Concentrating PV at $1/watt. Field tests of a disruptive approach to reduce cost

Roger Angel Steward Observatory University of Arizona

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

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

Inspection during stressed-lap polishing. Honeycomb cells visible beneath the surface.

Polishing an 8.4 m diameter mirror at the Lab

Two of the 8.4m diameter mirrors on a tracking mount make the world’s largest single astronomical telescope

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

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, size-appropriate, 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 wind2-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

Unique receiver optics take in strongly focused sunlight energy and apportion it equally to cells

UA solution allows use of large dish collectors

the ball lens stabilizes against tracking error

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

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)

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

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

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

• 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