october 8, 2014the promise of solar energy 1 the promise of solar energy the promise of solar energy...

October 8, 2014October 8, 2014 The Promise of Solar EnergyThe Promise of Solar Energy 11

The Promise of Solar The Promise of Solar EnergyEnergy

Joe O'GallagherJoe O'Gallagher Adjunct Professor of Physics, Governors State UniversityAdjunct Professor of Physics, Governors State University

University Park, IllinoisUniversity Park, Illinois

andand Lead Scientific Officer, Solargenix EnergyLead Scientific Officer, Solargenix Energy

Sanford, North CarolinaSanford, North Carolina

Formerly: Formerly: Senior Lecturer and Executive OfficerSenior Lecturer and Executive Officer

Department of Physics andThe Enrico Fermi InstituteDepartment of Physics andThe Enrico Fermi InstituteUniversity of Chicago University of Chicago

(now retired)(now retired)

Wichita State ColloquiumOctober 8, 2014

Why “Promise” ?Why “Promise” ? I Have worked in this field for nearly 40 years.I Have worked in this field for nearly 40 years.

Progress has been somewhat disappointing due in part toProgress has been somewhat disappointing due in part to Poor implementation of early concepts Poor implementation of early concepts Lack of understanding by the general public about what can and Lack of understanding by the general public about what can and

cannot be donecannot be done Other economic obstacles and market conditionsOther economic obstacles and market conditions

The vision of a renewable energy driven sustainable energy The vision of a renewable energy driven sustainable energy economy has not been achieved.economy has not been achieved.

The original “promise” remains unfulfilled, but that theme The original “promise” remains unfulfilled, but that theme provides a context for what I want to talk about today.provides a context for what I want to talk about today.

There has been much progress. There has been much progress. New technologies and techniques have been developedNew technologies and techniques have been developed Performance is improving and costs are coming down.Performance is improving and costs are coming down.

It is inevitable that the promise will be fulfilled!It is inevitable that the promise will be fulfilled!



ACKNOWLEDGMENTSACKNOWLEDGMENTS::

I would like to thank my colleague of over 30 years, I would like to thank my colleague of over 30 years, Professor Roland Professor Roland WinstonWinston, formerly of the University of Chicago and now at the University of , formerly of the University of Chicago and now at the University of California, Merced. Roland is the inventor and primary developer of most of California, Merced. Roland is the inventor and primary developer of most of the concepts belonging to the new optical subdiscipline now called the concepts belonging to the new optical subdiscipline now called “nonimaging optics” which led to the development of so-called “Compound “nonimaging optics” which led to the development of so-called “Compound Parabolic Concentrators” and related devices for solar energy concentration.Parabolic Concentrators” and related devices for solar energy concentration.

The Development of the Compound Parabolic Concentrator and other The Development of the Compound Parabolic Concentrator and other nonimaging optical devices at the nonimaging optical devices at the University of Chicago University of Chicago between 1975 and between 1975 and 2005 was supported largely by: the 2005 was supported largely by: the U. S. Department of Energy U. S. Department of Energy through the through the Office of Basic Energy Sciences, The Office of Energy Efficiency and Office of Basic Energy Sciences, The Office of Energy Efficiency and Renewable Energy, the National Renewable Energy Laboratory, Sandia Renewable Energy, the National Renewable Energy Laboratory, Sandia National Laboratory, and the National Laboratory, and the Jet Propulsion LaboratoryJet Propulsion Laboratory. .


Outline of TalkOutline of Talk General IntroductionGeneral Introduction

Background and MotivationBackground and Motivation The Solar ResourceThe Solar Resource

Properties, Problems, and EconomicsProperties, Problems, and Economics Overview of Solar Applications and Collection StrategiesOverview of Solar Applications and Collection Strategies

The Thermodynamic Limit and the Concentration of The Thermodynamic Limit and the Concentration of SunlightSunlight The Role of ConcentrationThe Role of Concentration Review of fundamental concepts (“The Sine Law”)Review of fundamental concepts (“The Sine Law”) Consequences – (Theoretical limits for Solar Consequences – (Theoretical limits for Solar

Concentration)Concentration) Introduction to “Nonimaging Optics”Introduction to “Nonimaging Optics”

Examples and Applications (Mostly a Slide Show)Examples and Applications (Mostly a Slide Show) The “Compound Parabolic Concentrator” (CPC)The “Compound Parabolic Concentrator” (CPC) Two-Stage Concentrators for solar-thermal and Two-Stage Concentrators for solar-thermal and

photovoltaic generation of electricityphotovoltaic generation of electricity Ultra-high concentration: Demonstration and exotic Ultra-high concentration: Demonstration and exotic

applicationsapplications

Summary and ConclusionsSummary and Conclusions

IIBackground and Background and

MotivationMotivation



Motivation Motivation Why Solar Energy?Why Solar Energy?

