Steve Boddeker's Energy and Society Ch 7 homework



CHAPTER 7 Energy Conservation

Questions and Problems

1. Find the number of Btu conducted through a wall in 8 hours. The wall is 8 feet high by 24 feet long and has a total R-value of 16.2 hr⋅°F⋅ft2 /Btu, including the effects of the inner and outer air layers. The inside temperature is 68°F and the outside temperature is 16°F.

2. Find the number of degree days in the heating season at a location where the average outside temperatures are distributed as follows: 60 days at 55°F, 50 days at 45°F, 30 days at 35°F, and all the other days above 65°F.

3. A 1500 ft2 wall has an R-value of 11 hr⋅°F⋅ft2 /Btu, including the effects of inside and outside air layers. How many Btu are lost through this wall in a 5600 degree day heating season?

4. Calculate the R-value of a brick-faced wall that has the following layers: 4 inches of brick, 0.75 inch of insulating sheathing, 3.5 inch of urea foam, 0.25 inch of plywood, inside air layer

5. BONUS A house in Reno, Nevada (6040 degree days), consists of the following, neglecting the heat lost through the concrete slab floor, air infiltration, and solar energy input, calculate the total number of Btu needed to heat the home for one heating season:

a. 544 ft2 of single-pane windows.

b. 544 ft2 of walls with total R-value, including air layers, of 16.31.

c. 480 ft2 of stone walls, 13 inches thick. Add air layers.

d. 1632 ft2 of roof with R value of 8.0, including air layers

6. BONUS Calculate the cost of heating the Reno home in #5 for one season with a natural gas furnace. Assume that the cost of natural gas is $13.00/106 Btu, and the furnace is 65% efficient.

7. BONUS Calculate the savings in fuel cost for one season if the house in problems 5 and 6 has the roof insulation increased to an R-value of 19, including air layers. Would the purchase of such insulation appear to be a wise choice if it costs $0.45/ft2?

8. What would be the payback period for installation of double-paned windows (originally single paned) in Fairbanks, Alaska, if fuel costs $15.50/106 Btu, the furnace is 75% efficient, and the cost of double-paning is $4.00/ft2?

9. BONUS Estimate the percentage savings in the fuel bill if the thermostat is set back from 68°F to 50°F for 12 hours of the day, and left at 68°F for the other 12 hours, assuming an outside temperature of 15°F all day.

10. How many Btu would be contributed to the heating of a house by the metabolism of four occupants over the six-month heating period? Assume each person generates 100 W and they are in the house 50% of the time.

11. BONUS List the ways in which your home could be made more energy efficient.

12. Enumerate some changes you could make in your personal life that would lead to appreciable savings in energy.

13. BONUS Estimate the life-cycle energy costs for a few common household appliances and compare them to the initial appliance cost. SeeTable Table 7.5.

14. BONUS Estimate the annual energy cost in dollars for an electric blanket. Use $0.30 / KW-hr for electricity cost. Assume that it has a 135-watt heating element, which is on 50% of the time for each 8-hour night, 160 nights each year. Does it make sense to cover an electric blanket with ordinary blankets?

15. BONUS How could you improve the design of the typical gas-fired residential water heater? Consider electric ignition, a flue damper, insulation, tempering tank, insulated pipe connections, separate “instant” heater, and so forth.

16. BONUS Consider the relative ease and cost-effectiveness of measures you might take to conserve energy in your present means of transportation as compared to your home.

17. You can purchase a 15-watt compact fluorescent lamp for $1 that is rated to last 10,000 hours. This lamp provides the same light output as a 75-watt incandescent lightbulb that costs $0.50 and lasts 1000 hours. Based on an electric energy cost of $0.30/kWh, calculate the money saved over the life of the fluorescent lamp by using it instead of the conventional lightbulb. Include the cost of the lightbulbs.

18. BONUS How much would be saved over an eight-year period by each household that signed up for the Niagara Mohawk Conservation Program kit?

Multiple Choice Questions

1. For a 20-day period the outside temperature is 20°F and the inside temperature is 65°F. How many degree days is this? (Show Work)

a. 1800

b. 900

c. 450

d. 45

e. 9000

f. 4500

g. 200

h. 2000

2. For a typical house and ordinary weather conditions, the inside air is completely replaced by outside air about once every ___ due to infiltration around doors, windows, and through other cracks.

a. 1 minute

b. 2 minutes

c. 100 hours

d. 2 days

e. 1 day

f. 30 seconds

g. 1 hour

h. 20 hours

3. In the United States space heating represents about how much of our residential energy consumption?

a. 1%

b. 5%

c. 24%

d. 48%

e. 80%

f. 98%

g. 99%

h. 100%

4. What is the exact meaning of the designation R-20 as used in describing quality of insulation?

a. 20 (Btu/hr⋅ft2 ⋅°F) 

b. 20 (hr/Btu⋅ft2 ⋅°F) 

c. 20 (hr⋅ft2 ⋅°F/Btu)

d. 1/20 (hr⋅ft2 ⋅°F/Btu

e. 20 (watt⋅hr/day)

f. 20 (cal/sec)

g. 20 (hr⋅ft2 ⋅K/Btu)

5. How many Btu/hr are lost through a 100-square foot wall that is made up of an 8-inch thick brick wall with an R value of 1.6 and 2 inches of foam insulation with an R value of 10.5? The inside temperature is 65°F, and the outside temperature is 35°F. (Ignore air layers.)

a. 290 Btu/hr  (Show Work)

b. 29 Btu/hr 

c. 537 Btu/hr 

d. 248 Btu/hr

e. 35 Btu/hr 

f. 112 Btu/hr 

g. 224 Btu/hr 

h. 1750 Btu/hr

6. For usual winter conditions, the dominant thermal resistance of a single-pane window is in __

a. the glass

b. the internal air layer

c. the external air layer

d. infiltration

e. none of the above

7. The efficiency of the furnace averages about ___% in existing homes, but the best new furnaces have efficiencies of about ___%.

a. 25, 75

b. 55, 70

c. 90, 95

d. 85, 98

e. 15, 50

f. 40, 60

g. 30, 70

h. 60, 95

8. The pilot light in a gas-fired furnace or water heater uses 1 ft3/hr of natural gas. In terms of heat energy, this is equivalent to a ___ watt lightbulb burning continuously.

a. 10 (Show Work)

b. 30

c. 100

d. 150

e. 300

f. 500

g. 3000

h. 10,000

9. On a cloudy winter day with an outside temperature of 0°F, what percent savings in fuel is realized if the thermostat setting is lowered from 75 to 65°F?

a. 5% (Show Work)

b. 15%

c. 45%

d. 85%

10. The energy equivalent of how much oil is saved, according to Table 7.7, by recycling a case (24 cans) of aluminum beverage cans rather than using them once and throwing them into the trash is ________

a. 42 barrels 

b. 0.8 gallons

c. 0.0194 gallons 

d. 13.3×10-6 barrels 

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