Solar Power Stations - Part 3
    Solar Power Stations   List of Solar Power Stations  
 
150 MW, CSP stations in Spain, 3 Parabolic trough of Solnova and 2 Power Towers - PS10 and PS20.
12-05-08 AS1.JPG
Andasol Solar Power Station in Spain,
 Parabolic trough, 150 MW
 
 1- Current Technology - Concentrated Solar Power  technology, CSP technology
 

CSP (Concentrated Solar Power) is used to produce electricity (sometimes called solar thermoelectricity, usually generated through steam). Concentrated-solar technology systems use mirrors or lenses with 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). 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.

Concentrating technologies exist in 5 common forms, namely

  1. Parabolic Trough,
  2. Solar Power Tower,
  3. Fresnel Reflector,
  4. Dish Stirling/Engine,
  5. Enclosed Trough.
    Although simple, these solar concentrators are quite far from the theoretical maximum concentration. For example, the parabolic-trough concentration gives about 1/3 of the theoretical maximum for the design acceptance angle, that is, for the same overall tolerances for the system. Approaching the theoretical maximum may be achieved by using more elaborate concentrators based on nonimaging optics.

    Different types of concentrators produce different peak temperatures and correspondingly varying thermodynamic efficiencies, due to differences in the way that they track the sun and focus light. New innovations in CSP technology are leading systems to become more and more cost-effective.

 
  A- Parabolic Trough
   

  Reference:
    1- Simplified Methodology for Designing Parabolic Trough Solar Power
        by Ricardo Vasquez Padilla
        University of South Florida, rsvasque@mail.usf.edu
        2011
    2- Parabolic Trough Collector Overview
        by Dr. David W. Kearney
        National Renewable Energy Laboratory, Golden CO
        Workshop 2007

 
   

 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 filled with a working fluid. The reflector follows the sun during the daylight hours by tracking along a single axis. A working fluid (e.g. molten salt) 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.

 
 1- Gallery, Parabolic Trough Solar Stations
Part of the 354 MW SEGS solar complex in northern San Bernardino County, California. Parabolic trough at a plant near Harper Lake, California
12-05-08 AS1.JPG
Andasol Solar Power Station in Spain,  Parabolic ttrough, 150 MW Nevada Solar Parabolic Trough in USA, 64 MW
 
  1- Design of  PTC

PTCs are composed of parabolic trough-shaped mirrors, which reflect the incident radiation from the sun on the solar receiver tube. The receiver tube is located at the focus of a parabola in whose sides the mirrors are located.
As shown in Figure A, the circulating Heat Transfer Fluid (HTF) passes through the receiver and is heated up by the radiant energy absorbed. Then, the HTF is collected to be sent to the power block, where it passes through a series of heat exchangers and produces superheated steam at high temperatures. The superheated steam flows through a steam turbine where rotational mechanical work is then converted into electricity.
A solar field consists of hundreds of Solar Collector Assemblies, which are independently tracking assemblies of parabolic trough solar collectors.
Each SCA has the following components: metallic support structure, mirrors, solar receiver, and collector balance of the system.
Figure B
shows different parts of a Solar Collector Assembly (SCA) for a LS3 solar collector. In order to reach the operational conditions, the Solar Collector Assembly (SCA) are arranged in a series configuration normally known as a loop.
 The length of the loop depends on the PTC performance, but as shown in Figure C, it usually has a U shape to minimize the pressure drop through the pipe header. Usually the PTCs are oriented North-South and tracking the sun from east to west, but this also depends on the land constraints.
The economic feasibility of PTC solar plants is based on finding the optimum size for a given electric output. The preliminary analysis is performed by the integration of the complex models and components, which are integrated to simulate real operating conditions.
This dissertation proposes the development of a simple methodology for the initial design of parabolic trough solar systems based on physical models. The methodology is focused in obtaining a preliminary optimum design through a simplified methodology based on correlations obtained from detailed component simulations. This methodology is expected to be great help for engineers for the design and performance analysis of parabolic trough solar systems.

 
 
 
   2- Schematic Diagrams
 A-

  The troughs are usually designed to track the Sun along one axis, predominantly north–south. A heat transfer fluid (HTF), such as synthetic thermal oil or a mixture of Molten Salt, runs into the absorber tube and transfers the thermal energy to a conventional steam turbine power cycle, generating electricity. In particular, the fluid is heated to approximately 400° C by the sun’s concentrated rays and then pumped through a series of heat exchangers to produce superheated steam.
The steam is converted in to electrical energy in a conventional steam turbine generator, which can be either part of a conventional steam cycle or integrated into a combined steam and gas turbine cycle.
 
