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Wichita State Colloquium October 8, 2014 The Promise of Solar Energy Joe O'Gallagher Adjunct Wichita State Colloquium October 8, 2014 The Promise of Solar Energy Joe O'Gallagher Adjunct Professor of Physics, Governors State University Park, Illinois and Lead Scientific Officer, Solargenix Energy Sanford, North Carolina Formerly: Senior Lecturer and Executive Officer Department of Physics and. The Enrico Fermi Institute University of Chicago (now retired) October 8, 2014 The Promise of Solar Energy 1

Why “Promise” ? n I Have worked in this field for nearly 40 years. Why “Promise” ? n I Have worked in this field for nearly 40 years. n Progress has been somewhat disappointing due in part to n n n 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. n n n New technologies and techniques have been developed Performance is improving and costs are coming down. It is inevitable that the promise will be fulfilled! October 8, 2014 The Promise of Solar Energy 2

ACKNOWLEDGMENTS: n I would like to thank my colleague of over 30 years, Professor ACKNOWLEDGMENTS: n 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. n 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. October 8, 2014 The Promise of Solar Energy 3

Outline of Talk § General Introduction § § Background and Motivation The Solar Resource Outline of Talk § General Introduction § § Background and Motivation The Solar Resource § § § 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) § § Overview of Solar Applications and Collection Strategies Thermodynamic Limit and the Concentration of Sunlight § § Properties, Problems, and Economics 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 October 8, 2014 The Promise of Solar Energy 4

I Background and Motivation October 8, 2014 The Promise of Solar Energy 5 I Background and Motivation October 8, 2014 The Promise of Solar Energy 5

Motivation Why Solar Energy? n The world economy and standard of living are strongly Motivation Why Solar Energy? n The world economy and standard of living are strongly coupled to energy availability. n Solar Energy research is an exciting, interesting, dynamic, and satisfying endeavor. n n The byproducts of energy production threaten the quality of life on the planet n n n 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. October 8, 2014 The Promise of Solar Energy 6

October 25, 2012 Global Warming: Fact or Fiction? October 8, 2014 The Promise of October 25, 2012 Global Warming: Fact or Fiction? October 8, 2014 The Promise of Solar Energy 7

Land-Ocean data through 2012 The World is definitely getting Warmer!! There’s been about a Land-Ocean data through 2012 The World is definitely getting Warmer!! There’s been about a 0. 8 o -0. 9 o Celsius (1. 4 o 1. 6 o Farenheit) increase in the last 130 years. October 8, 2014 The Promise of Solar Energy 8

GLOBAL CLIMATE CHANGE The World IS getting warmer -- warmer than it has been GLOBAL CLIMATE CHANGE The World IS getting warmer -- warmer than it has been in at least the last 2, 000 years n Mankind’s activities to produce energy are definitely a major part of the cause!! n Carbon Dioxide levels in the atmosphere are higher than they have been in the last 600, 000 years. n Our continuous combustion of fossil fuels is affecting the health of the planet!! n IPCC AR 5 Synthesis Report (SYR) – Due out 31 October 2014 Solar Energy 9 October 8, 2014 The Promise of n

What about “Peak Oil” ? October 8, 2014 The Promise of Solar Energy 10 What about “Peak Oil” ? October 8, 2014 The Promise of Solar Energy 10

OIL SUPPLIES ARE LIMITED q q q Geological Deposits of Fossil Fuels were produced OIL SUPPLIES ARE LIMITED q q q 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). q q q 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 to 10 years, if it hasn’t already It’s the beginning of the end of abundant energy! October 8, 2014 The Promise of Solar Energy 11

Production Lags Discovery October 8, 2014 The Promise of Solar Energy 12 Production Lags Discovery October 8, 2014 The Promise of Solar Energy 12

Spaceship Earth: The Only Planet we’ve got! October 8, 2014 The Promise of Solar Spaceship Earth: The Only Planet we’ve got! October 8, 2014 The Promise of Solar Energy 13

