*CONCENTRATING SOLAR COLLECTORSPortland State University

Solar EngineeringSpring 2008

Carolyn Roos, Ph.D.Washington State UniversityExtension Energy Program

*OUTLINEA review of six concentrating solar technologies and current projects.Basics of ray tracing.Sketch of a thermal analysis example

*Solar Concentrating SystemsConcentrate solar energy through use of mirrors or lenses.

Concentration factor (number of suns) may be greater than 10,000.

Systems may be small:e.g. solar cooker .... or large: - Utility scale electricity generation (up to 900 MWe planned) - Furnace temperatures up to 3800oC (6800oF)

*Concentrating Solar Power:A Revived IndustryUtility Action on ~3,000 MW in 2005-06CSP for Commercial & Industrial FacilitiesIndustrial Solar Techs Roof SpecsMore planned since 2006

*States Creating a Market for CSPAZ: 15% RE by 2025, 30% Distributed GenerationCA: 20% by 2010 & 33% by 2020 plannedCO: 10% by 2015NV: 20% by 2015, 5% SolarNM: 10% by 2011TX: 4.2% by 2015

*In a Carbon Limited FutureCarbon limits will close the cost gap.

CSP can scale up fast without critical bottleneck materials. (e.g. silicon)

Costs will come down with increase in capacityexpected to fall below natural gas in the next few years.

In the very near future, the CSP market in the SW US can grow to 1 to 2 GW per year.

From: http://www.nrel.gov/csp/troughnet/pdfs/2007/morse_look_us_csp_market.pdf

*Examples of CSP ApplicationsPower Generation:Utility Scale: 64 MW Nevada Solar One (2007)Buildings: 200 kW Power Roof

Thermal Needs:Hot Water and Steam (Industrial & Commercial Uses)Air Conditioning Absorption ChillersDesalination of seawater by evaporationWaste incineration

Solar Chemistry Manufacture of metals and semiconductorsHydrogen production (e.g. water splitting)

Materials Testing Under Extreme Conditionse.g. Design of materials for shuttle reentry

*Primary Types of Solar CollectorsParabolic TroughCompact Linear Fresnel Reflector newSolar FurnaceParabolic Dish & EngineSolar Central Receiver (Solar Power Tower)Lens Concentrators

Can be used in conjunction with PV:Use lenses or mirrors in conjunction with PV panels to increase their efficiency. (http://seattle.bizjournals.com/seattle/stories/2006/04/24/focus2.html)

*PARABOLIC DISH & ENGINESOLAR FURNACECENTRAL RECEIVERSOLAR FURNACEPARABOLIC DISHPARABOLIC TROUGHFRESNEL REFLECTORLENS CONCENTRATORS

*Major Components of Solar Collector SystemsConcentrating mirror(s) May use primary & secondary concentrators.

Absorber within a ReceiverReceiver contains the absorber. It is the apparatus that receives the solar energy; e.g. evacuated tube. Absorber absorbs energy from concentrator and transfers to process being driven (engine, chemical reactor, etc.); e.g. the pipe within an evacuated tube.

Heliostats Flat or slightly curved mirrors that track the sun and focus on receiver or concentrator. Used with solar furnaces and power towers.

*Parabolic TroughsMost proven solar concentrating technologyThe nine Southern California Edison plants (354 MW total) constructed in the 1980s are still in operation

*Parabolic Troughs - OperationParabolic mirror reflects solar energy onto a receiver (e.g. a evacuated tube).

Heat transfer fluid such as oil or water is circulated through pipe loop. (250oF to 550oF)

Collectors track sun from east to west during day.

Thermal energy transferred from pipe loop to process.

*Parabolic Trough System- Can be hybrid solar / natural gas- New systems include thermal storage.

*Thermal StorageUses high heat capacity fluids as heat transfer storage mediums12 to 17 hours of storage will allow plants to have up to 60% to 70% capacity factors.From: http://www1.eere.energy.gov/solar/pdfs/csp_prospectus_112807.pdf

*Thermal Output of Hybrid Plant with Thermal Storage

*What Have Been the Technical Challenges?Development of MaterialsHeat transfer tubes that are less prone to sagging & breaking.Improved surface material of heat transfer tubes. High absorptivity, low emissivity and long-term stability in air. Low cost mirrors that have reflectivity and washability of glass.

