HELIOSTAT DESIGN NILS BJÖRKMAN Master of Science Thesis Stockholm, Sweden 2014 HELIOSTAT DESIGN Nils Björkman Master of Science Thesis MMK 2014:03 MKN097 KTH Industrial Engineering and Management Machine Design SE-100 44 STOCKHOLM Examensarbete MMK 2014:03 MKN097 Heliostatkonstruktioner Nils Björkman Godkänt Examinator Handledare 2014-05-30 Ulf Sellgren Prof. Kjell Andersson Uppdragsgivare Kontaktperson Indian Institute of Science, Prof. Ashitava Ghosal Robotics Lab Sammanfattning En heliostat är en motordriven spegel som används i tornsolkraftverk, kända som Solar Power Tower, även kallade Central Receiver system. Tekniken har funnits sedan 1970-talet och går ut på att hundratals eller tusentals heliostater speglar solstrålarna till toppen av ett högt torn, där stålningsenergin omvandlas till värmeenergi, som t.ex. kan användas till att driva ångturbiner och producera elektricitet. Demonstrationsanläggningar har byggts i bland annat USA och Spanien, och ett flertal nya installationer har tillkommit sedan år 2005. För att verkligen nå ett kommersiellt genombrott måste tekniken göras billigare så att solelen kan produceras till minst lika bra pris som andra alternativ, så som t.ex. solceller, kärnkraft och kolkraft. En kritisk komponent för tornsolkraftverkens ekonomi är kostnaden för heliostaterna, som beräknas stå för ungefär 50 % av anläggningens totala investeringskostnad. Den här rapporten avhandlar heliostaten ur ett mångfacetterat perspektiv där olika konstruktionsspår förklaras. Vidare behandlar rapporten spegelgeometrier, och en Matlab-kod som genererar tillverkningsmått för en rotationssymmetrisk paraboloidformad spegelyta finns bifogad. Att undersöka vindlaster är bland det viktigaste i ett heliostatutvecklingsprojekt, eftersom dessa är de dimensionerande lasterna för designarbetet. Här används en vindlastberäkningsmetod utgiven av Sandia National Laboratories, som kortfattat går ut på att man multiplicerar det dynamiska vindtrycket med en korrigeringsfaktor som baserats på emiriska studier av heliostatmodeller i vindtunnel. En dimensioneringsprocess för heliostater föreslås och utvecklingsgången för två Azimut-Elevation heliostater i storlek 25 m2 resp. 49 m2 demonstreras. FEM-mjukvara nyttjas som det främsta verktyget för att dimensionera heliostatkonstruktioner som kan stå emot vindlasterna. Slutligen ges förslag på innovativa tekniska lösningar för spegelmontering, glidlager, montering av elevation- motorerna, och en unik azimut-motormodul, vilken använder stålvajrar som remmar och har en integrerad broms. Med all denna information bör Robotics Lab på IISc ha en god grund att stå på inför vidare forskning inom konstruktion och styrning av heliostater. Nyckelord: Solenergi, Heliostat, Termisk solenergi, Solar Power Tower, Tornsolkraftverk Master of Science Thesis MMK 2014:03 MKN097 Heliostat Design Nils Björkman Approved Examiner Supervisor 2014-05-30 Ulf Sellgren Prof. Kjell Andersson Commissioner Contact person Indian Institute of Science, Prof. Ashitava Ghosal Robotics Lab Abstract A heliostat is a motorized mirror used in a Solar Power Tower plant. The technology has been around since the 1970’s, and involves hundreds or thousands of heliostats reflecting the sun’s rays to the top of a high tower, where the incident solar energy is converted to heat energy, which in turn is used to drive steam turbines and produce electricity. Demonstration plants have been built in the U.S.A. and Spain, and a number of new facilities have been added since 2005. To achieve a commercial breakthrough, the technology must be made more efficient so that electricity can be produced at prices comparable with other options such as wind, photovoltaic, nuclear and coal. A critical component in the economics of the Solar Power Tower plant is the cost of the heliostat, which is estimated to account for approximately 50 % of the investment cost of the total plant. This report deals with the heliostat from a multifaceted perspective. Different design approaches are explained. First, mirror geometries, with manufacturing dimensions, for a rotation-symmetric paraboloid- shaped mirror-assembly are obtained for mirrors with different sizes with a Matlab code. Investigating wind loads are among the most important tasks in a heliostat development project, as large wind loads radically complicates the design work. A wind load calculation-method based on previous work by Sandia National Laboratories and finite element method (FEM) is used in this work to design heliostats to withstand expected wind loading. The design of the geometry and the structure to withstand wind loading is illustrated with two azimuth elevation heliostats (25 m2 and 49 m2). Finally, a number of innovative technical solutions are suggested. These are a new