Steam Power Plants and the Second Law of Thermodynamics

In summary, a steam power plant is a heat engine that operates between two thermal energy reservoirs, with steam as the working fluid. In a basic steam power cycle, heat from burning fuel is used to convert water into high temperature, high pressure steam, which is then used to produce work in a turbine before being cooled and pumped back into the boiler for another cycle. However, there is potential for waste heat recovery by directly pumping low temperature, low pressure steam from the turbine back into the boiler. This would reduce the need for fuel and save on the energy required to drive a larger pump, but it may not be as efficient as a standard steam engine cycle.
  • #1
Soumalya
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2
We know that a steam power plant is basically a heat engine with steam as the working fluid.

According to the second law of thermodynamics a heat engine must operate between two thermal energy reservoirs one at a higher temperature(source) than the other(sink) to produce net work in a thermodynamic cycle.

In a basic steam power cycle,heat received from burning fuel in a furnace is utilized to convert water into high temperature and high pressure steam,which when passes through blades of a turbine produces shaft work with low pressure low temperature steam leaving the turbine exit.The low temperature low pressure steam is then made to reject heat in a condenser before it is pumped back into the boiler for another cycle of operation.

While it is understandable that the low temperature low pressure steam must be condensed to the state of liquid water initially fed into the boiler to complete a thermodynamic cycle in doing so we are basically wasting the latent heat condensation of water along with the sensible heat required to manufacture high temperature high pressure steam from liquid water again in the boiler!

What if the low temperature low pressure steam from the turbine exit was directly pumped to the boiler such that less fuel would be required to restore high temperature high pressure steam at the turbine inlet thus recovering the waste heat as mentioned?
 
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  • #2
The volume of steam exhausting from the turbine is about 1000 times the volume of condensate coming out of the condenser. This means a vapor pump must be this much bigger than a condensate pump, and require a proportionately larger amount of power to drive it. You might recover some of the latent heat which is lost condensing the steam, but this recovered energy could not be turned into useful output work from the plant as a whole.
 
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  • #3
Soumalya said:
We know that a steam power plant is basically a heat engine with steam as the working fluid.

According to the second law of thermodynamics a heat engine must operate between two thermal energy reservoirs one at a higher temperature(source) than the other(sink) to produce net work in a thermodynamic cycle.

In a basic steam power cycle,heat received from burning fuel in a furnace is utilized to convert water into high temperature and high pressure steam,which when passes through blades of a turbine produces shaft work with low pressure low temperature steam leaving the turbine exit.The low temperature low pressure steam is then made to reject heat in a condenser before it is pumped back into the boiler for another cycle of operation.

While it is understandable that the low temperature low pressure steam must be condensed to the state of liquid water initially fed into the boiler to complete a thermodynamic cycle in doing so we are basically wasting the latent heat condensation of water along with the sensible heat required to manufacture high temperature high pressure steam from liquid water again in the boiler!

What if the low temperature low pressure steam from the turbine exit was directly pumped to the boiler such that less fuel would be required to restore high temperature high pressure steam at the turbine inlet thus recovering the waste heat as mentioned?

You don't need us to help you address this question. Just do some calculations to see how the scenario you are describing would play out. Do it on a sample system, by defining the operating conditions at the end of each step. This is how modeling can help you resolve uncertainties and doubts. (First, of course, to get your feet wet, do the calculations for a standard steam engine cycle).

Chet
 
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  • #5
This is the answer I was looking for and I am glad I came across it :smile:
 

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  • #6
Soumalya said:
This is the answer I was looking for and I am glad I came across it :smile:

Which book is it from?
 
  • #7
Jano L. said:
Which book is it from?

"Thermodynamics:An Engineering Approach" by Yunus A. Cengel and Michael A. Boles
 
  • #8
How to make it intuitive:...

if you've ever pumped up a bicycle or automobile tire with an old fashioned hand pump you know how much work it takes to pump a compressible fluid.
 
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  • #9
jim hardy said:
How to make it intuitive:...

if you've ever pumped up a bicycle or automobile tire with an old fashioned hand pump you know how much work it takes to pump a compressible fluid.

Very well said!
 

1. What is a steam power plant?

A steam power plant is a type of power generation system that uses the process of converting heat into mechanical energy to produce electricity. This is achieved by heating water to create steam, which then drives a turbine to generate electricity.

2. How does a steam power plant work?

A steam power plant works by using a heat source, such as coal, natural gas, or nuclear energy, to heat water and create steam. The steam then flows through a turbine, which spins and generates electricity. After passing through the turbine, the steam is condensed back into water and recycled back into the system.

3. What is the Second Law of Thermodynamics and how does it relate to steam power plants?

The Second Law of Thermodynamics states that in any energy conversion process, some energy will be lost in the form of heat. This means that not all of the energy produced by a steam power plant can be converted into usable electricity. The Second Law also sets limits on the efficiency of the power plant, as it is impossible to convert all of the heat energy into mechanical energy.

4. What are the main components of a steam power plant?

The main components of a steam power plant include a boiler, turbine, condenser, and a generator. The boiler is responsible for heating the water to create steam, which then flows through the turbine. The turbine drives the generator, which produces electricity. The condenser then cools the steam back into water for recycling in the system.

5. What are the advantages and disadvantages of using steam power plants?

The advantages of steam power plants include their ability to produce large amounts of electricity, their relatively low cost compared to other forms of power generation, and their use of readily available resources such as coal and natural gas. However, steam power plants also have some disadvantages, such as their impact on the environment through emissions and the limited availability of non-renewable resources. Additionally, the efficiency of steam power plants is limited by the Second Law of Thermodynamics.

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