How Much Water?

Regarding my previous post, how much water would a typical home have to pump into its own virtual dam in order to provide energy during a typical night while the water flows back out and through a generator?

Are we talking swimming pool size?

How big a container would you need to store the compressed air for the same purpose? Refrigerator size? Garage size?

I have the smartest readers in the world. Someone can probably answer that question on the back of an envelope.

Comments

[Chris]

Most people live near coasts. My plan is to build whole towns (but perhaps starting with just novelty tourist shops and cafes) on floating pontoons with some sort of hydraulic pistons fixed to both the pontoon and the sea or river bed. As the tide rises and falls (a distance of around two metres where I live) a constant flow of high pressure water would be forced through a turbine to supply power to the buildings on the pontoon.

I believe such things have been tried before and it turns out to be extremely difficult to build sufficiently robust hardware. But still, it should work in theory. I think.

[Jacko]

If we say we have: 50 kW hours / day, 20% of which needs to be stored overnight (more if you are in a cold climate), with a 1/3 efficiency in energy recovery, and a drop of 20m from the storage tank to the generator.
The volume of water needed is 550m^3. If we assume that the house has a footprint of 2000 square feet (185 m^2), then a storage tank under the floor would have to be 3 meters high, with the generator another 20m below the bottom of that.

On the plus side, the 550 tonnes of water under your house will save on heating overnight if you just warm it during the daylight hours.

Better yet, make it double as a giant heated indoor pool (unfortunately, no swimming before dawn).

[phosphorious]

"If this was such a wonderful idea, dont you think water which falls down those huge dams and and generates hydro electricity would have been pumped back up in a perpetual cycle?"

According to wikipedia, this is exactly what's done: hydroelectric plants sometimes use their output to draw water into resevoirs, which act as batteries, storing the energy for later use. You get something like eighty percent of the energy used to fill the resevoir back when you empty it.

Assuming scott is right about the future cheapness and effective ness of solar cells, this might be a useful method for storing energy in a hiusehold situation.

But I have no idea how big such a resevoir would have ot be to be useful.

[Mike]

As a first step, how about just an adobe (or brick) house? The Spanish used these in California and the Southwest because they are suited to the climate. During the day, they tend to be cooler than outside because the heat takes a while to penetrate the material, then at night they are warmer as the walls release the pent up heat. I lived in a brick house in New Mexico and we didn't have air conditioning and did fine. Kept the house closed up during the day, opened it up at night and ran a fan to bring in cool air.

[Arno]

There is a better idea - you already almost mentioned it. When solar power is that cheap, everybody will want to drive an electric car anyway. Electric cars have batteries and if everybody plugs in their car when they are not driving, all this storage capacity can be used to stabilize the net. Your car won't get charged overnight - it will instead put power back into the grid, while it is charged during the day, during work time when you don't use your car anyway.

[Satyam]

this is utopian .... can't be done .... Scott I can't believe you fell for such childish ideas.... the amount of energy required by your motor to pump all the water up into the reservoir would be equal to the amount of potential energy released by the water while falling down .... and factor in the inefficiencies and the fact that you cannot capture all the potential energy released, you end up expending more energy in getting all the water up than you get.
If this was such a wonderful idea, dont you think water which falls down those huge dams and and generates hydro electricity would have been pumped back up in a perpetual cycle?
Thats why if u want to use potential energy for your good then you need to take advantage of some natural phenomenon for the refilling.... maybe collect rain water or sth .....

[I once was a sailor named bob...]

To all of the anti-nuke people...

The US Navy has more than a 40 year safety record with Nukes. AND they have to float and work while being shot at. I think we are at a level of technology were we can do it safely.

Think - Chernobyl was before we even knew what the hell happened in a reactor, and 3 mile island was just a scare. but we know more now.

[Andrew]

Have you considered ice plants instead? They offer a similar energy storage function, except they can also provide direct cooling without an additional conversion step.

Underneath Stanford University is the third largest ice storage facility in the world:
http://facilities.stanford.edu/energy/iceplant/index.html

I toured the inside of the tank before it was filled...it's almost like going into the Grand Canyon.

(Ok, i exaggerate a bit)

[Mr. Wampus]

All we need is for someone to invent a sort of "Gravity Mirror." That would be a device that reflects gravity. The effect would be like when you put 2 magnets together with the magnetic poles reversed so they repel each other. The gravity mirrow would not require an external power source, it would simply reflect existing gravitational pull and therefore push against it. I'm sure it would have to be made of some sort of exotic material that would itself be controversial, like baby kittens. Nothing is entirely free, but we'd learn to live with it.

You could adjust the lifting power by changing the angle of the gravity mirror to the source of gravity. To keep it stable you would probably have an array of smaller gravity mirrors that reflected in different directions. If you pointed them all straight down at the earth, the platform would accelerate at 9.8 M/sec squared away from earth. That would solve a bunch of problems for NASA. Come to think of it, it would solve a bunch of problems for the airline industry too. In fact, gravity would become our chief source of energy. It is abundant and it would be essentially free.

Now all we need to do is figure out exactly what gravity is and how it could be "reflected" and we'd be in business.

