Solar-thermal-power-plant-report

In our project, we are going to study how solar energy can be used to generate heat, and how this heat is converted to electricity.By using mirrors and lenses to concentrate the rays of

Lebanese University Faculty of Engineering II

Final year project

submitted in fulfillment of the requirements for the

Diploma of Mechanical Engineering

by

Alain KARAM Abdo CHAMIEH Alain FAKHRY

Solar Thermal Power Plant

Parabolic Trough Technology

Project supervisor : Dr Francois KHOURY

2011

Acknowledgements

Apart from our efforts, the success of any project depends largely on the encouragement and guidelines of many others

We would like to thank Phoenix Member of Indevco group for proposing this challenging

project and offering their help even if we didn’t manage to continue working with them

We wish to thank the Lebanese University – Faculty of Engineering – Branch II, and we would like to offer our sincerest gratitude to our supervisor, Dr Francois Khoury, who has supported

us throughout our thesis with his patience and knowledge

Needless to mention Dr.Khalil Khoury for his able guidance and useful suggestions which helped

us in starting our project

Many thanks go to Mr Bernard Ammoun for offering his help and advice concerning markets in and outside Lebanon that sells solar technology parts required for the project, and taking care

of transporting the steam engine from the United States of America

Special thanks for Engineer Souad Chamieh for sharing her experience in programming and spending many hours working on the software with us

We would like to express our heartfelt thanks to our beloved parents for their blessing, help, and wishes for the successful completion of this project

The biggest thanks go to God who made all things possible

Abstract:

Solar Thermal Power Plant:

Solar thermal power is a relatively new technology which has already shown enormous

promise Solar thermal power uses direct sunlight, so it must be sited in regions with high direct solar radiation

The purpose of our project (CSP) is producing electricity without the use of fossil fuel and only using the sun’ s power

The first step of the project is to prove that Lebanon is suitable for solar applications

The second step is generally focusing on several technologies used for producing electricity and showing the advantages and disadvantages of each one including the differences between them

The third step is to study the parabolic trough technology including the solar field, the usual thermodynamic rankine cycle, and all the formulas needed to complete the calculation Briefly, how much solar power needed to produce a certain amount of electricity?

The fourth step is to make a software in which the user enters some needed parameters, for example the electrical power and gives how many collectors is needed in the solar field as an output

The final step is to make a prototype showing the conversion of sun’s heat to electrical power

Central Solaire:

L'énergie solaire thermique est une technologie relativement nouvelle qui a déjà montré une énorme puissance de promise L'énergie solaire thermique utilise la lumière solaire directe, donc il doit être situé dans les régions à forte radiation solaire directe Le but de notre

projet (CSP) est la production d'électricité sans l'utilisation de combustibles fossiles et en

utilisant uniquement la puissance du soleil

La première étape du projet est de prouver que le Liban est adapté pour les applications

solaires

La deuxième étape est généralement axé sur plusieurs technologies utilisées pour la production d'électricité et de montrer les avantages et les inconvénients de chacun, y compris les

La troisième étape consiste à étudier la technologie cylindro-paraboliques dont le

domaine solaire, le cycle thermodynamique d'habitude Rankine, et toutes les

formules nécessaires pour terminer le calcul En bref, combien d'énergie solaire nécessaire pour

La quatrième étape est de faire un logiciel dans lequel l'utilisateur saisit quelques

paramètres nécessaires, par exemple l'énergie électrique et donne le nombre

de collectionneurs qui est nécessaire dans le domaine solaire comme une sortie

La dernière étape consiste à réaliser un prototype montrant la conversion de la chaleur du soleil en énergie électrique

