High Concentrating Photovoltaics: Past, Present, and Future By: Shawn Lamb ASU – Dept. of Physics

The Promise of Concentrators Concentrator = any system that uses concentrating sunlight to harness energy. We will focus on Concentrating Photovoltaic Systems (CPV) Concentrator = any system that uses concentrating sunlight to harness energy. We will focus on Concentrating Photovoltaic Systems (CPV) Since 1970s concentrators have been developed and researched. Since 1970s concentrators have been developed and researched. From 1975 to 1992 the total of research funds for concentrator technology exceeds $40M worldwide. From 1975 to 1992 the total of research funds for concentrator technology exceeds $40M worldwide. Today only a handful of companies pursue research in concentrators. Today only a handful of companies pursue research in concentrators.

Concentrating Photovoltaics Of all types of Solar concentrating systems, CPV will have the most immediate impact on energy production. Of all types of Solar concentrating systems, CPV will have the most immediate impact on energy production. Uses optics to concentrate incident sunlight onto small PV cell. Uses optics to concentrate incident sunlight onto small PV cell. Concentrations from 2 to suns. Concentrations from 2 to suns. Must track the sun precisely (<0.1 degree) Must track the sun precisely (<0.1 degree) Cheaper because system uses less PV material (Silicon, AlGaAs, GaSb, etc.) Cheaper because system uses less PV material (Silicon, AlGaAs, GaSb, etc.)

Technology of CPV Lightweight, cheap, plastic Fresnel lenses are used to concentrate light. Lightweight, cheap, plastic Fresnel lenses are used to concentrate light. Light must be dispersed over cell not focused onto one spot. Light must be dispersed over cell not focused onto one spot. Cells mounted into receiver with built in heat- sink to dissipate heat. Cells mounted into receiver with built in heat- sink to dissipate heat. Ephemeris or autoguider used to track the sun so that solar rays are incident to panels. Ephemeris or autoguider used to track the sun so that solar rays are incident to panels. Exotic materials used for PV cell. Economically viable since less material is used. Exotic materials used for PV cell. Economically viable since less material is used.

Why Concentrate? Efficiency increases with concentration. Record set at 35% efficiency by NREL using a multi-junction GaInNAs substrate Efficiency increases with concentration. Record set at 35% efficiency by NREL using a multi-junction GaInNAs substrate More common materials used in structure (such as metal, plastic, glass, steel, etc.) More common materials used in structure (such as metal, plastic, glass, steel, etc.) Cheaper overall: Cheaper overall: –Currently Flat Plate PV ~ $0.39 / kWhr –CPV ~ $0.13 / kWhr (over the lifetime of the project)

Applications of CPV 3rd world water pumping for drinking or agriculture 3rd world water pumping for drinking or agriculture Grid-connected power farms (medium to large scale) Grid-connected power farms (medium to large scale) Off-grid homes (ex/ mountain homes) Off-grid homes (ex/ mountain homes) Hydrogen production: store energy Hydrogen production: store energy Backup generators: reliability beyond the grid (large battery banks) Backup generators: reliability beyond the grid (large battery banks) Disaster relief units: after hurricane, flood Disaster relief units: after hurricane, flood Remote hospitals & refugee camps (tent cities in desert) Remote hospitals & refugee camps (tent cities in desert)

Flat Plate vs. CPV Flat-Plate industry is well established Flat-Plate industry is well established Well developed production lines have brought overall costs down for F.P. Well developed production lines have brought overall costs down for F.P. F.P. doesnt need to track: uses diffuse, non- incident sunlight F.P. doesnt need to track: uses diffuse, non- incident sunlight Small flat-plate structures are easily transported and installed Small flat-plate structures are easily transported and installed CPV has better future prospects but a bad (un- warranted) reputation CPV has better future prospects but a bad (un- warranted) reputation

Weaknesses of CPV Not well established market (yet) Not well established market (yet) Must track the sun accurately (only incident rays used) Must track the sun accurately (only incident rays used) PV material tends to overheat (thus reducing the efficiency gained by concentrating) PV material tends to overheat (thus reducing the efficiency gained by concentrating) Needs cooling: passive or active (water flow over surface or Peltier cooling) Needs cooling: passive or active (water flow over surface or Peltier cooling) PV substrate has a shortened lifetime (typically ~ 5 years) PV substrate has a shortened lifetime (typically ~ 5 years)

Players in the Game Amonix (Torrence, CA) – large scale grid-tied power applications. High concentration Fresnel Lenses Amonix (Torrence, CA) – large scale grid-tied power applications. High concentration Fresnel Lenses ENTECH (Keller, TX) – Space applications ENTECH (Keller, TX) – Space applications BP Solar (Linthicum, MD) – PV solar apps BP Solar (Linthicum, MD) – PV solar apps Sunpower (Sunnyvale, CA) – R&D Sunpower (Sunnyvale, CA) – R&D NREL (Golden, CO) – R&D NREL (Golden, CO) – R&D Sandia Labs (Albuquerque, NM) – R&D, Grid stability Sandia Labs (Albuquerque, NM) – R&D, Grid stability National Center for Photovoltaics (DOE) – R&D National Center for Photovoltaics (DOE) – R&D Energy Efficient and Renewable Energy (DOE) Energy Efficient and Renewable Energy (DOE)