OK, I've done the hard part of thinking up the idea. Someone else can thake this and implement it and pay me a small but endless royalty.

[rjf]

What about flushing toilets?

[Mark]

Interesting thought, but too much political hype like ethanol. Instead of feeding corn to livestock or making cornmeal, the US makes hooch out of it, and helps to drive up the price of food.

Instead of ensuring clean water supply for everyone, rich nations hoard water so they can heat/cool their homes while using all the power hungry devices with all the lights on 24/7.

[cvantuss]

You are all wrong. The answer is 42.

[Drew]

Scott, you aren't trying to put together a super army of geniuses, because you know they aren't any good in a combat situation, right? Are you going to get all of your smarter readers into MENSA with you? I always get stuck in the shallow end of the dream pool...

[Stomper]

To all advocates of nuclear power:

It is misleading to refer to nuclear energy as "clean," unless:

1. You have figured out what to do with all the primary and secondary radioactive waste (not in general terms, but in specific terms that account for the NIMBY problem); and

2. You have figured out how to eliminate the kinds of engineering errors, human errors, "deferred maintenance" issues and earthquakes that can lead to Three-Mile Island, Chernobyl, and other disasters (real or potential).

I have yet to see any practical solution to the first, and the second is impossible to solve. Nuclear energy may still be the answer, but let's not delude ourselves about the costs and risks involved.

--Stomper

[AC]

"...but wouldnt' creating that many windmills cause dragon on the earth resulting in higher tides and possibly messing up how fast our days are?"

We have ripped down so many trees all over the planet which has reduced the natural drag. Adding some drag that was eliminated would be a good thing.

[Tibor]

Instead of water you could lift and lower your ENTIRE HOUSE to store energy:
During the day use an electric motor to slowly lift your entire home one store, making the basement become first floor.
Then during the night let your house slowly lower again, making it spin the electric motor backways becoming a generator...
Just remember to use the correct door when going to work on the morning ;-)

[Roger]

Some interesting reading here on perpetual motion / free energy.
http://www.surphzup.com/index.html
http://www.overunity.com/index.php/topic,4540.new.html#new

[Richard]

Scott, what about a global power grid for solar energy. We already have national power grids. Seeing that it is always daylight somewhere in the world, we could just hook all the grids together and have constant solar power, day and night.

[Yes.]

I'd be half crazy too if I learned to draw poses out of a Penthouse magazine. That's not art training, that's some crazy $hit. Hell– I'd feel ripped off if that were my childhood.

[Sillyones]

If I told a plumber that I was hiring an architect to design my new house, I doubt he'd be offended I didn't ask him to do it. LOL!

[henry]

did very deep holes in the ground until you get to the hot bits, pour in water. Job done.

[SkiBumPaul]

On a larger scale, it's been done already. See here:

http://www.nypa.gov/facilities/blengil.htm

[Jaime Bakulic]

...not a lot, not a few.

[Greg]

Why would each house need its own reservoir? In NSW, Australia, excess power generated within in the grid can be used to pump water back up the hydro electricity system. If all houses had the solar panels, excess power during the day could be fed into the grid and do the same. A few large projects would provide economy of scale and probably better efficiency than many small systems.

[WCE]

You forgot to provide an envelope.

OK, really, the right answer is "none of the above".

You want heat at night, you want cool during the hot part of the day, and you have lots of water. Pumping it isn't the right approach, using it as a heat-sink is. Water has one of the highest specific heats of any material (meaning it takes more energy to heat it up than just about anything else). So your swimming pool out back is average temperature of the day, minus a few degrees from evaporation. That's usually cool enough to use to replace your air conditioner with a simple heat exchanger. (Think "big car radiator + fan, plus a pump to circulate the water.) Your A/C bill just dropped to almost zero. At night, you use the same system to pump air through the water you warmed up all day while cooling your house. Your heating bill just dropped to almost zero.

If you don't have a pool and you aren't living where it gets hot during the day, then you buy a septic tank and byry it in the backyard and do the same trick, except you add a heat pump and use the water in the septic tank (not hooked up to your toilets, a separate dedicated heat reservoir with brine to keep it from freezing) to store the heat from the day for the following night to keep you warm. Heat pumps are more energy-efficient than A/C units or furnaces, but do require a fraction of the energy produced to run, and the buried-brine-tank method is the most energy-efficient approach. (On second thought, you'd probably want something other than brine, as it's corrosive to metals. Antifreeze would be bad for the groundwater if it leaks, so maybe somebody with a chemistry background could provide a good alternative.)

You lose energy when you compress air because air gets hot when it compresses, and that's heat that's generally lost by the time you want to use the compressed air for energy. When the compressed air is released, it's cold air at lower pressure than it was in the tank when you compressed it.

Water pumps are not efficient, either to get the water up into the attic or to recover the energy later.

You lose energy whenever you convert it, so the fewer conversions the more efficient the system. Just save the heat from the hot part of the day for night, when you need the heat. Water's cheap and good for storing heat. Underground concrete tanks are already available cheap (septic tanks cost about $500), and you really can use car radiators as viable heat exchangers. The rest is plumbing, and PVC pipe is about as cheap as anything gets.

