Photovoltaics Outlook for Minnesota

Steve Campbell scampbell@umn.edu

University of Minnesota

Department of Electrical and Computer Engineering

Saving dollars, not polar bears

Outline

• Why solar?

• Solar technologies and how they work

• Utility versus distributed generation

• On the horizon

Types of Solar Power

Photovoltaics Concentrated Solar Power

The Lede

We are rapidly approaching an era when the choice to install solar energy will be primarily driven by cost, even without subsidies. One can expect to see significant distributed and utility-scale deployment over the next decade. Large-scale energy storage is an unsolved problem.

There’s Power and There’s Power

• A solar installation is rated in the power it would produce in watts at a standard level of illumination: AM1.5.

• To compare different technologies, one uses the levelized cost of energy (LCOE) must take into account many factors

– Availability

– Operating costs

– Depreciation

Cost of Electricity in the US

• Grid Parity depends of location

– Hawaii

– West Texas

– Parts of CA depending on usage

Trends for PV Modules

• Price drops as efficiency and manufacturing improve

• They will be free in three more years

Jelle et al. Solar Energy Mater. Sol. Cells 100, 69-96 (2012).

04/03/13 Solar Cell Module Spot prices High ($/Wp) Low($/Wp)

Si Module 0.99 0.55

Thin Film Module 0.94 0.52

The Result of Falling Costs

At the current rate, we will have 1 TW of capacity in 10 years and ~4 TW in 20 years. The later would be about

15% of the total energy supply.

PV production doubles every ~ 2.5 years

• Solar Irradiance

• Installed cost

– Solar modules and balance of system

– Current BOS is about 65% for utility-scale

– Module cost and most of the BOS cost scale with module efficiency

To Reach Grid Parity at AM1.5

www.pveducation.org

Why US Solar?

http://www.nrel.gov/gis/solar.html

Germany

~8 kWh/m2/Day

~3 kWh/m2/Day

Why Minnesota Solar?

MN electric power use ~ 8 GW

In spite of the latitude, Minnesota is sunny: SW Minnesota

receives 80 to 90% of the irradiance of Arizona

12 miles x 12 miles

140 W/m2

170 W/m2

*144 sq. miles

(< 2% of all lakes)

could power the state

5.8 kW-hr/m2-day

7.1 kW-hr/m2-day

PV Technologies

Semiconductors

• Pure semiconductor is semi-insulating

• Light can free an electron

– Creates two carriers

– Energy that depends on the semiconductor (Si: 1.1 eV, in the near IR)

• Electric field separates the charge for collection

Ɛ

Creating a Field: Doping

+

+

-

-

ƐBI

pn Junction

n-type

p-type

p-n Junction Diode

I

VA

p n

o

o

o

o e

e

e

e

e

e

o

o

o

+ - VA > 0

e

e

e

e

e

e

e

e

e

oe

oe

Voc

.

. Isc

• PV current is in the opposite direction of the forward-bias dark current

• If e-h pairs are generated in the depletion region collection efficiency is high

ƐBI

Solar Cell Figures of Merit

Current, I (mA/cm2)

Voltage, V (Volts)

Isc ~ 20-40 mA/cm2

Voc ~ 0.4-0.8 Volts

Voc

Isc

Solar Radiation Variations

Space

Universe AM0

AM1

AM1.5

q

Global = Direct + Diffuse Direct light

Scattered

diffuse light

1msin

q

m 1.5 for q 41.8o

1324 - 1417 W/m2

~1000 W/m2

m

m amoI Ie

~1125 W/m2

Im = Iamo e-at

a is the inverse absorption length

Air Mass 1.5 (AM1.5) Solar Spectrum

0

0.5

1

1.5

0 500 1000 1500 2000

Wavelength, l (nm)

Irra

dia

nce (

W m

-2 n

m-1

) 2

solar

0

I ()d1000W/m

ll

where Isolar(l) is the solar irradiance in W m-2 nm-1

• If 1/a is of order the depletion width (~1 mm), device behaves as described

• If 1/a is >> than the depletion depth, we rely on diffusion of the generated holes and electrons

• In that case the material has to be very good (i.e. single crystal). This is the case with Silicon

Light Absorption and Efficiency

p-type

n-type

PV Technologies

Crystalline Silicon PV

1/a ~ 70 mm Performance depends strongly on crystal quality and purity

Cutting Si boules and polishing the wafers leads to a loss of 50 to 70% of the material

