Central Receiver System Power Plants - A Description of the Technology

The PS20 Solar Tower Plant at Sanlucar la Mayor, Seville, Spain. © Markel Redondo Photography

ENB456 - Energy

Lecturer - Farhad Shahnia

Queensland University of Technology

By Mads Hellegaard Andersen, Thomas Schmidt and Rune Wiben

October 10, 2011

Contents

1 Introduction 1

2 Description of The CRS Power Plant 3

3 Heliostats and Receiver 5

3.1 Receivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.2 Control of Heliostats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Heat Transfer Circuit 10

4.1 Heat Transfer Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.2 Thermal Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 The Heat Engine 12

5.1 The Rankine Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.1.1 Reversible and Irreversible Processes . . . . . . . . . . . . . . . . . . . . . . 12

5.1.2 The Rankine Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.1.3 Deviation From Ideal Cycle in Real Power Plants . . . . . . . . . . . . . . . 14

5.2 Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.3 Boiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.4 Turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.5 Condenser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

6 The Future of CRS Power Plants 18

Bibliography 18

2

Abstract

Mads Hellegaard Andersen, Thomas Schmidt and Rune Wiben

This paper describes the technology of Central Receiver Power Plants. Initially the technology iscategorized among other solar power technologies. A brief description of the overall plant structureis made followed by an elaborating description of each subsystem of the plant. Throughout thisdescription different possibilities for increasing the efficiency of the plant are pointed out anddiscussed. Finally the future of Central Receiver Power Plants are discussed and it is concludedthat this technology is well suited to be part of an increasingly renewable power generation in thefuture due to its energy storage system which reduce the implications of the fluctuating powergeneration.

Chapter 1

Introduction

This report is concerned with electricity generation using Central Receiver System (CRS) PowerPlants. A CRS Power Plant is based on a sustainable, green energy source as it uses energy fromthe sun.Solar energy is used in a variety of heating and power generating applications. The diagramin Figure 1.1 shows the categorization of CRS Power Plants in the wide area of solar energyapplications.

Figure 1.1: Categorization of a CRS Power Plant.

The broadest categorization of a CRS Power Plant in the Solar Energy Applications is thus PowerGeneration opposed to Heating which covers water and space heating used widely in both com-mercial and residential applications. The Power Generation using solar energy can be split in twomajor categories: Photo-Voltaics (PV) and Concentrated Solar Power (CSP) where PV applica-tions use photo-voltaic cells to directly convert light into electricity by means of the photoelectriceffect. PV systems have been widely used in space applications due to its high power capacity perunit weight and the technology is spreading to other areas - both residential and commercial insmaller and larger scales as the prices of PV cells decrease [1, ch. 8] [2].

In CSP applications reflectors are used to concentrate solar energy to a small area thereby achiev-ing a high energy concentration. This energy can then be used to heat a working fluid used to drivea turbine. CSP plants share characteristics with fossil fuel power plants but the heat originatesfrom concentrated solar energy and not from the burning of fossil fuels.CSP plants make use of different reflector shapes illustrated in figure 1.2.

1

CHAPTER 1. INTRODUCTION

Figure 1.2: a Trough shaped reflector, b Central Receiver with heliostat reflectors and c Discshaped reflector [1, ch. 9].

