Exercise 6. Suppose that the same home as in exercise 1 also uses E = 2200 [kWh]

Question

Exercise 6.

Suppose that the same home as in exercise 1 also uses E = 2200 [kWh] per year of electricity. Suppose that the roof area is covered with solar cells (photovoltaic cells) instead of the solar heat collection system. Assume that the solar cells operate with an efficiency of 10% at the output of the electricity generating system. Calculate the number of Joules the solar cells would effectively produce in a year, and compare this with E, the number of electric Joules used in the home during a year. Comment on the comparison.

Note: the energy storage problem for photovoltaic cells is larger than for a solar heat collection system. Indeed the solar cells electricity output should probably be hooked into the commercial electrical grid to mitigate the problem of storing energy in the form of electricity. The storage problem complicates a choice between renewable hot water and renewable electricity. Other complications include the initial (capital) cost of purchasing and installing either system, any maintenance costs, and the fair market cost to the consumer, both of natural gas and of electricity. These considerations lead to the idea of a “payback” period for the renewable energy technology. This concept is explored in your environmental economics classes.

Explanation / Answer

I kWh = 3.6 * 10^6 Joules

So, 2200 kWh = 2200 * 3.6 * 10^6 Joules

= 7.92 * 10^8 Joules

Now, efficiency of solar cells = 10%

So, it needs to produce 7.92 * 10^9 Joules of energy.

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