Bachelor of Science Thesis
KTH School of Industrial Engineering and Management Energy Technology EGI-2018
TRITA-ITM-EX 2018:421 SE-100 44 STOCKHOLM
Hybridization with CSP in a Cuban
sugar mill
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Bachelor of Science Thesis EGI-2018 TRITA-ITM-EX 2018:421
Hybridization with CSP in a Cuban sugar mill Iris Vesterberg Sofia Westerlund Approved 2018-06-08 Examiner Anders Malmquist Supervisor Anders Malmquist Commissioner UCLV, Cuba Contact person
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Acknowledgment
We would like to thank the Linnaeus Palme programme for granting us a scholarship which made this bachelor thesis possible. Our greatest gratitude goes to all the people who have helped us along the way during this project, both with guidance and information as well as accommodation and transport. First and foremost, we would like to thank Dr. Prof. Idalberto Herrera Moya for all his invaluable help and support with everything regarding our project, preparations and stay in Cuba. Without him this report would not have been what it is today, and our stay in Cuba would not have been as pleasant as it has. We would also like to thank Dr. Manuel Alejandro Rubio Rodríguez for accompanying us to the sugar mill factory Carlos Baliño and for all the help he has provided. Great thanks go to the employees at Carlos Baliño for taking the time to show us around the factory and provide us with necessary data and information. We are very grateful to our host Hector Treto Fernández for taking excellent care of us and helping us get the most out of our stay in Cuba. We are also grateful to Regina Leandersson and Malin Lönnqvist for all the time we spent together in Cuba. We would also like to thank Anders Malmquist for introducing us to this project and Cuba, as well as providing us with support during this whole process.
Iris Vesterberg and Sofia Westerlund
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Abstract
Cuba is currently highly dependent on imported oil, mainly from Venezuela, to meet their
growing electricity demand. This dependence makes Cuba sensitive to changes in oil price as well as the political climate. The current crisis in Venezuela has a large impact on Cuba’s electricity generation. By expanding its renewable energy sources Cuba could decrease their dependence on other countries and diversify their energy supply. Moreover, it would have a positive climate impact by reducing the country’s CO2-emissions.
Geographically, Cuba has ideal conditions for renewable energy utilization, such as solar power. Solar energy is constantly progressing and is considered a great source of energy. Concentrating Solar Power (CSP) is a technology which applies mirrors and/or lenses to concentrate the sunlight onto a small area which converts the sunlight into heat, possible to use in a
thermodynamic cycle. There are mainly two problems with the implementation of CSP in Cuba. Firstly, CSP is a non-dispatchable power generating system since it is dependent on the
instantaneous weather conditions. Secondly, it has high investment costs. One way of solving these problems is by implementation CSP in an already existing power plants with a dispatchable source of energy, making it a hybrid power plant. Accordingly, the hybrid power plant would be dispatchable and the investment costs would be significantly lower.
Existing power plants can be found in Cuban sugar mills. This study investigates the possibility to implement solar power in the sugar mill Carlos Baliño, located in Villa Clara, Cuba. The factory is currently self-sufficient electricity wise on a yearly basis, using a co-generation Rankine cycle to generate electricity and process heat used in the sugar production. The fuel used is bagasse, a rest product obtained after the sugar juice has been pressed out of the sugar canes.
Four CSP-technologies and three implementation layouts were examined, resulting in the parabolic trough-technology and feedwater heating being considered the optimal solution. Furthermore, two different scenarios for CSP was investigated; implementation of CSP in the mill at the current state (scenario 1) or after investing in a Condensing-Extraction Turbine (CEST) (scenario 2).
The results show that Carlos Baliño should invest in a CEST before considering implementation of CSP. Off-season operation is not available for scenario 1, leading to a vast amount of solar potential being unexploited. The maximal investment allowed for scenario 1 is 3.7 MUSD, which is not a realistic number. The maximal investment allowed for in scenario 2 is
5.9 – 7.2 MUSD, depending on bagasse import availability. If bagasse import is unlimited, it is not recommended to invest in solar power. Implementation of CSP in scenario 2 regarding bagasse import limits would yearly lead to an additional electricity generation at Carlos Baliño of 5.4 – 7.3 GWh, decrease the oil usage with 16,100 – 21,800 barrels and the CO2-emissons with 1,200 – 1,600 tonnes. Carlos Baliño’s annual yield would increase with 0.5 – 0.6 MUSD/year and the Cuban states annual yield would increase with 0.7 – 0.9 MUSD/year. Future work is
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Sammanfattning
Kuba har i dagsläget ett högt beroende av importerad olja, för att tillgodose sin växande efterfrågan på elektricitet. Importen sker främst från Venezuela. Detta beroende gör Kuba känsligt för ändringar i oljepriser samt det politiska klimatet. Den nuvarande krisen i Venezuela har haft en betydande inverkan på Kubas elproduktion. Genom att utöka landets förnybara energikällor kan Kuba minska sitt beroende av andra länder och diversifiera sin energiförsörjning. Detta kommer även att leda till en positiv miljöpåverkan då landets CO2-utsläpp minskar.
Kubas geografiska läge har ideala förhållanden för förnyelsebar energigenerering, så som solkraft. Solkraft utvecklas konstant och innehåller en hög potential. Concentrating Solar Power (CSP) är en teknologi där speglar och/eller linser används för att koncentrera solljus till en liten yta som konverterar solljuset till värme. Denna värme kan sedan användas i termodynamiska cykler. Det finns två huvudsakliga problem med implementering av CSP på Kuba. För det första är CSP beroende av momentana väderförhållanden, vilket leder till en oregelbunden elproduktion. För det andra har CSP höga investeringskostnader. För att adressera dessa problem, är det möjligt att implementera CSP i ett redan existerande kraftverk med regelbunden energikälla, d.v.s. skapa ett hybridkraftverk. På så vis uppnås regelbunden elproduktion med signifikant lägre
investeringskostnad.
Ett sådant existerande kraftverk kan hittas hos många av Kubas sockerbruk. Den här studien undersöker möjligheten att implementera solkraft i sockerbruket Carlos Baliño, beläget i Villa Clara, Kuba. Fabriken är självförsörjande av elektricitet på årlig basis. De använder en Rankine-cykel för att generera el och processvärme som används i sockerframställningen. Bränslet som används är bagasse, en restprodukt efter att sockerjuicen pressats ut ur sockerrören.
