Superheated steam generation in a fresnel solar field for hybridization of a coal power plant

SUPERHEATED STEAM GENERATION IN A FRESNEL SOLAR FIELD FOR HYBRIDIZATION OF A COAL POWER PLANT

Florianópolis 2015

SUPERHEATED STEAM GENERATION IN A FRESNEL SOLAR FIELD FOR HYBRIDIZATION OF A COAL POWER PLANT

Thesis Project presented to Federal University of Santa Catarina in partial fulfillment of the requirements for the degree

of Bachelor of Science in Mechanical Engineering

Professor Advisor: Dr. Eng. Eduardo Burin Professor Advisor: Ph. D. Andrés González

Florianópolis 2015

To Mabel, Hernán and Celina.

“Let the beauty of what you love be what you do”

To my mom who taught me to fly and to believe I actually can. Thank you for illustrating me that one lives for others and there is no other way to aim our dreams than hard work and honesty. You decide who you want to be.

To my grandma who illustrated me in real time what are love and kindness.

To my grandpa who is my best example of honesty and discretion. Thank you for your company, supporting and most of all, complicity. You are the adventure mate that everyone could desire. To my mom, my grandpa, my grandma and Micky for giving me my first chance.

To my family for your affection, company and example of honesty and responsibility.

To my uncle Et and Tata for adopting me before I started the project. Thank you for all your sweetness and for teaching me the basis of a family.

To my aunt Nazly and my uncle Boris for your company from distance.

To my cousin Daniela for the company, confidence and teaching me the example as a tool of change.

To R.S. Zoraida for the friendship, the listening, the advices and being my adoptive mother. To Ángela Dubois for the company despite distance, the friendship, the understanding and every laugh.

To Eliana Jácome for showing me there is no fear in order to live with kindness. Thanks for the company, listening and contagious happiness.

To Claudia Vanegas for teaching me the worth of silence and that everyone you meet is fighting a battle you know nothing about and I should not forget to be kind always.

To Eliana and Claudia who remembered me how real home looks like and allowed me to have siblings.

To Ricardo Velasco, Felipe Castro, Andrés León and Pablo Cortés for the company from distance, the support throughout the whole career, the friendship, the listening and the faithfulness.

To all my friends in Colombia and Europe for the company from distance, the affection and the confidence.

To every professor I met at the University and his/her teachings, personal and academic. Especial acknowledgement to Professor Orlando Porras for his advice, confidence and the opportunity to develop my thesis in a topic of my preference.

To Professor Eduardo Burin for the opportunity, all the advices, patience, cooperation, company and sharing me peace.

To Professor Andrés González for the opportunity, acceptance of this project and the support. To my colleagues from LabCet in Florianópolis. Especial acknowledgement to Nury Nieto for the company, advices, help and listening.

To people from Florianópolis who made my stay more enjoyable. Thanks to all Colombians, Brazilians and other foreigners for the experiences. Especial acknowledgement to David Velasco and Andrea Jiménez for the company, collaboration, advices and care, to Diego Suárez for the company, friendship, maturity and affection, to Andrés Chamorro for the company and friendship and to Crislaine Flor for the spiritual counseling.

ABSTRACT

The current work develops two simulations over a hybrid power plant in order to study its performance and to predict the technical feasibility of proposed hybridization layout. The hybrid power plant consists of a coal thermoelectric power plant that it is assisted by a Fresnel concentrator system at the process of generation of vapor. The power plant has the capacity of 10 MW in order to meet the local power demand of a coal mine located in the South of Santa Catarina State. Simulations are solved with assistance of software EES. The first simulation evaluates the performance of the hybrid power plant at design point and the characteristics of Fresnel system are identified. The second simulation evaluates the power plant with the data of a typical weather year in Florianópolis. This simulation compares the performance of plant in summer and winter days. Furthermore, the results allow estimating the advantages of hybridization. Finally, the project concludes about the performance of hybrid plant in comparison to conventional power plants, the advantages of hybridization, the differences between summer and winter days and the convenience of the location.

LIST OF FIGURES

Figure 1. Localization of Brazilian coal region in Santa Catarina. ... 2

Figure 2. Global cumulative installed concentrating solar power capacity (MW), 2007-2012 ... 4

Figure 3. Types of CSP ... 5

Figure 4. World Direct Normal Irradiation. ... 8

Figure 5. Sugarcane mills (a) and DNI (b) in Brazil. ... 9

Figure 6. Coal formation. ... 9

Figure 7. Relevant factor in a boiler selection for steam generation. ...12

Figure 8. Declination. ...13

Figure 9. Hour angle. ...14

Figure 10. Azimuth and solar altitude angles. ...14

Figure 11. Hybrid Plant layout. ...16

Figure 12. Detailed layout of Fresnel system. ...20

Figure 13. T-s diagram for Rankine cycle with regeneration for CFB and Fresnel concentrator hybrid power plant. ...21

Figure 14. Design Point simulation’s scheme with results. ...31

Figure 15. Temperatures at Fresnel Module for DNI variation...35

Figure 16. Power plant Layout for DNI∗ ηop<67 W/m2. ...36

Figure 17. Power plant Layout for 402W/m2> DNI ∗ ηop ≥ 67W/m2 ...37

Figure 18. Power plant Layout for 402W/m2≤ DNI ∗ ηop. ...37

Figure 19. Required parameters for solar angle’s calculus. ...38

Figure 20. Function of correction factors as a function of angles of correction. ...41

Figure 21. T-s diagram for DNI*ηop <67 W/m2 ...43

Figure 22. T-s diagram for 402 W/m²<DNI*ηop ≤67 W/m2. ...45

Figure 23. T-s diagram for 402 W/m2≤DNI*ηop. ...47

Figure 24. Range of coal demand. ...50

Figure 25. Range of solar share for the year of study. ...50

LIST OF TABLES

Table 1. Comparison Table of Concentrated Solar Power. ... 6

Table 2. Heating value for every coal type. ...10

Table 3. Coal Power Plants in Brazil by 2002. ...11

Table 4. Design Point Simulation Parameters. ...18

Table 5. Mass flow variables. ...22

Table 6. Resume of thermodynamic approach. ...27

Table 7. Configuration of mirrors for Fresnel System. ...29

Table 8. Pressure, temperature and thermodynamic properties for each point of circuit. ...30

Table 9. Heats and works extracted or required by each component of power plant. ...30

Table 10. Parameters of Evaluation. ...31

Table 11. Possible combinations of number of mirrors for module for converging at simulation. ...32

Table 12. Reference Power Plants around the world. ...33

Table 13. Calculation of number of day at the year. ...38

Table 14. Correction factors for model NOVA 1. ...40

Table 15. Thermodynamic parameters for annual simulation. ...42

Table 16. Used equations for thermodynamic model with DNI*ηop <67 W/m2. ...42

Table 17. Used equations for thermodynamic model with 402 W/m2<DNI*ηop ≤67 W/m2. ...44

Table 18. Used equations for thermodynamic model with 402 W/m2 ≤DNI*ηop. ...45

Table 19. Parameters of evaluation about coal consumption for Florianópolis location. ...49

Table 20. Peak of solar share and Average of Solar Share during operational time of Fresnel System. ...50

C Carbon

H Hydrogen

O Oxygen

S Sulfur

δ Declination [rad]

ω Hour Angle [rad]

θ Incidence Angle [rad]