The world economy and standard of living are The world economy and standard of living are strongly coupled to energy availability.strongly coupled to energy availability.

Solar Energy research is an exciting, interesting, Solar Energy research is an exciting, interesting, dynamic, and satisfying endeavor.dynamic, and satisfying endeavor.

Technically challenging (thermodynamics, optics, Technically challenging (thermodynamics, optics, semiconductor physics, materials science, etc. ) semiconductor physics, materials science, etc. )

InterdisciplinaryInterdisciplinary Very broad based (involves economics, politics, sociology, Very broad based (involves economics, politics, sociology,

etc.)etc.)

The byproducts of energy production threaten the The byproducts of energy production threaten the quality of life on the planetquality of life on the planet

Atmospheric pollutionAtmospheric pollution Greenhouse gases/global warmingGreenhouse gases/global warming

Conventional energy sources are limited and Conventional energy sources are limited and being consumed at an every increasing rate.being consumed at an every increasing rate.


Global Warming: Global Warming: Fact or Fiction?Fact or Fiction?

October 25, 2012


Land-Ocean data through 2012Land-Ocean data through 2012The World is definitely getting Warmer!!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.


GLOBAL CLIMATE CHANGEGLOBAL CLIMATE CHANGE The World The World IS IS getting warmer -- warmer getting warmer -- warmer

than it has been in at least the last 2,000 than it has been in at least the last 2,000 yearsyears

Mankind’s activities to produce energy Mankind’s activities to produce energy are definitely a major part of the cause!!are definitely a major part of the cause!!

Carbon Dioxide levels in the atmosphere Carbon Dioxide levels in the atmosphere are higher than they have been in the are higher than they have been in the last 600,000 years.last 600,000 years.

Our continuous combustion of fossil fuels Our continuous combustion of fossil fuels is affecting the health of the planet!!is affecting the health of the planet!!

IPCC AR5 Synthesis Report (SYR) – Due IPCC AR5 Synthesis Report (SYR) – Due out 31 October 2014out 31 October 2014

What about “Peak Oil” ?What about “Peak Oil” ?



OIL SUPPLIES ARE LIMITEDOIL SUPPLIES ARE LIMITED Geological Deposits of Fossil Fuels were Geological Deposits of Fossil Fuels were

produced about 300 million years ago !produced about 300 million years ago !

There’s only so much that was ever produced.There’s only so much that was ever produced.

We are now (or will be soon) reaching the Peak We are now (or will be soon) reaching the Peak of world oil production (often referred to as the of world oil production (often referred to as the “Hubbert Peak” after M. King Hubbert). “Hubbert Peak” after M. King Hubbert). The U. S. peaked in 1970 and has been in a sense The U. S. peaked in 1970 and has been in a sense

“running out” of oil ever since“running out” of oil ever since The world will peak (begin to “run out”) in the next The world will peak (begin to “run out”) in the next

5 to10 years, if it hasn’t already5 to10 years, if it hasn’t already It’s the beginning of the end of abundant energy!It’s the beginning of the end of abundant energy!

Production Lags DiscoveryProduction Lags Discovery



Space-Space-ship ship

Earth:Earth:The The Only Only

Planet Planet we’ve we’ve got!got!


What about Alternative Energy What about Alternative Energy Sources?Sources?

Solar Energy Solar Energy The Source of almost all energy on earth The Source of almost all energy on earth Fossil fuels are stored Solar energyFossil fuels are stored Solar energy Capacity dwarfs all the other so-called renewablesCapacity dwarfs all the other so-called renewables Can be thought of as “the mother of all renewables”Can be thought of as “the mother of all renewables”

Wind Energy Wind Energy Is an indirect form of SolarIs an indirect form of Solar Is economical today in many locationsIs economical today in many locations Still has aroused some practical concerns Still has aroused some practical concerns

Biomass Biomass Also has considerable promiseAlso has considerable promise

Nuclear– Nuclear– Not usually thought of in this context Not usually thought of in this context Has problems but probably will have to play a role Has problems but probably will have to play a role

IIIIThe Solar ResourceThe Solar Resource



The SunThe Sun(X-ray Image in False Color)(X-ray Image in False Color)

-The Source of (amost) all energy on earth

- The driver of all climate on earth


The Promise of Solar EnergyThe Promise of Solar Energy It’s abundant (very) It’s abundant (very) It’s evenly distributed (sort of)It’s evenly distributed (sort of) It’s forever (for all intents and purposes)It’s forever (for all intents and purposes)

But…But… It’s highly variable in timeIt’s highly variable in time It’s very dilute (relatively low intensity spread It’s very dilute (relatively low intensity spread

out over large areas)out over large areas) It’s expensive to collect (at least now)It’s expensive to collect (at least now) Difficult and expensive to convert to major “end Difficult and expensive to convert to major “end

uses”uses”


The Solar ResourceThe Solar Resource Very Large Thermonuclear Fusion Reactor

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)

Surface is almost perfect Black Body Radiator T = 6000o K max = 500 nm (5000 Angstroms)

Power Output 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.