Fyra CSP-teknologier och tre implementeringslayouts undersöktes, vilket resulterade i att parabolic trough-teknologin och förvärmning av vatten ansågs vara de bästa alternativen för Kuba och Carlos Baliño. Vidare undersöktes två olika scenarier för CSP. Scenario 1 innefattar implementering av CSP i sockerbruket under rådande skick och Scenario 2 består av
implementering av CSP efter en investering gjorts i en Condensing Extraction turbin (CEST). Resultatet visar att Carlos Baliño bör investera i CEST innan de implementerar CSP, det vill säga Scenario 2. Detta beror på att i scenario 1 är det inte möjligt att generera elektricitet utanför sockersäsongen, vilket leder till att en stor del av solpotentialen inte kan utnyttjas. Den maximala investeringskostnaden för scenario 1 är 3,7 MUSD, vilket inte är en realistisk kostnad. Den maximala investeringskostnaden för scenario 2 beror av tillgänglig bagasseimport och är 5,9 – 7,2 MUSD. Att investera i CSP rekommenderas ej om bagasseimporten är obegränsad. Givet att bagasseimporten är begränsad skulle CSP-implementeringen leda till en utökad elproduktion av 5,4 – 7,2 GWh/år, en årlig minskning av oljeanvändandet med
16 100 – 21 800 tunnor och minskade CO2-utsläpp med 12 00-16 00 ton årligen. Carlos Baliños ekonomiska resultat skulle öka med 0,5 MUSD/år och den kubanska statens med
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Table of Content
1 Introduction ... 13
1.1 Problem formulation ... 13
2 Background ... 14
2.1 Energy supply and usage in Cuba ... 14
2.2 Sugar production and earlier studies at Carlos Baliño ... 15
2.2.1 Sugar production process ... 15
2.2.2 Co-generation cycle... 16
2.2.3 Results from previous studies ... 18
2.3 Concentrating solar power ... 19
2.3.1 Solar collectors ... 20
2.3.2 Sun as a source of energy ... 23
2.4 Hybridization ... 24
2.4.1 Layout 1 ... 24
2.4.2 Layout 2 ... 24
2.4.3 Layout 3 ... 25
2.5 Previous studies of solar implementation in sugar mills ... 25
3 Methodology introduction ... 27
4 Part 1 – Initial evaluation ... 27
4.1 Method ... 27
4.1.1 Model overview ... 28
4.1.2 System boundaries ... 28
4.1.3 Assumptions ... 28
4.1.4 Conditions for solar power ... 28
4.1.5 Hybridization evaluation ... 29
4.2 Results ... 30
5 Part 2 – Designing a solar field and performance evaluation ... 31
5.1 Method ... 31 5.1.1 Model overview ... 31 5.1.2 System boundaries ... 32 5.1.3 Assumptions ... 32 5.1.4 Scenario 1 ... 32 5.1.5 Scenario 2 ... 33
5.1.6 Solar field dimensioning ... 33
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5.1.8 Economical evaluation ... 38
5.1.9 Environmental evaluation ... 39
5.1.10 Technical sensitivity analysis ... 39
5.1.11 Financial sensitivity analysis ... 41
5.2 Results and discussion ... 42
5.2.1 Results – Scenario 1 ... 42
5.2.2 Technical sensitivity analysis ... 43
5.2.3 Financial sensitivity analysis ... 44
5.2.4 Discussion ... 44
5.2.5 Results – Scenario 2 ... 45
5.2.6 Technical sensitivity analysis ... 47
5.2.7 Financial sensitivity analysis ... 47
5.2.8 Discussion ... 50
6 Comparison and final assessment ... 50
6.1 Comparison ... 50
6.2 Discussion ... 51
6.3 Sustainability analysis ... 51
7 Conclusions and recommended future work ... 53
7.1 Conclusions ... 53
7.2 Recommended future work ... 53
References ... 55
Appendix ... 58
Appendix A – Current operating scheme ... 58
Appendix B – Solar field dimensioning ... 60
Appendix C – Meteorological data... 62
Appendix D – CEST operating scheme ... 63
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List of figures
Figure 1 – Overview of the sugar milling process (Erlich, 2009)... 16
Figure 2 – An ideal co-generation cycle (Cengel & Boles, 2002) ... 17
Figure 3 – A simplified scheme of the cycle in Carlos Baliño ... 18
Figure 4 – Concentrating Solar Power Fundamentals (Lovegrove & Pye, 2012) ... 20
Figure 5 – Main technologies for CSP (Guédez, 2016a) ... 21
Figure 6 – A typical parabolic trough collector (Zarza Moya, 2012) ... 22
Figure 7 – Average DNI in Cuba, (World Bank Group, 2017) ... 23
Figure 8 – Schematic view of layout 1 (Jin & Hong, 2012) ... 24
Figure 9 – Schematic view of layout 2 (Jin & Hong, 2012) ... 25
Figure 10 – Schematic view of layout 3 (Jin & Hong, 2012) ... 25
Figure 11 – Additional solar equivalent electricity generated due to solar hybridization and comparison of land area in the Brazilian sugar mill (Burin, et. al., 2015b) ... 26
Figure 12 – Investment, O&M costs and LCOE costs of solar hybridization in the Brazilian mill (Burin, et. al., 2015b) ... 26
Figure 13 – Model overview, part 1 ... 28
Figure 14 – An example of how the electricity production in a sugar mill could increase if CSP were to be implemented. (Burin, et. al., 2015a) ... 30
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List of tables
Table 1 – Solar field dimensioning ... 35
Table 2 – Technical indicators and calculations, bagasse ... 36
Table 3 – Technical indicators and calculations, electricity, scenario 1 ... 36
Table 4 – Technical indicators and calculations, electricity, scenario 2 ... 38
Table 5 – Economic indicators and calculations ... 39
Table 6 – Environmental indicators and calculations ... 39
Table 7 – Technical sensitivity analysis, scenario 1 ... 40
Table 8 – Technical sensitivity analysis, scenario 2 ... 41
Table 9 – Financial sensitivity analysis ... 41
Table 10 – Results scenario 1 ... 42
Table 11 – Financial results scenario 1 ... 43
Table 12 – Results technical sensitivity analysis, scenario 1 ... 43
Table 13 – Results technical sensitivity analysis, scenario 1, best and worst case ... 43
Table 14 – Results financial sensitivity analysis, scenario 1 ... 44
Table 15 – Results financial sensitivity analysis, scenario 1, best and worst case ... 44
Table 16 – Results scenario 2 ... 46
Table 17 – Financial results scenario 2 ... 47
Table 18 – Results technical sensitivity analysis, scenario 2 ... 47
Table 19 – Results financial sensitivity analysis, scenario 2, Cuba ... 48
Table 20 – Results financial sensitivity analysis, scenario 2, best and worst case, Cuba... 48
Table 21 – Results financial sensitivity analysis, scenario 2, Carlos Baliño ... 49
Table 22 – Results financial sensitivity analysis, scenario 2, best and worst case, Carlos Baliño .. 50
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Nomenclature
Symbol Denomination Unit
𝐴𝑐𝑜𝑙𝑙 Collector area [m2]
𝐴𝑑𝑒𝑚 Area demand of solar field [m2]
𝐵𝑐𝑜𝑛𝑠,𝑜𝑓𝑓 Bagasse consumption, off-season [kg]
𝐵𝑖𝑚𝑝 Bagasse imported [kg]
𝐵𝑖𝑚𝑝,𝑑𝑒𝑐 Decrease in bagasse imported [kg]
𝐵𝑙𝑒𝑓𝑡 Bagasse not used during crushing season [kg]
𝐵𝑠𝑎𝑣𝑒𝑑 Bagasse saved [kg]
𝐵𝑎𝑟𝑐𝑜𝑛𝑡 Content in one barrel [litre]
𝐶𝐼 Concentrating Index
𝐶𝑖𝑛𝑣 Investment costs [USD]
𝐶𝑂&𝑀 Operation expenditures [USD]
𝐶𝑘,𝐶𝐵 Annual yield Carlos Baliño [USD]
𝐶𝑘,𝐶𝑢𝑏𝑎 Annual yield Cuban state [USD]
𝐶𝐹𝑠𝑢𝑔𝑎𝑟 Capacity factor for sugar production [%]
𝐶𝑂2,𝑜𝑖𝑙 CO2-emissions/kWh produced with oil [kg]
𝐷𝑎𝑑𝑑 Additional operating days [days]
𝐷𝐶𝑆 Duration crushing season [days]
𝐷𝑏 Duration off-season, no SP [days]
𝐷𝑑 Duration off-season, SP [days]
𝐷𝑆𝐹,𝑐 Full days with energy from solar field, crushing season [days]
𝐷𝑆𝐹,𝑑 Full days with energy from solar field, off-season [days]
𝐸𝑎 Electricity generation crushing season, no solar power [Wh]
𝐸𝑏 Electricity generation off-season, no solar power [Wh]
𝐸𝑐 Electricity generation crushing season, solar power [Wh]
𝐸𝑑 Electricity generation off-season, solar power [Wh]
𝐸𝑔𝑒𝑛 Electricity generation, no solar power [Wh]
𝐸𝑔𝑒𝑛,𝑆𝑃 Electricity generation, solar power [Wh]
𝐸𝑟𝑒𝑝 Electricity from oil replaced with renewable electricity [Wh]
𝐸̇𝑎 Power generation crushing season, no solar power [W]
𝐸̇𝑏 Power generation off-season, no solar power [W]
𝐸̇𝑐 Power generation crushing season, solar power [W]
𝐸̇𝑑 Power generation off-season, solar power [W]
∆𝐸𝑖ℎ𝑑 Electricity in-house demand, change [Wh]
ℎ Enthalpy [J/kg]
ℎ1 Enthalpy of water before solar heating [J/kg]
ℎ2 Enthalpy of water after solar heating [J/kg]
𝐼𝑅𝑅 Internal Rate of Return [USD]
𝑘 Years since investment [years]
𝐿𝐻𝑉𝐵,50 Lower heating value of bagasse with moisture content 50 % [J/kg]
𝐿𝐻𝑉𝑜𝑖𝑙 Lower heating value of oil [J/kg]
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𝑚̇ Mass flow of water [kg/s]
𝑁 Lifetime of investment [years]
𝑁𝑃𝑉 Net Present Value [USD]
𝑛𝑐𝑜𝑙𝑙 Number of collectors [collectors]
𝑛𝑐𝑜𝑙𝑙,𝑝 Number of collectors in parallel [collectors]
𝑛𝑐𝑜𝑙𝑙,𝑠 Number of collectors in series [collectors]
𝑂𝑖𝑙1𝑘𝑊ℎ Oil needed to produce 1kWh [kg]
