Renewable Energy Technology MJ2411

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1 Energy Technology EXAMINATION Renewable Energy Technology MJ /01/ This exam paper contains five problems. You need to attempt all these problems. Total points allocated for this examination are 60 points. This is equal to 60% of the total of RET evaluation. (The remaining 40% is accounted from Quiz Examinations, Quiz 1 AND Quiz 2 conducted previously) A help document (9 pages) is provided with the examination. It contains information such as equations, formulas and property data. You are permitted use the following during the exam, Provided help document A scientific calculator Pens, pencils and erasers A dictionary This is a closed-book exam. Therefore, you are NOT permitted to use any other materials; books, notes or electronic resources or any other help materials. You need to write your solution in the provided papers. Please write only on one side of the paper. When you start a new problem always start with a new page. Write your name, your student number, question number and page number on each page. 1

2 The Exam grade of the course is decided based on your aggregated total of this examination and the quizzes. This part of the examination gives 60 points Quizzes (previously held) account 40 points Total 100 points Exam Grade: A B C D E Fx 0-46 F A E, are exam pass grades. If you receive the grade Fx you can upgrade your grade to E by completing additional assignment. In such a case you need to contact the course responsible not later than one week after the result is released. If there is no intention from you, Fx will become F automatically 2 weeks later. The Final grade of the course is decided based on the Exam grade and Project grade of each individual and determined according to the following matrix. PROJECT EXAM A B C D E A A B B C D B A B C C D C A B C D D D B B C D E E B C C D E Good luck! 2

3 Q1. Bio Energy Question (15 points) Gasification is regarded as one of the practically effective energy conversion processes in renewable energy applications. Q1 Part 1- Gasification a) Briefly describe the process of gasification (only a short answer is required) (1 point) b) Briefly describe the advantage of having gasification in energy conversion. (only a short answer is required) (1 point) c) Name two practical applications, where gasification is a part of the energy conversion process (1 point). With the interest of transforming fossil fuel-based energy conversion systems to be more renewable energy systems, a farming community in southern Sweden has decided to convert their diesel engine-based electricity generation system to a wood-fired system. Since the same diesel engine is to be used, wood fuel has to be transformed into an acceptable form of energy for the engine. Therefore, the process of gasification is introduced for the fuel conversion process. The sketch below illustrates the overall process of the proposed bioenergy conversion system to generate electricity. Moist wood entering the system is first subjected to a drying process to reduce the moisture content. Thereafter, the dried wood is fed into the gasifier. Through the gasifer, the solid wood is converted into the gaseous fuel and fed to the diesel engine. Water vapor released Dried wood (with Engine exhaust reduced mositure content) Gasified gases Mechanical coupling Moist wood input (25 % moisture) Wood dryer Gasifier Engine Electricity Generator Electricity to the Grid Energy for the mechanical drying Gasification agent (Air gasification) The following information are available regarding the fuel as well as the equipment used in the system. 3

4 Fuel: Biomass (wood fuel) Physical properties Moisture Content before dryer = 25% Moisture content after dryer = 15% Fuel ultimate analysis Carbon (C) = 45 % wt (daf = dry ash free) Hydrogen (H) = 6.5 % wt (daf) Oxygen (O) = 48.5 % wt (daf) Fuel: Diesel Fuel properties Lower heating value= 43.5 MJ/kg Density = 840 kg/m3 Equipment Data: Electricity Generator Power output of the generator: 50 kwel Efficiency of the generator: 98% Diesel Engine Efficiency of the diesel engine: 30% (assume efficiency of the engine doesn t change when changing fuel type) Dryer Gasifier Specific electricity consumption for mechanical drying process =1.38 kwh / kg water content Gasifier Capacity: designed to serve the 50 kwel Gasifier Efficiency =48% Product gas LHVgas = 4.6 MJ/mn3 System operation: Capacity 8760 hours/year (at 100% capacity) Q1 Part 2- Retrofitting energy system Calculate the following information related to the system operation. d) Calculate the water content of the wood entering to the dryer as a percentage of mass (1 point) 4