[Michal Malkowski]

"I have the smartest readers in the world(...)" ...among some, that don't know how to roll to 4th panel...

[Dave]

[Hardcore leftist? If anything, he's Libertarian, which is more right than left. But for the most part, he doesn't play the whole political game, and is rather neutral in that regard.]

Yes, that certainly explains all the posts he has made supporting Obama. Excellent point.

[daniel]

Hmmm... "smartest readers in the world" = "DMD"

It just sounds nicer... and makes us/them dance differently...

[dan]

Of COURSE this won't be "efficient" in terms of energy used to store, vs. energy returned. BUT if the energy used is FREE, and there is enough, then efficiency is not the issue.

If you have sufficient energy during the daylight hours to store enough energy to get you thru the night, and the energy is free and plentiful (wind, sun), then the efficiency concerns matter much less or not at all, compared to use of a limited consumable energy resource (fossil fuels).

And if you add in some consideration of by-products, pollution, etc., this could be attractive.

So the question is not really how efficient this is, but instead what is required to store enough energy to get thru the night. If it requires enough water to fill a lake, and 10 acres of solar or wind energy collectors, then it is not a practical home solution. But if a roof covered in solar cells can pump water into some sizable attic water tanks to do this, more interesting, independent of efficiency.

[neoviky]

The theory of Stupid People:

http://neoviky.blogspot.com/2008/04/stupid-people-part-1.html

[Nathan]

Well I don't know anything about water but when people mention windmills I have a good question...There was a program on TV that said in the US, we have enough wind energy in the plains to power the whole country. but wouldnt' creating that many windmills cause dragon on the earth resulting in higher tides and possibly messing up how fast our days are? As for free energy, I have always wondered where the electricty from lightening goes. Static electricity goes into the ground but then what??? To power the dirt?

[obiDonWan]

I'm no engineer, but you gotta fix the Sunday strips. The new model doesn't permit me to read more than the first 3 panels. I had to go elsewhere to view the complete strip.

[Annabelle]

"thats ok, as long as they do good work"????

first dilbert cartoon without a punchline, is it?

[D. Mented]

Sterling engines. Nevada, Arizona, etc.
Wind turbines. Montana, Wyoming, etc.
Water turbines. Already in use in rivers, could be put to use on coasts - leaving only short slack-tide periods where batteries must be used.
Energy is feasible - it's water that's a limited resource...That and arable land...and nice red uniforms - no- strike that...
D. Mented

[Inquizitor]

While an interesting mental exercise alot of this is impractical regardlesss of how much water you have to store. It will take more energy to get the water into position than it is worth.

Although... I suppose technically you could have some sort of pump that would operate during the day ia solar power and pump the water that fell the previous night back up to the top. Then as the sun sets a solenoid could trip and open teh floodgates (as it were) to provde hydropower at night. Alternatily you could always save power ona treadmill and carry the water yourself but the numbers I am seeing people spout off here so far indicates that that would be ALOT of water.

[Thomas]

Dinorwig power station works on this basis.

http://en.wikipedia.org/wiki/Dinorwig_power_station

http://www.fhc.co.uk/dinorwig.htm

[Adrian monk]

Congratulations to William SMith, you have just invented a perpetual motion machine.

[Leora]

Check this email. The solution is on the back of the envelope I sent it in. Hope the postmark didn't cover any of the important numbers.

For some reason this topic made me have to pee.

[Jnny]

I think Robert X. Cringely has the smartest blog readers in the world, but yours are close.

Scott's a leftist? Hah!

Flywheels made of composite materials on magnetic bearings are the safest and most efficient way to mechanically store excess energy, not water or air reservoirs. The structural requirements to build even a modestly powerful liquid tank system would completely alter the way houses are built and add tens of thousands of dollars to the cost. Gaseous storage tanks increase the danger of explosive failures. A flywheel could be bolted to any of the existing structures that are already used in normal house construction and take up about the same volume as a heat pump and air handler. Because the flywheel is composite, failures could be contained within a modestly armored enclosure.

[Dwight Schrute]

[As far as I can tell, your readers are either hardcore leftists like yourself, and people who like to mock hardcore leftists.]

Hardcore leftist? If anything, he's Libertarian, which is more right than left. But for the most part, he doesn't play the whole political game, and is rather neutral in that regard.

[Jason Dumler]

According to what I've heard, the electric company charges less money for power during off hours - like say midnight through 8 am or something like that. I haven't confirmed if this is true. However, if it is, you could buy power during that time to charge your battery (gravity battery, efficient capacity battery, whatever), and then bleed that power off during the day when power is more expensive, saving money and lightening the load on the power infrastructure.

[Armando Esteban]

Thank you on the smart part!

[jerry w.]

I've found that running a wire underground to a nearby unoccupied house,

and connecting it to their lines produces basically free energy. Of course,

this only starts when the house gets sold and new owners move in

and turn on the power.....