What about Alternative Energy Sources? n Solar Energy The Source of almost all energy What about Alternative Energy Sources? n Solar Energy The Source of almost all energy on earth n n Wind Energy n n Is an indirect form of Solar Is economical today in many locations Still has aroused some practical concerns Biomass n n 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– n n Not usually thought of in this context Has problems but probably will have to play a role October 8, 2014 The Promise of Solar Energy 14

II The Solar Resource October 8, 2014 The Promise of Solar Energy 15 II The Solar Resource October 8, 2014 The Promise of Solar Energy 15

The Sun (X-ray Image in False Color) -The Source of (amost) all energy on The Sun (X-ray Image in False Color) -The Source of (amost) all energy on earth - The driver of all climate on earth October 8, 2014 The Promise of Solar Energy 16

The Promise of Solar Energy n It’s abundant (very) It’s evenly distributed (sort of) The Promise of Solar Energy n It’s abundant (very) It’s evenly distributed (sort of) It’s forever (for all intents and purposes) n But… n n n 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” October 8, 2014 The Promise of Solar Energy 17

The Solar Resource n Very Large Thermonuclear Fusion Reactor n n Surface is almost The Solar Resource n Very Large Thermonuclear Fusion Reactor n n Surface is almost perfect Black Body Radiator n n n T = 6000 o K lmax = 500 nm (5000 Angstroms) Power Output n n n 1. 4 x 106 km (870, 000 miles) in diameter 1. 5 x 108 km (93, 000 miles) away Subtends a half-angle of about 4. 7 milliradians (0. 27 o) 3. 8 x 1026 watts (1. 3 x 1027 BTU’s/hr) 13 trillion Quad’s*/hr Power Intercepted by the Earth n n 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. October 8, 2014 The Promise of Solar Energy 18

The Solar Resource (Cont’d) n The The Solar Resource (Cont’d) n The "Solar Constant" - Imax = 1370 watts/m 2 ( in space near earth) 1000 watts/m 2 ( at noon in Albuquerque) 170 watts/m 2 ( 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 - 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 n Simple Economics (Conventional Energy sources are still very inexpensive!) 1 M 2 - year of sunlight is worth (depending on local climate and fuel displaced) ~ $20 - $200 !! (That’s roughly $2 to $20 per square foot!) October 8, 2014 The Promise of Solar Energy 19

III Solar Collection and Conversion Technologies October 8, 2014 The Promise of Solar Energy III Solar Collection and Conversion Technologies October 8, 2014 The Promise of Solar Energy 20

Direct Solar Energy Conversion (“Active” strategies) THE SUN Photovoltaic Electricity (PV) Heat Hot water 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 October 8, 2014 The Promise of Solar Energy 21

The Direct Conversion of Sunlight to Electricity: ”Photovoltaics” or PV n n One of The Direct Conversion of Sunlight to Electricity: ”Photovoltaics” or PV n n 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/ft 2 to $100/ft 2 October 8, 2014 The Promise of Solar Energy 22

Flat Plate Photovoltaic Panel October 8, 2014 The Promise of Solar Energy 23 Flat Plate Photovoltaic Panel October 8, 2014 The Promise of Solar Energy 23

Photovoltaic Technologies n Single crystal silicon cells (about 95% of today’s market) Moderate performance Photovoltaic Technologies n Single crystal silicon cells (about 95% of today’s market) Moderate performance (h ~ 12% - 20%) n Expensive ($50/ft 2 - $100/ft 2 => $3/wp - $7/wp) (The “Peak Wattage” of a system is its power output under an insolation of 1000 watts/M 2. ) n n Thin film(e. g. Cadmium Telluride) or amorphous silicon n n lower performance (h ~ 6% - 12%)) Less expensive Can be deployed as roofing shingles Multi-junction cells n n n High performance (h >~ 40%) Very expensive (factors of 10 to 100 more than single crystal) Need concentration to be economical October 8, 2014 The Promise of Solar Energy 24

Solar Thermal Energy n n Absorb radiant energy as heat and transfer to a Solar Thermal Energy n n Absorb radiant energy as heat and transfer to a working fluid. Applications n n 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) October 8, 2014 The Promise of Solar Energy 25

