Science Vale UK energy event renewable energy technology - solar

Renewable Energy (Solar)

Nicholas M Harrison

Imperial College London

Daresbury Laboratory

The Rutherford Appleton Laboratory

Scale of the Problem: Supply

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) !

Hydroelectric

Geothermal

BiomassSolar

Ocean

Wind

Renewable Capacity

HydroelectricGross: 4.6 TW

Technically Feasible: 1.6 TW

Economic: 0.9 TW

Installed Capacity: 0.6 TW

Renewable Capacity

Geothermal Mean flux at surface: 0.057 W/m2

Continental Total Potential: 11.6 TW

Biomass50% of all cultivatable land:

7-10 TW (gross)

1-2 TW (net)

Solarpotential 120,000 TW;

practical > 600 TW ?

6 Boxes at 3.3 TW Each (graphic courtesy of Nate Lewis)

Solar Land Area Requirements

Electricity Production Costs

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

Price learn-curve of crystalline Si PV-

modules

Slide courtesy of G Willeke

DESRTEC-EUMENA

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

STFC

Current collaborative international projects:

– High efficiency photovoltaics (inorganic)

– Fundamentals of solar hydrogen production

– Dye sensitised nano-oxides

– Rectenna arrays

LightFuel

Electricity

Photosynthesis

Fuels Electricity

Photovoltaics

SC

e

SC

CO

Sugar

H O

O

2

2

2

Semiconductor/LiquidJunctions

H2O

O H22

SC

Energy Conversion

Performance of photovoltaic and photochemical solar cells

Type of cellEfficiency (%)*

Cell ModuleResearch and technology needs

Crystalline silicon 24 10-15Higher production yields, lowering of cost

and energy content

Multicrystalline silicon 18 9-12 Lower manufacturing cost and complexity

Amorphous silicon 13 7Lower 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 materials10-11 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

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)

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

junctions1 sun Max conc.

1 30.8% 40.8%

2 42.9% 55.7%

3 49.3% 63.8%

68.2% 86.8%

High-efficiency ISE triple-junction solar cells

Ga0.65In0.35P

tunnel diode

Ga0.83In0.17As

tunnel diode

Ge substrate

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

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)

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.

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

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.

Rectenna Arrays

An array of nanostructured antennas for

supported on metal-insulator-insulator-metal

diodes

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