The trough reflectors are the most widely used of the three different types. In these systems aliquid (usually oil) absorbs the concentrated sunlight through a glass tube running along the focalline of the parabolic trough. The hot oil is then used to heat water in order to produce steamwhich is used in a steam turbine [1, ch. 9]. In order to absorb as much energy as possible thetroughs must be controlled to track the sun which usually is done by one- or two-axis trackingmechanism using electric motors to align the troughs according to the position of the sun [2, p.31-32]. The parabolic trough power plants are used in high power applications of up to severalhundred MW [2, p. 40].In CRS Power Plants the sunlight is reflected using up to several thousand mirrors - called he-liostats - to a very small area - called a reciever - situated in a tower. The resulting temperatureat the receiver is higher than for parabolic troughs due to the high absorption area reflecting ontoa very small area. This makes it possible to achieve higher efficiencies than for parabolic troughplants [1, ch. 9]. The alignment of heliostats that reflect the sunlight onto the receiver must becontinuously controlled in order to hit the target area. This must be done by a two-axis trackingmechanism. As for parabolic trough plants CRS Power Plants in the several hundred MW rangeexist [1, ch. 8].In disc reflector applications one large disc reflects the sunlight to a receiver placed at the focusof the disc. The heat is then used in a sterling heat engine-generator unit [1, ch. 8]. The discreflectors are usually used in relatively small applications of tens of kW due to the limitations ofavailable engines and limitations of disc size due to the wind load [1, ch. 8]. The temperature atthe receiver is even higher than for CRS systems yielding a higher possible efficiency.Some characteristics of the three reflector types are listed in table 1.1.

Technology Operating temperature Thermodynamicon the hot side [◦C] cycle efficiency

Parabolic Trough 300-500 LowCentral Receiver 500-1000 Moderate

Disc 800-1200 High

Table 1.1: Comparison of different reflector types [1, ch. 8].

In this report the focus will be on CRS Power Plants due to the moderately high efficiencycombined with the possibility of high power applications.

2

Chapter 2

Description of The CRS PowerPlant

This section is based on [1, ch. 9]. In a CRS Power Plant solar energy is collected by sun-trackingmirrors, called heliostats, and is reflected to a single receiver on top of a tower. The energyconcentrated in the receiver is used to heat an energy transferring fluid. The heated fluid is usedfor energy production right away but also stored so that energy is available during cloud covers.When energy is needed the fluid is passed through a boiler to heat a working fluid used to driveturbine-generator application which is connected to the grid. After delivering thermal energy inthe boiler the “low” temperature heat transfer fluid is stored in another storage tank and pumpedback to the receiver to be reheated in a new thermal cycle. A simplified schematic of a CRS PowerPlant is seen in Figure 2.1, in this figure the heat transfer fluid is molten salt while the workingfluid is water/steam.

Figure 2.1: Schematic of a CRS Power Plant [1, ch. 9].

Evident from the schematic of a CRS Power Plant seen in figure 2.1 the plant has several subsys-tems that must be considered. Besides the Generator and Grid Connection part, the subsystemslisted below will be described in individual sections:

3

CHAPTER 2. DESCRIPTION OF THE CRS POWER PLANT

• Heliostat

• Receiver

• Heat Transferring Circuit

• Heat engine

– Pump– Boiler– Turbine– Condenser

• Generator and Grid Connection

As the plant has several subsystems the overall efficiency of the plant is determined by the efficiencyof all the individual subsystems. Higher overall efficiency of the plant yields lower constructioncosts. Furthermore some of the subsystems are subject to constraints especially regarding thetemperature range in which they can operate. Therefore it is not always possible to increase theoverall power output by increasing the plant size and by absorbing more energy from the sun.

4

Chapter 3

Heliostats and Receiver

The purpose of the heliostats and the receiver is to convert the electromagnetic energy of thesunlight into internal energy in the heat transfer fluid. The fluid is described in the Section 4. Apicture showing a Central Receiver System (CRS) plant is given in Figure 3.1.

Figure 3.1: Example of a CRS Plant. The plant is a project plant from Barstow in Californiabuild in 1982. [3].

The heliostat tracks the suns position across the sky and controls the rotation and inclinationangles of the mirror so it reflects the sunlight to the receiver in the top of the central tower. Thereceiver absorbs the concentrated sunlight and turns it into heat which is transfered away from thereceiver and to the Heat Engine by the Heat Transfer Fluid. By combining heliostats it is possibleto reach temperatures of up to 1000◦C with development of receiver technologies allowing for1200◦C. [3]. The high temperature allows for high efficiency in the Heat Engine which is describedin Section 5.