Parabolic Trough plants, then, consist of 2 parts:
 
A Solar Field, constituted by reflecting panels that concentrates the solar energy. Receiving tubes convert this energy into high-temperature heat which can be stored for some hours. 
Power Generation Plant where a traditional turbine power generation system produces electricity.
 
  B-

 
Parabolic Trough technology consists of installing rows or loops of parabolic trough-shaped mirror reflectors that are used to collect the solar radiation and concentrate it onto a thermally efficient receiver tube placed in the trough’s focal line. The fluid is heated up to approximately 400°C by the concentrated sun’s rays and then pumped through a series of heat exchangers to produce superheated steam. The steam is converted to electrical energy in a conventional steam turbine generator, which can either be part of a conventional steam cycle or integrated into a combined steam and gas turbine cycle. This fluid can also be used to heat a storage system consisting of two tanks of molten salt.

 

 3- Efficiency

The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day. Alternatively, the trough can be aligned on an east-west axis; this reduces the overall efficiency of the collector due to cosine loss but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method approaches theoretical efficiencies at the spring and fall equinoxes with less accurate focusing of the light at other times during the year. The daily motion of the sun across the sky also introduces errors, greatest at the sunrise and sunset and smallest at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower concentration acceptance product.

Parabolic trough concentrators have a simple geometry, but their concentration is about 1/3 of the theoretical maximum for the same acceptance angle, that is, for the same overall tolerances of the system to all kinds of errors, including those referenced above. The theoretical maximum is better achieved with more elaborate concentrators based on primary-secondary designs using nonimaging optics which may nearly double the concentration of conventional parabolic troughs and are used to improve practical designs such as those with fixed receivers.

A diagram of a parabolic trough solar farm (top), and an end view of how a parabolic collector focuses sunlight onto its focal point.
 4- Commercial plants 

Most commercial plants utilizing parabolic troughs are hybrids; fossil fuels are used during night hours, but the amount of fossil fuel used is limited to a maximum 27% of electricity production, allowing the plant to qualify in the US as a renewable energy source. Because they are hybrids and include cooling stations, condensers, accumulators and other things besides the actual solar collectors, the power generated per square meter of area varies enormously.
As of 2014, the largest solar thermal power systems using parabolic trough technology include, the 354 MW SEGS plants in California, the 280 MW Solana Generating Station that features a molten salt heat storage, the 250 MW Genesis Solar Energy Project, that came online in 2014, as well as the Spanish 200 MW Solaben Solar Power Station, the 200 MW Solnova Solar Power Station, and the Andasol 1 solar power station, using a Eurotrough-collector.

Array of parabolic troughs.
 
 
  B- Solar Power Tower

 Reference, Technical Paper:
    1- Progress Towards Cost-Competitive Solar Power Tower Plants
        by K.L. Santelmann, D.T. Wasyluk, and B. Sakadjian
        Babcock & Wilcox Power Generation Group, Inc., Barberton, Ohio, U.S.A.
        Presented to:  Power-Gen Middle East, 2014, Abu Dhabi, United Arab Emirates

 
 
   In Solar Thermal Tower Power plants, hundreds or even thousands of large two-axis tracked mirrors are installed around a tower. These slightly curved mirrors are also called heliostats; a computer calculates the ideal position for each of these, and a motor drive moves them into the sun. The system must be very precise in order to ensure that sunlight is really focused on the top of the tower. It is here that the absorber is located, and this is heated up to temperatures of 500–1000 °C (773–1,273 K (932–1,832 °F)) . Hot air or molten salt then transports the heat from the absorber to a steam generator; superheated water steam is produced there, which drives a turbine and electrical generator. Power-tower development is less advanced than trough systems, but they offer higher efficiency and better energy storage capability.
 
