Solar Energy Conversion: A very brief introduc8on. Chem 204 April 7, 2014

Transcription

1 Solar Energy Conversion: A very brief introduc8on Chem 204 April 7, 2014

2 Worldwide energy usage

3 Worldwide energy usage Now: about 15 TW (15 x J/s)

4 From Chemistry: the Central Science, 12 th ed.

5 Quick calcula8on 168 W/m 2 of sunlight strikes Earth s surface Surface area of Earth s land: 149 x 10 6 km 2 Therefore, 2.50 x W sunlight hits Earth s landmass

6 Quick calcula8on 168 W/m 2 of sunlight strikes Earth s surface Surface area of Earth s land: 149 x 10 6 km 2 Therefore, 2.50 x W sunlight hits Earth s landmass The energy from the Sun that hits Earth s landmass in one hour (9.0 x J) is sufficient to power the planet for about 3 months at present rate of consump8on

7 Photovoltaics, solar hea8ng, solar fuels Photovoltaic: direct sunlight to electricity Solar thermal power plant: sunlight drives steam turbine Solar fuels: sunlight into chemicals ( ar8ficial photosynthesis ; ar8ficial leaf )

8 Wavelengths of sunlight that reach Earth s surface

9 Wavelengths of sunlight that reach Earth s surface Want material that absorbs a lot of solar photons!

10 Wavelengths of sunlight that reach Earth s surface Silicon! Earth- abundant, bandgap 1.1 ev (about 1100 nm)

11 Silicon photovoltaics: pn junc8on energy Upon contact: Fermi levels equilibrate, leading to a space- charge region distance

12 Silicon photovoltaics: pn junc8on

13 Shockley- Quiesser Limit At best, for single- junc8on solar cells: 33.7% efficiency of photon- electron conversion

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15 A UIUC Startup: Semprius Tiny silicon solar cells that are about 500 nm x 500 nm ac8ve area; mounted on stands that track the sun; 37% efficient in mul8junc8on mode!

16 Dye- sensi8zed solar cells Michael Graetzel, EPFL

17 Dye- sensi8zed solar cells

18 Dye- sensi8zed solar cells Dye can be blackberry juice!

19 Dye- sensi8zed solar cells Sony prototype

20 Solar Fuels Concept: shine light, promote chemical reac8on to store energy in the form of chemical bonds Everybody s favorite reac8on: water splihng (2 H 2 O + sunlight à 2H 2 + O 2 )

21 Redox poten8als and band edges If you want to use only one material: Shine light, get photoexcited electrons to reduce protons, and photogenerated hole to oxidize water. But you have to worry about regenera8ng material.

22 Fujishima and Honda, 1972

23 Catalysts to speed up hydrogen and oxygen produc8on

24 Kanan and Nocera, 2008

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26 An ar8ficial leaf? H. Gray, Nature Chemistry The blue por8on of sunlight is absorbed by the metal oxide anode (red), where it oxidizes water to release protons. The red por8on passes through to be absorbed by the silicon nanorod cathode (blue), which combines these protons with electrons to make hydrogen for fuel. The membrane (light blue) must be permeable to the generated protons and conduct electrons between the two electrodes. The spheres in the region of the anode represent oxygen molecules; spheres in the region of the cathode represent hydrogen molecules. The hydrogen to oxygen ra8o is 2:1.

27 More solar fuels. Beqer anodes/catalysts Beqer cathodes/catalysts Beqer membranes Other reac8ons: carbon dioxide as a feedstock, like photosynthesis?

28 People and places in solar research Na8onal Renewable Energy Lab (US) Fraunhofer Ins8tute of Solar Energy (Germany) UIUC: John Rogers, Ralph Nuzzo Caltech: Harry Gray, Nate Lewis Harvard: Dan Nocera UNC Chapel Hill: Tom Meyer EPFL: Michael Graetzel Oxford: Henry Snaith (perovskite solar cells are hot!)