By the way, this energy costs you the same day or night, and there's no

water storage mess, maintenance, or noisy pumps for you to deal with.

http://boskolives.wordpress.com/

[Bob]

Hmm. What about a large triangular-shaped container, flatish and longish, suspended in mid air from a windmill frame and attached to a sail directly above it or something else to encourage a rocking motion? It would have to be small enough to fit in the average back yard. The container would be filled with a foot or so of water that would move from side to side inside the container with every wind shift and run over lots of mini-generators as they have in some of the newer hydroelectric dams.

[LA Clay]

If you lived in a bubble under the ocean, you would have all of the water and pressure needed to complete this. You also have the lifestyle for this to work.

[Arby]

No need to have a battery or energy storage system for each individual home or business. These storage devises are intrusive, expensive, high-maintenance, depreciative, and can be dangerous.

Most homes are perfectly well served with a working in-place utility power grid. The ideal setup for the average individual home and business owner is a grid-tie solar electric system (or hybrid solar/wind). The money spent on a power storage system is better invested in more solar panels (and/or wind generator) that increase productivity during the high-peak demand of daytime.

What is missing though is a mass produced, affordable medium sized car (hopefully American built) with a hybrid power system with batteries that are re-chargeable from any 120 volt AC socket. The homeowner is then almost completely free of the gas pump. If the car hybrid runs on natural gas with a home gas compressor they are completely free of Middle East oil.

Sure all these systems are expensive, but the price of fossil fuel in terms of dollars and an untenable world wide geopolitical situation are dramatically going up. Therefore, the payback for the investment gets shorter and shorter. China and India alone will gobble up any new fossil fuel production.

Of course, all this is predicated on a massive mandated conservation and recycling agenda. Detroit and the American public seem never to learn the lesson of the 70s when Toyota and other Asian car companies kicked their collective butts with smaller more efficient cars.

The biggest waste in America are all the SUVs going on sale now that the prices for fuel rise dramatically and demand for them is dropping thru the floor. Big home, big cars, big buildings, big lawns, big everything is stupid and only will lead to the haves and the have nots at each other’s throats.

Even people who can afford solar systems are not putting them in. Wonder when the “light bulb” will go on over their heads (hopefully a compact florescent one).

[Nimh]

Hey,
BTW, I just wanted to let you know that you're Daily aren't being sent out as regularly as they used to since the changeover. I missed April 29th + 30th (I think), and the last 3 days worth of strips came in all at once. Just in case you had missed that bug.

- Nimh

[Bob Paddock]

The Amish have been using RAM or "Thromp" pumps for hundreds of years
to pump flowing water to higher heights. The kinetic energy of the water
itself pumps the water to higher storage tanks.

http://www.unusualresearch.com/Pump/bellocq.htm
Bellocq, Toribio, Compression Wave Pump and Thromp Pumps

http://www.grit.com/article/2008/03/Move-Water-with-Water.html

To Mark Thorson's comment on Human Power, there are practical limits
to that:

http://blog.obscureresearch.net/bicycle_generator/20071122-103044-Putting-Bicycle-Power-in-perspective

[Bloodboiler]

I'm pretty confident that a fridge size hydrogen reactor (fuel cell etc.) has better chance of working than huge pools or compressed air.

[Rabbit]

Um...

It would take more energy to run a pump to fill the reservoir than one would get back in hydroelectric power. First you need to take into account moving the mass of water from a lower elevation to a higher elevation. Say if the water is provided directly from your house. I don't think that your backyard would be forming a natural reservoir. If it was you would have bigger problems than the need to generate free electricity. The depth of the reservoir will directly determine its potential to generate electricity because water pressure increases proportianly with water height. The problem is that you need to fill the tank and that is where you would loose more energy than you gain.

[cky625]

Three 150kw generator is able power that 80% pool for individuals.

[Azi]

Dave: why should leftish/anti-leftish leanings have any impact on smartness?

[velkairiwyth]

Well, look at it this way - I shall make up some figures that may be miles off - some are estimates from whats in my head at the time of typing. Small dams generate about 10Mw of electricity - think of the size of them. Perhaps it is possible to just use something similar to a watertower and have one in each neighbourhood - powering maybe 20-30 houses - and it will only be used at night. (or a tower sticking out of your roof some 50ft up - doesnt sound safe or cheap to me though :p)

The energy generated is from a form of pressure called "head" and the only variable is the height at which it descends from - producing force which spins a generator.

Im not 100% clear on the functioning or how much energy is needed - this is something I did study at college some 5 years ago though.

Nuclear power plants are used to pump water back up Dams at night when they are producing an excess of energy that is not being used - basically using the Dams as a big battery, I believe this is what you are theorizing here and it will work - provided you can get solar cells efficient enough that there is excess energy left over that can be used to pump the water.

Theoretically - lets say your solar panel generated 2Mw of energy (thats a hell of a lot but its just a random number)

say 50% of this was used that leaves 1Mw

your pump is not likely to be more than say 50% efficient so thats 0.5Mw that is required. Your generator would then be say 50% efficient again, thats 0.25Mw - minus losses from the lines and any other parts involved lets call that 0.2Mw - thats 10% efficient.

One way of looking at it is over the course of a 10 hour period where the solar panels are just being used to store this water energy - you get 1 hour use when required.

