Systems and SunPower
ELEG 620
Electrical and Computer Engineering
University of Delaware
April 22, 2010
ELEG 620 Solar Electric Power Systems
April 22, 2010
ELEG 620 April 22
1. Richard Corkish, UNSW, April 23, 3 pm, 103 Gore
2. Michael Mackay, What Does Solar Energy Mean?
006 Kirkbride, April 29, 4 pm
3. System design
4. Design, construction and test of a solar power
system
5. SunPower and other solar cells
ELEG 620 Solar Electric Power Systems
April 22, 2010
ENERGY INSTITUTE
Seminar Series
April 23, 2010
3:00 – 4:00 pm
103 Gore Hall
Reception to follow
P. S. DuPont Hall Lobby
“Photovoltaic and Renewable Energy Engineering
Education and Research Programs at UNSW”
Professor Richard Corkish
Head of School
The School of Photovoltaic and Renewable Energy Engineering
University of New South Wales (UNSW), Australia
The University of New South Wales is the leading solar electricity research and education
university. The most recent class admitted 200 students. UNSW has carried out silicon
solar cell device research since the mid 1970s. The UNSW solar cell research group has
led international commercialization and, since 2000, pioneered specialized undergraduate
education in photovoltaics engineering. That research and teaching is now included in the
School
of
Photovoltaic
and
Renewable
Energy
Engineering.
UNSW
is
a
leading
engineering school and the leading academic Solar Power program in the world.
They
are also a partner on the program and the new UD Solar Electricity, NSF-IGERT proposal.
The School’s photovoltaics devices research currently has four main strands. Firstly, it
continues to improve the commercially dominant technology of silicon wafer solar cells.
For example, strong advances are being made in novel front contact technologies in
collaborative work with Asian companies. In the second research strand the School is
able to produce thin film material of similar or slightly better quality than the market
leader. Third generation solar cells research uses advanced physics to investigate
structures, including some based on silicon quantum dots, to try to affordably exceed the
fundamental limits that apply to any of the above methods. Fourthly, work at UNSW on
the emission of (infrared) light from silicon some years ago led to the development of
photoluminescence
as
a
contactless
characterization
method.
New
tools
for
the
laboratory and production line are in use or under development. Aside from photovoltaics
devices, the School also carries out research into building integrated photovoltaics, solar
energy systems, energy efficiency of water pumping, combustion modeling, energy
policy and solar crop drying.
Richard Corkish is the Head of School at the School of Photovoltaic and Renewable
Energy Engineering, UNSW.
He graduated with distinction as a Communications
Engineer from the Royal Melbourne Institute of Technology in 1986 then worked with the
CSIRO Division of Radiophysics on satellite earth-station antenna design and testing
before studying for the PhD degree under the supervision of Professor Martin Green at
the University of New South Wales’ Centre for Photovoltaic and Renewable Energy
Engineering.
Co-Sponsored by:
ELEG 620 Solar Electric Power Systems
April 22, 2010
ELEG 620 Outcomes
1. Understanding the nature of Solar Radiation
2. Design of a solar cell from first principles
3. Design of a top contact system
4. Design, construction and test of a solar
power system
ELEG 620 Solar Electric Power Systems
April 22, 2010
ELEG 620 Solar Electric Power Systems
April 22, 2010
Typical Solar System
DC
Output
Photons
In
+
DC
AC
+
AC
Output
Storage
Battery



Highest reliability premium power solution
Unlimited backup time
No fuel, no maintenance
ELEG 620 Solar Electric Power Systems
April 22, 2010
Village System
ELEG 620 Solar Electric Power Systems
April 22, 2010
Pick Your Load (1-2 pages)
1.
2.
3.
4.
5.
6.
Pick a load. Available PV Power is 50W-800W (non full
time graduate students can go as low as as 1W)
Identify what you will measure, starting with the oad.
Identify time intervals over which you will measure – i.e: #
of days
Draw a diagram to show the energy flow and the
components in the system for your specific load.
List the input, the output and the methods for your design
part. (What information do you need, what information do
you want, and how are you going to relate the two?)
List the methods and the tools you will use for your system
test. (How to test whether the system is working as
expected? How to identify the problems if it’s not?)
ELEG 620 Solar Electric Power Systems
April 22, 2010
Photovoltaic Systems
• System Design:
– To make a successful system need to:
• Well-designed system.
• Reliable, appropriate and well-matched components.
• Suitable maintenance regimes.
• Conforming to legal, social, etc expectations, including
relevant standards
– Ensure that expectations and maintenance is realistic
through education.
– Well designed system:
• Appropriate choice of basic system topologies.
• Choice of array size, tilt angle, battery size and other
components to give “best” performance
ELEG 620 Solar Electric Power Systems
April 22, 2010
Photovoltaic Systems
• Parameters to judge system performance
– Availability: fraction of time that energy is available compared to
time load is required.
– Utilization of incident solar energy:
• Solar fraction: fraction of available solar energy which is
utilized by the system.
• Array-to-load ratio: Has units of Wp /Wh per day.
– If the (Wh per day) is from the load, then this is the hybrid
indicator.
– If the (Wh per day) is the net available to the load, it is a
measure of the system location.
ELEG 620 Solar Electric Power Systems
April 22, 2010
Photovoltaic Systems
•
Types of Systems
–
Direct-coupled PV system
–
DC PV system with storage
–
DC-AC PV systems
–
Hybrid PV systems
–
Grid-connected PV systems
•
Increasing components in a system
decreases reliability, decreases
efficiency
•
Adding additional power sources
increases the availability (usually) and
increases fraction of solar power used.
•
Increasing components increases cost
of systems, but not for same
availability.
ELEG 620 Solar Electric Power Systems
April 22, 2010
Photovoltaic Systems
• Impact of variability in solar resource
– A key element in renewable energy systems is the design of one
component that has inherent variation (the solar resource) to drive
another component (the load) in which the variation should be
minimized as much as possible.
– The larger the variation in the resource compared to the load, the
more difficult the trade-offs.
• Some loads have a match to solar resources, but often higher
loads are encountered in months with lower solar insolation.
– Large variation in solar radiation means that in order to get higher
availability, the system has:
• a lower solar fraction
• a substantial storage component.
• Higher cost.
ELEG 620 Solar Electric Power Systems
April 22, 2010
Photovoltaic Systems
• System Design:
– Goal is to produce a system within specified cost and power specifications that
has the highest availability and reliability
– In addition to availability, cost and reliability, the fraction of solar used and the
system losses are used to guide to the design process.
– Key issues and trade-offs
• Theoretically, power from array over year = load over year +
losses
• Needed availability = battery capacity to power load during
periods without solar irradiance
• Not valid because storage can’t be large enough, so need to
over design in one portion of year
• No way around problem of unused capacity, but can introduce a
secondary system that is either more predictable, lower cost or
well-matched
to complement solar resource.
ELEG 620 Solar Electric Power Systems April 22, 2010
Photovoltaic Systems
• Types of design procedures:
– Several different types of design procedures, depending on
availability of radiation data and time period over which
calculations are performed.
1. Determine feasibility/select system topology: rough calculations,
no specific location dependant parameters, and usually no
comparison, iteration or checking
2. Indicative analysis: look at key trade-offs (tilt, battery size, array
size) determine suitability system topology: location and load
dependant parameters, usually averaged over a month.
Different methods have different methods for choosing battery
storage.
ELEG 620 Solar Electric Power Systems
April 22, 2010
Screen-Printed Silicon Solar
Cell
 This device structure is used by most manufacturers today
•
The front contact is usually formed by POCl3 diffusion
•
The rear contact is formed by firing screen-printed Al to form a back-surface field
 The cell efficiencies for screen-printed multicrystalline silicon cells are
typically in the range of 14 – 16%
ELEG 620 Solar Electric Power Systems
April 22, 2010
Fabrication Process of
Screen Printing Silicon Solar Cells
Texturing
POCl3 Diffusion
PECVD SiNx AR
P-Si
Al Screen-printing
Ag Screen-printing
Belt Co-firing
Senergen Devices
February 26, 2009
ELEG 620 Solar Electric Power Systems
April 22, 2010
Issues for High Efficiency SP Solar
Cells
• Screen printed front contact
- Broad and low conductivity Ag
- High contact resistance
• Emitter diffusion
- High Joe and low Jsc due to high surface
concentration for low contact resistance
• Bulk : Conventional Multi-Si and CZ
- Low lifetime
• Screen printed Al rear contact
- High surface recombination velocity
- Low reflectivity
• New Structure (Back Contact Cell)
Senergen Devices
February 26, 2009
ELEG 620 Solar Electric Power Systems
April 22, 2010
High-Efficiency Cell Designs
High-sheet-resistance emitter cell
Ag
gridlines
Si heterojunction cell
(in collaboration with NREL)
Grid
n+ emitter
~100  /
ITO
n-type a-Si
intrinsic a-Si
p-type FZ Si
p-Si
Al-BSF
Al-BSF
Al rear contact
Al Contact
Gridded back contact cell
Ag gridlines
n+ emitter
p-Si
Interdigitated back contact cell
SiN/SiO2
SiN/SiO2
p-Si
