Energy equiv. for lifting water

[tired old white man]

Hello,

 

For some time now, I have been doing my best to conserve water. It dawned on me the other day that saving water also saves "energy." By this I mean, every gallon of water used from our city's system requires a replacement gallon of water to be pumped back up to the water tower.

 

Assuming a gallon of fresh water weighs 8.34 US pounds and the water tower is, say, 200 ft high, how much energy is required to get that gallon of water back in the tower?

 

I'm looking for an eventual energy equivalent of a gallon of gasoline (or diesel)...

 

Your assistance is sincerely appreciated.

 

Tired Old White Man

[Miles Guidon]

 

Assuming a gallon of fresh water weighs 8.34 US pounds and the water tower is, say, 200 ft high, how much energy is required to get that gallon of water back in the tower?

 

 

 

Potential energy of a gallon of water at 200 feet = mass of gallon of water * acceleration due to gravity * height

 

I like to do calculations in units of the metric system, so its [8.34lbs*(1kg/2.2lbs)] *[9.8 m/s^2]* [200ft*(0.3048 meters/foot)] = energy required = 2264 Joules

 

Lets say you use 10 gallons of water a day. That's [2264 Joules * 10] =22,640 Joules in a day due to lifting the water, or [22,630 Joules/day * (1 day/86400 seconds)] = 0.262 Watts (this is somewhat of a false statistic, because this assumes you're slowly lifting small amounts of water at a constant rate, adding up to 10 gallons at the end of the day). Watts are the number of Joules per second.

 

But there you have it.

[CaptainPanic]

Continuing the calculation of 1 gallon lifted 200 ft...

 

The actual answer to your question is: You require 0.000015 gallons of gasoline to lift that 1 gallon of water by 200 ft...

... because 1 liter of gasoline contains approximately 40 MJ... so, 1 gallon of gasoline contains about 150 MJ of energy.

And, assuming you use 10 gallons of water per day, you require 0.00015 gallons of gasoline to lift that water, per day... or 0.06 gallons of gasoline per year.

 

Note (1): the majority of the energy of cold tap water is required for purification, not for transportation to the water tower.

Note (2): Hot water for a shower costs much more energy. If you heat 1 gallon up from its cold temperature of 10 deg Celsius to a comfortable 40 deg Celsius for a shower, then you will use 474,000 J... or 0.003 gallon of gasoline... That's 210 times as much as for lifting it up into that water tower.

 

So... the conclusion is: moving water around (also up or down) doesn't really matter... heating water up, or cooling it down, really matters a lot.

 

Finally: then why do we use hydro-power? Answer: it's only useful because those really use thousands of cubic meters (tens of thousands of cubic feet) per second.

Edited November 25, 2010 by CaptainPanic

[insane_alien]

not to mention, if there was no water tower and the pressure was provided solely from pumps, it would require the same amount of energy as lifting it to a height that produces a similar pressure.

[tired old white man]

Many thanks...(to ALL)...

 

Agreed. The energy required to lift the water is minuscule by comparison to the other energy factors. As you mentioned, heating water is one of, if not the, largest energy hogs...

 

However, my one gallon of water is not the sole issue, its everyone's one gallon. "Municipal planners assume each individual (regardless of whether or not it is a family) uses about 150 gallons of water a day."

http://wiki.answers.com/Q/How_much_water_does_the_average_American_family_use_in_a_day#ixzz16VcdjZ93

A 10,000 population city would then consume 15 million gallons of water each day. That's 225 gallons of gasoline per day per 10,000 population. Or, 82,125 gallons per year. Each flush of the toilet likely uses an average of 3 gallons of water. I'm guessing here, but each person likely flushes the toilet, what, four times a day? That same 10,000 population then goes through 120,000 gallons of water each day just flushing the commode; a paltry 1.8 gallons of gasoline equivalent per day just as a result of flushing the john. Cut the number of flushes in half and you can realize a potential savings in excess of 325 gallons of gasoline equivalent per 10,000 people per year. The population of the US is estimated to be 300,000,000; or 9.8 million gallons of gasoline equivalent per year. When you look at numbers in that perspective, the savings really adds up.

 

So...the conclusion is: regardless how tiny the energy savings is on an individual basis, the energy savings are significant across the entire population. Every joule counts; a joule saved is a joule earned...

 

When it's yellow let it mellow. When it's brown send it down.

 

Amuse toi bien and thanks to all who answered...

 

Continuing the calculation of 1 gallon lifted 200 ft...

 

The actual answer to your question is: You require 0.000015 gallons of gasoline to lift that 1 gallon of water by 200 ft...

... because 1 liter of gasoline contains approximately 40 MJ... so, 1 gallon of gasoline contains about 150 MJ of energy.

And, assuming you use 10 gallons of water per day, you require 0.00015 gallons of gasoline to lift that water, per day... or 0.06 gallons of gasoline per year.

 

Note (1): the majority of the energy of cold tap water is required for purification, not for transportation to the water tower.

Note (2): Hot water for a shower costs much more energy. If you heat 1 gallon up from its cold temperature of 10 deg Celsius to a comfortable 40 deg Celsius for a shower, then you will use 474,000 J... or 0.003 gallon of gasoline... That's 210 times as much as for lifting it up into that water tower.

 

So... the conclusion is: moving water around (also up or down) doesn't really matter... heating water up, or cooling it down, really matters a lot.

 

Finally: then why do we use hydro-power? Answer: it's only useful because those really use thousands of cubic meters (tens of thousands of cubic feet) per second.