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Energy Conversion Efficiency and Practical Consequences of Lower Efficiency for Renewable Resources

## Efficiency of Energy Conversion Sequences using Renewable Sources

1. Question #2 of Assignment 9 involved an electrical generation system including a generator with an efficiency of 97%, a transmission line with a 92% efficiency, and finally a light bulb with a 40% conversion efficiency. Suppose that instead of coming from coal, the primary energy for the same system is captured by water turbines that have a 93% efficiency, wind turbines that have a 44% efficiency, or solar cells with a 24% efficiency.

a) Does the full energy conversion sequence using any of these renewable sources make significantly more efficient use of the primary energy available? Explain why or why not

Generator- 97- 92 =40

Water turbines:

93-92=40

Wind turbines and solar cells.

44-92=40

b) For non-renewable sources of primary energy, a consequence of lower efficiency is that in order to provide a given amount of useful energy to customers much more primary energy, in the form of raw resources, has to be extracted from the ground. For example, to deliver 1000 MJ of electrical energy using coal requires an input of roughly 3000 MJ of primary energy in the form of coal. What is a similarly practical consequence of lower efficiency for renewable resources like water, wind and solar? In other words, what needs to be done in order to deliver 1000 MJ of electrical energy to customers using water, wind or solar facilities given unavoidable conversion losses? Does it matter? Explain.

2. You’ve stumbled across a box containing a radioisotope and you open it. As you stare at the visually unimpressive lump of material in the box about 40 cm from your face, it occurs to you that you could be causing serious harm to yourself. What specific information do you need to know about the material in the box in order to determine whether you’re safe or whether you need to seek medical attention? Explain

3. The Fukushima nuclear reactor accident caused the environmental release of significant amounts of radioactive caesium (Cs) isotopes. Specifically, caesium-134 and caesium137, which have half-lives of, respectively, two years and 30 years. The survey of soil radioactivity conducted in late May, 2011 revealed soil surface Cs-134 activity of about 4600 kBq/m2 in the town of Okuma (roughly 10 km from the stricken power plant) and Cs-137 activity of roughly 5000 kBq/m2 . Disregarding any other changes or cleanup efforts, what would you expect the radioactivity due to these caesium isotopes to be in Okuma today compared to their original levels? (Semi-quantitative or qualitative statements are sufficient.) Explain.

4. Your friend is terribly afraid of radiation, so naturally her wealthy and mischievous younger brother offers her a choice for her birthday: a day-long walking tour of Okuma (during which she may wear a mask), where the residual radiation dose (from the 2011 accident) averages roughly 4 micro-Sieverts (or 0.004 mSv) per hour above normal background, OR dinner in far-from-Fukushima Tokyo where fresh fish caught off the coast of Fukushima prefecture and rice grown near Okuma will be served. In the absence of any other information, which option would you advise your friend to choose, and why? (There is no secret third option...)

5. In your own words, explain why spent nuclear fuel needs continuous cooling for at least ten years after its removal from the core of a reactor. What is the source of ongoing heat?

6. Genetically-modified sugar beets are being investigated as a possible source crop for the production of ethanol biofuel. Some studies suggest that sugar beets could yield more ethanol per kilogram, and for a lower cost, compared to corn.

a) In your own words, explain the basic reason for which biofuels such as ethanol (and biodiesel) are renewable.

b) Under what conditions would ethanol produced from sugar beets be carbonneutral?

7. Explain why further advances in energy-storage technologies will be crucial for the continued expansion of utility-scale wind and solar photovoltaic power? List at least two possible energy storage options.

8. The technology now exists to produce reliable and efficient electric cars that do not require fossil fuels and that produce no carbon dioxide or other atmospheric emissions.

a) Explain one reason why the use of an electric car might not eliminate your transportation carbon footprint

b) Find out which Canadian provinces are most likely to see real reductions in carbon emissions as a result of the introduction of electric cars. (Cite your sources, as usual.) Remember from the course notes that coal emits at least twice as much carbon dioxide for the same amount of released thermal energy as does gasoline or natural gas. (HINT: How does each province produce its electricity?)