γs Solar Azimuth [rad]

α_s Solar Altitude [rad]

lat Latitude [°]

mer Meridian [°]

lon Longitude

CFB Circulating Fluidized Bed CSP Concentrated Solar Power

DNI Direct Normal Irradiance [kW/m²]

x Quality

ṁ_T Total steam mass flow [kg/s]

ṁR Regeneration steam mass flow [kg/s]

ṁc Steam mass flow generated by CFB boiler [kg/s]

ṁ_f Steam mass flow generated by Fresnel system [kg/s]

T Temperature [K]

P Pressure [bar]

h Enthalpy [kJ/kg]

s Entropy [kJ/kg]

ẆT Power generated at turbine [MW]

η_t Isentropic Efficiency of Turbine

ηp Isentropic Efficiency of Pump

Q̇_l Required Heat flow at Heat Exchanger [kW]

Q̇sat Required Heat rate at saturator module of Fresnel system [kW] Q̇SH Required Heat rate at superheated module of Fresnel system [kW]

LHV Low Heat Value [kJ/kg]

η_c Efficiency of CFB boiler

ṁ_coal Required coal mass flow [kg/s]

Asat Total area of saturator module [m²]

P_lossSat Loss Heat rate at saturator module [kW]

η_th Thermal Efficiency of power plant

Ẇout Output Work rate [kW]

Q̇_in Inlet Heat rate [kW]

w_p Power Pump demand ratio

R Recirculation ratio

S Spray ratio

b Number of mirrors at saturator module c Number of mirrors at superheater module

d Number of mirror at superheater module before spray point

A Area [m²]

ηop Optical Efficiency B Factor of day of the year LH Local hour in numerical format

β Inclination of mirror [rad]

γ Superficial Azimuth [rad]

θ_z Zenith [rad]

θtrans Transversal Angle of incidence [°]

θi Longitudinal Angle of incidence [°]

K_⊥ Transversal Factor of correction

K∥ Longitudinal Factor of correction

plant

m_ccp Required coal for a conventional power plant of same dimensions

t_saved Time of operation for power plant with the saving coal of

previously year [days]

ss̅ Average solar share

1.1. Objectives ... 2

1.1.1. Main Objective ... 2

1.1.2. Specific Objectives ... 2

1.2. Structure of Document ... 3

2. BACKGROUND ... 4

2.1. Concentrated Solar Power ... 4

2.2. Mineral Coal ... 9

2.3. Relevant Solar Angles ...13

2.4. Solar time ...15

3. DESIGN POINT SIMULATION ...16

3.1. Description of Power Plant ...16

3.1.1. Conditions of Design Point ...17

3.1.2. Steam Turbine ...18

3.1.3. Condensing System ...19

3.1.4. Pumps ...19

3.1.5. CFB Steam Generator ...19

3.1.6. Fresnel Concentrator ...19

3.2. Thermodynamic Modelling ...21

3.2.1. Main Equations Definition ...22

3.2.2. CFB Boiler and Coal Mass Flow Determination ...23

3.2.3. Fresnel Concentrator System...23

3.3. Evaluation Parameters ...28

3.3.1. Thermal Efficiency ...28

3.3.2. Pumps Power Demand ...29

3.3.3. Recirculation ...29

3.3.4. Spray Ratio ...29

3.4. Results ...29

3.5. Discussion ...32

4. ANNUAL SIMULATION ...35

4.2.2. Thermodynamic Modelling ...42

4.3. Evaluation Parameters ...47

4.3.1. Coal Consumption ...47

4.3.2. Solar Share ...48

4.4. Results ...49

4.5. Discussion ...51

5. CONCLUSIONS ...53

6. BIBLIOGRAPHY ...54

7. APENDIX ...56

7.1. Design Point Simulation...56

1. INTRODUCTION

Our times requires new technologies for energy production and the present offers a wide range of possibilities. In this way, the implementation of green energy does not mean the abatement of fossil resources based plants. Instead, it means the development of auxiliary processes that minimize the emissions and increase the efficiency. First, it is challenging to pretend a world industry depended 100% of renewable energy because the actual demand of energy exceed its potential. Additionally, the high number of applications conditioned for fossil fuels makes a radical energetic transition to renewable energy non-economical. Therefore, it is necessary to promote the diversification of electricity production matrix considering alternative renewable sources.

In that context, the hybridization of conventional power plants with solar energy has been considered under different projects in order to minimize emissions related to the electricity production. This consists on using an array of solar panels to provide heat for the steam production. The first hybrid plants have been tested in USA and Europe (DUFOUR, 2011) since the seventies. In Latin America, academic works and projects related to solar hybridization of conventional plants have been developed (BERNARDELLI, 2010). As an example, in Brazil, the laboratory of combustion and thermal systems engineering from Federal University of Santa Catarina is working about hybrid thermoelectric plants fueled with coal and sugarcane bagasse. In Colombia, there is still no project for hybrid thermoelectric plant. However, there is much to learn about the other Latin-American cases.

Inside the context, the state of Santa Catarina is a Brazilian region with important reserves of coal. This mineral resource is extracted at mines located at the south of the state, as it is exposed at Figure 1. Coal contributes around 25% of PIB of Santa Catarina (CENTRO DE TECNOLOGIA MINERAL CETEM/MCT, 2001) and is used mainly for electricity production. Once the run-of-mine coal is processed in coal mines, it remains the wastes. According to legislation, the coal mine companies are obligated to store wastes accordingly and this represents an important charge for coal companies. Yet, because of the ashes strength properties, it is not possible to use the space where wastes are disposed in order to construct any building. Therefore, the SATC, an institution located at Criciúma (SC) and sustained by the mines of Santa Catarina,

proposes the construction of a hybrid thermoelectric for the region with generation of superheated vapor from Fresnel solar concentrator. The main idea is that solar mirrors are installed over waste disposal areas, what might represent a utility for this space.

The present project looks for simulation of a 10 MW thermoelectric plant with direct generation of superheated vapor from Fresnel solar concentrators and the design of the solar field system.

Figure 1. Localization of Brazilian coal region in Santa Catarina.

Source: (MARTINS, 2005)

1.1. Objectives

1.1.1. Main Objective

To evaluate the process of superheated steam generation in Fresnel concentrators field in order to perform the hybridization of a coal thermoelectric plant.

1.1.2. Specific Objectives

1. To implement a thermodynamic model for a coal thermoelectric plant equipped with a fluidized bed reactor and a Fresnel concentrators field with direct steam generation. 2. To study the characteristics of the superheated steam generation process in the Fresnel

concentrator field at design point condition and for distinct irradiation incidence levels. 3. To perform the simulation of the hybrid plant operation over typical summer and winter

days and over a typical meteorological year for the city of Florianópolis-SC. 4. To calculate performance for the hybrid thermoelectric plant.

1.2. Structure of Document

The Chapter 2 is divided in two main parts. The first one describes the motivation for the development of this project. On the other hand, the second part details the theoretical background of project. This part contains the definition of solar concentrated power, the generalities of mineral coal, the details about the Brazilian coal, the description of used solar angles at simulation and the definition of solar hour. The state of art is described implicitly at the solar concentrated power section.

The Chapter 3 has the description of the hybrid power plant. The chapter integrates the thermodynamic modelling, based on a design point, with the description and then, defines the parameters of evaluation for the simulation. Once the evaluation is exposed, the chapter exposes the results and analyzes them over the point of design.