 1- Gallery, Solar Power Tower Stations
Aerial photographs of the PS10 and PS20 solar power towers.
PS10 solar power tower, 11 MW, in Spain, has 624 mirrors reflecting the sun to the 40 storey tower, with some night-time storage capacity. 20 MW, CSP in Spain - PS10 Solar Power Plant in the foreground, with PS20 in the background.
5 MW Sierra SunTower, located in Lancaster, California The THEMIS solar power tower in the Eastern Pyrenees - France, 2 MW, has 201 mirrors reflecting the sun  which heated a boiler (a cavity lined with coolant tubes) at the top of a 100 m tower where the coolant (molten salts) carried the thermal energy to a vapour generator, itself powering an electric turbine.
solar technology using a central power tower the middle of a circle of mirrors, 10 MW, in Daggett California. Small sized Concentrating Solar Power system 

Ivanpah solar towers station  in the Mojave Desert - California, 400 MW,  uses an array of small, flat mirrors on heliostats that track the sun and focus its rays on a central "power tower" where the generator is located.

 
  2- Schematic Diagrams
 2.1- Power Tower System Plant


Power tower systems consist of numerous large, flat, sun-tracking mirrors, known as heliostats that focus sunlight onto a receiver at the top of a tower. The heated fluid in the receiver is used to generate steam, which powers a turbine and a generator to produce electricity. Some power towers use water/steam as the heat-transfer fluid. Individual commercial plants can be sized to produce up to 200 megawatts of electricity.
   
 
 2.2- Solar Power Tower System Plant without Thermal storage, Single or Multiple Receivers

 
 In this arrangement, solar energy is concentrated by a field of heliostats (mirror assemblies) onto one or more tower-mounted solar receivers which absorb the energy and convert feedwater into superheated steam. The superheated steam, at high temperature and pressure, is piped directly to a steam turbine-generator to produce electrical power.
 
The Sierra SunTower plant in Lancaster, California is a direct-steam plant with two power towers capable of supplying up to 5 MW of clean, renewable energy to the grid.
This direct-steam plant design changes the cost structure dramatically via thousands of independently controlled heliostats organized into modular collector fields.

 
 2.3- Schematic of two types of Tower Receiver of solar thermal tower power plant
 2.3a- Solar Tower, Open Volumetric Air Receiver Concept

 

 This  type of solar tower is the open volumetric receiver concept (2-2a). A blower transports ambient air through the receiver, which is heated up by the reflected sunlight. The receiver consists of wire mesh or ceramic or metallic materials in a honeycomb structure, and air is drawn through this and heated up to temperatures between 650°C and 850°C. On the front side, cold, incoming air cools down the receiver surface. Therefore, the volumetric structure produces the highest temperatures inside the receiver material, reducing the heat radiation losses on the receiver surface. Next, the air reaches the heat boiler, where steam is produced. A duct burner and thermal storage can also guarantee capacity with this type of solar thermal power plant.

 

  2.3a- solar thermal tower power plant, Open Volumetric Air Receiver
 2.3b- Solar Tower, Pressurized Air Receiver Concept


 

 The volumetric pressurized air receiver concept (2.2b) offers totally new opportunities for solar thermal tower plants. A compressor pressurizes air to about 15 bar; a transparent glass dome covers the receiver and separates the absorber from the environment. Inside the pressurized receiver, the air is heated to temperatures of up to 1100°C, and the hot air drives a gas turbine. This turbine is connected to the compressor and a generator that produces electricity. The waste heat of the gas turbine goes to a heat boiler and in addition to this drives a steam-cycle process. The combined gas and steam turbine process can reach efficiencies of over 50%, whereas the efficiency of a simple steam turbine cycle is only 35%. Therefore, solar system efficiencies of over 20% are possible.

 

  2.3b- solar thermal tower power plant, Pressurized Air Receiver (with combined gas and steam turbine cycle)

 

3- Economic

By varying the size of the solar field, solar receiver, and size of the thermal storage, plants can be designed with annual capacity factors ranging between 20 and 65%.
 

Economic studies have shown that levelized energy costs are reduced by adding more storage up to a limit of about 13 hours (~65% capacity factor). While it is true that storage increases the cost of the plant, it is also true that plants with higher capacity factors have better economic utilization of the turbine, and other balance of plant equipment

 
 
  C- Fresnel Reflector
Design and Analysis of Rooftop Linear Fresnel Reflector Solar Concentrator       (372 KB pdf file)   download
References:
    1- Advanced CSP Teaching Materials,  Chapter 6 - Linear Fresnel Technology
        by Matthias Günther       

    2- Linear Fresnel Reflector based Solar System, Operations & Maintenance Manual
        by UNDP-GEF Project on Concentrated Solar Heat
        Ministry of New & Renewable Energy Government of India

    3- AUGUSTIN FRESNEL 1 PROJECT: Design, Construction and Testing of a Linear Fresnel Pilot Plant in the Pyrenees
        by David Itskhokine, Patrick Lécuillier
       
 
 
Linear Fresnel solar CSP technology produces energy as heat through the use of long and narrow segments of mirror that pivot to reflect the sunlight onto a fixed absorber tube located at the common focal line of the reflectors called Linear Fresnel Reflectors. This energy can be used either directly in the form of heat, either in the form of electricity after conversion.

