CONCENTRATING SOLAR COLLECTORS
Portland State University
Solar Engineering
Spring 2008
Carolyn Roos, Ph.D.
Washington State University
Extension Energy Program
1
OUTLINE
• A review of six concentrating solar
technologies and current projects.
• Basics of ray tracing.
• Sketch of a thermal analysis example
2
Solar Concentrating Systems
• Concentrate solar energy through use of mirrors or lenses.
• Concentration factor (“number of suns”) may be greater than
10,000.
• Systems may be small:
e.g. solar cooker
.... or large:
- Utility scale electricity generation (up to 900 MWe planned)
- Furnace temperatures up to 3800oC (6800oF)
3
Concentrating Solar Power:
A Revived Industry
• Utility Action on ~3,000 MW in 2005-06
• CSP for Commercial & Industrial Facilities
Industrial Solar Tech’s Roof Specs
More planned since 2006
4
States Creating a Market for CSP
•
•
•
•
•
AZ: 15% RE by 2025, 30% Distributed Generation
CA: 20% by 2010 & 33% by 2020 planned
CO: 10% by 2015
NV: 20% by 2015, 5% Solar
NM: 10% by 2011
• TX: 4.2% by 2015
5
In a Carbon Limited Future…
• Carbon limits will close the cost gap.
• CSP can scale up fast without critical bottleneck materials.
(e.g. silicon)
• Costs will come down with increase in capacity
– expected to fall below natural gas in the next few years.
• In the very near future, the CSP market in the SW US can
grow to 1 to 2 GW per year.
From: http://www.nrel.gov/csp/troughnet/pdfs/2007/morse_look_us_csp_market.pdf
6
Examples of CSP Applications
Power Generation:


Utility Scale: 64 MW Nevada Solar One (2007)
Buildings: 200 kW “Power Roof”
Thermal Needs:




Hot Water and Steam (Industrial & Commercial Uses)
Air Conditioning – Absorption Chillers
Desalination of seawater by evaporation
Waste incineration
“Solar Chemistry”


Manufacture of metals and semiconductors
Hydrogen production (e.g. water splitting)
Materials Testing Under Extreme Conditions

e.g. Design of materials for shuttle reentry
7
Primary Types of Solar Collectors
1.
2.
3.
4.
5.
Parabolic Trough
Compact Linear Fresnel Reflector new
Solar Furnace
Parabolic Dish & Engine
Solar Central Receiver
(Solar Power Tower)
6. Lens Concentrators
Can be used in conjunction with PV:
Use lenses or mirrors in conjunction with PV panels to
increase their efficiency.
(http://seattle.bizjournals.com/seattle/stories/2006/04/24/focus2.html)
8
FRESNEL REFLECTOR
LENS CONCENTRATORS
PARABOLIC TROUGH
PARABOLIC DISH
PARABOLIC DISH
& ENGINE
SOLAR FURNACE
9
SOLAR FURNACE
CENTRAL RECEIVER
Major Components of
Solar Collector Systems
•
Concentrating mirror(s)
May use primary & secondary concentrators.
•
Absorber within a Receiver
Receiver contains the absorber. It is the apparatus
that “receives” the solar energy; e.g. evacuated
tube. Absorber absorbs energy from concentrator
and transfers to process being driven (engine,
chemical reactor, etc.); e.g. the pipe within an
evacuated tube.
•
Heliostats
Flat or slightly curved mirrors that track the sun and
focus on receiver or concentrator. Used with solar
furnaces and power towers.
10
Parabolic Troughs
• Most proven solar concentrating technology
• The nine Southern California Edison plants
(354 MW total) constructed in the 1980’s are
still in operation
11
Parabolic Troughs - Operation
•
Parabolic mirror reflects solar energy onto a receiver (e.g.
a evacuated tube).
•
Heat transfer fluid such as oil or water is circulated
through pipe loop. (250oF to 550oF)
•
Collectors track sun from east to west during day.
•
Thermal energy transferred from pipe loop to process.
12
Parabolic Trough System
- Can be hybrid solar / natural gas
- New systems include thermal storage.
13
Thermal Storage
• Uses high heat capacity fluids as heat
transfer storage mediums
• 12 to 17 hours of storage will allow plants to
have up to 60% to 70% capacity factors.