The largest expense in a Central Receiver System is the cost of the heliostat field. [4, p.79].Therefore it is of great interest to optimize the efficiency of the field so that the size of it can beminimized. In order to do so the engineers must design the layout of the heliostat field, the optimalheight of the tower, the shape and size of each heliostat, choose an efficient control algorithm for

5

3.1. RECEIVERS CHAPTER 3. HELIOSTATS AND RECEIVER

focusing the light and choose the best receiver type for the system. It is a complicated task witha lot of local optimums in the design space.

3.1 Receivers

Early receiver types were made out of steel tubes containing the heat transfer medium. Theproblem with this type were inadequate heat transfer and local overheating of tubes. Today tubesare still used but has been undergoing radical design changes. The most promising type of receiveris a volumetric receiver which consist of a wire mesh - made of ceramic or metallic materials - ina honeycomb structure, with air flowing through the mesh. The air reaches temperatures of upto 850◦C. An improvement of this concept is being prototyped and is called “the pressurized airreceiver” concept. The principle is the same as the volumetric, but the receiver is encapsulated ina pressurized chamber isolating it from the environment. Instead of pulling in air from the frontof the receiver, this type gets its air-supply from a compressor driven by the turbine.

Figure 3.2: Illustration of a pressurized volumetric receiver. [3].

Using this receiver the air temperature can reach 1200◦C which is high enough for supplying a gasturbine. The exhaust from the gas turbine is then re-heated and used in the heat engine describedin Section 5. A diagram showing the process is seen in Figure 3.3.

Using this hybrid configuration of both a gas- and a super heated steam turbine it is possibleto reach efficiencies of more than 50% of the Heat Engine. Therefore solar systems with a totalefficiency of over 20% is possible. [5].

6

3.2. CONTROL OF HELIOSTATS CHAPTER 3. HELIOSTATS AND RECEIVER

Figure 3.3: Central receiver system using a pressurized receiver and a steam heat engine extendedwith a gas turbine. [5].

3.2 Control of Heliostats

As long as the mirrors of the heliostats are symmetric and "target aligned" the control of themirrors are described by the equations presented here. A target aligned heliostat is a heliostatthat has two axis of rotations defined as in Figure 3.4 and has its first axis pointing to the targetat the tower.

Figure 3.4: Illustration of a target aligned heliostat. [6].

As the sun moves the heliostat rotates around its first-axis so the horizontal/meridian plane ofthe heliostat coincide with the incident vector of the sun light. The incident angle of the mirrorcan be found using the equation in (3.1).

θ = cos−1{√

22 [sin(α)cos(λ)− cos(φH −A) cos(α) sin(λ) + 1]−1/2

}(3.1)

7

3.2. CONTROL OF HELIOSTATS CHAPTER 3. HELIOSTATS AND RECEIVER

It is derived by defining two vectors: The Incident Vector which points towards the sun (shown infigure 3.5) and The Reflection Vector which points towards the target (shown in figure 3.6). Thevectors are defined in Equation 3.2 and 3.3. The incident angle is half the angle between the twovectors.

Figure 3.5: Illustration of the incident vectorpointing towards the sun from the heliostat[6].

Figure 3.6: The reflection vector pointing to-wards the target [6].

cos(αi)cos(βi)cos(γi)

=

cos(A) cos(α)sin(A) cos(α)

sin(α)

(3.2)

cos(αr)cos(βr)cos(γr)

=

− cos(φH) sin(λ)− sin(φH) sin(λ)

cos(λ)

(3.3)

The incident angle is in an earth fixed frame. In order to know the reference points for the twoactuators on a particular heliostat the angle must be transformed to auxiliary coordinates fixedto the heliostat. To relate the two reference-frames three rotational transformation matrices aredefined as follows:

M1 =

sin(φH) − cos(φH) 0cos(φH) sin(φH) 0

0 0 1

(3.4)

M2 =

1 0 00 cos(λ) sin(λ)0 − sin(λ) cos(λ)

(3.5)

M3 =

cos(ωH) − sin(ωH) 0sin(ωH) cos(ωH) 0

0 0 1

(3.6)

8

3.2. CONTROL OF HELIOSTATS CHAPTER 3. HELIOSTATS AND RECEIVER

The angles of rotations are seen in figure 3.7. By multiplying the three matrices the relationshipbetween the base frame and the auxiliary frame is established, and the Incident Vector (pointingtowards the sun) can be described in both coordinate frames and equated like in Equation (3.7).

