roger angel - rehnu€¦ · cells @ 1000x concentration cost $0.15/watt ... and higher efficiency...
TRANSCRIPT
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Concentrating PV at $1/watt. Field tests of a disruptive approach to reduce cost
Roger Angel Steward Observatory University of Arizona
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outline1. Heritage - making astronomical telescopes at the
University of Arizona2. Solar as a renewable electricity source, cost-competitive
with fossil fuels 3. Comparison of solar to electric conversion strategies:
– flat photovoltaic (PV) – Concentrating thermal (CSP)– Concentrating photovoltaic (CPV)
4. Arizona’s disruptive concept for large scale, low-cost CPV5. Field demonstration6. Next steps and commercialization
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1. Heritage
making astronomical telescopes at the University of Arizona
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Spin-casting liquid glass to make an 8.4 m diameter glass telescope mirror at the University of Arizona Mirror Lab
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Inspection during stressed-lap polishing. Honeycomb cells visible beneath the surface.
Polishing an 8.4 m diameter mirror at the Lab
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Two of the 8.4m diameter mirrors on a tracking mount make the world’s largest single astronomical telescope
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25 m paraboloidal reflector made from seven 8.4 m segments for the Giant Magellan Telescope
The future:
3 m square paraboloidal reflectors to concentrate sunlight on photovoltaic cells
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RRep Gabrielle Giffords with UA President Robert Shelton at experimental 3-m solar dish made at the Mirror Lab from back-silvered glass segments
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2. Solar as a renewable electricity source, cost competitive with fossil fuels
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Context for work• Eliminate carbon dioxide emission as a by
product of electricity generation• Reduce dependence on foreign fuel
• Generate electricity from sustainable sources, solar and wind
• Goal– Electricity delivered at cost parity with fossil
fuel– Method suitable for the required very large
scale, 100,000 km2 worldwide
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Basic challenges in meeting cost parity goal
• Conversion cost for wind and solar– Need ~$1/watt installed cost
• Storage to deal with intermittent sources – Combine direct solar (day) with wind and
stored solar heat (night)– Pumped hydro storage for time shift
• Transmission– up to 2000 miles needed from best solar and
wind resources to population centers
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Storage and transmission • Storage
– 50 GW of pumped hydro storage is already in US and Europe, and making a profit
– Note that while hydro requires large river flow, pumped hydro does not. Its volume can be greatly expanded with relatively little environmental impact
• Transmission– US example, Pacific Intertie 1000 miles, ± 0.5 MV.
2500 miles with 10% loss viable (± 1MW, 14 grams of aluminum per watt)
• Conclusion: storage and transmission costs should not preclude sustainable transcontinental grid at parity cost. Conversion of sunlight to electricity is the area where cost reduction is most critical
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Sun and wind - sources and needs
• Both have similarly low power density– Solar flux = 1 kW/m2
– Wind kinetic energy at 10 m/sec = 0.5 kW/m2
• And similar intermittency, ~ 30% duty cycle
• Allowing for 30% intermittency and 30% conversion efficiency, replacing today’s 10 TW of 24 hr power from fossil fuel will require harvesting over ~100,000 km2
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Wind • Wind generation currently provides 30 times
more power than solar, because of lower cost. Why does it cost less?