Solar Hot Water October 8, 2014 The Promise of Solar Energy 26 Solar Hot Water October 8, 2014 The Promise of Solar Energy 26

Flat Plate Geometry is very simple and can also collect reflected light October 8, Flat Plate Geometry is very simple and can also collect reflected light October 8, 2014 The Promise of Solar Energy 27

IV The Role of Concentration October 8, 2014 The Promise of Solar Energy 28 IV The Role of Concentration October 8, 2014 The Promise of Solar Energy 28

Tracking Parabolic Trough October 8, 2014 The Promise of Solar Energy 29 Tracking Parabolic Trough October 8, 2014 The Promise of Solar Energy 29

Part of the 30 Mega. Watt Solar Thermal Electric system in California October 8, Part of the 30 Mega. Watt Solar Thermal Electric system in California October 8, 2014 The Promise of Solar Energy 30

A 30 Megawatt Solar Power Plant in Southern California October 8, 2014 The Promise A 30 Megawatt Solar Power Plant in Southern California October 8, 2014 The Promise of Solar Energy 31

Large Concentrating Parabolic Dish October 8, 2014 The Promise of Solar Energy 32 Large Concentrating Parabolic Dish October 8, 2014 The Promise of Solar Energy 32

Central Receiver Test Facility Sandia Albuquerque October 8, 2014 The Promise of Solar Energy Central Receiver Test Facility Sandia Albuquerque October 8, 2014 The Promise of Solar Energy 33

Role of Concentration* * Definition: Solar concentration is the process of collecting sunlight (solar 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. October 8, 2014 The Promise of Solar Energy 34

V “Nonimaging Optics” October 8, 2014 The Promise of Solar Energy 35 V “Nonimaging Optics” October 8, 2014 The Promise of Solar Energy 35

Background n Nonimaging Optics n n n New approach to the collection, concentration and Background n Nonimaging Optics n n n 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”) n n The prototypical nonimaging “ideal” light collector invented by Roland Winston Generic name for whole family of similar devices October 8, 2014 The Promise of Solar Energy 36

Importance for Solar Energy Collection n n Achieves widest possible angular field of view Importance for Solar Energy Collection n n Achieves widest possible angular field of view for given geometric concentration Permits useful concentration without tracking 1. 1 -2 x for totally stationary collector n 2 x – 10 x with seasonal adjustment n > 10 x – 40, 000 x with relaxed optics and tracking requirements n October 8, 2014 The Promise of Solar Energy 37

Concentration and the Thermodynamic Limit Collecting Aperture, A 1 q Cgeom = A 1/A Concentration and the Thermodynamic Limit Collecting Aperture, A 1 q Cgeom = A 1/A 2 s ptic O Absorbing Aperture, A 2 For Cgeom >1 ( i. e. for A 2 < A 1) the optics must limit the field of view October 8, 2014 The Promise of Solar Energy 38

Concentration Limit In two dimensional (trough-like) geometry In three dimensional (cone-like) geometry n is Concentration Limit 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 October 8, 2014 The Promise of Solar Energy 39

The CPC BC October 8, 2014 The Promise of Solar Energy 40 The CPC BC October 8, 2014 The Promise of Solar Energy 40

Additional CPC Designs for different absorber shapes based on “edgeray principle” October 8, 2014 Additional CPC Designs for different absorber shapes based on “edgeray principle” October 8, 2014 The Promise of Solar Energy 41

Early Argonne XCPC Design (Evacuated Dewar-type Absorber tube with selective surface) October 8, 2014 Early Argonne XCPC Design (Evacuated Dewar-type Absorber tube with selective surface) October 8, 2014 The Promise of Solar Energy 42

CPC Solar Geometry n n Achieves widest possible angular field of view for given CPC Solar Geometry n n Achieves widest possible angular field of view for given geometric concentration Permits useful concentration without tracking n n 1. 1 x -2 x for totally stationary collector 2 x – 10 x with seasonal adjustment Collects large fraction of diffuse component of sunlight Higher Concentration (> 10 x – >40, 000 x) requires tracking with multi-stage system but allows relaxed optics and tracking tolerances October 8, 2014 The Promise of Solar Energy latitude angle q q S 43

