temperature, and high color rendering index

Results

Conclusion

• Visual aspects of LSC studied

• Most luminophores have low color

temperature, and high color rendering index

Acknowledgements

Climate-KIC and TKI-Urban Energy for financial support

Faculty of Geosciences Copernicus Institute of Sustainable Development

Group Energy and Resources

Modelling visual performance in luminescent solar concentrators

Gijs van Leeuwen ¹ , Panos Moraitis ² , Wilfried van Sark ³

Utrecht University, Copernicus institute of Sustainable Development, Princetonlaan 8A, 3584 CB, Utrecht, The Netherlands

1 E: g.e.vanleeuwen2@students.uu.nl, ² E: p.moraitis@uu.nl, ³ E: w.g.j.h.m.vansark@uu.nl,

Introduction

• Increased interest for building integrated photovoltaics (BIPV)

• Aesthetics, freedom of form and color

• Luminescent solar concentrators are good BIPV candidate

• Transparent windows: visual comfort and well- being of the occupants of the building

Color

• Prediction of color required

• Calculate CIE chromaticity diagram [3], correlated color temperature (CCT) and color render index (CRI) based on absorption and emission spectra of luminophores

Potential use

Restaurant, café Bus shelter

Aim

• Predict color of LSC device with different luminophores

• Use nanocrystals as luminophores [1]

Luminophores

Results

• Luminophores are located at different points on the chromaticity diagram

• Color temperature values are in accordance with the colors in the chromaticity diagram.

The blue-colored PbS dots have a very high value: all other luminophores have a lower

color temperature, ranging from 3,000-5,500 K, which corresponds to their red colors.

• Color rendering index is high for most

samples, mostly above 85. Only PbS and Si chromophores have values below 80. This is consistent with the observation that these two types yield the strongest coloration in light.

• Low CRI values are the result of non-uniform absorption of the solar spectrum:

luminophores that emit high energy photons (absorb more in the blue) have also lower CRI values

References

[1] P. Moraitis, R. Schropp, W. van Sark, “Nanoparticles for Luminescent Solar Concentrators—A review,” Opt.

Mater., vol. 84, pp. 636–645, 2018.

[2] D.J. Farrell, “pvtrace: optical ray tracing for

photovoltaic devices and luminescent materials”, 2014.

dx.doi.org/10.5281/zenodo.12820

[3] Commission Internationale de l’Eclairage, “Method of Measuring and Specifying Colour Rendering Properties of Light Sources”, CIE 013.3-1995, Vienna, Austria, 1995.

Luminescent Solar Concentrator principle

• Luminophores absorb photons and emit red- shifted photons

• Total internal reflection causes ¾ of emitted photons to remain in light guide

• Solar cells attached to sides, ideally with band gap matched to emission wavelength (high

efficiency)

solar cell

escaping photons

~3/4 of photons captured

incident photons

luminophore

transmitted photons mirror

plastic

Method

• Ray trace simulation, pvtrace [2]

• LSC size 10x10x0.5 cm ³ , with 8 different luminophores

• 1 million photons from AM1.5 spectrum

• Transmitted spectrum is modified AM1.5,

depending on absorption of luminophore, P(l)

• Calculate CIE chromaticity diagram X, Y [3]

using

with color matching functions

Luminophore Emission peak (nm)

FWHM

(nm) Quantum Yield

(%) CdSe/CdS-giant

shell 640 60 45

ZnSe/ZnS Mn ²⁺

doped 590 80 50

CdSe Cu doped 705 110 70

PbS/CdS 890 160 50

CuInSeS/ZnS 960 180 40

AgInS/ZnS 900 290 30

Silicon QDs 830 120 45

Carbon QDs 550 110 40

25

Fig. 17: A visual comparison of the correlated colour temperature of all chromophores.

Fig. 18: A visual comparison of the colour rendering index of all chromophores.

25

Fig. 17: A visual comparison of the correlated colour temperature of all chromophores.

Fig. 18: A visual comparison of the colour rendering index of all chromophores.

Materials 2019, 12, 885 10 of 22

known R,G,B (red, green, blue) system, in order to become more practical. X,Y,Z can be computed from the measured SPD and the color matching functions ¯x ( l ) _{, ¯y} ( l ) _{, ¯z} ( l ) , which represent the response of the human eye to certain wavelengths, also known as the photopic curve (Figure 5). The X, Y, and Z tristimulus values are given by the following formulas [50]:

X = _K

Z

vis P ( l ) _¯x ( l ) _dl, (10)

Y = _K

Z

vis P ( l ) ¯y ( l ) _dl, (11)

Z = _K

Z

vis P ( l ) ¯z ( l ) _dl, (12)

where the value P ( l ) represents the radiant power at each wavelength interval and the values ¯x ( l ) ,

¯y ( l ) _{, and ¯z} ( l ) represent the values of the three color-matching functions for each wavelength interval.

The value K = 683 lm/W is a constant.

Figure 5. Spectral response of color matching functions ¯ x ( l ) , ¯y ( l ) , and ¯z ( l ) [27].

The chromaticity coordinates are the ratio of each of the tristimulus values, X,Y, and Z, divided by their sum. As the sum of the chromaticity coordinates is equal to 1, only two of them are necessary to describe a color. They are used to visualize the color of the given stimulus in a 2D chromaticity diagram [51]:

x = ^X

X + _Y + _Z ^{, (13)}

y = ^Y

X + _Y + _Z ^{. (14)}

While the xy chromaticity diagram, as depicted in Figure 6, is very commonly applied, it suffers from a serious downside: The distribution of colors is very non-uniform [27]. This is better illustrated when trying to depict a perceptual color difference of the same magnitude between different colors.

Ideally the identical color differences should have been depicted by lines with the same length.

Although there is no chromaticity diagram that can totally solve this problem, the development of the u ⁰ v ⁰ chromaticity diagram created a more uniform color space. The xy coordinates can be easily translated to u ⁰ v ⁰ coordinates, using the following equations [51].

u ⁰ = ^4x

2x + _12y + 3 , (15)

Figure 7.2: Concept of a train station using Electric Mondrian windows as decorative elements.

References

[1] John P. Ra↵erty. Anthropocene Epoch. Encyclopedia Britanicca, 2018.

[2] Graeme Wearden. David attenborough tells davos: the garden of eden is no more. The Guardian, 2018.

[3] IPCC. AR5 synthesis report: Climate Change 2014. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. 2015.

[4] B. E. Schirrmeister, J. M. de Vos, A. Antonelli, and H. C. Bagheri.

Evolution of multicellularity coincided with increased diversification of cyanobacteria and the great oxidation event. Proceedings of the National Academy of Sciences, 110(5):1791–1796, jan 2013.

[5] FCCC/INFORMAL/84, editor. United Nations Framework Conven- tion on Climate Change. UN General Assembly, 1992.

[6] IEA. World energy outlook 2018. 2018.

[7] Wilson Peter R. Peter R. Wilson. Solar and Stellar Activity Cycles.

Cambridge University Press, 2003.

126

enough, smart buildings with the ability to manage energy generation with

high variability and on site storage are necessary.

The results of this thesis o↵er comprehensive knowledge on the perfor- mance, scalability and integration of LSCs in the building environment and the dependence of solar energy generation on the urban landscape. The com- bination of results is valuable not only to gain insight in the technology itself, but as the di↵erent parameters are interrelated, are showing a new path of BIPV solutions that could fundamentally alter the way we perceive sustain- able energy generation. The combination of methods as employed in this work can be extended to any new type of luminophore or LSC device.

Figure 7.1: The concept of the Electric Mondrian as it could be implemented as a solar window in a cafe.

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