ASSESSMENT 7
EAE3701_2022_2
Question 1
The average annual electricity consumption by a household refrigerator has decreased
from 1800 kWh in 1974 to 450 kWh today. Consider a country with 10 million households
with a market penetration of 100 percent for modern refrigerators. If the refrigerators in this
country were to continue to consume electric power at the 1974 levels, how much of
additional installed power would be needed to meet this extra demand? Assume that the
load factor (average fraction of installed power load that is actually produced) of the power
plants is 0.8 and the power plants operate 90 percent of the time on an annual basis.
[6]
Question 2
Water flows in a 10-cm-diameter pipe at 15°C at a rate of 7 kg/s. Determine the velocity and
volume flow rate of water?
[6]
Question 3
An aluminum pan whose thermal conductivity is 237 W/m·°C has a flat bottom whose
diameter is 20 cm and thickness 0.6 cm. Heat is transferred steadily to boiling water in the
pan through its bottom at a rate of 750 W. If the inner surface of the bottom of the pan is
108°C, determine the temperature of the outer surface of the bottom of the pan
[4]
Question 4
Electricity is generated and transmitted in power lines at a frequency of 50 Hz (1 Hz = 1
cycle per second). Determine the wavelength of the electromagnetic waves generated by
the passage of electricity in power lines. (c = 2.998x108 m/s)
[2]
Question 5
Consider a location with an average WPD (wind power density) of 200 W/m2. What is the
corresponding average wind speed? Another location has a WPD of 400 W/m2. What is the
average wind speed for this location? Take the density of air to be 1.18 kg/m3
[4]
Question 6
The measurements over an entire week period indicates that a wind turbine with a blade
diameter of 40 m has produced 11,000 kWh of electricity. If the overall efficiency of the wind
turbine is estimated to be 28 percent, determine the average wind velocity during this
period. Take the density of air to be 1.16 kg/m3
[10]
Question 7
A run-of-river plant operates on a water stream whose cross-sectional area is 30 m2. Water
is flowing at a speed of 4 m/s. If the overall efficiency of this plant is 40 percent, what is the
electrical power output from the plant? (p = 1000 kg/m3)
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[4]
ASSESSMENT 7
EAE3701_2022_2
Question 8
A certain biogas consists of 75 percent methane (CH4) and 25 percent carbon dioxide
(CO2) by volume. If the higher heating value (HHV) is 55.530 kJ/Kg and the lower heating
value (LHV) is 50.05 kJ/Kg, what are the HHV and LHV of this biogas?. Consider the molar
masses of C, H2, and O2 to be 12 kg/kmol, 2 kg/kmol and 32 kg/kmol
[10]
Question 9
List four (4) classifications of power takeoff systems for the wave energy
[4]
Total
(50)
@ UNISA 2022
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