Improved ComponentsFlex hoses used to join sections of pipe loop were prone to failure Replaced with ball joint design.Ability to track on tilted axis

Improved Processese.g. Generate steam directly instead of running heat transfer fluid through heat exchanger - Improves efficiency but more difficult to control.

*

Saguaro Solar Generating Station (north of Tucson) 1MW - Compared to 395MW in natural gas fired generating capacity at same site Broke ground March 24, 2004 and started generating power December 2005 Built by Solargenix, subsidiary of ACCIONA Energy of Spain Arizona has goal of 15% renewable energy by 2025. $6 Million ProjectFirst Solar Thermal Parabolic Trough Power Plant Built in The U.S. In Nearly Two Decades to Be Dedicated On Earth Day (2005)

*Saguaro Solar Generating Station 1MW - 2005

*Nevada Solar One64 MW - 2007Now producing 64 MW on 140 hectaresLocated in Eldorado Valley (south of Las Vegas)One of the world's largest CSP plants. Cost: $262 million Developed by Solargenix Energy.SHOTT North America provided receivers.Groundbreaking in February 2006On line in June 2007.

*Nevada Solar One64 MW - 2007

*Around the WorldGranada, Spain. Two 50 MW plantsDeveloped by Solar Millenium AG

Negev desert of Israel150 MW facility to be expanded to 500 MW Developed by Solel (successor company to Luz) Cost $1 billion

*Parabolic Troughs on a Smaller Scale:SolarGenix Power Roof (2002)Parker Lincoln Building (demonstration)Design point of 176 kWProvides 50 tons of absorption cooling

*Parabolic TroughsLinks for More Info

http://www.iea-ship.org/index.htmlhttp://www.solarpaces.org/solar_trough.pdfhttp://www.nrel.gov/docs/fy04osti/34440.pdf

Heat Transfer Analysis: http://www.nrel.gov/docs/fy04osti/34169.pdf

Ball Joint Design:http://www.eere.energy.gov/troughnet/pdfs/moreno_sf_interconnections_with_salt_htf.pdf

*Linksto Parabolic TroughProjects and Technology Examples

http://www.solargenix.com/power_plant_tech.cfmhttp://www.solargenix.com/building_products.cfmhttp://www.us.schott.com/solarthermal/english/index.htmlhttp://www.us.schott.com/solarthermal/english/products/receiver/details.htmlhttp://www.inderscience.com/search/index.php?mainAction=search&action=record&rec_id=6745http://www.sete.gr/files/Ebook/2006/Hospitality_Day_Lokurlu.pdfhttp://www.eere.energy.gov/troughnet/pdfs/lewandowski_vshot.pdfhttp://www.capitalsungroup.com/files/rmt.pdfhttp://www.industrialsolartech.com/

*PreviewSketch of thermal analysis and design for parabolic trough system at the end of this presentation.

*Compact Linear Fresnel ReflectorsAusra, Inc.http://www.ausra.com/Makes moot some of the design challenges and weaknesses of parabolic troughs.

*Compact Linear Fresnel Reflectors

A series of long, shallow-curvature mirrorsFocus light on to linear receivers located above the mirrors.

*Compact Linear Fresnel ReflectorsLower costs compared to parabolic troughsSeveral mirrors share the same receiverReduced tracking mechanism complexity

Stationary absorberNo fluid couplings requiredMirrors do not support the receiver

Denser packing of mirrors possibleHalf the land area

*

6.5-megawatt demonstration power plant under construction in Portugal (as of September 2007)Ausra and PG&E announce purchasing agreement for 117 MW facility located in central California (November 2007)Compact Linear Fresnel ReflectorsProjects

*Parabolic Dishes

- Plataforma Solar de Almeria DISTAL I and II- Dish with receiver for Stirling Engine

*Parabolic Dish/Engine - OperationSolar energy drives a Stirling engine or Brayton cycle engine (gas turbine.)