The Solar Resource The Solar Resource (Cont’d)(Cont’d) The "Solar Constant"The "Solar Constant"

- - Imax = 1370 watts/m2 ( in space near earth) Imax = 1370 watts/m2 ( in space near earth) 1000 watts/m2 ( at noon in Albuquerque) 1000 watts/m2 ( at noon in Albuquerque)

170 watts/m2 ( global yearly average)170 watts/m2 ( global yearly average)- Yearly total solar incident on U. S. land area = 40,000 quads- Yearly total solar incident on U. S. land area = 40,000 quads- 0.5 % of U.S. land area @ 50% efficient = total U.S.- 0.5 % of U.S. land area @ 50% efficient = total U.S. useuse- Solar Energy is - Solar Energy is abundant!abundant!

ProblemsProblems- Dilute- Dilute- Intermittent (it would help to have storage! – “beyond the - Intermittent (it would help to have storage! – “beyond the

scope scope of this talk”) of this talk”)

- Source is highly collimated and constantly moving- Source is highly collimated and constantly moving- Predominantly low grade thermal- Predominantly low grade thermal

Simple EconomicsSimple Economics (Conventional Energy sources are still (Conventional Energy sources are still very inexpensive!)very inexpensive!)1 M1 M22 - year of sunlight is worth (depending on local climate and fuel - year of sunlight is worth (depending on local climate and fuel displaced) displaced)

~ $20- $200 !! (That’s roughly $2 to $20 per square foot!) ~ $20- $200 !! (That’s roughly $2 to $20 per square foot!)

IIIIIISolar Collection and Solar Collection and

Conversion TechnologiesConversion Technologies



Direct Solar Energy Direct Solar Energy ConversionConversion

PhotovoltaicElectricity(PV)

Heat

Cooling(A/C andRefrigeration)

Solar Thermal Electricity

Hot water and space heating

IndustrialProcessHeat

Fuels and ChemicalsProduction(Hydrogen!)

THESUN

SolarCooking

(“Active” strategies)


The Direct Conversion of The Direct Conversion of Sunlight to Sunlight to

Electricity:”Photovoltaics” or Electricity:”Photovoltaics” or PVPV One of the Cleanest and neatest One of the Cleanest and neatest

forms of solar energyforms of solar energy Easy to install and useEasy to install and use Probably one of the most expensive Probably one of the most expensive

forms as wellforms as well Photovoltaic panels are about 12% to Photovoltaic panels are about 12% to

20% efficient and cost about $50/ft20% efficient and cost about $50/ft22 to to $100/ft$100/ft22

Flat Plate Photovoltaic PanelFlat Plate Photovoltaic Panel


Photovoltaic TechnologiesPhotovoltaic Technologies Single crystal silicon cells (about 95% of today’s Single crystal silicon cells (about 95% of today’s

market)market) Moderate performance (Moderate performance ( ~ 12% - 20%) ~ 12% - 20%) Expensive ($50/ftExpensive ($50/ft22 - $100/ft - $100/ft22 => $3/w => $3/wpp - $7/w - $7/wpp))

(The “Peak Wattage” of a system is its power output under an (The “Peak Wattage” of a system is its power output under an insolation of 1000 watts/Minsolation of 1000 watts/M22.) .)

Thin film(e.g. Cadmium Telluride) or amorphous siliconThin film(e.g. Cadmium Telluride) or amorphous silicon lower performance (lower performance ( ~ 6% - 12%)) ~ 6% - 12%)) Less expensiveLess expensive Can be deployed as roofing shinglesCan be deployed as roofing shingles

Multi-junction cellsMulti-junction cells High performance (High performance ( Very expensive (factors of 10 to 100 more than single crystal)Very expensive (factors of 10 to 100 more than single crystal) Need concentration to be economicalNeed concentration to be economical

OctoberOctober 8, 20148, 2014 The Promise of Solar EnergyThe Promise of Solar Energy 2424

Solar Thermal EnergySolar Thermal Energy Absorb radiant energy as heat and transfer to Absorb radiant energy as heat and transfer to

a working fluid.a working fluid. ApplicationsApplications

Domestic Hot Water Domestic Hot Water Space heatingSpace heating

Use concentration to get high temperatures Use concentration to get high temperatures and run an engine to generate electricity!and run an engine to generate electricity!

