Flux Mapping of the Beam Down Solar Thermal Concentrator at Masdar City

1. Flux Mapping of the Beam Down Solar Thermal Concentrator at Masdar City Steven Meyers, Marwan Mokhtar, Peter Armstrong, MatteoChiesa Laboratory of Energy and Nano-Science (LENS) Solar Energy Group

2. Outline • Flux Mapping Basics • Beam Down Solar Thermal Concentrator • Initial Results • Forward Optical Model • Bidirectional Reflectivity Distribution Function • Convolution • Model Results • Discussion

3. Flux Mapping Basics • Flux Mapping – A method to determine the distribution and quantity of concentrated solar radiation generated by a CSP facility • Three Basic Tools • CCD Camera • Diffuse Reflector (Lambertian) • Heat Flux Sensor (HFS) • CCD Camera –Luminance Map (cd/m2) • 300-750 nm • HFS – Discrete flux measurement (kW/m2) • 300-3000 nm • Conversion ratio (kW/cd) • Yields a continuous flux map of (kW/m2) Kaluza and Neumann, 1998 SolarPACES 2011

4. Diffuse Surface • Lambert’s Cosine Law (Lambertian surface) • The measured radiation intensity by an ideal diffusing surface is directly proportional to the cosine of the angle between the observer’s line of sight and the surface normal (Photometria, 1760). • If the surface obeys Lambert’s Cosine Law, the radiation angle of incidence (AOI) and azimuth are inconsequential • However, if the surface does not perfectly follow the Law, the reflected radiation to a stationary observer will change based on the radiation AOI and azimuth Source: www.odforce.net SolarPACES 2011

5. The Beam Down Solar Thermal Concentrator (BDSTC) E W 3 Central Reflector Structure 7 5 1 CCD Camera Location N HFS Location Heliostat Field • Designed by Tokyo Tech, constructed by MES • 100 kW/m2 peak flux at a net incident energy of 100 kWt • Flux measurement instrumentation • Thermally regulated CCD camera (Konika Minolta CS-2000) • Eight in-situ calibrated (Mokhtar et al. 2011) Head Flux Sensors (Medtherm - Gardon, Schmidt-Boelter) • Diffuse Reflecting Target (sandblasted unglazed tile) • Goal – Determine the conversion coefficient to generate a flux map (HFS/CCD) Target Receiver SolarPACES 2011

6. Initial Results • Increasing trend due to spectral sensitivity differences between CCD camera and HFS • (Kaluzaand Neumann. 1998, Ulmer et al. 2002) AM (blue) PM (red) SolarPACES 2011

7. Beam Down Optics • The 360 degree field of heliostats contribute significantly different radiation quantities over the day • Not observed in north field dominant towers and dishes due limited changes in cosine loss • Needed to quantify the changing levels of radiation contribution from each heliostat over the day SolarPACES 2011

8. Optical Model CR Mirror • Combined heliostat efficiency • Radiation Angle of Incidence on the Receiver • Radiation Azimuth direction • Mean AOI and Azimuth • Assumed Gaussian flux profile (no astigmatism) Z CR X CR Y CR CCD Camera X HFS Y HFS AOI rec φrec HFS SolarPACES 2011

9. Optical Model SolarPACES 2011

10. BRDF • Lambertian assumption? • A Bidirectional Reflectance Distribution Function (BRDF) was constructed Source: NIST SolarPACES 2011

11. BRDF • Results indicated a significant backscatter reflectance, consistent with a rough diffuse surface • (Oren-Nayar model) SolarPACES 2011

12. Sun – Afternoon BRDF CCD Camera Peak Reflection Reflected Ray Measured by CCD Camera θ θ North West South East L L SolarPACES 2011

13. Convolution • By convoluting the optical forward model with the BRDF, we can estimate the quantity of light which is measured by the CCD camera (Fmodel)and compare that to predicted light assuming a true Lambertian surface (Flambertian) SolarPACES 2011

14. Results Fmodel/ (Flambertian) Original Data West East West East AM (blue) PM (red) SolarPACES 2011

15. Discussion • The AM/PM trend correlates to the HFS located on the East or West side of the Diffuse Surface • By applying this methodology to many points across receiver (x,y), compensation for any non-Lambertian reflections allows for proper extraction of the flux levels on the surface and not the light levels reflected to and measured by the CCD camera • A homogeneous Diffuse Surface BRDF is critical to the success of this method SolarPACES 2011

16. Limitations • Assumed Gaussian flux distribution and no astigmatism • Modeled BRDF function valid for only one location, not entire surface • Inconsistent inter-HFS/CCD ratios due to non-uniform tile surface (poor conditioning) • HFS 2 had minimal AM/PM difference • Measured BRDF was very Lambertian • HFS 5 had significant AM/PM difference • Measured BRDF showed large back scattering reflection • Due to non-uniform tile surface, we were unable to generate an accurate flux map SolarPACES 2011

17. Conclusion • Method can be used to minimize errors caused by non-ideal Lambertian surfaces • Simple forward optical model and BRDF can provide significant insight into the changing radiation measured by the CCD camera • Allows for less precise (cheaper, more rugged) surfaces to be used for flux analysis, as long as they are homogeneous SolarPACES 2011

18. Thank you Questions?