The Chapter 4 presents the simulation of power plant throughout a year of operation (typical meteorological year). The chapter has the description of power plant functioning as a function of solar conditions and the variation of thermodynamic modelling in comparison with the one corresponding to the design point simulation. In this chapter it is also defined the evaluation parameters for the performance of plant in different periods of the year. These parameters contemplates the differences between typical summer and winter days of operation. Finally, there is an exposition of obtained results and the corresponding discussion around the benefits of a year of use and the differences between summer and winter days.

Finally, the conclusions are presented in chapter 5 and they are presented in accordance with projected objectives. This chapter also proposes some recommendations for future work at the main project.

2. BACKGROUND

Inside this context of project, the present work looks for simulation of a 10 MW thermoelectric plant with direct generation of superheated vapor from Fresnel solar concentrators and the design of solar field system. Thereby the results will provide technical information that will be useful in order to study the feasibility of project.

2.1. Concentrated Solar Power

The Concentrated Solar Power (CSP) is the denomination for the system that uses solar radiation in order to generate steam for a power plant. These plants are based on mirrors that are used to concentrate solar irradiation in a point or over a line (in case of line focusing systems). The generated steam is used to operate a steam turbine in case of Rankine cycles. CSP systems can be also designed in order to increase the temperature of air in Bryton gas cycles. In this regard, the gas consumption can be minimized at hybrid operation.

According to the International Energy Agency, the world CSP installed capacity is today near to 4,5 GW (CSP TODAY, 2015). However, until 2012 the installed capacity was just 2.8 GW. It is expected yet that CSP installed capacity might reach 800 GW by 2050.

Nowadays the largest producer of electricity based on CSP is Spain. The following diagram shows the increase of CSP installed capacity between 2007 and 2012.

Figure 2. Global cumulative installed concentrating solar power capacity (MW), 2007-2012

Source: (SCHLUMBERGER-SBC ENERGY INSTITUT, 2013).

In this way, the sector needs more investment for reaching the required capacity. In 2011, CSP received 18 USD billion in contrast with 125 USD billion for solar photovoltaic and 84 USD billion for wind.

2.1.1. Classification

CSP systems can be classified in two groups. The one that concentrates the sun rays along a focal line and the one that concentrates sun rays around a single point. The first group includes parabolic trough and Fresnel plants. The second group is integrated by solar tower and Stirling dish technology.

Figure 3. Types of CSP

Source: Author.

Parabolic Trough Systems

It is a system formed by an array of parabolic mirrors that reflect the sun light into a receiver located in the focal line. The assembly of modules approaches a normal size of 100 m long and the average width is between 5 m and 6 m. The mirrors are aligned North-South direction and a control system regulates their orientation as a function of the sun position from east to west.

Fresnel Concentrator

This system is similar to parabolic trough, but it uses parallel flat mirrors mounted on a single basis. The mirrors orientation is regulated in order to focalize the central tubular receiver. In contrast with parabolic trough systems, the Fresnel concentrator have an astigmatism problem that is solved with a second curved mirror over the central receiver. This reflects again the wrong refracted rays.

Solar Tower

A large camp of mirrors (heliostats) reflects the sun rays to a receiver located in the top of a tower. This system can reach higher temperatures in comparison with parabolic trough or Fresnel concentrators. A sophisticated control system is necessary for regulating the heliostats orientation.

CSP

Line Focus

Parabolic Trough

Linear Fresnel

Point Focus

Solar Tower

Parabolic Dish

Stirling Dish Technology

A parabolic disk reflects the sunrays to its own central focal point. A Stirling engine receives the high temperature working fluid and produces the electricity.

At Table 1 it is possible to compare the relevant characteristics and advantages of each system.

Table 1. Comparison Table of Concentrated Solar Power. Parabolic

Trough

Fresnel Concentrator

Solar Tower Stirling Dish

Typical Capacity

(MW)

10-300 10-200 10-200 0.01-0.025

Technology Providers Sky Fuel. Abengoa Solar. Solar Millennium. NextEra. ACS. Nihon Tokushu Kogaku Jushi. Hengwen Optics. Novatec Solar. Areva. eSolar. Abengoa Solar. Solar Tower Systems GmbH. Aalborg CSP. United Sun Systems. Clean Energy. Operating Temperature (°C)

350-550 500 250-565 550-750

Cycle Superheated

Rankine steam Cycle Saturated Rankine Cycle Superheated Rankine steam cycle Stirling

Source: (INTERNATIONAL RENEWABLE ENERGY AGENCY (IRENA), 2012)

In summary, every CSP technology might be appropriate depending on dimensions of production, available budget and local conditions. Thereby everyone has its own advantages. The parabolic through is special because of its experience at the market. The linear Fresnel technology requires less investment in flat mirrors, lighter concrete structure for supporting and it is ideal for places with strong wind velocities. On the other hand, solar tower allows higher temperatures and this comes to higher efficiencies. This technology also makes easier energy storage. Finally, stirling-dish technology reduces heat losses and it is practical for distributed generation. This last technology is also useful for slopes or uneven terrain because its orientation could be easily controlled.

2.1.2. Power Plants Hybridization

There are two reasons that explain why hybridization plants are starting up. First, world is living a fossil fuel shortage. Therefore, the development is limited at present and the searching for alternative energy sources that supply the work of fossils is imperative. On the other hand, the

progressive increase of 𝐶𝑂₂ emissions asks for an energy source change. In this way, the European Union have proposed a reduction of 20% by 2020. Countries like Germany have reduced yet more than the planned reduction (30% by 2020) (KATHER, 2013).

Many hybridization possibilities have been developed as an answer for exposed problems. The main alternatives for the hybridization of conventional fossil plants are

 Biomass

 Solar

 Geothermal

These alternatives can be integrated with fossil fuel power plants and offer a wide catalog of advantages. First of all, having the option of using two different kind of energy source avoids the tribulations tied to fossil fuel availability and its increase of cost. On the other hand, a study from Carnegie Mellon University (MOORE, 2013) had evaluated the effects of a solar hybrid power plant and it founded the feasibility of PV and CSP power plants. According to the study, the implementation of a solar hybrid power plant is feasible depending on the DNI of the place and the global radiation. In summary, the application of a hybrid plant is a function of the local environmental conditions and costs involved.

There are many projects developed around the world, which are based on the hybridization of plants with CSP. In Spain it was developed an investigation in order to build a hybrid solar power plant with biogas (COLMENAR, BONILLA, et al., 2015). The main idea was to compare a CSP plant equipped with a molten salts energy storage system in contrast with the use of biogas in order to improve the system’s capacity factor. With this study it was identified that the biogas hybrid system represented a better option in comparison with the conventional layout based on molten salts heat storage.

According with Figure 4, the countries with the highest DNI potential are Australia, Chile, Mexico, India, United States and those located in the north and south of Africa.

Figure 4. World Direct Normal Irradiation.

Source: (INSTITÜT FÜR TECHNISCHE THERMODYNAMIK, 2008)

In Brazil, there is an important potential for both biomass and solar power plants. Figure 5 shows the localization of the cogeneration plants of sugarcane region and DNI incidence. It can be seen that both bagasse and solar resources matches and in this sense it can be identified a potential for the implementation of hybrid plants. At Central-South region the DNI reaches 2000 kW/𝑚2-year, where most sugarcane mills are located (BURIN, 2015).