𝑂𝑖𝑙𝑟𝑒𝑝 Oil replaced [kg]
𝑃𝐵 Price of bagasse [USD/tonne]
𝑃𝐸𝑛𝑒𝑡 Price of electricity [USD/kWh]
𝑃𝑜𝑖𝑙 Price of oil [USD/barrel]
𝑃𝐵𝑇 Payback time [years]
𝑄𝐵 Heat from bagasse [J]
𝑄𝑐𝑜𝑙𝑙 Heat provided by one collector [J]
𝑄𝑠𝑓 Heat provided by solar field [J]
𝑄𝑠𝑜𝑙 Potential heat from the sun [J]
𝑄𝑠𝑞𝑚 Heat achieved per m2 solar field [J]
𝑄̇ Heat demand from solar field [J/s]
𝑟 Interest rate [%]
𝑆𝐺𝐼 Steam Generation Index [tonnes vapor]
𝑇𝑖𝑛 Temperature of water entering the boiler [°C]
𝑡𝑜𝑓𝑓 Operating hours off-season [hours]
𝑡𝑠𝑜𝑙 Hours of sun [hours]
𝜂𝑏𝑜𝑖𝑙 Boiler efficiency [%]
𝜂𝑐𝑜𝑙𝑙 Collector efficiency [%]
Abbreviations Subscript
CB Carlos Baliño
CEST Condensing Extraction Turbine
CSP Concentrating Solar Power
DNI Direct Normal Radiation
DSG Direct Steam Generation
ET-150 EUROTrough-150
GHI Global Horizon Irradiation
HTF Heat Transfer Fluid
LCOE Levelized Cost of Electricity
LFR Linear Fresnel Reflector
O&M Operation and maintenance
PD Parabolic Dish
PTC Parabolic Trough Collector
PV Photo Voltaic
ST Solar Tower
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1 Introduction
Cuba is currently heavily dependent on imported oil, especially from Venezuela, to meet their increasing electricity demand. The current crisis in Venezuela makes it more important than ever for Cuba to decrease their dependency on the outside world regarding their electricity generation. One way of doing this is by expanding their renewable energy sources. Cuba’s sugar industry has great potential regarding electricity generation. Their rest product bagasse is currently used in several mills as fuel to generate electricity.
The conditions on Cuba are optimal for renewable energy sources such as solar power (EIA, 2016). The investment cost of solar power is currently high, limiting the implementation of it in Cuba and around the world. A way to decrease the investment costs are by implementing solar power in an already existing power plant, making it a hybrid power plant. By combining solar power with another energy source, such as biomass, also the problem with solar power being non-dispatchable can be solved. A combined solar power and biomass energy plant makes is possible to generate electricity from a renewable source which can meet current demand without using electricity storage.
An increased electricity generation from the Cuban sugar mills by implementation of solar power would decrease Cuba’s dependency on the outside world and possibly lower their electricity cost. It would also have a positive effect on the environment by decreasing the oil use and
CO2-emissions. In summary, hybridization with solar power could be an alternative for sustainable development.
The aim of this study is to research the feasibility of integration of biomass and solar energy to increase the electricity generation in the Cuban sugar mill, Carlos Baliño (CB).
1.1 Problem formulation
While energy demand as well as renewables are growing globally, Cuba relies heavily on imported oil to meet their demand. Not only is the burning of fossil fuels causing global warming and negative environmental effects, it also makes Cuba vulnerable and dependent on the outside world. Their main oil provider has been Venezuela, now facing their own political crisis, which leads to an urgent need for Cuba to develop and secure their sources of energy.
Electricity production in sugar mills has been considered a secondary product. A useful way to disposal of excess bagasse, while at the same time generate the electricity to operate the mill. Some electricity is exported to the grid but is considerably limited by the seasonal operation of the mill. By hybridization with solar power, the mills could both increase their electricity generation as well as provide a more reliant source of energy for Cuba, not dependant on the seasonal variations.
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1.2 Aim
The aim of this study is to research the feasibility of integration of biomass and solar energy to increase the electricity generation in the Cuban sugar mill, CB. In particular, the study intends to investigate how Concentrating Solar Power (CSP) can best be implemented in the bagasse cogeneration power unit by hybridization. Three integrations layouts as well as different CSP-technologies will firstly be evaluated, where the optimal solution from an economic and technical perspective will be suggested. Then, a solar field will be dimensioned for the integration layout. The solar field will thereafter be evaluated based on technical, economic and
environmental performance and discussed with regards to sustainable development. The results of this study aim to provide necessary information when considering investing in a solar field at CB, mainly from the perspective of the Cuban state as investor. However, the study will
hopefully provide results which can be used in other mills within the Cuban sugar industry as well.
The study will deliver:
- A comparative analysis between three integration layouts and different solar collector technology, with a suggestion of a suitable integration.
- A dimensioning of a solar field at the site.
- Calculations of additional electricity generated by renewable energy.
- Performance evaluation of the suggested integration in terms of technical, economic and environmental impact.
2 Background
2.1 Energy supply and usage in Cuba
Cuba is highly dependent on imports, mainly oil, which currently covers the majority of their energy needs. The electricity consumption in Cuba is increasing as the country is developing. In the year 1990 the electrical power consumption in Cuba was 1,214 kWh/capita. 26 years later, 2016, the consumption had increased by over 18 percent to 1,434 kWh/capita. (World Bank Group, 2017) To meet their growing electricity demand and at the same time decrease their dependency on other countries, Cuba needs to expand its own electricity production within the country. Cuba is currently trying to both find new and expand their current sources of energy. One alternative is drilling for offshore oil and gas. Another alternative is expanding their renewable energy sources, which would also help diversifying Cuba's energy supply. Location wise Cuba has ideal conditions for renewable energy utilization. (EIA, 2016)
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40 percent from the first half of 2015 to the same period 2016. (EIA, 2016) The Cuban
government has since then ordered cuts in state-run company’s electricity and fuel consumption in an attempt to avoid blackouts as a direct consequence of the decrease in imported oil. To make up for some of the lost oil from Venezuela, Cuba has recently started importing oil from Russia. This is only a short-term solution since Russia is not willing to subsidize oil to Cuba. The weak economy in Cuba will therefore not be able to pay for the Russian oil in the long term, and another more permanent and sustainable solution is needed. (Business Insider, 2017)
2.2 Sugar production and earlier studies at Carlos Baliño
The sugar production in Cuba experienced a reduction of 80 percent, from 8 to 1.6 M tonnes, between 1990 and 2013, but remains an important export for the country (Faostat, 2015). Political events such as the fall of the Soviet Union and the, still standing, U.S. trading embargo against Cuba has had a great negative impact on Cuban sugar industries and the Cuban economy. Financing investments is difficult due to Cuban attempts to receive credit from international organizations being blocked by the U.S. The difficulty to finance investments in combination with poorly managed factories have led to high production costs, making it hard for the Cuban sugar industry to compete internationally. Brazil, the world’s currently largest sugar producer, has production costs half the amount of the Cuban ones. (Alonso-Pippo et al., 2008)
CB is a sugar mill located in the province Villa Clara and was founded in 1903. It started
producing organic sugar in 2001 and is currently the only Cuban sugar mill that produces certified organic sugar. (Birru et al., 2015) The mill normally operates between December and April. Over a whole season, CB is self-sufficient electricity wise, with a positive net trading balance. In other words, CB both imports and exports electricity during the year, with the total export to the national grid being larger than the total import.
