University of Strathclyde Faculty of Engineering

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1 University of Strathclyde Faculty of Engineering Energy Systems and the Environment: Part A Examination Monday 19 January , M329 Full-time students should attempt FOUR questions, 1 from each theme or you may omit one of themes 1, 3 or 4 and instead attempt an additional question from theme 2. Exam duration is 3 hours. Part-time students should attempt ONE question from EACH theme studied. Exam duration 45 minutes per question. Exchange students should attempt TWO question from EACH theme studied. Exam duration 45 minutes per question. Each question is worth 25 marks in total, with the marks for individual parts as shown. Statement on calculator use: while you may use any calculator type in this examination, you may not store formulae or text within its memory. The invigilator has the right to randomly delete the calculator s memory contents. Any communication capability must be disabled. Theme 1: Energy Resources and Policy 1.1(a) The power, P, dev eloped by a marine current turbine is given by the equation P = 0. 5C p ρπ R 2 V 3 and the tip speed ratio is defined as λ = ω R/V, where ω is the angular velocity of the rotor in radians/second. Identify the other quantities in these equations. (5) ES&E: Part A, 2003 Please turn over... Page 1 of 6

2 (c) For a turbine of diameter 22m, the characteristic curve of C p against λ is shown in Figure Q1.1. The turbine is intended to rotate at a steady speed of 6 rev/min in current speeds up to 3.5 m/s. Investigate the way in which the power, P, varies as the current speed rises towards maximum, and comment briefly on the significance of the results obtained. (12) The turbine will be stopped when the power output falls below 50 kw. Use a trial and error or graphical method to estimate the current speed at which this takes place. (8) 1.2(a) A monocrystalline photovoltaic (PV) module has the following characteristics at Standard Test Conditions (T module = 25 C and G = 1000W/m 2 ): V OC = V I SC = 5A V max = 18V I max = 4. 72A (i) Calculate the rated power at STC and fill factor. (ii) If the above module has a power drop-off due to temperature = 0.34 W/ C from STC, calculate the %/ C power reduction the module would experience. (iii) Using the value calculated in ii), establish the daily electrical energy supplied from a 2500 W module PV generating station if the mean daily irradiance is 230 W/m 2 and the mean daily cell temperature is 43 C. Assume: P G = 2 (G/1000). (15) 1.2 Describe what impact an increase in irradiance and cell temperature is likely to have on the voltage, current and power outputs from a crystalline photovoltaic system and discuss what measures can be taken to maximise the power output from a PV system when used in building installations. (10) Theme 2: Energy Systems Analysis 2.1 (a) Sketch typical fluid temperature variations along a counter-flow heat exchanger and write expressions for the following terms: (i) logarithmic mean temperature difference, (ii) the effectiveness. (5) A shell-and-tube heat exchanger with one shell pass and two tube passes is to be used to condense 1.8 kg/s of saturated steam at 1.2 bar to saturated liquid. Condensation occurs on the outer surface of the tubes and the corresponding convection heat transfer coefficient is 10,350 W/m 2 K. Cooling water flows through the tubes. Each tube pass consists of 50 thin-walled tubes of 25mm ES&E: Part A, 2003 Please turn over... Page 2 of 6

3 internal diameter. The cooling water is supplied to the exchanger at 17 C and is to leave it at 33 C. Using the tables provided and the data given below, determine: (i) the heat transfer coefficient for flow inside the tubes, (ii) the overall heat transfer coefficient, (iii) the required tube length per pass. (20) For flow inside the tubes, Nu = Re 0.8 Pr The layout of a gas turbine plant is shown in Figure Q2.2. Two stages of compression are used, with intercooling between the compressors. There are two stages of expansion, with a reheat combustion chamber between the turbines. The HP turbine drives the HP compressor. The LP turbine drives the LP compressor and provides the power output from the plant. Air enters the LP compressor at 1bar and 20 C. The intercooling between the compressors is perfect and the pressure at entry to the HP turbine is 9 bar. The temperature at entry to both turbines is 650 C. The pressure ratios of both compressors are equal. The isentropic efficiencies of the compressors and turbines are 0.8 and 0.85 respectively. Calculate (i) the pressure at entry to the LP turbine, (12) (ii) the power output from the plant for a flow rate of 120 kg/s, (8) (iii) the thermal efficiency and work ratio of the plant. (5) Assume air is the working fluid throughout, for which γ = 1. 4 and C p = kj/kgk. 2.3 In an ammonia refrigeration system, one evaporator provides a refrigeration capacity of 180 kw at a temperature of -10 C. A second evaporator provides a refrigeration capacity of 200kW at a temperature of 10 C. The system uses two compression stages with flash intercooling and is arranged as shown in Figure Q2.3. The discharge pressure of the LP compressor and the suction pressure of the HP compressor are the same as the pressure in the 10 C evaporator. The ammonia leaves both evaporators as dry saturated vapour. The condensing pressure is 1500 kn/m 2 and the ammonia leaving the condenser is subcooled to 30 C. Both compressions can be taken as isentropic. (i) Draw the cycle on the pressure-enthalpy chart provided and evaluate the enthalpy value for each state point. (6) Also determine: (ii) the mass flow rate through each compressor, (10) (iii) the coefficient of performance of the plant, (6) ES&E: Part A, 2003 Please turn over... Page 3 of 6

