Solar 2 - Solar Power Plants

8/17/2019 Solar 2 - Solar Power Plants

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MJ2411: Renewable Energy Technology – Concentrated Solar Power

Institutionen för Energiteknik: Kraft och Värmeteknologi

Solar Power Technologies

Concentrated Solar Power

James D. Spelling, KTH-EGI [email protected]

MJ2411: Renewable Energy Technology

mailto:[email protected]:[email protected]


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Solar Thermal Power

A solar thermal system is any process which harnesses solarradiation as a power source through the conversion of the incidentsolar flux to u s e f u l h e a t

Solar thermal power systems can be divided into two types, based onthe level of temperature at which the heat is to be delivered

06/09/2012 James Spelling 2

Non-Concentrating

Harnesses the Incoming Flux Directly

Low Temperature (


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Concentrating Solar Power

Concentrating solar power systems generate a high-temperatureheat source, which can be used to drive a conventional power plant

Thermal energy can be easily stored in large quantities, allowingsolar thermal plants to be d i s p a t c h a b l e



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Early Attempts at Solar Power

First demonstration of concentrated solar power in 1878 by AugustinBernard Mouchot at the Universal Exhibition in Paris

"Eventually industry will no longer find in Europe the resources tosatisfy its prodigious expansion... coal will undoubtedly be used up.What will industry do then?“ - Augustin Mouchot, 1876


Image Source: Wikipedia, 2012 Output: 140 kg/min of saturated steam


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Early Attempts at Solar Power

First power producing solar thermal power plant was built in Egypt in1913, using parabolic trough technology

First patent deposited in 1907

Steam production to drive a 40 kWreciprocating steam engine

Payback time of 2 years against coalfrom England at 13 $/ton

“One thing I feel so sure about, and thatis either the human race must finallyutilize direct sun power or revert tobarbarism” -Frank Shuman, 1914


Image Source: Wikipedia, 2012


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Part I: Solar Concentration Systems

Solar Thermal Power Technologies


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Why Concentrate?


Image Source: J. Spelling, 2011

Concentration increases the density of the radiant energy flux,allowing more power to be absorbed for a given surface area

Increased concentration means lowers areas for heat loss, allowingeffective receiver operation at higher temperatures

In a concentrating system two surfaces are defined:• The solar collector intercepts the incident solar

radiation, concentrates and redirects it

• Collector design fixes the aperture area Aa

• The receiver: intercepts the concentrated

radiation and converts it to high temperature heat

• Receiver design fixes the receiver area Ar

I b,a


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Why Concentrate?




Increased concentration means lowers areas for radiative heat loss,allowing effective receiver operation at higher temperatures

Only beam radiation can be harnessed bythe solar collector, as the focusing systemrequires that incident rays have a clearly-defined direction

I b,a: Beam irradiation at the aperture

I r ( x ): Flux distribution at the receiver

I b,a


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Concentration Ratio




The key parameter that determines the level of temperature that canbe reached is the solar concentration ratio

Two different definitions exist:• Ge om e t r i c Co n c e n t r a t i o n R a t i o :

A simple ratio of receiver area to

aperture area

• O p t i c a l Co n c e n t r a t i o n R a t i o :

A more accurate value based on

the intercepted solar flux

r

ag

A

ACR =

ab

r r

r o

I

A I

ACR,

1∫

=

δ

I b,a


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Concentration Ratio




The key parameter that determines the level of temperature that canbe reached is the solar concentration ratio

Two different definitions exist:• Geometric Concentration Ratio: CRg

• Optical Concentration Ratio: CRo

Linked by the o p t i c a l e f f i c ie n c y of the collector:

gopt o CRCR η =

I b,a


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Concentration Technologies

Currently four key solar thermal power technologies:


Parabolic Trough Central Receiver

Linear Fresnel Parabolic Dish


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Key Solar Technologies

Each solar collector technology has its own specific range ofpracticably achievable concentration ratios