Receiver absorbs solar energy and transfers it to the engines working fluid.

Systems are easily hybridized since Stirling engines can run on any external heat source.

*State of Dish Technology

Mature and Cost Effective Technology: Large utility projects using parabolic dishes are now under development.

Technical Challenges Have Been: Development of solar materials and components Commercial availability of a solar-izable engine.

Advantage: High Efficiency Demonstrated highest solar-to-electric conversion efficiency (still true with advances in CPV? No.) Potential to become one of least expensive sources of renewable energy. (still true with development of Fresnel reflectors?)

Advantage: Flexibility Modular - May be deployed individually for remote applications or grouped together for small-grid (village power) systems.

*Stirling Energy Systems, Inc.

*Stirling Engines

Stirling engines are simple, have high efficiency (25% for industrial heat), operate quietly, have low O&M costs (~$0.006/kWh)Waste heat can easily be recovered by the engine, as well as from the engineAccording to one manufacturer: $1000-2000/kW installed ButThey have higher costs for materials and assembly, are larger for same torque, have longer start up time (needs to warm up)

*Relatively small units are available.e.g. Stirling Danmarkhttp://www.stirling.dk/default.asp?ID=121 though these are designed for biopower

*Infinia Corphttp://www.infiniacorp.com/applications/Prod_Spec.pdf

*Stirling Engine ManufacturersStirling Denmark: http://www.stirling.dk/

STM Power: http://www.energysolutionscenter.org/distgen/AppGuide/Manf/STMPower.htm

QRMC

Infinia: http://www.infiniacorp.com Stirling Cycles has been acquired by Infinia.

ReGen Power Systems: http://www.rgpsystems.com/

Stirling Energy Systems: http://www.stirlingenergy.com/. Currently manufacturers large utility-scale Stirling engines for use with solar concentrating systems. Has plans to produce engines for use with combustible fuels in the future.

Stirling Biopower: http://www.stirlingbiopower.com/.In the start up phase (as of July 2007)

*Receiver Tubes for Stirling Engine

Located at focus of dish to absorb heat.

*From: www1.eere.energy.gov/solar/pdfs/csp_prospectus_112807.pdf

*SDG&E (2005) 300 MW From 12,000 Stirling Solar Dishesin Imperial Valley, Southern CA

San Diego Gas & Electric entered 20-year contract with SES Solar Two, an affiliate of Stirling Energy Systems in 2005.

12,000 Stirling solar dishes providing 300 MW on three square miles

Two future phases possible that could add 600 MWAt 900 MW would be one of the largest solar facilities in the world.

*SCE (2005) 500 MW from 20,000-Dish Arrayin Mojave Desert

Southern California Edison will construct 500 MW solar generating station on 4500 acres:Approved by CPUC in Dec 2005Using SES dishes

First phase: 20,000-dish array to be constructed over four years

Option to expand to 850 MW.

*A news story on these two projects

SAN DIEGO, California, US, September 14, 2005 (Refocus Weekly) An electric utility in California will buy 300 MW of solar power from a new facility that uses Stirling solar dishes. San Diego Gas & Electric will buy the green power under a 20-year contract with SES Solar Two, an affiliate of Stirling Energy Systems of Arizona. The 300 MW solar facility will consists of 12,000 Stirling solar dishes on three square miles of land in the Imperial Valley of southern California. SDG&E has options on two future phases that could add another 600 MW of renewables capacity and, if the plant grows to 900 MW within ten years, it would be one of the largest solar facilities in the world. The utility also announced the purchase of 4 MW of energy from a local biogas landfill project. SES says the contract is the second record-breaking solar project it has signed in the past month, following a contract with Southern California Edison for construction of a 4,500 acre solar generating station in southern California. That 20-year power purchase agreement, which also must be approved by the CPUC, calls for development of 500 MW of solar capacity in the Mojave Desert, northeast of Los Angeles. The first phase will consist of a 20,000-dish array to be constructed over four years, with an option to expand the project to 850 MW.

