The Role of Solar Electricity in Sustainable Building (Smart Windows)

Quantum Photovoltaic Group Keith Barnham, Ian Ballard, Andreas Ioannides, David Johnson,

Marianne Lynch, Massimo Mazzer, Tom Tibbits,(Cell design, cell characterisation and modelling)

Experimental Solid State Physics, Imperial College London, London SW7 2BW, UK http://www.sc.ic.ac.uk/~q_pv

John Roberts, Geoff Hill, Cath Calder

(Sample growth and fabrication)EPSRC National Centre for III-V Technology, University of Sheffield, Sheffield S1 3JD, UK

Tim Green (Systems, local power networks, storage)

Electrical Engineering, Imperial College London, SW7 2AZ

Optical Products Ltd, SolarStructure, Permasteelisa Centre for Integrated Photonics

Growth in World PV Capacity

PV installations World-wide increased by 57% in 2004

UK PV one of lowest in EU but a 40%/yr increase would generate 23% by 2023

P.Maycock, Renewable Energy World, Aug. 2005.

The Three Generations of PV First Generation

Crystalline and poly-crystalline Si ~15% efficiency, ~ $3/Wp

Second Generation Thin film cells CdTe, CuInSe2 (10-15)% effic., ~ $(1-2)/Wp

Third Generation expensive - III-V cells (400-1000)x concentration for

large scale power < $1/Wp. Our Target (1000x) Our Present State

(200x) Silicon - no concentration

M.A.Green, “Photovoltaics for the 21st Century II”,

Electrochemical Soc. Proc. Vol. 2001-10, 1, (2001).

http://www.pvsystem.net/ Shibuya, Japan

The First BIPV Building in Japan

First Generation cells in BIPV

Cell Efficiency ~ 15%

Power Glass™ Power Glass™ represents a new breed of solar cell design that balances solar cell efficiencies and manufacturing costs with broad applications and uses. Power Glass™ solar cells operate at as much as 50%, or half, the efficiency of conventional opaque amorphous solar cells yet costing as little as 25%, or one fourth, to produce.

http://www.xsunx.com/tech-power.htm

XsunX, Inc.Aliso Viejo, CA (USA)

Second Generation Thin Film Cells in BIPV

Cell Efficiency ~ 7%

Efficiency versus band- gap

GaAs cells highest single junction efficiencies

Lower Eg => higher efficiency

No lower Eg III-V alloys lattice matched to GaAs or Ge substrates

Multi-junction approaches also would like bulk-cell with band-gap ~ 1.1 mm ~ 1.1 eV

Can grow InyGa1-yAs bulk cells on virtual substrates but never dislocation free

SB-QWSC lowers GaAs absorption edge without dislocations

GaAsP/InGaAs Strain-Balanced QWSC

Advantages:

Can vary absorption band-edge and absorb wider spectral range without strain-relaxation

No dislocations

Higher barriers than GaAs

=> reduced recombination

single junction so can cope

with varying spectra

Balance stress between layers to match lattice parameter of the

substrate

17

19

21

23

25

27

29

0.1 1 10 100 1000Concentration (suns)

Eff

icie

nc

y (%

)

Control pn cell

pn prediction

GaAs record

SB-QWSC

SB-QWSC predicted

SB-QWSC Efficiency vs. Concentration

50 well SB-QWSC ~ 2% higher efficiency than p-n control

65 well cell should achieve World record at 500x

Single Junction Cell World Record

What is the Electricity DEMAND in Buildings?

63% of electricity in UK used in buildings

Sunlight on buildings ~ 7x electricity consumption in the buildings

DEMAND similar through year, peaks daily ~2x BASELOAD –

Nuclear only provides baseload

3rd Generation cells on 25% of S-facing walls replace all nuclear contribution

At 500x a UK semiconductor facility could produce cells

= 5 nuclear reactors in 10 years

00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 24:00

5

6

7

8

9

10

Po

we

r D

em

and

[MW

]

Time of Day

Mar 20th Aug 13th Feb 11th

Imperial College London

Novel Application of 3rd Generation CellsSolarstucture Ltd– new company for Building Integrated Concentrator PV

Curtain walls (glass facades) => Use tracking blinds to cut direct

sunlight to remove glare and

reduce air conditioning demand

Our cells are very small (~ 1mm)high efficiency (~ 30%) need 500 x concentrators to reduce costsThe concentrators must track the sun

Third Generation Cells in Concentrators (Smart Windows)

Unique advantages: No transmission of direct sunlight Reduce a/c requirementsMax diffuse sunlight - for illumination(2 – 3) x power from Silicon BIPV Provide electricity at peak times Cell cooling provides hot water

(500 – 1000)x concentration Transparent modules 1 mm solar cells Cell efficiency ~ 30% innovative 2-axis tracking adds ~ 20% to façade cost

Heat

Diffuse Daylight

Diffuse Daylight

Solar Cells

Direct Sunlight

Lenses

Front Glass

ElectricityHeat

Diffuse Daylight

Diffuse Daylight

Solar Cells

Direct Sunlight

Lenses

Front Glass

Electricity

65%

21%

22% (cell cooling)

6%

DirectSunlight

Diffuse Sunlight Available for

Illumination

Electricity

31% (IR rad.)

Smart WindowsEnergy Budget

Thin 2nd Generation Cells

Beneficial Detrimental

11%

6%

Electricity

80%

80%

Usable Heat

Heat Dispersion

Heat Dispersion

Light Reflection

DirectSunlight

Diffuse Sunlight

10%

10%

Annual electricity generation as a function of the curtain wall slope

90° = vertical wall

The maxima correspond to the latitude angle, i.e.:

37.8° for S.Francisco51.5° for London

San Francisco

0

50

100

150

200

250

300

350

30 45 60 75 90Window Tilt/deg

Ele

ctri

c E

ner

gy

(kW

h/m

2 /y)

SolarSkin - South SolarSkin - SE/SWSharpST - South SharpST - SE/SW

London

0

20

40

60

80

100

120

140

30 45 60 75 90Window Tilt/deg

Ele

ctri

c E

ner

gy

(kW

h/m

2/y

)

SolarSkin -South SolarSkin - SE/SWSharpST - SE/SW SharpST - South

Comparison with 2nd Generation BIPV

SB-QWSC + Ge 32%

vertical S - W facing wall in London

World record 3-junction cell

35% harvests same

electrical energy over year

Compare SB-QWSC + Ge with multi-junction Cell

MJ cell optimised for one spectrum, one temperature.

Tunnel Junctions problematic at high conc. high current

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0 28 56 84 112 140 168 196 224 252 280 308 336 364

Day

En

erg

y/(

kW

h/m

2/d

ay

)

QWSC+Ge Triple Junction

Conclusions

• 3rd generation cells, higher efficiency-lower cost in concentrators can help maintain > 40% per year PV market expansion

• Smart-windows alone could replace the nuclear component quicker than new-build

• Smart windows – new active architectural component