As such, each technology is adapted to one or more types oftemperature range and thus power generation cycles

Other technologies do exist, but are significantly less developed

Concentration Tracking Focal Spot Temperatures Scale

Linear Fresnel 15 – 60 One-Axis Line < 500°C unlimited

Parabolic Trough 30 - 100 One-Axis Line < 600°C unlimited

Heliostat Field 500 - 1’000 Two-Axis Point < 1200°C < 360 MWth

Parabolic Dish 1’000 - 10’000 Two-Axis Point < 750°C < 100 kWth


Data Source: C. Philibert, 2005


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Line Focusing Systems


Line focusing systems employ single-axis tracking and reach mediumtemperatures (typically between 120°C and 600°C)

They can be used for both power production as well as high-temperature process heat in industrial applications

Parabolic Trough Concentrators Linear Fresnel Concentrators

Image Source: RISE Information Portal, 2004


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Line Focusing Systems


Line focusing systems employ single-axis tracking and reach mediumtemperatures (typically between 120°C and 600°C)

They can be used for both power production as well as high-temperature process heat in industrial applications

Parabolic Trough Concentrators

• Fully parabolic in one axis to provide

high optical efficiency

• Parabolic shape requires complex

molding increasing cost• Large mirror surface results in high

wind loading, thus stronger structures

Linear Fresnel Concentrators

• A number of linear mirrors approximate

parabolic concentration resulting in

lower optical efficiencies

• Planar mirrors are simple and cheap to

manufacture• Gaps between mirrors, coupled with a

lower centre of gravity result in lower

loading and lighter structures

i i fö i k ik f h k l i


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Point Focusing Systems


Point focusing systems employ dual-axis tracking and can reach hightemperatures (typically between 600°C and 2000°C)

They are used mainly for power production, as well as solar chemistryand high-temperature materials testing

Heliostat Field Concentrators Parabolic Dish Concentrators

Image Source: RISE Information Portal, 2004

I i i fö E i k ik K f h Vä k l i


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Point Focusing Systems


Point focusing systems employ dual-axis tracking and can reach hightemperatures (between 600°C and 2000°C)

They are used mainly for power production, as well as solar chemistryand high-temperature materials testing

Heliostat Field Concentrators

• Many planar mirrors focus to a small

receiver area, approximating full 3D

concentration

• Large number of mirrors can be focused

to one receiver, allowing multi-MWsystems to be designed

• Planar mirrors are cheap to mass-

produce

• Central power system benefits from

economies of scale

Parabolic Dish Concentrators

• True parabolic shape gives 3D

concentration at high concentration

ratios and high efficiencies

• Power output limited to ~25 kWe by

maximum dish diameter of ~15m due tooptical precision and support

• Parabolic dish is a complex 3D geometry

which is expensive to manufacture

• Dishes can be deployed modularly to

increase the power output

I tit ti fö E it k ik K ft h Vä t k l i


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Energy Balance at the Receiver


The energy balance at the receiver can be established as function ofthe operating temperature of the receiver

At higher temperatures the key losses will be by radiation from thesurface of the receiver

The useful energy extracted is function of the temperature, theconcentration ratio, the incident flux and some material properties:

α : surface absorptivity [-]

ε: surface emissivity [-]

σ : Stephan-Boltzmann constant

T surf : surface temperature [K]

Ar : receiver surface area [m2]

( )44 asurf r r r use T T A I AQ −−= εσ α

( )( )44, asurf abor use T T I CR AQ −−= εσ α

r


I tit ti fö E it k ik K ft h Vä t k l i


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Maximum Temperature



The maximum temperature that can be reached is when the usefulenergy extracted from the receiver is equal to zero

The incident solar flux is totally dissipated by the radiation losses

From the energy balance equation this gives:

Re-arranging, T m ax can be found:

( ) 044,

=−−= asurf r abor use T T A I CR AQ εσ α

4,

4

max ab

g

opt a I CR

T T σ ε

α η +=r

I tit ti fö E it k ik Kraft och Värmeteknologi


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Example 1: Temperature

What is the maximum operating temperature fora parabolic trough collector with a concentrationratio of 120 at standard conditions?

