Energy Efficiency

Schematic of a Coal fired Power Plant

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Schematic of a Coal fired Power Plant

Audio with animation: Coal or oil or gas has chemical energy stored in the chemical bonds of the fuel. When the fuel is burned, the chemical bonds in the fuel are broken and new bonds are formed releasing thermal energy . This thermal energy is transferred to water that turns into high pressure, high temperature steam. The high pressure, high temperature steam turns the turbine and converts the thermal energy into mechanical energy. The steam after turning the turbine will still have some energy but not enough to turn the turbine. The low pressure steam is condensed into water and the water is sent back to the boiler. The turbine is connected to a generator and in the generator the mechanical energy is converted into electricity by turning a conductor in a magnetic field.

New Screen

Video demo goes here – will record on 12/16/04

The basic energy conversions in the three main components in a power plant are shown quantitatively in the image below:

(Can we make this a little bigger?)

Energy Conversions in a Power Plant

Audio with animation: Let’s say the boiler takes in 100 Btus of chemical energy and produces 88 Btus of useful thermal energy. The 88 Btus of thermal energy from the boiler goes into the turbine and 36 btus equivalent of mechanical energy (movement of turbine blades) is produced. This 36 Btus of mechanical energy is transferred to the generator which converts it to 10.26 Wh of electrical energy.


Same exercise as with car engine (hot, cold and graph), only use these ranges

3.8 Overall Efficiency

Using the energy efficiency concept we can calculate the component and overall efficiency.

Here the electrical energy is given in Wh and Chemical Energy in Btus. So Wh can be converted to Btus knowing that there are 3.412 Wh in a Btu.

This overall efficiency can also be expressed in steps as follows

Applying this method to the above power plant example


It can be seen that the overall efficiency of a system is equal to the product of efficiencies of the individual subsystems or processes. What is the implication of this?
We have been looking at the efficiencies of an automobile or a power plant individually. But when the entire chain of energy transformations from the moment the coal is brought out to the surface to the moment the electricity turns into its final form, true overall efficiency of the energy utilization will be revealed. The final form at home could be light from a bulb or sound from a stereo. The series of steps are
Need an image of this from Sarma: Production of coaltransportation to power plant  Electricity Generation  Transmission of electricity  Conversion of electricity into light
If efficiency of each step is known, we can calculate the overall efficiency of production of light from coal in the ground. The table below illustrates the calculation of overall efficiency of a light bulb.

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