CONCENTRATING SOLAR COLLECTORS Portland State University Solar Engineering Spring 2004 Carolyn Roos

Solar Concentrating Systems Concentrate solar energy through use of mirrors – or, on a small scale, 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 ~300 MWe - Furnace temperatures up to 3800oC (6800oF)

Examples of Applications Power Generation Example: 9 solar plants in Mojave Desert in California generate total of 354 MW for Southern Calif. Edison. High Temperature Processes Manufacture of metals and semiconductors Hydrogen production (e.g. water splitting, methane reformation) Desalination of seawater Waste incineration Materials Testing Under Extreme Conditions e.g. Design of materials for shuttle reentry

Primary Types of Solar Collectors Parabolic Trough Solar Furnace Parabolic Dish & Engine Solar Central Receiver (Solar Power Tower) Other arrangements exist, e.g. Photovoltaic concentrators: Use of lenses or mirrors in conjunction with PV panels to increase their efficiency.

SOLAR FURNACE PARABOLIC TROUGH CENTRAL RECEIVER PARABOLIC DISH & ENGINE SOLAR FURNACE

Major Components of Solar Collector Systems Concentrating mirror(s) May use primary & secondary concentrators. Receiver Absorbs energy from concentrator and transfers to process being driven (engine, chemical reactor, etc.) Heliostats Flat mirrors that track the sun and focus on receiver or concentrator. Used with solar furnaces and power towers.

Parabolic Troughs Sandia National Laboratory test facility

Plataforma Solar de Almeria – Test Facility - Located in the Desert of Tabernas in Spain - PSA is the The European Test Centre for Solar Energy Applications

Utility Scale Parabolic Trough Facility Luz “Solar Electric Generating System” (SEGS) - 14 MWe to 80 MWe Photo from http://www.nrel.gov/analysis/docs/2003/hank_price_presentation.ppt Photo from www.nrel.gov/analysis/docs/2003/hank_price_presentation.ppt

Parabolic Troughs - Operation Parabolic mirror reflects solar energy onto absorbent pipe. 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. Examples: Generation of steam for Rankine turbine/generator cycle. Distillation of seawater for desalination

Components of the 3rd Generation Luz SEGS Photo from www.nrel.gov/analysis/docs/2003/hank_price_presentation.ppt

Parabolic Trough System - Hybrid solar / natural gas - New systems include thermal storage.

of Hybrid Plant with Thermal Storage Thermal Output of Hybrid Plant with Thermal Storage

Parabolic Trough System with Thermal Storage                                                                                                                                             FIGURE 3. Schematic of a concentrated solar thermal trough power plant with thermal storage

Parabolic Troughs – State of Technology Most proven solar thermal electric technology. The nine Southern California Edison plants constructed in 1980’s continue to operate today providing 354 MWe. Arizona’s largest electric utility APS broke ground for new solar trough plant in March 2004 (1 MW) Most research and development is taking place in Europe. APS noted that smaller scales plants such as 1 MW are now cost effective – receiver technology is developed and smaller generating plants intended for geothermal facilities are commercially available. 1MW provides electrical needs of 200 homes.

Parabolic Troughs – Technical Challenges Development of Materials Heat transfer tubes that are less prone to sagging. Improved surface material of heat transfer tubes.  Absorptivity, emissivity and long-term stablility in air. Low cost mirrors that have reflectivity and washability of glass. Improved Components Flex hoses used to join sections of pipe loop are prone to failure  Replace with ball joint design. Ability to track on tilted axis Improved Processes e.g. Generate steam directly instead of running heat transfer fluid through heat exchanger - Improves efficiency but more difficult to control.

Solar Furnaces Centre National de Recherche Scientifique - Odeillo, France Largest solar furnace in the world (1 MWt)

CNRS Solar Furnace at Odeillo, France - Mirror is 10 stories high and forms one side of the laboratory. - Maximum temperature is 3800oC.

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.) Liberating aluminum from bauxite, for example, is a chemical process that can be run using electricity or solar energy.

Electricity through Solar Chemistry Water splitting: 2H2O → 2H2 + O2 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 1000°C - 2500°C. 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. Methane Reforming: CH4 + H2O→ 3H2 + CO

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

Receiver Examples Vaporization experiment with 2kW furnace at Odeillo.

Receiver Examples Solar reactor used in production of Al-Si alloys.

Receiver Examples Reactors with controlled atmospheres.

Receiver and Attenuator Plataforma Solar de Almeria: Attenuator – Louvers control sunlight entering furnace

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

Why Run Processes in a Solar Furnace? Energy Sustainability Use of renewable energy for industrial processes. Higher Temperatures 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.

Solar Furnaces Around the World Solar furnaces in Spain, Switzerland, Germany, Israel, France... Paul Scherer Institute - Switzerland (45 kW)

Building the PSI Concentrator Stretched film concentrator at Paul Scherer Institute - Switzerland

...and some little ones in the U.S. Sandia National Laboratory test facility

Solar Central Receivers - “Power Towers” Plataforma Solar de Almeria, Spain Absorber heats air to 700oC. Generates steam or goes to thermal storage.

Heliostat with Tower and Field Reflected at Plataforma Solar de Almeria Power Tower: 1.8 MW steam generator Produces steam at 340oC and 45-bar to drive steam turbine Thermal storage: 18-tons of Al2O3

Solar One Located near Barstow, California 1986 – when the U.S. was at the forefront of solar research

Solar One Moonrise over the Solar One Heliostat Field Photo from http://www.menzelphoto.com/gallery/big/altenergy3.htm

Solar Two Solar Two improved upon Solar One by adding thermal storage Photo from http://ucdcms.ucdavis.edu/solar2/

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

Receiver Tubes for Stirling Engine Located at focus of dish to absorb heat.

Parabolic Dish/Engine - Operation Solar energy drives Stirling or Brayton cycle engine. Receiver absorbs solar energy and transfers it to the engine’s working fluid.

Parabolic Dishes/Engine – State of Technology Technology is still in development. Technical Challenges Development of solar materials and components Commercial availability of a solarizable engine. High Efficiency Demonstrated highest solar-to-electric conversion efficiency Potential to become one of least expensive sources of renewable energy. Flexibility Modular - May be deployed individually for remote applications or grouped together for small-grid (village power) systems.

Parabolic Dish with Solar Cookers Using concentrators on a smaller scale...

Environmental Impacts Deserts have sensitive ecosystems and low water availability.   Land Use   The 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. 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.

Receiver Design Using Raytracing Computer modeling technique: Incoming rays created according to profile of primary concentrator. Define surfaces of windows, reflectors and absorbers. 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

Raytracing Example Designing front of receiver for reflection back to absorber

Raytracing Example Edge ray example: Designing receiver so that no rays are reflected away from absorber. Send in rays at the extreme angle of the primary concentrator across the aperture. Ensure no rays are turned back. In cone, some rays are. In CPC all rays reach outlet.

SOLAR FURNACE PARABOLIC TROUGH CENTRAL RECEIVER PARABOLIC DISH & ENGINE SOLAR FURNACE