Deck 9: Electricity From Solar Energy

Question 1

What is the maximum wavelength of the light that produces voltage in a CdS
photovoltaic device? Discuss this in terms of the spectrum of sunlight as presented in
Chapter 8.

Answer 1

From equation (9.3)

Question 2

For a photovoltaic efficiency of 20% and an average peak insolation of 350
W/m2, calculate the additional number of km2 of solar collectors that would need to be
installed in the United States to equal the per-capita photovoltaic capacity of Germany.

Answer 2

From the table the installed solar photovoltaic capacity in the Unitted States is

Question 3

If the focal area in Example 9.1 is made of a material with an emissivity of
0.9 and this loses heat only by radiation from the 5 cm diameter area, calculate its
equilibrium temperature

Answer 3

The area of the dish is

Question 4

Assume that the portable solar panel shown in Figure 9.25 has an efficiency

Question 5

Consider a typical family vehicle powered by photovoltaics similar to the
vehicle shown in Figure 9.27 where the top surface is covered with 20% efficient cells.
Compare the power available in such a vehicle on a sunny day in comparison with a
typical gasoline-powered vehicle. Some typical vehicle characteristics are given in Table
19.3.

Question 6

A photovoltaic panel with an efficiency of 19% receives an average

Question 7

Water flows through a 5 cm diameter pipe at the focus of a 2 m wide
parabolic trough reflector that is 5 m long at a rate of 0.2 L/s. If the insolation is 500 W/m2
and all of the solar energy is converted into heat, what is the increase in the temperature of
the water after it has traversed the length of the reflector?

Question 8

Compare the photovoltaic capacity for the United States given in Table 9.4
with the energy use shown in Figure 2.5. Comment on these results.

Question 9

Using the energy of electrons in a hydrogen atom as a function of n, as
given by equation (9.1), and following along the lines of the graph in Figure 9.13,
determine all combinations of values of n for which electronic transitions will produce
optical photons.

Question 10

Assume that all of our energy is obtained from solar by means of
photovoltaic devices with an efficiency of 18% using horizontal panels. If all energy is
produced in the United States on a state-by-state basis, calculate the percent of land area
needed to supply energy in Rhode Island and Idaho. Use an average annual per-capita
energy requirement of 350 GJ. Comment on the suitability of this approach.

Question 11

Consider a 1-m wide parabolic trough reflector with a 1-cm inside diameter

Question 12

What is the maximum frequency of light that will produce an output from a
Ge-based photovoltaic cell?

Question 13

A house at 40ºN latitude has a roof of dimensions 8 m by 15 m. This area is

Question 14

A 15 m diameter parabolic dish utilizes a Stirling engine for electricity
generation. On a sunny day the dish receives an insolation that is 47% of the value of the
solar constant. Estimate its output in We.

Question 15

A metal plate is placed on a horizontal surface on a sunny day when the solar
insolation is 500 W/m2. Calculate its equilibrium temperature as a function of its
emissivity, and make a plot of these results for values of the emissivity from ε = 0.1 to ε =
1.0.

Question 16

What is the maximum wavelength of light (in nanometers) that would
produce an output from a photovoltaic device manufactured from a semiconductor with an
energy gap of 1.02 eV?

Question 17

The northern part of the United States, as Figure 8.2 shows, has a yearly 24-
hour average solar insolation which is typical of the average value worldwide. Using this

Question 18

If all of the energy for humanity is generated using horizontal solar
photovoltaic arrays with an efficiency of 15% and which occupy 1% of the land area of the
earth, what is the maximum population density that can be supported? Assume that the
world's average per-capita energy use is the same as at present.

Question 19

Calculate the energy of an electron in the 1s energy level in a hydrogen
atom.