Solar thermal refrigeration and Air Solar thermal refrigeration and Air Conditioning (also requires higher Conditioning (also requires higher temperatures)temperatures)


Solar Hot WaterSolar Hot Water



Flat Plate Geometry is very Flat Plate Geometry is very simplesimple

and can also collect reflected and can also collect reflected lightlight

IVIVThe Role of ConcentrationThe Role of Concentration



Tracking Parabolic TroughTracking Parabolic Trough

Part of the 30 MegaWatt Solar Part of the 30 MegaWatt Solar Thermal Electric system in Thermal Electric system in

CaliforniaCalifornia

October 8, 2014October 8, 2014 3030The Promise of Solar EnergyThe Promise of Solar Energy

A 30 Megawatt Solar Power A 30 Megawatt Solar Power Plant in Southern CaliforniaPlant in Southern California

October 8, 2014October 8, 2014 3131The Promise of Solar EnergyThe Promise of Solar Energy


Large Concentrating Parabolic Large Concentrating Parabolic DishDish


Central Receiver Test Central Receiver Test FacilityFacility

Sandia Albuquerque Sandia Albuquerque


Role of Concentration*

To Improve PerformanceTo Improve Performance- Reducing the relative area of the hot thermal - Reducing the relative area of the hot thermal absorber reduces the heat losses ( ~ 1/C) and absorber reduces the heat losses ( ~ 1/C) and allows higher temperatures to be achieved.allows higher temperatures to be achieved.

-- Increased photon flux on solar cell increases Increased photon flux on solar cell increases conversion efficiency slowlyconversion efficiency slowly

To Reduce CostsTo Reduce Costs- Reduces the required area of expensive - Reduces the required area of expensive absorber (PV or Thermal) and replaces it with absorber (PV or Thermal) and replaces it with (presumably) less expensive optics. (presumably) less expensive optics.

* 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.

VV“Nonimaging Optics”“Nonimaging Optics”



BackgroundBackground Nonimaging OpticsNonimaging Optics

New approach to the collection, concentration and New approach to the collection, concentration and transport of light originally developed by Roland transport of light originally developed by Roland Winston, myself, and our group at the University Winston, myself, and our group at the University of Chicago of Chicago

Relaxes the constraints of point-to-point mapping Relaxes the constraints of point-to-point mapping of imaging opticsof imaging optics

Achieves or approaches the maximum geometrical Achieves or approaches the maximum geometrical concentration permitted by physical conservation concentration permitted by physical conservation laws for a given angular field of view. laws for a given angular field of view.

Focusing optics always fall short of this limit by a Focusing optics always fall short of this limit by a factor of ~ 2 to 4. factor of ~ 2 to 4.

The CPC (“Compound Parabolic The CPC (“Compound Parabolic Concentrator”)Concentrator”) The prototypical nonimaging “ideal” light collector The prototypical nonimaging “ideal” light collector

invented by Roland Winstoninvented by Roland Winston Generic name for whole family of similar devicesGeneric name for whole family of similar devices


Importance for Importance for Solar Energy CollectionSolar Energy Collection

Achieves widest possible angular field Achieves widest possible angular field of view for given geometric of view for given geometric concentrationconcentration

Permits useful concentration without Permits useful concentration without trackingtracking

1.1 -2x for totally stationary collector1.1 -2x for totally stationary collector 2x – 10 x with seasonal adjustment2x – 10 x with seasonal adjustment > 10x – 40,000x with relaxed optics and > 10x – 40,000x with relaxed optics and

tracking requirementstracking requirements


Concentration and the Thermodynamic LimitConcentration and the Thermodynamic Limit

Collecting Aperture, A1

Absorbing Aperture, A2Optics

Cgeom = A1/A2

For Cgeom >1 ( i.e. for A2 < A1) the optics must limit the field of view


Concentration LimitConcentration Limit

2

max 2sin

nC

In two dimensional (trough-like) geometrymax sin

nC

In three dimensional (cone-like) geometry

n is index of refraction at absorber surface, 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


The CPCThe CPC

BC


Additional CPCAdditional CPCDesigns for Designs for

different different absorber absorber

shapes based shapes based on “edge-ray on “edge-ray

principle”principle”


Early Argonne XCPC DesignEarly Argonne XCPC Design(Evacuated Dewar-type Absorber tube(Evacuated Dewar-type Absorber tube

with selective surface) with selective surface)