Figure 3.7: Illustration of the three rotational matrix transformations, transforming from earthfixed reference frame to an auxiliary reference frame on the heliostat. [6].

0sin(2θ)cos(2θ)

= M3M2M1 ·

cos(A) cos(α)sin(A) cos(α)

sin(α)

(3.7)

Solving the first row of the matrix equation we find the rotation angle about the first axis of theheliostat ωH :

ωH = tan−1[

cos(α) sin(θH −A)cos(λ) cos(α) cos(θH −A) + sin(λ) sin(α)

](3.8)

When the heliostat has been rotated about the first axis with the angle ωH , the angle that tiltsthe heliostat in the right position is denoted EH . The right position is when it is angled in themiddle of the vector pointing towards the sun and the vector pointing towards the target, as for asymmetric heliostat the angle of incident equals the angle of reflection, and this is the same angleas the previously derived incident angle θ:

EH = θ (3.9)

9

Chapter 4

Heat Transfer Circuit

The thermal solar energy concentrated in the central receiver (described in Section 3.1) is absorbedby a Heat transfer Fluid (HTF). Dependent on the design of the plant different fluids have provencapable of transferring and storing the thermal energy.

4.1 Heat Transfer Fluids

The HTF’s used in the majority of experimental CRS-plants are: water/steam, oil, air and moltensalt [3, p. 48] [7, ch. 10.1.2]. In table 4.1 the characteristics and advantages of these 3 fluids arehighlighted.

FluidOperatingTemperature(High)

Meltingpoint(at 1 [atm])

Advantages Disadvantages

Water/Steam 540 [◦C] 0 [◦C] Low cost and simpleimplementation.

Difficult to use as energystorage medium.

Oil 425 [◦C] -10 [◦C] Effective as energystorage medium.

Safety requirements dueto highly flammablefluid and high cost.

Air 1000 [◦C] N/ALow Cost and highefficiency due to ∆Tacross turbine.

Energy storage very dif-ficult.

Molten salt 570-600 [◦C] 98-220 [◦C] 1 High volumetric heatcapacity.

Requires heating beforeplant startup.

Table 4.1: Characteristics and advantages of Heat Transfer Fluids [7][8]

The melting temperature of the fluids in table 4.1 are of importance because a HTF with highmelting temperature requires reheating before startup of the plant [7, ch. 10.1.2]. Also in theHT-circuit some pipes will require additional heating to make sure that the HTF does not solidifyin the pipes used on the “cooled” side of the circuit 2.The operating temperature of the fluid is a primary criterion when choosing HTF [7, ch. 10.1.2].The volumetric heat capacity of the HTF is also of interest as this determines the possibilitiesfor thermal storage. Therefore molten salt has been used in the latest experimental CRS-plantsamong others the “Solar Two” experimental plant located in California [8, p. 5-9].

2This is done using electric heating with wires on the outside surface of the pipes[8, p. 5-16]

10

4.2. THERMAL STORAGE CHAPTER 4. HEAT TRANSFER CIRCUIT

4.2 Thermal Storage

For the turbine of the plant to deliver a constant power output, the energy from the HTF is storedso that fluctuations in solar energy due to clouds does not effect the electricity production [8,p.5-6]. The Plants are designed so that the thermal capacity of the heliostat fields exceeds thethermal requirements of the steam turbine - the ratio is called the solar multiple [9, p. 4]. With asolar multiple larger than 1, the CRS Power Plant will be able to store energy when the turbine isat maximum capacity and thus the Plant can operate at full capacity for a couple of hours aftersundown (or during cloud cover) [8, p. 5-7].Three storage techniques have been used in Solar Thermal Power Plants [10, p. 13]:

• Sensible heat storage - The HTF is stored directly or used to heat another medium with ahigher thermal capacity and then stored in a tank or a cave.

