Outline

Electricity Use Around the World

36.8% of world-wide energy use is in the form of electricity (42.1% in the USA) 97% of all energy world-wide is produced by heat engines Trivia - Humans change 55 km3 of water into steam every year to meet electricity needs Types of heat engines  External Combustion Engines (Steam Engine) Rankine Cycle (wiki) Primary "Fuel" - Coal, Solar Thermal, Nuclear, Biomass Gas Turbine Engine Brayton Cycle (wiki) Primary "Fuel" - Natural Gas

Further information about the history of electricity and the current electrical systems can be found in Electricity Systems.

Thermodynamics and Combustion Review

Laws of Thermodynamics

     0^th    You must play the game.
     1^st      You can't win.
     2^nd     You can't break even.
     3^rd      You can't quit the game.

Zeroth Law of Thermodynamics - If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other

First Law of Thermodynamics - 

• Energy can be neither created nor destroyed. It can only change forms.

• In any process in an isolated system, the total energy remains the same.

• For a thermodynamic cycle the net heat supplied to the system equals the net work done by the system.

Second Law of Thermodynamics - Consider two isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium in itself (but not in equilibrium with each other). Then let some event break the isolation that separates the two systems, so that they become able to exchange matter or energy. Wait till the exchanging systems reach mutual thermodynamic equilibrium. Then the sum of the entropies of the initial two isolated systems is less than or equal to the entropy of the final exchanging systems. In the process of reaching a new thermodynamic equilibrium, entropy has increased (or at least has not decreased). Both matter and energy exchanges can contribute to the entropy increase.

• heat cannot travel from a colder to a hotter body

Third Law of Thermodynamics - As temperature approaches absolute zero, the entropy of a system approaches a constant minimum.

• if all thermal motion of molecules could be removed, absolute zero would take place

Combustion Chemistry

Combustion chemistry relates chemical potential energy to  the heat that results from a chemical reaction.   Hydrocarbons from the fuel is mixed with oxygen and heat is added. Air is made up of approximately 21% oxygen (O2), 78% nitrogen (N2), and small amounts of other inert trace gasses. The hydrocarbons primarily react with oxygen to produce carbon dioxide (CO2), water (H2O) and heat. However it also reacts with nitrogen and sulfur to create the undesirable gasses, nitrogen oxides (NOx) and sulfur oxides (SOx). These gasses are criteria air pollutants while carbon dioxide contributes to climate change.    Example:

Carnot Efficiency

• The Carnot efficiency of a system undergoing a reversible power cycle while operating between thermal reservoirs at temperatures T_H and T_C is represented by the equation below.  The Carnot efficiency is the maximum/ideal thermal efficiency of a cycle.  Temperatures are in absolute scale.

Example: Shows how to solve the Carnot efficiency of an engine.

Thermal Efficiency

• The thermal efficiency of a cycle takes into account work required for parasitic loads, the heat input from a fuel source, and system losses.  The thermal efficiency of a cycle is represented by the equation below where W is work and q is the heat input into the system.

Example: Shows how to solve the Thermal efficiency of an engine.

System Efficiency

• The efficiency of a system can be represented by the product of several efficiencies where,  η_IDEALis the ideal efficiency of a thermodynamic cycle (i.e. brayton or rankine), η_REAL factors in losses due to the turbine and heat exchanger, and η_COMBUSTOR and η_GENERATOR factor in the losses due to the combustor and generators respectively.

Types of Cycles

Rankine Cycle

In the rankine cycle, steam is the driving fluid for the turbine.  The rankine cycle is used for power plants using combusion (coal or biomass) or fissionable material as the fuel.

Brayton Cycle

The brayton cycle uses the combustion gases as the driving fluid for the turbine. The brayton cycle is used for power plants operating on natural gas. 

Combined Cycle

The combined cycle uses elements from both the brayton cycle and rankine cycle, and closes the open loop of the brayton cycle.  Natural gas plants can operate on the combined cycle with much higher efficiencies, but the capitol costs for combined cycle are also significantly higher.

IGCC

The IGCC (Integrated Gassification Combined Cycle) turns coal into syngas so that impurities can be easily removed. The heat from the gasification process is used to generate steam with the rankine cycle.

Thermodynamic Properties

The following website has tables of thermodynamic properties that help solve different thermodynamic problems; such as the examples given in this section: Thermodynamic Properties Tables

The following websites have thermodynamic tables that can be downloaded to your I-Phone or computer (Excel format):

(I-Phone)

http://www.dofmaster.com/steam.html 

(Exel)

http://www.chemgoodies.com/SteamCalculator.aspx?gclid=CMfBvpq3m6QCFZRa2godcCgWDw

New Combustion-based Energy System Goals

• Reduce CO2 emissions until renewable and sustainable technologies develop into mature energy resources that are reliable and can makeup a large portion of the energy market.
• Increase the penetration of biofuels and biomass use in electricity and heat generation.  This is a carbon-neutral alternative to burning fossil fuels.
• Carbon capture and sequestration, as a technology, has the ability to reduce CO2 emissions until the world reaches the point where sustainable energy holds a large portion of the market share.

References

1. Cengel, Yunus and Michael Boles, Thermodynamics: An Engineering Approach. 6th ed. McGraw-Hill: New York, 2008.

2. Moran, Michael and Howard Shapiro. Fundamentals of Engineering Thermodynamics 5^th ed.  John Wiley: New York, 2004.

3. Siemens web site 8/01; TechPro DTE Energy Bob Fegan 2002

4. Vanek, Francis M.; Albright, Louis D. Energy Systems Engineering - Evaluation and Implementation. McGraw-Hill 2008.