Table of contents

Acknowledgements 1

Abstract: 2

1 Introduction 6

2 Concentration Solar Power Technology overview: (CSP) 9

2.1 Parabolic Trough: 9

2.2 Parabolic dish concentrators (Dish Stirling): 10

2.3 Central Receiver or Power Tower: 11

2.4 Fresnel solar power system: 12

3 Parabolic trough 14

3.1 Solar field: 14

3.2 Collector: 15

3.3 Receiver 16

3.4 Heat Transfer Fluid: (HTF) 17

3.4.1 Impact of salt HTF on the power plant performance 19

3.4.2 Advantages and disadvantages of using molten salt as a HTF: 20

3.4.3 Molten salt system Issues: 20

3.4.4 Routine freeze protection operation: 20

3.4.5 Solar Field Preheat Methods: 21

3.4.6 Collector Loop Maintenance with an Impedance Heating System: 22

3.4.7 Materials consideration: 23

3.5 losses and efficiencies 25

3.5.1 Optical losses: 25

3.5.2 Thermal losses: 27

3.5.3 Geometrical losses: 28

3.6 SOLAR ANGLES: 28

3.8 Fluid pump: 31

3.8.1 Materials and Temperatures: 32

3.8.2 Standard High Temperature Pump Designs: 33

3.8.3 Mounting and Sealing Molten Salt Pumps: 34

3.9 Tracking system: 34

3.10 Thermal Storage Systems for Concentrating Solar Power 35

3.10.1 Two-Tank Direct System 36

3.10.2 Two-Tank Indirect System 36

3.10.3 Single-Tank Thermocline System 37

4 Industrial Process 38

4.1 Basic Process and Components: 38

4.2 Steam Turbine: 40

4.2.1 Types of Steam Turbines: 41

 Non-Condensing (Back-pressure) Turbine: 42

 Extraction Turbine: 43

4.3 Heat Exchangers: 44

4.3.1 Exchanger Equation: 46

4.3.2 Types of Exchangers: 46

5 Designing an 8.5 Mwe solar thermal power plant 50

6 Practical study 53

Appendix VB Programming 60

References: 67

1 Introduction

Solar energy as a power from the sun is a vast and inexhaustible resource Once a system is in place to convert it into useful energy, the fuel is free and will never be subject to the ups and downs of energy markets Furthermore, it represents a clean alternative to the fossil fuels that currently pollute our air and water, threaten our public health, and contribute to global

warming Given the abundance and the appeal of solar energy, this resource is poised to play a prominent role in our energy future

So the advantages of using solar energy are:

 Unlimited alternative fuel

 Free alternative fuel

 Clean alternative fuel that means no pollution and health threat

 Fighting the global warming

Solar energy supports all life on Earth and is the basis for almost every form of energy we use The sun makes plants grow, which can be burned as biomass fuel or, if left to rot in swamps and compressed underground for millions of years, in the form of coal and oil Heat from the sun causes temperature differences between areas, producing wind that can power turbines Water evaporates because of the sun, falls on high elevations, and rushes down to the sea, spinning hydroelectric turbines as it passes But solar energy usually refers to ways the sun's energy can

be used to directly generate heat, lighting, and electricity

In our project, we are going to study how solar energy can be used to generate heat, and how this heat is converted to electricity.By using mirrors and lenses to concentrate the rays of the sun, solar thermal systems can produce very high temperatures, generating steam to produce electricity using steam turbines.One of the greatest benefits of large scale solar thermal

systems is the possibility of storing the sun’s heat energy for later use, which allows the

production of electricity even when the sun is no longer shining.Properly sized storage systems, commonly consisting of molten salts, can transform a solar plant into a supplier of continuous base load electricity Solar thermal systems now in development will be able to compete in output and reliability with large coal and nuclear plants

To decide what if a country is suitable to a solar application, we should define the solar

radiation and its value in this country So Insolation is a measure of solar radiation energy received on a given surface area in a given time It is commonly expressed as average irradiance

in watts per square meter (W/m2) or kilowatt-hours per square meter per day (kWh/(m2·day)) (or hours/day) Figure 1.1 shows that Lebanon is one of the best countries in solar radiation in this region which means that is a suitable country for solar applications

FIGURE 1.1: Solar irradiation map [1]

To be sure, an approximate solar data of Lebanon should be discussed and the results are in table 1.1 As we can see 8.15 average sunshine hours in the coastal area and 8.81 average sunshine hours in the interior area, it is about 34-36.7% of the year just sunshine and we know that we can use solar energy even if it is cloudy So as a result, Lebanon is an excellent country

to use solar energy

Month Coastal

insolation, kWh/m2/day

Interior insolation, kWh/m2/day

Coastal sunshine hours

Interior sunshine hours

2 Concentration Solar Power Technology overview: (CSP)

CSP technologies are solar thermal plants (STP) that produce electricity in the same way as

conventional power stations, except that they obtain part of their thermal energy input in a

different way by using mirrors to reflect and concentrate sunlight onto receivers where a

working fluid passes from low to high temperatures The resulting heat energy is used to power

a steam turbine, or alternatively to move a piston in a sterling engine Tracking techniques are

required in these systems to ensure that the maximum amount of sunlight enters the

concentration system Essentially, STP plants include four main components: the concentrator,

receiver, transport-storage, and power conversion Many different types of systems are

possible using variations of the above components, combining them with other renewable and

nonrenewable technologies, and in some cases, adapting them to utilize thermal storage The

four most promising solar power architectures are:

A parabolic trough system consists of many long parallel rows of curved mirrors that

concentrate light onto a receiver pipe positioned along the reflector’s focal line, as shown in

Figure 2.1 The troughs follow the trajectory of the sun by rotating along their axis to ensure

that the maximum amount of sunlight enters the concentrating system The concentrated solar

radiation heats up a fluid circulating in the pipes, typically synthetic oil or molten salt, to

temperatures of up to 750°F (approximately 400°C)