mirror facet mounting technique, sliding bearings, elevation drive, and a very unique azimuth motor module, which uses steel wires as belts with an integrated brake. It is hoped that the results and designs presented in this thesis will be a good foundation for further research in the heliostat structures and heliostat controls at the Robotics lab in IISc and elsewhere. Keywords: Solar Power, Heliostat, Thermal solar power, Solar Power Tower, Central Receiver System Master Thesis - Heliostat Design N.Björkman Solar energy glossary Azimuth drive The actuator and gear system for the azimuth axel Azimuth-Elevation heliostat A serial heliostat with two actuators; one at azimuth axis and a second at (Az-El) the elevation axis. The axes are perpendicular to each other. Canting The tilt of the mirror facets regards a plane common for the mirror assembly of one heliostat. Canting (as well as focusing) defines the shape of the mirror assembly when the assembly is built up of many mirror facets. Central Tower A tower structure located at the center of a Solar Power Tower plant, on which the sun radiation is focused and where the radiation is collected. CSP Concentrating Solar Power; refers to use of mirrors or lenses to focus sun rays on a small collector surface aiming to produce electrical power or process heat DOF Degrees of freedom Elevation angle, α The angle between the ground surface and the mirror surface, i.e. the tilt of the mirror relative the ground surface; extremes are parallel to the ground (flat, stowed; α = 0 deg) resp. perpendicular to the ground surface (α = 90 deg). Elevation drive The actuator and gear system for the elevation axel Focusing The curvature of an individual mirror facet, which is obtained by bending the mirror facet. Heliostat A device in which a mirror is automatically moved so that it reflects sunlight in a constant direction. Heliostat field The population of heliostats in a Solar Power Tower plant Parallel type of Heliostat Analog to parallel manipulator. Two or more actuators are mounted parallel between the ground and the mirror. Photovoltaic (PV) Using the photovoltaic method (solar cells) to convert light into electricity, a principle explained by quantum mechanics (interactions between photons and electrons in a collector material) Receiver A fluid-filled container that collect solar radiation and transform it into heat. In a Solar Power Tower plant, the solar receiver is located on top of the Central Tower. Serial type of Heliostat Analog to serial manipulator where two or more actuators are mounted in series. For example the first actuator connects the ground to a second actuator, and the second actuator connects first actuator to the mirror. Master Thesis - Heliostat Design N.Björkman Solar Power Tower / A type of thermal solar power plant, where solar radiation is Central receiver system / concentrated and used to heat fluid. The heated fluid is typically used Heliostat field-technology for electricity production in a thermal power process. The concept is a Central Tower that is surrounded by Heliostats. Target aligned heliostat A new type of serial heliostat with two actuators that has better optical properties than the Az-El-type. The azimuth-angle is fixed, while the mirror has rotation-freedom on the elevation axis and on the mirror- normal axis. Thermal solar power Sun rays are used to heat a collector far above ambient temperature. Electricity is gathered trough a thermal process (e.g. steam turbines or Stirling engines) where temperature difference is the key principle of operation. The thermal solar energy harvest method is typically contrasted to the PV-approach. Master Thesis - Heliostat Design N.Björkman Notations Symbol Unit Description A [m2] Area E [Pa] Module of Elasticity, Young’s Module F [N] Force φ [Rad] Angular distance φ' [Rad/s] Angular velocity φ’’ [Rad/s2] Angular acceleration Fx [N] Wind induced drag force Fx’ [N] Wind force perpendicular to mirror surface Fx’g [N] Weight of mirror array, perpendicular to mirror surface Fx’g [N] Weight of mirror array, parallel to mirror surface Fz [N] Wind induced lift force Fz’ [N] Wind force parallel to mirror surface G [m/s2] Gravity acceleration constant (= 9.81) M [Nm] Moment m [kg] Mass Mhy [Nm] Wind induced hinge moment My [Nm] Wind induced base overturning moment MZ [Nm] Wind induced azimuthal moment p [Pa] Pressure P [W] Power t [s] Time V [m3] Volume η [-] Efficiency Master Thesis - Heliostat Design N.Björkman μ [-] Friction factor ρ [kg/m3] Density σ [Pa] Stress τ [Nm] Torque Master Thesis - Heliostat Design N.Björkman Acknowledgments During the project several people contributed to this thesis in one way or another. Special thanks must be given to Prof. Ghosal, supervisor at the IISc, for always taking time