• Blade concentrates wind energy over large area (~10,000m2) for conversion by dynamo
• Advantage: blade area << capture area– Reduced cost– Stow in extreme wind eases survival– Steel mass is low, 150 kg/kW (land), 250 kg/kW
(sea)
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Solar comparison
• Harvest requires sunlight capture with PV panel or reflector extending over full area
• Mechanical support must be robust enough to survive large mechanical load on full area under extreme wind
• For current tracking systems, steel mass can exceed 300 kg/kW. Mass drives cost
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more than enough desert sunshine to power the world
NREL map of solar resource at direct incidence
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3. Comparison of solar to electric conversion strategies:
• Photovoltaic flat panel - PV– fixed or – single axis tracking
• Concentration with thermal conversion - CSP– single axis (trough) and dual axis (dish)– with/without thermal storage
• Concentration with photovoltaics - CPV– single axis (trough, low concentration)– dual axis (dish, high concentration)
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Solar dish powers a printing press in late 1880s
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PV and thermal are complementary
• Thermal storage has unique capability to handle late afternoon and evening load
• CPV likely to be cheaper during the day
• Solution may be separately optimized farms whose entire harvest goes to either daytime CPV production or to thermal storage CSP
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Different challenges for PV and CPV to reach $1/watt
• PV - Direct illumination of large areas of semiconductor– Challenge is to manufacture huge areas of
semiconductor of reasonable efficiency
• CPV - Optical concentration onto much smaller semiconductor areas– Semiconductor cost is 10x less – Challenge is to reduce the optomechanics cost
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Triple junction PV cells• Cells in 3 layers on germanium substrate• Blue photons absorbed in upper layer give higher voltage• Highest conversion efficiency of any method
– Best triple junction cells now give 42.5%, increasing 1%/year
• Least expensive cells @ 1000x concentration
Cost $0.15/watt
• Cells already in commercial production
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CPV and CSP comparison for daytime generation
• Both use optical concentration to address basic problem - sunlight energy is dilute - expensive to convert
• Both require tracking• CSP needs large engines for efficient conversion -
premium on bringing large power to a single focus, from a collecting area of 10 m2 to 10,000 m2
– Leads to higher costs/m2 for dish collector or heliostat fields with reduced collection efficiency
• CPV allows huge flexibility in concentrating geometry and higher efficiency – Collecting area being explored in current commercial
implementations varies from 1 mm to 10 m diameter i.e. 10-3 m2 – 100 m2 in area and energy collected
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CPV has enormous promise
• Most energy per unit power– 2300 kWh/kW/year from 2-d tracking in SW
• Longest hours of direct production - throughout the day• Least environmental impact
– small area (4 acres/MW), no blading of land, no water consumed
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But CPV volume currently < 1% of PV• Cost for balance of system (BOS) >10x cost
of cells (optical, mechanical, thermal, tracking)
• System architecture development neglected – R&D has strongly favored cell development, not
complete installed system
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4. Arizona’s disruptive concept for large scale, low-cost CPV
• Disruptive approach that does not exist in today’s energy market
• Complete rethinking of opto-mechanical system for lowest cost concentration in large scale mass production
• Uses fact that in HCPV the collectors are inherently very much larger than the small cell converters
• System structured to separate large and small, for mass production by proven, size-appropriate, high-volume methods
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Solar reflector heritage from CSP• Large back-silvered
primary trough reflectors validated by 20 years of CSP experience– High specular reflectivity
maintained over 20 years– Damage rate
• 0.3%/year (untempered)• 0.01%/year (tempered)
• Float glass inexpensive, high volume cost projected to be $0.05/W
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UA design uses back-silvered paraboloidal glass reflectors in spaceframe module
Aimed at lowest mass and cost/m2 to survive in 80 mph wind2-axis tracker has eight 3 m dishes each focusing 9 kW sunlight Steel mass including foundation is 100 kg per kW of output
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Early tests of dish manufacture -segmented 3 m reflector prototype
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15 sec exposure at focus
– melts a quarter-sized hole in ¼” thick steel – don’t try this at home!