The hot oil is pumped to heat exchangers to generate steam, which is used to drive a

conventional steam turbine generator A schematic diagram of a solar power plant using

parabolic trough concentrators is shown in Figure 2.2

Figure 2.1: schematic of a parabolic trough collector [3]

Solar power is intermittent and is not available overnight; therefore, some solar power plants are designed to operate as hybrid solar/fossil plants As hybrids, they have the capability to generate electricity during periods of low solar radiation The new parabolic trough plants use molten salt for the heat transfer medium, which is cheaper and safer than oil Also, because salts are an effective storage medium, the spare solar power is used in the form of heated molten salt in storage tanks, for use during periods when solar power is not available This makes the CSP technology truly dispatchable

Figure 2.2: Schematic diagram of a solar power plant with parabolic trough concentrators [3]

2.2 Parabolic dish concentrators (Dish Stirling):

A parabolic dish concentrator consists of a parabolic dish-shaped mirror that reflects solar radiation onto a receiver located at the focal point of the dish The dish structure is designed to track the sun on two axes, allowing the capture of solar energy at its highest density A

schematic of a parabolic dish concentrator is shown in Figure 2.3

Figure 2.3: schematic of a parabolic dish concentrator [3]

The solar dish concentration ratio is much higher then solar trough, typicaly over 2000, with a working fluid temperature to over 1300°F (approximately 750°C) The thermal receiver consists

of a bank of tubes filled with a cooling fluid, usually hydrogen or helium, which is heat transfer medium and also the working fluid for an engine The thermal receiver absorbs the solar energy and converts it to heat that is delivered to an electric generator to generate electricity Similar

to parabolic trough, a concentrator can be made up of multiple mirrors that approximate a parabolic dish to reflect the solar energy at a central focal point The main advantage of the solar dish technology is that the Stirling system requires no water for heating or cooling

2.3 Central Receiver or Power Tower:

A schematic of a central receiver system is shown in Figure 2.4

Central Receiver systems use a circular array of heliostats (large individually-tracking mirrors) to concentrate sunlight onto a central receiver mounted at the top of a tower The central receiver absorbs the energy reflected by the concentrator and by means of a heat exchanger (air/water) produces superheated steam Alternatively thermal transfer medium (molten nitrate salt) is pumped through the receiver tubes, which is heated to approximately 560°C and pumped either to a ‘hot’ tank for storage or through heat exchangers to produce superheated steam The steam is converted to electric energy in a conventional turbine generator (Rankine-

cycle/steam turbine or Brayton-cycle gas turbine) or in combined cycle (gas turbine with

bottoming steam turbine) generators

Figure 2.4: Schematic of a central receiver system [4]

2.4 Fresnel solar power system:

The linear solar Fresnel solar power system, often called compact linear Fresnel reflector (CLFR) technology, uses a series of linear solar mirrors, but each solar mirror is flat instead of the more expensive parabolic shape used for a parabolic trough solar plant Figure 2.5 shows a schematic

of Fresnel system

This is one of the solar concentrator technologies being developed to improve the efficiency for conversion of solar power to electricity A Fresnel solar plant uses an array of single axis, linear flat mirrors to reflect sunlight onto a receiver tube The compact linear Fresnel solar power system, however, uses a 'parabola' made up of ten flat mirrors that each rotate to follow the sun and this type of system allows the flat solar mirrors to remain near the ground, avoiding wind loads Current designs for the linear solar Fresnel system heat water to produce steam at

545oF (approximately 285°C) in the absorber tubes The steam is used directly to drive a turbine

in a standard Rankine cycle to produce electricity, avoiding the need for a heat exchanger to produce steam from some other high temperature fluid

Figure 2.5: schematic of a Fresnel system [5]

From all the following information regarding the types of concentrators used to generate

electricity, a conclusion can be summarized in the table 2.1 that shows the applications,

advantages, and disadvantages of the four types discussed in this chapter

Parabolic Trough Central Receiver Parabolic Dish Fresnel Applications -Grid-connected

plants;

-Mid to high process heat

-Grid-connected plants;

-High temperature process heat

-Stand-alone applications

or small off-grid power

systems

-Grid connected plants, or steam generation to be used in

conventional thermal power plants

Advantages =Commercially

available (over 9 billion kWh operational experience), -Solar collection efficiency

up to 60%, peak solar to

electrical conversion of 21%;

-hybrid concept proven;

-storage capability

-Good mid-term perspective for high conversion efficiencies;

-Solar collection efficiency approx 46% at temps

up to 565°C, peak solar to electrical conversion of 23%;

