generated. The electrical output of the generator is fed to a storage battery.
(a) Considering only the wind turbine–electric generator as the system, identify locations on the system boundary where the
system interacts with the surroundings. Describe changes occurring within the system with time.
(b) Repeat for a system that includes only the storage battery.
S O L U T I O N
Known: A wind turbine–electric generator provides electricity to a storage battery.
Find: For a system consisting of (a) the wind turbine–electric generator, (b) the storage battery, identify locations where the
system interacts with its surroundings, and describe changes occurring within the system with time.
Assumptions:
1. In part (a), the system is the control volume shown by
the dashed line on the figure.
2. In part (b), the system is the closed system shown by
the dashed line on the figure.
3. The wind is steady.
Analysis:
(a) In this case, there is air flowing across the boundary of the control volume. Another principal interaction between the
system and surroundings is the electric current passing through the wires. From the macroscopic perspective, such an interaction
is not considered a mass transfer, however. With a steady wind, the turbine–generator is likely to reach steady-state operation,
where the rotational speed of the blades is constant and a steady electric current is generated.
(b) The principal interaction between the system and its surroundings is the electric current passing into the battery through
the wires. As noted in part (a), this interaction is not considered a mass transfer. The system is a closed system. As the battery is charged and chemical reactions occur within it, the temperature of the battery surface may become somewhat elevated
and a thermal interaction might occur between the battery and its surroundings. This interaction is likely to be of secondary
importance.
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