• Latent heat storage - Energy is stored by phase change of a medium keeping the temperatureconstant.

• Thermochemical heat storage - Thermal energy is used to drive an endothermic chemicalprocess of a fluid which can then be stored an used in a reverse exothermical process whenenergy is needed.

The Sensible heat storage techniques is used in most Thermal Power Plants today as this is thesimplest way of storing the energy absorbed [10, p. 13].Latent heat storage is a relatively new concept and is first expected to be used commercially in 7years [10, p. 14].The advantage of Thermochemical heat storage is that the medium can be stored long term andtherefore stored energy can be transported off site and transformed where the energy is needed[10, p. 15].

11

Chapter 5

The Heat Engine

The heat engine is an essential part of a thermal power plant as it is responsible for the thermo-dynamic energy conversion from thermal to mechanical energy on the generator shaft. The heatengine consists of the pump, the boiler, the turbine and the condenser.

5.1 The Rankine Engine

An ideal vapor heat engine is called a Rankine engine as it undergoes a Rankine Cycle [11, ch. 10.2]- a schematic of a Rankine Engine is seen in Figure 5.1. In a Rankine Cycle the fluid undergoesthe following 4 internally reversible processes:

• Isentropic compression in a pump

• Isobaric heat addition in a boiler

• Isentropic expansion in a turbine

• Isobaric heat rejection in a condenser

5.1.1 Reversible and Irreversible Processes

A reversible process is a process that can be reversed without leaving any trace on its surroundings- that is, both the system and its surroundings are returned to their initial state after the reversecycle [11, ch. 6.6]. Such processes are idealizations and cannot occur in reality. However it ispossible to return the system to its initial state after the reverse cycle by letting its surroundingsperform some amount of work on it. This is not a reversible process due to the fact that thesurroundings are not returned to the initial state - such a process is called internally reversiblebecause no irreversabilities occur within the system and the reverse process pass through the sameequilibrium states as for the forward cycle.Reversible processes always have higher efficiency than irreversible processes and the irreversibil-ities are therefore undesirable. Irreversibilities include [11, p. 297-298]:

• Friction

• Unrestrained expansion of a gas

• Heat transfer through a finite temperature difference

As mentioned the Rankine Cycle is internally reversible but not externally reversible due to thefact that heat and work is supplied from outside the closed system. The internal reversibility ofthe process is however an idealization - therefore the term "ideal" vapor heat engine is used to

12

5.1. THE RANKINE ENGINE CHAPTER 5. THE HEAT ENGINE

Figure 5.1: Schematic of a Rankine heat engine.

describe the Rankine Engine.The entropy change of a system during a cycle is a measure of the irreversibility of the system.Therefore a lot of information about a system efficiency and how to increase it is included in thetemperature-specific entropy diagram (T-s diagram) of the system cycle.

5.1.2 The Rankine Cycle

The (T-s diagram) of the Rankine Cycle is seen in Figure 5.2.