-storage at high temperatures; hybrid operation possible

-Very high conversion efficiencies (peak solar to

electric conversion

of about 30%);

modularity;

hybrid operation;

-flat solar mirror less expensive than parabolic mirror

-absorber tube is simpler and less expensive than that of the parabolic trough -can be hybridized with fossil fuel backup

-structurally simpler than the parabolic trough and parabolic dish systems

Disadvantages -Lower

temperatures (up

to 400°C) restrict output to

moderate steam qualities due to temperature limits

of oil medium

-Capital cost projections not yet proven

-Hybrid systems have low combustion efficiency, and reliability yet

to be proven

-doesn't produce a fluid temperature

as high as the parabolic trough or parabolic dish solar

concentrators, so its thermal efficiency for conversion of solar power to electricity

is lower

 Receiver(Vacuum or Evacuated tube)

 Heat transfer fluid(HTF)

 losses and efficiencies

 Fluid pump

 Tracker

 Thermal storage systems

A simple relation between the different components of a solar field is illustrated in the

following diagram:

3.2 Collector:

Before we start designing a collector, one should know the role of a collector (mirror,

concentrator) and how it converts the sun’s thermal heat to electrical energy at the end of the process The main idea of a collector is to transfer heat as much possible as it can to the heat transfer fluid, so in order to accomplish this, the collector should have a bigger area than the absorber tube, and the bigger the aperture area of the collector and the smaller the area of the absorber tube would lead us to a bigger concentration So it’s clear that the geometric

concentration ratio is the area of the collected sun over the area of the receiver tube [12]

Figure 3.2: Collector and receiver shape [7]

Where: C : geometric concentration ratio

Ac: collector aperture area

Ar : receiver area

La: aperture length

L: mirror length

d: outer diameter of the receiver pipe

The rim angle, ф, is another parameter that should be discussed when designing a parabolic trough mirror The rim angle as shown in figure 3.2 has usually a value between 70o and 110 o Decreasing this value lower than 70 will lead to a smaller aperture length leading to a smaller aperture area then to a smaller concentration ratio; however, increasing the rim angle will result in increasing the reflecting surface without increasing the aperture length thus increasing the material cost without affecting the concentration ratio

The relation between the focal distance and the rim angle is: [12]

The latest power plants uses LS-3 collector, it is the third generation collector after LS-1 and

LS-2 In our project we choose to work on LS-3 collector with some changes in dimensons After

using equation 3.2 and 3.3 the characteristics of this collector are shown in Table 3.1:

Table 3.1: Characteristics of Power plant’s collector [12]

3.3 Receiver

The definition of a receiver is any material that receives the heat from several sources In a

solar field, the receiver is usually a tube that receives all the heat from the concentrators In the

parabolic trough concentrator, the receiver is a vacuum tube (evacuated tube) This vacuum

tube is composed of an inner steel pipe surrounded by a glass tube to reduce convective heat losses from the hot steel pipe The steel pipe has a high absorptivity (greater than 90%), and a low emissivity(less than 30% in the infrared) coating that reduces radiative thermal losses

Receiver tubes with glass vacuum tubes and antireflective coating achieve higher thermal

efficiency and better annual performance, especially at higher operating temperatures

Receiver tubes with no vacuum are usually for working temperatures below 250˚C, because thermal losses are not so critical at these temperatures The maximum length of a single

receiver pipe is less than 6 m due to manufacturing constraints In our case the length is 4 m The complete receiver tube of a PTC is composed of a number of single receiver pipes welded in series up to the total length of the PTC A typical parabolic trough concentrator vacuum

receiver pipe is:

Figure 3.3 Typical receiver tube [8]

The glass-to-metal welding used to connect the glass tube and the expansion joint is a weak point in the receiver tube and has to be protected from the concentrated solar radiation to avoid high thermal and mechanical stress that could damage the welding An aluminum shield

is usually placed over the joint to protect the welds Several chemical getters are placed in the gap between the steel receiver pipe and the glass cover to absorb gas molecules from the fluid that get through the steel pipe wall to the annulus [12]

3.4 Heat Transfer Fluid: (HTF)

The HTF flows through the receiver, collects heat from the sun to exchange this heat with water

In order to choose what type of HTF we use in the power plant, a feasibility investigation of

The most popular heat transfer fluid used in CSP power plants are:

 Therminol VP-1 (Known as solar oil)

Table 4.2 shows the characteristics of the Nitrate salts and Therminol VP-1, a quick look at this table reveals the following results:

 The freezing point is a major importance in solar thermal power plant because of the difficulties and the high cost of freeze protection system due to the need for extensive heat tracing equipment on piping and collector receiver For this reason the

conventional oil (Therminol VP-1) with 13o C freezing point is the best HTF, but between salts calcium nitrate mixture with 120o C is the most favored