T a = 25°C, AM = 1.5 (i.e. 850 W/m2)

The optical efficiency of the trough is 90%, andthe absorber is non-selective.


ηopt: optical efficiency

α: receiver absorptivity

ε: receiver emissivity

CRg: concentration ratio

σ: Stefan-Boltzmann cst.

Ib,a : beam irradiation

4,

4

max ab

g

opt a I CR

T T σ ε

α η +=

N.B σ = 5.67e-8



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Example 1: Temperature

What is the maximum operating temperature fora parabolic trough collector with a concentrationratio of 120 at standard conditions?

T a = 25°C, AM = 1.5 (i.e. 850 W/m2)

The optical efficiency of the trough is 90%, andthe absorber is non-selective.

T max = 1129 K = 856°C








4,

4

max ab

g

opt a I CR

T T σ ε

α η +=

N.B σ = 5.67e-8



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Collector Efficiency


The efficiency of the solar collector is the ratio of energy input tou s e f u l heat output:

At a given temperature, efficiency can be increased by:

• Increasing the concentration ratio

• Increasing the absorptivity of the receiver

• Reducing the emissivity of the receiver

• Increasing the optical efficiency of the collector

( )( )aba

asurf abor

sol

usesol

I A

T T I CR A

Q

Q

,

44

, −−

== εσ α

η

( )abg

asurf

opt sol

I CR

T T

,

44 −−=

εσ α η η



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Example 2: Efficiency

What concentration ratio is needed to operate asolar collector at 500°C with 75% efficiencyunder nominal conditions:

T a = 25°C, AM = 1.5 (i.e. 850 W/m2)

The optical efficiency is 90%, and the absorbercan be considered as a black-body.








( )abg

arecopt sol

I CR

T T

,

44 −−=

εσ α η η

N.B σ = 5.67e-8



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Example 2: Efficiency

What concentration ratio is needed to operate asolar collector at 500°C with 75% efficiencyunder nominal conditions:

T a = 25°C, AM = 1.5 (i.e. 850 W/m2)

The optical efficiency is 90%, and the absorbercan be considered as a black-body.

• CRg = 159






σ: Stefan-Boltzmann cst.Ib,a : beam irradiation

( )abg

arecopt sol

I CR

T T

,

44 −−=

εσ α η η

N.B σ = 5.67e-8

( )( ) abopt sol

arecg

I

T T CR

,

44

αη η

εσ

−

−=

ε = α = 1



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Collector Efficiency


The strongest parameter influencing the efficiency of the solar collectoris the concentration ratio of the system

( )abg

arecopt sol

I CRT T

,

44

−−= εσ α η η


Example Graph has following data:

I b,a = 850 W/m2, nopt = 0.9, ε =α=1.00



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Power Cycle Efficiency



Receiver efficiency decreases at higher temperatures

However, the efficiency of the power conversion equipment increaseswith temperature, with the limit set by the Ca r n o t e f f i c ie n c y :

Trade-off -> optimum?

rec

arecreccar

T

T −=Θ= 1,η


I b,a = 850 W/m2, nopt = 0.9, ε =α=1.00



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System Efficiency



Can combined the efficiencies to get the system efficiency:

For each concentration ratiothere exists an o p t i m u m operating temperature…


I b,a = 850 W/m2, nopt = 0.9, ε =α=1.00

( )

−

−−=Θ=

rec

a

abg

arecopt recsolsys

T

T

I CR

T T 1

,

44εσ α η η η



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Power Generation Cycles



The choice of which power generation cycle to use is closely linked tothe level of temperature that is achieved

Three main cycle types are considered: Rankine, Stirling and Brayton


I b,a = 850 W/m2, nopt = 0.9, ε =α=1.00



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Part II: Solar Thermal Power Plants

Solar Power Technologies



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SEGS Power Plants

First modern solar thermal power plants, the Solar EnergyGenerating Systems (or SEGS) were built in California in the 1980s

Initial built to hedge against high oil/gas prices after the oil crises ofthe 1970s


SEGS 3-7, Kramer Jct.

