Concentrating PV at $1/watt. Field tests
of a disruptive approach to reduce cost
Roger Angel
Steward Observatory University of Arizona
outline
1. Heritage - making astronomical telescopes at the
University of Arizona
2. 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 CPV
5. Field demonstration
6. Next steps and commercialization
1. Heritage
making astronomical telescopes at the
University of Arizona
Spin-casting liquid glass to make an 8.4 m diameter glass
telescope mirror at the University of Arizona Mirror Lab
Polishing an 8.4 m diameter mirror at the Lab
Inspection during stressed-lap polishing. Honeycomb cells visible beneath the surface.
Two of the 8.4m diameter mirrors on a tracking mount
make the world’s largest single astronomical telescope
The future:
25 m paraboloidal reflector
made from seven 8.4 m
segments for the Giant Magellan
Telescope
3 m square paraboloidal
reflectors to concentrate
sunlight on photovoltaic cells
RRep Gabrielle Giffords with UA President Robert Shelton at experimental
3-m solar dish made at the Mirror Lab from back-silvered glass segments
2.
Solar as a renewable electricity source,
cost competitive with fossil fuels
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
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
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
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
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)
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
more than enough desert sunshine to power the world
NREL map of
solar resource at
direct incidence
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)
Solar dish powers a printing press in late 1880s
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
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
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
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
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
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
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, sizeappropriate, high-volume methods
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
UA design uses back-silvered paraboloidal
glass reflectors in spaceframe module
Aimed at lowest mass and cost/m2 to survive in 80 mph wind
2-axis tracker has eight 3 m dishes each focusing 9 kW sunlight
Steel mass including foundation is 100 kg per kW of output
Early tests of dish manufacture segmented 3 m reflector prototype
15 sec exposure at focus
– melts a quarter-sized hole in ¼” thick steel
– don’t try this at home!
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
UA solution allows use of large dish collectors
Unique receiver optics take in strongly focused
sunlight energy and apportion it equally to cells
the ball lens stabilizes against tracking error
Summary –
Arizona separation architecture
Concentration by 3 m square glass dishes, massproduced 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
5.
Field demonstration
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
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
DOE Undersecretary Zoi inspects one of the 4 reflector segments
Eight triple junction PV cells used at the focus
@ 1000x concentration
Eight cells and funnels mounted on a cooled, faceted cup
(ball lens removed)
On-sun
data from
the 8-cell
receiver
I-V curve showing a
maximum power point of
511 W
The 8 cells are connected
in series in the receiver
Off-axis response measured for the 8-cell receiver is
very broad, given the 1200x geometric concentration
Consistent power > 500W over 100 hours of sun-tracking
Tracking advantage: > 80% of max power for 8 hours,
7 weeks before the winter solstice. More kWh per kW
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.
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
3.1 m square mold (right) to shape the flat float glass sheets
(back) in the furnace (left)
Initial fabrication test at the Mirror Lab of a 3.1 m
square glass dish, made from a single sheet of glass
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
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
Bottom up cost estimate for production at GW scale:
$0.80/watt installed, leaving $0.20 margin
Steel
components
$0.10
Margin $0.20
Reflectors
$0.04
Cells $0.16
Assembly &
installation
$0.12
Silica balls
$0.04
Remaining
receiver $0.09
Inverter &
controls $0.15
Cooling &
wiring $0.10
• 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.
• 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
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