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Systems, Energy, & Efficiency

Systems, Energy, & Efficiency. EGR 1301: Introduction to Engineering. Systems. System A particular subset of the universe specified in time and space by a boundary (Ch 17, p. 484). System boundary. t initial = start time t final = stop time. Source: Professor Thomas. System Definition.

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Systems, Energy, & Efficiency

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  1. Systems, Energy, & Efficiency EGR 1301: Introduction to Engineering

  2. Systems • System • A particular subset of the universe specified in time and space by a boundary (Ch 17, p. 484) System boundary tinitial = start time tfinal = stop time Source: Professor Thomas

  3. System Definition • Rules the engineer must follow: • Once a system is specified, it cannot be changed midway through a calculation. • The system boundary can be any shape, but it must be a closed surface. It must also be closed (or bounded) in time. • The system boundary can be rigid (defining a volume of space) or it can be flexible (defining an object).

  4. Importance of System Definition Source: Foundations of Engineering, Holtzapple & Reece, 2003

  5. Intensive vs. Extensive • Extensive quantities • Change with size of the system • Intensive quantities • Remain constant, regardless of size X X X X

  6. What is Energy? • “The capacity for doing work” OR • Unit of exchange (Ch 22, p. 572) • Examples: • Electricity  light or heat • Chemical energy in gasoline  torque in car or heat • Natural gas  electricity or hot water Source: Webster’s New Collegiate Dictionary

  7. Units of Energy Source: Foundations of Engineering, Holtzapple & Reece, 2003

  8. 1st Law of Thermodynamics • “Law of Conservation of Energy” • Energy can neither be created nor destroyed • Therefore, energy must be conserved • Energy can only be transformed • Work can be converted into another form of work • Work can be converted into heat • Need to keep track of, or “account” for, these changes

  9. Money Accounting • Can “account” for the money in your bank: • Ex: • Start with $1000 • Pay you $500 for coming to class • Spend $800 on new laptop • How much do you have (i.e. final balance)? Final balance – Initial balance = Deposits - Withdrawals Accumulation = Net input Final balance = Initial balance + Deposits - Withdrawals

  10. Energy Accounting • For any system, the same relationship is true: Final energy – Initial energy = Input - Output Accumulation = Net input State quantities = Path quantities System Boundary Energy in/out (Path Quantities) Accumulated Energy (State Quantities)

  11. State Quantities • Kinetic Energy • Energy associated with motion • Potential Energy • Energy associated with position, either against a field (e.g. gravity or electric field), compressed spring, or stretched rubber band • Internal Energy • Energy associated with atoms, such as temperature, phase changes, or chemical reactions

  12. Path Quantities • Work • Energy flow due to a driving force other than temperature: mechanical (shaft, hydraulic), electrical, photonic (laser, solar PV) • Heat • Energy flow due to temperature: conduction, blackbody radiation • Mass • Energy flow due to mass crossing the boundary: fuel

  13. Universal Accounting Equation • Mathematical version of the accounting equation: • All have the form: Change in kinetic energy Change in potential energy Change in internal energy Change = Energy at tfinal - Energy at tinitial

  14. Universal Accounting Equation • Mathematical version of the accounting equation: • Heat and Mass have the form: Work input = work done on the system from its surroundings Work output = work done by the system to its surroundings Energy added to system – Energy removed from system

  15. Joule’s Experiment tinitial = mass is raised tfinal = after mass falls and propeller and water stop moving System boundary Assume perfect insulation. How are variables related? Source: Foundations of Engineering, Holtzapple & Reece, 2003

  16. 2nd Law of Thermodynamics • Naturally occurring processes are directional • Closely tied to idea of reversibility • Reversible processes have no directionality • Entropy • Ex: balloon, car, office

  17. Energy Conversion • A system converts energy from one form to another • The process is not always perfect Energy Conversion Device (System) Energy In Energy Out Wasted Energy (often heat)

  18. Efficiency • Measure of how well a system can convert energy • Greek letter eta, η

  19. Example • If a system outputs 70,000 J and η = 0.7, what is the input energy? • How much was wasted? J 30,000 J

  20. Cascaded Conversion • Can connect multiple systems together and do several conversions Natural gas Rotating shaft Electricity Light Gas turbine Generator Light bulb E2 E3 η1 η2 η3 E1 E4 Waste 1 (heat) Waste 2 (heat) Waste 3 (heat)

  21. Overall Efficiency • Treat multiple conversions as a single process ηoverall η1 η2 η3 E4 E1 Total waste (heat)

  22. Recap • Systems – boundary (time & space) • Energy – unit of exchange • Intensive vs. Extensive Quantities • State vs. Path Quantities • Universal Accounting Equation • Efficiency • Cascaded systems • Next: examples

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