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

1. 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

2. 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.

3. The Three Generations of PV M.A.Green, “Photovoltaics for the 21st Century II”, Electrochemical Soc. Proc. Vol. 2001-10, 1, (2001). • 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

4. First Generation cells in BIPV Cell Efficiency ~ 15% Shibuya, Japan The First BIPV Building in Japan http://www.pvsystem.net/

5. Second Generation Thin Film Cells in BIPV Cell Efficiency ~ 7% XsunX, Inc.Aliso Viejo, CA (USA) Power Glass™ http://www.xsunx.com/tech-power.htm 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.

6. Efficiency versus band- gap • GaAs cells highestsingle 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

7. GaAsP/InGaAsStrain-Balanced QWSC Balance stress between layers to match lattice parameter of the substrate 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

8. 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

9. 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 What is the Electricity DEMAND in Buildings? Imperial College London

10. Novel Application of 3rd Generation Cells Solarstucture Ltd– new company for Building Integrated Concentrator PV • Our cells are very small (~ 1mm) • high efficiency (~ 30%) • need 500 x concentrators to reduce costs • The concentrators must track the sun • Curtain walls (glass facades) => • Use tracking blinds to cut direct sunlight to remove glare and reduce air conditioning demand

11. Third Generation Cells in Concentrators (Smart Windows) • (500 – 1000)x concentration • Transparent modules • 1 mm solar cells • Cell efficiency ~ 30% • innovative 2-axis tracking • adds ~ 20% to façade cost • Unique advantages: • No transmission of direct sunlight • Reduce a/c requirements • Max diffuse sunlight - for illumination • (2 – 3) x power from Silicon BIPV • Provide electricity at peak times • Cell cooling provides hot water

12. Smart Windows Energy Budget Electricity Beneficial Detrimental 21% Direct Sunlight 22% (cell cooling) Usable Heat 31% (IR rad.) 11% Heat Dispersion Diffuse Sunlight 65% Available for Illumination Thin 2nd Generation Cells 10% Direct Sunlight 80% Heat Dispersion 6% 10% Diffuse Sunlight 80% 6% Light Reflection Electricity

13. Comparison with 2nd Generation BIPV 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.Francisco 51.5° for London

14. Compare SB-QWSC + Ge with multi-junction Cell SB-QWSC + Ge 32% vertical S - W facing wall in London World record 3-junction cell 35% harvests same electrical energy over year MJ celloptimised for one spectrum, one temperature. Tunnel Junctions problematic at high conc. high current

15. 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