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Chapter 5 Home Energy Cons .

Chapter 5 Home Energy Cons. Thermal Resistance: Q/ D t = k A D T/L =: A D T/R R:= L/ k

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Chapter 5 Home Energy Cons .

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  1. Chapter 5 Home Energy Cons. • Thermal Resistance: Q/Dt = kADT/L =: ADT/R R:= L/k • This is useful because it folds in both the material property (k) and the thickness of the insulating layer (L), AND if you combine layers, then the thermal resistances (R) simply add, as shown on the next slide.

  2. R-value for a typical wall See table 5.2 in H&K for typical values of building materials

  3. Some typical R values

  4. Infiltration Q/Dt = 0.018 Btu/ft3.Fo V K DT Here K is the number of “Air exchanges per hour” and V is the interior volume of the house/building. Note: some exchange of air is necessary (you need to breath!), and this is not readily apparent in this figure.

  5. Exam I Review • EXTRA OFFICE HOURS: • WED 11:00 TO NOON AND 2:45 TO 5:00 • THURS: 8:00 TO 9:00 • Covers Chapter 1 section A through Chapter 5 section D • 23 questions, 5 points each • More than half involve no calculation, only about 5 or 6 involve calculations with more than 1 step. • Only 2 questions from chapter 5. 4-6 questions from each other chapter. • Exam cover page is on ONCOURSE (two modifications since yesterday). Does NOT give simple formulae that are really just definitions (e.g. Power=W/Dt; DS=Q/T) • CLOSED BOOK, CLOSED NOTES • Calculators are allowed (but not cell phones!).

  6. Key points so far • Chapter 1 • Working with numbers, units, uncertainties • Energy: definition, patterns of use, types, sources • Exponential growth: annual and continuous compounding, doubling time, • Hubbert’s curve • Renewable/non-renewable energy resources • Energy conservation • Chapter 2 • Energy Conversions • Newton’s laws • Mechanical Energy: Kinetic, Potential • Work • Power • Electrical suppliers cost structure

  7. Key points so far (cont.) • Chapter 3 • First law of Thermodynamics • Efficiency of energy conversions • Energy content of fuels and unit conversions • Lighting (as an example of new technology giving better efficiency) • Chapter 4 • “Zeroth” law of Thermodynamics (the real meaning of temperature) • Second Law of Thermodynamics • Heat Capacity and specific heats • Latent Heats (fusion and vaporization) • Heat transfer mechanisms: conduction, convection, radiation • Radiant transfer (higher T, more light, shorter wavelength). • Reversibility • Heat Engines and Carnot Efficiency

  8. Key points so far (cont.) • Chapter 5 • Conduction as a source of inefficiency in building climate control • Control of heat transfer as a way of improving efficiency • Infiltration

  9. Example questions • (5 points) Name two types of electrical lighting fixture that are more efficient than incandescent lights. • (5 points) What is the dominant form of heat transfer between the Sun and the Earth? • (5 points) What is the change in the potential energy of a crate whose mass is 9.3kg when it is raised from the floor onto a shelf at a height of 2.4m? • (5 points) In chapter 1 of the text, the authors describe a gas-fired combined-cycle power plant that runs with an efficiency of 57%. If the gas flame provides a boiler temperature of 1200oC, and assuming that the system releases heat at ambient temperature (25oC), what fraction of the maximum possible efficiency does this plant achieve?

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