*Salt River Landfill Demonstration ProjectFour 22 kW SunDishes

Each 'SunDish' is 50' high.

Stretched-membrane faceted dishes deflected to convex form by vacuum.

Reflective surface is made of sheets of 1.0 mm low-iron glass. Stirling engines and generators manufactured by STM Corporation.

Electricity is used by the landfill facilities.

Efficiency is 20% higher than other solar systems of a similar size.

Hybrid system: Stirling engines can run on solar energy, landfill gas or other liquid or gaseous fuels.

*STMs Sun Dish SystemFrom: http://www.energysolutionscenter.org/distgen/AppGuide/DataFiles/STMBrochure.pdf

*Small Scale & Low Tech Parabolic Dish with Solar Cookers

Using parabolic dish concentrators on a smaller scale...

*Solar FurnacesCentre National de Recherche Scientifique - Odeillo, FranceLargest solar furnace in the world (1 MWt)

*Solar Furnaces - Operation

Solar furnaces are used for: - High temperature processes Solar Chemistry - Materials testing

A field of heliostats tracks the sun and focuses energy on to a stationary parabolic concentrator which refocuses energy to the receiver.

Receivers vary in design depending on process: Batch or continuous process Controlled temperature and pressure Collection of product (gas, solid, etc.)

*Why Run Processes in a Solar Furnace?

Higher Temperatures (up to 3800oC) Higher temperatures are possible in solar furnace than in conventional combustion furnace or electric arc furnace.

Cleaner Processes e.g. Electric arc furnaces use carbon electrodes which often contaminate product.

Energy Sustainability Use of renewable energy for industrial processes.

*Electricity through Solar ChemistryExample: Water splitting: 2H2O 2H2 + O2

*Solar Furnaces Technical Challenges

From test bench to commercial scale processes Development of continuous processes from batch experiments

Material Development Materials suitable for very high temperatures.

Process Control e.g. Accurate measurement of high temperatures

*CNRS Solar Furnace at Odeillo, FranceMirror is 10 stories high and forms one side of the laboratoryMaximum temperature is 3800oC

*The Furnace

Inside the focal zone of the 1 MW mirror at Odeillo.

*Receiver Example

Vaporization experiment with 2kW furnace at Odeillo.

*Receiver and Attenuator

Plataforma Solar de Almeria: Attenuator Louvers control sunlight entering furnace

*Other Solar Furnaces

Solar furnaces in Spain, Switzerland, Germany, Israel, France...

Paul Scherer Institute - Switzerland (45 kW)

*Paul Scherer Institute, Switzerland

Stretched film concentrator

*Solar Central Receivers Power Towers

Plataforma Solar de Almeria, Spain

*Solar One

Located near Barstow, California Operated from 1982 to1986

*Solar One

Moonrise over the Solar One Heliostat Field

Photo from http://www.menzelphoto.com/gallery/big/altenergy3.htm

*Solar Two

Solar Two improved the thermal storage of Solar One

Photo from http://ucdcms.ucdavis.edu/solar2/

*Plataforma Solar de Almeria

1.8 MW steam generator

Produces steam at 340oC and to drive steam turbine

Thermal storage: 18-tons of Al2O3

Notice the heliostat field and the central tower reflected in this heliostat.

*Concentrating Solar Photovoltaics 500 kW now installed in Arizona (APS) Concentrating sunlight 250x to 500x reduces cell cost Amonix CPV cells are 26% efficient.Most efficient in world for silicon until (see next slides) With multi-junction cells, efficiency can be increased to 40%

*http://www.cc.state.az.us/utility/electric/EPS-USPAPS.pdf

*Lens ConcentratorsIn this example, energy is concentrated on to PV cells with lenses(but lens systems dont necessarily have PV cells.)40% efficiency for CPV achieved.

* Comparison of Technologies(2006)http://tomkonrad.wordpress.com/2006/12/07/they-do-it-with-mirrors-concentrating-solar-power/

*Environmental Impacts

Deserts have sensitive ecosystems and low water availability.Land UseThe heliostat field occupies a large area of land, shading areas where the ecosystem is accustomed to full sun.- Water Use Wet cooling towers used in power generation have high water consumption.