VI Early Applications October 8, 2014 The Promise of Solar Energy 44 VI Early Applications October 8, 2014 The Promise of Solar Energy 44

Selected Applications of Nonimaging Optics in Solar Energy n Nontracking Collectors n n n Selected Applications of Nonimaging Optics in Solar Energy n Nontracking Collectors n n n 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 October 8, 2014 The Promise of Solar Energy 45

Evacuated CPC Concentrators n Goal n n Combine n n to reduce the heat Evacuated CPC Concentrators n Goal n n Combine n n 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 October 8, 2014 The Promise of Solar Energy 46

CPC with evacuated Receiver Energy Design Collectors installed on U of C Physics building CPC with evacuated Receiver Energy Design Collectors installed on U of C Physics building in 1986 October 8, 2014 The Promise of Solar Energy 47

Integrated CPCs (Evacuated) October 8, 2014 The Promise of Solar Energy 48 Integrated CPCs (Evacuated) October 8, 2014 The Promise of Solar Energy 48

Ultra-High Flux Applications NREL Solar Furnace (Artists Conception) October 8, 2014 The Promise of Ultra-High Flux Applications NREL Solar Furnace (Artists Conception) October 8, 2014 The Promise of Solar Energy 49

Two-Stage Dish Thermal Concentrators October 8, 2014 The Promise of Solar Energy 50 Two-Stage Dish Thermal Concentrators October 8, 2014 The Promise of Solar Energy 50

Ultra-High Flux n Potential Applications for Ultra High Solar Flux Concentration n n n Ultra-High Flux n Potential Applications for Ultra High Solar Flux Concentration n n n 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 October 8, 2014 The Promise of Solar Energy 51

Ultra-High Flux Applications 3/18/2018 Solar Energy 52 Ultra-High Flux Applications 3/18/2018 Solar Energy 52

Ultra-High Flux Applications October 8, 2014 The Promise of Solar Energy 53 Ultra-High Flux Applications October 8, 2014 The Promise of Solar Energy 53

VII Recent Developments October 8, 2014 The Promise of Solar Energy 54 VII Recent Developments October 8, 2014 The Promise of Solar Energy 54

New Design for e. Xternal reflector CPC with evacuted tube (XCPC) Confidential - Do New Design for e. Xternal reflector CPC with evacuted tube (XCPC) Confidential - Do not Circulate!!

Modified Cusp XCPC-47: 58 mm dewar absorber Absorber Radius = 23. 5 mm; gap Modified Cusp XCPC-47: 58 mm 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. 50 X Aperture Width=221. 7 mm: Avrg. Gap. Loss (untruncated)=0. 0 210 Left. Branch (untruncated) 190 Right. Branch (truncated 1. 50 X) 170 Absorber Y- Coordinate (millimeters) 150 Vright 130 110 90 70 50 30 10 -135 -115 -95 -75 -55 -35 -10 5 25 45 -30 -50 X-Coordinate (millimeters) 65 85 105 125

Geometry for 1 mm thick glass tube Geometry for 1 mm thick glass tube

New XCPC Profile October 8, 2014 The Promise of Solar Energy 58 New XCPC Profile October 8, 2014 The Promise of Solar Energy 58

New XCPC Prototype Test Data Cleveland, T. and M. Ross, “High Temperature Performance Evaluation 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 October 8, 2014 The Promise of Solar Energy 59

High Concentration Photovoltaic Applications October 8, 2014 The Promise of Solar Energy 60 High Concentration Photovoltaic Applications October 8, 2014 The Promise of Solar Energy 60

Concentrating PV system Facetted Dish, C 1 = 116 X October 8, 2014 The Concentrating PV system Facetted Dish, C 1 = 116 X October 8, 2014 The Promise of Solar Energy 61

Present Limitations of Concentrating PV n n In a series string of cells, the Present Limitations of Concentrating PV n n 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. October 8, 2014 The Promise of Solar Energy 62