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Most CPV systems do not use large dish concentration
• Typical systems use many 25 cm concentrator optics with individual 1 cm small cells– Ensures equal power per cell, as needed for efficient
series chain – Concentrator/cell units are packaged into modules
with aluminum heat sinks behind each cell for passive cooling
• Disadvantage:– Modules emulate flat panels, but are more complex
and must be tracked
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Unique receiver optics take in strongly focused sunlight energy and apportion it equally to cells
UA solution allows use of large dish collectors
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the ball lens stabilizes against tracking error
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Summary –Arizona separation architecture
Concentration by 3 m square glass dishes, mass-produced at float glass factory @ $0.05/W
• Cells are packaged in compact receiver at 9kW (sunlight) focus
• Unique receiver optics ensure uniform high concentration illumination (1000x) over 36 cells
• Active cooling, using automobile and CPU technology, gives low - 1% - parasitic loss
• Module has multiple reflectors and receivers in balanced lightweight spaceframe, completely integrated as the elevation structure of alt-az tracker
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5. Field demonstration
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Current state of construction of 20 kW prototype at the University of Arizona. The full scale mechanical tracker weighs 2 tons including foundation, and tracks 99% of the time within 0.1° accuracy
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Prototype with ball-lens receiver and radiator at the focus of a partial (4-segment) reflector
The ball lens images the segments onto a partially populated receiver array with 4 pairs of optical funnels and cells
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DOE Undersecretary Zoi inspects one of the 4 reflector segments
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Eight triple junction PV cells used at the focus @ 1000x concentration
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Eight cells and funnels mounted on a cooled, faceted cup (ball lens removed)
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I-V curve showing a maximum power point of 511 W
The 8 cells are connected in series in the receiver
On-sun data from the 8-cell receiver
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Off-axis response measured for the 8-cell receiver is very broad, given the 1200x geometric concentration
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Consistent power > 500W over 100 hours of sun-tracking
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Tracking advantage: > 80% of max power for 8 hours, 7 weeks before the winter solstice. More kWh per kW
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Summary of current status• Dish shaping technology proven in back-silvered segments
• Prototype receiver with eight 15 mm triple junction cells at 1200x (geometric). First test 9/2010 with partial segmented reflector gave– 500+ watts (25A, 20V)– Cell temperature 20C above ambient– 25% end-to-end sun to DC efficiency (with 2 year old cells). – 30% efficiency projected for current cells and better coated optics.
• Spaceframe tracker for 8 reflectors (20 kW) shows– excellent pointing stability (99% < 0.1°) – very low mass. Total steel mass including foundation measured at 2
tons, i.e. 100 kg/kW.
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6. Next steps and commercialization
• joint development by the University of Arizona and REhnu LLC
• next 12 months– June 2011- implement and test on-sun full 3
m dish with full 36 cell 2.5 kW receiver– Jan 2012 – populate existing tracker with 8
dishes and receivers to demonstrate full module operating at 20 kW
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3.1 m square mold (right) to shape the flat float glass sheets (back) in the furnace (left)
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Initial fabrication test at the Mirror Lab of a 3.1 m square glass dish, made from a single sheet of glass
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Evolutionary path to 1 dish/minute for 1 GW/year
• Technology evolution from– Dish construction: segmented → monolith
– Furnace heat transfer: convective → radiative
– Silvering: chemical → sputtering
– shaping and coating: batch processing → in-line
• Next year build an in-line sputter coating plant and a shaping furnace, both rated for 2 – 8 MW/year
• Later our deep dish shaping technology will be combined with existing high volume trough reflector technology
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20 kW modules will be assembled on-site from separate shipments of dishes, steel struts, receivers etc. and transported out for mechanized installation
Assembly facility for generator
units
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Steel components
$0.10
Reflectors $0.04
Cells $0.16
Silica balls $0.04
Remaining receiver $0.09
Cooling & wiring $0.10
Inverter & controls $0.15
Assembly &installation
$0.12
Margin $0.20
Bottom up cost estimate for production at GW scale: $0.80/watt installed, leaving $0.20 margin
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• If the 20 kW, 3200 kg module units were built at the same cost per kg as a pickup truck ($10/kg), the cost of power would be $1.60/watt.
• Spaceframe modules are structurally much simpler than pickups, so $1/watt is credible.
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• Startup formed in 2009• Holds exclusive license
to UA CPV technology• Will build ten 20 kW
modules next year followed by 100 module (2 MW) farm in 2013
• Website: rehnu.com
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Commercialization path• Large commercial impact
– Potential for lower energy cost in daytime at utility scale than CSP and flat PV panels
• REhnu LLC startup formed specifically to develop the technology – Exclusive license for commercialization of University of Arizona
technology– REhnu’s goal >1 GW/year @ <$1/watt installed by 2018– Spaceframe and receiver risks mostly retired
• Key R&D to ensure rapid investment and commercialization– Demonstrate clear technology and manufacturing path to GW scale
• Key challenge– Quickly prove reliability to attract major investment