1. Is there enough sunshine? We can Now estimate in somewhat more detail: The US economy uses about 100 quad btu/yr =10 17 btu/yr = 10 20 joules/yr Available energy per unit area = 1354 joules/second meter 2 x 3x10 7 seconds/yr X(0.015 efficiency)= 6x10 8 joules/meter 2 yr Area needed = 10 20 joules/yr/(6x10 8 joules/meter 2 yr) =1.67 x 10 11 meter 2 Square area 4 x10 5 meters=400 km=250 miles on a side

3. Take home message: THERE IS PLENTY OF SUNSHINE. There is much more than enough energy coming directly from the sun to power our entire energy economy. There are three basic ways to use that energy: 1. By using it to heat things up. That is to convert it to thermal energy and then use it, either directly or by further converting the thermal energy to other forms. 2. By converting the electromagnetic energy in the sunlight directly to electricity. 3. By converting the energy from the sun to chemical energy, either in plants or artificially. This week, we discuss the first way.

4. Why is it warmer in the summer than in the winter? A. The sun is closer to the earth In the summer than in the winter B. The northern hemisphere tilts more toward the sun in the summer than in the winter C. The heated material In the interior of the earth emits more heat in summer than in winter. D. The earth's orbit is elliptical instead of circular.

5. Answer: B.

6. Fig. 6-4, p. 166

7. There are two effects arising from the tilt: 1.) The days are shorter in winter:

8. 2. The incident energy per square meter Is less in winter.

9. Fig. 6-5, p. 167 What fraction of the solar constant 1354watts/m 2 is a Horizontal Plate collecting at this latitude on June 21? 1btu/hr/ft 2 =3.16 watts/meter 2 A. 275/1354 B. 3.16x275/1354 C.275/(1354x3.16) C. 40x275/1354 40 o latitude, Horizontal collector

10. Answer B. about 64%

11. Fig. 6-6, p. 168

12. Which of the following is true about horizontal and vertical collectors? Vertical collectors collect more energy than horizontal ones A. in summer and in winter B. In winter but not in summer C. In summer but not in winter D. Both vertical and horizontal collectors collect more energy in summer than in winter.

13. Answer B. Discussed in next slides

15. Fig. 6-8, p. 169

16. Fig. 6-9, p. 169

17. Table 6-3, p. 170

18. Use the chart to get the average A. 2040 btu/ft 2 hr insolation per hour in Dodge B. 2400 btu/ft 2 hr City in June in btu/ft 2 hr on a C. 100 btu/ft 2 hr horizontal collector D. 85 btu/ft 2 hr

19. Answer: C.

20. Why is that answer (100 btu/ft 2 hr) so much smaller than the estimate (275 btu/ft 2 hr) from Figure 6.5 if Dodge city is at 38 0 latitude? A Dodge City is at 38 0 not 40 0. B. The data sources for the table and the figure are different. C. 100 btu/ft 2 hr is an average over the whole day. D. The units are wrong in Figure 6.5 Figure 6.5

21. Answer: C

22. Thermal energy collectors

23. p. 172

24. Solar PS10 'Power Tower' near Seville, Spain. 11 Megawatts. 624 mirrors.

25. Law of Reflection from mirrors: Indicated angles are equal

26. The optimal shape for a focussing mirror (a parabola) is designed so each ray, obeying the law of reflection, reflects to the pipe (the focus of the parabola)

27. A BC Where should the collector go for this reflector? A. A B. B C. C D. line from A to C

28. Answer D. Unlike the previous example, here the reflection angles (which are shown at C’) are not only equal to each other at each point on the mirror but also have the same value at each point on the mirror (like at B’) so that rays like C’-C and B’-B are parallel and don’t converge to the same point. CB C’ B’

29. Fig. 6-13, p. 173

30. Fig. 6-14, p. 174

31. Fig. 6-15, p. 175

32. Heat pumps

33. Fig. 6-17, p. 177

34. Fig. 6-20, p. 182

35. Fig. 6-22, p. 183 What makes the water circulate in this one? A. There is a pump which isn’t shown. B.The hot water rises and the cold water falls. C. A siphon effect D. A chemical is added to the water.

36. Answer. B.

37. Passive solar heating of houses

39. Fig. 6-24, p. 185

40. If the winter sun is to go to the back wall and the summer sun is not to enter the window one must A. Adjust the width of the eaves only B. Adjust the width of the floor C. Adjust the height of the eaves D. Adjust all three E. Adjust two of the three

41. Answer E. There are two conditions (that the rays come in in summer and winter as shown) and three unknowns (the widths of the floor (l)and eaves (w) and the height(h) to the eaves). This will give 2 equations for 3 unknowns and you can fix one of the unknowns and then the other two will be fixed by the 2 conditions. Less abstractly: h w h l+w summer winter The angles fix the ratios R s =h/w and R w =h/(l+w)

42. Fig. 6-25, p. 186

43. Table 6-5, p. 194

44. Consider a house in which the ‘heat load’ (amount of heat leaking out to the outside) is 30,000 btu/hr. If the sun warms a slab of concrete in the house to 160 F during the day, how much volume of concrete is needed if the slab is to cool to 90F in 12 hours at night while matching the heat load? heat capacity of concrete=32 btu/ft 3 F A. 12x30000/(32x70) cubic feet=160.7 cubic feet B. 12x30000/(160) cubic feet=225 cubic feet C.32x30000/(12x70) cubic feet=1142.9 cubic feet D. 30000/(160x32) cubic feet=5.86 cubic feet

45. Answer A. The amount of thermal energy leaking out of the house at night is (12 hours)x(30,000 btu/hr) If the volume of the concrete is V, the amount of heat coming out of the slab is 32 btu/ft 3 Fx(160-90) FxV Equate these and solve for V: V= 12x30000/(32x70) ft 3 =160.7 cubic feet