 Solar salt is the most practical HTF for Tower technology applications because of his high upper operating temperature limit (600o C) which allows using the most advanced

Rankine cycle turbines, but the disadvantage is its high freezing point of 220o C Calcium nitrate salt has a good operating limit of about 500o C

Table 3.2: Characteristics of Nitrate salts and Therminol VP-1 [9]

The High operation temperature of a molten salt is not enough to prove that is more efficient than oil, especially that a high freezing point leads to important issues that increases the capital

As shown in table 3.3 the temperature rise in the solar field was varied from 100oC to 200oC It

is clear that the cost of any salt is a lot lower than oil, same as storage cost

The calcium nitrate salt is significantly less expansive in terms of energy capacity than oil at the

same solar field temperature rise and over 70% lower if used at 200oC rise

What type of salt to choose? The Hitec look like is the best option because of its lower cost than

the Calcium Nitrate and lower freezing point than solar salt; however, it does require an N2

cover gas in the thermal storage tanks at atmospheric pressure to prevent the Nitrite from

converting to Nitrate, thus raising its freezing point So our choice is now limited between Solar

salt and Calcium Nitrate salt

Table 3.3: Effective fluid storage cost [10]

3.4.1 Impact of salt HTF on the power plant performance

The use of salt as HTF in the solar field has the following main effects:

 Molten salt can operate at higher temperature than Therminol VP-1 used in

conventional plants Consequently, higher steam temperature can be achieved in the

Rankine cycle leading to higher cycle efficiency

 Because of the higher operation temperature in solar field, higher heat losses is

achieved and the solar field efficiency decrease

 The freezing point of Calcium Nitrate is high (120oC) Therefore, more thermal energy is

consumed in freeze protection operation The solar field temperature must be kept well

above 120oC throughout the night That also leads to additional heat losses

3.4.2 Advantages and disadvantages of using molten salt as a HTF:

-ΔT for storage up to 2.5x greater

-Salt is less expensive and more

environmentally benign than present HTF

-Thermal storage cost drops 65% compared

to VP-1 HTF plant

-No need for an expensive oil-to-salt heat

exchanger

-High freezing point of candidate salts, leads

to significant O&M challenges and to innovative freeze protection concepts

-More expensive materials required in HTF system due to higher possible HTF

temperatures

-Solar field heat losses will rise

-Preheat concepts are required

Table 3.4: advantages and disadvantages of using molten salt

3.4.3 Molten salt system Issues:

Issues of using molten salt as a HTF are:

 Routine freeze protection operation

 Solar field preheat methods

 Collector loop maintenance

 Materials consideration

3.4.4 Routine freeze protection operation:

Since the freezing point of molten salt is considerably higher than the freezing point of VP-1, special attention has to be dedicated to freeze protection operation The strategy used for freeze protection overnight is:

 The HTF is circulated through the solar field during the whole night By this means the piping will be kept warm, thus avoiding critical thermal gradients during start up

 If the HTF temperature falls below a certain value, an auxiliary heater is used to maintain a minimum temperature of 150oC

According to the results of annual performance calculation the annual fuel consumption for freeze protection will be about 0.3 million m3 of Natural Gas for a 8 MW plant with molten salt

as HTF Assuming a gas price of $0.081/m3, freeze protection will cost $24300 per year This is

This procedure can be modified and improved slightly for system, with thermal storage Instead

of using fossil energy to heat up the salt, salt from the cold tank can be taken to keep the solar field and the whole system warm Of course, the cold tank has to be heated up again at the beginning of the operation, which consumes solar thermal energy On the other hand, fossil fuel can be saved According to annual performance calculation a storage capacity of one hour

is enough for freeze protection operation during the night In the case without thermal storage the solar field riches the critical temperature after six hours Then a fossil heater has to

maintain the temperature at 150oC In the case of thermal storage, the energy of the cold tank

is used for freeze protection For a normal winter day the minimum temperature of the storage

at start-up in the morning will be 250oC for one hour storage and 280oC for six hour storage The inlet temperature in the solar field is the same as the cold storage tank temperature

Hence, routine freeze protection can be done by the thermal storage However, an auxiliary heater still must be installed in configuration with thermal storage in case of emergency

3.4.5 Solar Field Preheat Methods:

The heat collection elements and piping within a solar collector assembly require an electric heating system to perform the following functions: preheating prior to filling with salt to

minimize transient thermal stresses; and thawing frozen salt following a failure in the salt circulation equipment

Heat Collection Elements:

Two methods have been proposed for the heat collection elements The first is an impedance system, which passes an electric current directly through the heat collection element