2.2.1 Sugar production process
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Figure 1 – Overview of the sugar milling process (Erlich, 2009)
2.2.2 Co-generation cycle
In CB, there is a thermodynamic cycle to produce electricity and process heat. It is a vapor power cycle, which means work is extracted when steam is expanded over a turbine. In particular, it is a co-generation Rankine steam cycle.
A Rankine cycle is the ideal for a vapor power cycle, and involves a pump, a steam generator, a turbine and a condenser. The working fluid is water. (Granryd & Ekroth, 2006) However, in CB they use a modified version of the Rankine to obtain a co-generation cycle. A co-generation cycle is an integrated process where one energy input leads to two useful energy outputs. (Cengel & Boles, 2002) The energy input is the burning of bagasse, where bagasse is the waste material from the sugar production. As previously stated, the two outputs in CB are electricity and process heat. Electricity is used both for in-house demand e.g. operating machinery as well as exported to the national grid. Process heat is the denotation for heat required in an industrial process. In CB, the process heat is used to thicken the sugar juice into a sugar syrup which is later crystallized into sugar. (Carlos Baliño, 2018a)
In Figure 2 an ideal co-generation cycle is shown. In step 1 → 2 saturated liquid water is compressed isentropically to obtain a higher pressure. From 2 → 3 heat is supplied at constant pressure. In general, there is a boiler causing a phase change from liquid to saturated steam followed by a superheater to rise the temperature and dry the steam. In step 3 → 4 the
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Figure 2 – An ideal co-generation cycle (Cengel & Boles, 2002)
In CB, the cycle is demonstrated in Figure 3. The pump work can be approximated as zero.In the steam generator, there is an economizer, a boiler and a superheater. The economizer provides initial heat from the flue gases. In the boiler and superheater, heat is supplied by the burning of bagasse. The live steam exits the steam generator at the temperature and pressure of
308 degrees Celsius and 15.6 bar, which is the max capacity of the turbines. (Jonsson & Mörk, 2016)
There are two back-pressure turbines in CB with a max capacity of 4.5 MW. Though the electrical generators max capacity is 3 MW, which limits the turbines’ work. (Jonsson & Mörk, 2016) A back-pressure turbine is ideal for delivering process heat since it does not expand fully in the turbine, thus creating more available process heat. (Kamate & Gangavati, 2009) After the turbines, the steam-liquid mixture has the pressure 2 bar and the temperature is between 127-130 degrees Celsius. Those properties are needed for the process heat in the sugar production. (Herrera Moya, 2018).
The steam generating index (SGI) in CB is 2.34. It means one tonne of bagasse, generates 2.34 tonne of vapor at the given quality (temperature and pressure) of the supplied bagasse. (Carlos Baliño, 2018a)
𝑺𝑮𝑰 = 𝒎𝒗𝒂𝒑𝒐𝒖𝒓
𝒎𝒃𝒂𝒈𝒂𝒔𝒔𝒆 (1)
A co-generation cycle has a higher overall efficiency since the heat that is left after the steam has passed the turbines is being used in the sugar process, instead of being let out to the surrounding air. There are losses throughout the cycle. (Granryd & Ekroth, 2006) For instance, in the process heater it occurs losses to the environment due to leakages. This renders in total losses of
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Figure 3 – A simplified scheme of the cycle in Carlos Baliño
2.2.3 Results from previous studies
Several previous studies have been made at the sugar mill CB. Results and conclusions that may come to use in this study will be presented below.
The sugar mill factory CB currently both import and export electricity to and from the national grid to meet their instant demand. They have a positive net trading balance, meaning that they are self-sufficient electricity wise seen to a whole season. CB has the potential of producing even more electricity, but is currently limited by its turbine and generator. The amount of electricity produced in the factory is tied to the process steam demand, making the back-pressure
technology a limitation. The turbine and generator can combined only produce 3 MW, which is not enough to make use of all the steam produced during operation at full capacity in the factory. Jonsson and Mörk suggested an investment in a Condensing Extraction Turbine (CEST) and increasing the parameters of the steam at the turbine inlet from the current 15.6 bar and
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future work should investigate how unplanned stoppages can be reduced and how they affect the sugar production as well as the electricity production. Lundberg and Öhman concludes that the factory, considering current stoppages, has the potential to generate 2.58 GWh per season with bagasse as direct fuel. An electricity export of this size lets Cuba reduce their oil usage with 1,080 tonnes per year and gives CB an expanded revenue of 387,000 USD. The study investigated the possibility to produce pellets from the excess bagasse. Pelletization of current excess bagasse at CB was calculated to be increasing the factory’s revenues with 653,000 USD if sold, and could generate 8.79 GWh net electricity and thereby reduce Cuba’s oil consumption by 3,700 ton. The most suitable use of the excess bagasse, from both an economical and environmental point of view, would be to maximize the pelletization at the factory. (Lundberg & Öhman, 2016) Calculations in a previous study by Hallersbo and Onoszko shows that CB during current circumstances produce 37,000 ton/season of nominal excess bagasse not needed in the sugar production. The large amount of excess bagasse should be considered a valuable asset, and not as waste, for the factory. The practical amount of bagasse has been calculated to 11,500 ton/season. A large amount of excess bagasse is wasted due to inefficient usage combined with unplanned production stoppages. Also Hallersbo and Onoszko suggests that unplanned production stoppages should be investigated since a large amount of bagasse is required to start the boilers again after each stop, causing significant losses of excess bagasse. An investigation of different boilers to maximize the amount of excess bagasse is also suggested. (Hallersbo & Onoszko, 2015) The study by Nylund and Puskoriute also suggests a replacement of the current back-pressure turbines with CEST in order to maximize the electricity generation. An extraction condensing turbine generates more electricity than a back-pressure turbine for the same steam inlet parameters, making it a better option. To increase the electricity generation the study suggests drying the bagasse. Drying bagasse increases the heating value of the bagasse, the boiler efficiency and the generated electricity. It is also suggested to investigate the possibility of generating
electricity during off-season to achieve larger revenues for CB. (Nylund & Puskoriute, 2015) A study by Villardefrancos Bello and Acosta Álvarez shows a solar field dimensioning at CB for the use of the parabolic trough technology EUROTrough-150 (ET-150) and feedwater heating. They calculated the average hours of sun for each month as well as the average heat collected by one collector. The conclusion was that 5-9 collectors are needed in three rows depending on the month of the year. (Villardefrancos Bello & Acosta Álvarez, 2017)
2.3 Concentrating solar power
During the last years, solar energy has been progressing and is now considered a great source of energy with varying technologies and applications. Concentrating Solar Power (CSP) is a
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The main solar collector technologies regarding CSP are Solar Tower (ST), Linear Fresnel Reflector (LFR), Parabolic Trough Collector (PTC) and Parabolic dish (PD). PTC is the dominated technology in the market, nevertheless ST is increasing the most. Guédez
prognosticated that the market shares in December 2017 would be changed so that PTC would have 74 percent, ST 22 percent, LFR 3 percent and PD 1 percent(Guédez, 2016). All systems have some principles in common, which can be illustrated in Figure 4. As seen by the Figure 4, all technologies begin with a concentrator where the mirror and lenses in different setups
concentrates the sunlight onto a receiver. The receiver then converts the sunlight, usually to thermal energy. The thermal energy is then transported to the power generating cycle. The thermal energy can either be carried by a heat transfer fluid or generate steam directly. The conventional denotation is to either classify the plant as a DSG (direct steam generation, shown by the arrow “Direct connection” in Figure 4) or a HTF (heat transfer fluid). DSG has been proven with all the concentrating technologies and has the benefit of not needing a heat exchanger since the steam can be applied directly in the power cycle. HTF has generally been used with thermal oil or molten salt, where molten salt provides with the greater opportunity to thermal storage to increase dispatchability. (Lovegrove & Pye, 2012) The “Heat to environment” in Figure 4 could also, if needed, be used to for example drive an absorption chiller or for direct heating instead of being wasted to the environment. The main challenge within CSP-technology is the large investment cost. To this day, it is not competitive against other power generating systems. (Aichmayer, 2016b) It is therefore important to carefully assess the economic feasibility before investing.
