Energy & Environment I AQ01

Final Exam

100 points

Closed book and notes

5-page personal “crib sheet” permitted

ANSWERS IN RED.

QUESTION #1 (13 points)

A. The world annually uses about 400 quads of primary energy. Of the several primary energies, which one has the greatest use?

ANSWER: OIL

B. What percentage of the 400 quads is used by the USA?

ANSWER: About 25%

C. Note the percentage of the world’s population that resides in the USA. On a per capita basis, how many times more energy does the USA consume than the whole world?

ANSWER: USA has almost 5% of the world’s 6 billion population.

25%/5% = 5. That is, per capita the USA uses about 5 times as much energy as the whole world.

D. The percentage contributions of the primary energies to total energy consumption are about the same for the USA as for the world. For example, nuclear energy accounts for about 7% of both the USA’s and the world’s total energy consumption. However, one of the primary energies has a significantly greater percentage contribution in the world than in the USA. What is it?

ANSWER: BIOMASS

E. For the USA, what percentage of total primary energy is used to generate electricity?

ANSWER: ABOUT 1/3 (or about 33%)

F. On average, what percentage of the primary energy used to generate electricity is received by the consumer as electrical energy?

ANSWER: ABOUT 1/3 (or about 33%)

G. For the USA, which of the fossil fuels is used the least to generate electricity?

ANSWER: OIL

QUESTION #2 (20 points)

This question is about thermodynamics and heat engines.

A. Two blocks, each of the same mass and the same material, initially have temperatures of 300K and 600K, respectively. The two blocks are brought into contact and transfer heat with one another (and only with one another). (Neglect any volume changes of the blocks.) Some possibilities (cases) for the final temperatures of the blocks are listed in the following table. By answering YES or NO, indicate whether of the possibilities satisfy the First and Second Laws of Thermodynamics.

	Temperature of Block #1	Temperature of Block #2	First Law Satisfied?	Second Law Satisfied?
Initial	600K	300K	xxxxxxxxx	xxxxxxxxxxxx
Case 1	700K	200K	YES	NO
Case 2	500K	400K	YES	YES
Case 3	450K	450K	YES	YES

B. Write the Carnot cycle efficiency equation:

h_CARNOT = 1 – T_REJECT/T_ADD

C. What does the Carnot cycle efficiency teach (or tell us) about the design of practical heat engines? That is, what guidance does it provide?

ANSWER: It tells us to try to design heat engines that add heat at as high of temperature as possible and reject heat at as low of temperature as possible.

D. All heat engines have efficiencies less than 100%. That is, it is impossible to design a heat engine for which the only processes are heat addition and work generation (with the work generated equal to the heat added). In terms of DISORDER (or chaos), explain why a heat engine must also REJECT heat:

ANSWER: The disorder or chaos of the universe cannot decrease. This is the second law of thermodynamics. That is, the operation of our heat engine cannot cause the disorder of the universe to decrease. When heat is pulled from a heat source into the engine, the disorder of the surroundings decreases. In order to add disorder back into the surroundings, the heat engine must reject heat to the heat sink.

E. In terms of the heat added (Q_ADD) and the heat rejected (Q_REJ), write the net work (W) of the heat engine and the efficiency (h).

W = Q_ADD - Q_REJ

h = 1 – Q_REJ/Q_ADD

F. For each of the heat engines in the following table, give the typical efficiency.

	Efficiency
Steam-Electric Power Plant	ABOUT 35%
Gas Turbine Engine	ABOUT 35%
Combined Cycle Power Plant	ABOUT 55% (new ones almost 60%)

QUESTION #3 (12 points)

The automotive gasoline engine, as routinely driven, has a low efficiency. Only about 20% of the chemical energy put into the engine as gasoline is converted into work at the output shaft of the engine. 20% is quite low compared to the efficiencies of other heat engines. New technologies that could improve the efficiency of the gasoline engine are stated below. For each, briefly state why the efficiency of the engine would improve.

A. From fuel science, a gasoline that does not “knock”.

ANSWER: If the fuel doesn’t knock, the compression ratio of the engine could be increased. It is well known that by increasing the compression ratio of a piston engine, the efficiency is improved (h = 1 – 1/r_c^g-1).

B. From chemical science, an exhaust emission control catalyst that can reduce NOx to N₂ when the engine is operating with excess air.

ANSWER: Current engines run at the chemically correct ratio, since only at this ratio are all three pollutants (CO, UHC, NOx) removed by the catalyst. With a new lean-NOx catalyst, the engine could run lean, that is, with excess air, all three pollutants would still be removed. It is well know that a lean (though not so lean that misfire occurs) gasoline engine is more efficient than a chemically correct engine.