Transmitting a current through an electrical conductor incurs a loss in power due to the

resistance of the circuit For a direct current, the impedance losses are given by the familiar expression:

Where:

I: the current

R: the resistance

The losses are manifested by a temperature rise in the conductor Impedance heating has been used in thousands of reliable pipe heating systems over the past 30 years In addition, a 5.4 kWe impedance heating system on a 16 m section of nitrate salt piping was successfully tested

at Sandia National Laboratories in 1996 [reference]

The second method is a resistance heating system, which uses a resistance heating cable placed inside the heat collection element The cable consists of an Inconel tube 9.5 mm in diameter, two Nichrome heating wires inside the tube, and mineral insulation separating the wires from

the tube The cables are available for a number of commercial applications, and were used successfully on the 10 MWe Solar Two central receiver project near

Barstow, California

The beneficial features of the impedance concept include the following:

 Heating occurs uniformly around the circumference of the pipe In contrast, the resistance systems depend on conduction and radiation to distribute the thermal energy around the pipe

 Power densities up to 250 Watts per meter are possible due to the distribution of the current across the full cross section of the pipe In comparison, the power density of mineral insulated cables is limited to about 165 W/m to prevent

corrosion of the Inconel tube in contact with the cable The principal benefit of a high power density is a shorter preheats time

 No heating elements are placed inside the heat collection element Thus, the flow characteristics and pressure losses in the solar collector assembly remain

unaffected Also, there are no penetrations through the wall of the heat collection element or connecting piping to act as potential leak sites

The principal liability of the impedance systems is the size of the electric equipment The

stainless steel in the heat collection elements has a low electric resistance; therefore, to

achieve a reasonable preheat period, high electric currents are required The high currents, in turn, require large transformers, cables, and switchgear

Collector Field Piping:

The wall thickness of the collector field piping is much greater than the wall thickness of the heat collection elements Thus, the mass per meter of the field piping is much higher, and the electric resistivity is much lower, which makes an impedance heating system impractical As a result, the field piping uses conventional resistance heating equipment to trace heat the piping Since the heating cables are notoriously brittle, the heat trace zones are arranged to begin and end at the piping ball joints

3.4.6 Collector Loop Maintenance with an Impedance Heating System:

The field wiring for the impedance system power distribution and the resistance heat tracing would be a permanent installation, while the step-down transformer for the impedance heating system would be carried on a loop maintenance truck The entire field, baring unforeseen problems, would need to be filled perhaps 15 times during the life of the project As a result, the capital investment in a permanent transformer and associated supply wiring for each loop probably cannot be justified, and portable engine-generators and transformers are used

A collector loop would be filled, as follows:

 The maintenance truck would park at the end of a loop, and the electric connections to the permanent wiring buses would be made

 The permanent electric heat tracing on the fixed piping would be activated

 A 300 kW engine-generator on the maintenance truck would be started, supplying electric power to the transformer After about 30 minutes, the temperatures of the fixed piping and the heat collection elements would reach 200°C The isolation valves at the inlet to, and the outlet from, the loop would be opened, and a flow of salt

established in the loop

 The set point for the electric heat tracing on the fixed piping would be reduced to 150°C for emergency freeze protection

 The engine-generator would be stopped, the electric connections to the wiring buses would be removed, and the truck would move to the next collector assembly loop

A collector loop would be drained, as follows:

 The maintenance truck would park at the end of a loop, and the electric connections to the permanent wiring buses would be made The engine-generator would be started, and electric power delivered to the heat collection elements to maintain a minimum temperature of 200°C

 The permanent electric heat tracing on the fixed piping would be activated

 The isolation valves at the inlet to, and the outlet from, the loop would be closed A temporary line would be installed between the loop drain valve and a vacuum tank on the maintenance truck, and a vacuum would be established in the tank A vent valve, on the opposite end of the loop from the drain valve, would be opened, and the flow of air through the loop would push the salt into the vacuum tank

All of the salt will probably not drain from the loop; however, this is an acceptable condition for maintenance of, and restarting, the loop As long as void spaces are established everywhere in the loop, there is little danger of plastically deforming the heat collection elements or the piping when the electric preheating system is activated and the salt is thawed in preparation for refilling the loop

3.4.7 Materials consideration:

Table shows that ferric steel is necessary because temperature of molten salt could reach

500oC, A 335 grade P91 for pipes, A 234 grade WP91 for fittings , A 217 grade WP91 for valves,

A 213 grade T91 for heat exchanger tubes and A 387 grade 91 for thermal storage tanks

a) American society for testing and materials designations

b) For steam generator heat exchangers

c) For thermal storage tanks and heat exchanger shells

Table 3.5: Material specifications [11]

Final overview:

 Feasible solutions have been put forward for system charging, freeze protection,

recovery from freezing, and routine maintenance tasks Selective surface and ball joints present greater challenges

 There is no compelling economic advantage to using molten salt in a trough solar field for a system without thermal storage

 There appears to be significant economic advantages for a molten salt HTF with storage compared to VP-1 with and without storage

 In our project we will use calcium nitrate molten salt as a heat transfer, and a design for two storage tank is needed

The calcium nitrate salt density as a function of temperature in degrees Celsius is:

The calcium nitrate salt enthalpy in kilojoules per kilogram as a function of temperature in

degrees Celsius is:

The calcium nitrate salt temperature in degrees Celsius as a function of enthalpy in kilojoules per kilogram is:

The calcium nitrate salt specific heat in joules per kilogram as a function of temperature in degrees Celsius is:

3.5 losses and efficiencies

When direct solar radiation reaches the surface of a PTC, a significant amount of it is lost due to different factors

The total loss can be divided into three types:

 Optical losses

 Thermal losses from the absorber pipe to the ambient

 Geometrical losses

3.5.1 Optical losses:

The optical losses are summarized in figure 3.4

Figure 3.4: optical losses in a PTC

Optical losses are divided into several losses which are: reflectivity

intercept factor transmissivity absorptivity

 Reflectivity (ρ):

When a direct solar radiation hits the surface of a parabolic trough, a part of it is absorbed by

the parabola, while the other part is reflected towards the receiver tube The higher the

reflectivity ρ of the material, the better it is There are several types of reflectors in which we

will mention silvered glass mirror having a reflectivity 0.93 When washing the mirrors, their

reflectivity continuously decreases as dirt accumulates until the next washing Usually, they are

washed when their reflectivity is about 0.9

 Intercept factor ( Ύ):

A parabolic trough as seen in the drawings has its parabolic shape perfectly in theory, but in

reality, there is no such thing as a parabola even with the highest industrial technologies This

leads to the existence of an intercept factor which means that a fraction of the direct solar

radiation will not reach the glass cover of the absorber tube This intercept factor Ύ is typically

0.95 for a collector properly assembled

 Transmissivity (τ):

The metal absorber tube is placed inside an outer glass tube in order to increase the amount of

absorbed energy and reduce thermal losses A fraction of the direct solar radiation reflected by

the mirrors and reaching the glass cover of the absorber pipe is not able to pass through it The

ratio between the radiation passing through the glass tube and the total incident radiation on

it, gives transmissivity, τ, which is typically τ=0.93

 Absorptivity (α):

This parameter indicates the amount of energy absorbed by the steel absorber pipe, compared

with the total radiation reaching the outer wall of the steel pipe This parameter is typically 0.95

for receiver pipes with a cermet coating, whereas it is slightly lower for pipes coated with black

nickel or chrome

Multiplication of these four parameters (reflectivity, intercept factor, glass transmissivity, and

absorptivity of the steel pipe) when the incidence angle on the aperture plane is 0˚ gives what is

called the peak optical efficiency of the PTC, ηoptical,0˚ :

ηoptical,0˚ is usually in the range of 0.70–0.76 for clean, good-quality PTCs

3.5.2 Thermal losses:

Due to radiative heat loss from the absorber pipe to ambient, and convective and conductive

heat losses from the absorber pipe to its outer glass tube, the thermal losses could be

calculated using the heat transfer formula:

∆T: difference in temperature between absorber pipe and ambient

(∆T =Tr-Tamb) The heat loss coefficient depends on absorber pipe temperature and is found experimentally by

performing specific thermal loss tests with the PTC operating at several temperatures within its

typical working temperature range Variation in the thermal loss coefficient versus the receiver

pipe temperature can usually be expressed with a second-order polynomial equation like the

following equation, with coefficients a, b, and c experimentally calculated:

( )

It is sometimes difficult to find values for coefficients a, b, and c valid for a wide temperature

range When this happens, different sets of values are given for smaller temperature ranges

This is an example of an experiment made where values of coefficients a, b, and c were

experimentally calculated:

Table 3.6: Values of coefficients a, b and c for a LS-3 collector [12]

Figure 3.5: Geometrical losses at the end of a collector [12]

3.6 SOLAR ANGLES:

The earth revolves once each day around an axis that passes through the North and South poles That axis is tilted 23.44o with respect to the orbital plane of the earth around the sun, as

shown in figure 3.6 The tilt of the earth’s axis is called the declination angle It’s the reason that

day length changes as the earth makes its annual orbit around the sun It also significantly affects the intensity of solar radiation striking a fixed surface at any location on earth We observe this effect as a change in the sun’s path across the sky, as seen in figure 3.7 The sun’s position in the sky can be precisely described using two simultaneously measured angles The

solar altitude angle is measured from a horizontal surface up to the center of the sun The solar azimuth angle is measured starting from true north (0 o) in a clockwise direction (i.e., true south would have a solar azimuth of 180 o) These angles vary continuously as the sun moves across the sky At any given time they are also different at different latitudes and longitudes These angles have been precisely measured, and when needed, can be calculated for any time and location on earth