Square TIR Optical Mixer October 8, 2014 The Promise of Solar Energy 63 Square TIR Optical Mixer October 8, 2014 The Promise of Solar Energy 63

Comparison No Mixer/Mixer October 8, 2014 The Promise of Solar Energy 64 Comparison No Mixer/Mixer October 8, 2014 The Promise of Solar Energy 64

Dish with Concentrating Truncated Pyramidal TIR mixer sslope = 3 milliradians C 1 = Dish with Concentrating Truncated Pyramidal TIR mixer sslope = 3 milliradians C 1 = 800 X C 2 = 2. 5 X C 1 C 2 = 2000 X October 8, 2014 The Promise of Solar Energy 65

High Concentration PV Applications n n Nonimaging concentrator/mixers very effective in making the irradiance High Concentration PV Applications n n 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 October 8, 2014 The Promise of Solar Energy 66

Progress in PV technologies October 8, 2014 The Promise of Solar Energy 67 Progress in PV technologies October 8, 2014 The Promise of Solar Energy 67

One Possible Long-term Vision - The use of ultra-high solar concentration for the production 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. October 8, 2014 The Promise of Solar Energy 68

Overview of Problem The Need for High Concentration n Splitting water requires temperatures in Overview of Problem The Need for High Concentration n Splitting water requires temperatures in the range 1500 K - 2000 K n In turn requires a net average concentration of 3000 – 4000 suns n The “ideal” concentration limit (for achievable optical errors) is about 10, 000 suns n n 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 n The only option for achieving required fluxes in a central receiver design is to use some kind of nonimaging secondary at the reactor. n This concept of has been around for some time but has not been seriously investigated until relatively recently. October 8, 2014 The Promise of Solar Energy 69

n The simplest geometry for a two-stage central receiver is a central tower (height n 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. October 8, 2014 The Promise of Solar Energy 70

Secondary Concentrator Options WIS beamdown secondary October 8, 2014 Source: Timinger, et. al. , Secondary Concentrator Options WIS beamdown secondary October 8, 2014 Source: Timinger, et. al. , Solar Energy 69(2), 2000 The Promise of Solar Energy 71

Findings n The highest possible concentrations can only be achieved with an axially symmetric Findings n The highest possible concentrations can only be achieved with an axially symmetric circular field surrounding a central tower with a CPC looking vertically downward. n 80% of the ideal limit can be achieved in this configuration with a tower height to field diameter ratio of about 1. 0. n The optimum configuration without a secondary is always very different from that for the optimum with a secondary. n 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. October 8, 2014 The Promise of Solar Energy 72

VIII n October 8, 2014 Present Status The Promise of Solar Energy 73 VIII n October 8, 2014 Present Status The Promise of Solar Energy 73

Present Status n n n Economics of Solar Energy is still problematic CPCs and Present Status n n n 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: n n 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 n n n 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! October 8, 2014 The Promise of Solar Energy 74

Final Thought n Energy from the sun must eventually play a major role in Final Thought n Energy from the sun must eventually play a major role in providing a "sustainable" source for mankind's needs. October 8, 2014 The Promise of Solar Energy 75

“Pale Blue Dot” October 8, 2014 The Promise of Solar Energy 76 “Pale Blue Dot” October 8, 2014 The Promise of Solar Energy 76

October 8, 2014 The Promise of Solar Energy 77 October 8, 2014 The Promise of Solar Energy 77

Concentration Proofs of Limit Based on Thermodynamic Argument If C could be made larger Concentration Proofs of Limit Based on Thermodynamic Argument If C could be made larger (Absorber A 2 smaller) there would not be enough area to radiate away incident energy and its temperature would begin to rise in violation of 2 nd 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) October 8, 2014 The Promise of Solar Energy 78

Increasing Appetite for Energy While the developed world has been limiting growth in energy Increasing Appetite for Energy While the developed world has been limiting growth in energy demand, the developing nations want their turn! October 8, 2014 The Promise of Solar Energy 79

Spectrally Selective Absorber Surface October 8, 2014 The Promise of Solar Energy 80 Spectrally Selective Absorber Surface October 8, 2014 The Promise of Solar Energy 80