Figure 5. Sugarcane mills (a) and DNI (b) in Brazil.

Source: a) (JANK, 2011); b) (SOLARGIS, 2014)

2.2.Mineral Coal

Mineral coal is the most abundant fossil fuel in the earth. It is estimated that its use can last more 120 years (PORRAS, 2015). It is composed of elements such as carbon, oxygen, hydrogen, nitrogen, sulfur, water and ashes of metallic and mineral type. Its formation begins with peat and organic residuals that are settled on waterlogged environments. Through bacterial action more complex carbonic components are formed. Finally, depending on coal age, it can be classified as lignite, sub-bituminous, bituminous, anthracite, or graphite (a pure carbon mineral) (KENTUCKY GEOLOGICAL SURVEY, 2012). This process is dependent on time, pressure and temperature.

Figure 6. Coal formation.

Source: (KENTUCKY GEOLOGICAL SURVEY, 2012)

Every coal type has its own carbon content and heating value. The following table specifies these characteristics for each one.

Table 2. Heating value for every coal type.

Type of coal Group % of Carbon

(dry, without minerals)

Volatile Material, %

Calorific Power kJ/kg

I. Anthracite

1. Meta-anthracite 98 2

2. Anthracite 92-98 2-8

3. Semi-Anthracite 86-92 8-14

II. Bituminous

1. Low volatile 78-86 14-22

2. Medium volatile 69-78 22-31

3. High Volatile, A <69 >31 32500

4. High Volatile, B 30200-32500

5. High Volatile, C 26700-30200

III. Sub-bituminous

1. Sub-bituminous, A 24400-26700

2. Sub-bituminous, B 22100-24400

3. Sub-bituminous, C 19300-22100

IV. Lignite 1. Lignite, A 14600-19300

2. Lignite, B … 14600

Source: Adapted of (PORRAS, 2015)

In Brazil, the most important coal reserves are situated in two states: Rio Grande do Sul and Santa Catarina. They represent, respectively, 89,25% and 10,41% of Brazilian coal reserves (CARBONIFERA CATARINENSE). The most coal of Rio Grande do Sul is high volatile bituminous B and the most coal from Santa Catarina is high volatile bituminous A. Hence, the coal mined in Santa Catarina has a higher heating value in comparison with coal mined in Rio Grande do Sul.

In 2000 it was mined in Brazil 13.8 million tons of coal, but the demand was of 16,2 million tons. Therefore, the country had to import mineral from United States (33%), Australia (31%), South Africa (9%) and Canada (8%). Nowadays 85% of coal used in Brazil is destined for thermoelectric plants, 6% for cement industry, 4% for paper industry and 5% for ceramics, food and grain drying.

According with a CAEEB (Auxiliary Company of Electrical Brazilian Studies in Portuguese) study, the standard composition of Brazilian coal is (dry, mass fractions):

𝐶 → 59% 𝐻 → 4% 𝑂 → 7% 𝑆 → 3% 𝐴𝑠ℎ𝑒𝑠 → 27%

2.2.1. Thermoelectric Electricity Generation in Brazil

As it was previously mentioned, the south of Brazil has the most important coal reserves. This is the reason why most thermoelectric plants based on coal combustion are located in this region.

The coal-based power plants in Brazil are described in Table 3. Table 3. Coal Power Plants in Brazil by 2002.

Enterprise Owner City State Power (MW)

Figueira Copel Geração Figueira PR 20

Alegrete Centrais Geradoras do Sul do Brasil S/A

Alegrete RS 66

Charqueadas Centrais Geradoras do Sul do Brasil S/A

Charqueadas RS 72

Jorge Lacerda A Centrais Geradoras do Sul do Brasil S/A

Capivari de Baixo SC 232

Jorge Lacerda B Centrais Geradoras do Sul do Brasil S/A

Capivari de Baixo SC 262

Jorge Lacerda IV Centrais Geradoras do Sul do Brasil S/A

Capivari de Baixo SC 363

Pres. Médici A/B Companhia de Geração Térmica de Energia

Elétrica

Candiota RS 446

Source: (AGÊNCIA NACIONAL DE ENERGIA ELÉTRICA, 2002) 2.2.2. Coal Steam Generators

Steam generator refers to the complete equipment to generate vapor. The components of a steam generator are (ELLIOT, CHEN e SWANKAMP, 1997):

 Furnace

 Steam superheater (primary and secondary)

 Steam reheater

 Boiler

 Steam drum

 Steam temperature control system

The most important models of coal fueled steam generators are the pulverized coal, the stoker-fired and the fluidized bed steam generators. Normally, the decision of the most appropriated steam generator model is based on the evaluation of factors described in Figure 7.

Figure 7. Relevant factor in a boiler selection for steam generation.

Source: (ELLIOT, CHEN e SWANKAMP, 1997)

The project related to the hybridization of a 10 MW coal power plant with linear Fresnel solar field proposed in a cooperation between Labcet and SATC is based on the use of a fluidized bed system since it offers the widest fuel flexibility of the three mentioned models. It is suitable for high ash content coals and waste fuel because its long combustion time and the high intensity bed mixing process. The control of SO₂ and NO_x emissions is done through limestone help and temperature control. Key Factors Boiler Selection Fuel Characteris tics Ash characteris tics Unit size and steam flow Steam use (cycling, load range, i.a.) Steam temperatur e and pressure Environme ntal requireme nts Fuel price and availability Operation and maintainan ce costs Site factors and Governme nt incentives

2.3. Relevant Solar Angles

In order to simulate the operation of solar concentrators it is necessary to calculate the solar angles. These angles are described in next sections.

2.3.1. Declination

Declination δ [rad] is the angle measured between the sun rays vector and the equatorial plane. The declination angle is showed in Figure 8 along a year.

Figure 8. Declination.

Source: (GROSSMONT COLLEGE, 2014) 2.3.2. Hour Angle

The hourly angle ω [rad] is the angle between the plane formed by local zenith and north-south axis with the sun rays vector. This angle determines the position of the sun from east to west due to the rotation of earth around its own axis. Its value is equal to zero rad at solar noon.

Figure 9. Hour angle.

Source: (ACADEMIC AND RESEARCH NETWORK OF SLOVENIA (ARNES), 2001) 2.3.3. Incidence Angle, Solar Azimuth and Altitude

The incidence angle determines the direction of sun rays in the concentrator. This angle depends on the azimuth angle and solar altitude 𝛼𝑠 [rad]. The azimuth angle is formed by the

projection of sun rays vector in the horizontal plane and the axis aligned to the north-south direction. On the other hand, the solar altitude is the counterpart of zenith angle.

Figure 10. Azimuth and solar altitude angles.

Source: (TWCARLSON, 2012) 2.3.4. Other Angles

The latitude and longitude angles are related to the location where solar field is installed. In this way, the latitude is the distance between the equator and the plant site over the same meridian. This angle is measured in degrees [°] between 0° and 90°. For convention, the positive angles correspond to north hemisphere and negative ones to south hemisphere.

Longitude is the distance between the Greenwich meridian and the plant site over the same parallel. This is measured in degrees [°] between 0° and 180°. The convention will be positive for east hemisphere and negative for west hemisphere.