*Geometrical Optics: Law of Reflection and Refraction are the only physical laws required for geometrical optics.

The rest is geometry How rays of light are reflected off surfaces or refracted through materials.

Ray Tracing

*Law of ReflectionThe incident ray and reflected ray lie in a plane containing the incident normal, and this normal bisects the angle between the two rays.

ReflectionReference: Modern Geometrical Optics by Max Herzberger, 1958

*Refraction through a LensSnells Law

n is index of refraction of the material

*Ray Tracing Example

Secondary concentrator to spread energy evenly across a cylinder.with a front that reflects reemitted radiation back to the cylinder.

Reemission is not really a single normal ray as shown, Normal is center of distribution of reemitted rays.

*Miscellaneous Reflection ExamplesModern Geometrical Optics, Max Herzberger, 1958

*Miscellaneous Refraction ExamplesModern Geometrical Optics, Max Herzberger, 1958

*Edge Ray AnalysisEdge ray analysis is used to do ray tracing by hand. Select rays to establish bounds: Extreme anglesWith maximum error.

*Example of Edge Ray Analysis

Rays Enter CPC at Extreme AnglePerfect CPC:

Conical approximation:Some rays are reflected back out without striking the absorber.Select cone so rejection of rays is acceptable.

A Compound Parabolic Concentrator focuses rays onto an absorber without tracking.

*Example of Secondary ConcentratorRays from primary concentrator focus on a pipe imperfectly. Design secondary mirror so many of the rays that miss the front will reflect back to the pipe.Select rays that represent the error of the primary concentrator.

Ray 1 strikes front.Ray 2 misses the front, but is reflected back.Ray 3 misses the front and misses the back.

*Ray Tracing by ComputerRay tracing by hand, you are limited to selecting a small number of rays.Ray tracing by computer, you can send in many rays.Can look at distribution of rays across a surface.

*Example:Focal Point of an Imperfect Primary Concentrator

*Ray Tracing by Computer

Computer modeling: Incoming rays created according to the profile of primary concentrator. Define surfaces of windows, reflectors and absorbers mathematically. Follow path of incoming rays to absorber and reemission of rays from absorber back out of system Determine surface temperatures and available process heat from distribution of rays using energy balance.

Example design goals: Minimize reflection out of receiver Obtain even distribution across absorber surfaces

*NREL Thermal Analysis Examplehttp://www.nrel.gov/docs/fy04osti/34169.pdf

Consider a parabolic trough.Receiver - Pipe with and without evacuated tube.From: Heat Transfer Analysis and Modeling of a Parabolic Trough Solar Receiver Implemented in Engineering Equation Solver, R. Forristall, NREL, October 2003, http://www.nrel.gov/docs/fy04osti/34169.pdf

*Thermal Analysis Example

Evacuated tube

*Heat Balance on Receiverwith and without evacuated tube

*Heat Balance Equations on Receiver

*DesignIn your thermal analysis, you may be interested in considering:

Length and cross-section of troughDiameters of pipe and evacuated tubeVelocity of heat transfer fluidOptical properties of the pipe, glass and troughWeather data: Temperature, Insolation, Wind Temperatures of surfaces and heat transfer fluid.Energy absorbed by heat transfer fluid

Vary geometry, velocity and materials to meet your design criteria cost effectively.

*Thermal Analysis You may also want to include other losses such as heat loss through support brackets.

*Solar News LinksThe Energy Blogs Solar Thermal page:http://thefraserdomain.typepad.com/energy/solarthermal_/index.html

*PARABOLIC DISH & ENGINESOLAR FURNACECENTRAL RECEIVERSOLAR FURNACEPARABOLIC DISHPARABOLIC TROUGHFRESNEL REFLECTORLENS CONCENTRATORSThe End