CPC Solar GeometryCPC Solar Geometry Achieves widest possible Achieves widest possible

angular field of view for given angular field of view for given geometric concentrationgeometric concentration

Permits useful concentration Permits useful concentration without trackingwithout tracking

1.1x -2x for totally 1.1x -2x for totally stationary collectorstationary collector

2x – 10 x with seasonal 2x – 10 x with seasonal adjustmentadjustment

Collects large fraction of Collects large fraction of diffuse component of sunlightdiffuse component of sunlight

Higher Concentration (> 10x Higher Concentration (> 10x – >40,000x) requires tracking – >40,000x) requires tracking with multi-stage system but with multi-stage system but allows relaxed optics and allows relaxed optics and tracking tolerancestracking tolerances

latitudeangle

max sin

nC

S

VIVIEarly ApplicationsEarly Applications



Selected Applications of Nonimaging Selected Applications of Nonimaging Optics in Solar EnergyOptics in Solar Energy

Nontracking CollectorsNontracking Collectors Evacuated CPCsEvacuated CPCs The Integrated CPC (Evacuated)The Integrated CPC (Evacuated) Nonevacuated CPCSNonevacuated CPCS

High Concentration Tracking CollectorsHigh Concentration Tracking Collectors•Two stage Concentrators

•Solar Thermal Conversion•Solar Photovoltaic conversion

•Ultra- High Flux Solar Furnaces

Evacuated CPC Evacuated CPC ConcentratorsConcentrators

Goal Goal to reduce the heat losses at high operating to reduce the heat losses at high operating

temperatures (from the hot absorber to ambient) as temperatures (from the hot absorber to ambient) as much as possible).much as possible).

CombineCombine Vacuum insulation (eliminates conductive and Vacuum insulation (eliminates conductive and

convective heat losses)convective heat losses) Spectrally selective absorber surface (suppresses Spectrally selective absorber surface (suppresses

radiation loss)radiation loss) Nonimaging concentration (reduces surface area of hot Nonimaging concentration (reduces surface area of hot

absorber) absorber)

Achieves high temperature end uses with a Achieves high temperature end uses with a nontracking collectornontracking collector



CPC with evacuated ReceiverCPC with evacuated Receiver Energy Design Collectors installed on U of C Energy Design Collectors installed on U of C

Physics building in 1986Physics building in 1986


Integrated CPCs Integrated CPCs (Evacuated)(Evacuated)


Ultra-High Flux ApplicationsUltra-High Flux ApplicationsNREL Solar FurnaceNREL Solar Furnace (Artists Conception) (Artists Conception)


Two-Stage Dish Thermal Two-Stage Dish Thermal ConcentratorsConcentrators


Ultra-High FluxUltra-High Flux

Potential Applications for Ultra High Potential Applications for Ultra High Solar Flux ConcentrationSolar Flux Concentration Production of Exotic Materials (e.g. Fullerenes)Production of Exotic Materials (e.g. Fullerenes) Hydrogen Production (Direct water splitting)Hydrogen Production (Direct water splitting) Solar Pumping of LasersSolar Pumping of Lasers High Temperature Gas Turbine Solar Receivers High Temperature Gas Turbine Solar Receivers

(Weizmann Institute (Weizmann Institute for Science, Rehovath, for Science, Rehovath, Israel)Israel)

Solar Thermal Propulsion in SpaceSolar Thermal Propulsion in Space Solar Thermo - Photovoltaic ConvertersSolar Thermo - Photovoltaic Converters

04/21/2304/21/23 Solar Energy Solar Energy 5252

Ultra-High Flux ApplicationsUltra-High Flux Applications


Ultra-High Flux ApplicationsUltra-High Flux Applications

VIIVIIRecent DevelopmentsRecent Developments


New Design for eXternal New Design for eXternal reflector CPC with evacuted reflector CPC with evacuted

tube tube (XCPC)(XCPC)

Confidential - Do not Circulate!!Confidential - Do not Circulate!!

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

New XCPC New XCPC ProfileProfile


New XCPC Prototype Test New XCPC Prototype Test DataData

Cleveland,T. and M. Ross, “High Temperature Performance Evaluation of the XCPC Concentrating Collector”, Preliminary Report from the North Carolina Solar Center, August, 2012


High Concentration High Concentration Photovoltaic ApplicationsPhotovoltaic Applications



Concentrating PV systemConcentrating PV systemFacetted Dish, CFacetted Dish, C11 = 116X = 116X


Present Limitations of Present Limitations of Concentrating PVConcentrating PV

In a series string of cells, the current is limited to that In a series string of cells, the current is limited to that produced by the cell with the lowest illumination.produced by the cell with the lowest illumination.

