21

New Renewable Energy Alternatives

Answers to End-of-Chapter Questions

Testing Your Comprehension

  1. Renewable sources account for 12.7% of the world’s primary energy supply. Of that 12.7%, fuelwood is most prevalent, but hydropower produces nearly 90% of renewable electricity generation.
  2. The renewable market share is expanding because of fear of diminishing fossil fuel supplies, because of technological advances, and because of growing demand. Wind energy is growing the fastest, at 48% per year over the past three decades.
  3. Passive solar heating works by designing buildings to maximize their direct absorption of sunlight in winter while keeping the interior cool during the summer. In an active solar heating system, the sun’s energy is trapped and stored in a form that can be easily circulated throughout the house (e.g., electricity from solar photovoltaic panels).
  4. The photoelectric effect occurs when light strikes specially made metal plates, causing the release of electrons that can be channeled into a current or stored in batteries. Photovoltaic cells are made primarily of the semiconductor silicon, enriched on one side with boron, and on the other with phosphorus. These PV cells are often mounted together on rooftops to generate electricity for that building.
  5. Solar power is practically inexhaustible, requires no fuel, is quiet and safe, generates no pollution or emissions during operation, and promotes local, decentralized control over power. Its disadvantages are that not all regions are sunny enough, and the up-front costs of installing PV systems are still high.
  6. Wind blowing through the turbines turns the rotor blades, which rotate the gears in the gearbox, which transmits the rotation to an electrical generator. Wind speed increases the energy density and flux of the system. The energy density increases with the square of the wind velocity, and as the wind speed increases there is a linear increase in the volume of air moving past the turbines. Wind turbines are erected in locations where the topography and climate create strong, steady, and dependable winds.
  7. Wind power produces no emissions during operation, it is more energy efficient than conventional sources, and it also promotes local self-sufficiency. It does, however, require significant up-front costs for erection of turbines and transmission lines; it is an intermittent resource, varying in both time and space; some find turbines to be an eyesore; and the rotor blades may pose a threat to birds and bats.
  8. Geothermal energy is derived from the interior heat of Earth. The steam pressure of naturally heated water is used to turn turbines and generate electricity. It is renewable in the sense that the Earth generates interior heat faster than we use it. It is not renewable, however, when the water use locally exceeds the recharge rate of the aquifer, or on those occasions when geological hotspots move.
  9. Kinetic energy can be obtained from the tides, currents, and ocean waves, using them ultimately to turn turbines to generate electricity. Thermal energy from the water itself can also be used directly in an experimental, low-pressure system to generate steam to turn turbines.
  10. Hydrogen fuel (H2) is made from water or methane. Splitting water requires an input of electrical energy, which may have some pollution associated with its generation. Oxidizing methane produces carbon dioxide as a by-product.

Interpreting Graphs and Data

  1. Approximately 30 m2of photovoltaic cells would be needed to supply one person’s residential electrical needs for a year. If the efficiency was improved, approximately 7.5 m2would be needed.
  2. One person: approximately 20 m2 in April and 22 m2 in July. Four people: approximately 80 m2 in April and 88 m2 in July.
  3. 9¢ per kWh × 370 kWh/month = $33.30 per month. $20,000 divided by $33.30 per month = just over 600 months or 50 years. The 10-year payoff could be achieved at 9¢ per kWh if the system cost only $4,000, or a $20,000 system would pay for itself in 10 years if the cost of electricity were 45¢ per kWh. Other answers at moderate costs and electrical prices are also possible.

Calculating Ecological Footprints

Area of photovoltaic cells / Cost of photovoltaic cells
You / 25 / $20,000
Your class / Answers will vary / Answers will vary
Your state / Answers will vary / Answers will vary
United States / 7.5 × 109 m2 or 7500 km2 / $6 × 1012
  1. One would need to know the population distribution in order to estimate the weighted average for the incident solar radiation on the population.
  2. The Southwest clearly has the greatest amount of incident solar radiation per meter squared. It is, however, an area of generally low population density. The cost of equipping the entire United States is approximately equal to the annual federal budget, so it seems unlikely that the entire nation will move to solar power all at once.
  3. The system will eventually pay for itself in terms of avoided electricity costs. PV systems do not generate any greenhouse gases or cause other pollution during operation, and so those costs are avoided as well, even though they are hard to quantify.