MasterThesis Daniel Horst

8/10/2019 MasterThesis Daniel Horst

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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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Solar Power Plants And ExportScenario Analysis For North Africa

By

Daniel Horst


Faculty of Engineering at Cairo University

in Partial Fulfillment of the


MASTER OF SCIENCE

In


%&'() *+,()-./.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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Solar Power Plants And ExportScenario Analysis For North Africa

By

Daniel Horst


Faculty of Engineering at Cairo University

in Partial Fulfillment of the


MASTER OF SCIENCE

In


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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XI

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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XII

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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XIII

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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10

" =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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11

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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12

"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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13

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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14

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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15

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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16

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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17

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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19

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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21

"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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22

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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23

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]:


40/122


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25

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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26

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

MasterThesis Daniel Horst

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