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Performance Simulation For
Parabolic Trough Concentrating
Solar Power Plants And Export
Scenario Analysis For North Africa
By
Daniel Horst
A Thesis Submitted to the
Faculty of Engineering at Cairo Universityin Partial Fulfillment of the
Requirement for the Degree of
MASTER OF SCIENCE
In
MECHANICAL POWER ENGINEERING
FACULTY OF ENGINEERING, CAIRO UNIVERSITY
GIZA, EGYPT!"#!
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Performance Simulation For
Parabolic Trough Concentrating
Solar Power Plants And ExportScenario Analysis For North Africa
By
Daniel Horst
A Thesis Submitted to the
Faculty of Engineering at Cairo University
in Partial Fulfillment of the
Requirement for the Degree of
MASTER OF SCIENCE
In
MECHANICAL POWER ENGINEERING
%&'() *+,()-./.0& 01
Dr. Adel Kahlil Dr.-Ing. Jrgen
Schmid
Dr. Carsten Pape
Professor Professor
In Mechanical PowerEngineering
Department
In ElectricalEngineering
Department
Frauenhofer-Institutefor Wind Energy and
Energy System
Technology
Faculty of Engineering
Cairo University
Faculty of Engineering
Kassel University
FACULTY OF ENGINEERING, CAIRO UNIVERSITYGIZA, EGYPT
!"#!
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Performance Simulation For
Parabolic Trough Concentrating
Solar Power Plants And ExportScenario Analysis For North Africa
By
Daniel Horst
A Thesis Submitted to the
Faculty of Engineering at Cairo University
in Partial Fulfillment of the
Requirement for the Degree of
MASTER OF SCIENCE
In
MECHANICAL POWER ENGINEERING
Approved by the Examining Committee:
______________________________
Prof. Dr. Adel Khalil
______________________________________
Prof. Dr.-Ing. Jrgen Schmid
_______________________________________
Dr. Hany Nokrashy
FACULTY OF ENGINEERING, CAIRO UNIVERSITYGIZA, EGYPT
!"#!
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I
Table of Contents
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VI
Figure 52: Schematic drawing of evaporation cooling system with air-cooling
tower69
Figure 53: Schematic drawing of condenser connected to a dry cooling system...71
Figure 54: Plant performance calculation for SM1 equipped with dry cooling system
at a location of 31N 29E for the meteorological year 2007. Collector orientation is
north south..73
Figure 55: Plant performance calculation for SM4 equipped with evaporation cooling
system at a location of 31N 29E for the year 2007. Collector orientation is north
south ...74
Figure 56: Feed in series direct calculated and optimised and for CSPP SM 4 with
wet cooling and back cooling system located at 31N 29E. CSOMO-EU ambient
data for 2007..76
Figure 57: Schemata for the ELLC calculation [UVE].77
Figure 58: Method of recursive convolution, example for 3 power plants [DENA1]..78
Figure 59: Distribution of exporting CSP plants for Germany, with installed gross
capacity of 200 MWelfor each plant, in North Africa for 205080
Figure 60: SIM-EE model IWES for simulation of the residual load curve [UBA]...82
Figure 61: Annual load duration curve for Germany in 2050 with an energy supply by
100% renewable energy sources [UBA]84
Figure 62: Residual load curve from the UBA study 100% renewable electricity
supply by 2050 [UBA]..84
Figure 63: SIM-EE model IWES for simulation of the residual load curve. Changes
are under taken in the position of importing energy according to the simulation in this
thesis. [UBA]...85
Figure 64: Weekly average energy production from CSP plants SM1 based on the
weather data COSMO-EU 2007.87
Figure 65:Residual load curve without import energy and residual load curve
including import energy form CSP with SM2. Irradiation and ambient data fromCOSMO-EU model 200788
Figure 66: Partial view of figure XX residual load curve without imported energy and
residual load curve including imported energy from CSP with SM2. Irradiation and
ambient data from COSMO-EU model for 2007.88
Figure 67: Annual load duration curve for the residual load with and whiteout CSP
import SM289
Figure 69: Residual load curve without imported energy and residual load curve
including imported energy from CSP with SM3. Irradiation and ambient data from
COSMO-EU model 200790
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VII
Figure 69. Partial view part of figure XX residual load curve without import energy
and residual load curve including import energy form CSP SM3 as well as energy
production CSP. Irradiation and ambient data from COSMO-EU model for 2007..90
Figure 70: Annual load duration curve for the residual load with and whiteout CSP
import SM3.91
Figure 71. Residual load curve without imported energy and residual load curve
including imported energy form CSP SM4. Irradiation and ambient data from
CSOMO-EU model 2007.92
Figure 72. Partial view part of figure XX residual load curve without imported energy
and residual load curve including imported energy from CSP with SM4. Irradiation
and ambient data from COSMO-EU model for 2007..92
Figure 73: Annual load duration curve for residual load with and without CSP import.
Ambient data COSMO-EU 2007.93
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VIII
List of Tables
Table 1: Compulsive and optional criteria for the exclusion of land used for CSP
plants [SID].29
Table 2: Areas for CSP in km2
available in the MENA countries for different DNI
Classes [SID]..35
Table 3: Concentrated solar thermal potential in North Africa [MCSP]....42
Table 4: Basic input parameters for individual simulation of CSPP..49
Table 5: Input parameter for scenario analyze..50
Table 6: Design parameters for simulation of the solar field for CSP plant with gross
capacity of 200 MWel[DLS].57
Table 7: Design parameter for storage simulation [DLS]60
Table 8: Design parameters for simulation of the power block whit gross capacity of
200 MWel [DLS].64
Table 9: Design parameters for water steam calculation with a wet cooling system
[DLS]67
Table 10: Design parameters for water steam calculation evaporation cooling system
[DLS|69
Table 11: Design parameters for water steam calculation dry cooling system
[DLS]71
Table 12: Installed exporting CSP plants and gross capacity separated by countries
for North Africa 2050.81
Table 13: Unplanned outage probability [DENA1]...85
Table 14: PPN for ELCC simulation...86
Table 15: Summery of the results from export scenario analyze..97
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IX
List of Symbols
A = Area
C = Concentration ratio (chapter 2) / ventilator constant (chapter 4)c = specific heat capacity
D = distance collector row
F = area angle (chapter 2.2) / control function (chapter2.3)
f = focal length (chapter 2.) / solar field coefficient (chapter 4)
G = irradiation at absorber pipe (chapeter2.2) / Gibbs free energy (chapter2.3)
H = height of collector
h = specific enthalpy / probability density function (chapter 4.4)
I = global horizontal irradiation
Ib= global horizontal beam radiation
Id= global horizontal diffuse radiation
L = length of the collector
l = length of the not irradiated absorber part
m = mass flow
N = number of nodes
n = number of the day
P = power / black out probability (chapter 4.4)
p = pressure
Q = thermal energy
q = specific thermal energy
S = entropy
s = specific entropy
T = temperature in Kelvin
t = time
U = heat transfer coefficient (chapter2.2) / internal energy (chapter 2.3)
V = volume (chapter 2) / availability collector field (chapter 4)
v = specific volume (chapter 2) /wind speed at ground height (chapter 4)
W = work
w = specific work (chapter 2)/ speed (chapter 4)
x = distance (chapter 2) /water contend (chapter 4.3)
z = distance for water transportation
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X
Greek letters
!s= surface azimuth
!r= surface azimuth relative"= tilt angle
"s= solar elevation
#= storage capacity
$= angle of declination
%= emissivity
&= latitude
'= efficiency
(= relative humidity
)= density
*= incidance angle
*Z= zenith angle
+= solar field reduction factor
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List of Abbreviations
ALB_RAD: ground albedo
ASOB_S: short wave radiation at ground highCC: capacity credit
CSP: concentrated solar power
CSPP: concentrated solar power plant
DLR: German aerospace center
DNI: direct normal irradiation
DSMW: digital soil map of the world
ELCC: effective load carrying capacity
GDP: gross domestic product
GHG: green house gasses
GIS: global information system
GLCC: global land cover characterization
GNI: gross national income
HC3: HelioClime3
IDR: Incidence direct radiation
IEA: international energy agency
IWES: Frauenhofer-institute for wind energy and energy system technology
IPCC: Intergovernmental Panel on Climate Change
LOLE: loss of load probability
LOLP: loss of load probability
MENA: Middle East and North Africa
NDVI: normalized difference vegetation index
PPA: purchasing power parity
PPN: power plant network
RE: renewable energies
SM: solar multiple
SEGS: solar energy generation system
UBA: German Federal Environmental Agency
WBGU: German Advisory Council on Global Change
WCU: World conservation Union
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Abstract
Since the Intergovernmental Panel on Climate Change (IPCC) report 2007
established: it is very likely that global warming nowadays is man made[IPCC2007a], it becomes obviously that the emission of CO2 have to be reduced
drastically. The green house gas (GHG) concentration for 2010 was 39% above the
preindustrial level. Therefore the warming trend has increased significantly over the
last 50 years [IPCC2007a]. Still GHG emissions associated with the provision of
energy services are the major cause of climate change. Nevertheless, cold-fired
power plants are still the basis of electricity production all over the world. In order to
work against this trend research and political influence is necessary to avoid the
negative impact of the global warming. Based on the history of industrial
development the industrialized countries have the duty to reduce their own
emissions, as well as supporting developing countries by doing so. In this context,
options like exporting solar energy form the deserts of North Africa to Europe would
offer great possibilities and both sides might have a benefit. Because of the high
solar irradiation form 2500kw/m2/y in some parts of North Africa and the high share of
direct radiation concentrated solar power plants (CSP) are an excellent option for a
sustainable electricity production in this region.
The thesis is providing a model witch can calculate the electrical output of CSP
parabolic trough plants for several locations in North Africa. Furthermore it is
analyzed how these CSP plants can contribute to a 100% renewable energy supply
in Germany for the year 2050. Therefore the first part of this thesis presents a model
estimating suitable locations for CSP plants in North Africa and calculating the
electrical output of CSP plants. Several criterias for land use, annual irradiation and
infrastructure, are processed, and rules for depositing CSP plants were specified. In
alliance with weather prediction models an energy-balanced CSP model simulates
the electrical output of exclusive solar driven CSP parabolic trough plants. By using
different configurations the simulation can display various forms of storage sizes and
different cooling system.
The second part of the thesis is focused on how the CSP plants can support a 100%
renewable energy system in Germany in the year 2050. Therefore, the effective load
carrying capacity (ELCC) and the capacity credit (CC) for a number of CSP plants in
relation to the residual load in Germany is evaluated. The focus is on the influence
different storage sizes have on the possibility to supply energy on demand for
Germany.
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The work shows that even a high capacity credit of around 53% for solar multiple four
CSP plants is not necessarily helpful to reduce significant the high demand in
Germany during winter times. This can be mainly related to the seasonality noticed in
the CSP output even whit high storage capacities.
The thesis is outlined by the MED-CSP study [MCSP] and the study 100%
renewable electricity supply by 2050[UBA2050] of the German federal
environmental agency.
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1. Introduction
1.1 Renewable energy and climate change
The 2007 IPCC report states that Most of the observed increases in global average
temperature since the mid-20thcentury is very likely due to the observed increase in
anthropogenic greenhouse gas concentration. [IPCC, 2007a]
The concentration of CO2 has been increased to about 390 ppm CO2at the end of
2010, 39% above the pre-industrial level. This lends to an average global
temperature increase of 0.76 C. In order not to exceed a global temperature
increase of 2C with a probability of 67% the CO2concentration has to be limited at
450 ppm by 2050. This means that only 750 billion tons of GHG can be emitted until2050. For that reason the 2050 GHG emissions must be bisecting in relation to the
emissions of 1990. Taking into consideration historical reasonability as well as their
economic strength, developed countries must reduce their emissions by about 80%
to 95% by 2050. Countries in transition and developing countries have more time to
reduce their emissions. However, taking into consideration the growth of population
and the ongoing development in countries of transition and developing countries, a
significant increase in energy consumption along with GHG emissions will be
noticed. In 2011 the global energy consumption was around 510 EJ/year compared
to 340 EJ/year in 1990. In parallel to this trend, the yearly amount of GHG emissions
is increasing, reaching 30 Gt CO2 in 2010. If this trend is ongoing the limit of 750 Gt
GHG emissions will be reached before 2035 and it would cause the 2C goal to not
be met. Nevertheless all societies require energy services to meet basic human
needs like lighting, cooking, mobility, communication, and to serve productive
processes, but GHG emissions associated with the provision of energy services can
be seen as the major cause of climate change [IPCC, 2007a]. Consequently, other
ways of energy production have to be found.
1.1.1 Solar technology
The use of renewable energies (RE) offers a great chance to reduce the GHG
emissions in an economical way. The costs and the challenges for the integration of
RE into an existing energy supply system is mainly dependent on the actual system
characteristic, the current share of RE and the availability of RE resources. Based on
these circumstances the WBGU recommends establishing representative projects for
introducing RE on a large scale. In this way, incentives on a strategic level for a
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global change in energy policies can be set. A strategic partnership between the
European Union (EU) and the MENA region can be seen as a key element of such a
policy. In such cooperation the EU can offer technologies and finances in order to
address their national and international responsibility for climate protection. The
MENA region can benefit from transferring their renewable resources as an export
product along with benefiting from the accompanying economic growth. [DLR, MED-
CSP]
The MENA countries have the greatest potential for using direct solar energy on a
global scale, with a minimum of 412 EJ/year and a maximum of 11,060 EJ/year.
Solar energy especially offers a significant potential for the near-term (until 2020) and
the long-term (until 2050) climate change mitigation. The wide range of technologies
for harvesting the energy provided by the sun can be excellently implemented in
North African countries and the Middle East. The costs of these technologies have
considerably reduced over the last 30 years and political conditions as well as
technical development will offer additional cost reduction in the future. Besides
thermal energy for heating and cooling, PV systems and concentrating solar power
electricity generation can play an important roll in the MENA countries. The majority
of the worlds electricity production nowadays derives from nuclear, coal, gas, oil,
and biomass driven power plants. The CSP plants work on the same concept while
simply providing an alternative heat source. Therefore CSP can benefit from not only
the improvements made in the solar concentrator technologies but also from the
ongoing advantages in steam and gas turbine cycles. Further advantages of this
technology are that it does not need exotic materials and adding a thermal storage
for operations on grid stability is possible. Also the CSP technologies that can be
used are cheap. It can be installed in small-scaled applications with a few kW-
installed-capacities (dish/Stirling systems) up to multiple MWs (tower and trough
systems). [IPCC, SREEN]
Parabolic trough plants can be operated as hybrid plants, together with gas turbines,that can be used as desalination plants or operating as pure solar plants with
different storage sizes. Actualization of this technology already exists in the MENA
region like the Kuraymat power station. The focus in this work is with parabolic
trough technology installed in North African regions. The next part will give a rough
overview about the parabolic trough technology.
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1.2Parabolic trough technology
The solar field produces thermal energy by using direct normal irradiation (DNI), and
delivers this energy to a steam power plant. The solar field can be considered in a
first approach as a solar steam generator.
The glass mirror of the solar field has a parabolic shape and is reflecting the
incoming direct radiation with a concentration value of around 80 to the absorber
tube. One of the most modern Collectors nowadays is the type LS-3. One LS-3
collector consists of 224 mirror segments, where each segment has an area of 2.68
m2. Taking into consideration the bending of the mirrors, an area of 545 m2 is
reached with one LS-3 collector. In addition the collector contains 24 absorber tubes.
The complete construction is a lightweight metal structure, which normally is
equipped with a single axis tracking system.
Figure 1: Functional principle of a parabolic trough [REB]
A complete solar field contains several parallel rows of solar collectors, which get
connected in loops of normally 6 LS-3 collectors. The power block is located in the
center of the solar field. The distance between the collector rows is planned
according to minimizing the piping costs on the one hand and having a minimal
shading effect between the rows on the other. In general the design of the solar field
depends on plant and collector size, the temperature and pressure losses in the
piping system and the specific ambient conditions. Parabolic trough fields can be
erected in any direction, but erected in a north-south direction leads to the highest
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possible energy yield over the year while an east-west orientation smoothes down
the seasonal fluctuations.
Some of the components like the metal structure, the tracking system, the controllers
and other subsystems, which make up around 60% of the direct solar field costs, are
standard components and can be ordered from several countries and in different
forms. The reflectors and the absorber tube, however, are special components and
have to be produced specifically for the parabolictrough solar field.
1.2.1 Reflectors
The concentrators (see figure 1) consist of a heat-formed glass cake. It is carried by
the metal structure of the collector. By using special production techniques, like the
float-glass method, absolute evenness of the cake is guaranteed. Glass, which is
used in solar applications, must have very low iron content for getting a transmissivity
in the solar spectrum of around 91%. The iron content of a so-called White Glass is
around 0.015% compared to normal glass with an iron content of around 0.13%. The
binding of the reflectors is done under heat conditions. Several safety layer coatings
are added, giving additional protection for the mirror. Finally the contour accuracy is
tested using a laser beam.
1.2.2 Absorber
For the LS-3 collectors the absorber pipe consists of a stainless steel tube with a
length of 4 meters and a thickness of 70mm. A glass pipe surrounds the tube (see
figure 2). The glass tube allows evacuating of the area between the absorber tube
and the glass pipe in order to minimize convection and conduction heat losses.
Figure 2: Absorber tube of a parabolic trough collector [REB]
The vacuum also serves to protect the highly sensitive coating. Nowadays, such
selective coatings remain stable in temperatures of 450C upwards to 500C. On
average the solar absorption is currently above 95% and at an operational
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temperature of around 400C the emissivity is below 14%. This leads to an optical
efficiency of around 80% for upcoming perpendicular radiation. Furthermore the
hydrogen getter (see figure 2) absorbs the hydrogen, which is getting through the
glass pipe and the stainless steel pipe by diffusion. A membrane finally pumps the
hydrogen out of the vacuum. As a final point, glass/metal joints realize extension
bellows compensating the thermal expansion of the pipe, and the connection
between the glass pipe and the metal structure.
1.2.3 Tracking and controlling
Solar fields in a CSP plant use single axel tracking systems. The tracking is
according to the position of the sun and/or the requirement of the power block.
Therefore a solar sensor is used to evaluate the sun position. Sensors consisting of a
convex lens focus the sun light to a small photovoltaic cell, reaching a resolution of
around 0.05%. These kinds of sensors are used in the so-called SEGS plants, where
they prove most effective. The tracking system must have sufficient torque to operate
the collectors even at higher wind speeds. For LS-3 collectors normally electro-
hydraulic drives are used. In the design specs the movement can take place with a
speed of 9 m/s. For emergency reasons or for operation conditions, which are not
requiring a high optical efficiency, the speed can be increased up to 20 m/s.
In existing plants the controlling of the field takes place in two separate stages. The
overall control is located in the central control room and the second stage is placed
on each collector unit. The local units take care of the incoming irradiation; wind
speed and mass flow of heat transport medium. In case of emergency the local units
can shut down parts of the solar field. The overall unit operates the solar field
according to the overall plant requirements, mainly the electrical output in relation to
the actual solar radiation.
1.2.4 Heat transfer medium
At the moment high-boiling synthetic thermal oil has been applied as the heat
transfer medium in the absorber tubes. According to the thermal stability of this oil
the actual operation temperature of the solar field is approximately 400C. When
operating at this temperature, the oil has to be pressurized at around 12 to 16 bar.
The thermal oil is circulating in the collector tubes, where the driving forces are
speed adjustable pumps. For the purpose of thermal expansion during its heat up an
expansion vessel is installed.
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1.2.5 Thermal storage
The availability of thermal storages currently plays an important role for the economic
success of solar thermal power plants. In parabolic trough plants sensible heat
storages, operating with temperatures between 300C and 400C, are in use.The storage has a significant influence on the operating conditions of the solar
thermal plant. Changes in the solar radiation availability lead, without proper storage,
to a change in the electrical output. This not only leads to the plant having a reduced
supply security, but also to a reduced lifetime of the steam turbine itself. Frequent
changes to the load of the turbine lead to more thermal stresses, thus reducing the
lifetime of the turbine. Larger storages are able to support load shift to non-day times,
for example the evening hours when peak load demand is needed. In combination
with an over dimension of the solar field the yearly operational hours can beextended significantly. Therefore the operation time of around 2,000 hours per year
can be increased to 4,000 hours per year by doubling the solar field and storing the
produced energy over the day. The solar multiple can extend up to 8,000 operating
hours per year. This allows solar multiple (SM) plants to operate as base load plants.
SM is used as an indicator of how much the solar field is oversized. SM 2 means an
additional energy for around 2,000 operating hours per year, SM 3 for a sum of 6,000
operating hours per year and so on. The figure 3 displays one possible arrangement
of a CSPP with thermal storage.
Figure 3: Basic concept for the integration of thermal energy storage into a solar thermal
parabolic trough power plant [REB]
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1.3 Thesis objectives and outlines
The objective of the thesis is to develop a physical model; witch can predict the
electrical output of CSP parabolic thought plants in North Africa. Furthermore it have
to be analyzed how these CSP plants can contribute to a 100% renewable energy
supply in Germany for the year 2050. Consequently the model must be able to deal
with data sets out of the SIM-EE model [UBA2050]. Using Matlab as programming
software it is possibility to fulfill this requirement.
So far no exclusive solar driven CSP plants installed in North Africa. Realistic
predictions for the future have to be developed, and ways of distributing the CSP
plants in North Africa must be evaluated. Therefore, the first work for the thesis is to
find a way to estimating suitable locations for CSP plants in North Africa. Criterias forland use, annual irradiation and infrastructure, can be used to do so. Furthermore
weather prediction model must be added to the simulation for getting exact results.
Also different configurations for the plant like various forms of storage sizes and
different cooling must be considered.
The second part of the thesis is implementing process criterias from the SIM-EE
model in order to compare the results from part one, with results from the SIM-EE
simulation for 2050. The focus should be on the effective load carrying capacity witch
will allow to make statement related to the influence import energy from North Africahas on the 100% renewable energy system in Germany for the year 2050.
The thesis starts with a theoretical description of the physical processes, occurring in
a parabolic trough CSPP. In the next chapter the recourses and the potential for CSP
plants in North Africa, related to several criterias are analyzed. The program
development is described in the 4thchapter. This chapter contains also the results of
the simulation program and the added program parts from the SIM-EE model. The 5th
chapter finally, presents the outlines and the results for the export scenario analyze.
Chapter six summarizes the results and is giving an outlook for possible further
investigations.
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2. Theoretical performance calculation CSPP
2.1 Geometrical relations
The energy contained in the direct normal radiation is harvest by the concentration
collectors of a solar field. By reflecting the received normal direct radiation to the
absorber, thermal energy is produced and transported to the power block.
Conventional technology is used to convert this energy into electrical energy.
Therefore the incoming radiation can be seen as the fuel of a CSP plant. For a
precise calculation of the CSPP performance it is essential to understand the
geometrical relations between the sun and earth at any time of the day. In this
chapter these theoretical relations are described.
2.1.1 Sun earth geometry
The geometrical relationship between the incoming direct radiation and a plane of
any orientation at every hour can be described in terms of several angles. The
following schematic drawing gives an overview of these geometrical relations.
Figure 4: Geometrical relation between sunbeam and tilted surface [REG].
The most important angel, in this arrangement is the so-called incidence angle ! it
can be calculated out of [SETP; S.14]:
(2.1)
Here " is the tilt angle of the plane, and # the surface azimuth angle, with 0 in the
southern direction, then becoming negative when moving towards an eastern
direction or positive in a western direction. Both angles can be either fixed or
adjusted according to sun position by using a tracking system. The angle of
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declination $ can be found by the approximate equation of Cooper (1969) [SETP;
S.16]:
" = 23.45 sin 360284 + n
365
#
$%
&
'( (2.2)
This angle represents the angular position of the sun at solar noon in relation to the
plane of the equator. The number of days in a year is represented by n. The hour
angle %sets the angular displacement of the sun east or west of the local meridian.
This is done due to rotation of the earth with a value of around 15 per hour. The
angle will be negative in the morning and positive in the afternoon. The latitude in
equation (2.1) is mentioned with &and reveres to a certain location.
The zenith angle can be understood as the incidence angle of beam radiation on a
horizontal surface. Therefore equation (2.1) can be simplified to [SETP; S.16]:
cos"z= (cos#cos$cos%+ sin#sin$) (2.3)
The sun height "s shown in figure 4 is the complementary angle of !z. Due to this
reason it can simply be stated as:
"s=90 #$
z (2.4)
Tracking systems for the collector are important in terms of harvesting the maximum
amount of energy out of the sunlight. The standard tracking system for a parabolictrough collector is a single axis tracking system. The collector orientation is normally
either in a north-south or east-west direction. For a fixed east-west direction the
surface azimuth is defined as [SETP; S.21]:
" =0 if "s
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" =90 if "s >0
#90 if "s$ 0
%&'
(2.8)
Consequently, the collector tilt angle is evaluated according to the following equation
[SETP; S.21]:
tan"=tan#zcos($% $
s) (2.9)
According to equation (2.8) the surface azimuth angle will be 90 or -90 depending
on the sign of the solar azimuth angle.
The incidence angle for a plane rotating about a horizontal north-south axis, with
continuous tracking, is [SETP; S.21]:
cos" = (cos2"
z+ cos
2#sin
2$) (2.10)
Based on the angles described in this chapter it is possible to calculate the incoming
beam radiation on a collector field, with one axis tracking system for a certain
location and time.
2.1.2 Sun collector geometry
For the operation of a CSPP temperatures of around 400C are necessary. These
high temperatures cannot be reached with a flat plate collector. Therefore
concentrating collectors are used. One type of these concentrating collectors is theparabolic trough collector. The direct normal radiation reaching the collector is
concentrated on the absorber tube located in the focal point of the parabolic
collector. The most important characteristic factor therefore, is the concentration
ratio. It is defined as the aperture area in relation to the absorber area [SETP; S.327]:
C=A
a
Aabs
(2.11)
Here Aa is the aperture area and Aabs the area of the absorber. For a three
dimensional system like a parabolic dish system with a two axis tracking system,
which is focusing on one point, the maximum concentration ratio is around 45,000.
However, the parabolic trough is a two dimensional system, where a maximum
concentration ratio of 200 can be reached.
The most significant losses under some circumstances occurring in a solar field are
the shading losses. This reduction is happening when one collector row reflects their
shadow onto the next row. In well-designed CSP plants this effect only shows up in
the morning or evening hours with shadowing due to low tilt angle. However at thesetimes shading losses can reach a maximum of up to 100%, where the rest of the day
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these losses are close to zero. The part of the collector area that is not in the shaded
region, can be approximately calculated according to [DLS; S.10]:
"SL
=1# 1# D
H cos$+cos%
r
cos$s
sin$&
'(
)
*+
&
'
((((
)
*
++++
1#
Hsin$sin%
tan$sD
H cos$+cos%
r
cos$s
sin$&
'(
)
*+L
&
'
((((
)
*
++++
(2.12)
If the result of the equation above is smaller or equal to zero the collector is
completely in the shadow. If the result is greater or equal to one, no shading losses
are occurring on the collector. It is obvious that the shading effect is dependent on
the collector size and the distance between the collector rows. In equation (2.12) D
represents the distance between the collector rows in meters, H the height of the
collector in meters and L the length of a collector row also in meters. The relative
azimuth angle #rcan be calculated as the absolute value of the different between the
solar and the surface azimuth angel.
Another loss factor occurs because, the incoming radiation to the collector is not
exact perpendicular and the absorber tube has a finite length. At the end of each
collector a certain part of the absorber tube will be not be irradiated. This
displacement of the sun image is shown in the following figure
Figure 5: Displacement of the sun image [SLP].
In the northern hemisphere these effects can be noticed especially during wintertime.
Normally these losses are under two percent in feasible areas for CSP plants.
Nevertheless, the reduction factor can be calculated according to [SLP; S.22]:
l = f tan"sin #$ #s( ) (2.13)
In formula (2.13) l represents the length of the not irradiated part, displayed in figure
5. The focal length f depends on the collector design. Consequently, the losses in
percentage can be calculated as:
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"EV
=1# l
L (2.14)
Further losses that occur depend on the finite earth sun distance where the beams
reaching the collectors are also not exactly parallel. As a result the sun image on the
absorbers is not precisely circular. The image can be seen in the form of an ellipse,
which changes the frame, depending on the angle of incidence. Only for!is zero the
image will be circular. By increasing the incidence angle the performance
characteristic of the sun image becomes worse. This happens because the absorber
is designed for a perfect circular sun image. This effect is called incidence angle
modifier and can be predicted according to Marco (1995) as follows [SLP; S.23]:
"IAM
= cos#(1+ sin3#) (2.15)
From formula (2.15) we can observe that the losses occurring are negligible.
Nevertheless, they do increase with distance from the equator.
Finally the irradiation reaching the collector can be separated in two parts. One is
exactly perpendicular the other one is horizontal to the collector. The collector
however only can reflect the perpendicular part of the radiation. This leads to the so-
called cosin-effect. Here the amount of useful irradiation can be calculated by
[DLS; S.9]:
"COS
= cos# (2.16)
Finally the amount of energy, which can be reflected per square meter of collector
area to the absorber, can be calculated [DLS; S.10]:
IDR ="COS
"IAM
"SL"EV
(2.17)
IDR stand for Incident Direct Radiation and represents the useful part of energy
provided under ideal conditions to the absorber tube.
2.2 Solar field
Inside of the absorber tube a heat transport medium, mainly synthetic oil, is used to
collect the thermal energy and transport it to heat exchangers for producing steam in
order to operate a steam turbine. During the transportion of the oil thermal losses can
be recognized, and also further losses during the concentration process have to be
taken into account.
2.2.1 Optical losses
Energy losses not only occur because of geometrical reasons like shading or
unirradiated absorber parts, but also from the material properties of the mirror, the
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hull pipe, and the absorber. Therefore, some operating figures will be defined. They
are all dependent on the design quality of the collector elements being used [DLS;
S.12]:
Reflectivity of the mirror:
A small part of the incidence radiation is not reflected to the absorber, because the
reflectivity of the mirror being finite. The mirror absorbs a part of the incoming
radiation. As an average the reflection coefficient of a mirror used in solar thermal
abdications can be set to '=0.93 [DLS; S.12].
Contamination of the mirror:
The part of irradiation, which is absorbed by the mirror, is increasing with ongoing
contamination of the mirror surface. Taking into consideration frequently washing
procedure the contamination factor can be estimated with $=0.98 [DLS; S.12].
Transmission factor of the mirror:
The glass on the top of the mirror also partly absorbs the irradiation. The irradiation
has to pass the glass cover two times, which leads to a transmission coefficient of
around (s=0.99 [DLS; S.12].
Quality factor of the mirror:
The quality factors depending on the production processes as well as the erection on
site. For example the absorber tube is not exactly mounted in the focal point of the
mirror additional losses will occur. Also different focal length of the mirror plats will
lead to additional losses. Nowadays a quality factor of )=0.90 is assumed. [DLS;
S.12].
Transmission factor of the hull pipe:
A small part of the reflected irradiation is again reflected by the glass pipe, which
surrounds the absorber tube. This transmission coefficient can be set here to (H=0.95
[DLS; S.12]. Absorption factor of the absorber pipe:
At the absorber tube not all of the reflected radiation will be absorbed. Due to
physical conditions a part of the radiation will always be reflected. The absorption
factor can be estimated with #=0.95 [DLS; S.12].
All the quality and material factors mentioned above are factored for an LS-3
collector type.
Taking into consideration all the additional factors above it is finally possible to
calculate the amount of energy received per square meter absorber pipe:
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GA =
IDR"#$%H%S2=IDR&opt (2.18)
As displayed in equation (2.18) the combination of all quality factors can be
summarized as the optical efficiency of the mirror *opt. In the formula above GA
represents the irradiation reaching the absorber pipe per square meter. Now the partof how the transformation into thermal energy and consequently, the losses occurring
during transportation in the absorber pipe is described.
2.2.2 Heat losses
Until now it has been described how the energy of the direct solar radiation reaches
the surface of the absorber. Here the form of energy is now changed into thermal
energy. This can be described by constructing an energy balance equation like [SLP;
S.23]:
GAA"=Q
n+Q
losses (2.19)
Here A is the absorber area and + is the absorbance of the absorber. Qn identifies
the useful thermal energy collected in the thermal oil and the thermal losses are
combined in QLosses.
Physically there are three different types of heat transportation, occurring naturally.
These are heat transport according to convention, conduction and radiation.
However these phenomena are the reason for the thermal losses in the solar field.The losses depending on conduction and convection can be set in a first approach in
a linear relation with the difference between the average absorber temperature and
the ambient temperature [SLP; S.24]:
Q
CC =UCCA(Ta"TU) (2.20)
UCC is the heat transfer coefficient, which is adjusted by measurement results. The
average absorber temperature is labeled Ta , and Tu is the ambient temperature. In
addition to the convection and conduction losses, the thermal radiation losses mustbe taken into consideration. Therefore the heat flux between two surfaces by thermal
radiation is described as [VDW; S.255]:
Q
rad =
"(T24#T1
4)
1#$1
$1A
1
+
1
A1F
1,2
+
1#$2
$2A
2
(2.21)
T represents the temperature for each of the two surfaces, F is the area angle
between the surfaces, A is the area of the surfaces, + is the Stefan Boltzmann
constant, and ,is the emission coefficient for each surfaces. In case of a solar field it
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can be assumed that the absorber area is relatively small compared to the ambient
area. These allows to simplify equation (2.21) to [SLP; S.24]:
Q
rad = "1A1#(T24$T1
4) (2.22)
In the formula above the emissivity of the environment is considered with one. A1
displays the area of the absorber. Now the energy used for heating up of the
collector, for example in the morning hours can be calculated by [SLP; S.25]:
Q
C =AUC(Tc"Ta ) (2.23)
UC shows the heat transfer coefficient of the collector and Tc is the collector
temperature. Putting all these thermal losses together and combining it with formula
(2.19) the amount of thermal energy produced by the solar field can be calculated
out of :
Q
n =G"A # AUCC(Tc# Ta ) #$%cA(Tc4#T
a
4) # AU
c(T
c# T
a) (2.24)
Consequently, the amount of energy collected in the absorber must be transported
either into storage or to the power block. Here additional losses in the absorber pipe
will be occurring. Finally these piping losses can be calculated according to [SLP;
S.33]:
Q
P =UP (TF"
Ta ) (2.25)
Now all necessary relations for the description of how the energy provided by the sun
is harvested and distributed either into storage or to the power block are described.
The point of interest is now the storage and power block arrangements.
2.3 Thermal Storage
There are two general types of thermal storage mechanisms. The first one is based
upon the use of sensible heat in various forms of solid and/or liquid materials. The
other storage type involves the latent heat of phase change reactions.
Sensible heat is added to a material simply by heating it up. Generally all energy that
is involved in changing the temperature of a medium is called sensible heat, and it
amounts simply to the product of the specific heat and the temperature change. [ES;
S.22]:
q ="cpV#T (2.26)
In equation (2.26) ' is the density of the storage material, cp the specific heatcapacity at constant pressure of the storage material and V the volume of the
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storage. A different mechanism for storing thermal energy involves a phase transition
with no change in the chemical composition of the storage material. A simple
example therefore is water. At low temperatures under 0C it is solid, at temperatures
between 1C and 99C it becomes liquid and at temperatures above 100C it
converts into gas. Thus, it can undergo two transition phases, with associated
changes in entropy and enthalpy. The Gibbs free energy, or chemical potential, for
the two phases are in equilibrium with each other at the transition temperature.
Therefore it can be evaluated according to [ES; S.23]"
"G = "H#T"S=0 (2.27)
At the temperature, the change in heat content -H at the transition temperature is
equal to T-S. The slope of the temperature dependence of the Gibbs free energy
and is proportional to the negative value of the entropy, which is different in diverse
phases and materials. Considering that state-of-the-art sensible heat storages are
used in CSP plants, here two different ways of calculating this kind of thermal
storage will be described.
2.3.1 Stratification Storage the Multi-node-model
Liquid storage tanks are operated at a significant degree of stratification. The degree
of stratification in real operation is strongly dependent on the design of the tank and
its location.
In the multi-node model heat storage is modeled and divided into N nodes in order to
display the stratification layers. In order to formulate the necessary equations, it will
be appropriate to set some assumptions about how the liquid will be entering the
tank and how the distribution to the several nodes takes place. The density of the
liquid is dependent on the temperature. Therefore we can assume that the storage
material will find its way to the node with the same temperature as the liquid. As it is
displayed in figure 6 the mass flow coming from the collector field mcfinds its way to
a node according to its temperature between Ts,1and Ts,3. The same physical effect
can be discovered when the liquid enters the tank upstream.
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Figure 6: Three-node stratification liquid storage tank [SETP].
Inthe three node-model displayed in graph 6 the nodes are counted from the top to
the bottom. The liquid flow inside of the tank however, can run from the bottom to the
top or the other way around depending on the thermodynamic conditions of the
different layers. A collector control function Ficcan be defined in order to determine,
which layer receives the thermal energy coming from the solar field. [SETP; S.384]:
FiC=
1 if i =1 and T c,0 > Ts,1
1 if Ts,i"1# Tc,0 > Ts,i
0 if i = 0 or if i =N+10 otherwise
$
%
&&
'&&
(2.28)
During the operation of the solar field the control functions can be non-zero only for
one node. The liquid returning from the power block can be controlled in the same
way like the mass flow coming from the collector field. Therefore the control function
FLiis established [SETP; S.384]:
FiL=
1 if i =N and T L ,r < Ts,N
1 if Ts,i"1# TL ,r > Ts,i0 if i = 0 or if i =N+1
0 otherwise
$
%
&&
'
&&
(2.29)
Equation (2.29) follows the same assumption as for equation (2.28). Therefore only
one control function can be set 1 during an operation.
The net flow between the nodes can now be either up or down. This depends on the
magnitudes of the load flow rates and the values represented in the two control
functions (2.28) and (2.29) at any particular instance. It is appropriate to define a
mixed flow rate in order to describe the net flow rate into node I from node i-1 in the
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multi-node model. The equations for calculating these flow rates can be presented as
[SETP; S.385]:
m
m,1 =0 (2.30)
m
m,i =m
c Fjc "mL Fj
L
j=i+1
N
#
j=1
i"1
# (2.31)
m
m.N+1 = 0 (2.32)
By using the control functions (2.28 and (2.29) it is now possible to describe the
energy balance for each node i. This function can be expressed as [SETP; S.385]:
midTs,i
dt
=
UA
cp
"
#$
$
%
&'
'(Ta (Ts,i) + Fi
c m
(Tc,0( Ts,i)+ FiL m
c(TL,r( Ts,i)
+
m
m,i(Ts,i"1" Ts,i) if m
m,i > 0
m
m,i+1(Ts,i" Ts,i+1) if m
m,i+1 < 0
#
$%
&%(2.33)
Here the first term representing the heat losses to the to the environment with the
temperature Ta. By increasing the number of nodes the model can be used for
describing processes in highly stratified tanks. Therefore equation (2.33) can be seen
as the basic description of the energy balance. Furthermore the model allows adding
a heating system in one or more nodes or going into more details for the description
of the thermal losses. For solving equation (2.33) numerical integration can be
performed by techniques like the explicit Euler or the Runge-Kutta methods.
However, for this type of storage model very little experimental evidence for
supporting the results are available. Nevertheless, the assumptions are all based on
physical facts.
2.3.2 The Plug-Flow Model
The plug flow model is another possibility for describing a stratification tank. Here
different layers, flowing around in the tank are the focus of interest. When the heat
transport fluid is entering the tank a new layer is simply added to the model. On the
other hand when liquid is leaving the tank a layer will be removed from the model.
The size of each segment varies depending on the flow rate and the time increments
set for the calculation.
Figure 7 presents a heat storage tank according to the plug and flow model with four
layers. On the x-axis the temperature will be marked, while the y-axis represents the
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height of the storage. V ipresents the volume of each storage layer.
Figure 7: Plug and Flow model whit 4 layers [SLP].
Here, losses on the surface of the tank occurring. The temperature change of eachlayer i can be detected with the following equation [SLP; S.30]:
mi
dTs,i
dt=
UA
cp
"
#$$
%
&''i
(Ta ( Ts,i) (2.34)
The definition mirepresents the mass at each layer, Ts,Ithe Temperature of the layer,
U represents the heat transfer coefficient, A is the outer area of the storage and T a
stands for the ambient temperature.
Both models have their advantages and disadvantages. The decision which model is
used in the simulation has to be determined by considering the required accuracy,
the available data sets, and the available computational capacity.
2.4 Power Block
The heart of the CSP plant is the power block: here the thermal energy delivered
either from storage or from the solar field is transferred into electrical energy. In
previous chapters, the process has been described for how to harvest and transfersolar energy, as well as the transport and storage of thermal energy. Here a closer
look of how the thermal energy is transformed into electrical energy is undertaken.
For CSP plants, well-established techniques are used. These techniques are
depending theoretically on the Clausius-Rankine cycle.
2.4.1 Carnot Cycle
Cycle processes play a very important roll in the mean of transferring thermal energy
into mechanical energy. The ideal thermodynamic cycle is the Carnot process. It has
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the maximum thermodynamic efficiency, the Carnot efficiency *c. This is because all
changers of states are reversible. In practical applications this efficiency can never
be reached but is used for indicating the thermodynamic quality of a real process.
Figure 8 shows the Carnot cycle in a p,V and a T,S diagram it can be seen that all
parts of the cycle are reversible.
Figure 8: Carnot cycle p,V and T,s Diagram [TDK].
As it is displayed in the graph above, the process consists of two isothermal and two
reversible adiabatic changes of state. The isothermal expansion form state (1) takes
place by adding thermal energy Q12and performing work W12. A reversible adiabatic
expansion takes place from state (2) to state (3). The isothermal compression
between states (3) and (4) releases the thermal energy Q34and must be supplied by
work W34. Lastly, by reversible adiabatic compression the working gas is broughtback to state (1).
By using the first law of thermodynamics the process can be described as [TDK;
S.66]:
dU= "W + "Q### (2.35)
Taking into consideration, that the change of energy in a closed cycle is zero, the
formula (2.35) can be simplified to [TDK; S.66]:
"W =# "Q$$ (2.36)
Equation (2.36) shows that the sum of all added and removed work is equal to zero.
Therefore, it can be defined as [TDK; S.66]:
W1,2+W
2,3+W
3,4+W
4,1+Q
1,2+Q
3,4= 0 (2.37)
From equation (2.37) the Carnot efficiency can be explained only by the added and
removed thermal energy in the cycle [TDK; S.66]:
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"th =1#(#Q3,4 )
Q1,2=1#
T3
T1 (2.38)
By using the entropy difference the thermodynamic efficiency can be also descript as
the relation of temperature T3 and T1. This efficiency finally can be used forevaluating the quality of any other circular process.
2.4.2 Clausius-Rankine Cycle
In the CSP plants the water steam cycle is operated on the basic concept of the ideal
Clausius-Rankine cycle. The primary working medium here is water/steam.
!"# %'( )('*+%*+,-'./%.# )01(#
Figure 9 represents all essential parts of a power block, and the T,s diagram for the
related Clausius-Rankine cycle.
Figure 9: Schematic drawing of a steam power plant and the T,s Diagram of a Clausius-
Rankine cycle [TDK].
The cycle starts with a reversible adiabatic compression (1.2) by the feed water
pump (P), the compression takes place in the liquid phase. Followed by an isobaric
heat addition (2.3) in the evaporator. The heat source in case of a CSPP is the
thermal energy provided by the solar field or the thermal storage. The heat switch
over takes place in several heat exchangers (KE). When reaching the evaporation
line (3) an isobaricisothermal heating up of the steam is taking place (3.4).
Followed by another isobaric heat adding (4.5) in the superheated (). After
superheating the steam a reversible adiabatic expansion (5.6) takes place in the
steam turbine (T), where the thermal energy is transformed into mechanical energy
and again transformed by the generator (G) into electrical energy. Finally the water
steam mix is isobaric condensates (6.1) in the condenser (Ko) until it reaches the
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evaporation line (1). Bringing the thermal energy into the cycle between point (2) and
(5) can be described as [TFI; S.188]:
Q
add =m
(h5" h
2) (2.39)
Where m represents the mass flow of the water/steam, and h5 and h2are the specific
enthalpy values at point (5) and (2) out of graph 8. The enthalpy values can be either
taken from the water steam table or form the h,s diagram.
The transferred energy in the turbine (5.6) can be calculated by [TFI; S.187]:
P = m
(h6" h5) (2.40)
In the formula above m stand for the mass flow of the steam, h 6 and h5 are the
enthalpy values in state (5) and (6). In the condenser the thermal energy of the
exhaust steam will be removed according to the function [TFI; S.188]:
Q
0 =m
(h1" h6) (2.41)
The energy consumption of the feed water pump can be calculated by the
water/steam mass flow m together with the specific volume of the condensate and
the pressure increase between (1.2) [TFI; S.189]:
PP =m
v1(p
2" p
1) (2.42)
In formula (2.42) Ppstands for the power consumption of the feed water pump, and
v1is the specific volume of the condensate. From equations (2.30) through (2.42) it is
possible to calculate the thermodynamic efficiency of the ideal Clausius-Rankine
cycle. In most of the cases the power used by the feed water pump compared to the
energy generated in the steam turbine is very small. Therefore this power is not
taken into consideration when calculating the thermal efficiency [TDK; S.120]:
"th =P
Q=1
#
Q
0
Q (2.43)
It becomes obvious that even in a reversible process cycle the thermal efficiency in
the Clausius-Rankine cycle is lower the efficiency in the Carnot cycle (equation 2.38).
!""#$#"%&'(# *+,-.# /0 %1,1# &- 1+# 2(,3%&3%45,-6&-# *7*(#
So far all processes in the cycle were considered as being reversible, but in reality
these processes are irreversible. The change of state in the working medium
(water/steam) in the feed water pump, the steam generator, the turbine and thecondenser are irreversible. This leads to differences in the operation points. Figure
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10 displays this difference between a reversible and an irreversible cycle.
Figure 10: Real Celsius Rankine Process T,s Diagram [KWT].
The dotted line presents the real process, where the continuous line shows the idealprocess. The main differences, which can be acknowledged in figure 10, are:
a: irreversible compression in the feed water pump
b: pressure losses in the steam generator
c: irreversible expansion in the steam turbine
d: pressure losses in the condenser
The increase of pressure in the feed water pump is irreversible. Because of this an
increase of the entropy can be noticed. The change of state taking place in reality is
not from (1.2) it is from (1.2r). Here the pressure increase from p2 to p2r is thesame as the pressure reduction of the working medium in the steam generator,
caused by friction in the pipe. In modern steam plants the parasitic load of the feed
water pump can be assumed with around 2%-3% of the generated power. The
irreversible behavior in the steam generator is mainly dependent on the temperature
difference between the working medium and the heat transport fluid from the solar
field. Furthermore, the movement of the working medium through the steam
generator leads to a pressure drop. The lost energy of the working medium h2r-h2has
to be added also by the feed water pump.
Depending on the irreversible expansion in the steam turbine only state (4r) is
reached at the outlet of the steam turbine. The additional heat, which has to be
removed in the condenser, logically is increasing according to:
"Q
=h4 r# h
4 (2.44)
Consequently, the efficiency of the real Clausius-Rankine cycle in reduced by [TFI;
S.192]:
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in the high-pressure stage of a steam turbine. The following T,s-diagram shows the
thermodynamic consequences of this procedure.
Figure 12: Water steam cycle with reheating [KWT].
The process (3.4) presents the expansion in the high-pressure stage of the steam
turbine (4.5) is the reheating process in the steam generator and (5.6) is again the
expansion in the intermediate and low-pressure stage of the turbine. The
thermodynamic efficiency, not including the feed water pump, can now be defined as
[KWT; S.70]:
"th=1#
T6(s6# s1)
h3# h1 + h5# h4
(2.46)
Equation (2.46) shows that for increasing the thermal efficiency by using a reheat
system the average temperature of the added heat in the reheater has to be higher
than the average temperature of the cycle without a reheating system. The average
temperature of the basic process can be calculated as [KWT; S.70]:
Tzu,1
=
h3" h
1
s3" s
1
(2.47)
Where the average temperature for the reheating process is calculated [KWT; S.71]:
Tzu,2 =
h5 " h4
s5 "
s4
# 0.5(T4 +T5) (2.48)
In normal operation conditions the temperature for the reheating system is equal to
the temperature of the main steam temperature (T5=T3). Therefore, the efficiency will
be increased if Tzu2>=Tzu1. This lead to [KWT; S.72]:
T4>2T
zu,1" T
5 (2.49)
Another advantage of the reheating stage can be seen in the reduced amount of wetsteam in the low-pressure part of the steam turbine.
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Increasing the cycle efficiency is also possible by adding a preheating system. It
heats up the feed water above the condensation temperature, and therefore the
efficiency of the Clausius-Rankine cycle gets closer to the efficiency of a Carnot
cycle. In practical use the steam for heating up the feed water comes from several
stages of the steam turbine.
Figure 13: Regenerative feed water preheating [KWT].
Figure 13 gives a schematic overview of the preheating process. It is clearly shown
that the heat is shifted from b to a by using the preheating system. In order to avoid
evaporation effects in the feed water the water is pressurized. The thermal efficiency
of the water steam cycle can now be calculated as [SLP; S.33]:
"th=1#
h3#
h1'
T1(s4# s1) (2.50)
Increase in efficiency mainly depends on the reduction of the mass flow through the
condenser. This results in a reduction of the condenser losses.
Another conclusion of the preheating system is that the steam mass flow to the high-
pressure part of the steam turbine is increasing where the steam flow in the low
pressure and intermediate pressure part is decreasing. This leads, depending on
sealing losses in the turbine, to an increase of the internal turbine efficiency.
2.4.3 Steam Turbine
A steam turbine is an axial turbo engine, in which the thermal energy stored in the
steam is converted into mechanical, mainly rotational energy. For the transition of the
enthalpy into kinetic energy the working medium is seeded up in the nozzles. These
nozzles are composed by the outlines of the guidance wheels. After this a switch of
flow direction of the working medium takes place by using the rotating wheels. As a
reaction of the impulse forces occurring now on the wheels, a torque is created andtransferred to the turbine shaft. The set of a guidance wheel and a rotational wheel is
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called turbine stage. Figure 14 gives an overview of the construction concept of a
modern steam turbine.
Figure 14. Schematic drawing of a high-pressure turbine [KWT].
The construction schema above shows the inner cover a, the outer cover b, the
labyrinth sealing c and the turbine shaft d.
Furthermore, figure 14 is giving an overview of how the thermal energy stored in the
steam is transferred into the kinetic energy of the shaft. Simplified the power
produced by a steam turbine can be calculated out of the enthalpy drop over the
turbine, the steam mass flow, and the turbine efficiency [KWT; S.261]:
PT ="Tm
#h (2.51)
The turbine efficiency *T is depending on several losses occurring during the energy
conversion in the turbine. For an easier understanding of the turbine efficiency we
can calculate the efficiency like [KWT; S261]:
"T="
i"
mech (2.52)
In formula (2.52) *i is considered as the inner efficiency. It is depending on losses
occurring on the wheels, which are manly friction losses. Gap losses depending on
the design of the turbine and ventilation losses are also recognized in this factor.
Furthermore, losses depending on wet steam and losses occurring during the steam
are leaving the turbine because of rearrangement in the flow direction. As a
benchmark the inner efficiency for modern turbine can be assumed to be between
93% and 95%.
The mechanical efficiency *mech includes steam losses in the labyrinth sealing as well
as friction losses between shaft and bearings. Because of using modern hydraulic
bearings for carrying the turbine shaft the efficiency here can be assumed whit 98%or 99%. All the assumptions above are related to the design point of the steam
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turbine, which is normally at the maximum rated power of the turbine.
The turbine power however is controlled by the steam mass flow. This is affecting the
efficiency as well as the net power output of the generator. The relationship between
the steam mass flows for the different operation modes (full load or part load) are
described by the cone law of Stodola [KWT; S.261]:
m
T
m0
=
p"T
2# p
$T
2
p"0
2#p
$0
2
T"0
T"T
(2.53)
Here # stand for the entrance of the turbine and % stands for the turbine outlet, 0
characterizes the full load operation, where T represents part load behavior. Over the
years, several possibilities like fixed pressure operation, sliding control or equivalent
sliding pressure have been established for the control of the steam turbine. Based onequations (2.53), the operation mode of the steam turbine, and also the part load
behavior of a CSPP can be assumed.
This chapter has outlined theoretical relations used for the developed model
explained in chapter 4. However not only were technical factors taken into
consideration for the simulation but also a recourse assessment for CSP plants in
North Africa is undertaken in the next.
.
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3. Recourse Assessment for CSPP
3.1 Land Recourse Assessment
Using numerous criteria such as ground structure, water bodies, slope, shifting sand,
protected and/or restricted areas, forest, and agricultural covered areas allows for the
detection of land resources, which would permit the placement of concentrating solar
collector fields. For collecting these data sets from the DLR are used, witch finally all
combined in order to yield a map of usable areas. Some of the used criteria can be
seen as optional. For instance, tourist areas or agricultural areas can be transformed
into potential sites for CSP plants. Other information like slope of the terrain or water
availability can be understood as compulsory criteria. For example if the slope of the
terrain is greater than 2.1% the placement of a CSP plant will be, considering the
state of the current technologies, impossible. Table 1 shows the compulsive and the
optional criteria an area must fulfill for being considered as a possible construction
site for a CSP plant. [SI; S.39]
Table 1: Compulsive and optional criteria for the exclusion of land used for CSP plants [SI|.
Exclusion Criteria Compulsive Optional
Slope of Terrain
>2.1% X
Land Cover
Sea X
Inland Water X
Forest X
Swamp X
Agriculture X
Rice Culture X
Hydrology
Permanent Inland Water X
Non-Permanent Inlet Water X
Regularly Flooded Area X
Geomorphology
Shifting Sand, Dunes X
Security Zone for Shifting Sands 10km X
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Salt Pans X
Glaciers X
Security Zone for Glaciers X
Land UseSettlements X
Airport X
Oil or Gas Fields X
Mine, Quarry X
Protected Area X
Restricted Area X
In this work, all criteria, whether compulsive or optional, are used for evaluating thecapability of an area. Out of table 1 it can be noticed that the criteria can be
summarized into five major topics, which will be described in the following
subchapters. Furthermore a minimum direct irradiation as well as the existing grid
system is discussed as possible site exclusion criteria.
3.1.1 Slope
The digital elevation map shown in figure 15 presents the slope of the terrain. As it
was mentioned previously a slope of more than 2.1% excludes a site for installationof a CSP plant. Providing elevations in a 1 x 1 km2 resolution, the digital elevation
model from GLOBE is used for determination of the slope. [SI; S41]
Figure 15: Digital elevation map for determination of the slope [SI].
The figure 15 highlights a slope of greater than 2.1% in bright red. Smaller slopes are
displayed in different tones from white, which shows flat terrain to dark blue, which
stands for a slope of 2.1%. Beside of a few areas in Morocco and Egypt no restriction
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by the slope of the terrain can be noticed in North Africa.
3.1.2 Land cover
The information about the land cover is extracted from the land cover
characterization (GLCC) database. This database uses the normalized differencevegetation index (NDVI). The NDVI divides the land cover into ten classes, which are
comprehensive to the global ecosystem's classification. [SI; S43]
Figure 16: The land cover in the Euro-Mediterranean Region [SI].
Figure 16 displays the 10 classes of land use for the EU-Mediterranean region. The
area of interest in North Africa is largely covered by desert and semi desert. In
relation to table 1 numerous sites can be see as accessible areas for a CSPP
according to the land cover information.
3.1.3 Hydrology
Data sets for rivers, lakes etc. are also extracted from the GLCC database.
Figure 17: The Hydrology of the Euro-Mediterranean Region [SI].
Map 17 shows the most significant hydrological features in the Euro-Mediterranean
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region. Small rivers are not taken into account simply because of the fact that shifting
a plant site of a maximum of 500 meters in any direction is consider as suitable. [SI;
S43] Large rivers, however, mostly near the sea-confluence are taken into account.
Regions where no data sets are available are marked red. As a final point the areas
prone to flooding are displayed in the satellite image. Likewise subtracted from the
map above, the hydrology will not lead to a great exclusion of obtainable sites in
North Africa.
3.1.4 Geomorphologic features
Due to their properties and physical conditions some areas due to their soils are not
suitable for erecting concentrating solar collectors. Dynamic structures like shifting
sand dunes can be taken as prohibiting areas for CSP plants simply not providing a
compound strong enough for the erection of the pylons needed for the CSPP.
Taking into consideration the capability to provide additional safety zones around
glaciers and sand dunes has to be accounted for. The movement of a sand dune can
be assumed to be 200m/year. If the operation time of the CSP plant is judged with 50
years a security zone of at least 10 km around sand dunes must be maintained. The
restricted area around the glaciers must be reviewed individually for each glacier and
is not part of this work. Spatial information about sand dunes and salt areas are
extracted from the Digital Soil Map of the World (DSMW). The spatial resolution of
the map amounts to approximately 10 x 10 km2. The DSMW includes 26 groups of
soil types, 106 soil types, as well as showing some non-soil features, including the
sand dunes and salt areas of interest. [SI; S44].
Figure 18: Geomorphologic exclusion criteria in the Euro-Mediterranean region [SI].
Figure 18 gives an impression of the geomorphology characteristic in the Euro-
Mediterranean region. Shifting sand dunes are marked yellow. These regions areconsidered as exclusion areas for CSPP plants. Here for the first time a larger
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amount of restricted area in the North African region can be noticed.
3.1.5 Protected areas
According to the definition published by the World Conservation Union (WCU) a
protected area is: An area of land and/or sea especially dedicated to the protection
and maintenance of biological diversity, and of natural and associated cultural
resources, and managed though legal or other effective means. [SI; S45]. Areas,
which meet the universal guidelines contained in this definition, can be seen as
eliminated areas for CSP plants and presented in figure 19.
Figure 19: Protected areas of the Euro-Mediterranean Region [SI].
However in practice the precise purpose for witch protected areas are managed
differs greatly. Therefore the WCU has defined a series of six protected area
management categories, listed in the legend of map 19. The six area types are strict
nature reserved area, wilderness area, national parks, natural monuments,
habitat/species management areas, protected land/seascape, and managed
resource protected areas [SI; S.46]. The restrictions for erecting the CSP plants in
North Africa therefore will cause it to be limited to a few sites in Morocco, Algeria,
Libya and Egypt.
3.1.6 Industry and Population
Figure 20 displays data about industry and highly populated places. This map also
includes oil or gas fields, mines, quarry airports and desalination plants. The data set
is based on the Digital Chart of the World ASCII.
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Figure 20: Industry and population of the Euro-Mediterranean region [SI].
Still a great restriction for available CSP sides in North Africa based on industry andpopulation criterias can be not noticed. [SI; S47]
3.1.7 Technical potential
The data sets explained previously are used to develop maps showing possible
restricted areas for concentrated solar power plants based respectively on land
configuration. Another powerful criteria is the yearly average of direct normal
irradiance at a certain location. This is displayed in the following EU-Mediterranean
map.
Figure 21: Annual direct normal irradiation in kWh/m2/y on non-excluded areas in the Euro-
Mediterranean Region [SI].
A site is considered to have a technical potential for a CSP plant when the yearly
average direct irradiation is at least 2000 kWh/m2/year. In general solar thermal
electricity generation is also possible with a lower irradiation. Taking into
consideration economical factors, CSP first start to become feasible with a higher
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irradiation. However, the 2000 kwh/m2/year represents a suitable value for operating
a CSP plant. Furthermore, the image 21 can be seen as the resultant map including
all criteria mentioned previously. Based on this map it is possible to calculate the
accessible area in km2for the countries of interest. In this thesis a closer look at five
countries of North Africa, Morocco, Algeria, Libya, Tunisia and Egypt is under taken.
In the following description only these countries and their potential are taken into
consideration for any calculation. Table 2 presents the maximum available areas with
a direct normal irradiation of greater than 2000 kWh/m2/year. [SI; S48]
Table 2: Areas for CSP in km2available in the MENA countries for different DNI Classes [SI].
DNI Classes Morocco Algeria Tunisia Libya Egypt
2000-2099 6083 6237 9288 7773 206
2100-2199 5650 34142 6445 25331 1481
2200-2299 10875 29006 9864 109712 16846
2300-2399 17194 39462 19464 176659 40969
2400-2499 34348 222860 22823 152875 41347
2500-2599 30569 384570 11637 183342 44613
2600-2699 18930 428487 240 155513 98004
2700-2800+ 48074 277580 373665 354972
TOTAL 171724 1422344 79761 1184870 598439
All data sets presented in this chapter are also available on a global scale and
collected be the German Aerospace Center.
Figure 22: Annual direct normal irradiation on non-excluded areas in global scale [SI].
For further scenario analyses the North African parts out of map 22 are used (see
chapter 4). It can be seen that sometimes areas whit even a high irradiation are not
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available because of other excluding criterias.
3.1.8 High voltage Grid
In order to supply the electrical energy produced by a CSPP plant a connection point
to the transmission grid must be within a certain distance. This DESERTEC scenarioincludes high voltage direct current transmission lines. In this work only the actual
existing high voltage and extra high voltage grid systems are taken into
consideration. The transmission grid not only consists of high voltage lines, but also
of transformers, switchgear and other electrical equipment. If power plants must be
connected to high voltage grids over a long distance enormous investment costs
must be taken into account. But not only equipment like cables, transformers and
switchyards are expensive: the transmission losses over long distances in AC grid
systems have to be taken into consideration. With the idea to minimize the
connection costs and minimizing the transmission losses, the distance from the
power plant to the existing high voltage Grid in North Africa has to be very short.
Therefore the existing high voltage grid like it is presented in figure 23 gives another
possibility of excluding possible areas for erecting concentrated solar power plants.
Figure 23: Electrical Transmission System Network of North Africa [GEMI]
For a more detailed description of the national grid systems in Morocco, Algeria,
Tunisia, Libya and Egypt see appendix B. The images presented in the appendix are
used for the simulation explained in chapter 4.2. The existing grid offers already the
possibility to export energy from North Africa to Europe. Syria as well as Morocco
having connection points to European transmission systems. Figure 24 giving a
schematic overview of the interconnection points between the MENA region and theEU.
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Figure 24: Envisage Mediterranean interconnections with Europe [GEMI].
For this work it is assumed that the transmission capacity of the existing grid s
sufficient for transporting the electrical energy produced by CSP plants between
North Africa and Europe.
The finale side availability is now based on the access to infrastructure, ground and
land characteristic as well as a minimum availability of direct normal irradiation.
3.2 MED-CSP SCENARIO CG/HE
The scenario of the MED-CSP study is used in this work for further calculations. The
study analyzes the demand of electricity and water in the MENA countries