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Energy Resources 100 - Fall 2006 - Midterm

# Energy Resources 100 - Fall 2006 - Midterm - ER 100 Fall...

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ER 100, Fall 2006 Solutions to Midterm Exam General comment: This was a challenging exam and most people could have used more time. If you ran out of time, we tried to give you at least partial credit if you were on the right track. Part I. Short Answers [6 questions. Suggested time: 15 minutes] Warm-up and Unit Conversions (7 points) 1. You run your 1200 W microwave for 6 minutes. How much energy did you just use, in kWh? (2 pts) 1200 W " 0.1 h = 120 Wh = 0.12 kWh _ 2. Suppose everyone in the Bay Area (pop. 5 × 10 6 ) simultaneously turned on their microwaves at the same time you did. How many additional 1-GW power plants would be needed to meet this surge in demand? (2 pts) 1200 W " 5 # 10 6 = 6.0 # 10 9 W = 6 1-GW power plants _ 3. A light bulb with a power input of 60 watts emits waste heat at the rate of 200 kJ per hour (in addition to light). What is the first-law efficiency of the light bulb? (3 pts) Power input = 60 W 60 W " 3600 J / h 1 W = 216,000 J/h = 216 kJ / h Heat output = 200 kJ / h. Therefore, we can deduce that light output = (216 – 200 kJ / h) = 16 kJ / h. The first law efficiency is η I = 16 kJ / h 216 kJ / h = 7.41% = 7%. _ Short Answer Questions—#4,5: one or two sentences; #6: three or four sentences (7 points) 4. Is a coal-fired power plant in Minnesota more efficient in February or August? Why? (2 pts) The plant will be more efficient in February, because Carnot efficiency increases with the difference between T low and T high . In the winter, T low (the temperature of the cold sink) will be lower and therefore η C will be higher. 5. Explain why a combined cycle power plant increases the overall efficiency of a plant (refer directly to the thermodynamics of the plant in your answer). (2 pts) A combined cycle plant takes the output of a Brayton (gas) cycle and uses it as the input of a Rankine (steam) cycle. The steam cycle is able to use heat energy that is too low-quality for

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ER 100 Fall 2006 Midterm solutions Page 2 of 6 the gas cycle to capture ; without the steam cycle, the heat energy would simply be wasted and therefore the system would be less efficient. 6. What is Friedman’s key hypothesis in the article “The First Law of Petro-Politics?” Provide a concise critique of his argument—what connections are oversimplified or missing? (4 pts) The First Law of Petro-Politics is: when oil prices rise , political and economic freedoms in oil-producing states decline . Conversely, when oil prices fall, freedoms increase. (This is different than simply saying that oil-rich countries have low levels of freedom.) Many critiques could be made of Friedman’s argument. Rather than providing a comprehensive analysis of the relationship between oil prices and levels of freedom, he instead focuses on several case studies, and assures us that the other data are similar. Friedman also omits any discussion of the long-term development history of “petrolist” states. For example, there is evidence that foreign interests have played a key role in hampering the democratization of oil-rich states. To blame all failures of democracy and freedom on current money-hungry regimes ignores the complexity of history.
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Energy Resources 100 - Fall 2006 - Midterm - ER 100 Fall...

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