Future Objectives for CPV R&D More efficient, large-scale production is needed to stay cost competitive with flat plate PV More efficient, large-scale production is needed to stay cost competitive with flat plate PV Tracking systems must be improved and made cheaper (currently ~ $20K) Tracking systems must be improved and made cheaper (currently ~ $20K) Cooling systems must be integrated into the modules themselves Cooling systems must be integrated into the modules themselves Optics: anti-reflection using thin films or water flow Optics: anti-reflection using thin films or water flow Higher efficiency substrate made cheaply Higher efficiency substrate made cheaply

Tracking the Sun Cheaper, more accurate tracking systems are needed for CPV technology. Cheaper, more accurate tracking systems are needed for CPV technology. Autoguiding techniques, borrowed from astronomy, using a CCD or CMOS imager can be developed into a closed-loop tracker for < $100 (relatively disposable). Autoguiding techniques, borrowed from astronomy, using a CCD or CMOS imager can be developed into a closed-loop tracker for < $100 (relatively disposable). Decentralizing the CPV modules into several independent tracking systems working coherently could drastically reduce downtime for the system. Decentralizing the CPV modules into several independent tracking systems working coherently could drastically reduce downtime for the system. An ephemeris can be used for times when the autoguider cannot find the sun (cloudy day). An ephemeris can be used for times when the autoguider cannot find the sun (cloudy day).

Cooling / Anti-reflection Energy is lost by reflection of solar rays off of glazing layer (glass) also by decreased efficiency due to substrate heating Energy is lost by reflection of solar rays off of glazing layer (glass) also by decreased efficiency due to substrate heating Thin films have been used but are very expensive. It has been shown that a thin dynamic layer of water flowing over the surface of the PV panels will reduce reflection as well as cool the system Thin films have been used but are very expensive. It has been shown that a thin dynamic layer of water flowing over the surface of the PV panels will reduce reflection as well as cool the system Passive cooling includes designing modules to draw heat away from substrate (heat sink) Passive cooling includes designing modules to draw heat away from substrate (heat sink)

Economics Currently medium sized, silicon CPV generators produce electrical energy (fed to grid) at $0.21 / kWhr Currently medium sized, silicon CPV generators produce electrical energy (fed to grid) at $0.21 / kWhr Currently Fossil / Nuclear / Hydro costs about $0.08 / kWhr (will only increase) Currently Fossil / Nuclear / Hydro costs about $0.08 / kWhr (will only increase) Gallum Arsenide has near future prospects of ~$0.07 / kWhr Gallum Arsenide has near future prospects of ~$0.07 / kWhr Future projections show silicon CPV systems at <$0.09 / kWhr Future projections show silicon CPV systems at <$0.09 / kWhr By 2020 CPV is predicted at ~$0.06 / kWhr By 2020 CPV is predicted at ~$0.06 / kWhr

Grid Connected Power Farms Net metering - credit for power produced by PV (paid by power companies) Net metering - credit for power produced by PV (paid by power companies) Sell green tags to independent brokers (label stating that electricity was produced by renewable source). Sell green tags to independent brokers (label stating that electricity was produced by renewable source). Federal / State subsidies for renewable energy power farms, including PV Federal / State subsidies for renewable energy power farms, including PV Several test facilities: 480 kW Euclides Project in Canary Islands, Spain by BP Solar. Entechs 300 kW project in Austin, TX Several test facilities: 480 kW Euclides Project in Canary Islands, Spain by BP Solar. Entechs 300 kW project in Austin, TX Investors needed for future development Investors needed for future development ~20 year lifetime (if cells are maintained and or replaced) ~20 year lifetime (if cells are maintained and or replaced)

Future of World CPV Market Medium to large sized solar farms for rural power generation Medium to large sized solar farms for rural power generation Water pumping applications are most likely as well as water pasteurization Water pumping applications are most likely as well as water pasteurization Could be used as a form of revenue for isolated villages to sort of link together separate PV power farms into a quasi-stable grid Could be used as a form of revenue for isolated villages to sort of link together separate PV power farms into a quasi-stable grid Unlikely CPV will overtake flat-plate PV on world market Unlikely CPV will overtake flat-plate PV on world market

Disaster Relief / Humanitarian Aide Used to power refugee camps and remote hospitals in places where fuel is not readily available. Used to power refugee camps and remote hospitals in places where fuel is not readily available. Portable power trailers could be brought to the location upon demand and transported easily. Portable power trailers could be brought to the location upon demand and transported easily. Hybrid CPV, Wind, Diesel Generator system most probable (need to split up the electrical load). Hybrid CPV, Wind, Diesel Generator system most probable (need to split up the electrical load). Would pay for itself (if maintained correctly) in a short amount of time. Would pay for itself (if maintained correctly) in a short amount of time.

Conclusions Under-exploited renewable energy resource Under-exploited renewable energy resource Needs maturity in public opinion / knowledge and industry production capabilities Needs maturity in public opinion / knowledge and industry production capabilities Future economics looks good, has potential to overtake most forms of renewable electrical production Future economics looks good, has potential to overtake most forms of renewable electrical production Technology: Tracking, cooling, anti- reflection can be improved Technology: Tracking, cooling, anti- reflection can be improved

Further Information The National Renewable Energy Lab: The National Renewable Energy Lab: For information on how PV has helped in disaster relief visit: For information on how PV has helped in disaster relief visit: National Center for Photovoltaics: National Center for Photovoltaics: U.S. Dept. of Energy, Energy Efficient and Renewable Energy (EERE): U.S. Dept. of Energy, Energy Efficient and Renewable Energy (EERE): Sandia National Labs PV: Sandia National Labs PV: