2 Identify the ultimate energy source for the following types of renewable energy: wind power, geothermal power and biofuel.

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1 Problems and Solutions Chapter 0 The world average ecological footprint is about.7 global hectares per capita, while biocapacity is. global hectares per capita. xplain what this means in simple terms. cological footprint is a simple estimate of human demand on the biosphere, and on the biosphere s regenerative capacity and ability to absorb waste, while biocapacity is the amount of biologically productive land and sea available for human use. If the ecological footprint is greater than the biocapacity, then the population is living unsustainably. Identify the ultimate energy source for the following types of renewable energy: wind power, geothermal power and biofuel. Wind power is driven by the ; geothermal power is heat produced by the earth; and the energy available in biofuel is obtained from the. The Philippines, Indonesia, Japan, New Zealand, and Iceland together supply a large portion of the total geothermal electricity in the world, although they are not big with respect to land areas. Why do these countries have such large geothermal electricity generation capacity? The natural heat flux near tectonic plate boundaries is much higher than the average value and the countries listed are located near these global hot spots. 4 xplain why offshore wind farms are often preferable for a littoral city with a high population density. Onshore wind farms require large areas of land. Although they are compatible with some land uses, such as agriculture, they may create adverse aesthetic problems including noise and visual impacts, and may disrupt wildlife and birds. In contrast, offshore wind farms take advantage of steadier and stronger ocean winds and have lower noise and visual impacts. However, they cost more for construction and maintenance. 5 Compare the pros and cons of a dry steam geothermal power plant and a dry hot rock geothermal power plant. A dry steam power plant is the simplest and oldest type of geothermal power plant. It is very efficient, but requires dry steam at 50 C or more and is only feasible at a limited number of hydrothermal system sites. Dry hot rock power plants produce geothermal electricity from previously unusable sites, where the naturally occurring water and rock porosity is insufficient to carry heat to the surface. Although more expensive to construct

2 than a dry steam power plant, dry hot rock power plants provide a promising approach for geothermal electricity in areas without natural hydrothermal systems. 6 Coal and biomass both remove CO from the air when they are being formed, but biomass is considered to be a carbon neutral form of energy, while coal is not. xplain why. The process by which coal is created removes large quantities of CO from the atmosphere over long geological periods of time, while the combustion of coal releases most of this CO back into the atmosphere instantaneously. In contrast, the process by which biomass removes CO from the atmosphere is on the order of a few years, meaning that, over a period of decades, the amount removed is essentially equal to the amount released when the biomass is burned, making the overall process carbon neutral. 7 How can landfills be used to provide energy, and how does this contribute to sustainability? Landfills produce methane. The gas can be collected and used to generate power, rather than allowing it to escape to the atmosphere. The combustion of methane converts it into carbon dioxide, which has a global warming potential that is 5 times lower than methane. Thus, combustion of landfill gas simultaneously generates energy and reduces global warming potential. 8 A horizontal axis wind is installed on a flat rural area with average wind speed of 8 m/s (measured at 80 m height). The tower is 0 m high, with the nacelle on the top of the tower. The airfoils are 5 m long. The electricity generator has an efficiency of 0%. Ignoring energy loss due to friction, estimate the power of this wind (density of air is.7 kg/m ) V 0 m V 80 m 8 m / s 6.84 m/s k AV.7kg / m 5 m 6.84m / s 9900W input C k W 8545W output 0 % input W 4764 W 4.8kW 9 An electricity company is planning to use the wind s in problem 8 to build a wind farm in a rural area. When constructing a wind farm, wind s cannot be packed too

3 densely because the up-wind s will cast wind-shadows on the downwind ones. xperts recommend that wind s should not be spaced closer than five times their diameter (see figure below). At this spacing, calculate the power that the wind s can generate per unit land area. If wind s with different length of airfoils are used, how does the result change? What is your conclusion? Power per wind land area per wind output (5D). W/m In fact, Power per wind land area per wind C D 4 (5D) V C 00 V 0 % 0.59 (.7 kg/m ) ( m/s) =. W/m Therefore, the result is independent of D or the length of the airfoils. 0 It is known that the solar energy received by the arth is about W at the outer surface of the atmosphere. How much solar energy is received by Pluto? (diameter of the Sun is m; effective blackbody temperature of the is 5,800 K; Pluto s

4 diameter is,00 km; and the average distance from Pluto to the Sun is m). How does the solar energy received by Pluto compare to the solar energy received by the arth? A T D T W A 4 R m Solar Constant for Pluto A 0. 9 W / m A junior student drives three miles to campus every day (six miles/day considering roundtrip). The fuel economy for his car is about 0 mpg (local). After learning about the importance of environmental sustainability, he decides to take the bus to campus instead of driving. Assuming this student goes to campus 60 days/year and the bus he would like to take follows the same route as that when he drives, estimate the cological Footprint reduction due to this change in square meters. Here are some equations and constants you may need. Fossil nergy Land Required for Bus = (0000/7000) Distance Traveled per Year quivalence Factor for Different Land Types Correction Factor for the USA. Fossil nergy Land Required for Bus: square meters. Distance Traveled per year: kilometers. Built-up Land Required for Bus = 0.0 Distance Traveled per Year quivalence Factor for Different Land Types Correction Factor for the USA. Built-up Land Required for Bus: square meters Distance Traveled per year: kilometers Fossil nergy Land Required for Gasoline = (0,000/7,000) 5.5 Gasoline Use per Year quivalence Factor for Different Land Types Correction Factor for the USA. Fossil nergy Land Required for Gasoline: square meters Gasoline Use per Year: Liters Built-up Land Required for Gasoline = 0.66 Gasoline Use per Year quivalence Factor for Different Land Types Correction Factor for the USA. Built-up Land Required for Gasoline: square meters Gasoline Use per Year: Liters gallon=.785 Liters mile=.6 kilometers

5 Correction factor for the USA I) Fossil Transportation II) Arable III) IV) V) VI) Pasture Forest Built-up sea quivalence factor for different land types I) Fossil II) Arable III) Pasture IV) Forest V) Built-up VI) Sea Total distance traveling to campus: 6 miles/day 60 day/year = 960 miles = 56 km Gasoline needed = 960 miles/0 mpg = 48 gal = 8 L Fossil nergy Land for Bus = (0,000/7,000) = 57 m Built-up Land for Bus = = 8 m Fossil nergy Land for Gasoline = (0,000/7,000) =,600 m Built-up Land for Gasoline = = m So the reduction of cological Footprint due to the change is: (,600 + ) (57 + 8) =,580 m