Figure 4 – Concentrating Solar Power Fundamentals (Lovegrove & Pye, 2012)
2.3.1 Solar collectors
Different kinds of solar collectors can be used in CSP systems. Which kind of collector that is preferred may vary in different cases, depending for example on sizing and economic conditions. There are currently four different main technologies for CSP: Linear Fresnel reflector, Solar tower, Parabolic dish and Parabolic trough. The main idea for each of the technologies are being shown in Figure 5 and will all shortly be introduced below.
-21- 𝐶𝐼 = 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝑎𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑎𝑟𝑒𝑎
𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑟 𝑎𝑟𝑒𝑎 (2)
where linear-focusing has a concentrating ratio of 100, and point-focusing technology between 1000-4000 (Hoffschmidt, et al., 2012). Parabolic trough is, as previously mentioned, currently the most used technology even though Solar Tower is rapidly expanding its market share.
Figure 5 – Main technologies for CSP (Guédez, 2016a)
2.3.1.1 Linear Fresnel reflector
Linear Fresnel reflector (LFR) is a technology in the early commercial stage with some ongoing projects.(Guédez, 2016a) LFR is a linear-focus solar collector where long rows of flat or slightly curved mirrors are used to reflect solar radiation on a downward facing fixed receiver. The mirrors rotate independently on one axis, following the suns movements under the day so that the reflected sun radiation hits the receiver. A fixed receiver as the one in LFR helps reducing the convection losses and has the advantage of not needing rotary joints for the heat transfer fluid. LFR has potential in its development and in becoming a relatively inexpensive CSP technology to implement, but is still a young technology. (Mills, 2012)
2.3.1.2 Solar Tower
Solar Tower (ST) is a proven technology with some larger projects already ongoing. It is currently the second largest CSP technology and the fastest expanding one. (Guédez, 2016a)The ST technology contains a tower with a fixed receiver at the top and an array of heliostats, large mirrors with two-axis tracking, around the tower reflecting the sunlight onto the receiver. The two-axis tracking lets the mirrors move around to maximize the solar radiation collected during the day and year. ST is a point-focus solar collector, allowing high efficiency energy conversation at a single larger receiver point. Point focusing systems achieve higher concentration ratios than linear focusing systems, and thereby making it possible to operate at higher temperatures with reduced losses. (Vant-Hull, 2012) The higher temperatures allow the system to be able to also create superheated steam. The technology is still young and have room for innovation. It is currently a relatively expensive technology. (Guédez, 2016b)
2.3.1.3 Parabolic dish
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maturity (Garrido, 2016). Dish systems contains of three-dimensional paraboloids that reflects solar radiation to a point focus receiver. Similar to tower systems it can achieve very high temperatures, up to over 1 000 degrees Celsius. The three-dimensional constructions allow the Parabolic dish to always be directed straight towards the sun beams and avoid the “cosine loss effect”. By avoiding these losses the Dish systems are able to offer the highest potential solar conversion efficiencies out of the CSP technologies. (Schiel & Keck, 2012) The PD technology is currently in need of further research and development in order to be implemented in a
hybridization power plant. (Garrido, 2016)
2.3.1.4 Parabolic trough collector
Parabolic trough collector (PTC) is a proven technology that currently is the most mature one and holds the largest market share of the mentioned technologies. A typical PTC can be seen in Figure 6. (Guédez, 2016a) PTC is a linear-focus solar collector and works by placing parabolic trough-shaped mirrors so they produce a linear focus on a receiver tube that is placed along the parabola’s focal line. Direct solar radiation reflects from the larger mirror area onto the smaller outer surface of the receiver tube, heating the fluid circulating inside it. Thereby, solar radiation transforms into thermal energy that can be used in Rankine cycles to produce electricity. Most PTC fields are composed of parallel rows of collectors that also are connected in series. The frame on which the mirrors and the receiver is placed tracks the suns daily movement and will spin around one axis in order to maximize the sun energy being collected. It is not possible to rotate the mirrors in any other way once they have been installed, resulting in that seasonal movements of the sun will lead to some spill at the row ends. The main limitation for how high temperatures can be achieved is linked to the working fluid. Thermal oil is currently the most common working fluid and can reach a temperature of 398 degrees Celsius, but research in new fluids may allow even higher temperatures in the future. Once a PTC field has been installed its main cost will be the one of manpower for plant operation and system maintenance. (Zarza Moya, 2012)
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The sun provides planet Earth with 885 million TWh each year (IEA, 2014). It is a
non-dispatchable source of energy, meaning the output cannot be provided on demand. The generation of electricity is reliant on the instantaneous weather conditions. With the aim of determining if the location is feasible for solar power, the dataset TMY (Typical Meteorological Year) is used for calculations. A TMY consists of meteorological values and data of solar
radiation for each hour. The months are selected from different years, which are compiled into a full year. (Kalogirou, 2012) In terms of CSP a TMY should have clear days with low aerosol optical depths (IEA, 2014). The ground should be flat to avoid tilting collectors, which causes an inclination against the sun and therefore reduces the ability to absorb the sun rays. The soil should be firm, and there should be a minimum of trees and constructions which could shadow the collectors. Access to water is important to cover a CSP-plants water needs. Water is needed partly for the steam cycle, but also for mirror-washing to keep the collectors at free sight for the radiation from the sun.
The output of CSP is proportional to the direct normal irradiation (DNI). While Photo Voltic (PV) is calculated by Global Horizon Irradiation (GHI), DNI is a less researched subject leading to a lack of information on DNI forecast accuracy. In order to optimize the system, e.g. planning the operation to minimize heat losses and maximizing the value of saved thermal storage, DNI forecast information contributes with important parameters. (Law et al, 2014) The recommended amount of DNI should exceed 1,700 kWh/m2/year (Peterseim et al, 2013). As seen in Figure 7, the area around Santa Clara where CB is located exceeds the recommended amount of DNI. Appendix C, showing the average solar radiation at CB during different months of the year and from different angles calculated by the Metrological institute of Cuba, also indicates that the DNI at the location is sufficient.
