1. Solar Energy is incident on the top of Earth\'s atmosphere at the rate of abo
ID: 223 • Letter: 1
Question
1. Solar Energy is incident on the top of Earth's atmosphere at the rate of about 1,364 watts on every square meter (W/m^2). This energy effectively falls on the cross-sectional area of the planet. From this fact, calculate the total rate at which solar energy arrives at Earths. You'll need to find Earth's radius.
2. Use the answer to Exercise 1, along with the percentage of Earth's total energy supplied by the geothermal flow shown in Figure 1.8, to estimate the total geothermal energy flow.
3. Suppose you got all your energy from human servants turning hand-cranked electric generators, each producing energy at the rate of 100 W. If you paid those servants $8 per hour, a bit more than the U.S. minimum wage, what would be your cost for 1kWh of energy? (The typical cost of 1 kWh of electrical energy is actually around 10C).
4. Find the cost per kilowatt-hour of electricity in your area, and the cost of a gallon of gasoline. Considering that gasoline contains about 40 kWh/gal, compare the price per kilowatt-hour for gasoline versus electrical energy.
5. The average daily human diet has an energy content of about 2,000 kcal. Convert this 2,000 kcal per day into watts.
6. Using appropriate conversions between pounds and grams, and degrees Celsius and Fahrenheit, show that 1 Btu is equivalent to 1,054 J.
7. Verify that the 5,000 Btu/h heat flow through the basement wall described in Section 4.3 is equivalent to about 1,500 W and therefore 15 energy servants.
8. My solar hot-water system has a 120-gallon storage tank with a surgace area of 4.3 m^2. It's insulated with 5-cm thick urethane foam, and the water in the tank is at 76 degrees Celcius. It's in the basement, where the ambient temperature is 15 degrees Celcius. What is the rate of heat loss from the tank?
Explanation / Answer
My solar hot-water system has a 120-gallon storage tank with a surgace area of 4.3 m^2. It's insulated with 5-cm thick urethane foam, and the water in the tank is at 76 degrees Celcius. It's in the basement, where the ambient temperature is 15 degrees Celcius. What is the rate of heat loss from the tank?
thermal conductivity of .022 (W/m x K)
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