From: http://www1.eere.energy.gov/solar/pdfs/csp_prospectus_112807.pdf
14
Thermal Output
of Hybrid Plant with Thermal Storage
15
What Have Been the
Technical Challenges?
Development of Materials



Heat transfer tubes that are less prone to sagging & breaking.
Improved surface material of heat transfer tubes.
 High absorptivity, low emissivity and long-term stability in air.
Low cost mirrors that have reflectivity and washability of glass.
Improved Components


Flex hoses used to join sections of pipe loop were prone to
failure
 Replaced with ball joint design.
Ability to track on tilted axis
Improved Processes

e.g. Generate steam directly instead of running heat transfer
fluid through heat exchanger - Improves efficiency but more
difficult to control.
16
“First Solar Thermal Parabolic Trough
Power Plant Built in The U.S. In Nearly Two
Decades to Be Dedicated On Earth Day”
(2005)
 Saguaro Solar Generating Station (north of Tucson)
 1MW - Compared to 395MW in natural gas fired
generating capacity at same site
 Broke ground March 24, 2004 and started generating
power December 2005
 Built by Solargenix, subsidiary of ACCIONA Energy
of Spain
 Arizona has goal of 15% renewable energy by 2025.
 $6 Million Project
17
Saguaro Solar Generating Station
1MW - 2005
18
Nevada Solar One
64 MW - 2007
•
•
•
•
•
Now producing 64 MW on 140 hectares
Located in Eldorado Valley (south of Las Vegas)
One of the world's largest CSP plants.
Cost: $262 million
Developed by Solargenix Energy.
SHOTT North America provided receivers.
• Groundbreaking in February 2006
On line in June 2007.
19
Nevada Solar One
64 MW - 2007
20
Around the World
Granada, Spain.
• Two 50 MW plants
• Developed by Solar Millenium AG
Negev desert of Israel
• 150 MW facility to be expanded to 500 MW
• Developed by Solel (successor company to Luz)
• Cost $1 billion
21
Parabolic Troughs on a Smaller Scale:
SolarGenix “Power Roof” (2002)
• Parker Lincoln Building (demonstration)
• Design point of 176 kW
• Provides 50 tons of absorption cooling
22
Parabolic Troughs
Links for More Info
http://www.iea-ship.org/index.html
http://www.solarpaces.org/solar_trough.pdf
http://www.nrel.gov/docs/fy04osti/34440.pdf
Heat Transfer Analysis:
http://www.nrel.gov/docs/fy04osti/34169.pdf
Ball Joint Design:
http://www.eere.energy.gov/troughnet/pdfs/moreno_sf_interconnecti
ons_with_salt_htf.pdf
23
Links
to Parabolic Trough
Projects and Technology Examples
http://www.solargenix.com/power_plant_tech.cfm
http://www.solargenix.com/building_products.cfm
http://www.us.schott.com/solarthermal/english/index.html
http://www.us.schott.com/solarthermal/english/products/recei
ver/details.html
http://www.inderscience.com/search/index.php?mainAction=s
earch&action=record&rec_id=6745
http://www.sete.gr/files/Ebook/2006/Hospitality_Day_Lokurlu.
pdf
http://www.eere.energy.gov/troughnet/pdfs/lewandowski_vsh
ot.pdf
http://www.capitalsungroup.com/files/rmt.pdf
http://www.industrialsolartech.com/
24
Preview…
• Sketch of thermal analysis and design
for parabolic trough system at the end
of this presentation.
25
Compact Linear Fresnel Reflectors
Ausra, Inc.
http://www.ausra.com/
Makes moot some of the design challenges and
26
weaknesses of parabolic troughs.
Compact Linear Fresnel Reflectors
• A series of long, shallow-curvature mirrors
• Focus light on to linear receivers located
above the mirrors.
27
Compact Linear Fresnel Reflectors
Lower costs compared to parabolic troughs
• Several mirrors share the same receiver
– Reduced tracking mechanism complexity
• Stationary absorber
– No fluid couplings required
– Mirrors do not support the receiver
• Denser packing of mirrors possible
– Half the land area
28
Compact Linear Fresnel Reflectors
Projects
• 6.5-megawatt demonstration power plant under
construction in Portugal
(as of September 2007)
• Ausra and PG&E announce purchasing
agreement for 117 MW facility located in central
California
(November 2007)
29
Parabolic Dishes
- Plataforma Solar de Almeria – DISTAL I and II
- Dish with receiver for Stirling Engine
30
Parabolic Dish/Engine - Operation
• Solar energy drives a Stirling engine or
Brayton cycle engine (gas turbine.)