Anyway, ill end my rant there - please note most of these figures are made up or approximations from memory - I like the idea though :)

There are other possible solutions such as using the panels to heat up some items that contain their energy for a long time, or for charging up simple battery cells - such as a prius battery - that should be enough to light your house for a night if your up surely?

rant over. Im late for work now :p

[Alex]

I know a scientist who's working on energy reclamation (specifically, recovering energy from wasted heat in smoke stacks). He says that the most promising current method of short-term energy storage is paraffin wax. Solar energy or any form of heat energy melts the wax, and energy can be reclaimed as the wax melts.

The Wikipedia entry on "Solar Energy" discusses wax briefly. I'm not entirely sure how long the wax takes to cool completely, but you I would imagine that a giant hunk of wax being heated all day would stay hot for quite a while...

Wax seems like a great solution because:
a - it's probably dirt cheap
b - it contains no moving parts
c - it lasts a very long time

I haven't really researched it in depth, but it's probably worth looking into.

[Martin]

Forget compressed air.
The efficiensi of compressed air is only a few percent.
run a compressor with 6kWh and you you could take out at most 600 Wh from a generator. you lose a shit lode of energy to heat.
And as for water i think it would be to bulky.

[Rob Landley]

Yeah, about the compressed air thing:

UT has been trying to find ways to store the energy from windmills and solar for years. (Austin has hippies and a big university. The reason we've got such a big software industry in this city in the first place is the hippies aggressively recruited software development companies back in the early 1980's because that industry doesn't pollute. Who knew there was _money_ in it? The money attracted republicans the way sugar attracts ants, and now we have toll roads everywhere. Oh well.)

Anyway, we've been doing alternative energy stuff here for decades, and they've tried a bunch of things to store electricity, or mitigate the need for it. (The biggest electricity user in the summer in texas is air conditioning, but they couldn't get the land to freeze a lake to run a cooling loop through downtown. The terrain around here's all wrong for it, and land prices are way too high.)

But one thing Texas _does_ have is old empty oil wells. And those are under granite around here, sealed airtight bedrock. (If it wasn't, the oil would have leaked up to the surface millions of years ago.)

So several years ago they put lots and lots of compressed air down one of these an old oil well, so they could let it out again to spin a turbine. And they forgot about Boyle's law: compressing the air squeezed all the heat out of it, and when it got really hot all the heat leached into the bedrock (50 degrees farenheit or so once you get a few hundred feet down). Then when they let the air out, as it expanded again it got cold. Really cold. In fact it turned liquid.

Somewhat disappointed when liquid air came out of their pilot hole. Couldn't spin the turbine with that. They ended up burning it 50/50 with natural gas in the pilot project to heat it back up to a gas to spin the turbine, but I haven't heard of them trying it again since.

Lifting water to store energy might work, but good luck finding enough water. (Heck, if you've got that much, electrolyze it to produce hydrogen and put it into a fuel cell to get the electricity back. Lots and lots of work going on that. Only problem is that water's turning into a limiting factor. No, you don't want to electrolyze salt water, chlorine gas is not much fun.)

[Lyon]

You wouldn't want to vaporize the water in order to elevate it, since that would take even more energy than simply pumping the water up would. Water has a higher specific heat than just about any substance. (specific heat is the amount of energy required to raise one gram of a substance one degree Centigrade). Plus you'd have to re-condense it afterwords.

[Daniel Von Fange]

We've got a pair pumped storage plants in my area:

http://www.duke-energy.com/power-plants/pumped-storage-hydro/bad-creek.asp

[wkwillis]

Graphite block. Carbon doesn't melt till 4000 degrees, you can run some of your solar cells (4000 degrees is infrared radiation that matches your solar cell pretty well) on the heat all night. Heat it up again in the morning. When all your solar cells are working.
You lose 80% of the power round trip, though.
Sometimes nuclear makes sense. Night is one of those times.

[chat]

Thanks

[Nicevil]

Space Based Solar Power FTW. :D

[kls]

Some good inputs, but and here's one more point of view - it's helpful to think about all the reasons you need to store energy at night.

To have hot water on tap? Why not heat some extra water during the daytime (like in a batch solar heater), effectively using the thermal mass of the water as energy storage?

For heating or air conditioning? After doing a proper job of insulation and isolation, can you heat or cool some other thermal mass during the day and release the energy as needed?

Those are usually the two biggies, and most the rest are lighting and electrical appliances, for which you need some non-solar source of nighttime energy, like wind or geothermal, or finally storage, like batteries, potential energy of elevated water or flywheels or compressed air.

[burt]

i wish i was as smart as most of you people...

its always sunny somewhere right? why not have everyone connected by a wire. or maybe train some gerbils. but seriously, nuclear energy is amazing. Everyone is talking about some miracle substance that produces an enormous amount of energy. why not this?

damn russians and their chernobyl

burt (clueless) trub

[Mike D]

Wow, Scott, you live a life isolated from reality, don't you?

[geopax]

If you are looking for efficient mechanical energy storage, why not consider a large flywheel? One or more high-mass, high-rpm flywheels with a low-friction mountings connected to motor/generators could store and release energy efficiently. They could take up less space than water reservoirs and would not be plagued with problems associated with pumping and storing water (corrosion, leaks, evaporation, etc.)

