## Wednesday, July 16, 2014

### Energy Needs

How easy is it to supply our typical energy needs? The electric power utilities do that routinely these days. Should we be surprised by that?

There are two pieces to this puzzle. The first: is our typical energy usage a good approximation to our actual needs? The energy we typically use is just what is needed to support our present lifestyle. Many societies, with lifestyles quite different than ours, have survived over many generations on much smaller rates of energy consumption. Do we need to maintain our present lifestyle?

The second puzzle piece: how much energy do we typically use? In the United States, a typical household consumes about 10 kilowatthours each day. Since there are 3600 seconds in an hour, that is 36,000 kilowattseconds, or 36 million joules. How much is that?

To get a feel for it, suppose we wanted to store that energy in a household pumped hydroelectric facility. Suppose we built a reservoir 10 meters high, on top of the roof of the house. How big would the reservoir need to be?

The energy in the reservoir is mgh, where m is the mass of the water, g is the acceleration of gravity, and h is the height of the reservoir. g is about 10 meter/sec^2 and we have h as 10 meter. So 100m = 36,000,000 or m = 360,000 kg. A liter of water weighs a kilogram, so that would be 360,000 liters of water, or about 90,000 gallons.

A typical backyard swimming pool contains about 15,000 gallons of water. I.e. our household hydroelectric reservoir needs to hold about six swimming pools of water in order to supply a day's worth of electricity.

Maybe better not to put that on top of the roof!

#### 1 comment:

1. Take a look, too, at this chart:
https://flowcharts.llnl.gov/content/energy/energy_archive/energy_flow_2013/2013USEnergy.png

The 10 KWh per day is residential electricity. Total energy consumption is about eight times larger, though the accounting is tricky.