02_Collectors and Concentrators (4)

7/28/2019 02_Collectors and Concentrators (4)

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Technische Universitt Mnchen

2nd Chapter Collectors andconcentrators

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1. General Overview

2. Parabolic trough collector

3. Fresnel collector

4. Heliostat collector

5. Paraboloidal collector

6. Solar tower collector

Content


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1. General Overview

Parabolic trough collector

Fresnel collector

Heliostat collector

Paraboloidal Collector

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1. General Overview

Parabolic trough collector

Fresnel Collector

Heliostat collector

Paraboloidal Collector

Temperature: 50-400 CAbsorber efficiency: -Costs: +Concentration factor: 15-45

Status: matureTracking: 1-Axis

Temperature: 300-2000 CAbsorber efficiency: +Costs: -Concentration factor: 150-1500

Status: developmentTracking: 2-Axis

Temperature: 50-300 C

Absorber efficiency: -Costs: ++Concentration factor: 10-40Status: matureTracking: 1-Axis

Temperature: 150-1500 C

Absorber efficiency: +Costs: --Concentration factor: 150-1500Status: developmentTracking: 2-Axis

3Note: The concentration factor C is based on the absorber perimeter!


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1. General Overview

State of the art Technology Comparison

Source: www.nrel.gov

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Numberofp

rojects


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2.1 Reflector Mirror Geometry

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2.1 Reflector

Defocusing of collector :

Effect of wind on large mirror surfaces

Bending due to wind influence

Solution

Reinforcement of the support structure

Reinforcement of the mirrors by using fibreglass

vWind

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2.1 Reflector Mirror surface

Source: www.flabeg.de

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2.1 Reflector Mirror surface

Mirror cleaning

Necessary to maintainreflectivity

Non-contact methods

In order to avoid scratches

Methods

High-pressure process

Deionized water

Use of detergents

Source: Alpert 1990

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2.1 Collector field

Collector field

The outer regions are exposedto larger wind forces than the

innter ones, which leads to

different designs

95 % of the collector segmentsin the interior are of alightweight construction

2.5 % of the collector elementin transition section

2.5 % the collector element inthe outer zone consists ofreinforced construction

Source: DLR-Forschungsbericht 2001-10

vWind

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2.2 Receiver

Construction

Consists of?

A black metallic tube (Absorber)

With a selective coating

High absorption and low emission

Surrounded by a glass envelope (minimization of convectivelosses)

Anti-reflective coating

Increase in transmission

Vacuum between the glass envelope and the absorber

Reduction of convective losses

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2.2 Receiver Coated glass

Anti-reflective coating

Solar glass

High transmissivity

But: 8-9 % reflective losses

Reduction by means of

Anti-reflective coatings

Source: Wang, 2010

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2.2 Receiver Vacuum

A B C

Pressure [mbar]

Heattransport

Source: www.bmgs.info

A: RadiationB: Radiation + conductionC: Radiation + conduction + convection

Coarse Vacuum 1000-1 mbar

Medium Vacuum 1-10-3 mbar

High Vacuum (HV) 10-3 10-7 mbar

Ultra high vacuum (UHV) < 10-7 (upto10-11) mbar

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2.2 Receiver Vacuum

Operational Problem: O2/N2 and H2 are diffusing into the vacuum

Getter

A getter is a chemically reactive material (e.g. barium,titanium, platinum), in order to complete and maintain the vacuum in

an enclosed space

Application

Getter materials are applied during manufacturing

Activation of getter material by means of heating

Vapours absorb the remaining molecules

Gas molecules are retained by chemisorption or physisorption

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2.2 Absorber

Source: LIU, 2011

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2.3 Steam generation concepts

Why direct steam generation?

No limitations in temperature (compared to heat transfer oil)

No cost-intense components from the HTF-cycle needed

Heat exchanger (HTFWorking medium)

Pumps

Operating costs

No losses due to heat transfer

Terminal temperature difference of the heat exchanger (HTF = working

medium)

Simple System

Low maintenance costs

High thermal efficiency

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Direct steam generation: Once Through

Location for saturated steam changes

Thermal stresses on the components are inhomogeneous

Difficult control strategy

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Once Through


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Direct steam generation: Recirculation

Location for saturated steam is fixed (drum/phase separator)

Thermal stress on the components is homogeneous

Easy control strategy

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Recirculation


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Direct steam generation: Injection cooling

Position of saturated steam can be influenced (by means ofinjections)

Thermal stress on the components is homogeneous

Difficult control strategy

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Injection cooling


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Challenges in Direct steam generation (1)

Higher steam parameters

Higher temperatures New materials needed

Higher pressures Higher wall thicknesses needed

Higher total weight Reinforced construction

Still necessary: Flexible connection between absorber and collector

Evaporation

Horizontal arrangement Phase seperation Tilts in thermal loads

Tracking of mirrors vs. gravity Tilts in thermal loads

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Challenges in Direct steam generation (2)

Fluid flow

Heat transfer for superheating defines a

maximum flow rate

Long Tubes

Steam plugs in regions with local superheating

Control is only possible by mass flow

Defocussing of mirrors should be avoided because of economicaspects

System reacts very slowly

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2.4 Model: Heat transfer (Sun Fluid)

0

A

x

Reflector

Absorber

Solar radiation

Absorber tube

100%

81%

94%

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2.4 Models Thermodynamic absorber model