• Receiver absorbs solar energy and transfers
it to the engine’s working fluid.
• Systems are easily hybridized since Stirling
engines can run on any external heat source.
31
State of Dish Technology
Mature and Cost Effective Technology: Large utility projects
using parabolic dishes are now under development.
Technical Challenges Have Been:
 Development of solar materials and components
 Commercial availability of a solar-izable engine.
Advantage: High Efficiency
 Demonstrated highest solar-to-electric conversion efficiency
(still true with advances in CPV? No.)
 Potential to become one of least expensive sources of
renewable energy. (still true with development of Fresnel reflectors?)
Advantage: Flexibility
 Modular - May be deployed individually for remote
applications or grouped together for small-grid (village power)
systems.
32
Stirling Energy Systems, Inc.
33
Stirling Engines
• Stirling engines are simple, have high efficiency (25% for
industrial heat), operate quietly, have low O&M costs
(~$0.006/kWh)
• Waste heat can easily be recovered by the engine, as well as
from the engine
• According to one manufacturer: $1000-2000/kW installed
But
• They have higher costs for materials and assembly, are larger
for same torque, have longer start up time (needs to warm up)
34
Relatively small units are available.
e.g. Stirling Danmark
http://www.stirling.dk/default.asp?ID=121
35
… though these are designed for biopower
Infinia Corp
http://www.infiniacorp.com/applications/Prod_Spec.pdf
36
Stirling Engine Manufacturers
• Stirling Denmark: http://www.stirling.dk/
• STM Power:
http://www.energysolutionscenter.org/distgen/AppGuide/Manf/STMPow
er.htm
• QRMC
• Infinia: http://www.infiniacorp.com
– Stirling Cycles has been acquired by Infinia.
• ReGen Power Systems: http://www.rgpsystems.com/
• Stirling Energy Systems: http://www.stirlingenergy.com/.
– Currently manufacturers large utility-scale Stirling engines for use with solar
concentrating systems. Has plans to produce engines for use with combustible fuels
in the future.
• Stirling Biopower: http://www.stirlingbiopower.com/.
– In the start up phase (as of July 2007)
37
Receiver Tubes for Stirling Engine
Located at focus of dish to absorb heat.
38
From: www1.eere.energy.gov/solar/pdfs/csp_prospectus_112807.pdf
39
SDG&E (2005)
300 MW From 12,000 Stirling Solar Dishes
in Imperial Valley, Southern CA
• San Diego Gas & Electric entered 20-year contract with
SES Solar Two, an affiliate of Stirling Energy Systems in
2005.
• 12,000 Stirling solar dishes providing 300 MW on three
square miles
• Two future phases possible that could add 600 MW
– At 900 MW would be one of the largest solar facilities in the world.
40
SCE (2005)
500 MW from 20,000-Dish Array
in Mojave Desert
• Southern California Edison will construct 500
MW solar generating station on 4500 acres:
– Approved by CPUC in Dec 2005
– Using SES dishes
• First phase: 20,000-dish array to be constructed
over four years
• Option to expand to 850 MW.
41
A news story on these two projects…
•
•
SAN DIEGO, California, US, September 14, 2005 (Refocus Weekly) An
electric utility in California will buy 300 MW of solar power from a new
facility that uses Stirling solar dishes.
San Diego Gas & Electric will buy the green power under a 20-year contract with
SES Solar Two, an affiliate of Stirling Energy Systems of Arizona. The 300 MW
solar facility will consists of 12,000 Stirling solar dishes on three square miles of
land in the Imperial Valley of southern California.
SDG&E has options on two future phases that could add another 600 MW of
renewables capacity and, if the plant grows to 900 MW within ten years, it would
be one of the largest solar facilities in the world. The utility also announced the
purchase of 4 MW of energy from a local biogas landfill project.
SES says the contract is the second record-breaking solar project it has signed
in the past month, following a contract with Southern California Edison for
construction of a 4,500 acre solar generating station in southern California. That
20-year power purchase agreement, which also must be approved by the
CPUC, calls for development of 500 MW of solar capacity in the Mojave Desert,
northeast of Los Angeles.