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2.4 Hybridization
Hybridization is when there are two or more sources of energy to generate useful energy services. There are several advantages with hybridization. In terms of concentrating solar power,
hybridization can increase dispatchability when combined with a dispatchable source of energy. Thus, creating a reliant generation of electricity. (Jin & Hong, 2012) Solar power is also a renewable and emissions-free source of energy, which could decrease the use of fossil fuels in traditional plants. This would decrease CO2-emissions and reduce global warming. There are also economic values. Concentrating solar power is a complex system on its own, however there are several plants and industrial factories with the system and technology accessible. It is therefore suitable to integrate the solar power to existing systems, consequently decreasing the initial investment costs and at the same time acquire the beneficial values from hybridization. (Burin, et al., 2015b) If the current system is operating under high fuel costs, the CSP integration would reduce the demand for the fuel and accordingly decrease the operating costs. It is also possible to increase the current power generation if there is capacity, or to increase the cycle efficiency. In CB, there is already a co-generation Rankine cycle, and therefore there are some options for how to implement CSP in the mill. This study will evaluate three different integrations layouts, described below.
2.4.1 Layout 1
Additional solar-aided feedwater heating is the concept of layout 1. It is an integrated process with the current fuel, bagasse, which means it is not possible to operate the mill without the bagasse. The main idea with layout 1 is to increase the steam generating index (SGI). A higher SGI would decrease the demand for bagasse. The excess bagasse can then be sold or used to operate the mill during off-season. Solar-aided feedwater heating is the most mature technology, which has been tested and validated in various locations. It is also the most inexpensive and easy to implement. Besides the solar collectors, it is only needed to add feedwater heaters. The existing feedwater heaters remains, in order to still being able to operate the mill during times when solar power is not available.
Figure 8 – Schematic view of layout 1 (Jin & Hong, 2012)
2.4.2 Layout 2
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feedwater it will need a more powerful CSP-system to both increase the temperature and cause a phase change. The saturated steam will lead to less bagasse needed to superheat the steam to the right properties. The generation index will be higher in Layout 2 than in Layout 1, but so will also the initial investment cost.
Figure 9 – Schematic view of layout 2 (Jin & Hong, 2012)
2.4.3 Layout 3
Solar-aided with superheated steam. The solar field in Layout 3 produces in parallel with the steam generator superheated steam which can be ejected directly into the turbine. This kind of layout would usually use a central receiver (solar tower) since a powerful CSP-system is needed to produce superheated steam directly. Layout 3 is the only layout which is independent of bagasse and can run the cycle on its own. A CEST with a condenser and cooling system has the
possibility to condensate steam leaving the turbines without being used as process heat. This layout would therefore also be able to generate electricity all year-around if a CEST would be installed, making it possible to condense the steam also under off-season. Layout 3 would save the largest amount of bagasse and has the highest potential in generating electricity if turbines and generators with higher capacity would be installed in the factory, but needs a larger initial investment than the other layouts. It is also a more technical advanced layout than layout 1 and 2.
Figure 10 – Schematic view of layout 3 (Jin & Hong, 2012)
2.5 Previous studies of solar implementation in sugar mills
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The study mentions that they choose to look at LFR and PTC technologies for implementations similar to layout 1 and 2 in this study, since relatively low temperatures are needed. Layout 3 needs higher temperatures, making it more suitable to invest in solar towers, raising the cost of investment. Neither of the first layouts enables solar only operation, bagasse is needed to superheat the steam.
As shown in Figure 11, layout 3 with solar towers would give the largest additional electricity production, due to the possibility to produce electricity without bagasse, and during off-season. In both layout 1 and 2 PTC is the technology which produces the highest amount of additional electricity. Layout 1 generates the lowest amount of additional electricity. Figure 11 also shows the solar multiple and needed land area for the different layouts and technologies. Layout 1 requires the least land and mirror area. The LFR technology needs even smaller areas than the PTC technology, but the PTC technology is still preferred due to it being more mature and easier to produce and implement. As shown in Figure 12, layout 1 has the highest Levelized Cost of Electricity (LCOE) but a significantly lower investment cost.
Figure 11 – Additional solar equivalent electricity generated due to solar hybridization and
comparison of land area in the Brazilian sugar mill (Burin, et. al., 2015b)
Figure 12 – Investment, O&M costs and LCOE costs of solar hybridization in the Brazilian mill
(Burin, et. al., 2015b)
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Nixon, Dey and Davies concluded in their report “The feasibility of hybrid solar-biomass power plants in India” that hybrid solar-biomass power plants currently are feasible options for the combined generation of electricity, cooling and heat in small-mid scale applications (2-10 MW). Only electricity production would work better in larger scales. (Nixon, et. al., 2012)
3 Methodology introduction
A methodology including a literature study, a field study, collection, modelling and analysis of data as well as calculations were used in order to meet the goal of the study. The literature study was conducted to review the previous work on CB and collect important data to use for this study. The literature study also provided knowledge of the subject, e.g. a deepened understanding of concentrated solar power, hybridization and the co-generation cycle used in CB. However, the literature study lacked the necessary information to consider the local properties at CB as well as fully understanding the conditions within the Cuban sugar industry, in terms of economic and technical possibilities. Therefore, the field study can be considered the most important part of the methodology, consisting of visits to the sugar mill CB where necessary data and information were collected. The data consisted of technical sheets, solely obtainable from actual visits as well as discussions with the personnel at the mill and a possibility to evaluate the current equipment and machinery. This data and information was later used for modelling and calculating possible scenarios, as well as the feasibility for different scenarios of the chosen technology and layout. While it would be possible to conduct a similar study without the field study, the local properties as for example operating parameters, technological limitations, etc. would not be regarded in the same extent. The study would then deliver results based in a greater extent of theoretical values. Theoretical results can provide with interesting insight, however, this study aimed to suggest the optimal solution at CB with the local properties regarded and by extension, grounds for
investment with the Cuban state as investor. As shown in the literature study, CSP is both a complex and expensive technology and there is no “best practice” for how to implement it, especially when in a hybridization layout. The proven experience within the field, shows there are multiple aspects needed to be considered, rendering in different outcomes, and that CSP still is a developing technology lacking standardized applications. To achieve realistic results, it is
therefore crucial to include a field study in the methodology.
4 Part 1 – Initial evaluation
An initial evaluation will take place in part 1 in order to find the CSP-technology as well as implementation-layout most suitable for Cuba and CB. The method, model overview, system boundaries and assumptions listed below applies exclusively to Part 1.
4.1 Method
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The model for Part 1 – initial evaluation is presented in Figure 13. The literature study provides with a deepened understanding of the subject; however the field study is essential in order to establish which option is optimal for CB and Cuba. In particular, the operating conditions will provide with important information to be able to make a realistic suggestion.
Figure 13 – Model overview, part 1
4.1.2 System boundaries
Cuba is a developing country with technical and economical limitations. The Cuban government has set a cost of capital on their investments to 8 percent and uses 20 years as the lifetime of their projects. Their internal rate of return is set at a minimum of 11 percent. This will be used
throughout the financial calculations in this thesis. (Herrera Moya, 2018) This limits the options for CB in terms of the choices of CSP-technologies.
4.1.3 Assumptions
• Only four CSP-technologies to choose from. • The solar field and collectors will be built in Cuba. • The Cuban state is supposed to finance the investment. 4.1.4 Conditions for solar power
Some conditions at the site, in this case CB, needs to be fulfilled in order for solar power to have the possibility to be implemented. These conditions are listed below and evaluating them at CB is the first step in the model.
• Location. The first step is evaluating the site location’s properties. The assessment will take place during the field studies in CB to verify that the site has satisfying location properties.
• Metrological data. The meteorological conditions will be evaluated from the data from the Typical Metrological Year (TMY) collected by the Metrological Institute of Cuba. It will mainly be assessed according to the DNI, where values over 1,700 kWh/m2/year is
Evaluate conditions for
solar power
Field study visit to assess site conditions Collect meterological data Evaluate solar collector technology
Field study visit to assess operating conditions Analysis of previous studies Evaluate integration layouts Field study visit
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recommended for implementation of Concentrating Solar Power (Peterseim et al, 2013). This data can be found in Appendix C.