[Jeremy Miles]

No one seems to have mentioned this yet:
http://en.wikipedia.org/wiki/Pumped_storage

[JimG]

If you limit the discussion to household use, then how much power do you need anyway? A standard deep cycle lead acid (automobile) battery with an inverter would provide all of the lighting I would need from sunset to bedtime (except on party nights - then I might use candles anyway). If I didn't raid the refrigerator at night it could probably be off until morning. Refrigeration including AC are the two biggest power hogs for home use. Why not tackle the problem directly? Use intense insulation so that neither AC or refrigeration are required to consume power at night. At present, I'll still need about 10kW to comfortably air condition my house during the day plus use all of my "accessories". I would need to spend $15,000 to $30,000 and wouldn't get an even pay back for 10-15 years - not much of a motivator.

[Albert]

Why not chill a substance during they day and then use the thermoelectric effect to generate electricity at night?

[Albert]

Why not chill a substance during they day and then use the thermoelectric effect to generate electricity at night?

[Dave]

>I have the smartest readers in the world.

As far as I can tell, your readers are either hardcore leftists like yourself, and people who like to mock hardcore leftists.

The smartness of these people varies widely.

[Joshua Jacobsen]

Weird. When I looked up the average American Household Energy Consumption, I found the number to be 40 kwh per day. Everyone else seems to be using a number half as big. I forgot to take into account that it was only night-time hours that need to be accommodated by this type of energy storage... so... half an olympic pool 6 meters above a second.

[Brad K.]

It looks like the more height the better - but then you get into friction of the water against the side of the pipe, and turbulence effects on higher water speeds through the pipe to the turbine - which reduce efficiency and mean more water.

I remember that people that used solar power to pre-heat the water into the water tank saw the most savings. Combine your reservoir with solar to preheat (or vaporize water to be lifted?) water to be used for heating, and you might lower the kwatts needed enough to pay off.

The other thing to remember is that you wouldn't have to replace the entire day's electricity to pay off - even 20% would likely pay out in time. Plus, if you restrict clothes washing and drying to night time, and much of cooking, you can reduce your day time requirements further.

People worried about inefficient pumps and generators need to recall the premise - that we pump when when rates are low, and generate when rates go up. In 1998 west of Phoenix, the day rate from 9 am to 9pm was 14.4 cents/kwh, 4.4 cents at night. That kind of rate difference will cover a lot of inefficiency.

[DWH]

Forget the water,which evaporates and is becoming scarce.Just hook all your alternate energy sources to flywheels.

[Josh Powell]

One of the best things about solar is that it produces power when power is most needed, during the day. Most energy is consumed during the day, much less at night. Houses use very little power at night, so a very small battery system can easily supply that power, while excess electricity created during the day can be sold on the grid, offsetting any costs needed at night.

Another thing I've noticed when these questions come up is that people are always looking for a single energy solution, when multiple means could work better. For example, on cars why not have solar/plug in/gasoline/hydrogen/ethanol power. Car runs first on electricity generated from solar because it is the cheapest fuel and doesn't lose energy for storage, next it pulls energy from the battery where solar or plugin charges it. Next, it pulls energy from hydrogen because it is the next cleanest solution, and finally from ethanol/gasoline.

For houses it could mean a solar/hydrogen/wind/gravity/grid combination. I know, each one of these adds more cost to the equation, so pick your favorite three.

[Basil]

There are cheaper, cleaner, less complicated ways to store energy. If you're dead set on storing potential kinetic energy (as in with the pool), a flywheel is far more efficient. If you're willing to store electrical energy, you can either use batteries (like the ones in a hybrid) or capacitors (like in a loud car stereo system). Capacitors are becoming larger these days and might someday replace batteries, but they're not there yet. If you're looking to store thermal energy, there are heatsinks and heat exchangers.

The reason most of these are not commonly used in houses is because in the end, the energy has to come from somewhere, and we can't store energy efficiently for long.

[HALiverpool]

How about using excess electricity to split water and store the hydrogen, then you just burn the hydrogen to run a generator when there is high demand or it's dark. This would probably be more efficient if all the houses in an area were linked to a grid with one hydrogen producing machine and one generator.

-HAL

[Mark Thorson]

The problem with storing
energy as water is that it's
dead weight. It's not moving
while in storage mode.

A flywheel is moving. That
allows you to make the storage
device much smaller. How
small depends on a lot of
assumptions about what
material it is made out of,
which drives other design
decisions like how big it is
and how fast it spins. But
it's surely much smaller and
lighter than a big tank of
water.

[William SMith]

Treadmills, exercise bikes, and ellipticals (my personal choice) all are relatively simple machines designed to help humans burn off calories. These are closed systems to some degree (most use electricity to power their resistance levels and nifty statistics programs). Would it make more sense, if one were to get a swimming pool sized hydroelectric dam, to combine the water pump to the exercise equipment as a way to generate a fraction of one's own power?

[Erwan]

Everybody agrees on different results, but in the end, water just takes too much space, so here is my take on it:
step 1: Use windmill to fill tank with compressed air
step 2: Use tank to blow air on windmill

Even simpler than the air car (http://en.wikipedia.org/wiki/Air_car).