Parabolic Troughs

Mirror Washing



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SEGS Power Plants

First modern solar thermal power plants, the Solar EnergyGenerating Systems (or SEGS) were built in California in the 1980s

Initial built to hedge against high oil/gas prices after the oil crises ofthe 1970s

A total of 354 MW of capacity was installed over a period of 6 years


Output Collector Field Storage Temperature Oil Type Location Completed

SEGS 1 14 MWe 82’960 m2 3 h 307°C Mineral Daggett 1984

SEGS 2 30 MWe

165’380 m2 - 316°C Mineral Daggett 1985

SEGS 3, 4, 5 30 MWe 230’300 m2 - 349°C Synthetic Kramer Jct. 1986, 86, 87

SEGS 6, 7 30 MWe 191’140 m2 - 391°C Synthetic Kramer Jct. 1988, 88

SEGS 8, 9 80 MWe 474’160 m2 - 391°C Synthetic Harper Lake 1989, 90

Data Source: National Renewable Energy Laboratory, 2004



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g g

Recent CSP Deployment

Solar thermal power development began in the 1980s with the SEGS

A new “solar renaissance” started in 2006 with new power plants




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g g

Spanish Solar Renaissance

In 2004 a royal decree equalized conditions for CSP and PV plants

Feed-in tariffs for solar energy were guaranteed, removing someeconomic barriers to the deployment of solar thermal technology

By early 2012, over 1’000 MW of solarthermal power had been deployed

Another 1’200 MW are currently underconstruction

Over 90% of all CSP plants built are ofthe parabolic trough type


Image Source: Protermosolar, 2011



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g g

Commercial Solar Plants


Solnova 1, 2 & 4

Commercial solar thermal power plants in Spain:

Andasol 1, 2 & 3

PS 10 & 20

Puerto Errado II



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g g

Parabolic Trough Plants


Over 90% of all installed solar thermal power plants are basedaround the use of parabolic troughs with Rankine-cycles

The technology was well-proven, making it easier to obtain fundingwhen the second wave of CSP construction started

• Continuous operation of the SEGS plants since 1984However, limited innovation in commercial plants…



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Types of Trough Plants


Two main types of parabolic trough plant have emerged:

• ‘SEGS-type’: daytime-peaking, no storage

• ‘Andasol-type´: dayload and evening peak, with storage

Power-plants based around standard steam-cycle technology

• Compatible temperature levels betweensolar collector and power block

• Lower risk: well understood technology

Plant design strongly affect by local regulationand incentive measures:

– USA: loan guarantees and tax credits

– Spain: limited to 50MW power block

» limited to 13% fossil co-firing



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SEGS-Type Power Plant


Designed primarily to meet midday peak electricity demands

Reheat steam cycle used to allow higher cycle efficiency at the lowsteam temperatures

• Operating temperatures limited by heat transfer fluid

Thermal Oil HTF-System

Medium: Therminol-72

Thermal Stability: 400°C

Power Block

Reheat Rankine-cycle

Steam Temperature: 390°C

Steam Pressure: 100 bar



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Andasol-Type Power Plant


Designed to meet two daily peaks, midday and early evening

Thermal energy storage tanks used to harness extra energy duringdaily hours, allowing production to be extended in the evening

• Larger solar field required to charge storage tanks

Molten-Salt Storage

Medium: NaNO3-KNO3Thermal Stability: 580°C

Power Block

Reheat Rankine-cycle

Steam Temperature: 390°C

Steam Pressure: 100 bar



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Molten-Salt Storage System


Thermal energy storage based on molten salts adds complexity to thesystem, as three separate fluid loops are required

Thermal Oil Molten Salt Water/Steam

Thermal Oil

Good heat transfer

Low freezing point

No phase-change

Molten SaltHigh heat capacity

Pre-available product

Inexpensive

Chemically inert



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Molten-Salt Storage System


Thermal energy storage based on molten salts adds complexity to thesystem, as three separate fluid loops are required

Complexity is outweighed by reduced cost and increased safety!

