California State Polytechnic University, Pomona



CHAPTER 4

RENEWABLE ENERGY SOURCES I: SOLAR ENERGY

Questions and Problems

1. BONUS Why are we still designing so many new homes with little regard for passive solar heating?

2. Skip, multiple choice #5

3. Skip

4. BONUS With the abundance of solar energy incident on our nation, what are the technical, economic, or political factors that must be overcome to break our reliance on fossil fuels?

5. Skip

6. BONUS What are some steps that could be taken by an individual or a family to lessen their dependence on nonrenewable energy?

7. Design an array of individual solar cells that would produce 120 volts DC. About how large an area of solar cells would be needed to produce 2 kWe of output power at noon on a clear day?

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8. BONUS What collector area is needed for the system shown in Figure 4.6to provide the necessary space heating for a home having 2000 ft2 of floor area? Assume an average daily insolation of 1000 Btu/ft2 and an efficiency of 50%. Also assume that the house requires 50 million Btu/1000 ft2 of floor area for the 180-day heating season.

9. A car wash needs 1200 gallons of warm water a day heated from 50°F to 100°F. How large a solar collector would be needed to do this? The incident solar energy is 1100 Btu/ft2 each day and the collector efficiency is 50%.

10. A passive solar home has energy stored in a concrete floor of 1,000 ft2 area. How thick should this floor be to store 200,000 Btu with a temperature swing of 20 °F?

Multiple Choice Questions

1. Skip

2. The solar spectrum has its maximum intensity in terms of watts per square centimeter at a wavelength corresponding to _____

a) the maximum of the ozone absorption spectrum

b) the ultraviolet region

c) the visible region

d) a wavelength of about 10−7 meters

3. The spectrum of solar radiation above the earth's atmosphere has its maximum intensity in the ___

a) ultraviolet region

b) region of 100×1006 meters

c) visible region

d) infrared region

4. Representative wavelengths for ultraviolet and infrared radiation, respectively, are _____

a) 0.1 and 1 microns

b) 1 and 10 microns

c) 10 and 100 microns

d) 100 and 1000 microns

5. BONUS You can easily feel the heat from the sun through a glass window, but behind a sheet of glass you do not feel the heat radiated from a fire in a fireplace. This is because _____

a) glass is transparent to visible light and opaque to infrared

b) the fireplace emits essentially pure ultraviolet radiation

c) glass is opaque to visible light and transparent to infrared

d) the sunlight is extremely infrared

6. BONUS Of the solar radiation incident on the earth's upper atmosphere, about _____, on average, is available at ground level.

a) 1/8

b) 1/4

c) 1/2

d) 100%

7. Your house foundation and basement floor are made up of 1000 ft3 of concrete. About how many Btu are required to raise the temperature of this mass from 0°C to 40°C?

Given: 22 Btu/ft3⋅°F for concrete.

a) 1,600,000

b) 900,000

c) 22,000

d) 40,000

e) 1,800

f) 40

8. BONUS A flat-plate solar collector has a useful upper temperature limit of about _____°C

a) 2

b) 10

c) 20

d) 100

e) 200

f) 1000

9. Skip

10. In Boulder (latitude 40° N), a flat-plate collector should be tipped at an angle from/above the horizontal of about _____ in order to maximize the annual direct plus diffuse sunlight that it collects.

a) zero degrees (flat on the ground)

b) 90 degrees (upright)

c) 55 degrees

d) 15 degrees

e) If find better, post here _____

11. At 40° N latitude a south-facing vertical surface, such as a window, has incident on it on a clear winter day roughly _____

a) 157 Btu/m2

b) 200 Btu/cm2

c) 20 cal/min⋅cm2

d) 20 Btu/m2

e) 1600 Btu/in2

f) 1600 Btu/ft2

g) 1600 Btu/m2

h) 1600 Btu/cm2

12. Skip

13. A parabolic reflector _____

a) can be used for solar collection systems only if the overall efficiency is less than 5%

b) focuses both the direct and diffuse components of sunlight

c) focuses only the diffuse component of sunlight

d) focuses only the direct component of sunlight

14. Good photovoltaic cells have an efficiency of approximately _____

a) 28 to 38%

b) 14 to 17%

c) 2 to 6%

d) 38 to 58%

15. BONUS It has not been practical to enhance the performance of silicon photovoltaic cells by using lenses with high concentration ratios because ___

a) the extreme magnification makes the sun's image too large to match the surface of the solar cell

b) the Carnot efficiency cannot exceed the maximum

c) the solar cell efficiency is reduced at the resulting high temperatures

d) the lenses are more expensive per square centimeter than are the silicon solar cells

16. Only about _____ of the power in the solar spectrum is in a wavelength region to which silicon photovoltaic cells are sensitive.

a) 47%

b) 57%

c) 67%

d) 77%

e) none of the above

17. The reason that photovoltaic cells are not generally competitive with fossil fuel–fired power plants is _____

a) the cost of raw silicon is too high

b) the expense of growing crystals and fabricating the cells is too high

c) the efficiency is only 2%

d) they cover too much land area

1 2C 3C 4A 5

6 C, (B -2%...online sources state 30%, book is 47%)

7A 8D 9

10 C (35 is better)

11 A. From Table 4.2, 1646 Btu/ft2

12 13D 14A (2/3 of class choose B, -2%)

15C 16D 17B

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