 Solar thermal energy is collected and concentrated with mirror reflectors equipped with tracking devices to maximize the amount of solar energy collected over the day. The reflected sunrays are focused on a receiver tube located on the focal axis of the reflectors. Inside the tube, a heat transfer fluid circulates - water in this case - which is heated, evaporated and superheated by the concentrated solar energy.
 With the control of a two-phase flow within the receiver tube, Solar Euromed’s Linear Fresnel solar CSP technology allows the use of water and steam directly as heat transfer fluid. It simplifies the overall solution, by preventing the need of a potentially toxic and pollutant fluid heat transfer fluid and of a steam generator. The steam produced can then be transferred to a Rankine thermodynamic cycle, wherein a steam turbine coupled to an alternator converts thermal energy into electricity
Linear Fresnel Reflectors for Direct Steam Generation
 
  1- Gallery, Linear Fresnel Solar Power Systems
The Fresnel Solar power Station, PE 1. PE 1 is situated in Murcia/Spain, 1.4 MW. PE 2, the largest Fresnel Solar power Station in Murcia, Spain, with 30 MW electric power, approximately 50 million kW hours of electricity per year.. The technology uses direct steam generation. Instead, this highly economical and proven concept utilizes compact almost flat glass mirrors, with a mirror surface of 302,000m2. The uniquely efficient solar boiler produces superheated steam directly at a temperature of up to 500 degrees Celsius and a pressure of 100 bar.
 

.
 

The Fresnel Solar power station in Bakersfield/California. Its electric power reaches 5 MW. small scale Fresnel Collector for Heat Generation.
 9 MW, Novatec Solar’s Fresnel technology station in Australia, uses parallel rows of flat mirrors to focus direct solar irradiation onto a linear receiver. Water that is conveyed through the absorber tubes is directly evaporated or superheated in a temperature range from 270°C to above 500°C depending on the steam application. The cleaning of the mirrors is done by cleaning robots using very less water to save valuable water in this arid region
 
  2- Schematic Diagram of Linear Fresnel Solar CSP
 
 Linear Fresnel Solar Systems

   An array of nearly flat reflectors concentrate solar radiation onto elevated inverted linear receivers. Water flows through the receivers and is converted into steam. This system is linear-concentrating, similar to a parabolic trough, with the advantages of low costs for structural support and reflectors, fixed fluid joints, a receiver separated from the reflector system, and long focal lengths which allows the use of flat mirrors. The technology is seen as a potentially lower-cost alternative to trough technology for the production of solar process heat and steam.
Linear Fresnel reflector (LFR) also based on solar collector rows or loops. However, in this case, the parabolic shape is achieved by almost flat linear facets. The radiation is reflected and concentrated onto fixed linear receivers mounted over the mirrors, combined or not with secondary concentrators. One of the advantages of this technology is its simplicity and the possibility to use low cost components. Direct saturated steam systems with fixed absorber tubes have been operated at an early stage of use of LFR technology. This technology eliminates the need for HTF and heat exchangers. Increasing the efficiency depends on superheating the steam. Superheated steam up to 500°C has been demonstrated at pilot plant scale and first large commercial superheated LFR plants have begun recently their operation.
  Note: 

    Fresnel Reflectors (FR) plants are similar to PT plants but use a series of ground-based, flat or slightly curved mirrors placed at different angles to concentrate the sunlight onto a fixed receiver located several meters above the mirror field.

    Each line of mirrors is equipped with a single axis tracking system to concentrate the sunlight onto the fixed receiver. The receiver consists of a long, selectively-coated tube where flowing water is converted into saturated steam (DSG or Direct Steam Generation). Since the focal line in the FR plant can be distorted by astigmatism, a secondary mirror is placed above the receiver to refocus the sun’s rays. As an alternative, multi-tube receivers can be used to capture sunlight with no secondary mirror. The main advantages of FR compared to PT systems are the lower cost of ground-based mirrors and solar collectors.
 