5 e) Calculate the lower heating value (LHV) of moist wood after the dryer in MJ/kg (2 points) f) Calculate the amount of diesel required for running the engine with 100% diesel in litres/year (2 points) g) Calculate the yearly wood consumption before the gasifier if it replaces 90% of diesel in ton/year (2 points) h) Calculate the gas flow rate out of the gasifier in m n 3 /h (1 point) i) What would be the amount of water removed in the dryer (ton/year) (1 point) j) What would be the annual electrical power consumption of the drying process (kwh/year) (1 point) k) What is the percentage of electricity consumed for drying process? (%) (1 point) l) What alternative is available to consider for reducing the internal energy consumption of drying process (considering that the electrically driven mechanical drying is currently employed)? (1 point) Q2. Wind Power Question (10 points) Wind power system Wind energy is one of the most attractive renewable energy sources and is growing largely in the current global energy context. To establish a wind turbine at a specific location, several technical parameters are necessary to be determined. Considering such a situation, answer the following. a) For a given location, the wind speed at 10 m and 50 m heights above ground has been measured to be 4 m/s and 6.8 m/s respectively. Find the wind speed at 90 m height. (5 points) b) A wind turbine has a 1 MW rated power output at standard air density (15 ᵒC, 1 atm). Estimate the change of rated power if the turbine operates at a freezing temperature of -10 ᵒC.(3 points) c) Again for the 1MW turbine, estimate the change of the rated power relative to the sea level conditions, if the turbine is installed at 2000 m altitude above the sea. Use the below graph for determining the variation of air density with altitude.(2 points) (Take R = J/kg K) 5

6 Q3. Solar Energy (15 points) Problem Description: The Energy Department at KTH plans to reduce its carbon dioxide emissions by maximizing its energy efficiency. As part of the design, solar energy will be used to produce hot water. For hot water production of the Energy Department, a solar thermosyphon system has been purchased. The system has a collector for heating the water with a frontal area of 100m². The system has a water tank with 4000 liters of water. The collector is a single glass type with an average transmission τ G of 0.9. The absorption coefficient of the collector α A is 0.95, the U-value on the front is 4.5 W/(m² K) and on the back 0.5 W/(m² K). The collector can be considered perfectly insulated (adiabatic) at the sides. The collector efficiency factor F can be assumed as 0.94 and the mean ambient temperature as 18 C. For the calculation of the thermosyphon measurement data of the DNI (direct normal irradiation = beam radiation) form May 1st 2004 as shown in Figure 1 should be considered. Due to an error in the measurement system the clock time is unknown. Use your knowledge of the solar time to relate solar and clock time. In order to simplify the calculations this day can be can be divided into morning, noon and afternoon (sections I, II, and III) with mean irradiance levels of 300 W/m², 625 W/m², and 350 W/m², respectively. The noon section can be considered twice as long as the morning and evening sections (these two are equally long). 6

7 The solar collector of the thermosyphon system is placed due south with an inclination angle βc to harness the maximum amount of solar energy during an entire year. As the direct beam radiation is quite strong on this particular day the impact of the diffuse radiation can be neglected. Additionally, it can be assumed that the cosine effectiveness of the solar collector is constant during the day and equal to the cosine effectiveness of the collector at solar noon. Important assumptions: 1. There are no losses in the water tank and no usage of water either. The water temperature in the morning is the same as outdoor temperature 18 C. 2. After each section of the day the water in the tank is mixed to obtain a uniform temperature (the end temperature of each segment represents the starting temperature of the next one). 3. The mean fluid temperature in the collector can be assumed as equal to the arithmetic mean value of the start and end temperature of the water in the tank. Figure 1 Direct normal irradiance for Stockholm, May 1st,