C. From mechanical science, an engine that does not need to “throttle” the air.

ANSWER: By not restricting (throttling) the air flow (which decreases the pressure of the air entering the engine) the engine does not need to work “as hard” as an air pump. Thus, efficiency improves. More of the energy of the gasoline goes into pushing the pistons instead of pumping the air.

QUESTION #4 (7 points)

For application in Western Washington and Oregon, the residential heat pump has a coefficient of performance of about two. Thus, with respect to the energy delivered to the residence, the heat pump is about two and one-half times as efficient as the conventional natural gas furnace.

A. Define the coefficient of performance of the heat pump.

ANSWER: COP = Q_{to warm space}/W_{to run the compressor}

B. Because of its efficiency, it would appear beneficial to encourage the use of the heat pump. If this is done as part of a policy on efficient use of energy, what factors (maximum of three) should be considered regarding heat pump use and explored as part of the policy-making process?

FACTOR #1: Heat pumps have a high capital cost compared to gas furnaces and conventional electric heating appliances. Some homeowners may choose not to spend the money.

FACTOR #2: More electrical generating power plants might have to built to accommodate the increased use of electricity for heating (presuming the heat pumps are replacing gas furnaces in homes).

FACTOR #3: There is a significant natural gas infrastructure in place. That is, the use of natural gas for residential heating has “inertia”. It might be difficult to overcome this inertia.

QUESTION #5 (5 points)

Use a McKelvey diagram to show how the cost of extracting (and processing) a fossil fuel and the uncertainty of finding it distinguish the resources from the reserves.

McKelvey diagram

SHOW FIGURE 7.1 OF TEXT HERE

QUESTION #6 (20 points)

In earlier questions, the efficiencies of several heat engines were examined. This question deals with the air impacts of some heat engines.

A. Complete the following table. For each heat engine, list only one fuel – the predominant primary fossil fuel. For each pollutant, state only YES or NO, the heat engine is a significant source of the pollutant, and thus, it requires the significant application of the appropriate pollution abatement technology.

	FUEL	CO	NOx	SOx	Particulates
Steam-Electric Power-Plant	COAL	NO	YES	YES	YES
Combined Cycle Power-Plant	NAT GAS	NO	YES	NO	NO
Automotive Gasoline Engine	OIL	YES	YES	NO	NO
Diesel Engine	OIL	NO	YES		YES

B. The annual worldwide emission of carbon into the atmosphere from fossil fuel burning is about 5 Gte. [Note: 1 Gte = 10¹² kg.] Additionally, deforestation and wildland burning release significant carbon into the atmosphere, and one sink is predominant in removing CO₂ from the atmosphere.

Approximately, how many Gte’s of carbon are annually contributed by deforestation and wildland burning?

ANSWER: ABOUT 2 GteC

What is the major sink, and approximately how many Gte’s of (net) carbon does it annually absorb?

ANSWER: The Oceans, about 2 GteC

C. One large combined cycle power plant has a rated electrical power output of 250 MW. The heating value of the fuel is 45 MJ/kg. Assume the power plant runs at rated power 8000 hours per year. Calculate the annual emission of CO₂, as kg of C.

CALCULATION: (250 MJ/s)x(3600 s/hr) x (8000 hr/yr) x (0.75 fraction of C in methane by mass) / (45 MJ/kg) / (0.55 efficiency) = 218 million kg C per year.

D. Within a few years, the Pacific Northwest will have about 20 such combined cycle power plants operating. Calculate the percentage of the nation’s annual carbon emission these power plants will contribute.

CALCULATION: 218 million x 20 power plants = 4.4 billion kg C per yr.

USA emits about ¼ of the world’s C, or about 5/4 = 1.25 GteC per yr.

4.4 x 10⁹ kg C x 100 / 1.25 x 10¹² = 0.35%

E. It is interesting to compare electrical generation by large installations of wind turbines to the combined cycle power plant. The Stateline wind turbine project on the border of eastern Washington and Oregon is a good example of a large wind turbine system.

How many Stateline projects would it take to provide Seattle with all of its electricity? How many large combined cycle power plants would it take?

ANSWER: Seattle on average requires about 1200 MW of electricity. The Stateline wind turbine project, when fully built will have a rating of almost 300 MW. However, the wind doesn’t blow all the time (even in eastern WA). The average power output of Stateline will be about 100 MW. Thus, about 12 Statelines would be required to power Seattle. On the other hand, about 5 large combined cycle power plants could provide Seattle’s electricity.