4 (iv) the swept volume in m 3 /min of the HP compressor if the volumetric efficiency is (3) 2.4(a) Air at a barometric pressure of 1.04 bar has a dew point temperature of 8 C and a thermodynamic wet bulb temperature of 16 C. Using the tables provided, calculate: (i) the dry bulb temperature, (ii) the degree of saturation, (iii) the specific volume. (10) θ * = θ h* fg C pas (g * s g) (c) Define the term Lewis Number and explain its significance in the correlation of the measured psychrometer wet bulb temperature with the temperature of adiabatic saturation. (5) A heating panel 3m wide by 1m high is fitted to the wall of a large room in which the air and surfaces are at 20 C. The surface of the panel is maintained at 86 C and its emissivity is 0.8. Using the tables provided and the information given below, calculate the rate of heat emission from the panel. (10) For convection, Nu = 0. 13(GrPr) 0.33 ; Gr = β g( T )l 3 ρ 2 /µ 2 For radiation, σ = W/m 2 K 4 and Q = Theme 3: Electricity Generation and Supply σ A 1 T 1 4 T2 4 1 ε A 1 1 ε 2 ε 1 F 12 A 2 ε 2 3.1(a) Electrical power systems constitute the largest man-made structures and supply electrical energy as a dynamic product, i.e. the electrical generation at any instant must equal the electrical demand at the same instant to maintain a stable electrical supply. Conventional large scale electrical generation is facilitated through the use of AC synchronous machines operating as generators, as these machines offer distinct advantages for providing a fixed voltage, fixed frequency electrical supply. Briefly discuss the main features of a synchronous generator that enables a fixed voltage, fixed frequency electrical supply to be easily generated and controlled. (3) ES&E: Part A, 2003 Please turn over... Page 4 of 6

5 A 3 phase, 200 MVA, 20 kv, two pole, 50 Hz, star connected synchronous generator has negligible stator winding resistance and a synchronous reactance of 0.55 Ω/phase at rated terminal voltage. (i) Calculate the synchronous speed of this generator. (1) (ii) Determine the excitation voltage and rotor power angle when the machine is delivering rated MVA at 0.95pf lagging; draw the associated phasor diagram. (5) (iii) If the excitation voltage is now increased by 20% (without changing the prime mover power), find the stator current, power factor and reactive MVA supplied by this machine. (5) (iv) If this generator were now to be operated at its stability limit (steady state or static stability limit), calculate the new value of excitation voltage required to maintain its real power output as in part ii) above. (4) v) Explain why the situation described in iv) above, could not be permitted to occur. (1) (c) Derive a simple expression for calculating the present worth of a future sum of money incorporating the interest (discount) rate and period of time and clearly state the present worth factor. Use this expression to calculate the net present value for the following power project, Table 3.1c, using an interest rate of 8%. (6) Table 3.1c Year Annual Costs ( k) Annual Revenues ( k) (a) Explain, using sketches if necessary, the operation of a circulating current unit protection scheme. Show how the relay remains stable for external fault conditions. (5) Substation A is connected to an 11 kv radial feeder supplying two substations, B and C. At C, there is a sensitive load which requires the protection to isolate a fault in 0.25s. At A, B and C, standard overcurrent protection relays with a rating of 5 A are used. The CT ratios are 400/5 at A and 300/5 at B and C. Load current at C does not exceed 200A. The fault levels at A, B, and C are 225 MVA, 215MVA and 180MVA. Determine the plug setting and time multiplier settings for the relays at A, B, and C, assuming a time margin for discrimination of 0.5s. No plug setting multiplier should exceed 20, the available plug settings are between 50% and ES&E: Part A, 2003 Please turn over... Page 5 of 6

6 200% in 25% steps and the time multiplier is variable between 0.1 to 1.0 in steps of (20) Theme 4: Energy/ Environment Modelling and Monitoring 4.1(a) (c) Describe the issues confronting the first time user of advanced energy simulation programs. (7) Give your opinion, with reasons, of how user training could best be achieved. (7) Assume you are working for an engineering consultancy and are requested to undertake a modelling study to evaluate energy-related options for a new hotel. Describe firstly the information you would need in order to develop the model(s) and, secondly, the modelling strategy you would adopt to determine the energy consumption of the various design options. (11) 4.2(a) Assume you are working for a Regional Council and have been asked to produce a discussion report on the setting up of a regional fuel monitoring and targeting system for all the properties owned by the council (schools, council offices etc). Outline the main points regarding the merits of such a system and the problems that are likely to be encountered. Include in your answer an account of the problems associated with: (i) data capture and quality assurance; (ii) data analysis to assess the benefits of energy saving measures. (13) Assume that you have been asked to give general advice to the community council in a small village in a rural location in Scotland. They are interested in increasing the amount of renewable energy that the community uses. Outline how you would approach the task in terms of the information you would need and the analyses you would undertake. (12) END OF PAPER ES&E: Part A, 2003 Please turn over... Page 6 of 6

7 ES&E: Part A, 2004 Figure Q1.1

8 Figure Q2.2 Figure Q2.3 ES&E: Part A, 2004

9 ES&E: Part A, 2004 Pressure-Enthalpy Chart