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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)