African solar systems part 4: A problem on energy usage

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A person does not only require electrical energy. Consider the following daily typical energy usage:

  • Reading and working for four hours after sunset in the light of a 10 Watt LED lamp.
  • 3 L of sterilized drinking water. Sterilization of drinking water minimally requires the water temperature to be raised to 10 0 ∘ 100^\circ C from the ambient temperature of 2 5 ∘ 25^\circ C.
  • 1 hour of cell phone or radio use at 0.5 Watts.

Given these energy requirements, what percentage of the total energy usage per day is made up of heat?


The answer is 86.595.

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David Mattingly by David Mattingly

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1 solution

Thaddeus Abiy
Apr 4, 2014

Energy for LED lamp = 10 × 4 × 60 × 60 = 144000 J =10\times 4\times 60\times 60=144000J

Energy for water = m c Δ T = 3000 × 4.186 × 75 J = 941850 J =mc\Delta T=3000\times 4.186\times 75J=941850J

Energy for cell = 0.5 × 60 × 60 = 1800 J =0.5\times 60\times 60=1800J

Since heat energy is used only for sterilizing water

x = 100 ( 941850 941850 + 144000 + 1800 ) = 86.595 x = 100(\frac{941850}{941850+144000+1800}) = 86.595

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