One “dark” cell in such a string effectively “kills” the string One “dark” cell in such a string effectively “kills” the string outputoutput

Thus, for acceptable performance, PV cells wired together in Thus, for acceptable performance, PV cells wired together in an array require near uniform illumination on all cells. an array require near uniform illumination on all cells.

One solution is that the entire concentrator can be scaled up One solution is that the entire concentrator can be scaled up and coupled to a larger array of cells, if and coupled to a larger array of cells, if optical mixing optical mixing can be can be employed to distribute the flux nearly uniformly over a multi-employed to distribute the flux nearly uniformly over a multi-cell array. cell array.


Square TIR Optical MixerSquare TIR Optical Mixer


Comparison No Mixer/MixerComparison No Mixer/Mixer

0.00

6.00

12.00

18.00

24.00

30.00

Flux MapEntrance of Optical Mixer

24.00-30.00

18.00-24.00

12.00-18.00

6.00-12.00

0.00-6.00

0.00

0.20

0.40

0.60

0.80

1.00

Flux MapExit Aperture - Refractive Mixer

0.80-1.00

0.60-0.80

0.40-0.60

0.20-0.40

0.00-0.20


Dish with Concentrating Dish with Concentrating Truncated Pyramidal TIR Truncated Pyramidal TIR

mixermixer

slope = 3 milliradians

C1 = 800X

C2 = 2.5X

C1C2 = 2000X


High Concentration PV High Concentration PV ApplicationsApplications

Nonimaging concentrator/mixers very Nonimaging concentrator/mixers very effective in making the irradiance highly effective in making the irradiance highly uniformuniform

Can boost geometric concentration by Can boost geometric concentration by factor of 2 to 4. factor of 2 to 4.

Symmetry breaking critical to function: Symmetry breaking critical to function: (e.g., Square cross section mixer, not (e.g., Square cross section mixer, not cylindrical mixer)cylindrical mixer)

TIR is preferable for high throughputTIR is preferable for high throughput

Progress in PV technologiesProgress in PV technologies



- 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.

One Possible Long-term Vision


Overview of ProblemOverview of Problem The Need for High ConcentrationThe Need for High Concentration

Splitting water requires temperatures in the range 1500K - 2000KSplitting water requires temperatures in the range 1500K - 2000K

In turn requires a In turn requires a netnet average concentration of 3000 – 4000 suns average concentration of 3000 – 4000 suns

The “ideal” concentration limit (for achievable optical errors) is The “ideal” concentration limit (for achievable optical errors) is about 10,000 sunsabout 10,000 suns

Conventional single-stage focusing dish systems fall short of this Conventional single-stage focusing dish systems fall short of this limit by a factor of 3 – 4, and conventional central receiver limit by a factor of 3 – 4, and conventional central receiver systems fall even farther short of these requirements.systems fall even farther short of these requirements.

Bottom line: We can’t hope achieve the required concentrations Bottom line: We can’t hope achieve the required concentrations with a conventional single stage central receiver with a conventional single stage central receiver

The Need for Nonimaging SecondariesThe Need for Nonimaging Secondaries The only option for achieving required fluxes in a central receiver The only option for achieving required fluxes in a central receiver

design is to use some kind of nonimaging secondary at the design is to use some kind of nonimaging secondary at the reactor.reactor.

This concept of has been around for some time but has not been This concept of has been around for some time but has not been seriously investigated until relatively recently.seriously investigated until relatively recently.


The simplest geometry for a two-stage central The simplest geometry for a two-stage central receiver is a central tower (height H) surrounded receiver is a central tower (height H) surrounded by a circular heliostat field. The secondary is a by a circular heliostat field. The secondary is a simple CPC with acceptance angle simple CPC with acceptance angle cc. (Note that . (Note that c c = the “rim angle” of the system.) The optimum = the “rim angle” of the system.) The optimum field is circular with radius R = H*tanfield is circular with radius R = H*tancc = L* sin = L* sincc..

H L

CPC

A

Surround Field Circular Area for heliostats

R

c


Secondary Concentrator Secondary Concentrator OptionsOptions

Source: Timinger, et.al., Solar Energy 69(2), 2000

WIS beamdown secondary


FindingsFindings

The highest possible concentrations can only be achieved The highest possible concentrations can only be achieved with an axially symmetric circular field surrounding a with an axially symmetric circular field surrounding a central tower with a CPC looking vertically downward.central tower with a CPC looking vertically downward.