[Shawn Y.]

Can't the grid itself be your electrical storage?

I think most states require by law that your electric company buy up, at the going rate, any excess electricity you generate back onto the grid. Meaning you should be able to use the national grid as your own personal "battery" for your house.

For example, if there was enough excess solar power flowing into the grid, somebody rich would build a huge pair of lakes seperated by a dam, buy up everybody's solar power to pump the water into the high lake, then run the water back into the low lake when later needed, generating electricity to be sold at a profit.

But that's not your problem as a homeowner, I'm thinking. All you'd need to do is generate enough power, on average, to make up for your average power usage. You wouldn't ever need to worry about where that power is stored because the grid is there to store it for you.

[Kevin]

Not sure if this would work or not, or if it makes sense at all, but... What if you did it in reverse? Use solar energy during the day to power pumps that make the water go to the top of the dam, and then use the power produced by the water to power the house. All of this reduces or eliminates the need for inefficient batteries.

[Maurs]

Natural gas is heavily compressed and stored in massive underground reservoirs. There are already plans to do the same with air, with solar-powered compressors in the daytime (I think I saw that in Scientific American a couple months back). There's no real reason to use water/gravity or compressed air energy storage on a home by home basis; it's much more efficient to do it at a central location and then distribute the energy over the grid at night. I mean, if you really want all homes to be self-sufficently solar powered, they can power themselves during the day while selling excess energy back to the grid to be used to compress air, which then powers them at night. Same idea, just distributed.

[Gsu]

Given that the average home uses 8900 kW-hr = 32 gigajoules of energy annually (http://hypertextbook.com/facts/2003/BoiLu.shtml) and thus 88 megajoules per day:

Arbitrarily assume average night energy usage is 35 megajoules.

A square tank of water with sides of length s meters has potential energy 4900*s^4 joules. A tank to store 35 MJ would measure over 9 meters per side.

The potential energy of gas in a container is the pressure times the volume. To get a fridge-sized container to hold 35 MJ, one would need a pressure of (35*10^6) / (1.5*1.5*2) = 7800000 N/m^2 = 78 atmospheres.

...Plus or minus an order of magnitude to account for rounding, friction, celestial alignment, etc.

[Peter Stern]

I think the solution is to live in an area with lots of rain, with a swimming pool on the roof of the house to catch the water, with a drain pipe at the bottom that would provide enough water pressure to turn the turbine when it does rain. Some extra auxillary drains near the top would be needed in the unlikely event of a really heavy rain to prevent it from overflowing and/or getting too heavy for the structure of the house to support.

[Vlad]

Assuming the existing efficiency of small hydros (70 liters/second for 1 kW - http://www.navitron.org.uk/page.php?48) it would take about 252 cubic meters of water for each kWh (that's a cube with each side ~6.3 meter for each kWh). So yes... you're talking "HUGE swimming pool on top of your dream house" size.

[Aric]

I see a common error popping up in some post's while increasing the hiegt would technically increase the potential power from the water you cant just lower the amount of water you have to account for the minimum amount fo water you need to keep the generator going all night so while a cup of water that is lifted to the stratosphere would power the house in P.E. all would do for a convential wter turbine is get it wet. oh and to all you neigh sayeres there are more ways then electric pumps to fill a water tower tradionally water towers have been filled by wind driven pumps witch means that in reasonbly windy area you wouldnt stop filling the tank at night so it would have to have less capacity. If you look at as a aid to solar cells and city electric it is viable in small amounts

[Rand]

Given all the talk about water volumes, I wonder if the better solution is to take water and use the solar power to split the water into hydrogen and oxygen using electrolysis. Then use a fuel cell to power the home with the hydrogen as its fuel source, and water as its byproduct. Repeat ad infinitum. How much water would that take compared to the gravity battery? I'm not smart enough to figure it out (and I'm at work so I don't have time to try).

[Aric]

We need alittle more info than that Scott. I'am a electrical engineer and right off the cuff I would have to say that depending on the family and the efficiency of the generator if you had a water tower about 20 ft off the ground you would need abotu 2 thousand gallons if your talking about a active family surfing the net watching tv and blasting the radio. Now a quiet early to bed early to rise family would need about 100 gallons just enough to light there way to bed and power there alarms. This of course will get all thrown off if they want to do things like take a long hot shower or wash 5 loads of diaper(my friends have twins and they use cloth diaper) They could drastically change the amount of water needed by changeing the height of there water tower.

[Actuary]

6.

But why bother. It's always light somewhere - spend more time perfecting the electrical grid. Then everyone that has private solar cells and windmills gets their evening or windless-day power from the grid, and water can be used for it's proper purposes, like making ice for scotch.

[Joshua Jacobsen]

Well, if my calculations are correct, an olympic sized swimming pool six meters (19 feet) above a second olympic sized swimming pool could store the energy used by one average american household.

Since the energy equation uses distance (height) that the water travels, you could half the size of the container by doubling the height.

I don't have the kind of knowledge necessary to describe the efficiency of this system, but I have always understood that pumps and compressors are remarkably inefficient.

