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Exam 1 Monday

Exam 1 Monday. Chapters 1-4 Open book, no notes Front cover data to screen Bring a simple calculator Multiple choice, number problems, story problems, essay questions. HW #5 also due. Solar energy in practice. Passive heating. Photovoltaic electricity Solar thermal electricity.

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Exam 1 Monday

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  1. Exam 1 Monday Chapters 1-4 Open book, no notes Front cover data to screen Bring a simple calculator Multiple choice, number problems, story problems, essay questions. HW #5 also due

  2. Solar energy in practice Passive heating. Photovoltaic electricity Solar thermal electricity

  3. Passive solar heating --4.3, 4.4 • Radiative heat from a source at 5800 deg C comes in, hits something black, which absorbs some of that energy. • The hot material radiates, but at a lower temperature---80 deg C. See text. • This re-radiation is less effective, and the black surface gets hot. • Useful for space heating.

  4. To store solar heat—Problem 10 Store 200,000 Btu in a concrete floor of area 1000 ft2, with temp. ‘swing’ of 20 deg F. How thick must it be? Use ‘English’ units. Btu = heat 1 pound of water 1 deg F. For today, water=concrete. Concrete(water) density=62.4 pound/ft3. Heat stored(Btu)=weight(pounds) x T(deg F) x1 Btu/pound deg F

  5. Weight=volume x density Volume(ft3)=heat(Btu) x 1/temp. change (deg F) x1/Btu/(pound-deg F) x1/(pound/ft3) =200,000 x 1/20 x 1/1 x 1/62.4 =160.3 ft3. If area = 1000 ft2, thickness=0.1603 ft =1.9 inches

  6. Photovoltaic (PV) Systems—4.6 • There are materials that release an electron at some voltage when hit by a quantum of light, a single photon. Each such reaction gives about ½ eV of electrical energy. It takes 1.2 x1019 per second of these to give one Watt. • The efficiency of photon energy to electrical energy is about 11%, average of Table 4.3 • Each cell provides fraction of a volt, DC (power = volts times amps) • Needs a ‘converter’ to make AC for the grid.

  7. Solar thermal-- 4.5 • Concentrate the sun’s rays to raise something to a high temperature, using mirrors. • Heat to steam, to turbines, to the grid. • Parabolic steerable trough (Fig. 4.12) • Thermal tower (Fig. 4.10), hundreds of ‘heliostats’ • About 25% efficient (better than PV)

  8. Example-- One m2 in Boulder: Max power? 1366 W/m2 atop the atmosphere X 0.47 for atmo. abs. X1 if pointed at the sun X22% (best in Table 4.3, or as solar thermal) =141 Watts

  9. Cost of solar power? Later- in a comparison among all power sources.

  10. Exam 1 • In class, Monday Feb. 8 • Chapters 1-4 • Multiple choice, numerical exercises, ‘essay’ answers • Final score = 25% HW, 20+20% midterms, 35% final, and a bit for clickers.

  11. How to take the exam Scan/triage—start with the easiest, and be sure of every point. Defer hardest to last. 2. Put down your pencil to start each question. Sketch? 3.Is an equation useful? Write in standard form, list the known/unknown ingredients. Do they match? Will the units work out? 4. Put in the numbers, step-by-step, with the thinking clear to the grader in case partial credit is due. Naked answers will not get full credit. 5.Look at your answer. Does it make sense? Are the units OK? Is it worthy of some relevant comment? 6.For an essay answer, make a mental or written outline of your argument. Give evidence to back your words. 7. In general—slow down! 8. Hardest last

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