Figure 3.6: Earth yearly orbit around the sun [13]

Figure 3.7: Azimuth and altitude angle [13]

The incidence angle depends on the PTC orientation and sun position which can be easily

calculated by equation 3.12 for horizontal north south orientation and equation 3.13 for east

west orientation

√

√

Where AZI: azimuth angle

ALT: sun elevation angle

The incidence angle modifier, which directly depends on the incidence angle, is usually given by

a polynomial equation so that it is equal to 0 for φ =90o, and equal to 1 for φ =0o Therefore, for

instance, the incidence angle modifier for an LS-3 collector is given by: [12]

The efficiencies: global efficiency (ηglobal), peak optical efficiency(ηoptical,0o), and thermal

efficiency(ηth), without forgetting the parameter K(φ) are used to describe the performance of

a PTC.A fraction of the energy flux incident on the collector is lost due to the optical losses,

while another fraction is lost because of an incidence angle φ>0, which is taken into account by

the incidence angle modifier, K(φ)) The remaining PTC losses are thermal losses at the

absorber tube Figure 3.8 is a simple diagram that explains what’s happening

Figure 3.8: PTC efficiencies and losses

Let’s explain the above mentioned terms:

ηoptical,0o considers all optical losses that occur with an incidence angle of φ =0o (reflectivity of

the mirrors, transmissibility of the glass tube, absorptivity of the steel absorber pipe and the

K(φ), incidence angle modifier, considers all optical and geometrical losses that occur in the PTC

Thermal efficiency, ηth, includes all absorber tube heat losses from conduction, radiation, and

Global efficiency, ηglobal, includes: optical, geometrical, and heat losses and can be calculated

using the following formula:

The global efficiency can be calculated in another method using the figure above where the

global efficiency is the ratio of the output power over the solar energy flux on the collector

P(HTF): power transferred to the fluid(vp1 oil, molten salt, )

η(optical): optical efficiency

3.8 Fluid pump:

The fluid pump, which is in our project a molten salt pump, is the heart of all the system due to

the fact that the whole circuit will not operate satisfactorily if the pump does not function

properly, so we should select the type of pump that will best meet all of the criteria for a

successful high temperature molten salt system Meeting the flow and head demands for a

given system is a very small portion of understanding the specifications for a high temperature

pumping system From a design standpoint, these pumps require simplicity, ruggedness, and

flexibility to meet longevity in life Everything, from how the pump is installed to the

maintenance that will be required, must be evaluated before purchasing it Understanding the total system requirements will ensure a safe and long life of a molten salt pump

3.8.1 Materials and Temperatures:

The starting point for selecting the materials of construction used for a molten salt pump is knowing the temperature The operating temperature range for molten salt is between 238oC and 1200oC, 238 being the melting point The molten salt will decompose at a certain

temperature so it is necessary to know the chemistry of the salt being used As well known, temperature weakens the strength of all materials, so the correct materials and designs are reviewed together For selecting a pump, one should understand the relationship between

materials being used and the design Either the materials can be used to compensate for

weaker designs, or the design can compensate for weaknesses in the materials due to high

temperatures The length of the pump and the operating temperature of the molten salt

greatly affect the basic design and material selection of the pump It is dangerous to select a material that falls into the marginal range or at the extreme high end of the temperature range High temperatures and basic materials can be divided into four major categories as shown in the following table:

Table 3.7: materials to be used according to the temperature range [14]

Before selecting a material, the correlation between the type of salt, temperature range or

ranges, velocities within the pump volute and discharge, thermal expansion of the pump shaft, column and discharge assemblies, and mounting arrangement of the pump must be evaluated The type of salts used in molten salt applications varies widely, from simple compound salts like sodium nitrates and potassium nitrates to blended complexes, such as fluoride based salts like FLiNaK Understanding their melting points, decomposition temperatures, corrosion

characteristics, need for agitation, fluid density at different temperatures and freezing points all help in selecting the proper materials Temperature ranges that vary can cause distortion and binding in the rotating elements of the pump For this reason, calculations of each range must

be performed to determine the thermal expansion effects on the pump’s rotating assembly and stationary components to ensure the proper materials are used in case these temperature

Temperature 240-350 deg C 350-600 deg C 700-930 deg C 930-1100 deg C

Basic Materials Carbon Steel 316, 321,

347SS

600,625 Inconel

TZM Moly