Compact Linear Fresnel Reflectors (CLFRs) to concentrate energy on water pipes, which produce steam to drive turbines
 
   Schematic Diagram of Linear Fresnel Solar CSP, in Frensh
 
  3- Examples, Linear Receiver for LFR Collector
Geometry Fresnel reflector
Schematic design of a Solar single-tube receiver Solar single-tube receiver with compound parabolic shaped secondary concentrator  
Schematic design of a Solar multipe tube receiver multiple tube receiver with simple trapezoidal secondary reflector
 
  4- Concentrated Linear Fresnel Reflectors CLFR, The next generation of Concentrating Solar Power

      Benefits of CLFR:

  • Sturdy, Low Cost Construction

  • Primary Components Steel, Glass, Water

  • Efficient Use of Land

  • Air Cooled; Minimal Water Use

  • No Toxic Materials

  • Easily Protected from Hail and Dust Storms

  • Can be hybridized with fossil fuel plants
CLFR for direct steam generation to be used in electric power production or process heat in Australia.
  5- Conclusion
 

   Compared to other technologies, the investment costs per square meter of collector field using linear Fresnel technology tend to be lower because of the simpler solar field construction, and the use of direct steam generation promises relatively high conversion efficiency and a simpler thermal cycle design. The Fresnel design uses less expensive reflector materials and absorber components. It has lower optical performance and thermal output but this is offset by lower investment and operation and maintenance costs.
 

   There are almost more than 200MW LFR plants in operation or under construction. After a first pilot scale application in Australia, a few new pilot plants have been tested in Spain and in the United States. In 2012, the first commercial 30-MW Puerto-Errado 2 plant began its operation in Spain. France has already implemented two linear Fresnel pilot plants and is currently building two new commercial plants with this technology of 9 and 12 MW respectively, named Llo and Alba Nova 1 plants constructed respectively by CNIM and SOLAR EUROMED. In Australia, there are two plants running with this technology of 6 and 9.3 MW respectively. There is also one 44-MW plant under construction at Kogan Creek. In India, Reliance has completed and connected to the grid in November 2014 at Dhursar in Rajasthan a 125MW CLFR plant designed and constructed by AREVA Solar.
 
 
  D- Dish Stirling/Engine
EuroDish, Stirling System Description       (854 KB pdf file)   download
 Reference:
    1- Price Model of the Stirling Engine
        by Isabelle Gadré, Johanna Maiorana
        KTH School of Industrial Engineering and Management, STOCKHOLM

    2- 30 kW Maintenance, Free Solar Dish Engine
        by Infinia FOA Program
        Austin, Texas, 2008
 
     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
approximately 250–700 °C (523–973 K (482–1,292 °F)). This fluid or gas is then used to generate electricity in a small piston or Stirling engine or a micro turbine, attached to the receiver. Parabolic-dish systems provide high solar-to-electric efficiency (between 31% and 32%), and their modular nature provides scalability.
Parabolic dish could be applied individually in remote locations, or grouped together for small-grid (village power, 10 KW) or end-of-line utility (100 MW) applications. The electricity has to be used immediately or transmitted to the gird as the system has no storage device.

 

  1- Gallery, Stirling/Engine Dish Power Systems

This solar dish-engine system collects the sun’s energy and concentrates it on a small receiver. The thermal receiver absorbs the concentrated beam of solar energy, converts it to heat, and transfers the heat to the engine/generator. 150 kW of grid-ready electricity. this dish unit consists of 82 mirrors.

Dish-Stirling prototype systems in Spain. A parabolic concave mirror (the dish) concentrates sunlight; the two-axis tracked mirror must follow the sun with a high degree of accuracy in order to achieve high efficiencies. In the focus is a receiver which is heated up to 650°C. The absorbed heat drives a Stirling motor, which converts the heat into motive energy and drives a generator to produce electricity. If sufficient sunlight is not available, combustion heat from either fossil fuels or biofuels can also drive the Stirling engine and generate electricity. Dish/engine system with stretched-membrane mirrors: this design allows wind to pass through to minimize the destructive force of wind
Dish/engine system with stretched-membrane mirrors: this design allows wind to pass through to minimize the destructive force of wind. Solar Dishes on a solar farm near Peoria, Arizona, USA Stirling Dishes System, 300 MW solar thermal power plant in California, USA. each dish unit consists of 82 mirrors.
 