4.1.5 Hybridization evaluation
The results from previously reviewed report in Brazil indicates that an implementation of layout 3 in combination with CEST in CB would lead to the lowest LCOE (Burin, et. al., 2015b), but since the initial investment cost is very high both to invest in CEST and in solar towers or to invest in a large enough solar field of parabolic trough to be able to produce super-heated steam it is currently not a realistic solution for Cuba and CB. Brazil has much more developed
renewable energy sources than Cuba, in 2011 87 percent of the country’s electricity was produced with renewable sources. (Burin, et. al., 2015b) This leads us to assume that Brazil easier and more affordable can access the technology needed to produce solar towers and/or larger solar fields. In layout 2 and 3 all or some of the water would be going directly to the boiler or turbine without passing the economizer. That would lead to a decrease in the efficiency off the economizer during the times when the solar field was operating. The loss of the use of the economizer will lead to the need for a much larger solar field in layout 2 and 3, than in layout 1. This leads us to believe that layout 1 would be the best option for CB. Layout 1 would be easier and cheaper to implement, increasing the chances of an investment in solar power taking place in the factory. Layout 3 needs higher temperatures, making it more suitable to invest in solar towers, raising the cost of investment and thereby making it an unrealistic option for the Cuban sugar mill. (Burin, et. al., 2015b) Under current conditions at the sugar mill, an implementation of CSP will let the mill save bagasse that can be sold and then used in all year-around production at a different location. That way CB will increase their revenues, and Cuba can increase their renewable electricity production and thereby decrease their oil usage. If a CEST would be installed in CB it would be possible to generate electricity during off-season, making all year-around generation possible, as long as they still have saved bagasse or buys extra bagasse or some other kind of biomass elsewhere. This would be the optimal solution seen to LCOE, but comes with a high investment cost.
Layout 1 generates the lowest amount of additional electricity, but due to the lower investment cost of a smaller solar field and the current economic situation in Cuba it is still the preferred option in CB’s case.
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Figure 14shows how a sugar mill with a CEST could extend its operation in to off-season by implementing CSP. In CB’s current state the saved excess bagasse could instead be sold to generate higher revenues for the factory and increase electricity generation somewhere else where off-season operation is possible.
Figure 14 – An example of how the electricity production in a sugar mill could increase if CSP were to
be implemented. (Burin, et. al., 2015a)
EUROTrough collectors, in this case EUROTrough-150 (ET-150), are according to professor Herrera Moya the best option for Cuba seen to investment cost and accessibility in Cuba. It is difficult for Cuba to import technology and materials. The use of ET-150, a mature and
consolidated technology, in a pilot project like CB, will provide Cuban engineers with experience with solar technology. The parabolic trough collector seems to be suitable to produce in Cuba with technology already available in the country. (Herrera Moya, 2018) PTC is a proven and previously well used method making it easier to and cheaper to implement production of the needed technology in Cuba. Also, the solar field size is not limited as in Solar towers. PTC is easier to scale to the specific situation than the other mentioned technologies. It has been assumed to be the most feasible and realistically alternative for the current situation in Cuba and CB. EUROTrough uses synthetic oil up to temperatures as high as 300-400 degrees Celsius to heat up water and is a good option for obtaining low pressure saturated steam. (Geyer et.al., 2002)
4.2 Results
The ground as well as the sun availability at CB is evaluated to be sufficient for the implementation of a solar field (Carlos Baliño, 2018a). See Appendix C for solar data. An initial evaluation shows that the most suitable and realistic option for CB is Layout 1 in combination with the parabolic trough technology, in particular ET-150. This technology and layout currently imposes a relatively low cost as well as a developed technology, making it suitable for Cuba and the country’s current resources.
To fully evaluate this option, it will be assessed with both the current scheme as well as a scheme with a CEST. The CEST-scheme has in previous studies been concluded to be the next technical upgrade in CB, and will also be beneficial for solar power. It is therefore of interest when
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5 Part 2 – Designing a solar field and performance
evaluation
With the results from Part 1, it is concluded the integration layout should be evaluated both with the current scheme and with a CEST-scheme, now introduced as Scenario 1 and Scenario 2. The method, model overview, system boundaries and assumptions listed below applies exclusively to Part 2, and is valid for both scenarios. The scenarios will then be further presented, where
specific conditions, boundaries and assumptions, solely valid for respective scenario will be listed.
5.1 Method
A solar field is dimensioned with regard of the proposed layout. The proposed layout is then evaluated technically, economically and environmentally. In order to calculate the results, technical and economic data is collected from the field studies. The technical evaluation investigates potential increase in electricity generation, implementation and operation, and bagasse savings. The economical evaluation consists of calculations of the maximal investment, with regards to limitations for Pay Back Time (PBT), Net Present Value (NPV) and Internal Rate of Return (IRR) set by the Cuban government, of the suggested implementation. The
environmental evaluation consists of calculations of the potential reduction in CO2-emissions. Lastly a summary of the results and evaluations including a sensitivity analysis and a final implementation assessment is presented.
5.1.1 Model overview
The model for part 2 is showed in in Figure 15. The concept model applies for both scenarios, however will the numerical model differ since the scenarios contains different input parameters, see 5.1.4 and 5.1.5. The input parameters from scenario 1 is modeled from operating parameters obtained mostly from the field study, and scenario 2 is modeled from theoretical parameters from a previous study.
SUMMARY
Sensitivity analysis Final suggestion ENVIROMENTAL EVALUATION
Reduction of CO2-emissions Reduced oil usage ECONOMICAL EVALUATION
IRR PBT NPV
TECHNICAL EVALUATION
Eletricity generation Implementation and operation Bagasse savings SOLAR FIELD DIMENSIONING
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Figure 15 – Model overview, part 2
5.1.2 System boundaries
Being able to generate and export electricity is as a bonus, the main activity is the sugar
production. Therefore, it is of highest importance to fulfil the needs within the sugar production. One condition for the sugar production, the process heat demand, is directly connected to the electricity generating process. The process heat demand, currently at 45 tonnes/hour, needs to be fulfilled at all time, creating a system boundary for the model. This boundary, for example, means that the steam process needs to be in operation all the time during crushing season. It is not possible to stop the steam generation without it affecting the sugar production process.
The economical limitations stated in Part 1 still applies. The Cuban government has set a cost of capital on their investments to 8 percent and uses 20 years as the lifetime of their projects. Their internal rate of return is set at a minimum of 11 percent. This will be used throughout the financial calculations in this thesis and limits the maximum accepted investment. (Herrera Moya, 2018)
5.1.3 Assumptions
• A capacity factor of 85 percent for sugar production during crushing season, meaning that the factory on average is not in operation 15 percent of the crushing season due to unplanned stoppages.
• Crushing season goes from beginning of December to end of April. • Full year-operations are 350 days.
• Steady state operation. • Constant boiler efficiency.
• Moisture content of bagasse is 50 percent (Carlos Baliño, 2018a). • Storage for bagasse is infinite during one season.
• Bagasse can be bought or sold for 4.7 USD/tonne (Carlos Baliño, 2018a).
• A linearization of the energy provided by the sun during the day, giving a capacity factor for the solar field of 75 percent.
5.1.4 Scenario 1
CB is assumed to be operating under the same conditions after the implementation of solar power, as the current scheme. These conditions can be found in Appendix A, and is obtained from the technical data sheets from CB. In that regard, the process heat production as well as the electricity generation will not increase after implementation. However, CB’s demand for bagasse will decrease, which will be calculated as 𝐵𝑠𝑎𝑣𝑒𝑑. The amount of 𝐵𝑠𝑎𝑣𝑒𝑑 will then be used to determine the additional electricity generation, assumed to be generated at a different location, for instance, another factory. The calculations will be performed in Microsoft Excel with the data from field studies and literature studies as input parameters.