80% of the ideal limit can be achieved in this configuration 80% of the ideal limit can be achieved in this configuration with a tower height to field diameter ratio of about 1.0.with a tower height to field diameter ratio of about 1.0.

The optimum configuration without a secondary is always The optimum configuration without a secondary is always very different from that for the optimum with a secondary.very different from that for the optimum with a secondary.

In general, a pre-existing configuration that has been In general, a pre-existing configuration that has been originally designed for operation as a one-stage system originally designed for operation as a one-stage system should not be used as the starting point for designing a should not be used as the starting point for designing a two stage system.two stage system.

VIIIVIII

Present StatusPresent Status



Present StatusPresent Status Economics of Solar Energy is still problematicEconomics of Solar Energy is still problematic CPCs and other nonimaging devices hold CPCs and other nonimaging devices hold

promise of eventual simpler, less expensive, promise of eventual simpler, less expensive, higher performing collection technologieshigher performing collection technologies

Near Term Goals: Near Term Goals: Inexpensive commercial non-evacuated CPCsInexpensive commercial non-evacuated CPCs High Performance Evacuated CPCs for Solar High Performance Evacuated CPCs for Solar

Cooling and HeatingCooling and Heating Development of TIR terminal concentrators/mixers Development of TIR terminal concentrators/mixers

for PV applications with advanced high efficiency for PV applications with advanced high efficiency cellscells

Longer Term GoalsLonger Term Goals Mass production of low cost evacuated CPC for Mass production of low cost evacuated CPC for

widespread production of Solar Thermal Energywidespread production of Solar Thermal Energy Very High Concentration Systems for Hydrogen Very High Concentration Systems for Hydrogen

Production through water-splitting.Production through water-splitting. Towards a Solar Hydrogen Economy!Towards a Solar Hydrogen Economy!


Final ThoughtFinal Thought

Energy from the sun Energy from the sun mustmust eventually play a eventually play a major role in providing a major role in providing a "sustainable" source for "sustainable" source for mankind's needs.mankind's needs.

““Pale Pale Blue Dot”Blue Dot”



ConcentrationConcentration

Proofs of LimitProofs of LimitBased on Thermodynamic ArgumentBased on Thermodynamic Argument

If C could be made larger (Absorber AIf C could be made larger (Absorber A22 smaller) there smaller) there would not be enough area to radiate away incident energy would not be enough area to radiate away incident energy and its temperature would begin to rise in violation of 2and its temperature would begin to rise in violation of 2ndnd LawLaw

Based on Phase Space ConservationBased on Phase Space ConservationLiouville Theorem: Brightness is conserved along rayLiouville Theorem: Brightness is conserved along ray

Role of ConcentrationRole of Concentration Improved PerformanceImproved Performance

Reduced area of thermal absorber reduces the heat losses Reduced area of thermal absorber reduces the heat losses on an aperture basis ( ~ 1/C)on an aperture basis ( ~ 1/C)Increased photon flux on solar cell increases conversion Increased photon flux on solar cell increases conversion efficiency slowlyefficiency slowly

Reduced CostReduced CostReduces area of expensive absorber (PV or Thermal)Reduces area of expensive absorber (PV or Thermal)

Increasing Appetite for Increasing Appetite for EnergyEnergy


While the developed world has been limiting growth in energy demand, the developing nations want their turn!


Spectrally SelectiveSpectrally Selective Absorber Surface Absorber Surface


The Flow-The Flow-line or line or

“Trumpet” “Trumpet” ConcentratoConcentrato

rr

04/21/2304/21/23 Solar Energy Solar Energy 8585

Summary of Hi Flux Summary of Hi Flux MeasurementsMeasurementsDateDate LocationLocation SecondarySecondary Measured Measured

Flux Flux (suns)(suns)

Total Total PowerPower

FebruaryFebruary

1988 1988 ChicagoChicago Lens-Oil Lens-Oil

filled filled

Silver Silver vesselvessel

((n n = 1.53)= 1.53)

56,000 +/-56,000 +/-

50005000 44 watts44 watts

MarchMarch

19891989ChicagoChicago Solid Solid

SapphireSapphire

DTIRC ( DTIRC ( nn = =

1.76)1.76)

84,000 +/-84,000 +/-

35003500

72 watts72 watts

July AugJuly Aug

19901990NRELNREL

(Golden CO)(Golden CO)

Water CooledWater Cooled

Reflecting Reflecting SilverSilver

CPC - air filled CPC - air filled

((nn = 1.0) = 1.0)

22,000 +/-22,000 +/-

10001000 3.5 3.5 KilowattsKilowatts

March 1994March 1994 NRELNREL

(Golden CO)(Golden CO)

