# Homework Help: Water electricity pump, given efficiency percentage?

1. Mar 24, 2015

### Allison Barry

Changing the potential energy of water is frequently used as a means of storing excess renewable energy when it is not needed and using it when it is needed. An electrically-powered pump with efficiency 62 % raises a 10 kg mass of water a height of 10 m. The water is subsequently allowed to return to its original elevation while passing through an electricity-generating device with efficiency 84 %. Calculate the percentage of the electricity originally used to pump the water that is recovered by the entire process.

I haven't a clue where to begin, this is energy systems engineering. Would you derive an equation for the electric pump and then the electricity generator? In need of serious guidance

2. Mar 24, 2015

### SteamKing

Staff Emeritus
Start by asking yourself how much energy does it take to raise 10 kg of water by a height of 10 m. Apply the efficiency figure for the electric pump to this amount of energy to see what the pump must furnish.

3. Mar 25, 2015

### CWatters

You can do what steamking said but the question can be answered without calculating the power needed to raise the water...

If you have any two machines connected together and one is 62% efficient and the other is 84% efficient what is the efficiency of the two combined? Perhaps draw a block diagram and remind yourself what "efficiency" actually means in terms of the inputs and outputs.