The Energy of Science Vale UK - 26 May 2011. Solar Renewable Energy Technology presentation by Nicholas Harrison (Imperial College, London). More details at www.sciencevale.com

1. Renewable Energy (Solar)
Nicholas M Harrison
Imperial College London
Daresbury Laboratory
The Rutherford Appleton Laboratory

2. Scale of the Problem: Supply

3. Renewables and Climate Change
COP-15 is widely considered a failure, as it did not
result in binding CO2 - reduction targets.
Nevertheless, COP-15 lead to global acceptance of
the 2oC target as maximum permissible
warming; more will definitely result in climate-
disaster.
This means, the world cannot emit more than 750
Gt of CO2 during this century; it currently emits
about 35 Gt of CO2 per year (9.5 Gt C/a) !

4. Hydroelectric
Geothermal
BiomassSolar
Ocean
Wind
Renewable Capacity

5. Hydroelectric
Gross: 4.6 TW
Technically Feasible: 1.6 TW
Economic: 0.9 TW
Installed Capacity: 0.6 TW
Renewable Capacity

6. Geothermal Mean flux at surface: 0.057 W/m2
Continental Total Potential: 11.6 TW

7. Biomass
50% of all cultivatable land:
7-10 TW (gross)
1-2 TW (net)

8. Solar potential 120,000 TW;
practical > 600 TW ?

9. 6 Boxes at 3.3 TW Each (graphic courtesy of Nate Lewis)
Solar Land Area Requirements

10. Electricity Production Costs

11. CO2 - free sources of energy
Nuclear energy - non-renewable feedstock, final storage ?, risks ?
Clean coal technologies - requires carbon sequestration, unproven
technology and energy inefficient
Wind - fluctuating production, limited number of suitable sites – offshore ?
Hydro - can be switched on instantaneously, suitable for storage, good sites
limited, production should be maximized
Biofuels – interesting liquid fuel for transport, production energy intensive
Geothermal - excellent where easily accessible
Solar energy (Photovoltaics, Solarthermal) - unlimited energy source
PV: continuous price reduction through savings of scale

12. Price learn-curve of crystalline Si PV-
modules
Slide courtesy of G Willeke

13. DESRTEC-EUMENA

14. Research Landscape
Large international investment in research
and development
Strong focus on optimisation of existing
systems
=> The opportunity is for step change in
cost and / or efficiency

15. STFC
Current collaborative international projects:
– High efficiency photovoltaics (inorganic)
– Fundamentals of solar hydrogen production
– Dye sensitised nano-oxides
– Rectenna arrays

16. Light
Fuel
Electricity
Photosynthesis
Fuels Electricity
Photovoltaics
SC
e
SC
CO
Sugar
H O
O
2
2
2
Semiconductor/Liquid
Junctions
H2O
O H
22
SC
Energy Conversion

17. Performance of photovoltaic and photochemical solar cells
Type of cell
Efficiency (%)*
Cell Module
Research and technology needs
Crystalline silicon 24 10-15
Higher production yields, lowering of cost
and energy content
Multicrystalline silicon 18 9-12 Lower manufacturing cost and complexity
Amorphous silicon 13 7
Lower production costs, increase production
volume and stability
CuInSe2 19 12
Replace indium (too expensive and limited
supply), replace CdS window layer, scale up
production
Dye-sensitized
nanostructure materials
10-11 7
Improve efficiency and high-temperature
stability, scale up production
Bipolar AlGaAs/Si photochemical cells 19-20 - Reduce material cost, scale up
Organic solar cells 2-3 - Improve stability and efficiency
M. Grätzel, Nature 415, 338 (2001)
Status

18. Ultimate Efficiency Limits
Thermodynamic limit of Carnot engine: η = 1 – T0/Ts ~ 95% (100% absorption)
Shockley-Queisser efficiency limit for single band semiconductor based on detail
balance eq.:
~31% (1 sun: Planck low) and ~41 (max conc.)
Origin of the solar cell losses:
a) Light with energy below Eg will not be
absorbed
b) The photons with excess energy above Eg is
lost in the form of heath
c) Single crystal GaAs solar cell ~ 25%(AM1.5)

19. Multijunction or tandem cells:
• First approach to exceed single
junction efficiency
• To achieve >50% efficiency need
3 or more tandems with different
Eg’s
• Significant technological
problem to relax strain
• 75% efficiency achieved with 36
tandems
Tandem solar cells
No of
junctions
1 sun Max conc.
1 30.8% 40.8%
2 42.9% 55.7%
3 49.3% 63.8%
 68.2% 86.8%

20. High-efficiency ISE triple-junction solar cells
Ga0.65In0.35P
tunnel diode
Ga0.83In0.17As
tunnel diode
Ge substrate

21. Intermediate band solar cells
Multi-junction solar cell
• Each junction  single gap
• N- junctions  N- absorptions
Multi-band solar cell
Single junction (no lattice mismatch)
N- bands  N(N-1)/2 (gaps)
Add 1 band  Add N- absorptions

22. Intermediate band solar cells
Intermediate band vs multi-junction solar cell
• Max. efficiency for 3 band cell ~66% (vs 55%)
• Max. efficiency for 4 band cell ~72% (vs 60%)
• Better performance than any other structure of similar complexity
A. Luque & A. Marti, Phys. Rev. Lett 78, 5014 (1997)

23. Requirements & Possible Realization
Designing a materials system:
Finite width IB to allow excitations
VB-IB, IB-CB
Narrow IB to reduce carrier transport
Predictive simulations yield QD arrays
as an excellent candidate
QD arrays produce an IB with zero density of states between VB
& IB & CB, which increases the radiative lifetime relative to the
relaxation time within bands.

24. Current technology
Vertical ordering is provided by strain driven alignment
Horizontal regularity of QD’s is observed on high Miller index surfaces
Q. Xie, et al., Phys. Rev. Lett. 75, 2542 (1995)
S. Tomic, NMH et al., J. Appl. Phys. 99, 093522 (2006)
Y. Okada, private communication

25. Solar Hydrogen
Detailed understanding of:
– Excitation
– Transport
– Surface dynamics
– Reduction reaction
EPSRC EP/G060940/1 Nanostructured Functional Materials for Energy Efficient Refrigeration, Energy Harvesting
and Production of Hydrogen from Water. Programme grant Oct 2009.

26. Rectenna Arrays
An array of nanostructured antennas for
supported on metal-insulator-insulator-metal
diodes

27. Conclusions
Solar energy will be a significant component of the
energy mix by 2050
Significant scientific / technological breakthroughs
required to ease the transition
Very large international research and development
effort – the current opportunity is in step change