Alternative Energy Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 11

1. The term "cut in" refers to the lowest wind speed at which a wind turbine will reliably function. Most large-scale wind turbines need at least a 7- to 9-mi/h wind to start working. The correct answer to this question is B (8 mi/h).

2. Potential energy equals mass multiplied by height (or altitude), multiplied by the acceleration of gravity. We can express mass in kilograms, height in meters, and the acceleration of gravity in meters per second squared. When we multiply all of these units, we get kilogram meters squared per second squared. The correct choice is B.

3. In a wind of constant speed, the power that we can get from a small wind turbine with a certain number of blades (say, three of them) varies in proportion to the square of the blade length, which corresponds to the turning radius. Radius and diameter are directly proportional to each other; if we double the radius, we also double the diameter. If we increase the diameter of a wind turbine's blade system from 7 to 10 feet in a constant 20 mi/h wind, we can expect to get (10/7)2 times, or about twice, the power output. The correct choice is A.

4. A pumped-storage hydroelectric system gets its power as a result of water falling (in a controlled flow) from a reservoir at a certain elevation to a reservoir at a lower elevation. The average elevation of the two reservoirs must significantly differ in order for such a system to work. The correct choice is C.

5. An impoundment hydroelectric system employs a dam to create a large reservoir and an artificial waterfall (that's where the water goes down inside the dam from the reservoir to the river below). Such a system won't work in a lake of any size, because lake water doesn't flow, and a lake maintains a constant surface level throughout. Therefore, choice A is wrong. Choice B is also wrong, because we can't build up an effective reservoir on a flat plain. (We'll flood out a lot of people, that's all!) Impoundment systems have nothing to do with the ocean, so choice D is wrong as well. That leaves us with choice C, which is the right answer here. Hoover Dam near Boulder City, Nevada, and its large reservoir called Lake Mead, provide an excellent and classic example of an impoundment hydroelectric system. Again, the answer is C.

6. A diversion type hydroelectric system works by channeling water from a fast-moving river into a secondary network of pipes or ducts, and using water turbines to get power from the flow. The correct choice is A. Diversion systems aren't used in the ocean, and they won't work in a slow-moving river, so choices B, C, and D are all wrong.

7. Littoral currents are ocean currents that generally run parallel to the shoreline. These currents result from the same forces that produce the tides. The correct choice here is D. All the other three choices describe systems used primarily in rivers, not in the ocean.

8. A large wind turbine should maintain a constant, or nearly constant, rotational speed throughout its entire working wind-speed range. The only choice that describes that sort of situation here is B.

9. Any stand-alone alternative power system for residential homes must include storage batteries. That's true for hydroelectric, wind-power, or solar-power systems. The correct choice is B. Stand-alone systems by definition have nothing to do with the electric utility, so choices A and C (and by extension, D as well) are wrong.

10. Ocean currents at a given speed can yield more power than winds at the same speed because water has greater mass per unit volume than air has. None of the three choices A, B, or C state this fact, or even remotely imply it. Therefore, the answer is D, "None of the above."