2.4. Solar time

The meridians are stablished as a convention of hours for a same longitude. However, the hour might be different for the same meridian. The real local time related to sun position is called solar time 𝑡𝑠𝑢𝑛 and it is based on two corrections. The first one is about the real location of the

place. Every place has a local hour associated to local meridian, but the location could be not exactly at the center of meridian. Therefore, the real location must be considered. The second correction considers the variation of earth’s rotation rate due to its elliptical trajectory around the sun. This last correction is associated to the equation of time.

3. DESIGN POINT SIMULATION

3.1. Description of Power Plant

The main components of the hybrid power plant are the following: 1. Turbine;

2. Condensation system; 3. The pump system;

4. Circulating Fluidized Bed (CFB); 5. Fresnel solar concentrators.

The steam cycle is also equipped with control valves, a steam drum which is necessary in order to separate saturated liquid and steam phases in Fresnel field, a deaerator tank and a condenser. The main components are showed in Figure 11.

Figure 11. Hybrid Plant layout.

■ Fresnel Concentrator ■ CFB ■ Turbine ■ Condensation System ■ Pumps Source: Author

3.1.1. Conditions of Design Point

In order to design the Fresnel system to meet the pre-defined parameters of superheated steam temperature, pressure and mass flow, it was developed a design point simulation.

The power output of power plant was fixed in 10 MW. This capacity was selected to meet the power demand of coal mine where the plant is to be installed. Additionally, the Direct Normal Irradiance (DNI) received by Fresnel mirrors was defined according with the radiation at solar noon at plant’s site. The design value of DNI was defined as 900 W/m2_.

On the other hand, the modules of Fresnel system had an associated optical efficiency. This depended on the solar incidence angle. The design point simulation was performed at solar noon and at this time the incidence angle was assigned as zero degrees. According with manufacturer specifications, the design point optical efficiency under zero degrees incidence angle and clean mirrors was 0,67.

Another important parameter defined was the fraction of vapor produced by solar system in comparison with the total mass flow of steam directed to steam turbine at design point. This was defined as the design point solar share and its value was set as 10%.

It is presented in Table 4 the additional parameters related to the performance of power plant at design point. The temperature T₁ and the pressure P₁ at the entry of the steam turbine were defined according with the specifications of manufacturer. The thermal efficiency of CFB boiler was provided by the manufacturer. This was assumed as a constant throughout the whole simulation. The low heat value (LHV) was provided by SATC. Finally, the environmental temperature was assumed.

Table 4. Design Point Simulation Parameters.

PARAMETER VALUE

DNI 900 W/m2

η_op 0,67

SOLAR SHARE (SS) 10%

W_t 10 MW

T1 789 K

P1 67 bar

P2 2,5 bar

P3 0,09 bar

η_t1 0,85

η_t2 0,78

ηp1 0,68

η_p2 0,62

η_c 0,9

LHV 10600 kJ/kg

AREA OF MODULE 513,6 m2

T_amb 292 K

Source: Author. 3.1.2. Steam Turbine

Steam produced in both solar field and coal steam generator was conduced to a steam turbine. According to the manufacturer specifications, the incoming steam should have a pressure of 67 bar and superheated temperature of 525 °C. The turbine had the capacity to produce 10 MW.

The turbine had one bleed-off steam extraction. Hence, part of main stream was directed to the deaerator at a pressure of 2,5 bar.

Deaerators were implemented in thermoelectric plants in order to perform the extraction of oxygen and other residual gases mixed with the steam. According with industry specifications (ALSTOM, 2012), in order to avoid the corrosion inside the boiler and steam turbine, it was only allowed a couple of micrograms of dissolved gases per liter of water. The deaerator had also the function of mixing the condensed water with bleed-off steam in order to increase the temperature of water until a saturated point (x=0) at constant pressure. In this regard, the feed water temperature was increased before it was directed to the steam generator.

Exhaust steam was directed finally to condenser which was operated at a vacuum pressure (0,09 bar for practical effects of simulation).

It is important to mention, finally, that the isentropic efficiency of steam turbine was provided separately for the two group of stages positioned before and after the bleed-off steam extraction to deaerator. These values were, respectively, η_t1 = 0,85 and η_t2 = 0,78, which were provided by steam turbine manufacturer.

3.1.3. Condensing System

A wet-cooling condenser was here considered. Inside this component the exhaust steam was turned saturated liquid (x=0) at constant pressure.

3.1.4. Pumps

The condensed water was directed to the deaerator in order to be pre-heated. In this regard, a condensate pump was required in order to increase the water pressure to 2,5 bar.

After deaeration process, pumped water was supplied to generate vapor to the Fresnel concentrator and to CFB boiler. This was performed by a feed water pumping system.

3.1.5. CFB Steam Generator

In order to generate superheated vapor based on coal combustion, a circulating fluidized bed boiler was considered.

The favorable points related to this technology are:

 Appropriated for high ash-coals.

 Versatility of fuels (all types of coal, oil, wood and other biomass fuels).

 Capture of SO₂ and limitation of NO_x reactions.

This component was here modelled considering constant thermal efficiency. In addition, the thermal load of steam generator was reduced up to 90 % once solar field was at maximum capacity (10 % solar share at design point). Under this condition, the coal steam generator was able to produce superheated steam at constant parameters (525°C and 67 bar).

3.1.6. Fresnel Concentrator

In case of Fresnel solar field, the complete heating process was performed at constant pressure (67 bar). The system was composed of two sections. The first one was implemented in order to produce the saturated vapor. This section received feed water mass flow from deaerator tank. The saturated steam generation process was based on the recirculation of water supplied

from the separator tank (called drum) (see Figure 12). The separator tank contained water in both phases: saturated liquid and vapor. The superheated steam was produced, finally, in the superheating section of solar field. As it can be observed in Figure 12, saturated liquid water was injected in between the first and second Fresnel modules in order to control the final superheated steam temperature.

Figure 12. Detailed layout of Fresnel system.

Source: Author.

It is important to notice that the superheated steam mass flow produced in solar field was dependent mainly on the number of Fresnel modules disposed in the boiler section. The superheating section was, in the other hand, configured according to the required final steam temperature.

3.2. Thermodynamic Modelling

According with power plant description, the T-s diagram is sketched in Figure 13. The numbers in green represent the Fresnel concentrator process and the numbers in blue are related to the CFB process.

Figure 13. T-s diagram for Rankine cycle with regeneration for CFB and Fresnel concentrator hybrid power plant.

Source: Author

Before defining the thermodynamic model, it is necessary to clarify some mass flow terms. The total superheated steam mass ṁ_T is directed to turbine. Part of this is directed to the deaerator tank at point 2. This flow quantity is called mass flow for regeneration ṁr. This means

that the steam mass that will be condensed is:

ṁ3 = ṁT− ṁr (1)

On the other hand, the feed water mass flow obtained from deaerator is divided into the mass flow boiled by CFB ṁ_c and mass flow boiled by the Fresnel solar field ṁ_f. Thereby

ṁ_T = ṁ_c + ṁ_f (2)

Table 5. Mass flow variables.