Would the cost of the electricity from the wind turbines be competitive with that from the combined cycle power plants? Explain.

ANSWER: Wind turbine electricity is becoming competitive. Seattle City Light is going to pay 4.8 cents per kwh for Stateline electricity. Electricity from natural gas fired combined cycle power plants is about 3 to 3.5 cents per kwh.

QUESTION #7 (23 points)

This question is about nuclear fission reactor safety and the future of nuclear fission reactors for electrical generation.

A. Three Mile Island was a “loss-of-coolant” accident. In loss-of-coolant accidents there is a danger of two types of explosions. Name the two explosions and briefly explain how they occur.

EXPLOSION #1: Water boils creating high pressure steam. If the steam pressure becomes very large, the reactor vessel can become damaged, or breached, thereby permitting steam (and radioactive nuclides) to escape and allowing air to enter the reactor core.

EXPLOSION #2: If the temperature in the core reaches about 900 degrees C, the zirconium cladding of the fuel rods can react with steam forming hydrogen (Zr + 2H2O ® ZrO2 + 2H2). If air enters the reactor, an explosion between the hot H2 and the O2 in the air might occur.

B. Nuclear fission reactors are designed with negative void coefficients. Thus, if the water coolant should boil (in regions of the reactor core where it shouldn’t boil), the fission reactivity will decrease, since fewer hydrogen nuclei are available to thermalize the neutrons. However, some reactors (such as the Chernobyl reactor) have positive void coefficients. In these reactors, the fission reaction can increase if there is boiling (and loss) of the water coolant. In terms of a property of the hydrogen nucleus, explain how a positive void coefficient is possible?

ANSWER: H nuclei capture thermal neutrons. Thus, as the water boils away, decreasing the number of H’s present, the capture decreases, more thermal neutrons remain the reactor, increasing the fission reactivity.

Chernobyl was both a “loss-of-coolant” and a “criticality” accident. What is meant by criticality accident?

ANSWER: Criticality accident means the fission process becomes run-away, the fission process may become super-critical.

C. Even after the fission reaction is shut off (by fully inserting the control rods) a reactor core remains thermally hot. The core may melt unless the flow of coolant is maintained. What is the source of the continuing heat?

ANSWER: Radioactive decay of the unstable fission products.

What information about the isotopes in the reactor must the nuclear engineer know in order to predict how long the heat will persist? (Assume a colleague of the engineer has already estimated the amounts of the isotopes in the reactor.)

ANSWER: The decay half-lives of the unstable fission products in the reactor.

D. What levels of U-235 enrichment are used in nuclear reactors

ANSWER: From about 0.7% by mass (natural uranium) to about 3% U-235 in the U.

E. Explain how an electrical generation nuclear reactor fueled with uranium produces fissionable plutonium.

ANSWER: This happens as a consequence of the capture of fast and thermal neutrons by U-238. The U-238 becomes U-239, which fairly quickly decays by beta-emission to become Np-239. This decays by beta-emission within a few days to Pu-239, the fissionable isotope of plutonium.

Explain: does the reactor produce weapons grade plutonium?

ANSWER: No, Pu-240 is also formed in the reactor. This acts erratically in plutonium weapons, and must be removed.

Explain: Are all electrical generation nuclear reactors equal in ability to generate fissionable plutonium?

ANSWER: No, depending upon the level of enrichment of the uranium fuel, and on the type of moderator used, the amount of Pu-239 produced varies. Some reactors are simply “burners” while some, producing more Pu-239, are “converters”.

F. Currently, there are over 400 electrical generation nuclear fission reactors in the world. Over what period of time were many of these nuclear power stations put into service, and when are they expected to reach the end of their lifetimes?

ANSWER: Fission reactors for electricity generation mainly came on line in the 1970-90 period. With normal lifetimes of 40 years, and extended lifetimes of 60 years, these reactors will be retiring in the 2010 to 2050 time-frame.

G. Your generation will need to decide on whether societies should build a new round of reactors and continue the dependency on nuclear fission power plants. What three points regarding the nuclear fuel cycle do you believe are crucial to this decision, and should be fully aired and discussed?

POINT #1: Safety of the reactor – will they be “inherently” safe?

POINT #2: The spent fuel (ie, the reactor waste). Reprocessing can open up concerns about the proliferation of plutonium. Long-term disposal is still not reality – the waste must be stored for thousands of years.

POINT #3: How expensive will the reactors be? Cost is of significant concern given the escalation of costs that occurred when the present generation of reactors was built. Will nuclear need a substantial tax-payer subsidy, or will it be able to economically compete on its own against the improving fossil fuel and renewable energy technologies?