Realising the Potential

of

Solar Power

Peter Weightman

Physics Department, University of Liverpool, Oxford Street, Liverpool

Liverpool Energy Institute

The Problem: Climate Change

Current 2050 EstimatesGlobal power need 13 TW 26 TWFossil Fuels ~ 11 TW 0 ???Nuclear Fission 1 TW 5 TW (optimistic)Fusion 0 TW ?Other renewables < 1 TW 4 TW (optimistic)Photovoltaics 3 GW Great potentialArtificial Photosynthesis 0 TW Great potential

Potential: Power reaching the earth from the sun 100,000 TW

(0.3 % of the sunlight reaching the Sarah Desert meets Europe’s power needs)

ALICE: A New Tool

“Scientific advance is more often driven

by the development of a

new tool than a new concept”

Freeman Dyson In a review of a biography of the mathematician George Green

1µ

10µ

100µ

1m

10m

100m

1

10

100

1000100101.1.01

Impatt

Gunn

III-V

's

Lead salts

Ou

tput

Pow

er (

Wat

ts )

SLED

Frequency (Terahertz )

RTD arrayRTD

HG QC Laser

Courtesy: J. Allen. M. Chamberlain

Accelerator Sources of Terahertz Radiation

Power of laboratory instruments At 1 THz ~ 100 watts

Average power ~ 24 mWPeak power ~ 70 kW

Short electron bunches When bunch length < wavelength

Coherent emission ---> massive output power

Daresbury ERLP/ALICE

Energy Recovery Linear Accelerator / ALICE

Daresbury

The most intense broad band source of THz in Europe and only the 3rd in the world.

5 years under construction now commissioning

NW Science Fund: Liverpool

Liverpool THz beamline

. .

Liverpool THz Beamline

1st Floor Tissue Culture Facility

Lower level hutch for THz energy experiments

Beamline funded and built by physics dept.

Improving Solar Cells Problem: Solar spectrum is broad, absorbing structures, band gaps, are narrow

h, Eg, wasted h>, Eg, wasted

Liverpool Energy Institute

h creates anelectron-hole pair (exciton)

Bulk semiconductors1 hi

Shockly-Queiser limitPhotovolatic energy collection < 32%Due to phonon emission

Nanocrystals: PbS(Klimov et al 2004)

For h> Eg can

create many excitons(MEG)

To get electron and hole out attach functional organic groups

Controversy (Science 322 1784 December 2008)

Reproducibility. Exciton lifetime. Do organic groups quench MEG production?

Key is to understand dynamics. Exciton energy levels are in the THz

Need a high intensity THz source with good time structure: ALICE

Artificial PhotosynthesisKey elements:

•A photo receptor, often a metal complex• Function: adsorb photons and release excited electrons

•A transducer, often organic ligands• Function: transport electrons from the photo receptor to the catalytic reactor

•A catalytic reactor, also often a metal complex• Function: reduce CO2 to CO, split H2 from H2O, or convert CO2 and H2O to formic

acid HCOOH

The goal is to use sunlight to create high-energy molecules which can then be recombined with other molecules to release the stored chemical energy. The principle is applied in living organisms (bacteria, plants). Harnessing it for technological applications has the potential to create cycles of energy production and consumption, which have no negative impact on the environment.

So why hasn’t this been done already?

Short answer: it turns out to be rather difficult.But the good news is: we know that it works.

Courtesy Werner Hofer

Scheme of a cell for artificial photosynthesis

e-

CRED

H+

½ H2

e- e- e-

Antenna

COX A

½ H2O

H+ +½ O2

C chromophoreA electron acceptorD electron donor

C D

h

H+

½ H2

½ H2O

H+ +½ O2

Photoelectrochemical Synthesis Cell (PES Cell)

Courtesy Werner Hofer

Recent advances in artificial photosynthesis (Julia Weinstein, University of Sheffield)

Generation of very stable charge separated state in d8 organometallic system

Combination of transient bond formation with long distance charge separation

Liverpool Energy Institute

ConceptLight induced S-S bonde transfer via Ptfrom S-S to N N

\Structural reorganisation in excited state traps the energy and prevents back electron transfer

Key issues

Chemical synthesis (electrodes?)

Fast light sources to monitor

transient changes in electronic and

geometrical structure in real time

ALICE and NLSJ.A. Weinstein, M.T. Tierney, E.S. Davies, K. Base, A.A. Robeiro and M.W. Grinstaff Inorg. Chem. 45 4544 (2006)