 2- Dish Design

 
 The solar dish design is relatively new and quite different from the other concentrated solar power designs due to the fact that it usually doesn’t use steam and a turbine to produce electricity. Instead, a reflective disc that is similar to the shape of a satellite dish, focuses sunlight onto something called a Stirling engine . The engine is positioned in front of the reflective dish by an arm attached to the dish (see images below). This is a very unique non-combustible engine that functions by using temperature differences . The Stirling engine has a fixed amount of gas sealed within . Heating one end of the Stirling engine with concentrated sunlight creates a large temperature difference from the opposite end of the engine. Therefore, the hot temperature at one end causes the gas to expand and move towards the cooler end. As a result, the gases flow within the engine chamber, which in turn makes the engine run. The Stirling engines can then be used to create electricity.
solar dish design

Stirling Energy Systems Solar Dish  Stirling Energy Systems Station in the Imperial Valley of Southern California, 300 MW, consists of 12,000 dishes

 3- Schematic Diagram

Dish/engine systems use parabolic dishes of mirrors to direct and concentrate sunlight onto a central engine that produces electricity. The dish/engine system produces relatively small amounts of electricity compared to other CSP technologies-typically in the range of 3 to 25 kilowatts.
The Solar Concentrator, or dish, gathers the solar energy coming directly from the sun. The resulting beam of concentrated sunlight is reflected onto a thermal receiver that collects the solar heat. The dish is mounted on a structure that tracks the sun continuously throughout the day to reflect the highest percentage of sunlight possible onto the thermal receiver.
The Power Conversion Unit includes the thermal receiver and the engine/generator. The thermal receiver is the interface between the dish and the engine/generator. It absorbs the concentrated beams of solar energy, converts them to heat, and transfers the heat to the engine/generator. A thermal receiver can be a bank of tubes with a cooling fluid—usually hydrogen or helium—that typically is the heat-transfer medium and also the working fluid for an engine. Alternate thermal receivers are heat pipes, where the boiling and condensing of an intermediate fluid transfers the heat to the engine.
The engine/generator system is the subsystem that takes the heat from the thermal receiver and uses it to produce electricity. The most common type of heat engine used in dish/engine systems is the Stirling engine. A Stirling engine uses the heated fluid to move pistons and create mechanical power. The mechanical work, in the form of the rotation of the engine's crankshaft, drives a generator and produces electrical power.

Dish/Engine Power plant - Concentrator and Power Conversion Unit.
 
 
  4- Solar Powered Stirling Engine, Parabolic Dish

   Parabolic Dish systems use satellite-like mirror dish(es) to focus the light onto a singlecentral receiver in front of the mirror. They so far have the highest heat-electricity conversion efficiencies among all CSP designs (up to 30 %). The size of the concentrator is determined by its engine. A dish/Stirling system’s concentrator with a nominal maximum direct normal solar insolationof 1000 W/m2 and a 25-kW capacity has a diameter ofapproximately 10 meters. It could also run on a single Brayton cycle, where air, helium or other gas is compressed, heated and expanded into a turbine.
  "Parabolic dish could be applied individually in remote locations, or grouped together for small-grid (village power, 10 KW) or end-of-line utility (100 MW) applications. The electricity has to be used immediately or transmitted to the gird as the system has no storage device".
 Intermittent cloud cover can cause weakening of highly concentrated receiver source flux. Sensible energy storage in single-phase materials was proposed to allow a cylindricalabsorber element not only absorb the energy but also store it in its mass, thus reducing the amplitude of cloud cover transients. Although this design only allows short period energy storage, potential longer time storage technology would make parabolic dish more appealing

Stirling Energy System Inc.’s 300 MW commercial solar thermal power plant in California
Dish/engine system schematic. The dish that follows the sun on two axes focuses the sunlight onto one single point on a receiver posed right in front of the mirror. 
 
 
  5- EuroDish Stirling System
   Dish-Stirling Systems are small power generation sets which generate electricity by using direct solar radiation. The capacity of a single unit is typically between 5 and 25 (50) kWel.

  Dish-Stirling Systems consists of the following components:
  • Parabolic solar concentrator
  • Tracking system
  • Solar heat exchanger (Receiver)
  • Stirling engine with generator

The parabolic concentrator reflects the incoming solar radiation onto a cavity receiver which is located at the concentrator’s focal point. The solar radiation is absorbed by the heat exchanger (receiver) and thus heats the working gas (helium) of the Stirling engine to temperatures of about 650 oC. This heat is converted into mechanical energy by the Stirling engine. An electrical generator, directly connected to the crankshaft of the engine, converts the mechanical energy into electricity (AC). To constantly keep the reflected radiation at the focal point during the day, a suntracking system rotates the solar concentrator continuously about two axes to follow the daily path of the sun. The electrical output of the system is proportional to the size of the reflector.

