Chapter 17: Problem 57
A river \(60 \mathrm{~m}\) wide and \(2 \mathrm{~m}\) deep flows at \(2 \mathrm{~m} / \mathrm{s}\). A hydro plant develops a pressure of \(300 \mathrm{kPa}\) gage just before the turbine. What maximum power, in MW, is possible? a) 72 c) 56 b) 64 d) 48 Electrical Circuits
Short Answer
Expert verified
a) 72 MW
b) 100 MW
c) 120 MW
d) 150 MW
Answer: a) 72 MW
Step by step solution
01
Find the flow rate
To find the flow rate of the river, we can use the formula: Flow rate = Area × Velocity. The area of the river can be found using the formula for the area of a rectangle: Area = Width × Depth
Area = 60 m × 2 m = 120 m²
Flow rate = Area × Velocity = 120 m² × 2 m/s = 240 m³/s
02
Calculate the mechanical energy
We can use Bernoulli's equation to find the mechanical energy per unit volume of fluid:
\(E = \frac{P}{\rho} + \frac{v^2}{2} + gh\), where \(P\) is the pressure, \(\rho\) is the density of water, \(v\) is the velocity of water, \(g\) is the gravitational acceleration, and \(h\) is the height of the fluid.
Since the pressure is given in kPa, we'll need to convert it to Pa: \(P = 300,000 \mathrm{~Pa}\)
The density of water is approximately \(1000 \mathrm{~kg/m³}\), and the gravitational acceleration is approximately \(9.81 \mathrm{~m/s²}\).
Since the problem asks for the maximum power, we'll assume that the height is 0 (i.e., the turbine is at the same level as the river).
Now, we can plug in the values into the Bernoulli's equation:
\(E = \frac{300,000 \mathrm{~Pa}}{1000 \mathrm{~kg/m³}} + \frac{(2 \mathrm{~m/s})^2}{2} + 0 = 300 \mathrm{~J/kg} + 2 \mathrm{~J/kg} = 302 \mathrm{~J/kg}\)
03
Calculate the power generated by the turbine
Now that we have the mechanical energy per unit volume, we can find the total mechanical energy and the power generated by the turbine:
Mechanical energy = Flow rate × Volume × Mechanical energy per unit volume
Mechanical energy = 240 m³/s × (\(1000 \mathrm{~kg/m³}\)) × (302 J/kg) = 72480000 J/s
Since 1 MW = 1000000 W, we can convert the mechanical energy to MW:
Power = 72.48 MW
04
Choose the correct answer
Based on our calculations, the maximum power possible is approximately 72.48 MW. As a result, the closest answer is:
a) 72 MW
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Fluid Mechanics
Fluid mechanics is the branch of physics that studies the behavior of liquids and gases at rest or in motion. It encompasses various principles that help in solving problems related to fluid flow, such as how rivers flow, the design of irrigation systems, and even how pollutants disperse in the air or ocean.
In the exercise, we examined a river's characteristics to determine the flow rate. The width and depth of the river are paramount to this calculation, as they allow us to determine the cross-sectional area through which the water flows. This is crucial because the flow rate of a fluid, in this case water, directly influences the potential power generation capability of a hydroelectric plant. It's also important to consider the velocity of the river, another fundamental aspect of fluid mechanics, which together with the area, gives us the flow rate that can be harnessed for energy.
Understanding fluid mechanics is critical in engineering education because it provides the foundational knowledge that engineers need to design and analyze systems involving fluids, such as pipes, pumps, and turbines.
In the exercise, we examined a river's characteristics to determine the flow rate. The width and depth of the river are paramount to this calculation, as they allow us to determine the cross-sectional area through which the water flows. This is crucial because the flow rate of a fluid, in this case water, directly influences the potential power generation capability of a hydroelectric plant. It's also important to consider the velocity of the river, another fundamental aspect of fluid mechanics, which together with the area, gives us the flow rate that can be harnessed for energy.
Understanding fluid mechanics is critical in engineering education because it provides the foundational knowledge that engineers need to design and analyze systems involving fluids, such as pipes, pumps, and turbines.
Bernoulli's Equation
One of the most fundamental principles in fluid mechanics is Bernoulli's equation. This principle provides insight into the balance between pressure, kinetic energy, and gravitational potential energy in a flowing fluid. It states that for an incompressible, frictionless fluid, the total mechanical energy remains constant along a streamline.
Application in the Given Exercise
Bernoulli's equation was instrumental in calculating the mechanical energy per unit volume in the context of the hydroelectric power problem we solved. The equation looks like this: \[\begin{equation}E = \frac{P}{\rho} + \frac{v^2}{2} + gh\end{equation}\]where:- \(P\) is the static pressure within the fluid,
- \(\rho\) is the fluid's density,
- \(v\) is the fluid's velocity,
- \(g\) is the gravitational acceleration, and
- \(h\) is the height above a reference point.
Hydroelectric Power
Hydroelectric power is a form of renewable energy that converts the energy of flowing water into electricity. It is considered one of the most efficient and environmentally friendly sources of energy. In our problem, we explored the potential of a river to generate power through a hydro plant, a typical application of hydroelectric power.
By using the flow rate determined in the fluid mechanics section and the mechanical energy derived via Bernoulli's equation, we estimated the maximum power output that the hydroelectric plant could generate. This was done by calculating the total mechanical energy being conveyed to the turbine by the flowing water and then converting this energy into power, presented in megawatts (MW).
For students aspiring to work in renewable energy engineering or environmental resource management, mastering the calculation of hydroelectric power as demonstrated here is crucial. It requires a deep understanding of how water flow can be converted into energy and the efficiency of the turbines that translate this potential into electric power.
By using the flow rate determined in the fluid mechanics section and the mechanical energy derived via Bernoulli's equation, we estimated the maximum power output that the hydroelectric plant could generate. This was done by calculating the total mechanical energy being conveyed to the turbine by the flowing water and then converting this energy into power, presented in megawatts (MW).
For students aspiring to work in renewable energy engineering or environmental resource management, mastering the calculation of hydroelectric power as demonstrated here is crucial. It requires a deep understanding of how water flow can be converted into energy and the efficiency of the turbines that translate this potential into electric power.