The Rankine Cycle seen in Figure 5.2 is now explained - the states 1,2,3 (3’) and 4 (4’) correspondto the markings in Figure 5.1:Water enters the pump at state 1 as saturated liquid. Work is applied to the pump from thesurroundings and the liquid is compressed to the operating pressure of the boiler. The watertemperature rises slightly during the compression because of the decrease in specific volume of thewater. At state 2 the pressurized water enters the boiler in which heat is transfered to the systemresulting in a constant pressure heat addition. Heat is added until a phase change occurs (thehorizontal line in the T-s diagram). Two different scenarios continue from this point. In a normalRankine cycle the fluid enters the turbine as saturated (or nearly saturated) steam (point 3) andundergoes an isentropic expansion in which work is produced by rotating the shaft connected toa generator. The temperature and pressure of the liquid drops during this process to the point 4.In order to increase the work done by the turbine the steam is often superheated before enteringthe turbine. In this scenario the boiler adds more heat to the system causing the fluid to go intothe superheated region where the steam is dry (the phase change from liquid to gas is complete)and the temperature rises further to the point 3’. The steam then enters the turbine at a highertemperature and higher pressure producing more work. During the isentropic expansion in theturbine the temperature and pressure falls and leaves the turbine at state 4’ - at this state thesteam is a high quality mixture of liquid and gas (high quality meaning that the liquid content

13

5.1. THE RANKINE ENGINE CHAPTER 5. THE HEAT ENGINE

Figure 5.2: T-s diagram of a Rankine Cycle with and without superheating.

is low compared to the gas content). At state 4 (or 4’ for the superheated case) the fluid entersthe condenser where heat is removed at constant pressure using active or passive cooling and isbrought back to the initial state 1.For internally reversible processes the area under the process curve in the T-s diagram) representsthe heat transfer. From figure 5.2 it is seen that the area under process curve 2-3 (or 2-3’) repre-sents the heat transfered to the fluid in the boiler and the area under process curve 4-1 (or 4’-1)represents the heat rejected in the condenser. The difference between these to curves (the areaenclosed by the cycle curve) is the net work produced during the cycle [11, p. 554].Using this information it is seen that the superheated scenario produces more net work, thus ahigh temperature of the salt - acting as a heat source - is needed in order to increase the efficiencyof the system.

5.1.3 Deviation From Ideal Cycle in Real Power Plants

Reversible processes are difficult to approximate in real life due to irreversibilities in the variouscomponents used in the heat engine. The major causes of irreversibilities are [11, ch. 10.3]:

• Fluid friction which causes pressure drops in the boiler, the pipes and the condenser. Tocompensate for this the water must be pumped to a significantly higher pressure than inthe ideal cycle. This demands more work input to the pump and thus lowers the overallefficiency of the plant.

• Heat loss to the surroundings which causes the temperature difference between the turbine

14

5.2. PUMP CHAPTER 5. THE HEAT ENGINE

inlet and outlet to decrease resulting in less output work and thus lower overall efficiency.

• Mechanical friction in pump and turbine which causes an entropy increase during the com-pression and expansion of the fluid - this converts some of the input and output work to beconverted into heat that escapes the system thus lowering the overall efficiency.

Figure 5.3 shows the T-s diagram of an ideal and an actual Rankine cycle.

Figure 5.3: T-s diagram of an ideal (black) and actual (red) Rankine cycle.

5.2 Pump

Different hydraulic pumps can be used to maintain a steady flow in the Rankine cycle (eg. gear-,vane- or piston-type pumps [12, ch. 2.1.1]). The total efficiency of the pump is determined by thevolumetric efficiency and the mechanical efficiency. The volumetric efficiency of the pump, ηvP ,depends on pressure across the pump, ∆pp, [12, ch. 2.2.2]:

ηvP = 1− ∆ppKlp

µQtP(5.1)

where,Klp, is a leakage constant for the pump, µ is the dynamic viscosity andQtP is the theoreticalpump flow (stroke replacement times rotational pump speed).The mechanical efficiency, ηhmP , is determined by the ratio between the theoretical input torqueto the pump, Mtp, and the actual input torque, Mp, [12, ch. 2.2.2]:

ηhmP = Mtp

Mp(5.2)

15

5.3. BOILER CHAPTER 5. THE HEAT ENGINE

Therefore the total efficiency of the circulation pump is:

ηP = ηhmP ηvP (5.3)

Choosing a pump with high efficiency would increase the total efficiency of the plant.Other factors which determines the pump choice are: The working fluid of the Rankine cycle [7,ch. 12.2.3], and the flow and pressure requirements of the boiler within the cycle.

