In a hydro-electric generating station, the mean head (i.e. the difference of height between the mean level of the water in the lake and the generating station) is 400 metres. If the overall efficiency of the generating stations is 70%, how many litres of water are required to generate 1 kWh of electrical energy ? Take one litre of water to have a mass of 1 kg
Output energy = 1 kWh = 36 × 105 J
Input energy = 36 × 105 /0.7 = 5.14 × 106 J
If m kg. water is required, then
mgh = 5.14 × 106 or m × 9.81 × 400 = 5.14 × 106
= 1310 kg.
