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Concentrated Solar Power

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 List of Contents
  1. Introduction to CSP Technologies
    1. Why should we take interest in harvesting energy from solar?
    2. What is CSP?
    3. History of CSP
    4. CSP First Principles
    5. Parabolic Trough System
    6. Parabolic Dish System
    7. Central Receiver System
    8. Linear Fresnel
    9. Hybrid System / ISCC
  2. Components of a CSP Power Plant
    1. The Solar Field
      1. Concentrator structure
      2. Mirrors or reflectors
      3. Linear receiver or heat collection element
      4. Pump system for the HTF
      5. Collector balance of system
    2. Thermal Energy Storage
      1.  The two-tank direct system
      2. Two-tank indirect system
      3. Single-tank thermocline system
    3. The Power Generation System
      1. Power cycles
        1. Steam Rankine
        2. Organic Rankine
        3. Combined
      2. Fossil-fired (hybrid) backup
      3. 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, 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 earth’s 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 “quad Btu” 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 race’s 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 system that 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. 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 Roman fleet 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 (1911–1980) designed and built the first concentrated-solar plant. which entered 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

 

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 trakcing 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 400°C, 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 transferred to the working fluid which may be air, water, oil, or molten salt. The upper termperature that can be achieved in solar thermal conversion depends on

  • the insolation
  • 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 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 of land. 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 (http://www.fplenergy.com/portfolio/contents/segs_viii.shtml). 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 capacity, due to the former plant's thermal energy storage system and larger solar field.
  • 553MW new capacity is proposed in Mojave Solar Park, California (http://www.ynetnews.com/articles/0,7340,L-3430085,00.html). Furthermore, 59MW hybrid plant with heat storage is proposed near Barstow, California (www.earthtoys.com/news.php?section=view&id=3361). Near Kuraymat in Egypt, some 40MW steam is used as input for a gas powered plant (http://www.bbj.hu/main/news_32058_iberdrola+to+build+150+mw+egyptian+thermal+solar+plant.html). Finally, 25MW steam input for a gas power plant in Hassi R'mel, Algeria.

 

 

Fig. Parabolic Solar Trough System at Kramer Junction, Mojave desert, California

Technology Constraints:

  • The upper process temperature is currently limited by the heat transfer thermal oil to 400°C.
  • 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 fliexible 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, but also sometimes a steam engine is used (www.stirlingenergy.com/solar_overview.htm).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 engines from 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 (http://www.bizjournals.com/phoenix/stories/2010/01/18/daily87.html). 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, (http://gigaom.com/cleantech/tessera-solar-sells-troubled-850mw-project-to-k-road/) 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 72,000 kilowatt-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

Technology Constraints:

  • The electricity output of single dish/Stirling unit is limited to small ratings due to geometric and physical reasons.
  • 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 of the 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 tracking system.
  • 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 receiver where 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. The heliostats 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 funding from 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 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 gigawatt over the next 10 years.
  • eSolar's proprietary sun-tracking software coordinates the movement of 24,000 1 meter-square mirrors per 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 π square mile parcels of land, resulting in a land-to-power ratio of 4 acres (16,000 m2) per 1 megawatt.
  • BrightSource Energy entered into a series of power purchase agreements with Pacific Gas and Electric Company in March 2008 for up to 900MW of electricity, the largest solar power commitment ever made by a utility (http://news.cnet.com/8301-11128_3-9907089-54.html). BrightSource 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, BrightSource Energy dedicated its 4-6 MW (http://www.brightsourceenergy.com/projects/sedc) Solar Energy Development Center (SEDC) in Israel'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 (http://cleantech-israel.blogspot.com/2008/06/brightsource-luz-ii-dedicate-negev.html).
  • A working tower power plant is PS10 in Spain with a capacity of 11MW.
  • The 15MW Solar Tres 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 m² (http://solar4africa.net/news/viewnews.php?ID=249). A 10MW power plant in Cloncurry, Australia (with purified graphite as heat storage located on the tower directly by the receiver) (http://www.theage.com.au/news/National/Cloncurry-to-run-on-solar-alone-Bligh/2007/11/04/1194117867247.html).
  • Morocco is building five solar thermal power plants around Ouasarzate. The sites will produce about 2000 MW by 2012. Over ten thousand hectors of land will be needed to sustain all of the sites (http://www.afdb.org/en/projects-and-operations/project-portfolio/project/p-ma-fac-012/).
  • Out of commission are the 10MW Solar One (later redeveloped and made into Solar Two) and the 2MW Themis plants.
  • A cost/performance comparison between power tower and parabolic trough concentrators was made by the 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 be 72.9% and 56.2% for parabolic troughs (http://www.nrel.gov/solar/parabolic_trough.html). There is some hope that the development of cheap, durable, mass producible heliostat power plant components could bring this cost down (http://www.google.com/intl/en/press/pressrel/20071127_green.html).

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

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

Technology Constraints:

  • 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 Solarmundo in Belgium.
  • The Solarmundo research and development project, with its pilot plant at Liège, was closed down after successful proof of concept of the Linear Fresnel technology. Subsequently, Solar Power Group GmbH (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 Ausra’s 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 18,000m². 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:

  • 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.

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.
    • 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 thermocline 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 tanks—one 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

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 Thermocline

Single-tank thermocline systems store thermal energy in a solid medium—most 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 thermocline. High-temperature heat-transfer fluid flows into the top of the thermocline and exits the bottom at low temperature. This process moves the thermocline downward and adds thermal energy to the system for storage. Reversing the flow moves the thermocline 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.

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 can be hybridized—other 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 feedwater heaters
    • Deaerator
    • Wet cooling towers.

 Organic Rankine cycles (ORCs):  use an organic fluid—such as butane or pentane—instead of water, like a steam Rankine cycle.

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.

 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

SUMMARY

Energy is no good unto itself; it is valued rather as a means of stisfying 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 radiative 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.