***Theoretical maximum concentration is 46,211 finite because sun is not a point radiation source.Suns surface temperature is 5500oC.*I first gave this presentation in 2004 prior to much of this activity. *http://www.nrel.gov/csp/troughnet/pdfs/2007/morse_look_us_csp_market.pdf*http://www.nrel.gov/csp/troughnet/pdfs/2007/morse_look_us_csp_market.pdf******Luz SEGS plants at Kramer Junction haveconcentration factor of ~80HTF is synthetic oil (flammable)HTF temperature in pipe loop is 400oCOverall efficiency is ~15%, largely due to the 35% efficiency of the steam cycle.*In contrast to photovoltaic systems, a hybrid solar thermal power plant can guarantee capacity. During bad weather or at night electricity is generated by natural gas, biomass or hydrogen, or by drawing on thermal storage.**Thermal storage system sized for 65% solar / 35% fuel usage over year has been more cost effective than oversizing thermal storage for 100% solar usage. Without thermal storage, solar provides only about 25% of power. *Notice how fresnel reflector makes many of these problems moot. **http://www.nrel.gov/csp/troughnet/pdfs/2007/morse_look_us_csp_market.pdf*http://www.power-technology.com/projects/solaronesolar/http://thefraserdomain.typepad.com/energy/2005/09/about_parabolic.htmlApril 20062006 News:In the US, a 1 MW plant is being built by Arizona Public Service's at its Saguaro Power Plant and a 64 MW plant is being built in Eldorado Valley near Boulder City, NV. Solargenix is the prime contractor on both projects in conjunction with EERE/NREL. SHOTT North America is supplying the receivers. The Boulder City plant is currently scheduled to go on line in June 2007. Funding for the American projects has been difficult, but on 10/4/04 the Western Governors Association and EERE announced funding and support of a 1000 MW goal of concentrating solar power for the region by 2010. Included in this program is support for the two afore mentioned projects. Nevada Power and Sierra Pacific Power Company have signed long term contracts to buy the power produced by the 64 MW plant.

*http://www.nrel.gov/csp/troughnet/pdfs/2007/morse_look_us_csp_market.pdf*http://thefraserdomain.typepad.com/energy/2005/09/about_parabolic.htmlApril 20062006 News:

Two identical 50 MW plants are being built by Solar Millinium AG in Granada, Spain. The first plant has received a 5 million (~$6.25 million) grant from the EUs Fifth Framework Program and financial support from the German Ministry for Environment. To encourage this type of technology the Spanish government has set up regional funding programs. This is coupled with a bonus of 12 Eurocents per kWh generated from solar power has been instrumental in bringing such projects to Spain. A 150 MW facility that is to be expanded to 500 MW In the Negev desert of Israel is being developed by Solel (the successor company to Luz). Solel will establish a consortium to finance and build the power station, which it estimates will cost $1 billion. The state has promised to buy the electricity under the terms of a contract to be signed by the parties.

*http://www.enr.state.nc.us/upclose/pages/parkerlincoln.html*Sagging of heat transfer tubes breaks glass.*Sagging of heat transfer tubes breaks glass.**http://www.askac.org/meetings/2003-09-10-Energy/Solar%20Concentrating%20Power%20Developments-David%20Mills.pdfhttp://jcwinnie.biz/wordpress/?p=2470http://thefraserdomain.typepad.com/energy/2007/09/on-sept-10-ausr.html*http://www1.eere.energy.gov/solar/pdfs/csp_prospectus_112807.pdf

*http://www1.eere.energy.gov/solar/pdfs/csp_prospectus_112807.pdf

*http://blogs.business2.com/greenwombat/2007/09/solar-startup-a.html****http://www.stirlingenergy.com/