Fused SilicaFused Silica

(Quartz) (Quartz)

((nn = 1.46 = 1.46 DTIRC with DTIRC with “extractor tip“extractor tip

50,000 +/-50,000 +/-

20002000 900 Watts900 Watts


Ultra-High Flux ApplicationsUltra-High Flux ApplicationsNREL Solar Furnace (Aerial NREL Solar Furnace (Aerial

View)View)


New Generation CPC’sNew Generation CPC’s

• We describe here some advances in the optical and thermal models for We describe here some advances in the optical and thermal models for non-evacuated CPCs and discuss in some detail, the development and non-evacuated CPCs and discuss in some detail, the development and prototype performance testing results for one new design, referred to prototype performance testing results for one new design, referred to here as CPC 2.0. here as CPC 2.0.

• We also review a proposed new eXternal reflector CPC (or “XCPC”) We also review a proposed new eXternal reflector CPC (or “XCPC”) design for optimum match with absorption air conditioning applications design for optimum match with absorption air conditioning applications

The “CPC 2.0”The “CPC 2.0” The overall scale of the design is determined by the outer diameter of The overall scale of the design is determined by the outer diameter of

the absorber tube, here 1.125 inches ( 2.858 cm).the absorber tube, here 1.125 inches ( 2.858 cm).

The design acceptance angle is The design acceptance angle is cc = ± 35 = ± 35oo. This allows the apparent . This allows the apparent position of the sun to be within the acceptance angle for at least 7 position of the sun to be within the acceptance angle for at least 7 hours a day throughout the year. hours a day throughout the year.

For a fully developed (untruncated) traditional CPC profile, this For a fully developed (untruncated) traditional CPC profile, this acceptance angle yields a maximum geometric concentration Cacceptance angle yields a maximum geometric concentration Cmaxmax = = 1/sin1/sinc c = 1.74X..= 1.74X..

To allow for mechanical tolerances and provide thermal isolation of To allow for mechanical tolerances and provide thermal isolation of the absorber, there must be a gap, g, between the reflector cusp the absorber, there must be a gap, g, between the reflector cusp underneath the absorber tube and the tube itself. Here, the design underneath the absorber tube and the tube itself. Here, the design gap was chosen to be 0.125 inches (3.18 mm). gap was chosen to be 0.125 inches (3.18 mm).

This introduces unavoidable optical throughput losses due to a fraction of This introduces unavoidable optical throughput losses due to a fraction of the reflected rays passing underneath the absorber. the reflected rays passing underneath the absorber.

However, these losses can be reduced by placing a small cavity in the 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 form of a “vee-groove” underneath the absorber and using some form of “modified cusp” CPC solution. “modified cusp” CPC solution.

October 8, 2014October 8, 2014The Promise of Solar EnergyThe Promise of Solar Energy 8888

CPC 2.0 Optical ProfileCPC 2.0 Optical Profile


Basic Geometry only – Normal Basic Geometry only – Normal Incidence Incidence

The Promise of Solar EnergyThe Promise of Solar EnergyOctober 8, 2014October 8, 2014 9090

CPC 2.0 on test stand CPC 2.0 on test stand


Comparison with non-concentrating Comparison with non-concentrating CollectorsCollectors



SummarySummary

Nonimaging Optics has changed our approach Nonimaging Optics has changed our approach to solar energy concentrationto solar energy concentration Useful concentration is possible w/o trackingUseful concentration is possible w/o tracking Combined with evacuated selective absorber Combined with evacuated selective absorber

delivers mid-temperature heat (200 – 300 C) from delivers mid-temperature heat (200 – 300 C) from stationary collector ( for air conditioning or stationary collector ( for air conditioning or industrial processes)industrial processes)

Nonimaging Secondaries promise high temperature Nonimaging Secondaries promise high temperature systems (>500C) with relaxed primary optics and systems (>500C) with relaxed primary optics and tracking requirements (e.g. lower cost)tracking requirements (e.g. lower cost)

Nonimaging Solar Furnaces now can produce Nonimaging Solar Furnaces now can produce concentrated fluxes dramatically exceeding concentrated fluxes dramatically exceeding previous levelsprevious levels


Small Solar Power TowerSmall Solar Power TowerSandia Nat. Lab., Albuquerque, NMSandia Nat. Lab., Albuquerque, NM

1.5x Geometric Only1.5x Geometric Only


1.5X Alum, Abs 0.95 (no 1.5X Alum, Abs 0.95 (no Fresnel)Fresnel)

With glass tube(1mm) (with With glass tube(1mm) (with AR)AR)


october 8, 2014the promise of solar energy 1 the promise of solar energy the promise of solar energy...

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