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2.5 Solar field

Tracking

1-axis tracking

East-West

North-South

East-West tracking More thermal energy/year

North-South tracking

More thermal energy/summer

Generally, an East-West tracking isfavoured

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3.1 Fresnel lens

Source: Wikipedia

Fresnel-Lens Convex-Lens

Fresnel lenses

plain approximation of a convex lens with Lower weight and a

Comparatively poor image quality

Used in

Light fires in light houses Spotlights

Overhead projectors

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3.1 Fresnel lens

Classifcation of Fresnel lens:

Imaging

Non-imaging Lens

Advantages of non-imaging lenses

Shorter focal point

Larger tolerances in geometry

Low volume

High concentration

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3.2 Fresnel collector

Properties of a Fresnel collector

Reflector

Modular design

Low production cost

1 or 2-axis tracking possible

Receiver

Height: 15 m

No Integration into the Reflector

Tracking is simple, exact and precise

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3.2 Fresnel collector

Properties of a Fresnel collector

Problem: Mutual mirror shading

Solution

Larger distances between mirrors

Height of the receiver

d h

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4. Heliostat collector Tower plant

Source: www.epochtimes.de

Heliostat field

Heliostat

Solar tower

Receiver/Absorber

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4.1 Heliostat Concepts

Stretched-membrane heliostats Glass-mirror heliostats

Source: ZAE-Bayern

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Glas-Mirror heliostat

Proven technology

Structure of the mirrors similiar to aparabolic trough

Simple construction Few different parts

Mass production is possible

Heliostat surface up to 150 m

Two geared motors for trackingnecessary

Source: Power from the sun

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Steel ring

Pressure

controlledvolumeFront membrane

with coating

Rearmembrane


Membrane heliostat

Light weight construction

Less material required

Membrane

Metal foil on the top and the bottom

Reflective coating on the top

Vacuum is used between the twomembranes to focus/defocus the

heliostat

Still in development stages

Source: Power from the sun

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4.1 Heliostat Tracking

Concepts for 2-Axis tracking

2-Axis tracking for each heliostat

High costs

For motors and

Gear box Wind effects on the edges of solar fields

Consideration in design

Optimization goal Reduction of the amount of motors and

gear boxes

Optimization in construction

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4.2 Solar field

Concepts of Solar fields

Unilateral Heliostat field Circulating HeliostatSuns orbit

Heliostat field

Solar tower

Suns orbit

Heliostat field

Solar tower

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4.2 Solar field

Unilateral heliostat field

Examples: PS10 and PS20

Circulating heliostat field

Examples: Torresol and Solar 2

Structure of Solar fields

Source: wikipedia Source: ME News Wire

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4.3 Receiver

External receiver Cavity receiver

Receiver Concepts

Source: Sener Source: PS10 Final Report

Source: Bader 2010

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4.3 Receiver

External receiver Cavity receiver

Receiver Concepts

+ Simple construction+ Large angle of aperture

360

+ No isolation necessary

+ Modular design

- High radiative and convectivelosses

- Temperature limitations due tomaterial

+ Low radiative and convectivelosses

+ High temperature

resistant materials can be used

- Complex constuction

- Small angle of aperture

- Isolation necessary

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4.3 Receiver


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4.3 Receiver


Volumetric Receiver

Constructive problems with receivers

Cracks during the heat-up phase

Up-scaling to areas >100m

Development of HiTRec Receivers

Ceramic material

SiC or SiSiC

High absorption

Thermal shock resistant

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4.4 Losses in Receiver

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Types of losses in Mirror field (1)

Blocking

Optical path between heliostat and the receiver is blocked

Shading

Optical path between heliostat and the sun is blocked

Cosine losses

Sun rays are not perpendicular to the surface of the heliostat

Losses in the optical path

Scattering of light with particles and water vapour

Mirror defects

Aberration due to manufacturing defects


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4.4 Losses in Receiver

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Types of losses in Mirror field (2)

Orientation error

Depends on tracking and control

Wind loads on the mirror

Wind loads cause deflection of the heliostats

Tower motion

Due to thermal expansion

Reflective losses

Depending on the quality of the mirror surface


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5.1 Reflector

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5.1 Reflector

Solar-dish reflector

Disadvantages

Construction is expensive because

High precision is necessary for reflectors

2-Axis tracking Heavy weights in the center (engine in the focus of parabola)

Optimization and new research approaches

Modularization of the parabolic shape

Cavity receiver with absorber tubes inside Mini dish with fiber optics for directing the light

Heat pipe receiver

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5.2 Electricity generation

Electricity generation

Electricity production

Central

Several collectors are connected together via a HTF

Engine is central Decentral

Collector as a stand-alone machine

Heat engine in the focal point

Heat dissipation must be ensuredEngine

Stirling (< 950 C)

Air turbine or combined cycle

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6. Solar chimney power plants

Concept of solar chimney thermalpower plant

View from inside the collector

Source: www.except.nlSource: www.climatetechwiki.orgl

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6.1 Chimney effect

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6.2 Solar Chimney - Construction

Collector

Transparent cover

Transparent for sunlight

Absorption of light by

Ground Coating

Storage of heat

Tanks

Heat exchangers in/on the ground

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02_Collectors and Concentrators (4)

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