8 Q3 Part 1 (Low temperature solar thermal and solar fundamentals) a) Sketch a thermosyphon system and explain its advantages and disadvantages. (1 point) b) How many hours of sunlight are registered on May 1 in Stockholm? What clock time does the solar noon correspond to and at which clock time does the sun rise and set, respectively. (3 points) c) What is the cosine effectiveness of the solar collector at solar noon? Therefore, it is necessary to calculate the solar elevation and azimuth angle at solar noon. (2 points) d) What will be the temperature of water in the tank at the end of section I, assuming a fully mixed tank? The heat capacity of water is 4200J/(kgK). (Hint: An analytic solution is preferred as the iterative solution requires at least 3 iteration steps) (2 points) e) Taking the fully mixed water temperature at the end of section I as starting condition (2 points), a. How long will it take until the water in the tank reaches a temperature of 60 C (assuming a fully mixed tank)? b. What will be the temperature of water in the tank at the end of section II, assuming a fully mixed tank? Q3 Part 2 - (Concentrating Solar Power Basics) a) What is Concentrating Solar Power? (3 lines maximum) (1point) b) Sketch the layout of a Parabolic Trough Concentrating Solar Power Plant with an indirect two-tank Thermal Energy Storage System, similar to the Andasol I Plant in Seville (1 point). Please identify the solar field, the thermal energy storage block and the power block together with the name of the main components in each of them (1 point). c) List one advantage and one disadvantage of solar tower CSP plants similar to Gemasolar when compared against parabolic trough CSP plants like Andasol. (1 point) d) In terms of the levelized cost of electricity, CSP is and will remain more costly than other renewable technologies. However, it is still expected that CSP will increase its share in the generation mix in the next future (by 2050). Please briefly explain why is this expected to occur? (3 lines maximum) (1 point) 8

9 NOTE: Stockholm coordinates: The latitude φ = N The longitude Ψ loc = E Q4. Hydropower (17 points) A hydropower plant is planned to be built using an old dam already in place and constructed for irrigation purposes. A 2m diameter concrete pipeline could be used as a penstock. The length of the penstock is 1100 m. the friction losses are 5% of the gross head. The gross head is 900 m. You are involved in a feasibility study as a consultant to establish an operation plan for the plant. For the project to be feasible the payback time should be less than 20 years. It is your task to evaluate if this will be the case. Assumptions: The electrical generator efficiency is 95% The plant operates 8760 h/year Parasitic losses are 1% Transmission losses are 1% Downtime losses are 4% A gas turbine using natural gas has a CO 2 emission factor of 56 kg/gj of fuel and an electrical efficiency of 40% The price of electricity is USD/kWh The capital cost of the turbine is COST= P 0.71 (H/0.3048) 0.42 where P is the net power output (MW) and H is the net head (in meters) The capital cost of the plant is twice the capital cost of the turbine The operational and maintenance cost of the plant is Cop = P 0.8 where P is the net power output (MW) Calculate the following. a) What is the design flow of the hydropower scheme (m 3 /s) (2 points) b) What is the net head of hydropower scheme (m) (1 point) c) What is the efficiency of the hydropower turbine (%) (3 points) d) What is the hydropower plant power output at design condition (MW) (2 points) e) What is the annual energy production (MWh/y) (2 points) f) What would be the annual revenue of the power plant in USD (2 points) g) What is the payback time (years) (2 points) h) What is the amount (annual) of CO2 emissions avoided by using the hydropower instead of fossil fuel plant i) What is your conclusion, is the project feasible? (1 point) 9

10 Q5. Geothermal Energy Question (3 points) A dry steam geothermal turbine operates at steam turbine inlet conditions, inlet pressure 6.5 bar, inlet temperature 163 C and steam flow rate of 20 kg/s. The turbine exhaust is directly sent to the atmosphere. The isentropic efficiency (Ƞis) of the turbine is known to be Data available: From steam property tables and Mollier diagram, enthalpy of steam at turbine inlet conditions (6.5 bar/163 C) is read as 2810 kj/kg and isentropic enthalpy at turbine exit is taken as 2490 kj/kg. a) Calculate the mechanical power produced by the turbine. Calculation Aid: h1 h2 η is = h1 h2s and, P = mass flow rate (kg/s) (h 1 -h 2 ) (kj/kg) Where, is η = the turbine isentropic efficiency h 1 = steam enthalpy at the turbine inlet (kj/kg) h 2 = steam enthalpy at the turbine exit (pressure=1.013 bar) (kj/kg) h 2s = isentropic steam enthalpy at turbine exit (kj/kg) P = Mechanical power output (MW) 10