Wichita State Colloquium
October 8, 2014
The Promise of Solar Energy
Joe O'Gallagher
Adjunct Professor of Physics, Governors State University
University Park, Illinois
and
Lead Scientific Officer, Solargenix Energy
Sanford, North Carolina
Formerly:
Senior Lecturer and Executive Officer
Department of Physics andThe Enrico Fermi Institute
University of Chicago
(now retired)
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Why “Promise” ?

I Have worked in this field for nearly 40 years.

Progress has been somewhat disappointing due in part to
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Poor implementation of early concepts
Lack of understanding by the general public about what can and cannot be
done
Other economic obstacles and market conditions
The vision of a renewable energy driven sustainable energy economy
has not been achieved.
The original “promise” remains unfulfilled, but that theme provides a
context for what I want to talk about today.
There has been much progress.

New technologies and techniques have been developed

Performance is improving and costs are coming down.
It is inevitable that the promise will be fulfilled!
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ACKNOWLEDGMENTS:

I would like to thank my colleague of over 30 years, Professor Roland Winston,
formerly of the University of Chicago and now at the University of California,
Merced. Roland is the inventor and primary developer of most of the concepts
belonging to the new optical subdiscipline now called “nonimaging optics”
which led to the development of so-called “Compound Parabolic
Concentrators” and related devices for solar energy concentration.

The Development of the Compound Parabolic Concentrator and other
nonimaging optical devices at the University of Chicago between 1975 and
2005 was supported largely by: the U. S. Department of Energy through the
Office of Basic Energy Sciences, The Office of Energy Efficiency and
Renewable Energy, the National Renewable Energy Laboratory, Sandia
National Laboratory, and the Jet Propulsion Laboratory.
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Outline of Talk
 General Introduction
 Background and Motivation
 The Solar Resource
 Properties, Problems, and Economics
 Overview of Solar Applications and Collection Strategies
 The Thermodynamic Limit and the Concentration of Sunlight

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The Role of Concentration
Review of fundamental concepts (“The Sine Law”)
Consequences – (Theoretical limits for Solar Concentration)
Introduction to “Nonimaging Optics”
 Examples and Applications (Mostly a Slide Show)
 The “Compound Parabolic Concentrator” (CPC)
 Two-Stage Concentrators for solar-thermal and photovoltaic
generation of electricity
 Ultra-high concentration: Demonstration and exotic applications
 Summary and Conclusions
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I
Background and Motivation
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Motivation
Why Solar Energy?

The world economy and standard of living are strongly
coupled to energy availability.

Solar Energy research is an exciting, interesting,
dynamic, and satisfying endeavor.

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The byproducts of energy production threaten the
quality of life on the planet
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Technically challenging (thermodynamics, optics, semiconductor
physics, materials science, etc. )
Interdisciplinary
Very broad based (involves economics, politics, sociology, etc.)
Atmospheric pollution
Greenhouse gases/global warming
Conventional energy sources are limited and being
consumed at an every increasing rate.
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October 25, 2012
Global Warming:
Fact or Fiction?
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Land-Ocean data through 2012
The World is definitely getting Warmer!!
There’s been
about a 0.8o -0.9o
Celsius (1.4o 1.6o Farenheit)
increase in the
last 130 years.
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GLOBAL CLIMATE CHANGE
The World IS getting warmer -- warmer
than it has been in at least the last 2,000
years
 Mankind’s activities to produce energy are
definitely a major part of the cause!!
 Carbon Dioxide levels in the atmosphere
are higher than they have been in the last
600,000 years.
 Our continuous combustion of fossil fuels
is affecting the health of the planet!!
 IPCC AR5 Synthesis Report (SYR) – Due
out 31 October
2014
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
What about “Peak Oil” ?
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OIL SUPPLIES ARE LIMITED
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Geological Deposits of Fossil Fuels were
produced about 300 million years ago !
There’s only so much that was ever produced.
We are now (or will be soon) reaching the Peak
of world oil production (often referred to as the
“Hubbert Peak” after M. King Hubbert).

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The U. S. peaked in 1970 and has been in a sense
“running out” of oil ever since
The world will peak (begin to “run out”) in the next 5
to10 years, if it hasn’t already
It’s the beginning of the end of abundant energy!
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Production Lags Discovery
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Spaceship
Earth:
The
Only
Planet
we’ve
got!
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What about Alternative Energy
Sources?

Solar Energy  The Source of almost all energy on earth
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Wind Energy
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Is an indirect form of Solar
Is economical today in many locations
Still has aroused some practical concerns
Biomass
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Fossil fuels are stored Solar energy
Capacity dwarfs all the other so-called renewables
Can be thought of as “the mother of all renewables”
Also has considerable promise
Nuclear–
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Not usually thought of in this context
Has problems but probably will have to play a role
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II
The Solar Resource
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The Sun
(X-ray Image in False Color)
-The Source
of (amost) all
energy on
earth
- The driver of
all climate on
earth
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The Promise of Solar Energy

It’s abundant (very)
It’s evenly distributed (sort of)
It’s forever (for all intents and purposes)
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But…
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It’s highly variable in time
It’s very dilute (relatively low intensity spread out
over large areas)
It’s expensive to collect (at least now)
Difficult and expensive to convert to major “end uses”
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The Solar Resource

Very Large Thermonuclear Fusion Reactor

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Surface is almost perfect Black Body Radiator
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T = 6000o K
lmax = 500 nm (5000 Angstroms)
Power Output
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1.4 x 106 km (870,000 miles) in diameter
1.5 x 108 km (93,000,000 miles) away
Subtends a half-angle of about 4.7 milliradians (0.27o)
3.8 x 1026 watts (1.3 x 1027 BTU’s/hr)
13 trillion Quad’s*/hr
Power Intercepted by the Earth


1.7 x 1017 watts (5.7 x 1017 BTU’s/hr)
590 Quads*/hr = ~10,000 total world energy use!
* One Quad = One Quadrillion (1015) BTU’s
The U.S. annual energy consumption is just under 100 quad’s
per year.
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The Solar Resource (Cont’d)

The "Solar Constant"
- Imax = 1370 watts/m2 ( in space near earth)
1000 watts/m2 ( at noon in Albuquerque)
170 watts/m2 ( global yearly average)
- Yearly total solar incident on U. S. land area = 40,000 quads
- 0.5 % of U.S. land area @ 50% efficient = total U.S. use
- Solar Energy is abundant!

Problems

Simple Economics (Conventional Energy sources are still very
inexpensive!)
- Dilute
- Intermittent (it would help to have storage! – “beyond the scope
of this talk”)
- Source is highly collimated and constantly moving
- Predominantly low grade thermal
1 M2 - year of sunlight is worth (depending on local climate and fuel displaced)
~ $20- $200 !! (That’s roughly $2 to $20 per square foot!)
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III
Solar Collection and
Conversion Technologies
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Direct Solar Energy Conversion
(“Active” strategies)
THE
SUN
Photovoltaic
Electricity
(PV)
Heat
Hot water and
space heating
Solar Thermal
Electricity
Cooling
(A/C and
Refrigeration)
Industrial
Process
Heat
Fuels and Chemicals
Production
(Hydrogen!)
Solar
Cooking
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The Direct Conversion of Sunlight
to Electricity:”Photovoltaics” or PV

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One of the Cleanest and neatest forms of
solar energy
Easy to install and use
Probably one of the most expensive forms
as well
Photovoltaic panels are about 12% to
20% efficient and cost about $50/ft2 to
$100/ft2
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Flat Plate Photovoltaic Panel
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Photovoltaic Technologies

Single crystal silicon cells (about 95% of today’s market)
Moderate performance (h ~ 12% - 20%)
 Expensive ($50/ft2 - $100/ft2 => $3/wp - $7/wp)
(The “Peak Wattage” of a system is its power output under an
insolation of 1000 watts/M2.)

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Thin film(e.g. Cadmium Telluride) or amorphous silicon
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lower performance (h ~ 6% - 12%))
Less expensive
Can be deployed as roofing shingles
Multi-junction cells
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High performance (h >~ 40%)
Very expensive (factors of 10 to 100 more than single crystal)
Need concentration to be economical
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Solar Thermal Energy
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Absorb radiant energy as heat and transfer to a
working fluid.
Applications
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Domestic Hot Water
Space heating
Use concentration to get high temperatures and
run an engine to generate electricity!
Solar thermal refrigeration and Air Conditioning
(also requires higher temperatures)
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Solar Hot Water
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Flat Plate Geometry is very simple
and can also collect reflected light
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IV
The Role of Concentration
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Tracking Parabolic Trough
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Part of the 30 MegaWatt Solar Thermal
Electric system in California
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A 30 Megawatt Solar Power Plant in
Southern California
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Large Concentrating Parabolic Dish
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Central Receiver Test Facility
Sandia Albuquerque
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Role of Concentration*
* Definition: Solar concentration is the process of
collecting sunlight (solar energy) from a large area and
delivering it to a smaller area. The “concentration ratio”
is the ratio of the collection area to the target area.
To Improve Performance
- Reducing the relative area of the hot thermal absorber
reduces the heat losses ( ~ 1/C) and allows higher
temperatures to be achieved.
- Increased photon flux on solar cell increases
conversion efficiency slowly
To Reduce Costs
- Reduces the required area of expensive absorber (PV
or Thermal) and replaces it with (presumably) less
expensive optics.
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V
“Nonimaging Optics”
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Background
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Nonimaging Optics
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New approach to the collection, concentration and
transport of light originally developed by Roland
Winston, myself, and our group at the University of
Chicago
Relaxes the constraints of point-to-point mapping of
imaging optics
Achieves or approaches the maximum geometrical
concentration permitted by physical conservation laws
for a given angular field of view.
Focusing optics always fall short of this limit by a factor
of ~ 2 to 4.
The CPC (“Compound Parabolic Concentrator”)
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The prototypical nonimaging “ideal” light collector
invented by Roland Winston
Generic name for whole family of similar devices
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Importance for
Solar Energy Collection

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Achieves widest possible angular field of
view for given geometric concentration
Permits useful concentration without
tracking
1.1 -2x for totally stationary collector
 2x – 10 x with seasonal adjustment
 > 10x – 40,000x with relaxed optics and tracking
requirements

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Concentration and the Thermodynamic Limit
Collecting Aperture, A1
q
Cgeom = A1/A2
Absorbing Aperture, A2
For Cgeom >1 ( i.e. for A2 < A1) the optics must limit the field of view
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Concentration Limit
Cmax
n

sin q
Cmax
n2

sin 2 q
In two dimensional
(trough-like) geometry
In three dimensional
(cone-like) geometry
n is index of refraction at absorber surface,
q is half-angle of acceptance
Any system that can attain these limits is referred to as “ideal”.
All conventional imaging systems fall short of this limit by
factors of at least 2 to 4
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The CPC
BC
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Additional CPC
Designs for
different
absorber shapes
based on “edgeray principle”
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Early Argonne XCPC Design
(Evacuated Dewar-type Absorber tube
with selective surface)
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CPC Solar Geometry

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Achieves widest possible angular
field of view for given geometric
concentration
Permits useful concentration
without tracking
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1.1x -2x for totally stationary
collector
2x – 10 x with seasonal
adjustment
Collects large fraction of diffuse
component of sunlight
Higher Concentration (> 10x –
>40,000x) requires tracking with
multi-stage system but allows
relaxed optics and tracking
tolerances
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latitude
angle
q
q
S
Cmax 
n
sin q
43
VI
Early Applications
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Selected Applications of Nonimaging Optics
in Solar Energy

Nontracking Collectors
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Evacuated CPCs
The Integrated CPC (Evacuated)
Nonevacuated CPCS
High Concentration Tracking Collectors
•Two stage Concentrators
•Solar Thermal Conversion
•Solar Photovoltaic conversion
•Ultra- High Flux Solar Furnaces
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Evacuated CPC Concentrators

Goal

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Combine
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to reduce the heat losses at high operating temperatures (from
the hot absorber to ambient) as much as possible).
Vacuum insulation (eliminates conductive and convective heat
losses)
Spectrally selective absorber surface (suppresses radiation loss)
Nonimaging concentration (reduces surface area of hot
absorber)
Achieves high temperature end uses with a nontracking
collector
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CPC with evacuated Receiver
Energy Design Collectors installed on U of C Physics
building in 1986
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Integrated CPCs (Evacuated)
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Ultra-High Flux Applications
NREL Solar Furnace
(Artists Conception)
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Two-Stage Dish Thermal Concentrators
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Ultra-High Flux

Potential Applications for Ultra High Solar
Flux Concentration
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Production of Exotic Materials (e.g. Fullerenes)
Hydrogen Production (Direct water splitting)
Solar Pumping of Lasers
High Temperature Gas Turbine Solar Receivers
(Weizmann Institute for Science, Rehovath, Israel)
Solar Thermal Propulsion in Space
Solar Thermo - Photovoltaic Converters
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Ultra-High Flux Applications
4/13/2015
Solar Energy
52
Ultra-High Flux Applications
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VII
Recent Developments
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New Design for eXternal
reflector CPC with evacuted tube
(XCPC)
Confidential - Do not Circulate!!
Modified Cusp XCPC-47:58mm dewar absorber
Absorber Radius = 23.5 mm; gap = 5.5 mm: virtual gap = 3.0 mm
Untruncated geometric concentration: 1.565 X; Truncated
Concentration: 1.50X
Aperture Width=221.7mm: Avrg. GapLoss (untruncated)=0.0
210
LeftBranch (untruncated)
Y- Coordinate (millimeters)
190
170
RightBranch (truncated
1.50X)
Absorber
150
Vright
130
Vleft
110
90
70
50
30
10
-135
-115
-95
-75
-55
-35
-15
-10
5
25
45
-30
-50
X-Coordinate (millimeters)
65
85
105
125
Geometry for 1mm thick glass
tube
New XCPC
Profile
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New XCPC Prototype Test Data
Cleveland,T. and M. Ross, “High Temperature Performance Evaluation of the XCPC Concentrating Collector”,
Preliminary Report from the North Carolina Solar Center, August, 2012
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High Concentration
Photovoltaic Applications
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Concentrating PV system
Facetted Dish, C1 = 116X
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Present Limitations of
Concentrating PV

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In a series string of cells, the current is limited to that produced by
the cell with the lowest illumination.
One “dark” cell in such a string effectively “kills” the string output
Thus, for acceptable performance, PV cells wired together in an array
require near uniform illumination on all cells.
One solution is that the entire concentrator can be scaled up and
coupled to a larger array of cells, if optical mixing can be employed to
distribute the flux nearly uniformly over a multi-cell array.
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Square TIR Optical Mixer
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Comparison No Mixer/Mixer
Flux Map
Exit Aperture - Refractive Mixer
Flux Map
Entrance of Optical Mixer
1.00
30.00
24.00
18.00
0.80
24.00-30.00
18.00-24.00
0.60
6.00
0.00
October 8, 2014
6.00-12.00
0.00-6.00
0.60-0.80
0.40-0.60
12.00-18.00
12.00
0.80-1.00
0.40
0.20
0.20-0.40
0.00-0.20
0.00
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Dish with Concentrating Truncated
Pyramidal TIR mixer
sslope = 3 milliradians
C1 = 800X
C2 = 2.5X
C1C2 = 2000X
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High Concentration PV Applications
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Nonimaging concentrator/mixers very effective
in making the irradiance highly uniform
Can boost geometric concentration by factor of
2 to 4.
Symmetry breaking critical to function: (e.g.,
Square cross section mixer, not cylindrical
mixer)
TIR is preferable for high throughput
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Progress in PV technologies
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One Possible Long-term Vision
- The use of ultra-high solar concentration for the production
of hydrogen by water-splitting.
- Hydrogen can be used as a fuel or to produce electricity in
a fuel cell.
- Obviously hydrogen is ultra-clean (by-product is water!)
- Solves the storage problem!
-The concept of doing this with a central receiver plant has
been under study at the Weizmann Institute in Israel for
some time.
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Overview of Problem
The Need for High Concentration

Splitting water requires temperatures in the range 1500K - 2000K

In turn requires a net average concentration of 3000 – 4000 suns

The “ideal” concentration limit (for achievable optical errors) is about
10,000 suns


Conventional single-stage focusing dish systems fall short of this limit
by a factor of 3 – 4, and conventional central receiver systems fall even
farther short of these requirements.
Bottom line: We can’t hope achieve the required concentrations with a
conventional single stage central receiver
The Need for Nonimaging Secondaries

The only option for achieving required fluxes in a central receiver
design is to use some kind of nonimaging secondary at the reactor.

This concept of has been around for some time but has not been
seriously investigated until relatively recently.
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CPC
Surround Field
Circular Area
for heliostats
qc
H
L
A

R
The simplest geometry for a two-stage central receiver
is a central tower (height H) surrounded by a circular
heliostat field. The secondary is a simple CPC with
acceptance angle qc. (Note that qc = the “rim angle” of
the system.) The optimum field is circular with radius R
= H*tanqc = L* sinqc.
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Secondary Concentrator Options
WIS beamdown secondary
October 8, 2014
Source: Timinger, et.al., Solar Energy 69(2),
2000
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Findings

The highest possible concentrations can only be achieved with an
axially symmetric circular field surrounding a central tower with a
CPC looking vertically downward.

80% of the ideal limit can be achieved in this configuration with a
tower height to field diameter ratio of about 1.0.

The optimum configuration without a secondary is always very
different from that for the optimum with a secondary.

In general, a pre-existing configuration that has been originally
designed for operation as a one-stage system should not be used
as the starting point for designing a two stage system.
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VIII

October 8, 2014
Present Status
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Present Status



Economics of Solar Energy is still problematic
CPCs and other nonimaging devices hold
promise of eventual simpler, less expensive,
higher performing collection technologies
Near Term Goals:




Inexpensive commercial non-evacuated CPCs
High Performance Evacuated CPCs for Solar Cooling
and Heating
Development of TIR terminal concentrators/mixers for
PV applications with advanced high efficiency cells
Longer Term Goals



Mass production of low cost evacuated CPC for
widespread production of Solar Thermal Energy
Very High Concentration Systems for Hydrogen
Production through water-splitting.
Towards a Solar Hydrogen Economy!
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Final Thought
 Energy
from the sun must
eventually play a major role
in providing a "sustainable"
source for mankind's needs.
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“Pale Blue
Dot”
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Concentration
Proofs of Limit
Based on Thermodynamic Argument
If C could be made larger (Absorber A2 smaller) there would not be
enough area to radiate away incident energy and its temperature
would begin to rise in violation of 2nd Law
Based on Phase Space Conservation
Liouville Theorem: Brightness is conserved along ray
Role of Concentration
Improved Performance
Reduced area of thermal absorber reduces the heat losses on an
aperture basis ( ~ 1/C)
Increased photon flux on solar cell increases conversion efficiency
slowly
Reduced Cost
Reduces area of expensive absorber (PV or Thermal)
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Increasing Appetite for Energy
While the developed world
has been limiting growth in
energy demand, the
developing nations want
their turn!
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Spectrally Selective
Absorber Surface
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The Flow-line
or “Trumpet”
Concentrator
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Two-Stage Dish Thermal Concentrators
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Two-Stage Dish Thermal Concentrators
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Two-Stage Dish Thermal Concentrators
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Summary of Hi Flux Measurements
Date
Location
Secondary
Measured
Flux (suns)
February
1988
Chicago
Lens-Oil
filled
Silver vessel
(n = 1.53)
56,000 +/5000
March
1989
Chicago
Solid Sapphire
DTIRC ( n =
84,000 +/3500
1.76)
July Aug
1990
NREL
(Golden CO)
Water Cooled
Reflecting Silver
CPC - air filled
Total Power
44 watts
72 watts
22,000 +/1000
3.5 Kilowatts
50,000 +/2000
900 Watts
(n = 1.0)
March 1994
4/13/2015
NREL
(Golden CO)
Fused Silica
(Quartz)
(n = 1.46
DTIRC with
“extractor tip
Solar Energy
85
Ultra-High Flux Applications
NREL Solar Furnace (Aerial View)
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New Generation CPC’s
• We describe here some advances in the optical and thermal models for nonevacuated CPCs and discuss in some detail, the development and prototype
performance testing results for one new design, referred to here as CPC 2.0.
• We also review a proposed new eXternal reflector CPC (or “XCPC”) design for
optimum match with absorption air conditioning applications
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The “CPC 2.0”
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The overall scale of the design is determined by the outer diameter of the
absorber tube, here 1.125 inches ( 2.858 cm).
The design acceptance angle is qc = ± 35o. This allows the apparent position
of the sun to be within the acceptance angle for at least 7 hours a day
throughout the year.
For a fully developed (untruncated) traditional CPC profile, this acceptance
angle yields a maximum geometric concentration Cmax = 1/sin qc = 1.74X..
To allow for mechanical tolerances and provide thermal isolation of the
absorber, there must be a gap, g, between the reflector cusp underneath
the absorber tube and the tube itself. Here, the design gap was chosen to
be 0.125 inches (3.18 mm).
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This introduces unavoidable optical throughput losses due to a fraction of the
reflected rays passing underneath the absorber.
However, these losses can be reduced by placing a small cavity in the form of a
“vee-groove” underneath the absorber and using some form of “modified cusp”
CPC solution.
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CPC 2.0 Optical Profile
35 degree Modified Cusp CPC
Absorber Diameter = 1.125 inches
physical gap = 0.125 inches,
6 = 0.094 inches
virtual gap
C = 1.55X: Average GapLoss = 2.14%
5
Y- Coordinate(inches)
4
3
2
1
0
-4
-3
-2
-1
0
1
2
3
4
-1
-2
X-Coordinate (inches)
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Basic Geometry only – Normal
Incidence
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CPC 2.0 on test stand
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Comparison with non-concentrating Collectors
Collector Efficiency
0.800
Measured Performance
y = -0.0028x + 0.6436
0.700
Predicted Performance
(48 square feet module)
0.600
0.500
0.400
Evacuated Tube
0.300
Predicted performance
(24 square feet module)
0.200
0.100
Predicted and Observed Optical and Thermal Performance
of the New Non-evacuated CPC 2.0 Collectors
0.000
-20
Good Flat Plate
0
20
40
60
80
100
120
Delta T (C)
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Summary
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Nonimaging Optics has changed our approach to
solar energy concentration
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Useful concentration is possible w/o tracking
Combined with evacuated selective absorber delivers
mid-temperature heat (200 – 300 C) from stationary
collector ( for air conditioning or industrial processes)
Nonimaging Secondaries promise high temperature
systems (>500C) with relaxed primary optics and
tracking requirements (e.g. lower cost)
Nonimaging Solar Furnaces now can produce
concentrated fluxes dramatically exceeding previous
levels
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Small Solar Power Tower
Sandia Nat. Lab., Albuquerque, NM
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1.5x Geometric Only
Confidential - Do not Circulate!!
1.5X Alum, Abs 0.95 (no Fresnel)
With glass tube(1mm) (with AR)
Confidential - Do not Circulate!!