The first phase will consist of a 20,000-dish array to be constructed over four
years, with an option to expand the project to 850 MW.
42
Salt River Landfill Demonstration Project
Four 22 kW SunDishes
• Each 'SunDish' is 50' high.
• Stretched-membrane faceted dishes deflected to convex form by vacuum.
• Reflective surface is made of sheets of 1.0 mm low-iron glass.
•
• Stirling engines and generators manufactured by STM Corporation.
• Electricity is used by the landfill facilities.
• Efficiency is “20% higher than other solar systems of a similar size.”
• Hybrid system: Stirling engines can run on solar energy, landfill gas or
other liquid or gaseous fuels.
43
STM’s Sun Dish System
44
From: http://www.energysolutionscenter.org/distgen/AppGuide/DataFiles/STMBrochure.pdf
Small Scale & Low Tech
Parabolic Dish with Solar Cookers
Using parabolic dish concentrators on a smaller scale...
45
Solar Furnaces
• Centre National de Recherche Scientifique - Odeillo, France
• Largest solar furnace in the world (1 MWt)
46
Solar Furnaces - Operation
Solar furnaces are used for:
- High temperature processes  “Solar Chemistry”
- Materials testing
A field of heliostats tracks the sun and focuses
energy on to a stationary parabolic concentrator
which refocuses energy to the receiver.
Receivers vary in design depending on process:
 Batch or continuous process
 Controlled temperature and pressure
 Collection of product (gas, solid, etc.)
47
Why Run Processes in a Solar Furnace?
Higher Temperatures (up to 3800oC)
 Higher temperatures are possible in solar furnace
than in conventional combustion furnace or
electric arc furnace.
Cleaner Processes
 e.g. Electric arc furnaces use carbon electrodes
which often contaminate product.
Energy Sustainability
 Use of renewable energy for industrial processes.
48
Electricity through Solar Chemistry
Example: Water splitting: 2H2O → 2H2 + O2
49
Solar Furnaces
Technical Challenges
From test bench to commercial scale processes
 Development of continuous processes from
batch experiments
Material Development
 Materials suitable for very high temperatures.
Process Control
 e.g. Accurate measurement of high temperatures
50
CNRS Solar Furnace at Odeillo, France
• Mirror is 10 stories high and forms one side of the
laboratory
• Maximum temperature is 3800oC
51
The Furnace
Inside the focal zone of the 1 MW mirror at Odeillo.
52
Receiver Example
Vaporization experiment with 2kW furnace at Odeillo.
53
Receiver and Attenuator
Plataforma Solar de Almeria:
- Attenuator – Louvers control sunlight entering furnace
54
Other Solar Furnaces
Solar furnaces in Spain, Switzerland, Germany, Israel, France...
Paul Scherer Institute - Switzerland (45 kW)
55
Paul Scherer Institute, Switzerland
Stretched film concentrator
56
Solar Central Receivers
“Power Towers”
Plataforma Solar de Almeria, Spain
57
Solar One
Located near Barstow, California
Operated from 1982 to1986
58
Solar One
Moonrise over the Solar One Heliostat Field
Photo from http://www.menzelphoto.com/gallery/big/altenergy3.htm
59
Solar Two
Solar Two improved the thermal storage of Solar One
Photo from http://ucdcms.ucdavis.edu/solar2/
60
Plataforma Solar de Almeria
• 1.8 MW steam generator
• Produces steam at 340oC and
to drive steam turbine
• Thermal storage: 18-tons of Al2O3
Notice the heliostat field and the
central tower reflected in this heliostat.
61
Concentrating Solar
Photovoltaics
• 500 kW now installed in Arizona (APS)
• Concentrating sunlight 250x to 500x reduces cell cost
• Amonix CPV cells are 26% efficient.
•Most efficient in world for silicon until… (see next slides)
• With multi-junction cells, efficiency can be increased to62
40%
http://www.cc.state.az.us/utility/electric/EPS-USPAPS.pdf
63
Lens Concentrators
• In this example, energy is concentrated on to PV cells with lenses
(but lens systems don’t necessarily have PV cells.)
• 40% efficiency for CPV achieved.
64
Comparison of
Technologies
(2006)
65
http://tomkonrad.wordpress.com/2006/12/07/they-do-it-with-mirrors-concentrating-solar-power/
Environmental Impacts
Deserts have sensitive ecosystems and low water
availability.