 Technical Data:

  1. Concentrator:
    Diameter                        8.5 m
    Projected area              56.7 m²
    Reflectivity                    94 %
  2. Tracking and Control:
    Drive                             servo motor
    Control system           PC, micro controller
    Remote control           telephone / WWW
  3. Stirling engine:
    Type                             single acting, 90° V-engine
    Swept volume             160 cm³
    Gross power output   9 kW
    Net power output        8.4 kW
    Grid connection         400 V, 50 Hz, 3 phase
    Receiver gas temperature     650 °C
    Working gas              helium
    Gas pressure              20-150 bar
    Power control             pressure control

 
 
 
 
  F- Enclosed trough
 Reference:
    1- Construction of an Enclosed Trough Enhanced Oil Recovery - EOR system in South Oman
        by B. Biermana
        SolarPACES 2013
 
 

       Enclosed trough systems are used to produce process heat. The design encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment to with stand the elements that can negatively impact reliability and efficiency of the solar thermal system. Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight. The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure. Water is carried throughout the length of the pipe, which is boiled to generate steam when intense solar radiation is applied. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors.

 
 
GlassPoint Brings 7 MW of Enclosed Trough CSP On-Line in Oman GlassPoint pilot plant, South Oman
robotic cleaning system automatically washes the roof of the glasshouse at night Lightweight mirror installation, 3 workers, no crane, no work at height
   
  1- Description
  • The enclosed trough is a variant of the technology parabolic trough
  • The design encapsulates the solar thermal system within a greenhouse-like glasshouse.
  • The glasshouse creates a protected environment to withstand the elements that can negatively impact reliability and efficiency of the solar thermal system.
  • Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight.
  • The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure. Water is carried throughout the length of the pipe, which is boiled to generate steam when intense sun radiation is applied. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors.
 
  2- Schematic

The Solar Steam Generation Pilot (SSGP) plant has a solar field footprint of 17,280 m2, with a peak output of over 7 MW thermal. The Enclosed trough system is protected by a glass structure, which is similar to an agricultural greenhouse. Lightweight parabolic mirrors (4 of Fig 1) are hung within the glasshouse (2 of Fig 1). The glasshouse provides structural support and isolates the solar collectors from wind and moisture.
Roof washer.
The enclosed trough glasshouse structure is fitted with an automated roof washing system (1 of Fig 1) capable of cleaning the entire roof surface each night. The majority of wash water is returned in the gutter system and can be recovered for re-use. Dust infiltration is minimized by positive pressure from an air-handling unit (6 of Fig 1), which provides filtered, dried air at slight overpressure within the structure in all conditions.
Reflector.
Due to the absence of wind forces acting on the collectors, a lightweight parabolic reflector can deliver consistently high optical accuracy. Total weight of the mirror and frame is only 4.2 kg/m.2 The lightweight reflector enables a simple cable drive aiming system.  The fully automatic control system delivers less than 0.5 mrad pointing error at hundreds of points within the glasshouse.
Receiver.
The low system weight allows the entire mirror system to be suspended from the fixed receiver system. This fixed receiver eliminates all moving parts from the high-pressure direct steam receiver system. Tubular glass shields minimize heat losses from convection (3 of Fig 1). The receivers are suspended from the structure by steel rods (5 of Fig 1).
Boiler.
This system is specifically designed to mimic a conventional oilfield OTSG boiler process, and accommodates feed water, with total dissolved solids as high as 30,000 ppm, and produces 80% quality steam at 100 bar.
Water tank.
An 80-m3 upright, insulated water tank was added to the system during the year to improve overall performance.

 
Fig 1 - Shematic of Enclosed trough solar system
 
   
  3- How Enclosed Troughs work  
  • Step One - There are no solar panels in a GlassPoint system. instead, large curved mirrors are suspended inside an agricultural glasshouse.
  • Step Two - The mirrors automatically track the sun throughout the day, focusing sunlight  on a stationary boiler tube containing water
  • Setp Three -  The concentrated sunlight heats the water to efficiently produce high-pressure steam
 
 
 
 
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