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Photovoltaics Outlook for Minnesota
Steve Campbell [email protected]
University of Minnesota
Department of Electrical and Computer Engineering
Saving dollars, not polar bears
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Outline
• Why solar?
• Solar technologies and how they work
• Utility versus distributed generation
• On the horizon
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Types of Solar Power
Photovoltaics Concentrated Solar Power
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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.
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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
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Cost of Electricity in the US
• Grid Parity depends of location
– Hawaii
– West Texas
– Parts of CA depending on usage
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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
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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
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• 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
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Why US Solar?
http://www.nrel.gov/gis/solar.html
Germany
~8 kWh/m2/Day
~3 kWh/m2/Day
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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
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PV Technologies
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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
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Creating a Field: Doping
+
+
-
-
ƐBI
pn Junction
n-type
p-type
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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
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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
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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
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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
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• 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
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PV Technologies
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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
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A Partial Solution: mc-Si
Reduced performance but reduced cost
Still hard to get to really low cost
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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
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Amorphous Silicon
• Unlike c-Si, a-Si has a large a
• Stability issues prevent high efficiency
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CdTe
• Leading thin film in manufacture (First Solar)
• Cd has environmental concerns – some countries restrict it use
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CIGS
• More complex material, making it more difficult to manufacture
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Fraunhofer ISE, Report July 2014, page 18
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Thin Film PV
a-Si
CdTe
CIGS
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Leading Manufacturers
c-Si mc-Si Thin Film
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Utility Versus Distributed Solar
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70 MW Rovigo Solar Plant in NE Italy
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Agua Caliente
Between Yuma and Phoenix 5.2 million modules Currently rated for 290 MW CdTe thin film (First Solar)
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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/
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PV Installations 2010-2014
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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
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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
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The Case Against Central PV: Distribution
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United States Power Grid
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National Stadium (Kaohsiung), Taiwan
Completed in 2009 capacity of 55,000
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BMW Building
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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
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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?
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PV Outlook: Mostly sunny with scattered clouds
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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
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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.
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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
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Cloud Two: Physical Limits
• Ultimately we are limited by Shockley-Queisser
CdTe
How to get low cost multi junction cells?