Question

1. Question #2 of Assignment 9 involved an electrical generation system including a generator with an efficiency of 98%, a transmission line with a 94% efficiency, and finally a light bulb with a 12% conversion efficiency. Suppose that instead of coal, the primary energy for the same system is provided by wind turbines that have a 45% efficiency, or solar cells with a 23% efficiency.

   a) Does the full energy conversion sequence using these renewable sources make significantly more efficient use of the primary energy available? Explain why or why not. [2 Marks]. b) For the fossil fuels, a consequence of low efficiency is that to provide a given amount of energy to customers much more 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 practical consequence of lower efficiency for renewable resources like wind and solar? In other words, how might 1000 MJ of electrical energy be delivered to customers using wind or solar facilities? Explain. [2 Marks]

3. The Fukushima nuclear reactor accident caused the environmental release of significant amounts of radioactive caesium (Cs) isotopes. Specifically, caesium-134 and caesium- 137, which have half-lives of, respectively, two years and 30 years. The survey of soil radioactivity conducted in June 2011 revealed soil surface Cs-134 activity of about 500,000 Bq/m2 in the town of Okuma (roughly 10 km from the stricken power plant) and Cs-137 activity of roughly the same level. Disregarding any other changes or cleanup efforts, what would you expect the radioactivity due to these caesium iso- topes to be in Okuma today compared to their original levels? (Semi-quantitative or qualitative statements are sufficient.) Explain. [2 Marks]

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...)

Answer 1

Answer 2:

Replace 700 GW of baseload coal-generated electricity with 2000 GWp of wind-generated electricity.

This requires the construction of "2 million 1-MW-peak windmills (50 times the current capacity) "occupying" 30 million hectares, on land or offshore". The difference in energy generation (addition of 2000 GWp wind vs. reduction of 700 GW of coal) arises from an accounting for the intermittent nature of wind power.

Wind turbines convert the kinetic energy of moving air masses (i.e. wind) into mechanical energy. This mechanical energy is then used to rotate an electricity-generating turbine. Modern wind turbines usually are composed of three long, narrow blades that are directly connected to a horizontally-mounted rotating electric generator. The largest functioning wind turbine in the world currently has a rotor diameter of 126 meters and generates over 7 megawatts of electricity under ideal wind conditions.3At that generation capacity, the expected energy output is roughly 20 million kilowatt hours per year - enough energy to power 1776 average American households.   

Answer 3:The Fukushima Daiichi nuclear power plant (DNPP) accident caused massive releases of radioactivity into the environment. The released highly volatile fission products, such as 129mTe, 131I, 134Cs, 136Cs and 137Cs were found to be widely distributed in Fukushima and its adjacent prefectures in eastern Japan. However, the release of non-volatile actinides, in particular, Pu isotopes remains uncertain almost one year after the accident. Here we report the isotopic evidence for the release of Pu into the atmosphere and deposition on the ground in northwest and south of the Fukushima DNPP in the 20–30 km zones. The high activity ratio of 241Pu/239+240Pu (> 100) from the Fukushima DNPP accident highlights the need for long-term 241Pu dose assessment, and the ingrowth of 241Am. The results are important for the estimation of reactor damage and have significant implication in the strategy of decontamination.