Variable Meaning

ṁ_T Total mass flow

ṁ_r Mass flow for regeneration

ṁ_c Mass flow boiled by CFB

ṁf Mass flow boiled by Fresnel concentrator

Source: Author 3.2.1. Main Equations Definition

The steam turbine mechanical power output was here calculated according to the energy balance equation presented below:

Ẇ = ṁ_{T T}(h1− h2) + (ṁT− ṁr)(h2− h3) (3)

The enthalpy of steam at points 2 and 3 was calculated as a function of the isentropic efficiency values indicated by the turbine manufacturer. The manufacturer specified the efficiencies of η_t1 = 0,85 η_t2 = 0,78. In this regard, Equations (4) and (5) were used in order to obtain, respectively, h₂ and h₃.

η_t1 = h1− h2 h₁− h_2s

(4)

η_t2 = h2− h3 h₂− h_3s

(5)

The mass flow 𝑚_𝑟 was condensed through process 3-4. The energy conservation equation was represented by:

Q̇ = (ṁl T− ṁr)(h3− h4) (6)

The condensed water was guided to the deaerator tank. In this regard, the required pumping power was calculated according to Equation (7).

W_p1̇ = (ṁ_T− ṁ_r)(h₅− h₄) (7)

The enthalpy of water at point 5 was calculated as a function of the isentropic efficiency of this component (Equation (8)), which was set as η_p1 = 0.68.

ηp1 =

h5s− h4 h5 − h4

(8)

The liquid water inside the deaerator system contains dissolved gases, which are released by an extraction system at the tank. The condensate is heated by the bleed-off steam until

saturation point is reached. This is an open heat exchanger and the energy balance is represented by following equation:

ṁ_rh₂+ (ṁ_T− m_r)h5 = h6(ṁr+ ṁT− ṁr) (9)

ṁ_rh₂ + (ṁ_T− ṁ_r)h5 = ṁTh6 (10)

In order to boil the water, the second pump guides liquid to the boil equipment, both CFB and Fresnel concentrator. The power required for the feed water pump is:

W_p2̇ = ṁ_T(h₇− h₆) (11)

The enthalpy of water at point 7 was calculated as a function of the isentropic efficiency of this component (Equation (12)), which was set as η_p2 = 0.62.

η_p2 =h7s− h6 h₇− h₆

(12)

3.2.2. CFB Boiler and Coal Mass Flow Determination

During the day, both boiler and solar field were able to produce superheated steam. The following expression represented the heat provided by the CFB boiler:

Q̇c = ṁc(h9− h8) (13)

where h₈was known because h₈ = h₇ and h₉ = h₁ because the superheated steam produced at CBF boiler was assigned with the same temperature as at turbine’s entry.

Once the required CFB heat was determined, the coal mass flow was calculated. This depended on the coal properties and thermal efficiency of CFB boiler. According with coal supplier information, the low heat value (LHV) of coal was 10600 kJ/kg. It was also considered a thermal efficiency of 90% for the CFB system based on LHV of fuel. Therefore, required coal mass flow was calculated as:

ṁcoal = Q̇_c LHV ∗ ηc

(14)

3.2.3. Fresnel Concentrator System

The Fresnel solar field was composed of two stages. The first one was implemented in order to provide the saturated steam production. In this regard, liquid water was turned saturated vapor (x=1). The second stage was finally necessary to increase the temperature of steam until

the required final temperature. In this way, the supplied heat of Fresnel concentrator was based on:

Q̇f= Q̇sat+ Q̇SH (15)

Obtaining the exact amount of heat required to generate the specified mass flow of superheated steam is difficult with an integer number of Fresnel modules in boiler and superheating sections. However, inside the simulation, and at the commercial industry, the user should define an integer number of modules for every solar field section.

Fresnel Tank (Drum)

According with power plant description, the tank should work with following flow mass relations:

ṁa2 = ṁf+ ṁa1 (16)

ṁ_a3+ ṁ_a4 = ṁ_f (17)

All the incoming and outgoing masses should be defined in order to control the temperatures at Fresnel modules and achieve the thermal equilibrium at point 1.

Boiler Section Modelling

The number of modules in boiler section, 𝑏, was defined in order to calculate the total mass flow of saturated steam. The Fresnel model used was the NOVA 1 (NOVATEC SOLAR, 2015). The Equations (18) and (20) were provided by manufacturer in order to reproduce the system’s performance.

Q̇_sat = A_sat(DNI ∗ η_op− P_lossSat) (18)

Asat = b ∗ área of single solar module (19)

P_lossSat = 0,056 W

m2_K(T12− Tamb) + 0,000213 W

m2_K2(T12− Tamb)2 (20) A_sat → Mirror Area for saturation modul circuit

DNI → Direct normal irradiation η_op→ Optical Efficiency

Tamb → Environmental temperature P_loss → Thermal loss

The temperature 𝑇₁₂ was known because this was the saturation temperature for the pressure at turbine’s entry 𝑃1. On the other hand, it was possible to do an energy balance over the

Fresnel boiler. With this, the required heat was also described by the Equation (21), which turned possible to calculate the saturated steam mass flow.

Q̇_sat = ṁ_a2(ha2− h10) (21)

where h_a2 was determined as the enthalpy of water at pressure P₁ and quality of x=0.75. This condition of quality was stablished based on the model proposed by SAM Software and developed in cooperation with NOVATEC (NREL, 2015). On the other hand, ṁ_a2 and h₁₀ were unknown. The first one was the only inlet of drum and the second one was the enthalpy at the entry of boiler section. Then it was necessary other equation more in order to solve both values.

The additional energy balance was performed between the point where recirculation mass

ṁa1 was added to feed water prevenient from deaerator. This relation was described as:

ṁa1ha1+ ṁfh7 = ṁa2∗ h10 (22)

With the last two equations and the mass balance written for the drum it was possible to determine ṁ_a1, ṁ_a2 and h₁₀. The enthalpy h_a1was the enthalpy of water at pressure P₁ and quality of x=0.

Superheating Section

The superheating process was performed in two steps. The first step occurred through the first Fresnel module. The first superheating process was calculated considering the following equations, which were obtained from literature (WAGNER e GILMAN, 2011) (Equations (25)-(29)) for evacuated tube.

Q̇_SH1 = ṁ_a3(h₁₅− h₁₂) (23)

Q̇SH1 = Area of single solar module ∗ (DNI ∗ ηop− PlossSH1) (24) PlossSH =

(Q_l1+ Q_l2+ Q_l3+ Q_l4) w ∗ (Tout− Tin)

(25)

Q_l1= (c₀+ c₅∗ √Vw) ∗ (Tout− Tin) (26) Ql2= (c1+ c6∗ √Vw) ∗ [

(T_out2 _{− T} in2)

Ql3= (c2+ c4 ∗ DNI ∗ IAM ∗ cos θ) ∗

(T_out3 _{− T} in3)

3 (28)

Q_l4= c₃ ∗(Tout4 − Tin 4₎

4 (29)

Vw → Wind speed at the place

T_out, T_in → Output and input temperatures

IAM → Incidence Angle modifier

cos θ → Cosene of incidence angle w → Length of solar modul

c0 to c6 are constants fitted to experimental data tested by manufacturer (BURKHOLDER

e KUTSCHER, 2009)

The temperature and enthalpy at point 15 were unknown. With both equations for Q̇SH1,

equation for P_lossSH and the relation between T₁₅ and h₁₅ it was possible to determine the thermodynamic state at point 15. For design point, it was assumed the maximum optical efficiency. Then, the IAM and cos θ were equal to 1.