Land Use
The heliostat field occupies a large area of land, shading areas where
the ecosystem is accustomed to full sun.
-
Water Use
Wet cooling towers used in power generation have high water
consumption.
66
Ray Tracing
• Geometrical Optics:
– Law of Reflection and Refraction are the
only physical laws required for geometrical
optics.
– The rest is geometry  How rays of light
are reflected off surfaces or refracted
through materials.
67
Reflection
• Law of Reflection
– “The incident ray and reflected ray lie in a
plane containing the incident normal, and
this normal bisects the angle between the
two rays.”
Reference: “Modern Geometrical Optics”
by Max Herzberger, 1958
68
Refraction through a Lens
• Snell’s Law
n1 sin 1  n2 sin 2
n is index of refraction of the material
69
Ray Tracing Example
Secondary concentrator to spread energy evenly
across a cylinder.
…with a front that reflects reemitted radiation back
to the cylinder.
Reemission is not really
a single normal ray as shown,
Normal is center of distribution
of reemitted rays.
70
Miscellaneous Reflection Examples
71
“Modern Geometrical Optics”, Max Herzberger, 1958
Miscellaneous Refraction Examples
72
“Modern Geometrical Optics”, Max Herzberger, 1958
Edge Ray Analysis
• Edge ray analysis is used to do ray
tracing by hand.
• Select rays to establish bounds:
– Extreme angles
– With maximum error.
73
Example of Edge Ray Analysis
Rays Enter CPC at Extreme Angle
• Perfect CPC:
A Compound Parabolic
Concentrator focuses rays
onto an absorber without
tracking.
• Conical approximation:
– Some rays are reflected back
out without striking the
absorber.
– Select cone so rejection of
rays is acceptable.
74
Example of Secondary Concentrator
• Rays from primary concentrator focus on a pipe imperfectly.
• Design secondary mirror so many of the rays that miss the front
will reflect back to the pipe.
• Select rays that represent the error of the primary concentrator.
Ray 1 strikes front.
Ray 2 misses the front,
but is reflected back.
Ray 3 misses the front
75
and misses the back.
Ray Tracing by Computer
• Ray tracing by hand, you are limited to
selecting a small number of rays.
• Ray tracing by computer, you can send
in many rays.
– Can look at distribution of rays across a
surface.
76
Example:
Focal Point of an Imperfect
Primary Concentrator
77
Ray Tracing by Computer
Computer modeling:
• Incoming rays created according to the profile of primary
concentrator.
• Define surfaces of windows, reflectors and absorbers
mathematically.
• Follow path of incoming rays to absorber
and reemission of rays from absorber back out of system
• Determine surface temperatures and available process heat
from distribution of rays using energy balance.
Example design goals:
• Minimize reflection out of receiver
• Obtain even distribution across absorber surfaces
78
NREL Thermal Analysis Example
http://www.nrel.gov/docs/fy04osti/34169.pdf
• Consider a parabolic trough.
• Receiver - Pipe with and without evacuated tube.
From: “Heat Transfer Analysis and Modeling of a Parabolic Trough Solar Receiver
Implemented in Engineering Equation Solver”, R. Forristall, NREL, October 2003,
http://www.nrel.gov/docs/fy04osti/34169.pdf
79
Thermal Analysis Example
• Evacuated tube
80
Heat Balance on Receiver
with and without evacuated tube
81
Heat Balance Equations on Receiver
82
Design
In your thermal analysis, you may be interested in
considering:
•
•
•
•
•
•
•
Length and cross-section of trough
Diameters of pipe and evacuated tube
Velocity of heat transfer fluid
Optical properties of the pipe, glass and trough
Weather data: Temperature, Insolation, Wind
Temperatures of surfaces and heat transfer fluid.
Energy absorbed by heat transfer fluid
Vary geometry, velocity and materials to meet your design
criteria cost effectively.
83
Thermal Analysis
You may also want to include other losses such as
heat loss through support brackets.
84
Solar News Links
The Energy Blog’s Solar Thermal page:
http://thefraserdomain.typepad.com/energy/solartherma
l_/index.html
85
FRESNEL REFLECTOR
LENS CONCENTRATORS
The
End
PARABOLIC TROUGH
PARABOLIC DISH
PARABOLIC DISH
& ENGINE
SOLAR FURNACE
86
SOLAR FURNACE
CENTRAL RECEIVER