Is efficiency the only important aspect to solar energy?

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Is efficiency the only important aspect to solar energy?

Michael G. Debije

Chemical Engineering and Chemistry Functional Materials and Devices (SFD)

Eindhoven University of Technology

April 21, 2012 Alumni Dag

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Meeting the solar challenge

Buildings residential 21% Buildings commercial 18% Industry 33% Transportation 28% 12% 10% 32% Lights Cooling Heating 7% 28% 13% 16% Ventilat… Lights Cooling Heating Buildings use 40% of our energy Our inability to control sunlight costs ~16% of worldwide energy consumption!!

Why has this not been addressed before?

• large areas need be covered with inexpensive systems that look

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State-of-the-art in solar cells

Silicon ‘blue’ “Type III-V”

Silicon ‘black’

Organic Dye-sensitized

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Even higher efficiency using concentrators: focusing

Decrease solar cell size Use high-efficiency cells

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Decrease solar cell size Use high-efficiency cells

Even higher efficiency using concentrators: reflecting

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Difficult to integrate into the built environment

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Another solution?

Bringing the light to the cell…. replace expensive semi-conductor materials

with inexpensive, colorful plastics that provide aesthetic advantage and adaptability which can be employed where standard

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The basic function of the

luminescent solar concentrator

Solar Cell Plastic “waveguide”

Luminescent dye

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Advantages

• Mostly inexpensive materials

• The waveguides may be of any color

• The waveguides may be cut to (almost) any shape • The device can be made flexible

• The waveguides could be transparent • The devices could be lower in weight • Work in both direct and indirect sunlight

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Increased options

Photo courtesy Harry Harkema Photo courtesy Eduardo Sentchordi

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Responsive signs that generate own power

LEDs Battery storage S ol ar C el l Waveguide Solar Cell Dye pattern

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…to light themselves at night

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How could we employ liquid crystals to improve LSC function?

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A bit about liquid crystals

reactive mesogen

liquid crystalline polymer

hν

planar homeotropic splay twisted

Liquid Crystalline Monomers & Polymers

crystal mesophase liquid (liquid crystal)

UV

A variety of conformations possible

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Taking advantage of anisotropy of light emission from

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Emission from fluorescent dyes

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Liquid crystal alignment of dyes

Host liquid crystal + guest dye

Dye oriented by liquid crystal (homeotropic) Alignment

layer

Dye oriented by liquid crystal (planar)

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21 Directed emission 1.6 1.5 1.4 1.3 1.2 1.1 1.0 O u tp u t R a tio ( E || /E ⊥ ) 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Dichroism Parameter, R_a

Verbunt et al, Advanced Functional Materials, 2009, 2714-2719

Perfect alignment: R_a = 1 Totally random: R_a = 0

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Another challenge

Around 50% of absorbed energy

is lost through the top and bottom surfaces!

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Reducing surface losses with liquid crystals

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Copying ideas from nature…

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Cholesterics

2µm

+ Chiral dopant =

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What the cholesteric does

* A right-handed cholesteric helix reflects a

bandwidth of right-circularly

polarized light of specific wavelength. * The reflected wavelengths depends on the pitch of the helix

* Left-circularly polarized light will pass through this cholesteric

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Reflection of the cholesterics

0 20 40 60 80 100 400 450 500 550 600 650 700 750 800 Wavelength (nm) Tr a ns m is s ion ( % ) 0 dgr 20 dgr 30 dgr 50 dgr 1.0 0.8 0.6 0.4 0.2 0.0 A bs or pt ion 800 700 600 500 400 Wavelength (nm) 1.0 0.8 0.6 0.4 0.2 0.0 E m is s io n

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Results of using cholesterics Lumogen

Red 305

With normal cholesterics, we are able to convert 35% of surface losses into useful emission

With new broadband cholesterics, we could be able to convert > 60% of losses into useful emission!

…with interesting visual effects

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Existing solutions

Photo- and electro-chromic windows:

Control light entrance but generates no electricity

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Transparent electrical generation

Organic and thin-film solar cells

Generate electricity but

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Using LCs: Tunable ‘smart’ windows

Unique design simultaneously generates electricity as well as controlling light influx, lessening air conditioning needs

Light Dark Privacy

MG Debije, Adv. Funct. Mater. 20, 1498 (2010) Pictures courtesy Peer+

Can be

automatic or manual

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Switchable windows, dark state

PV Glass I Dye 0V Host LC

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Switchable windows, light state

PV

Glass

10V

I

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35 Anisotropy in absorption/ emission 1.6 1.2 0.8 0.4 0.0 A bs or ba nc e 700 650 600 550 500 450 400 Wavelength (nm) 0.20 0.15 0.10 0.05 0.00 E m is si o n ( m W )

Incident light parallel to alignment direction

Incident light perpendicular to alignment direction

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Window goals

• Switch between 10% and 70% transmission

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What value is adaptability? • Record efficiencies

(ECN):

Type III-V cells: 7.1% Silicon cells: 4.3%

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Our challenge

• Improve the light-to-electricity efficiency • Match the design to the architect’s needs

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Ubiquitous to the built environment?

Is there room for a large-area, lower efficiency system?

Does the ability to add color and shape, or retrofit interest anyone besides designers?

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Acknowledgements

• At the TU Eindhoven

– Paul Verbunt Industrial partners: – Ties de Jong - Theo Hoeks

– Albert Schenning Sabic IP – Dick Broer - Casper v. Oosten – Cees Bastiaansen Peer+

– Dick de Boer – Shufen Tsoi

• Funding