Image Source: L. Hartley, 1999

SEGS FireOriginal oil-based storage

Damaged in fire

Never replaced

Parabolic Troughs

HTF Heaters



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Parabolic Trough Plants


Over 90% of all installed solar thermal power plants are basedaround the use of parabolic troughs with Rankine-cycles

HTF Headers

Collector Arrays

Molten Salt

Storage Tanks

Power Block

bl h l d l



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Parabolic Trough Collector


A large number of different parabolic collector designs have beenproposed but all share a similar structure

Parabolic MirrorAbsorber Tube

Support Structure

Drive Pillar

Flexible Joint

Intermediate Pillar

MJ2411 R bl E T h l C t t d S l P



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Parabolic Trough Collector


A large number of different parabolic collector designs have beenproposed but all share a similar structure

The central drive pillar provides tracking power and control for theentire solar collector assembly

Tracking device uses a PV-cell sensor to align the shadow created bythe central tub receiverParabolic Mirrors PV Panel

Tube Receiver Drive Axis

MJ2411 R bl E T h l C t t d S l P



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Alternative Trough Collectors


In addition to the conventional single-axis tracked parabolic troughcollectors, more advanced designs have been proposed

MAN Dual-Axis Tracking Collector

Azimuth/elevation trackingRemoves incidence angle losses

Increased power input

Increased cost/complexity did not

compensate for added power

-> Current focus mainly on increasing

aperture and reducing structural cost

MJ2411: Renewable Energy Technology Concentrated Solar Power



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Conventional Trough Evolution


There has been a steady evolution in the design of parabolic troughsfor the typical commercial solar thermal power plant

General trends towards increased aperture and length

• Reduction in end-losses as well as drives and tracking!

LS-1 LS-2 LS-3 Eurotrough HeliotroughUltimate

Trough

Aperture 2.5 m 5 m 5.8 m 5.8 m 6.77 m 7.8 mUnit Length 6.3 m 12m 15 m 12 m 19 m 24 m

SCA Length 50.4 m 48 m 99 m 148.5 m 191 m 242 m

Active Surface 128 m2 235 m2 547 m2 820 m2 1’263 m2 1’813 m2

Early LUZ Designs (1980s) Recent EU/FLABEG Designs (2000s)




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Parabolic Trough Receiver


The receiver tube placed at the focal point is a composite tubestructure consisting of different layers

• Stainless-steel tube covered with absorptive coating

• Glass envelope to reduce heat losses from tubes

Cermet coating: ε = 0.14, α = 0.97 (@400°C)





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Solar Tower Power Plants


A much wider array of power plant concepts is encountered whenconsidering solar tower (central receiver) systems

• No standard or “optimal” plant concepts has yet emerged

• Competing concepts between technology suppliers

Four main solar tower system concepts:

• Direct steam generation (solar boiler concept)

• Molten salt tower with direct thermal storage tanks

• Volumetric air receiver with packed-bed storage tanks

• Pressurised volumetric receiver with hybrid gas-turbines




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Solar Tower Concentrator



Heliostats

Solar Receiver

Power Block




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Heliostats


Image Source: Southern California Edison Co., 1982

A heliostat is a Sun-tracking mirror, mounted on a dual-axis trackingsystem, allowing it to be positioned freely and direct the solar flux

A number of issues must be addressed duringdesign of a heliostat:

• High reflectivity

• High optical precision

• High tracking accuracy

• Resistant structureAll of these serve to maintain a high optical

efficiency of the collections system:

ηopt T max




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Heliostat Designs


Currently, each solar tower power plant has had its own heliostatdesign, each with advantages and disadvantages




