-tank systems.

The only entirely proven and commercially employed technology is the two-tank molten salt system.

Future Storage Technologies

Scientists are also researching advanced heat-transfer fluids and novel thermal-storage concepts. The goal

is to increase efficiency and reduce costs for thermal energy storage.

Some of the concepts which are being tested are:

Direct Molten-Salt Heat Transfer Fluid

Using molten-salt in both the solar field and thermal energy storage system eliminates the need for

expensive heat exchangers. It allows the solar field to be operated at higher temperatures than current

heat transfer fluids allow. This combination also allows for a substantial reduction in the cost of the

thermal energy storage (TES) system. This technology is currently at research stage.

Indirect thermal energy storage:

Indirect Thermal systems heat an alternate medium to contain heat, including high temperature fluids

such as molten salts and solid media like cement and ceramics.

Concrete

Though only in experimental and in small scale commercial, high temperature concrete and castable

ceramics have been shown to be suitable thermal energy storage mediums. This method is extremely

simple and very low cost being even cheaper in the case of concrete.


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Phase Change Materials

Phase change materials can store large amounts of heat in small volume due to latent heat, resulting in

one of the lowest costs for storage media in thermal storage systems. Cascaded phase change materials

with slightly varying melt points were considered that take up heat and discharge upon flow reversal.

Raw materials, components and sub components in thermal storage system include:

Raw materials, sub-components and components of heat transfer fluids which carry thermal energy from

the collector to the storage media includes:

Power Generation System

A parabolic trough solar power plant uses a large field of collectors to supply thermal energy to a

conventional power plant. Because they use conventional power cycles, parabolic trough power plants


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can be hybridizedother fuels can be used to back up the solar power. Like all power cycles, trough

power plants also need a cooling system to transfer waste heat to the environment.

Parabolic trough power plant technologies include:

Power cycles

Steam Rankine

Organic Rankine

Combined

Fossil-fired (hybrid) backup

Wet and dry cooling

Power Cycles

There are a number of different power cycles that can be used for parabolic trough power plants. And

there are a number of options for how to integrate solar energy into the power cycle.

Steam Rankine: All of the SEGS (solar electric generating system) plants and most new projects are

planning to use steam Rankine power cycles. The power cycle uses a solar steam generator in place of the

conventional boiler fired by natural gas, coal, or waste heat from nuclear fission. Otherwise the power

cycle is very similar with the following components:

A surface condenser

Multiple low-pressure and high-pressure feed water heaters

Deaerator

Wet cooling towers.

Combined Cycle Power Plants: Some plants now integrate Solar Fields with an alternate energy source such

as gas turbines to ensure more constant production and sometimes even 24-hour steady operation.

Turbine waste heat is used for pre-heating or superheating the steam. Such systems can sometimes

double steam turbine capacity, but during non solar production, the large installed turbine will have to

run at part load, reducing efficiency. The cost of such a system is substantially lower than the installation

of a fresh standalone gas powered Rankine cycle station and it is the system of choice with several new

projects.

Fossil-Fired (Hybrid) Backup

Most existing parabolic trough power plants have hybrid backup capability. Because parabolic trough

power plants use conventional power cycle technologies, fossil-fired boilers or heaters usually can be

integrated to enable power plant operation at full-rated output during periods of low solar radiation,

such as overcast days and at night.


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Wet and Dry Cooling

Historically, parabolic trough power plants have used wet cooling towers. But now they can be designed

to use dry cooling technology for reducing water consumption. Utilization of dry cooling usually only

requires a modest increase in electricity cost.

Raw materials, sub-components and components of power block and cooling system are:

A complete flow diagram of raw materials to products


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SUMMARY

Energy is no good unto itself; it is valued rather as a means of satisfying important needs of a society. In

classical thermodynamics, energy is defined as the capacity to do work; but from a more practical point of

view, energy is the main stay of any industrial society. To maintain our present social structure, it is

desirable, that we supply an increasing portion of our energy needs from renewable sources and CSP can

be a one of those sources contributing a significant amount to the global energy production.

The radioactive solar energy reaching the earth during each month is approximately equivalent to the

entire world supply of fossil fuels. Thus, from a purely thermodynamic point of view, the global potential

of solar energy is many times larger than the current energy use. However, many technical and economic

problems must be solved before large-scale use of solar energy can occur. The future of solar power

deployment depends on how we deal with these constraints, which include scientific and technological

problems, marketing and financial limitations, and political and legislative actions including equitable

taxations of renewable energy sources.

introduction to csp technologies

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