5.3 Boiler

If the average temperature at which the boiler operates is increased the efficiency of the Rankineengine is increased as well. The problem for increasing the maximum temperature is that itdramatically increase the requirements to the mechanical structure. Special materials must beused to withstand the higher temperature and pressure which increases the initial expenditure ofthe plant. The increased efficiency should be high enough as to facilitate such an investment. It ispossible just to stop the cycle when the temperature reaches the same maximum, but if the entropyof the fluid at this point is too low the vapor will start turning into liquid in the turbine before itreaches the condenser. This dramatically increase wear on the turbine blades which can lead toa significant increase in operating expenses. If re-heating is used then the averaged temperaturecan be increased without increasing the maximum temperature, and the problem can be avoided.The Rankine cycles with and without re-heating is seen in Figure (5.4) and Figure (5.5) below.

Figure 5.4: Example of Rankine cycle withincreased boiling pressure but maintainedmaximum temperature. [11].

Figure 5.5: Rankine cycle using re-heatingto avoid moisture on last turbine blades.[11].

5.4 Turbine

Steam turbines are the preferred choice in CRS Power Plants because steam is a well known fluidand because the Rankine Cycle for steam is well documented and tested in fossil powered plants[7, ch 12.2.3] [13, p. 3]. Steam turbines are under constant development. Figure 5.6 shows a steamturbine from Siemens to be used in the Ivanpah Solar Power Complex, California. The turbine isone of three turbines designed for the complex, each has a power output of 123 [MW].

16

5.5. CONDENSER CHAPTER 5. THE HEAT ENGINE

Figure 5.6: The siemens SST-900 steam turbine. Has a rotational speed of up to 3600 [rpm] andan inlet steam temperature of up to 585 ◦C [14, p. 6]

.

5.5 Condenser

The condenser is responsible for rejecting heat from the fluid, which enters as a mixture of liquidand gas and leaves as saturated liquid. Condensers are basically large heat exchangers whichdissipate heat from the fluid to the environment. Condensers are split into active and passivecondensers with active condensers using fans.

In order to achieve a high efficiency of the thermodynamic cycle it is necessary to reject the heatat as low pressure as possible. In a two-phase saturation (a mixture of e.g. water and steam) alower condensing pressure also means that the temperature is lowered as it is fixed to the fluidssaturation level doing the condensing process [11]. The condensing temperature cannot be loweredto a value less than the temperature of the cooling medium, it actually has to stay well abovethis temperature as to allow for effective heat transfer. Therefore when planning the location fora concentrated solar power plant it is essential to have a cold cooling medium, which can be aproblem as they also needs to be placed in regions with a lot of solar radiation which often havea warmer environment.

17

Chapter 6

The Future of CRS Power Plants

Solar Power Towers is a relatively new technology, only two commercial plant exists [10, p. 4].However experimental plants have proven capable of delivering a constant power output. Theplants have also shown improvements in efficiency as the technology has evolved [15, p. 47]

If CRS Power Plants are to be a success the location of the Plants is essential. They shouldbe placed in regions with a global irradiance of more that 1800 [ kW h

m2 ] [5]. As figure 6.1 showsthis involves the earths Sunbelt. However this is an insignificant limitation of the technology. For

Figure 6.1: Annual average of daily direct normal irradiation of global solar exposure [16].

example if 1% of the Sahara desert is used for Solar Power Plants, they could theoretically coverthe total global electricity consumption [5].The advantages of Solar Power is that it offers thermal energy storage where other Renewabletechnologies such as wind turbines have fluctuating power production. Solar Power Plants pro-duce electricity when the consumer demand is largest. And finally the materials used in SolarPower Plants are well known and conventional materials which are not costly [10, p. 55].

All in all solar plant is an expanding technology and a possible part of the solution to a con-tinuously increasing global energy demand.

18

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