Main Design of Solar Thermal Power Plants 1st Edition 2019

Design of Solar Thermal Power Plants 1st Edition 2019

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DESIGN OF SOLAR
THERMAL POWER
PLANTS
ZHIFENG WANG

Academic Press is an imprint of Elsevier
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Notices
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and experience broaden our understanding, changes in research methods,
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Practitioners and researchers must always rely on their own experience and
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This book is applicable to the construction design of new solar thermal power
plants and existing facility expansions that use water and steam working
medium. Steam pressure parameters include secondary MP (medium pressure),
MP, and secondary HP (high pressure), the nominal evaporation capacity
corresponding to the output power of the receiver is 8e800 t/h, and the capacity of
the condensing steam turbine is 1e100 MW.
Heat transfer medium for solar collector fields in the book can be water/steam,
synthetic oil, molten salt, air/ceramic, etc. Concentration modes include parabolic
trough, power tower, and Fresnel reflectors for concentrated solar thermal
power plants.

v

Preface
SOLAR THERMAL POWER PLANT DESIGN
Since the beginning of China’s research on solar thermal power (also
known as concentrating solar power, CSP) generation in 1996, CSP generation technologies have gone through the entire development process
from inception to realization. Ever since “the 11th Five-Year Plan,” CSP
generation technologies have enjoyed rapid development throughout
the country, with the appearance of a large variety of experimental solar
thermal collection and storage systems and experimental power plants.
Breakthroughs have also been made in the manufacturing techniques of
core components and materials; professional CSP generation equipment
manufacturers have also appeared. The first China state standard for
CSP technology was released in September 2011. Along with a further
deepening of technologies during “the 12th Five-Year Plan” as well as
commercial development, this reference book for CSP plant design has
become a requisite work for understanding solar power plant commercialization. Currently, there is no other reference book in the world that
systematically describes the design methods of CSP plants.
The book mainly focuses on CSP technologies, and those mainly
consist of power tower and parabolic trough collector technologies and
thermal storage. The book not only describes the design of CSP systems,
but also explains the design methods and operation modes of key facilities in detail, such as the heliostat, heliostat field, parabolic trough
concentrator, parabolic trough receiver tube, and whole-plant distributed
control system, or DCS. It also discusses the fundamental basis of
designing CSP plants and system as well as the key design points that
should be considered.
CSP generation design mainly includes resource evaluation, site selection, design of the optical efficiency of the concentration field, thermal
control of the receiver and electrical design, thermal storage capacity, thermal storage charging and discharging design, heat exchanger and evaporation design, whole-plant electrics, whole-plant thermal control
instruments, power plant construction, and whole-plant security design.
By focusing on the contents just mentioned, the book offers calculation
and design methods separately in different chapters and provides

xi

xii

PREFACE

examples by integrating with practices of the author, so as to facilitate the
understanding of readers.
Solar irradiation serves as the basis for solar power utilization. The
evaluation of solar resources used by CSP generation is the most fundamental process for solar power plant design and site selection. Although
it has been stated in numerous articles, the author further discusses solar
resource evaluation by integrating his own research, especially stressing
its relationship with the site selection of thermal power plants.
Solar concentrators, solar receivers, and thermal storage are three major core components for CSP systems. The section of the book that discusses these topics mainly explains equipment application, evaluation
methods for equipment performance, and the thinking and methods for
equipment design.
In equipment performance evaluation, great difficulties have been
encountered in the facula error analysis of the heliostat. The book offers
mathematical approaches and test methods corresponding to facula error
analyses of various types of heliostats, which basically have been derived
from the research achievements of the author and his graduate students.
Heat loss of receivers is the subject at the core of receiver design, with a
variety of analysis methods. The book offers a relatively simple calculation method and corresponding examples.
The book was preliminarily defined as a reference book similar to a
design manual by specifically referring to China’s state standard of
“Code for Design of Small-size Thermal Power Plant” (GB 50049), which
may be recognized by readers from its general arrangement. However, as
writing progressed, the author discovered that books about and references for CSP generation technologies had rarely been seen in China,
while many methods were still under development and evolution.
Thus, it was determined that the major concepts and methods should
be explained more explicitly to benefit readers. By introducing these descriptions and analyses, the book is easier to be understand and greater
reference value as well. These gains were made by sacrificing its original
style as a “design manual.”
The book was mainly composed by Wang Zhifeng with coauthors Guo
Minghuan (2.4, 3.2.1, 3.2.2, 3.2.3, 4.2.2, 4.2.3), Li Xin (5.5), Yu Qiang (2.6),
Gong Bo (4.7), and others.
The book was summarized and made by the author, his colleagues and
students, and the coauthors based on multiple years of research and engineering practice in CSP technology. It is written by following the principles of sharing his own “fruits planted by himself” with readers and
thus striving for fewer examples extracted from the achievements of
others. The writing work started at the beginning of 2010 with a hope
that the book could be finished within a year and be capable of catching
up to “Chinese National 11th Five-Year Plan” 863 Program project

PREFACE

xiii

acceptance at the end of 2011. However, additional progress in research
work, especially steam production by the Beijing Badaling power tower
solar power plant in July 2011, its power generation in August 2012,
and its gradual commissioning, the author’s knowledge of CSP generation technologies deepened. The author discovered that content that
was as useful as possible would not be sufficient without the author’s
original theories and experiments. Particularly, CSP generation technologies still remain at the development stage, and many basic concepts and
terms have been expressed in diversified ways in the articles and writings
of different world-renowned scholars, such as the most important concept
in power plant design, namely the “design point.” As for similar major
content, the author expresses his own opinions in the book by conducting
in-depth theoretical and experimental research. Therefore, after 5 years,
this book can finally be dedicated to its readers. Henceforth, along with
the deepening of research work, the book may be modified regularly so
that upgraded “achievements”dfor example, improvements in the
design and operation mode of the thermal storage unitdcan be dedicated
to readers and the industry as well.
Special thanks should be given by the author to Mr. Xu Jianzhong,
Academician of the Chinese Academy of Sciences, and Mr. Huang
Ming, Chairman of Himin Solar Energy Group Co., Ltd., who have
made every endeavor possible, for more than a decade, to support the
author in conducting research on solar thermal power generation and
its practices; meanwhile, the author is also grateful for care and support
from his family. During research on solar thermal power generation
and its practices, the author has been deeply grateful for tremendous support from the national “863 Program” (2006AA050100), (2006AA050100),
“973 Program” (2010CB227100), “the National Natural Science Fund of
China,” “Beijing Municipal Science and Technology Project,” “Knowledge
Innovative Project of CAS,” “International Cooperation Program of the
Ministry of Science and Technology,” and the “6th and 7th” Framework
Programmes for Research of the European Union.
The book assimilates the development experiences and essence of CSP
technologies both at home and abroad and provides them to readers. Yet
due to the author’s limited knowledge, as well as insufficient practices in
CSP plant R&D and construction, many imperfections may exist. It would
be greatly appreciated if readers could provide the author with critiques
and corrections of these imperfections for use in future editions.
Zhifeng Wang
March, 2018

Contents
Preface

xi

1. Introduction

1.1 General Principles of Solar Thermal Power Plant Design
1.2 Brief Introduction to Solar Thermal Power Generation

2. The Solar Resource and Meteorological Parameters
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10

The Nature of the Solar Resource
The Solar Constant and Radiation Spectrum
Atmospheric Influences on Solar Irradiation
Calculating Methods for Solar Position
Distribution of the Solar Resource in Several Typical Areas of China
Solar Irradiance Prediction Methods
Distribution of Solar Direct Normal Radiation Resources in China
Various Special Climate Conditions in the Plant Area
Measuring Instrument
Global Direct Normal Irradiance Distributions

3. General Design of a Solar Thermal Power Plant
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10

Power Plant Design Point
Heliostat Field Efficiency Analysis for Power Plants
Thermal Performance of Parabolic Trough Collector
Basic Data Required by Power Plant Design
Major Parameters and Principles of Design
Description of General Parameters of the Power Plant
Calculation of Annual Power Generation
Determination of Thermal Storage Reserve
Main Points for Power Plant Site Plan
Notices for Concentration Field Layout

4. Design of the Concentration System
4.1
4.2
4.3
4.4
4.5
4.6
4.7

General System Description
Principles for Concentration Field Layout
Design of the Solar Tower Power Plant Concentrating Field
Control Design of the Heliostat Field of a Solar Tower Power Plant
Solar Field Design of Parabolic Trough Power Plant
Description of solar Concentrator
Instantaneous Efficiency

vii

3
7

47
48
50
52
60
83
89
100
107
112

117
119
150
192
193
194
194
214
216
223

225
229
243
256
264
273
276

viii

CONTENTS

4.8 Design of the Parabolic Trough Collector Field
4.9 Concentrator Field Control Design of the Parabolic Trough Power Plant
4.10 Wind Load Characteristics of the Concentrator

5. Design of the Receiver System
5.1
5.2
5.3
5.4
5.5
5.6
5.7

General Receiver System Description
Selection of Materials for the Receiver System
Selection of Pipes and Pumps for Receiver System
Receiver System Control
Design of the Operation Modes of the Receiver System
The Discharge System and Equipment of the Receiver
Vacuum Performance of the Parabolic Trough Receiver Tube

6. Thermal Storage Systems
6.1
6.2
6.3
6.4
6.5
6.6

General System Description
Technical Requirements of Thermal Storage Systems
Thermal Storage Materials and Modes
Categories and Constitutions of Thermal Storage Systems
Selection of Thermal Storage Materials and Tanks
Charging and Discharge Equipment of the Thermal Storage
Tank and Respective Process Design
6.7 Thermal Storage System Control
6.8 Facilities for Thermal Storage System Inspection

7. Site Selection, Power Load, and Power Generation Procedures
7.1 Site Selection
7.2 Power Load and Power Generation Procedures

8. Plant Layout Planning
8.1
8.2
8.3
8.4
8.5

Basic Rules
Layout of the Main Buildings and Concentration Field
Communication and Transportation
Vertical Layout
Pipeline Layout

9. Main Powerhouse Layout

9.1 Direction of Main Powerhouse
9.2 Main Powerhouse and Thermal Storage
9.3 Solar Thermal Storage System Layout

10. Water Treatment Equipment and System

10.1 Receiver and Evaporator Makeup Water Treatment
10.2 Calculation of Water Treatment Equipment
10.3 Feed Water and Boiler Water Modification and Thermal
System Steam Sampling
10.4 Anticorrosion

279
281
283

319
327
330
331
334
336
339

388
392
393
401
412
413
414
415

417
423

425
428
429
430
432

435
435
435

437
437
438
438

CONTENTS

11. Power System

11.1 Power Grid Connection of Power Plant
11.2 System Protection
11.3 System Communication

12. Electrical Equipment and System
12.1
12.2
12.3
12.4
12.5
12.6
12.7
12.8
12.9

High-Voltage Electrical Installations
Main Electrical Control Room
DC System
Electrical Measuring Instrument
Relay Protection and Automatic Safety Device
Lighting System
Cable Selection and Layout
Overvoltage Protection and Grounding
Electrical Installations in a Dangerous Environment with Potential
Explosions and Fire Hazards

13. Thermal Automation
13.1
13.2
13.3
13.4
13.5
13.6
13.7
13.8
13.9
13.10
13.11
13.12

Basic Rules
Control Mode
Thermal Inspection
Automatic Adjustment
Thermal Protection
Interlocking
Power Supply and Steam Supply
Control Room
Cables, Conduits, and Local Equipment Layout
Basic Rules for Building Space Heating
Solar Tower
Heating Network and Heating Station in the Plant Area

14. Architecture and Structure
14.1 Basic Rules
14.2 Fire Protection
14.3 Interior Environment

15. Auxiliary and Affiliated Facilities
16. Environmental Protection of the Concentrating Solar
Power Plant
16.1
16.2
16.3
16.4

Basic Rules
Requirements for Environmental Protection Design
Pollution Prevention and Treatment
Environmental Protection Facilities

References
Index

ix
439
439
439

441
441
441
442
442
442
443
443
443

445
446
446
446
447
447
447
448
448
449
449
450

451
452
453
455

457
457
458
458

459
463

C H A P T E R

1

Introduction
Since the beginning of the 21st century, energy and environmental
problems have become increasingly more noticeable. Due to limited
nonrenewable fossil energy resources and the severe influences of
excessive use of these resources on the environment, excessive greenhouse gas emissions, global warming, and severe deterioration of regional
climates and ecological environments appeared and have greatly endangered the living space of humans. The prominent advantages and
development potential of concentrating solar power (CSP)dalso known
as solar thermal power (STP) or concentrated solar powerdgeneration
technology have aroused widespread concern. The main challenge it faces
right now is to reduce its power generation costs so that it can compete
with fossil fuels. In the next two decades, it is estimated that stable and
economic CSP generation technology will gradually mature and become
strongly competitive commercially. CSP generation technology features
stable and constant power output, low costs, and outstanding technical
and economic advantages; the development strategy of this technology is
of great significance.
The basic process of CSP generation involves concentrators, receivers,
thermal storage, thermal power conversion, etc. Thermodynamics, heat
transfer, optics, mechanics, materials science, information science, and
many other disciplines and interdisciplinary studies serve as the theoretical
foundation of CSP generation technologies. Only by mastering these key
technologies can we greatly improve the efficiency of the system and
further reduce power generation costs so that we may further push forward its large-scale commercialization and development, and realize the
effective utilization of solar energy.
For power plant design and operational targets, the following two
questions are key points to be solved in terms of CSP research and engineering; they are also the main contents of this book:
1. Optical efficiency and cost of the concentrator. High-density
concentration of solar irradiation acts as the basic process of CSP
generation. Concentrator costs in solar tower and parabolic trough
Design of Solar Thermal Power Plants
https://doi.org/10.1016/B978-0-12-815613-1.00001-8

1

Copyright © 2019 Chemical Industry Press.
Published by Elsevier, Inc. under an exclusive license
with Chemical Industry Press. All rights reserved.

2

1. INTRODUCTION

systems account for 45%e70% of the primary investment; the annual
mean efficiency of a concentration field is normally 58%e72%, so
research on the concentration process greatly influences the efficiency
and cost of the system.
Energy losses in the concentration process mainly include cosine,
reflection, air transmission, and receiver interception losses caused by
concentrator errors. In addition, the limits of working environmental
conditions and concentrator shelf life, while still ensuring
concentrator precision, mean that concentrator cost reductions now
face great restrictions. Considering both of these factors, it is
necessary to carry out in-depth research on the collection of optical
energy and high-precision concentration using aspects of optics,
mechanics, and materials science and overcome the influences of
concentrator mirror shape aberration and tracking errors on energy
flow transmission efficiency as well as the problem of low CSP system
conversion efficiency caused by spatial and temporal distribution of
the energy flux failing to satisfy the requirement of receiver; an
integrated design method of solar beam concentration and thermal
absorption based on the highly efficient energy flow transmission
must be established.
2. CSP system conversion efficiency and reliability of devices. When
the efficiency of an CSP system is increased by 1%, the levelized
cost of electricity from CSP generation will decrease by 8%, and the
corresponding total capital investment will be reduced by 5%e6%.
System efficiency has significant impacts on CSP system costs.
Future technical developments shall be mainly based on stable
operation of the system, improvements in system efficiency, and
development of major technical equipment techniques, system
integration techniques, equipment performance evaluation
methods and their respective testing platforms, technical
standards, and regulations in the large-scale CSP generation
system. Conventional thermal power conversion efficiency was
improved along with increases in the parameters of the working
medium, and the basic approach to improving the efficiency of the
cycle was to increase the temperature and pressure of the working
fluid. During CSP generation, however, the efficiency of solar
receiver system conversion decreases with increases in the
temperature of the heat transfer medium, which is also
accompanied by intensive unsteadiness in time, nonuniformness in
space, and transient strong energy flow impact. Therefore,
improvements in thermal power conversion efficiency shall not be
accomplished by completely relying on the regular thermal cycle,
and the laws of fluid flow and heat transfer processes are also
distinguished from regular ones. To greatly improve the efficiency,

1.1 GENERAL PRINCIPLES OF SOLAR THERMAL POWER PLANT DESIGN

3

TABLE 1.1 Comparison of Solar Thermal Power Generation and Solar Photovoltaic
Power Generation
Items to Be
Compared

Solar Thermal Power

Photovoltaic Power
Generation

Power Generation
Mechanism

sunlight energy- thermal
energy-power, with working
process

Photoelectric effect of materials,
sunlight energy-power, without
working process

Definition of
Efficiency

Annual mean efficiency,
Generation energy/annual
solar irradiation, kWh/kWh

Peak value efficiency,
generation power/input
power, kW/kW

Capacity

No scale limits; dish system
is suitable for distributed
power generation; towertype and parabolic trough
systems are suitable for
large-scale projects

No scale limits; can be either
separately applied, or applied
in large-scale project

Solar Spectrum

300e3000 nm

300e600 nm

Power Quality

Small load fluctuation, high
quality

Large fluctuation without
power storage impacts on
power grid, poor quality

Cost

high

lower

it is not possible to simply apply conventional materials systems.
These technical considerations pose challenges to the conventional
techniques being applied right now. The development of largescale, highly efficient CSP cycle technologies requires new and
further research on highly efficient concentrator fields, unsteady
high-temperature heat transfer and thermal storage mechanisms,
materials design, reliability of the CSP-generation system and its
recurring effects on the overall system, etc.
3. Differences between CSP generation and solar photovoltaic power
generation. The two solar energy power generation modes are
compared in Table 1.1.

1.1 GENERAL PRINCIPLES OF SOLAR THERMAL
POWER PLANT DESIGN
Design of the CSP plant shall follow the general principles of (1)
tallying to national conditions, advances in technology, economic feasibility, and operating in a safe and reliable manner; (2) striving for economic and social benefits, saving energy, engineering investment, and
raw materials, and shortening the construction period; and (3) being in

4

1. INTRODUCTION

line with existing Chinese state standards and regulations for saving land,
water conservancy, and environmental protection, as well as exercising
requirements for labor safety and industrial hygiene.

1.1.1 Constitution of the Solar Thermal Power Plant
An CSP plant consists of three major units: solar energy collection,
thermal energy storage, and a thermal power generation unit. The first
two mainly include the irradiation concentrator, the receiver, thermal
storage, and the evaporator, whereas the last mainly includes the turbine,
the power generator, control of the power cycle, the electricity system,
water treatment, and the supply system.
Capacity of an CSP plant shall be determined according to the capacity
of the generator unit, which is irrelevant to solar irradiation resources,
environmental and meteorological conditions and concentrator power.
Power plants of equivalent capacity may correspond to concentration
fields (mirror fields) of different sizes.
An CSP plant can be constructed economically by using combined
heating and electricity based on solar direct normal irradiation (DNI)
resources, the current status of the local power load, and thermal load.
CSP can be complemented by coal, petroleum, or natural gas in a
mixed-fuel power plant constructed according to circumstances in areas
with an abundant solar resource and coal or petroleum resources.
According to the needs of thermal and power load development in
corporate planning, construction of a self-contained heating-type CSP
plant with an appropriate scale is suggested.

1.1.2 Selection of Pressure Parameters for Power
Generation Units
It is suggested that the water steam pressure parameters of generator
units be selected according to unified short-term and long-term construction plans while being in-line with the following rules:
1. For a generator unit with a stand-alone capacity of 1.5 MW and
below, a medium-pressure (MP) or lower MP steam turbine is
suggested. For one with a stand-alone capacity of 3 MW, a MP steam
turbine is suggested. For one with a stand-alone capacity of 6 MW or
above, an MP or secondary high-pressure (HP) steam turbine is
suggested.
2. For a condensing-type generator unit with a stand-alone capacity of
3 MW, lower MP parameters are suggested; for one with a standalone capacity of 6 MW or above, MP or lower HP parameters are
suggested.

1.1 GENERAL PRINCIPLES OF SOLAR THERMAL POWER PLANT DESIGN

5

3. For solar collectors within the same power plant, the same type of
collector with the same output parameters should be used;
generator units within the same power plant should also use the
same parameters. For a parabolic-trough-and-tower mixed-CSP
plant, the parabolic trough system should be used as the preheater
with the tower used for the superheated part.
4. When designing the concentration field, the influences of sun beam
shading and blocking between the reflectors on concentration
efficiency shall be considered; attention should also be paid to the
land use rate and future expansion needs of the concentration field
and thermal storage system. Normally the land coverage of a
parabolic trough concentration field is about 2.5 times that of the
total aperture area of the concentrators, whereas the land coverage
of a solar tower concentration field is 4e6 times that of the total
aperture area of the heliostats and also related to the height of tower.
In some countries, land quotas are quite complex.

1.1.3 Heat Transfer Fluid of the Receiver
Water/steam, synthetic oil, air, or molten salt can be selected as the heat
transfer fluid. The working medium of a steam turbine is water/steam.
For a CSP plant that uses steam as the corresponding working medium,
water pretreatment equipment must use desalinated water; otherwise,
permanent damage may be caused to the reverse osmosis water system.

1.1.4 Schedule Capacity of the Power Plant and Number of
Installed Units
New power plants can be designed and constructed all at once or in
sections according to incremental load speed based on scheduled capacity. Due to the comparatively large investment, the concentration field
corresponding to a power plant can be designed all at once but constructed in sections. The major loop for synthetic oil, the design and
construction of the parabolic trough collector field, and the height of the
receiver tower in the tower power plant shall be configured to match the
intended ultimate capacity of the power plant. A large-scale collector field
can be divided into different thermal collection modules, the thermal fluid
output of which will flow into the thermal storage unit. In the thermal
collection system that directly produces steam, the steam will be discharged to the main pipe of the power plant.
The number of condensing power turbines shall not exceed four in one
plant. For a power tower plant with an installed capacity of less than
100 MW, no more than one receiver corresponding to the concentration
field shall be installed [1]. For a large-capacity tower power plant, the

6

1. INTRODUCTION

multitower system shall be considered when designing the concentration
field. A single-tower system is recommended for a tower power plant that
uses molten salt as endothermic fluid because of great difficulties in the
high-temperature molten-salt transmission process, poor reliability, and
high pipeline cost.
The turbine and boiler configuration, model selection of main auxiliary
facilities, major production process system, and main powerhouse layout
in the power plant shall be determined through technical and economic
analyses. While satisfying the safe, economical, and reliable operation of
the power plant, the system and/or layout can be simplified in an
appropriate manner.

1.1.5 Control of Power Plant Influences on the Environment
In designing the power plant, it is necessary to indicate the disposal
plan for the concentrator as well as the thermal storage and heat transfer
materials, especially thermal storage medium to be used in large
amounts. The working medium of water/steam for heat transfer and
thermal storage is very environmental friendship.
If landscaping is damaged during concentration field construction, a
land restoration program shall be provided.
Wastewater, sewage, light pollution, noise, and all kinds of other pollutants shall be prevented, controlled, and discharged by implementing
and executing national laws, regulations, and standards for environmental protection, and the relevant rules for labor and industrial hygiene
must be tallied. These items can only be discharged by satisfying the
respective standards.
Engineering facilities for pollutant prevention and control as well as
labor and industrial hygiene facilities must be designed, constructed, and
placed into operation with the core work on a simultaneous basis.

1.1.6 Power Plant Seismic Resistance and Windproof Design
The solar collection system consists of concentrators and receivers.
Concentrators use optical equipment with high precision requirements.
Any deformation of the foundation or supporting structure of the
concentrator will greatly influence the precision of the concentrator and
have major impacts on the overall working conditions of the power plant
and could even result in scrapping of the concentration field. The seismic
design of the concentrator shall be based on the hundred-year earthquake.
The design of the receiver tower must be conducted by executing the
current China state standard. Seismic resistance shall also be considered
during design of the power plant’s concentrator.
Wind-resistant design for the concentrator and receiver tower of a CSP
power plant shall be conducted according to the hundred-year wind scale
of the plant’s applicable locality.

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

7

1.1.7 Principles of Concentration Field Design
The determination of the concentration field area serves as the key to
CSP plant design and is normally calculated by applying the design point
method.
Design point is a very important concept for CSP generation design
and can be used to determine the parameters of various segments of solar
concentration field, the receiver, thermal storage, and power generation.
Factors of a design point include time, solar DNI, ambient air temperature, and wind speed, etc. The time selection is normally midday during
the spring or autumn equinox; annual mean temperature can be used
as the ambient air temperature and annual mean wind speed can be used
as the respective wind speed.
In determining unit capacity, two methods can be used for selecting the
solar direct normal irradiance that corresponds to the design point:
1. Apply solar direct normal irradiance ¼ 1 kW/m2 when the
designed area of the concentration field is small; if the calculated
concentration field area has an irradiance of less than 1 kW/m2, it is
impossible for the needs of the power generation and thermal
storage systems to be directly satisfied by field output.
2. Apply the annual mean solar direct normal irradiance of the locality
when the designed area of the concentration field is large. The
output of the concentration field is normally sufficient to satisfy the
energy needs for thermal storage and the steam turbine. In cases
where solar irradiance exceeds the annual mean level, a portion of
the concentration field shall be closed.
To exert maximum functionality of the concentrator relative to a large
one-time investment, the first method is normally adopted for concentration field design.
The annual capacity factor of the CSP plant is determined by the design
point and operational mode of the power plant.
Thermal storage capacity is determined by generator unit capacity, the
annual capacity factor, and the operational mode of the power plant.

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL
POWER GENERATION
1.2.1 Basic Concepts of Solar Thermal Power Plants
With the gradual exploration and consumption of conventional energy
resources such as coal, petroleum, and natural gas, the fossil energy
sources used by humans and maintained for thousands of years are now

8

1. INTRODUCTION

facing exhaustion. In addition, with severe pollution in the global environment, more and more governments around the world are planning
and energetically exploiting various new energies to ensure a large-scale
energy supply and maintain and satisfy rapid economic growth and the
interests of their citizens. CSP generation technology is viewed as a
low-cost sustainable power supply clean-energy technology.
1.2.1.1 Basic Concepts of Solar Thermal Power Generation
Technology
CSP generation is a system that converts solar energy into thermal
energy and generates power through thermalework conversion. The
thermalework conversion system is similar to conventional thermal
power generation except that CSP generation also contains a solar-tothermal conversion process; it uses a solar radiation to thermalework
coupled system. A CSP plant normally consists of thermal collection,
thermal storage, and thermalework power conversion systems. Based on
different concentration modes for the CSP generation system, the CSP
generation [2] normally can be divided into solar tower (also known as
central receiver), parabolic trough, parabolic dish, linear Fresnel reflector,
etc. Those that have already reached the commercial application level are
mainly concentrating solar tower and parabolic trough types. CSP enjoys
the advantages of comparatively mature techniques, low power generation
costs, and minor impacts on the power grid; thus it has been deemed the
most promising among various renewable energy power-generation
modes. Meanwhile, CSP thermalework conversion is partially similar to
that of a conventional thermal generator unit. Existing mature techniques
can be utilized, and thus CSP is especially suitable for large-scale applications. In 2018, a total capacity of 5206 MW was connected to the power
grid all over the world, with 1048 MW under construction, and 3691 MW
under development.
As mentioned in “Technology Roadmaps Concentrating Solar Power,”
released by the International Energy Agency in September 2014 under a
proper policy support, it was estimated that by 2050 the cumulative
installed capacity of global CSP generation facilities would reach
1089 GW with a mean capacity factor of 50% (4380 h/year), an annual
power generation of 4770 TWh that would account for 11.3% of global
power production (9.6% of which derives from pure solar energy), and
China’s CSP generation would account for 4% of the global amount with
an annual power generation of about 190 TWh. In areas with excellent
solar resources, CSP generation is expected to become a competitive largecapacity power supply that will undertake peak modulation and medium
power load by the year 2020 and basic load power generation by 2025e30.
Based on geographic information system analysis, the potential capacity
for the installation of CSP generation in China that meets the basic
conditions for CSP generationddirect normal irradiation 5 kWh/(m2 day)

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

9

and surface slope 3%dapproximates 16,000 GW, which is similar to
that of the United States; the potential capacity to be installed with a
direct normal irradiation of not less than 7 kWh/(m2 day) approximates
1400 GW. In terms of annual power generation capacity, China’s potential annual CSP generation capacity is 42,000 TWh/year, which means
that even if all fossil fuel energy resources become exhausted, China will
still enjoy abundant CSP generation resources far beyond those required
for self-sufficiency.
China has abundant solar energy resources. Its annual solar irradiation
falls approximately in the range of 1050e2450 kWh/m2; on average, the
solar energy that irradiates the 9.6 million square kilometers of land in
China every year is equivalent to 1700 billion tons of standard coal. About
300,000 square kilometers of the Gobi Desert in China, which accounts for
about 23% of China’s total desert area, can be used to develop solar power
generation. Based on existing CSP generation technologies and annual
conversion efficiency, constructing power plants on China’s 70,000 square
kilometers of sand would result in annual power generation that would
satisfy power demands of China equal to those for all of 2018. China’s
extremely abundant solar and sand resources are especially prevalent
western China, where CSP technologies will play a significant role in
economic development, environmental protection, and resource protection. As constantly supported by the 8th, 10th, 11th, and 12th Five-Year
Plans of China on science and technology issued by the Ministry of
Science and Technology of the People’s Republic of China, numerous
achievements have been made on parabolic trough, solar tower, and
parabolic dish CSP generation systems. In July 2011, the Beijing Badaling
megawatt-level solar tower power plant was completed and started to
produce steam [3]; it started generating power in August 2012. The power
plant can generate power not only by pure solar energy, but also by being
connected with fossil fuels in a parallel manner. The Chinese first parabolic trough solar power plant was completed in the Yanqing district of
Beijing in 2017.
CSP generation is capable of applying two thermal cycles, namely
direct and indirect (double-loop). The former directly drives the steam
turbine unit for power generation (Fig. 1.1) by using the steam produced
by the receiver. The latter produces steam thermally using the working
media-water or fluid with a low boiling point in the auxiliary system
through thermal exchange during the thermal cycle of the main system
and thus driving the steam turbine unit for power generation (Fig. 1.2).
Compared with a conventional thermal power plant, the most intuitive
difference between the two is that the conventional boiler is replaced with
thermal collection and storage facilities in CSP generation, whereas the
thermal cycle mode and respective equipment applied for thermaleworkpower conversion are basically the same as those used in conventional
power plants. In comparing an CSP power plant’s acquisition mode with

10

1. INTRODUCTION

FIGURE 1.1 Schematic diagram of water/steam tower power generation system.

FIGURE 1.2 Schematic diagram of molten-salt tower power generation system.

that of a conventional thermal power plant, the biggest difference lies in
the unstable source of thermal energy. As solar irradiation itself features
time discontinuity, it may be greatly influenced by weather conditions.
Thus the thermal process demonstrates an unstable state, frequent variations, and complexity that lead to nonlinearity, time variation, and uncertainty of multivariable coupling and result in a variety and complexity
of operational modes and control means. Along with the development of
large-scale thermal storage technology, it is possible to realize large-scale
stable operation. For a tower power plant, a thermal collection system that
consists of concentrators and receivers does not exist in conventional
power plants; in contrast to a boiler, the collection system is quite complex
with multiple variables, loops, and operational modes.
A major characteristic of CSP generation technology is thermal collection. The concentration ratio is one of the most important parameters for

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

11

FIGURE 1.3 Relationship of solar thermal power generation system efficiency, thermalcollecting Temperature, and concentration ratio.

CSP generation system design. The greater of the concentration ratio, the
more possible it is to achieve a higher maximum temperature (Fig. 1.3).
The concentration ratio is the ratio of mean radiation flux density that
gathers on the surface of the receiver’s aperture to the solar direct normal
irradiance that enters the aperture of the concentration field. Annual power generation is a key factor that determines the benefits of an CSP plant.
The annual power generation of an CSP plant is the product of the CSP
plant’s annual efficiency and that solar direct normal irradiance that
has been cast on the aperture area of the concentration field. Thus the
CSP plant’s annual efficiency and solar direct normal irradiance at the
construction site of the CSP plant are two extremely critical factors. The CSP
plant’s annual efficiency (which can also be deemed the system efficiency)
is determined by the thermal collection efficiency and the efficiency of the
thermal engine. As shown in Fig. 1.3, based on a certain concentration
ratio along with increases in the thermal collection temperature, the
system efficiency curve will demonstrate a “saddle point,” which is
mainly caused by the increased efficiency of the thermal engine along
with the increment of the thermal collection temperature. However, due to
increased heat losses by the receiver, thermal collection efficiency decreases after reaching a certain level. Therefore, in the CSP generation
system, simply increasing the working temperature of the system is not
advised; instead, the concentration ratio and thermal collection temperature should be comprehensively considered by applying the high-ratio
daylight concentration and high-performance absorber techniques.
Based on the concentration mode, CSP generation technologies can be
divided into two systems, point focusing and line focusing, with the point

12

1. INTRODUCTION

focusing system mainly including solar tower (also known as central
receiver system) and parabolic dish/Stirling solar power generation, and
the line focusing system mainly including parabolic trough and linear
Fresnel reflector solar power generation. In these four forms of CSP
generation technology, parabolic dish/Stirling engine power generation
technology enjoys the highest concentration ratio (1000e3000), followed
by solar tower (300e1000), whereas the line focusing system’s parabolic
trough (70e80) and linear Fresnel reflector (25e100) concentration ratios
are comparatively low.
1.2.1.2 Characteristics of Solar Thermal Power Generation
Technology
CSP generation is by its nature a way to utilize solar thermal energy. Its
generation principle is a clean and green energy utilization method. The
development of CSP generation technology is of great significance for the
sustainable development of human economies and societies. Compared
with other energy utilization methods, CSP generation enjoys certain
unique development advantages:
1. Resource request: always available for use. Compared with other
renewable energies, the solar resource is inexhaustible and always
available for use. China is a nation with extremely abundant solar
resources. Solar irradiation received by its land areas approximates
the equivalent of 1700 billion tons of standard coal, resulting from an
annual sunshine duration that exceeds 2200 h. Total irradiance
exceeds 5000 MJ/m2 that is abundant or comparatively abundant
over a vast area including: Alxa League in western Inner Mongolia
and Ordos, Hexi Corridor in western Gansu, Qinghai, Tibet, and
Hami and Turpan of Xinjiang. Accounting for over two-thirds of the
total area of China, these areas have excellent conditions for solar
energy utilization. In particular, Gansu, Hexi Corridor, Qinghai, and
Tibet possess certain water resources and are sparsely populated,
thus enjoying the potential for development of large-scale CSP
plants. In addition, the Gobi Desert, deserted land, abandoned saline
land, and desert land in western China are vast in area. For example,
the portion of the Hanggin Banner of Inner Mongolia along the
south coast of the Yellow River that is suitable for the development
of CSP generation is as large as 10,000 ha; the area has rich surface
water resources, and construction of a two million-kW CSP plant
with annual power generation of up to 10 billion kWh is possible.
Dunhuang of Gansu has a flat Gobi Desert of over 5000 square
kilometers. After implementation of the “Transferring Water from
Dahaerteng River to Danghe River” project (a water conservancy
project), a one million-kW CSP plant can be constructed. Thus in

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

13

terms of developable and exploitable resources, solar resources are
superior to wind, biomass, geothermal, hydro, and other renewable
energies (Table 1.2); in terms of exploitable areas, solar availability is
broader than that of geothermal, oceanic, and like energies.
2. Environmental influences: extremely low. The entire CSP generation
process doesn’t produce pollutants or greenhouse gases; compared
with conventional fossil fuel power generation, it is a clean energy
utilization form. Meanwhile, during utilization and exploitation of
resources, the ecological environment is not damaged or influenced;
compared with wind, hydro, geothermal, oceanic, and similar
energies, it enjoys the advantage of being environmentally friendly.
Furthermore, by viewing the overall life cycle, energy consumption
level, and environmental influences of the entire process of CSP
generation from equipment manufacturing, to power generation, to
scrapping, it can be seen that they are equivalent to those of other
renewable energy utilization forms. Compared with the
manufacturing and scrapping of solar panels for solar photovoltaic
power generation, CSP generation’s energy consumption and
pollution levels are greatly reduced. The carbon dioxide emissions
of an CSP generation system during its life cycle are extremely low.
Based on 2009 technology, the carbon dioxide emissions of an CSP
plant during its life cycle were about 17 g/kWh, which is far
below those of coal (776 g/kWh) and natural gas combined-cycle
(396 g/kWh) power plants. Thus CSP generation uses renewable
energy power generation and utilization with minimal environmental
impacts.
TABLE 1.2 Exploitation Potentials of Renewable Energy Resources in China [4]
Type

Exploitable Resources

Solar energy

1700 billion tons of standard coal

Wind energy

1 billion kW, including 300 million kW on land

Hydroenergy

Economically exploitable ¼ 400 million kW
Technically exploitable ¼ 540 million kW

Biomass
energy

Biomass power
generation

300 million tons of straw þ 300 million tons of forest
waste

Liquid fuel

50 million tons

Methane

80 billion m3

Total

e

Geothermal, excluding medium
and low temperature

6 million kW

14

1. INTRODUCTION

3. Output characteristics of power generation: smooth. Due to different
power generation principles, the output characteristics of CSP
generation are superior to those of solar photovoltaic and wind
power generation. This is especially true for the thermal generator
unit through thermal storage units, as it is capable of generating
power on a significant, smooth basis while reducing output
fluctuations. Based on different thermal storage modes, the
utilization hours and power generation of the power plant can be
improved by varying degrees, which can improve the adjustment
performance of the plant. Furthermore, the output characteristics of
CSP generation are normally ameliorated through afterburning or in
combination with conventional thermal power generation so that it
can be used during the night for constant power generation,
including for stable output and undertaking basic load operation.
4. Characteristics for power grid connection: flexible and steady. An
CSP plant with thermal storage and afterburning facilities is
distinguished from other energy sources, such as wind power and
solar photovoltaics, that experience fluctuating power supply.
Thermal storage facilities can be used to generate power in a smooth
manner, improve the flexibility of the power grid, make up for the
fluctuation characteristics of wind power and photovoltaic power
generation, and improve the capability of the power grid by
eliminating fluctuating power supply. Meanwhile, an CSP
generation system that has been equipped with thermal storage
facilities converts partial solar energy during daytime into thermal
energy and stores it in a thermal storage system; at night or when
peak regulation of the power grid is required, it can be used to
generate power and satisfy power grid requests while ensuring
more stable and reliable power output. Photovoltaic power
generation directly converts optical energy into power, and the rest
of the energy can only be stored using batteries, for which the costs
are far greater than they are for solar-concentrating power
generation (which uses thermal storage). Thus it is easier to store the
excessive energy to realize constant and stable power generation
and peak regulation. It is the most important and obvious advantage
of CSP generation against wind power, solar photovoltaics, and
other renewable energy power generation modes; it is conducive to
stable operation of the power system and is easier to connect to the
power grid. Furthermore, because it drives a steam turbine for
power generation through the production of superheated steam,
CSP generation has the same power-generation mode as that of
conventional thermal power, so it will not have a negative impact on
the power grid. It provides reactive power using a generation mode
that is friendly to the existing power system.

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

15

1.2.1.3 Comparison of Solar Thermal Power and Solar
Photovoltaic Power Generation
1. Energy storage. Energy storage in CSP power generation has clear
advantages, in that thermal storage techniques are more mature
and much less expensive than other power storage techniques.
Currently, solar photovoltaic power plants are still unable to realize
the expectation of all-weather power generation, whereas CSP has
already accomplished this target. The remarkable feature of CSP
generation technology is the application of a thermal storage system,
which is also a major advantage of CSP generation versus
photovoltaic power generation. The thermal storage system
(Fig. 1.4) accumulates excess thermal output by the receiver under
intensive solar irradiation for use in case of clouds, overcast
conditions, and peak times while realizing: (1) generation capacity
buffering; (2) controllable power output; (3) stable power output; (4)
improvement in annual availability and the increment of fullcapacity generation hours; and (5) improvement in the effectiveness
of the CSP plant as well as a reduction in power generation cost.
Research has indicated that an CSP plant with a thermal storage
system can improve its annual availability from 25% (in the case of
no thermal storage) to 65%. Thus thermal storage technology serves
as a key factor in the competition between CSP generation and any
other renewable energy power generation. By applying a longduration thermal storage system, CSP generation is capable of
satisfying the requests of the basic load power market in the future.
Currently, the longest thermal storage time by a power plant has
exceeded 15 h.

FIGURE 1.4 Schematic diagram of thermal storage techniques in solar thermal power
generation.

16

1. INTRODUCTION

FIGURE 1.5 Kuraymat parabolic trough/natural gas combined-cycle testing facilities.
Picture: Iberdrola, 2011.

In addition to utilizing thermal storage technology, a CSP generation
system can conduct a combined cycle with coal, fuel, natural gas,
biomass, and similar forms of power generation (Fig. 1.5) to
overcome its disadvantages of discontinuity and instability, realize
all-weather uninterrupted power generation, and achieve optimal
technical economy. The Beijing Badaling CSP testing power plant
can perform complementary operational experiments between
gas/fuel power plants and parabolic trough and solar tower power
plants (Fig. 1.6) [5].
Also, CSP generation can be combined with the thermochemical
process to realize highly efficient solar thermochemical power
generation. Waste heat of CSP generation can be used for seawater
desalinization (Fig. 1.7), space heating, and other uses, resulting in
integrated utilization. Recently, some scientists have also proposed
using CSP generation technology for the gasification and
liquidation of coal and for forming long-distance gas or liquid
fuels:
2. Power grid access. The instability of photovoltaic power generation
has created major challenges for power grid operation, whereas CSP
power generation can be connected to the power grid just as in
conventional thermal power generation without any adverse effects.
CSP power generation is a green power supply for the basic load.

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

17

FIGURE 1.6 Complementary scheme of Beijing Badaling solar thermal power testing power plant. Provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2012.

FIGURE 1.7 Solar thermal power generation and seawater desalinization combinedcycle system. Picture provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2013.

18

1. INTRODUCTION

3. Power dispatching. CSP generation can also serve as a powerdispatching source for the power grid like a pumped-storage
power plant; under a considerable peakevalley price mechanism, it
will generate greater economic benefits. This also accelerates the cost
payback period of the power plant.
4. During the energy conversion process, photovoltaic power
generation only requires one opticalepower conversion, whereas
CSP power generation requires the secondary conversion of optical
to thermal to power. Although this has increased the difficulty of
system integration, thermal production as the intermediate link of
CSP plant operation has also expanded the application scope of CSP
power-generation techniques. For example, the superheated steam
produced in CSP generation can be used for combined power
generation with conventional coal, gas, and biomass power plants.
Furthermore, the thermal energy produced by CSP generation can
be deemed a by-product for use in seawater desalinization,
industrial thermal and air-conditioning, etc.
5. Raw material supply. Photovoltaic power generation mainly
consists of photovoltaic solar panels. Currently, those that have
been widely applied are crystalline silicon cells and CdTe thin-film
cells, the raw material supplies of which could be tight. Market
prices could rise, especially for those elements of CdTe cells that use
rare metals that are subject to triggering large price fluctuations.
Raw materials of the CSP power plant, on the other hand, are mainly
commodity items such as glass, steel, concrete, and other common
materials in sufficient supply; major price fluctuations in these raw
material supplies are unlikely.

1.2.2 Main Technical Forms of Solar Thermal Power
Generation
1.2.2.1 Solar Tower Power Generation
Solar tower power generation (Fig. 1.8) is a system that transmits solar
irradiation to the receiver mounted on the tower and acquires the hightemperature heat transfer medium through multiple heliostats by
tracking movement of the sun, generating power directly or indirectly
through the thermal cycle using a high-temperature heat transfer
liquid [6]. Solar tower power plants mainly include a heliostat, a receiver
tower, a receiver, thermal storage, and a generator unit. Under the
working state of the solar tower thermal power plant, all heliostats in
the concentration field reflect daylight through double-axis tracking of the
azimuthal angle and altitude angle to the receiver mounted on the solar
tower in order to thermal the heat transfer fluid inside the receiver of

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

19

FIGURE 1.8 Solar tower power generation of Gemasolar plant owned by Torresol
Energy, Spain. Picture provided by SENER, Spain, 2018.

receiver. The concentration ratio of solar tower power generation falls
into the range of 300e1000, and thus it is easy to realize a comparatively
higher system operation temperature. Furthermore, solar tower power
generation systems feature a short heat transfer path, small heat losses,
and high collection efficiency. Therefore, the solar tower power generation
system features comparatively higher comprehensive opticalepower
conversion efficiency.
Based on different thermal transfer media in the receiver, the system
operational mode and performance characteristics of the power plant
may be distinguished from each other. Thermal transfer media that are
currently available mainly include water/steam, molten salt, and air. In a
water/steam power plant system, high-temperature high-pressure steam
generated by the receiver can be directly used to drive the steam turbine
to generate power; it enjoys the advantage of a thermal-absorbing medium that is the same as the working medium, and annual mean efficiency can exceed 15%. A molten-salt power plant system uses an indirect
thermal cycle power generation system, which requires the use of a
molten-salt/steam generator to indirectly produce high-temperature
high-pressure steam to drive the steam turbine to generate power.
Compared with the water/steam power plant system, a molten-salt system can realize supercritical, ultrasupercritical, and other high-parameter
operational modes due to low pipeline pressure during high-temperature
operation and thus further improve the efficiency of the solar tower
thermal power generation system. It is also convenient for storing energy

20

1. INTRODUCTION

and thus is a technology that enjoys an extremely efficient standardization
prospect. An air receiver power plant normally applies the Brayton cycle
thermal power generation mode in which air passes through the receiver
and becomes 700 C above-high-temperature hot air before entering the
gas turbine; the hot air drives the compressor to work and realize power
output, which greatly reduces gas consumption, and its operation efficiency can exceed 30%. In addition, it can realize water-free operation and
serves as a major research direction for the development of a highly
efficient solar tower thermal power plant in the future.
1.2.2.2 Parabolic Trough Solar Power Generation
Parabolic trough solar power generation (Fig. 1.9) is a technology that
concentrates solar irradiation in the receiver tube mounted at the focal
line of the paraboloid through linear parabolic mirrors that track the
movement of the sun and thermal the heat transfer liquid for power
generation. Key equipment of a parabolic trough power plant mainly
includes a concentrator, a receiver tube, and thermal storage. The parabolic trough power plant is the first (1980s) thermal power generation
technology to realize commercial operation, with a maximum power
plant capacity of up to 80 MW while still ensuring stable operation.
Certain problems with the parabolic trough power generation technology
are the low concentration ratio of the paraboloid mirror (70e80), difficulty
raising the working temperature of the heat transfer liquid, and restrained
system efficiency.

FIGURE 1.9 Parabolic trough solar power generation. Picture provided by the Institute of
Electrical Engineering, Chinese Academy of Sciences, 2017.

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

21

FIGURE 1.10 Structural diagram of parabolic trough solar collector.

As shown in Fig. 1.10, a parabolic trough solar power collector consists
of a parabolic trough concentrator that tracks the movement of the sun and
a receiver tube mounted at the focal point of the paraboloid. The parabolic
trough concentrator uses a single-axis tracking concentrator, namely a
concentrator with a mirror element revolving in a one-dimensional manner
by surrounding a single axis to track the movement of the sun. The surface
of the parabolic mirror is the trajectory formed by a line moving along
a certain parabolic curve while parallel to the fixed line. Thus with a
parabolic trough concentrator that tracks the movement of the sun, DNI is
constantly concentrated on the surface of the receiver tube and creates a
focal line so that the heat transfer liquid inside the receiver tube can be
heated. High-temperature and high-pressure steam is then generated
directly or through an oilewater heat exchange system in order to participate the thermal cycle power generation system and drive the steam
turbine to function and generate power or provide the requested steam
for industrial processes. The heat transfer liquid of the system transfers
thermal energy and is normally water/steam, synthetic oil, or molten salt.
The parabolic trough concentrator is a key component that receives and
reflects solar radiation and consists of the base, bracket, mirror, power
machine, transmission system, and control system. A typical parabolic
trough concentrator is made up of multiple units connected in series
along the axis and equipped with a power, transmission, and control
system. Normally for a parabolic trough concentrator with small radiation areas, hydraulic or mechanical transmission can be applied; for one
with large radiation areas, only hydraulic transmission can be applied.
A bracket is connected to the mirror through fixtures to support and
ensure the stability of the parabolic mirror surface; its structure can be
categorized as torque tube, torque box, and space truss types; the materials are normally metals, such as steel or aluminum products, and the
processing pattern is mainly welding and punching.
Structures of the mirror can be categorized as single-layer or composite. The single-layer structure is an ultraclear glass hot-bending parabolic
trough surface that is coated with silver, whereas the composite structure

22

FIGURE 1.11

1. INTRODUCTION

Structural diagram of receiver tube of parabolic trough solar collector.

consists of a backboard and adhesive and reflective materials. The backboard functions to create a parabolic surface, and it can be made from
steel plate, aluminum plate, float glass, and fiberglass. Reflection materials can be thin glass mirror, metal film, or firm composite materials.
Adhesive materials can be PVB, neutral organic silicone, etc., in which the
aluminum reflector has a high reflection rate created with an aluminum
plate through the use of surface finishing and oxidation protection. A
silver-coated polymer mirror is a reflection surface with a high reflection
rate that is created by coating with silver on one side of the hightransmittance, strong weather-resistance polymer film; it is equipped
with multiple layers of protective film that are attached to the bottom of
the curved surface to create a curved mirror.
As shown in Fig. 1.11, the receiver tube of the parabolic trough solar
collector is a core component of the parabolic trough collector and is
typically about 4 m long. The interior tube is a commercial-type metal
receiver tube with an external diameter of 70 mm, whereas the exterior
tube is a glazed shield tube with an external diameter that falls in the
range of 115e125 mm. Due to the metal receiver tube and glazed shield
tube having different coefficients of expansion and thermal intensities
during operation, high-temperature-resistant glass and metal sealing
pieces are required as transition pieces to ensure an airtight connection. In
addition, a metal corrugated pipe is used as the thermal stress buffer
section to relieve the longitudinal thermal expansion difference between
the metal receiver tube and the glazed shield tube. To ensure degree of
vacuum degree in the vacuum interlayers of the receiver tube, a getter
must be mounted between the metal receiver tube and the glazed shield
tube. Furthermore, with any focusing solar irradiation, the seal undertakes great thermal stress that may easily invalidate the sealing of
glass and metal. Therefore, thin-walled materials with good reflection
performance are required as a solar shade to block radiation while
reflecting it to the metal receiver tube.
Both the thermal properties and life of the parabolic trough receiver
tube are determined by the vacuum degree of the vacuum interlayer. If the
vacuum environment is damaged, not only will the respective heat losses

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

23

rapidly increase, but also the selective receiver film of the metal receiver
tube surface will deteriorate due to oxidation, which may result in severe
reduction of the receiver tube’s optical efficiency. Under the special
working conditions of high temperature and strong radiation, CSP performance and vacuum life can only be ensured when the materials and
properties of these components satisfy certain requirements:
1. Glazed shield tube. Due to dayenight alternation and temporary
cloud occlusions, alternating stress may be generated at the seal,
which thus requires high hardness and thermal stability as well as
corrosion resistance. Materials that are widely applied at the present
include borosilicate glasses such as Pyrex glass, the expansion
coefficient of which is 3.3  106/K while featuring high hardness,
good optical properties, and acid and alkali corrosion resistance. It
also has the disadvantages of having no corresponding sealing
metal, and its softening temperature approximates 820 C, and thus
the temperature is extremely high during sealing operations.
2. Metal receiver tube. The temperature of the metal receiver tube
under concentration effect will be much higher than 400 C. Thus it
is necessary that it is equipped with high-temperature and corrosion
resistance. To eliminate the influences of axial expansion on the
collector bracket, the expansion coefficient shall be as small as
possible. Due to thermal and gravitational influences, downward
deflection may occur, so there must be a sufficient distance between the
exterior wall and the interior wall of the glass tube. Currently, hightemperature-resistant 316L stainless steel is normally used with an
external diameter of 70 mm, a wall thickness of 3e5.5 mm, a standard
length of 4060 mm, and a mean roughness of less than 0.5 mm.
3. Glass-metal sealing transition piece. A certain sealing alloy is
applied to solve the inconsistency of the expansion coefficients of
the interior metal tube and exterior glass tube. Therefore, both
expansion coefficients shall be as close to each other in value as
possible in order to satisfy matched sealing and easier welding to the
corrugated pipe.
4. Thermal stress buffer section. This buffer is required in order to
compensate the expansions of the metal receiver tube and the
glazed shield tube. Thus it is necessary that is has good flexibility,
excellent tension fatigue strength and life, high-temperature
resistance, and acid and alkali corrosion resistance. The respective
length shall be as short as possible to increase the effective
concentration length of the receiver tube.
5. Getter. A getter is used to absorb the residual gases in the vacuum
interlayer after sealing and the released gases of components
under high-temperature working status to ensure a satisfactory

24

1. INTRODUCTION

vacuum state. A getter accomplishes the target of absorbing residual
gases by mainly by relying on physical and chemical absorption.
6. Selective absorption film. According to its working mechanism, it
can be categorized as optical interference, intrinsic absorption,
metal ceramic, or multilayered gradient film. As a general
requirement, for temperatures below 400 C, its absorptivity shall be
not less than 95%, and its reflectivity shall be less than 14%. The most
widely applied selective absorption film is composite material
absorption film, including multilayered gradient metal ceramic film
and double-layered absorption film. The multilayered gradient metal
ceramic film has a metal substrate, and the absorption layer is made of
metal and dielectric gradient film, whereas double-layered absorption
film creates two absorption layers and one or two dielectric
antireflection layer(s) on the high-reflectance metal substrate to
achieve low reflectance without reducing the absorption rate.
As shown in Fig. 1.12, in 2017, a 9000-m2 parabolic trough solar collector was completed at the Beijing Badaling CSP experimental base. The
collector was arranged horizontally along the 3000-m2 northesouth axis
and 6000-m2 westeeast axis with a tracking length of 300 m. The bracket
was mounted by applying a torque tube structure and selecting an
independently developed sandwich-structure glass mirror, the technical
parameters of which are shown in Table 1.3.
As shown in Fig. 1.13, the collector is of torque tube-type, the support
arm is made from rectangular steel pipe by welding, and the parabolic
mirror is made by gluing together the hot-bending glass paraboloid and
the ultrathin glass mirror. A parabolic trough solar power collector contains 24 pieces of vacuum receiver tubes; the metal absorber pipe inside

FIGURE 1.12 Beijing parabolic trough solar power collector. Provided by the Institute of
Electrical Engineering, Chinese Academy of Sciences, 2017.

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

25

TABLE 1.3 Parameters for Parabolic Trough Solar Power Collector
Item

Parameters

Item

Parameters

Total area/m

9000

Glazed shield tube wall
thickness/mm

3

Aperture area/m2

9000

Single-piece receiver tube
length/mm

4060

Aperture width/m

5.76

Total length of receiver
tube/mm

97,440

Focal length/mm

1.71

Total length of collector/m

1500

2

precision/( )

0.1

Glass thickness of
mirror/mm

4

Tracking

Thickness of glass
mirror/mm

3.2

Maximum operating
temperature/ C

400

Exterior diameter of
metal receiver tube/mm

70

Maximum operating
pressure/MPa

1.6

Exterior diameter of
glazed shield tube/mm

120

Tracking axis direction

3000 m2
northesouth
6000 m2
westeeast

Provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2017.

FIGURE 1.13 Structural diagram of parabolic trough collector. Provided by the Institute of
Electrical Engineering, Chinese Academy of Sciences, 2010.

the cover glass tube is made from 316L stainless steel with a hightemperature-resistant metal ceramic selective absorption coating on the
exterior surface. The thermal stress buffer section is made from stainless
steel corrugated pipes, the displacement of which is calculated based on
the thermal expansion differences generated by the metal receiver tube at
450 C and the glazed shield tube at 0 C when the length is 4 m. Heat
transfer oil is used as the heat transfer fluid inside the receiver tube, the

26

1. INTRODUCTION

type of which is selected according to minimum ambient air temperatures
in different seasons: Dowtherm A by Dow Chemical is used, the main
ingredients of which are diphenyl and diphenyl ether.
1.2.2.3 Dish-Stirling Solar Power Generation
Dish-Stirling solar power generation (Fig. 1.14) is a system that concentrates solar beam radiation on the generator mounted at the focal point
by utilizing a parabolic dish concentrator to generate power through the
Stirling cycle. Key components of the dish-Stirling solar power generation
system include the parabolic dish concentrator, Stirling generator, and
transmission system. Both dish-Stirling power generation and solar tower
power use technology that incorporates point focusing concentration and

FIGURE 1.14 Dish-stirling solar power generation. Picture provided by the (Oriental
Great Ocean New Energy Technology Development Co., Ltd., CHINA, 2017).

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

27

a thermal collection method. For dish-Stirling, the concentration ratio is
600e3000, operational temperature is up to 750 C, and solar dish-Stirling
net efficiency of converting peak solar energy into power is up to 30%. The
dish-Stirling system features less power, normally 5e50 kW. Thus it can
be used independently as the distributed power generation system in
remote areas as well as being incorporated into an MW-level power plant
for grid-connected power generation.
1.2.2.4 Liner Fresnel Reflector Solar Power Generation
Linear Fresnel reflector (LFR) solar power generation (Fig. 1.15) is a
system that concentrates solar beam radiation into a receiver tube
mounted at the focal point of the Fresnel mirror through the FLR mirror
tracking of the movement of the sun and generates high-temperature
working media for thermal cycle power generation. Major components
of LFR power generation include the liner reflective mirror, receiver tube,
and transmission system. The LFR power generation system is a simplified parabolic trough power generation system. The parabolic trough
concentrator is replaced by a surface mirror; the mirror features a small
distance to ground, low wind load, a simple structure, an intensive layout,
and higher land-use efficiency; furthermore, vacuum treatment for the
receiver tube is not necessary, thus reducing technical difficulties and
costs. The total cost of the system is comparatively low. However, due to
the system’s low concentration ratio, the operational temperature stays
low, resulting in low system efficiency as well.
Multiple CSP generation modes are compared in Table 1.4.

FIGURE 1.15 Linear Fresnel reflector solar power generation. Picture provided by
Himin Solar Energy Co., Ltd., CHINA, 2010.

28

1. INTRODUCTION

TABLE 1.4 Comparative Performance of Three Concentrating Solar Thermal Power
Generation Systems
Performance Parameters

Parabolic Trough

Solar Tower

Dish-Stirling

50e600 MW

10e600 MW

5e25 kW

Working temperature of
receiver/ C

400

565e1200

750

Maximum efficiency/%

20

23

30

12e16

16e20

12e25

Commercialization
experiences

Rich

Yes

No

Thermal storage
conditions

Yes

Yes

No

Power of fossil fuel hybrid
thermal source

Yes

Yes

Yes

Potential of
BraytoneRankine
combined cycle

No

Yes

Yes

6

6

10

Installed capacity

Annual mean
efficiency/%

Future mean power
generation cost (levelized
electricity cost), US¢/(kWh)

1.2.3 Basic Terms
This section describes the basic concepts of CSP generation that are
frequently used, which will be helpful in providing designers with clear
knowledge of these concepts.
Firstly, “design point.” “Design point” is an important yet hard-tounderstand concept associated with CSP plant design. There is no design
point in the conventional thermal power and photovoltaic power
systems.
The design point is used in a solar power generation system to determine the parameters of the solar thermal collection and power generation
systems, including year, day, hour as well as the corresponding weather
conditions and solar direct normal irradiance.
The design point is associated with a specific hour and corresponding
solar irradiance and ambient air temperature. It can be used to clarify the
area of concentration field, capacity of the steam turbine generator, capacity of thermal storage on a quantitative basis, and relationship among
these crucial factors. Normally, a design point is not defined based on
peak value and extreme solar angle under local weather conditions, and
wind speed is not considered. For a large-scale power plant with a

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

29

thermal storage system, the design point is typically defined by considering the output capacity of the collector field, which is equivalent to the
thermal power of the steam turbine generator under full load operation.
Examples are as follows:
A. Design point of some CSP plant
Time: midday of spring equinox
Solar irradiance and environmental conditions: solar direct normal
irradiance ¼ the mean solar direct normal irradiance for multiple
years at the locality on the spring equinox; ambient air
temperature ¼ mean ambient air temperature for 30 years at the
locality
B. Design point of some CSP plant
Time: midday of spring equinox
Solar irradiance and environmental conditions: solar
irradiance ¼ 1000 W/m2; ambient air temperature ¼ mean ambient
air temperature for 30 years at the locality on the spring equinox
Differences between A and B are as follows:
1. When solar irradiance exceeds the set value of the design point, the
thermal output of the collector field in plan A can be transmitted to
thermal storage, which means that in cases of partial thermal output
of the collector field being transmitted to thermal storage, there will
be no impacts on full-load operation of the steam turbine. For plan B,
solar irradiance has already been defined as the maximum value
possible on the earth’s surface, which does not exist.
2. When solar irradiance is less than the set value of the design point,
the steam turbine cannot operate under full load. Thanks to the
design of plan B, collector field output can never directly drive the
steam turbine to operate under full load; instead, full-load
functioning of the steam turbine relies solely on the operation of the
thermal storage system.
Based on the above, plan A is more optimized than plan “B.” However,
with plan “B,” the collector field’s energy output will never be “excessive,” yet such an outcome is possible with plan A. For areas with an
extremely nonuniform seasonal distribution of solar irradiance, the
annual mean solar irradiance is low but the transient solar irradiance is
high, and this might result in collector field output that exceeds the
requested level of the steam turbine and thermal storage under certain
weather conditions. At that moment, part of the concentration field will be
closed, resulting in wasted investment; for example, in Hainan Province
in China. For arid and semiarid areas in northwestern China, the daily
mean solar direct normal irradiance is comparatively even, for which plan
“A” is suitable; for Hainan, plan “B” is more appropriate.

30

1. INTRODUCTION

1.2.3.1 Optic Terms
1. Absorber.
Element of the receiver absorbing radiant solar energy and
transferring it to a fluid in the form of heat.
2. Concentrator aperture area.
The maximum projected area of solar irradiation intercepted by the
concentrator, which is actually the sum of all mirror areas in a
concentrator.
This is different from the contour area. The concentrator contour
area contains the clearance between reflective glasses and is
normally larger than the aperture area.
3. Solar collector net aperture area.
The area of the perpendicular projection over the aperture plane of
the solar collector reflecting/refracting components. In a line
focusing system it is this surface plus the part of the perpendicular
projection of the steel receiver tube onto the aperture plane that
does not overlap, provided that the sun-oriented side of the
receiver is absorbing radiation. For LFR and heliostat: the net
aperture area of a Fresnel collector or heliostat is defined as the sum
of the net collecting areas of its mirror segments. The net aperture
area of a mirror segment is the perpendicular projection of the
reflective mirror area over its aperture plane when they are in
horizontal position.
4. Solar collector gross aperture area.
The area of the flat surface defined by the outer perimeter of the
collector, including the gaps between adjacent reflectors. This
definition may be used for modules, heliostats, heliostat fields,
parabolic dishes, LFRs, etc., as well as complete concentrating
collectors.
5. Optical concentration ratio.
Ratio of the average irradiance integrated over the receiver area to
irradiance incident on the solar collector aperture, also called as
flux concentration ratio.
6. Geometric concentration ratio.
The ratio of the collector aperture area to the receiver aperture area.
7. Receiver net collection area.
The maximum receiver flat area that accepts concentrated solar
radiation. It is given by the sum of the products of the active length
and diameter of the receiver elements that compose the receiver.
8. Solar field
The part of the CSP plant that collects and concentrates beam solar
radiation. In CSP plants with a parabolic trough collector or Fresnel
linear collectors, the solar field is composed of a set of solar collectors
and their piping interconnections and headers. In a central receiver

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

9.

10.

11.

12.

13.
14.
15.

16.

31

plant, the solar field is composed of the heliostats. In CSP plants with
parabolic dishes the solar field is composed of the parabolic dishes.
Parabolic trough collector
A line-focus solar collector that concentrates solar radiation by
means of a reflector with a parabolic cross section. It is composed
of a set of elements that altogether can track the sun as a single
unit.
Linear Fresnel collector
A line-focus solar collector that uses reflectors composed of at least
two longitudinal segments with parallel axes to concentrate solar
radiation onto a fixed receiver.
CSP plant
Synonymous with solar thermal power and concentrating solar
power plants. A facility that applies solar concentration and
thermodynamic processes to convert direct solar radiation into
electricity suitable for distribution and consumption. The facility
may include further sources of thermal energy, such as fossil fuel or
biomass, in parallel with solar radiation.
Concentrator
The part of a solar collector composed of reflecting or refracting
elements that concentrate and redirect beam solar radiation onto
the receiver.
Useful solar irradiation
The integral of the useful radiant solar power over the time interval
considered, measured in kWh (1 kWh ¼ 3.6 MJ).
Heat transfer fluid
Fluid used to carry heat from one system component to another in
a CSP plant.
Heliostat
A system that reflects beam solar radiation toward a
predetermined fixed target by means of a single reflecting element
or a set of reflecting elements (facets) controlled by a two-axis solar
tracking system.
Solar collector aperture area.
The maximum projected area that accepts solar radiation. The
geometrical dimensions of heliostat and parabolic trough
concentrators are separately shown in Fig. 1.16 (in which the side
length of a single heliostat is c) and Fig. 1.17. For a heliostat, the
concentrator aperture area ¼ 64  c2, and the concentrator’s
contour aperture area ¼ a  b. As shown in Fig. 1.17, a parabolic
trough concentrator consists of 7  12-meter-long units with a unit
length of c and a total length of a with concentrator aperture
area ¼ 7  b  c and concentrator contour aperture area ¼ a  b.

32

1. INTRODUCTION

b
c

a
FIGURE 1.16 Geometrical dimensions of heliostat.

a
b
c
FIGURE 1.17 Geometrical dimensions of parabolic trough concentrator.

17. Receiver aperture area. The maximum receiver flat area that
accepts concentrated solar radiation. This is the area of the flat
surface defined by the outer perimeter of the receiver, including
nonactive zones (if any) between adjacent receiver elements
composing the receiver. For receivers without a secondary
concentrator and composed of several parallel tubes, it is given by
the product of the total length of each tube and the total width of
the receiver. For receivers without a secondary concentrator and
composed of a single tube, it is given by the product of the total
length and the diameter of the receiver tube (excluding the glass
cover, if any). For receivers with a secondary concentrator, it is given
by the product of the total length of the receiver and the width of the
aperture area of the secondary concentrator. For cavity receivers, it is
the flat surface associated with the aperture of the cavity.
18. Concentrator performance requirements.
While the concentrator is receiving and reflecting solar energy,
there exist specular reflectance losses, including specular loss,
Cosine loss, shading and blocking loss, atmospheric attenuation loss,

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

33

and spillage loss. Based on this, the concentration field layout
design must consider the causes for these losses and mitigate them
through a reasonable layout of concentrators to collect more solar
irradiation through the receivers.
a. Specular loss. Based on the need for concentrating efficiency,
specular reflectance on the reflective surface of the concentrator
is normally high, about 0.93e0.94. However, as a heliostat is
exposed to atmospheric conditions while functioning,
environmental factors such as dust and humidity will contribute
to decreases in specular reflectance.
Fig. 1.18 shows the measured result of one heliostat in the Beijing
Badaling CSP plant, based on which we can see that influences
of dust accumulation contribute to a decrease in specular
reflectance from 94.6% on Aug. 23, 2011, to 45.5% on Oct. 10,
2011. On Oct. 13, 2011, due to rain, the specular reflectance
climbed back to 82.1%.
b. Cosine loss. In order to reflect sun beam onto a fixed target, the
surface of the heliostat is not always perpendicular to the
incident light and may create certain angles. Cosine loss is
generated because of the reduction of the heliostat surface area
against the sun beam projected area caused by such an
inclination. The value of cosine efficiency is in proportion to the
cosine value of the angle between the heliostat surface’s normal
direction and incident solar radiation. When arranging
heliostats in a concentration field, heliostats must be arranged in
areas with the greatest cosine efficiencies possible.

FIGURE 1.18 Influence of dust on the reflectance of the heliostat. Provided by the Institute
of Electrical Engineering, Chinese Academy of Sciences, 2012.

34

1. INTRODUCTION

FIGURE 1.19

Cosine loss of heliostat.

Fig. 1.19 shows the ratio of solar irradiation received on certain
areas of the surface to the maximum received solar irradiation,
which is equivalent to the cosine value of the angle between the
incident beam and the normal direction of the receiving surface.
c. Shading and blocking losses. As shown in Fig. 1.20, shading loss
occurs when the reflective surface of a heliostat is under the
shadow of one heliostat or several neighboring heliostats. Due to
the shading of the frontal mirror, the rear heliostat might not
receive any solar radiation. Such circumstances are especially bad
during winter when the height of the sun is comparatively low.

Reflected
light
Incident
light

Heliostat

Blocking
Shading

FIGURE 1.20

Shading and blocking losses.

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

35

FIGURE 1.21 Influence of blocking from frontal heliostat on rear heliostat’s reflected
light. Provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2013.

Shadow from the receiver tower or other objects will cause
certain shading loss for the heliostat concentration field as well.
Fig. 1.21 displays the influence of blocking from the frontal
heliostat on the rear heliostat’s reflective light. Fig. 1.22 shows
the influence of receiver tower shadow on the concentration
field. Although the heliostat is not under any shadow, the shade
of the rear side of neighboring heliostats may cause a situation
where the reflective solar irradiation is not received by the
receiver, the corresponding loss of which is referred to as blocking
loss. In Fig. 1.21, the bright band at the upper section of the
heliostat is caused by the rear heliostat’s reflective light being
blocked by the frontal heliostat. The frontal heliostat blocks the
path of solar radiation between the rear heliostat and the receiver.
Values of shading and blocking losses are relevant to the time
when solar energy is received as well as the position of the
heliostat itself, which are calculated mainly based on the
projected area of neighboring heliostats on the calculated
heliostat along the solar incident light direction or along the
reflected solar beam direction of the receiver mounted on the
tower. Normally, it is necessary to consider shading and
blocking on the calculated heliostat caused by several
neighboring heliostats. For partial heliostats, it might be
possible for the overlapping of shading and blocking losses,
which should be taken into consideration during calculation.
When designing a heliostat concentration field free of blocking,
the distance between heliostats necessarily increases, and the

36

1. INTRODUCTION

FIGURE 1.22

Influence of receiver tower shadow on concentration field. Provided by the
Institute of Electrical Engineering, Chinese Academy of Sciences, 2013.