p+
SiN/SiO2
Al/Ag rear
SiN/SiO2
contact
ELEG 620 Solar
Electric Power Systems April 22, 2010
n+
rear contacts
BP Solar Saturn Solar Cell
 The BP Solar Saturn
solar cell utilizes a
laser-grooved, buried
front contact
 The aluminum back
contact is heated to
form a back surface
field, which reduces
surface recombination
 Best lab efficiency =
20.1%
ELEG 620 Solar Electric Power Systems
April 22, 2010
Localized Emitter Cell Using
Semiconducting Fingers
 This type of cell was developed at the University of New South Wales
 Suntech may start production in the near future
ELEG 620 Solar Electric Power Systems
April 22, 2010
Sanyo HIT Solar Cell
Screen-printed Ag
collection grids
Textured n-type CZ Si
(200 m thick)
 The HIT cell utilizes amorphous Si intrinsic layers (~ 5 nm) as super-passivation
layers. The cell is symmetric except for the a-Si p+ emitter layer (~ 10 nm) on
the front and the a-Si n+ contact layer (~ 15 nm) on the rear. The transparent
electrodes are sputter-deposited indium-tin-oxide (ITO)
 Best lab efficiency = 22% (open-circuit voltages ~ 730 mV)
ELEG 620 Solar Electric Power Systems
April 22, 2010
SunPower Back Contact Solar Cell
 The SunPower cell has all its electrical contacts on the rear surface of the cell
 The diffusion lengths > twice the cell thickness
 Best efficiencies ~ 23% (SunPower is now using CZ-Si)
ELEG 620 Solar Electric Power Systems
April 22, 2010
Advent Solar Emitter-Wrap-Through Cell
 Advent Solar started selling EWT cells in the first quarter of 2007
 They need to laser drill ~ 45,000 holes per wafer
 They claim solar cell efficiencies of ~ 15%
ELEG 620 Solar Electric Power Systems
April 22, 2010
Metal-Wrap-Through Solar Cell
 Photovoltech is commercializing the MWT solar cell; efficiencies ~ 15%
ELEG 620 Solar Electric Power Systems
April 22, 2010
The CSG Solar Cell
 CSG Solar (Germany) is using laser patterning of thin polycrystalline silicon
to construct a metal-wrap-through type of back-contact cell.
 Their best cell
efficiencies are ~ 10%.
ELEG 620 Solar Electric Power Systems
April 22, 2010
The Sliver® Solar Cell
 Origin Energy (Australia) is commercializing the Sliver® Solar Cell
 They have demonstrated cell efficiencies > 20%
Solar Cell Technologies
•
Highest efficiencies are reached
by making tandem solar cells,
which consist of multiple solar cells
stacked on top of one another.
•
Each solar cell absorbs light with
energy close to its band gap,
allowing overall higher efficiency.
•
Maximum thermodynamic
efficiency is 86.8%, but material
limitations give maximum
efficiencies of just over 30%.
•
Used primarily in space markets
From Compound Semiconductor
ELEG 620 Solar Electric
Power
Systems February
April 26, 2009
Senergen
Devices
Physics of Solar
Cells
28
ELEG 620 Solar Electric Power Systems
April 22, 2010
Solar Cell Operation
Phosphorus doped
n-type silicon
Anti-reflection
coating
Top metal contact grid
Boron-doped,
p-type silicon
Bottom metal contact
Cell Cross-Section
29
ELEG 620 Solar Electric Power Systems
April 22, 2010
Solar Cell Operation (cont.)
1. Photon of sunlight
knocks electron loose
Top metal contact
N-type silicon
Attracts electrons
P-type silicon
attracts holes
Bottom metal contact
2. Free electron
goes to top metal
contact
3. Hole (broken bond) left behind
goes to bottom metal contact
30
ELEG 620 Solar Electric Power Systems
April 22, 2010
Conventional Solar Cell Loss
Mechanisms
Reflection Loss
I2R Loss
1.8%
0.4%
0.4%
0.3%
1.54%
3.8%
Recombination
Losses
2.0%
1.4% Back Light
Absorption
Limit Cell Efficiency
Total Losses
Generic Cell Efficiency
2.6%
29.0%
-14.3%
14.7%
31
ELEG 620 Solar Electric Power Systems
April 22, 2010
High-Efficiency Back-Contact Loss
Mechanisms
0.5%
0.8%
1.0%
0.2%
0.3%
0.2%
0.2%
1.0%
Limit Cell Efficiency
29.0%
Total Losses
-4.4%
Enabled Cell Efficiency
24.6%
I2R Loss
0.1%
32
ELEG 620 Solar Electric Power Systems
April 22, 2010
Pick Your Load (1-2 pages)
1.
2.
3.
4.
5.
6.
Pick a load. Available PV Power is 50W-800W (non full
time graduate students can go as low as as 1W)
Identify what you will measure, starting with the oad.
Identify time intervals over which you will measure – i.e: #
of days
Draw a diagram to show the energy flow and the
components in the system for your specific load.
List the input, the output and the methods for your design
part. (What information do you need, what information do
you want, and how are you going to relate the two?)
List the methods and the tools you will use for your system
test. (How to test whether the system is working as
expected? How to identify the problems if it’s not?)
ELEG 620 Solar Electric Power Systems
April 22, 2010