Energy and Environment II

HW #1

Due Friday, January 11, 2002

Suggested Answers in Red
  1. Figure 1.14 in the text is very important. It states that the average solar power striking the earth’s outer atmosphere is 173,000 TW (where TW = terawatt = 1012 watts). Using your knowledge of the sun’s diameter and average surface temperature, the distance from the sun to the earth, and the diameter of the earth, confirm the 173,000 TW value. Clearly show your calculation (note that sloppy, poorly organized calculations will not be graded). Not considering changes in the sun, by how much does the solar power striking the earth change over the year? When is it greatest, and when is it the least?

In class we learned the following:

1367 w/m2.

Multiple this value by the disk area of the earth, and one has the solar power striking the earth on average at any instant: pde2/4 x 1367 = 174,700 x 1012 watts, which is close to the text value of 173,000 TW.

Because of the eccentricity of the earth’s orbit, the solar power striking the earth changes by ± 3.5% over the year. It is greatest in late December, 3.5% greater than the average, and it is least in late June, 3.5% less than average.

  1. The caption of Figure 1.14 tells us that average primary power consumption by humankind is 13 TW. Calculate the annual primary energy consumption by humankind as EJ (where EJ = exajoule = 1018 joules). Also calculate the number of hours of solar power striking the earth required to equal the total annual primary energy consumption of humankind. According to the text, what are the main primary energies used by humankind? Do any of these meet our definition of renewable? If yes, which ones?

    2. Annual energy used by humankind = 13 x 1012 J/s x (3600 x 24 x 365) s/yr = 4.1 x 1020 J = 410 x 1018 J = 410 exajoules.

    Solving 173,000 x 1012 J/s x 3600 s/hr x NUMBER OF HRS = 4.1 x 1020 J gives 0.66 hr.

    According to Figure 1.1 in the text, the main primary energies used by humankind are: oil, coal, natural gas, biomass, hydro (hydroelectric), and nuclear (nuclear electric). Two of these are renewable: biomass and hydro.
  2. Based on Figure 1.14, how many TWs of long wave radiation (also termed infrared or thermal radiation) must leave the earth (and its atmosphere) for space.

    3. According to Figure 1.14 in the text, 173,000 TW of solar power strikes the earth and 52,000 TW is reflected back into space, leaving a net of 121,000 TW of solar power absorbed by the earth and its atmosphere. For the earth to be in steady-state with its surroundings, an equal power must leave the earth and its atmosphere. That is, 121,000 TW of long wavelength (or thermal or infrared) radiation must leave the earth at each instant.
  3. Which of the renewable energies discussed on pages 29-31 of the text are derived from sunlight, and which are not. For each of those not derived from the sun, what is the source of the energy.

Derived from the sun – solar direct or indirect:

Not derived from the sun:
  1. Which of the following do you believe is most important for the development of renewable energy? Pick only one, and then discuss your choice, providing rationale for your choice. Your discussion must be typed (two pages double spaced maximum). Your grade on this question will be based on the strength and clarity of your discussion – not on the choice picked.
    1. The finiteness of fossil fuels.
    2. Global climate change.
    3. Diversity in energy for national security.

Individual choice and discussion.