After the first superheating step, the superheated vapor was cooled with saturated water from the Fresnel tank in a process called Spray-desuperheating. The heat balance for desuperheating process was formulated as:

ṁ_a4h_a4+ ṁ_a3h₁₅ = ṁ_fh₁₆ (30)

where h_a4 was the enthalpy corresponding to pressure P₁ and quality of x=0. The enthalpy h₁₆ was the enthalpy of vapor directed to the next superheater module. The temperature 16, ṁa4 and ṁa3 were unknown. These variables could not still be determined by system of

equations previously defined. It was required first the temperature at the entry of second superheater module (point 16).

The vapor was further heated in the second part of superheating section. The complementary superheating process was determined by the number of Fresnel modules number,

𝑐:

Q̇_SH2 = ṁ_f(h₁₃− h₁₆) (31)

The loss heat was determined as it was previously described, according to Equation (25). The only difference is that now T_outis T₁₃ and T_inis T₁₆. T₁₃ is the temperature at the turbine’s entry. With both definitions of Q̇_SH2 it was possible to find the temperature at the entry of module

T16 and its respective enthalpy h16 because the losses PlossSH2 were a function of this temperature

and the enthalpy was directly related as a property to this temperature.

Once h₁₆ was known, ṁ_a4 and ṁ_a3were determined using energy balance at spray-desuperheating and the mass balance described at drum section.

For the stablished solar share, the Fresnel mass flow and CFB mass flow were:

ṁc = 0,9ṁT (33)

ṁ_f= 0,1ṁ_T (34)

In summary, it is illustrated in Table 6. the proposed set equations and the unknown variables. This system of equations was solved numerically by using the Engineering Equation Solver (EES) software.

Table 6. Resume of thermodynamic approach.

Number Equation Known Variables Unknown

Variables (3) W_Ṫ = ṁ_T(h₁− h₂) + (ṁ_T

− ṁr)(h2− h3)

Ẇ T T1, P1(h1)

ṁT, ṁr h2, h3 (6) Q̇ = (ṁ_{l T}− ṁ_r)(h₃− h₄) h₄ → f(P₃, x = 0) Q̇_l

(7) W_p1̇ = (ṁ_T− ṁ_r)(h₅− h₄) h₅, W_p1̇

(10) ṁ_rh₂+ (ṁ_T− ṁ_r)h₅ = ṁ_Th₆ h₆ → f(P₂, x = 0)

(11) W_p2̇ = ṁ_T(h₇− h₆) h₇, W_p2̇

(13) Q̇_c = ṁ_c(h₉− h₈) h₈ = h₇

h9 = h1

Q̇c, ṁc (14)

ṁcoal= Q̇_c LHV ∗ ηc

LHV, η_c ṁ_coal

(16) ṁ_a2 = ṁ_f+ ṁ_a1 ṁ_a1, ṁ_a2, ṁ_f

(17) ṁ_a3+ ṁ_a4 = ṁ_f ṁ_a3, ṁ_a4

(21) Q̇_sat = ṁ_a2(h_a2− h₁₀) h_a2 → f(P₁, x

= 0.75) Q̇sat, h10

(22) ṁ_a2h₁₀= ṁ_fh₇+ ṁ_a1h_a1 h_a1 → f(P₁, x = 0)

(20) P_lossSat

= 0,056 W

m2_K(T12− Tamb) + 0,000213 W

m2_K2(T12− Tamb)2

Tamb T12 → Tsat@P1

(23) Q̇_SH1 = ṁ_a3(h₁₅− h₁₂) Q̇_SH1, h₁₅

(24) Q̇_SH1

= Area of single solar module ∗ (DNI ∗ η_op− P_lossSH1)

PlossSH1

(25) P_lossSH1

= (Ql1+ Ql2+ Ql3+ Ql4) w ∗ (Tout− Tin)

Tout= T15 Tin= T12 Q_l1, Q_l2, Q_l3, Q_l4, w

T15

- T₁₅ → f(P₁, h₁₅)

(30) ṁ_fh₁₆ = ṁ_a4h_a4+ ṁ_a3h₁₅ h_a4 → f(P₁, x = 0) h₁₆

(31) Q̇_SH2 = ṁ_f(h₁₃− h₁₆) h₁₃= h₁ Q̇_SH2

(32) Q̇_SH2

= (c − 1)

∗ Area of single solar mirror ∗ (DNI ∗ ηop− PlossSH2)

h_a4 → f(P₁, x = 0) P_lossSH2

(25) P_lossSH2

= (Ql1+ Ql2+ Ql3+ Ql4) w ∗ (T_out− T_in)

Tout = T13 → f(P₁, h₁₃) Tin = T16 Ql1, Ql2, Ql3, Ql4, w

T16

- T₁₆ → f(P₁, h₁₆) (4)

ηt1 =

h₁− h₂ h1− h2s

η_t1

(5)

η_t2 = h2− h3 h₂− h_3s

ηt1

(8)

ηp1 =

h_5s− h₄ h5− h4

η_p1

(12)

ηp2 =

h7s− h6 h7− h6

ηp2

(33) ṁ_c = 0,9ṁ_T

(34) ṁ_f = 0,1ṁ_T

Source: Author

3.3. Evaluation Parameters

These following criteria of evaluation were implemented to measure the performance of the plant at design point operation.

3.3.1. Thermal Efficiency

Considering the heat load supplied by CFB and Fresnel system, the power plant thermal efficiency was calculated as:

η_th =Ẇout Q̇in

= 1 − Q̇h Q̇c+ Q̇ =f

Ẇt− Ẇp1− Ẇp2 Q̇c + Q̇f

(35)

This parameter allowed to compare the performance of power plant in contrast with other CSP plants.

3.3.2. Pumps Power Demand

According with proposed thermodynamic model, the power plant required the work of two pumps. The simulation determined also the total power. The ratio of power that power plant produce to the required for the pumping system was calculated as:

wp =

Ẇ_pumps

Ẇ_t (36)

3.3.3. Recirculation

The recirculation consisted on the mass flow of saturated water from boiler drum that was added at the inlet of the boiler section. The recirculation factor (Equation (37)) was here calculated and it consisted on the ratio of recirculation mass flow to the total mass flow in the boiler section.

R =ma2̇

m_a1̇ (37)

3.3.4. Spray Ratio

It represented the fraction of spray that was added to superheating line in comparison with the total superheated vapor flow mass generated by Fresnel system.

S =ma4̇

ṁ_f (38)

3.4. Results

The simulation was performed for a combination of Fresnel modules, as it is exposed in Table 7.

Table 7. Configuration of mirrors for Fresnel System.

Variable Symbol Value

Number of Fresnel modules at boiler section b 7

Number of modules at superheater section c 3

Number of Fresnel modules before the Spray-Desuperheating system

d 1

The simulation results regarding design point operation are summarized in Table 8, Table 9 and Figure 14. The presented points are related to the Figure 11, Figure 12 and Figure 13.

Table 8. Pressure, temperature and thermodynamic properties for each point of circuit.