5.1.4.1 System boundaries
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• The electricity generation at CB is limited to crushing season. • The electricity generation is a function of the process heat demand.
5.1.4.2 Assumptions
• Constant boiler efficiency at 0.825 (Carlos Baliño, 2018a).
• All bagasse sold will be used to generate electricity elsewhere, replacing electricity generate by oil.
• Current operating scheme applies when implementing solar power. 5.1.5 Scenario 2
A Condensing-extraction turbine is installed in CB allowing year-round operation. The operating conditions will change, such as steam parameters, boiler efficiency, bagasse consumption, etc. These conditions can be found in Appendix D and is obtained from a previous study on
CEST-implementation at the site. The process heat production will remain, since the demand has not changed, however will the mass flow of steam increase due to the CEST. The electricity generation will increase, both due to the CEST as well as the solar power. The increase will be calculated as a function of the saved bagasse, 𝐵𝑠𝑎𝑣𝑒𝑑. The value calculated will be the increase in electricity generation due to implementation of solar power, not the increase due to the CEST. The amount of imported bagasse needed, 𝐵𝑖𝑚𝑝, to generate maximum capacity of electricity all year around will be calculated. The calculations will be performed in Microsoft Excel.
• Case 2A: no bagasse available for import
• Case 2B: enough bagasse to operate during the full off-season only if solar power is implemented
• Case 2C: unlimited amount of bagasse available for import
5.1.5.1 System boundaries
• The electricity generation at CB is limited by its new generators to 11.8 MW. (Rubio Rodríguez & Rubio González, 2016)
5.1.5.2 Assumptions
• Constant boiler efficiency at 0.9 (Rubio Rodríguez & Rubio González, 2016).
• The price received for exporting electricity is assumed to be constant at 0.15 USD/kWh (Herrera Moya, 2018).
• All excess electricity generated can be exported to the national grid. • Three different cases of bagasse import available.
5.1.6 Solar field dimensioning
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Villardefrancos Bello and Acosta a solar field at CB needs three collectors in parallel, to achieve the desired mass flow. This study will then determine the number of collectors in series, 𝑛𝑐𝑜𝑙𝑙,𝑠. As previously reviewed in the literature study, there is a Rankine cycle at CB. This means the heating of water at constant pressure is an isobaric process where the heat demand is a function of the mass flow of water multiplied by the enthalpy change, see equation 1. (Havtun, 2014). For now, the heating occurs by the burning of bagasse, and increases the water temperature from 90 degrees to 360 degrees. (Carlos Baliño, 2018a)
When implementing solar power, a portion of the heat supplied by bagasse can be replaced by the heat supplied by the solar power, i.e. feedwater heating. In order to heat the water without causing a phase change, the highest allowed temperature, 𝑇𝑖𝑛, should not be higher than the saturation temperature. For scenario 1, the operating pressure is 15.6 bar which leads to a saturation temperature of 205 degrees. For scenario 2, the operating pressure is 65 bar (Rubio Rodríguez, 2018), which leads to a saturation temperature of 280 degrees. (Havtun, 2014) These thermal properties are then used to decide the enthalpies before, ℎ1 , and after, ℎ2 , the heat from the solar field is supplied. For scenario 1, the mass flow of water equals to the process heat demand. Accordingly, this is inserted to equation 10 to determine the heat demand. While in scenario 2, the modelled data from ASPEN concludes the heat demand to be 6111 kwh/h. (Rubio Rodríguez & Rubio González, 2016)
When the heat demand from the solar field is determined, the number of solar collectors in series can be calculated with regard to the climate data at the site. Firstly, the available solar energy is determined by equation 4, see Appendix B for calculated results and Appendix C for input data. The heat per square metre collector, 𝑄𝑠𝑞𝑚, is then a product of the available solar energy and the collector efficiency, shown in equation 5. A EUROTrough-150 collector has a collector efficiency of approximately 56 percent at the current location. (Villardefrancos Bello & Acosta Álvarez, 2017). By dividing the heat demand with the heat supplied per square metre collector, the area demand is derived. A ET-150 has an aperture area of 817.5 m2 which is then used to determine the number of collectors needed. (Geyer et al, 2002) By dividing with the number in parallel, the number in series, 𝑛𝑐𝑜𝑙𝑙,𝑠, is accomplished. These results will vary depending on the month of the year, which means a suggestion need to be made. The solar field dimensions have accordingly been determined and in the following chapters the technical, economical and environmental performance will be evaluated.
Solar field dimensioning Heat demand isobaric
process 𝑄̇ 𝑚 ⋅̇ (ℎ2 − ℎ1) (3)
Available solar energy 𝑄𝑠𝑜𝑙
𝐷𝑁𝐼
𝑡𝑠𝑜𝑙 (4)
Heat per sqm collector 𝑄𝑠𝑞𝑚 𝜂𝑐𝑜𝑙𝑙⋅ 𝑄𝑠𝑜𝑙 (5)
Area demand solar field 𝐴𝑑𝑒𝑚
𝑄𝑠𝑞𝑚 𝑄𝑠𝑜𝑙
-35- Number of collectors 𝑛𝑐𝑜𝑙𝑙 𝐴𝑑𝑒𝑚 𝐴𝑐𝑜𝑙𝑙 (7) Number of collectors in series 𝑛𝑐𝑜𝑙𝑙,𝑠 𝑛𝑐𝑜𝑙𝑙 𝑛𝑐𝑜𝑙𝑙,𝑝 (8)
Heat provided by one
collector 𝑄𝑐𝑜𝑙𝑙 𝐴𝑐𝑜𝑙𝑙 ⋅ 𝑄𝑠𝑞𝑚 (9)
Heat provided by solar
field 𝑄𝑠𝑓 𝑛𝑐𝑜𝑙𝑙⋅ 𝑄𝑐𝑜𝑙𝑙 (10)
Where,
Enthalpy ℎ 𝑓(𝑃, 𝑇)
Direct normal irradiation DNI 𝐴𝑝𝑝𝑒𝑛𝑑𝑖𝑥 𝐶
Sun hours 𝑡𝑠𝑜𝑙 𝐴𝑝𝑝𝑒𝑛𝑑𝑖𝑥 𝐶
Collector efficiency 𝜂𝑐𝑜𝑙𝑙 0.56
Collector area 𝐴𝑐𝑜𝑙𝑙 817.5 𝑚2
Table 1 – Solar field dimensioning
5.1.7 Technical evaluation
The technical evaluation will evaluate the saved bagasse and electricity from oil replaced with renewable electricity for both scenarios. For scenario 1 the electricity is assumed to be generated elsewhere where in scenario 2 the electricity replacing oil is produced at CB, allowing for all year-around operation.
Scenario 1
To calculate how much bagasse can be saved each crushing season for scenario 1, the heat provided by bagasse, 𝑄𝐵, is replaced with heat provided from the solar field, 𝑄𝑠𝑓. The same amount of 𝑄𝑠𝑓 added is equal to the reduction in 𝑄𝐵. To calculate heat generated by bagasse, 𝑄𝐵, equation 11 is used, which is a product of the lower heating value, the mass of bagasse, the boiler efficiency and the capacity factor for CB. The lower heating value of bagasse with a moisture content of 50 percent, which is 8816 MJ/kg, will be used to find the most accurate result
(Herrera Moya, 2018) (Jonsson & Mörk, 2016). By replacing 𝑄𝐵 in equation 11 to 𝑄𝑠𝑓, and solve for mass bagasse, equation 12 is achieved which renders into the savings in bagasse.
Scenario 2