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Trough vs. Tower Collector


Both trough and tower plants typically operate using Rankine steam-cycles but differ in a number of key aspects

Both are capable of utility scale but trough plants can be larger

• Largest plant under construction: Solana, 280 MWe (trough)

Parabolic Trough Central Receiver

Heat Collection Modular Centralised

Energy Transfer Heat via HTF circulation Light

Max. Size Almost unlimited Limited by efficiency

of heliostats furthest

from the tower

Temp. Limited By Heat transfer fluid Receiver materials




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gy gy

Solar Tower Power Plants



• No standard or “optimal” plant concepts has yet emerged

• Competing concepts between technology suppliers

Concept Power Plants Size Receiver Conditions Storage Status

Direct Solar Steam PS 10 &20

eSolar Tower

Ivanpah

11/20 MWe5 MWe131 MWe

265°C / 40 bar

440°C / 60 bar

550°C / 165 bar

Steam buffer

N/A

Salt tanks (opt.)

Operational

Operational

Under construction

Molten-Salt Tower GemasolarTonopah 20 MWe

110 MWe

565°C

550°C

Salt tanks

Salt tanks

Operational

Planning

Volumetric Air Jülich Tower 1.5 MWe 680°C Packed-bed Operational

Pressurised Air AORA Solar 100 kWe 1000°C N/A (hybrid) Operational




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gy gy

PS10/20 Solar Tower Plants


The first of the new generation of solar power plants to be built wasthe PS10 direct steam solar tower

Low-temperature (265°C) saturated-steam receiver demonstrated

Saturated Steam

Power Block

Heliostat

Field

Receiver

Tower Steam

Buffer

Turbine Capacity: 11 MWeStorage Capacity: 0.5 hrs




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PS10/20 Solar Tower Plants


The first of the new generation of solar power plants to be built wasthe PS10 direct steam solar tower

Low-temperature (265°C) saturated-steam receiver demonstrated

Power Block

Heliostat Field

Steam

Buffer

Central

Tower




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The Ivanpah Solar Complex


The next generation of direct steam solar thermal power is underdevelopment in California by the Brightsource company

The Luz Power Tower (LPT) technology allows production ofsuperheated steam at 550°C, efficiently driving steam turbines

Turbine Capacity: 131 MWe

Storage: optional molten-salt tanks

steam-salt heat exchange




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The Ivanpah Solar Complex


The next generation of direct steam solar thermal power is underdevelopment in California by the Brightsource company

When completed in 2013, the Ivanpah complex will total 393 MWe




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Gemasolar Power Plant


Molten-salt tower present the possibility to reach significantly highersteam temperatures than conventional parabolic trough

Compared to direct steam tower, molten-salt technology allowsintegration of large-scale thermal energy storage

Salt Storage Tanks

Reheat Steam

Power Block

Turbine Capacity: 20 MWeStorage Capacity: 15 hrs




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Gemasolar Power Plant


Molten-salt tower present the possibility to reach significantly highersteam temperatures than conventional parabolic trough

Heliostat Field

Central Tower

Solar Receiver

Storage Tanks

Power Block




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Jülich Solar Tower Plant


Volumetric air technology allows solar heat to be harnessed at evenhigher temperatures as air is chemically very stable

Air is a poor heat transfer fluid, so the volumetric concept is used toovercome this and provide a large surface area for absorption

Packed-bed Storage

Conventional HRSG-Type Boiler

1,5 MWe Turbine

Superheated Steam




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Volumetric air technology allows solar heat to be harnessed at evenhigher temperatures as air is chemically very stable

Air has a low thermal energy density so direct storage of air isuneconomical and cumbersome

• Regenerative storage technology allows storage in a secondmedium with better thermal properties and later recovered




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Volumetric air technology allows solar heat to be harnessed at evenhigher temperatures

Small plant size allows integration of entire plant into thecentral receiver tower -> efficient use of space

Volumetric Receiver

Storage Tank

Power Block

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Solar 2 - Solar Power Plants