Photovoltaics Outlook for Minnesota

Saving dollars, not polar bears

University of Minnesota Department of Electrical and Computer Engineering 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

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

Jelle et al. Solar Energy Mater. Sol. Cells 100, 69-96 (2012). The Result of Falling Costs

PV production doubles every ~ 2.5 years

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. To Reach Grid Parity at AM1.5

• 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

www.pveducation.org

Why US Solar?

~3 kWh/m2/Day

~8 kWh/m2/Day Germany

http://www.nrel.gov/gis/solar.html Why Minnesota Solar? 1405.8 kW W/m-hr/m22 -day

12 miles x 12 miles

*144 sq. miles (< 2% of all lakes) could power the state

2 7.1170 kW -W/mhr/m2-day 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 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

pn Junction

p-type

+ -

+ - n-type

ƐBI p-n Junction Diode

oe o p e n e o e oe o e e o o e e e e o o e e V e e . A e e ƐBI Isc . Voc + - VA > 0 • 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 Solar Cell Figures of Merit

Current, I (mA/cm2)

Voc Voltage, V (Volts)

Isc

2 Isc ~ 20-40 mA/cm

Voc ~ 0.4-0.8 Volts Solar Radiation Variations

Space

Universe AM0 1324 - 1417 W/m2

AM1.5 Scattered diffuse light ~1125 W/m2 AM1 q ~1000 W/m2

Direct light Global = Direct + Diffuse

m 1 II = I Ie e-at m m  1.5 for q  41.8o mm amo amo sinq

a is the inverse absorption length Air Mass 1.5 (AM1.5) Solar Spectrum



) 1.5 2 -1 I ()d1000W/mll solar

nm -2 1

0.5 Irradiance (W m Irradiance

0 0 500 1000 1500 2000 Wavelength, l (nm) -2 -1 where Isolar(l) is the solar irradiance in W m nm Light Absorption and Efficiency

• If 1/a is of order the depletion width (~1 mm), device behaves as described n-type • If 1/a is >> than the depletion depth, we rely on diffusion of the generated holes and electrons p-type • In that case the material has to be very good (i.e. single crystal). This is the case with Silicon 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 Transparent Conductor n-type Window formed by reaction with n-type Buffer ~1 mm P-type Absorber a vapor Back Contact • These absorber have a Substrate small 1/a and so are 1 to 2 mm thick 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

CdTe a-Si

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: 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

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

The Case Against Central PV: Distribution United States Power Grid Completed in 2009 capacity of 55,000

National Stadium (Kaohsiung), Taiwan 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 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?

Note that the installation shown at right is soft (conformal and lay on top of existing structure. They do not require extensive installation 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?1 TW of CIGS requires 55 years • Cells with Cd of Indium production, but cannot be In is heavily used deployed in some in touch screens, flat panels, etc. parts of the world. Opening for CIGS? • Material cost and availability for TW PV Is There a Limit to Efficiency?

) 1.5

-1 nm

-2 Photons have 1 just enough energy to remove electrons

Long wavelength light 0.5 can’t produce electron

Irradiance (W m Irradiance Photon energy above /hole pairs – no absorption the bandgap can’t be absorbed 0 0 500 1000 1500 2000 Wavelength, l (nm) Ideally VOC = EG-0.5 eV Ephoton = 1210 eV-nm/l Cloud Two: Physical Limits

• Ultimately we are limited by Shockley-Queisser

CdTe

How to get low cost multi junction cells?