A Partial Solution: mc-Si

Reduced performance but reduced cost

Still hard to get to really low cost

Thin Film Solar

• Typical stack involves an absorber layer that is vacuum deposited or formed by reaction with a vapor

• These absorber have a small 1/a and so are 1 to 2 mm thick

Substrate

Back Contact

P-type Absorber

n-type Buffer

n-type Window

Transparent Conductor

~1 mm

Amorphous Silicon

• Unlike c-Si, a-Si has a large a

• Stability issues prevent high efficiency

CdTe

• Leading thin film in manufacture (First Solar)

• Cd has environmental concerns – some countries restrict it use

CIGS

• More complex material, making it more difficult to manufacture

Fraunhofer ISE, Report July 2014, page 18

Thin Film PV

a-Si

CdTe

CIGS

Leading Manufacturers

c-Si mc-Si Thin Film

Utility Versus Distributed Solar

70 MW Rovigo Solar Plant in NE Italy

Agua Caliente

Between Yuma and Phoenix 5.2 million modules Currently rated for 290 MW CdTe thin film (First Solar)

Utility Scale (>100 MW) PV Facilities

• Agua Caliente Solar Project, (Arizona, 250 MW - to increase to 397 MW)

• California Valley Solar Ranch (250 MW)

• Golmud Solar Park (China, 200 MW)

• Welspun Energy Neemuch Project (India, 150 MW)

• Mesquite Solar project (Arizona, 150 MW)

• Neuhardenberg Solar Park (Germany, 145 MW)

• Templin Solar Park (Germany, 128 MW)

• Toul-Rosières Solar Park (France, 115 MW)

• Perovo Solar Park (Ukraine, 100 MW)

https://openpv.nrel.gov/

PV Installations 2010-2014

Utility Scale Problems

• Intermittancy of the source

– Variable source / variable load

– Short range planning: Microcasting

– Low cost energy storage

• Financing cost

• Long haul distribution

• Materials

Utility Scale Storage

• How to store the capacity of Agua Caliente operating for 24 hours at AM1.5?

• Energy ~ 2.5x1016 Joules or 25 Petajoules • State of the art energy storage options:

• For Pb-acid this is a 10 story building 3 to 5 km on a side

Technology Energy Density (MJ/kG) Requirement (kG/tons)

Lithium-Ion 0.875 26 B / 30 M

Alkaline Battery 0.67 38 B / 43 M

Lead-acid Battery 0.17 150 B / 160 M

Supercapacitor 0.018 1400 B / 1540 M

The Case Against Central PV: Distribution

United States Power Grid

National Stadium (Kaohsiung), Taiwan

Completed in 2009 capacity of 55,000

BMW Building

Costco, Richmond CA

PG&E Com Rates 1) 0.15 $/kW-hr 2) 0.18 $/kW-hr 3) 0.26 $/kW-hr 4) 0.32 $/kW-hr

Grid parity is much easier to achieve for Tier 3 or 4 usage

Note that the installation shown at right is soft (conformal and lay on top of existing structure. They do not require extensive installation infrastructure.

Distributed Generation and BIPV

• The patchwork of licensing requirements drives up BOS ‒ Permit to plug-in ~6 months

• Unresolved question – who pays for local transmission infrastructure?

PV Outlook: Mostly sunny with scattered clouds

Summary So Far

• PV module cost has dropped dramatically. This has been difficult for manufacturers, but great for users

– No end in sight to this long-term trend

– BOS cost reduction is lagging; needs to be solved

• As a result, supplementary power applications are growing rapidly

• Minnesota is quite viable for utility-scale solar, especially in the southwest

Cloud One: Materials

• It is hard to imagine a way to scale Si to and below $0.50/watt installed. Will TF mfg survive until then?

• Cells with Cd cannot be deployed in some parts of the world. Opening for CIGS?

• Material cost and availability for TW PV

1 TW of CIGS requires 55 years of Indium production, but In is heavily used in touch screens, flat panels, etc.

Is There a Limit to Efficiency?

0

0.5

1

1.5

0 500 1000 1500 2000

Wavelength, l (nm)

Irra

dia

nce (

W m

-2 n

m-1

)

Long wavelength light can’t produce electron /hole pairs – no absorption

Photons have just enough energy to remove electrons

Photon energy above the bandgap can’t be absorbed

Ephoton = 1210 eV-nm/l Ideally VOC = EG-0.5 eV

Cloud Two: Physical Limits

• Ultimately we are limited by Shockley-Queisser

CdTe

How to get low cost multi junction cells?