POINT PRESSURE

(bar)

TEMPERATURE (K)

ENTHALPY (kJ/kg)

ENTROPY

(kJ/kg) QUALITY

1 67,00 789,0 3452,0 6,87 SH

2 2,50 423,3 2644,0 7,17 SH

3 0,09 316,9 2370,0 7,52 0,91

4 0,09 316,9 183,3 0,62 0,00

5 2,50 317,0 183,6 0,62 SL

6 2,50 400,6 535,5 1,61 0,00

7 67,00 402,2 546,6 6,87 SL

8 67,00 402,2 546,6 6,87 SL

9 67,00 789,0 3452,0 6,87 SH

10 67,00 433,8 682,2 1,94 SL

12 67,00 556,1 2776,0 5,84 1,00

13 67,00 789,0 3452,0 6,87 SH

15 67,00 638,3 3068,0 6,34 SH

16 67,00 602,2 2957,0 6,13 SH

SH: Superheated state. SL: Saturated liquid. Source: Author

Table 9. Heats and works extracted or required by each component of power plant.

Turbine’s work (kW) 10000

Heat Exchanged at Condenser (kW) 18360

Work required by pump 1 (kW) 3

Work required by pump 2 (kW) 107,8

Heat supplied by CFB boiler (kW) 25094

Total Heat supplied by Fresnel System (kW) 3154

Figure 14. Design Point simulation’s scheme with results.

Source: Author

The calculated parameters defined in order to evaluate the performance of power plant are showed at Table 10.

Table 10. Parameters of Evaluation.

Parameter Evaluated

ηth 35,03%

w_p 1,1%

ṁcoal 2,665 kg/s

R 5,198

S 0,0491

3.5. Discussion

According with the simulation, the number of modules required for Fresnel system was small in comparison with traditional concentrated solar power plants. This was because the low demand of power. The low demand of power turned difficult to identify the correct number of modules in each section in order to reach the required superheated steam mass flow and temperature. In Table 11 are presented the configurations tested in this work.

Table 11. Possible combinations of number of mirrors for module for converging at simulation.

b c d

7 3 2 7 5 3 7 6 4 8 4 2 8 5 3 9 6 4 Source: Author

The number of modules in boiler section determined the saturated steam mass flow production. Therefore, if this number was too low, the system was not able to produce the required energy to evaporate the water. In this regard, the maximum number of modules, b, was determined by the number of modules at superheating section, c. If b<c, the capacity of heat production would be higher for the superheating section. This would be not correct because the demanded heat for the saturation process is higher than for the superheating process.

The number of mirrors for superheater section, c, was conditioned by the heat demand in order to conduce the steam to a superheated state. As the case of modules for saturator section, this number could not be too low because it would not be produced the needed heat. The superior limit was restricted by the number of mirrors at boiling section b, as it was explained in paragraph above. Thereby, the number b and c might be determined during design phase. The design always is performed in order reach the design parameters of steam mass flow and temperature.

At Table 11 it was showed that d could be 1 or 2 for b=7 and c=3. The preference for one of these options depends on temperature difference before and after spray’s point. In order to avoid pipeline’s wear due to the high temperature, it was desired the lowest possible difference of temperature between these points (15 and 16). With one module of superheater section before the spray point, the drop of temperature at spray was of 36,1°C and the maximal temperature before spray point is 638,3 K (365,3 °C). On the other hand, with two modules before the spray point,

the drop of temperature at spray is of 43,1°C and the maximal temperature before spray point is 737,5 K (464,5 °C). This means that option that is more convenient is one module before spray point because the heat shock is low for pipeline and the temperatures will be lower for this part of superheating process.

This restriction of allowed combinations was a product of the fact that the number of modules is an integer number and this represented compact units of surface areas. In this way, it is important to mention that the number of possible combinations of modules was determined by the surface area of a single module. Less surface area per module would allow more options of combination.

The thermal efficiency of current hybrid power plant was similar to the typical values for thermal efficiencies of conventional coal power plants. Typically, thermoelectric power plant efficiencies are around 35%. However, these can be higher in case of higher steam temperature and pressure parameters. Considering the simulated hybrid power plant at design point, results revealed a thermal efficiency of 35,03%. The following diagram compares the thermal efficiency of well-known coal power plants at the world with the current power plant.

Table 12. Reference Power Plants around the world.1

Power Plant Location Installed Capacity (MW) η th (%)

Taichung Taiwan 5500 -

Touketuo China 5400 -

Belchatów Polonia 5400 42

Çayırhan Turkey 940 38

Jorge Lacerda Brasil 857 -

Termotasajero Colombia 153 35,7

Source: Author

1

This table was elaborated with information based on the following sources:

 List of largest power stations in the world. Link:

https://en.wikipedia.org/wiki/List_of_largest_power_stations_in_the_world (01/09/2015)

 Cooling flue gas to maximize power plant efficiency. Link:

http://www.powerengineeringint.com/articles/print/volume-17/issue-8/features/cooling-flue-gas-to-maximize-power-plant-efficiency.html (01/09/2015)

 Thermodynamic and exergoeconomic analysis of Çayırhan thermal power plant. Bolatturk, A; Coskun, A;

Geredelioglu, C. Energy Conversion and Management. Vol. 105, 371-378. (2015).

 Complexo Termelétrico Jorge Lacerda. Link:

http://www.tractebelenergia.com.br/wps/portal/internet/parque-gerador/usinas-termeletricas/complexo-termeletrico-jorge-lacerda (01/09/2015)

 Termotasajero. Link:

http://www.termotasajero.com.co/page/index.php?option=com_content&task=view&id=17&Itemid=36

Taichung, Touketuo and Belchatów are the largest coal power plants in the world in that order. In contrast with the current power plant, the thermal efficiency of these are significantly higher. On the other hand, the Çayırhan power plant and Termotasajero are more efficient too. Additionally, these referenced thermoelectric power plants have implemented multiple sophisticated systems in order to increase the thermal efficiency. Then, the difference with current designed power plant should not be disturbing because this is a first and simple layout of power plant.

In order to approach the requirements of power plant, the power demand of pumps was taken into account. It represented just the 1,1% of total produced power. On the other hand, the needed of coal mass flow was reasonable. Coal for an operational hour at design point implied a cost of raw material of $532 US dollars ($2066 Brazilian Reals)2 if coal was acquired at region of Santa Catarina.

Finally, recirculation parameter revealed that the heated water through the saturation process was five times the mass flow injected before the entry of module. Furthermore, because the temperature drop at spray’s point was significantly low, the required quantity of mass flow from spray system represented just 4% of total superheated generated steam.

2

4. ANNUAL SIMULATION

4.1. Description of Power Plant Operation

The superheated steam mass flow production in the Fresnel solar field was dependent on the DNI incidence, as well as on the system’s optical efficiency. The generated heat of a Fresnel module was here calculated as:

Q̇ = A(DNI ∗ η_op− P_loss) (39)

In addition to the superheated steam mass flow, the water/steam parameters were also changed as a function of DNI and time of the days. In order to determine the behavior of solar field operation as long as important parameters were varied, firstly DNI was varied from a minimum level of 0 until a maximum level of 1000 W/m2. In Figure 15 are presented the water/steam temperatures in Fresnel solar field as a function of DNI. The solar field operation was based on a constant recirculation ratio R in boiler section. As it can be seen, above 100 W/m2 the Fresnel system was able to assist CFB boiler in steam generation. Between 100 W/m2 and 600 W/m2 the Fresnel system operated without spray-Desuperheating process. Once DNI was over 600 W/m2, the spray-Desuperheating was turned on and the superheated steam temperature was kept at design point reference value. The Figure 15 shows the temperatures for points before and after spray-Desuperheating and at the exit of Fresnel system.

Figure 15. Temperatures at Fresnel Module for DNI variation.