The Flow-line or “Trumpet” Concentrator October 8, 2014 The Promise of Solar Energy 81 The Flow-line or “Trumpet” Concentrator October 8, 2014 The Promise of Solar Energy 81

Two-Stage Dish Thermal Concentrators October 8, 2014 The Promise of Solar Energy 82 Two-Stage Dish Thermal Concentrators October 8, 2014 The Promise of Solar Energy 82

Two-Stage Dish Thermal Concentrators October 8, 2014 The Promise of Solar Energy 83 Two-Stage Dish Thermal Concentrators October 8, 2014 The Promise of Solar Energy 83

Two-Stage Dish Thermal Concentrators October 8, 2014 The Promise of Solar Energy 84 Two-Stage Dish Thermal Concentrators October 8, 2014 The Promise of Solar Energy 84

Summary of Hi Flux Measurements Date Location Secondary February 1988 Chicago Lens-Oil filled 56, Summary of Hi Flux Measurements Date Location Secondary February 1988 Chicago Lens-Oil filled 56, 000 +/Silver vessel 5000 (n = 1. 53) March 1989 Chicago Solid Sapphire DTIRC ( n = Measured Flux (suns) NREL (Golden CO) Water Cooled Reflecting Silver CPC - air filled 44 watts 84, 000 +/3500 1. 76) July Aug 1990 Total Power 72 watts 22, 000 +/1000 3. 5 Kilowatts 50, 000 +/2000 900 Watts (n = 1. 0) March 1994 3/18/2018 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) October 8, 2014 The Promise of Ultra-High Flux Applications NREL Solar Furnace (Aerial View) October 8, 2014 The Promise of Solar Energy 86

New Generation CPC’s • We describe here some advances in the optical and thermal 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 e. Xternal reflector CPC (or “XCPC”) design for optimum match with absorption air conditioning applications October 8, 2014 The Promise of Solar Energy 87

The “CPC 2. 0” n n The overall scale of the design is determined The “CPC 2. 0” n n 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 = ± 35 o. 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. 74 X. . 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). n n 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. October 8, 2014 The Promise of Solar Energy 88

CPC 2. 0 Optical Profile 35 degree Modified Cusp CPC Absorber Diameter = 1. CPC 2. 0 Optical Profile 35 degree Modified Cusp CPC Absorber Diameter = 1. 125 inches physical gap = 0. 125 inches, 6 virtual gap = 0. 094 inches C = 1. 55 X: Average Gap. Loss = 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) October 8, 2014 The Promise of Solar Energy 89

Basic Geometry only – Normal Incidence October 8, 2014 The Promise of Solar Energy Basic Geometry only – Normal Incidence October 8, 2014 The Promise of Solar Energy 90

CPC 2. 0 on test stand October 8, 2014 The Promise of Solar Energy CPC 2. 0 on test stand October 8, 2014 The Promise of Solar Energy 91

Comparison with non-concentrating Collectors October 8, 2014 The Promise of Solar Energy 92 Comparison with non-concentrating Collectors October 8, 2014 The Promise of Solar Energy 92

Summary n Nonimaging Optics has changed our approach to solar energy concentration n n Summary n Nonimaging Optics has changed our approach to solar energy concentration n n 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 (>500 C) with relaxed primary optics and tracking requirements (e. g. lower cost) Nonimaging Solar Furnaces now can produce concentrated fluxes dramatically exceeding previous levels October 8, 2014 The Promise of Solar Energy 93

Small Solar Power Tower Sandia Nat. Lab. , Albuquerque, NM October 8, 2014 The Small Solar Power Tower Sandia Nat. Lab. , Albuquerque, NM October 8, 2014 The Promise of Solar Energy 94

1. 5 x Geometric Only Confidential - Do not Circulate!! 1. 5 x Geometric Only Confidential - Do not Circulate!!

1. 5 X Alum, Abs 0. 95 (no Fresnel) With glass tube(1 mm) (with 1. 5 X Alum, Abs 0. 95 (no Fresnel) With glass tube(1 mm) (with AR) Confidential - Do not Circulate!!