mean distance between heliostats in the concentration field and
tower increases as well. Thus the heliostat’s optical efficiency
decreases, as does the annual efficiency of the entire
concentration field. Thus blocking-free design does not equate
to optimized design.
Considering the causes of shading and blocking losses, heliostats
should not be mounted too closely to each other. Based on this
point, it is possible to properly reduce mutual blocking by
restraining the distance between neighboring heliostats.
To realize comprehensive utilization of land, plants can be grown
at the bottom of the heliostat. In this case, it is necessary to analyze
the influence of the heliostat on the solar irradiation receiving of
the ground surface. As shown in Fig. 1.23, this is the mean
shading rate of the ground surface in the heliostat concentration
field of the Beijing Badaling solar tower power plant from 8:
00e16:00 on March 21. The shadow at the outer margin of the
figure indicates no shading.
d. Atmospheric attenuation loss. When solar radiation is reflected
from the heliostat to the receiver, the energy loss of solar
irradiation caused by attenuation during atmospheric
propagation is referred to as attenuation loss. The degree of
attenuation is normally relevant to the height of the sun (which

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

FIGURE 1.23

37

Analysis of heliostat concentration field shadow.

changes over time), local elevation, factors caused by
atmospheric conditions (such as dust, moisture, and carbon
dioxide content), and distance. The further the heliostat is from
the receiver, the greater the attenuation loss. Thus the heliostat
concentration field layout should be restrained to a certain range
that is not too distant from the receiver. Fig. 1.24 indicates the
solar radiation loss (atmospheric attenuation loss) when aerosol
concentration in the air is high at the Beijing Badaling solar
tower power plant, from which the solar beam caused by solar
radiation scattering can be clearly identified.
e. Spillage loss. This refers to the loss of solar radiation energy
reflected from the heliostat that overflows to the atmosphere
without reaching the surface of the receiver.
The size of a facula generated by a heliostat on the surface of the
receiver aperture is mainly relevant to the heliostat’s mirror
shape error, tracking control error, and solar cone angle.
Furthermore, it is related to the relative position of the heliostat
against the receiver and also changes with the position variation
of the sun. All of the above factors influence the concentration
effect of the heliostat, which is likely to result in the generation of
larger faculae by the reflective solar beam of the heliostat on the
surface of the receiver aperture and overflow from the receiver
aperture to the atmosphere (refer to Figs. 1.25 and 1.26).

38

1. INTRODUCTION

FIGURE 1.24

Atmospheric attenuation loss. Provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2011.

FIGURE 1.25 Spillage loss. Provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2013.

Therefore, a range of heliostat concentration field layouts should
be defined while considering the concentration performance of
the heliostat, the dimensions of the receiver aperture and other
factors to ensure that the heliostat on the ground is capable of
concentrating reflective solar beam radiation within the receiver
aperture to the largest extent possible.

39

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

FIGURE 1.26 Spillage loss of Gemasolar solar tower power plant owned by Torresol
Energy, Spain. Picture provided by SENER, Spain, 2018.

Various losses are calculated in Table 1.5.
Solar radiation obtained on the receiver in the end shall be the
sum of all the energy projected by the heliostat on the receiver in
the entire concentration field:
X
(1.1)
E¼
IAhcos href hS&B hatt hint
The optical efficiency of the heliostat concentration field is:
hfield ¼ hcos href hS&B hatt hint

(1.2)

Features of software for calculating the annual mean optical performance of the heliostat concentration field are compared in Table 1.6.
19. Mirror surface errors of concentrator. This is the error caused by the
inconsistency of the actual reflective surface and the theoretical
reflective surface of the concentrator, including canting position
TABLE 1.5 Calculation of Various Losses
Factor
Solar direct normal
irradiance/(W/m2)
Cosine loss (hcos )
Shadow and shade
losses (href ,hS&B )

Calculation

Factor

Calculation

I

Atmospheric
attenuation loss
(hatt )

Ihcos href hS&B hatt

Ihcos

Spillage loss (hint )

Ihcos href hS&B hatt hint

Ihcos href hS&B

Concentration field
aperture area (A)

IAhcos href hS&B hatt hint

TABLE 1.6 Feature Comparison of Softwares for Calculating Annual Mean Optical
Performance of Heliostat Concentration Fields
Model

RCELL

WINDELSOL

ASPOC

Arrangement
pattern

Radial staggered/
northesouth staggered/
Radial cornfield/
northesouth cornfield

Radial staggered

Radial staggered

Performance
calculation

Annual calculation time
points: 200
Minimum solar
altitude: 0

Annual
calculation time
points: 29
Minimum solar
altitude:
15 degrees

4-Degree
polynomial fit for
5-day energy data
4-Degree
polynomial fit for
a daily 9 h energy
value

Definition of
unit and
quantity

Northesouth square of
11  11 or 21  21 units
Dimensions of the unit
are related to the height
of the tower
Performance calculation
for the center of each unit
No quantity restriction
for unit

12  12 Units
with specified
space
Radial size of
each unit is
related to the
height of the
tower
Performance
calculation for
each unit

8  10 Units with
specified space
Energy
calculation for the
heliostat at the
center of each
unit

Annual
energy

University of Houston’s
energy flux model
Truncation factor is not
related to time
Shadow and shade losses
in annual mean values
(not related to the height
of the tower)

Improved
Houston energy
flow model
Use annual mean
truncation factor
Calculate annual
mean shadow
and shade losses
(not related to
the height of the
tower)

Gauss energy
flow distribution
model
Use the hour
truncation factor
Calculate the
hourly shadow
and shade losses
related to the
height of the
tower

Density

Different radial space and
circumferential space for
corresponding units
Circumferential spaces of
various units are
equivalent to each other
during optimization and
are determined through
minimum unit energy
cost optimization
Radial space is
determined according to
terrain and shading
design

To be provided
with space
parameter from
the external
source

Radial space and
circumferential
space are
determined by
three individual
variables

Memo

Calculate shadow and
shade losses of 48
neighboring heliostats

Consider
shadow and
shade losses of
12 neighboring
heliostats

Consider shadow
and shade losses
of neighboring
heliostats within
a radius of 50 m

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

41

FIGURE 1.27 Concentrator mirror surface errors. (A) Canting position error. (B) Mirror
slope error.

error and slope error, in which the incidence point position of solar
radiation is inconsistent with the expected position and is subject to
position error; this position error is mainly caused by installation,
namely support structure location error. Fig. 1.27A indicates an
incident solar direct radiation position error caused by altitude
angle error due to inappropriate installation of the support
structure of the reflective surface.
A surface slope at the incidence point inconsistent with the
theoretical value is subject to slope error, namely the normal error on
the reflective surface, which is related to the mirror manufacturing
process, field assembly, temperature, gravity deformation, wind
force, and other error factors (refer to Fig. 1.27B).
20. Efficiency of concentration field. This is the ratio of solar radiation
energy (kWh) reflected or transmitted by the concentration field
into the aperture of the receiver to the total direct normal solar
radiation energy (kWh) on the mirror surface of the concentration
field within unit time. According to Eqs. (1.1) and (1.2), due to the
position variation of the sun for the parabolic trough concentrator
and heliostat, the efficiency of the concentration field changes
according to different solar angles, whereas the efficiency of the
dish concentrator does not change with the solar angle.
21. Annual efficiency of the concentration field. This is the ratio of the
solar radiation energy (kWh) reflected or transmitted by the
concentration field into the aperture of the receiver within a year to
the total incident solar direct normal radiation energy (kWh) on the
mirror surface of the concentration field.

42

1. INTRODUCTION

1.2.3.2 Thermodynamic Terms
1. Parabolic trough surface. This is the trajectory of the line parallel to
the fixed line and moving along a certain parabolic curve, as shown
in Fig. 1.28.
2. Receiver efficiency. This is the ratio of the total energy in the
receiver obtained through heat transfer medium to the total
energy that enters into the aperture of the receiver within unit time.
3. Receiver peak flux density (unit: W/m2). This is the maximum
radiation energy flux density received on the receiver surface.
This value is crucial in receiver design, which determines the
material (allowable energy flux density), thermal transfer structure,
and mechanical structure of the solar receiver.
Figs. 1.29 and 1.30 indicate the flux density distribution of the solar
concentrator at different places against the focal point, in which the
peak flux density at a place 20 cm from the focal point is 80 kW/m2
and peak flux density at the focal point is 350 kW/m2.
The relationship between the position of the aperture of the
receiver and the absorber can also be determined by applying the
method indicated in Figs. 1.29 and 1.30.
4. Rated thermal power of receiver (unit: W). This is the output
thermal power of the receiver at the design point. Rated value
refers to the corresponding value at the design point.
This value depends on one of the most important parameters for
calculating the thermal balance of the system. The type of receiver,
thermal storage, heat exchanger, and steam turbine are selected by
using this data. The thermal storage operational mode of the
system is also related to this value.

–20

–20

0

0

20

0

20

FIGURE 1.28 Parabolic trough surface.

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION

43

kw/m2
80

Position /mm

60
50

200
40
30
100
20

100

200

300

0

500

400

Radiation flux density /(kw/m2)

70
300

Position /mm
FIGURE 1.29 Energy flux distribution of a solar concentrator on a target plane located
20 cm from focal point.

kw/m2
350

0

Position /mm

100

250
200

200
150
100
300
50
90% power circle

0

100

200

0

Radiation flux density /(kw/m2)

300

Center of the spot

300

Position /mm
FIGURE 1.30 Energy flux distribution of the solar concentrator on the target plane
located at focal point.

Table 1.7 shows data for the PS10 tower power plant located in
Spain. The rated optical efficiency of the concentration field is
distinguished from the annual mean optical efficiency of the
concentration field. When designing thermal storage according to

44

1. INTRODUCTION

TABLE 1.7 Relationship Between Rated Thermal Power and Annual Mean
Efficiency at PS10 Power Plant
Item

Data

Item

Data

Rated optical efficiency of
the concentration field

77%

Annual mean shading
and blocking efficiency
of the concentration field

>95.5%

Annual mean optical
efficiency of the
concentration field

64.0%

Rated output solar
radiation power of the
concentration field

55.0 MW

Mean cosine efficiency of
the concentration field

>81%

Annual mean output
solar radiation power of
the concentration field

45.7 MW

5.
6.
7.

8.
9.
10.
11.

the rated output and annual mean output, different capacities and
operational modes can be obtained, which normally are calculated
based on the rated thermal power of the receiver.
Besides the properties and thermal parameters of the concentrator
and receiver themselves, major external factors that influence this
value include position, irradiance, ambient air temperature, and
wind speed.
Receiver net thermal power (unit: W). This is the energy of the
receiver transmitted to the working fluid within unit time.
Thermal storage capacity (unit: J). This is the parameter for
describing the amount of