*http://www.stirlingenergy.com/photos/photo/SES2272x1704_2.jpghttp://en.wikipedia.org/wiki/Stirling_enginehttp://www.rgpsystems.com/low_temp.html*****http://www1.eere.energy.gov/solar/pdfs/csp_prospectus_112807.pdf*Signed September 2005: http://www.sparksdata.co.uk/refocus/redesign/showdoc.asp?docid=39811343&accnum=1Approved by CPUC December 2005: http://www.renewableenergyaccess.com/rea/news/story?id=40914*Signed September 2005: http://www.sparksdata.co.uk/refocus/redesign/showdoc.asp?docid=39811343&accnum=1Approved by CPUC December 2005: http://www.renewableenergyaccess.com/rea/news/story?id=40914*Signed September 2005: http://www.sparksdata.co.uk/refocus/redesign/showdoc.asp?docid=39811343&accnum=1Approved by CPUC December 2005: http://www.renewableenergyaccess.com/rea/news/story?id=40914*http://www.clean-air.org/sundish.htm****Liberating aluminum from bauxite, for example, is a chemical process that can be run using electricity (Hall process) or alternatively by solar energy.**IMP Laboratory studies the thermal methods for hydrogen production with solar energy among the various routes: electrolysis, photo-catalysis, biosynthesis... Typical range of working temperature is 1000C - 2500C. In a long-term vision, water decomposition into hydrogen and oxygen by closed thermochemical cycles is a very promising approach (Figure 2). In a medium term vision, solar decarbonization and up-grading of hydrocarbons are hybrid solutions that permit to reduce significantly the CO2 emission, as shown in Figure 3. Hybrid methods may lead to a reduction of 2/3 of CO2 emission.********Solar Power Towers improve upon efficiency of solar troughs due to increased operating temperatures (700 to 1000oC) Efficiency of around 20% overall.May circulate air or molten salt through receiver and pipe loop.Receiver may be open to the atmosphere (air blown through honey comb receiver) or may be sealed by glass dome for pressurization.To reduce radiation losses, cool air may be blown over front face of receiver honeycomb -- increases convection losses but reduces radiation losses by greater amount.

*Constructed as demonstration project, not optimized economically.1818 heliostats10MWe for 8 hours on summer solstice and 10 MWe for 4 hours on winter solsticeAnnual efficiency of 7%Solar One stored heat from solar produced steam in a tank filled with rocks and sand using oil as heat transfer fluid. But storage system was complex and thermodynamically inefficient.**Solar One and Solar Two both at 10 MWe are the largest power towers in the world. Solar Two was connected to the grid in 1996 and began routine power production in 1997.Storage tanks can be designed with sufficient capacity to power a turbine at full output for up to 13 hours. Hot tank cools slowly, only loses about 75oC over one month.Molten salt is circulated through receiver, avoiding heat exchanger.

MOLTEN SALT:Melting point is 220oC.Kept molten by a heat trace system on pipes. Trace system was problematic in start up of Solar Two.Mixture of 60% sodium nitrate and 40% potassium nitrateAt higher temperatures it is corrosive, must use stainless steel for hot side (565oC). On cool side (290oC) can use mild carbon steelMolten salt has viscosity similar to water.Wets metal surfaces extremely well and so is difficult to contain and transport.Must use pumps, valves, valve packing and gaskets materials that work with salt.Systems minimize gasketed flanges. Valves, fittings and instruments are welded in place.

POWER TOWERS TECHNICAL CHALLENGESHeliostats:Low cost manufacturing technique for heliostats. Heliostats are largest single cost of power tower plantAdvanced Receivers:Smaller simpler recievers to improve efficiency & reduce maintenance. Materials DevelopmentRequire materials capable of withstanding 1100 suns. Stainless steel can withstand only 800 suns.Molten Salt System:1. Heat trace system can be problematic. Improve system by:- Develop antifreeze to lower freezing point of molten salt- Develop components that can be cold started- Develop thermal management practices that are less reliant on heat trace.2. Valves can be troublesome in molten salt service.Steam GeneratorKettle-boiler design at Solar Two vs. recirculating drawn type boiler****http://solarpoweredtechnofix.blogspot.com/*This is a good summary of several technologies: http://tomkonrad.wordpress.com/2006/12/07/they-do-it-with-mirrors-concentrating-solar-power/*Land use: When land use associated with mining and drilling of fossil fuels is included in comparison, land use of solar power towers is less than for fossil fuels. Land use is also less than for wind, biomass and hydro.Water: Water consumption is the same as it is for any Rankine cycle power plant with wet cooling towers. Dry cooling can reduce water requirements by 90% but have cost and performance penalties.

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