[Dave Wombat]

Following up, water density is 1g/cm3 2 billion grams is
a tank 5 m deep by 25 m by 16 m wide so a pretty big pool.

[Zubin]

Wouldn't work. You would need to have enough elevation difference on a property to get that water flowing to turn the generators. The cost of pumping the water up to the higher elevation would null out any gains. And there's no way there's enough room either.

Zubin, P.E.
Civil Engineer

[Keith]

Someone check my math:
Wikianswers says avg (1600-2000 sq ft) home use 50 kwatts/day
50 kwatts 1 day = 4,320,000,000 joules
4.32 gigajoules = 160,000 tons at 10 feet at 1 earth gravity
160,000 tons water = 38,344,000 gallons
38 megagallons = 5,126,203 cubic feet of water
5 mega-cubic-feet = 10 feet * 715 feet * 715 feet

so a tank 10 feet deep, 715 ft * 715 ft with a 10 foot drop
of course you'd probably only be 50% efficient so you'd either need twice the water or twice the drop. Your best bet would be to bury the generator about 100 ft underground so you could reduce the water footprint. Of course, each house being a lake front lot wouldn't be bad either.

[Alsadius]

It'd depend on how high you could get the pool more than anything. Hydroelectric turbines are about 75% efficient at converting gravitational potential(E=mgh) to electricity in the context of a large dam, and we'll assume this carries forward to smaller scale projects. For every watt, you'd need 4/3 joules per second of potential energy. Assume a second-story reservoir(3 metre head), m = E/gh = (4/3)/(9.8)(3) = 0.045 kilograms per second, or about 45 millilitres per second. Assuming a 12-hour night, you've got just under two tons of water needed. Now, given that a single incandescent light bulb needs 100 times this much, you'd need a prohibitively large reservoir - in essence, this can't be done at home. But just for reference, one kilowatt hour, which will cost you about 10-20 cents coming from the wall, requires 163 tons of water to be run through a home turbine.

[Dave Wombat]

Per Wikipedia 1000kg of water droped 100m generates
.272kWh. For a house, lets use 10m as viable drop distance
which gives us .03kWh/1000kg of water.

Picking a fairly random number of 50kWh of usage per night
you need 2 billion grams of water stored per night.

[Peter Drier]

“The average household in America consumes 10,656 kilowatt-hours (kWh) per year, according to the Department of Energy,”

= ~29 kWh per day per household.

29 * 3600 * 1000 = ~104.4 million joules of energy per day

assuming 0 loss conditions & pool is 1000 meters above ground

104.4MJ / 100 meters / 9.8 = 10,650 kilograms of water

= 10,650 liters worth, to run the house for the whole day.

There would be loss in a real world situation (probably > 50%) but you also wouldn't have to store the whole day's load worth. probably roughly 1/3 of a day stored at night would cover the peaks of load during the day.

for the air version, see this:
http://en.wikipedia.org/wiki/Compressed_air_energy_storage

[Mike Powell]

I don't think this is gonna work. According to http://hypertextbook.com/facts/2003/BoiLu.shtml , the average home is drawing about 1000 watts at any given time.

To find the height and flow rate of water required to generate 1000 watts, take a look here: http://www.nooutage.com/hydroele.htm#How%20much%20power

So if the bottom of your pond is 10 feet higher than your turbine you'd need a flow rate of 1111 gallons per minute to get 1000 watts. That's almost 3 olympic-sized swimming pools per day..

If you built a 100-foot tower to hold the water, you'd still need 111 gallons per minute. At that rate you'd only need about 1/3 of an olympic pool, which is good since it will be 100 feet above your house.

And this isn't even accounting for surges where you might need more than 1000 watts at any given time; e.g. drying your hair, making toast, heating water...

[David Jacobs]

Potential energy = 1kg x 1m x 10m/s = 10 joules

watt = joules/sec

12 hours = 60*60*12 seconds = 43200 sec

So dropping 4,320 kg a meter would generate a watt of power for 12 hours. If we want a kilowatt for 12 hours we would need to drop around 4 million kilograms.

A large residential pool (say 26000 gallons) is around 100,000 kilograms. So 10 pools of water dropping 4 meters would have the necessary potential energy. Of course you can raise the pool higher to reduce the needed water.

This of course assumes no energy loss in the conversion of potential energy into electricity :-).

[Daniel]

Unfortunately the user group you are adressing never seems to be commenting your posts?

[Eric Miller]

Typical homes uses 8900 kWhr per year = 24 kwHr per day (http://hypertextbook.com/facts/2003/BoiLu.shtml).
24kwHr = 86,400,000 Watt seconds = 86,400,000 joules.
Gravity potential energy: Ug = mgh (http://id.mind.net/~zona/mstm/physics/mechanics/energy/gravitationalPotentialEnergy/gravitationalPotentialEnergy.html).
m = mass, g = gravity acceleration = 9.8 m/s*s.
Assume water (or stone) is lifted 10m.
Then m * 9.8 * 10 = 86,400,000 joules
m = 882,000 kg (= 972 tons).
For the uranium density, that would be 9 cubic meters according to your first link in the last post.
For water, that would be 882,000 liters = 233,000 gallons.
A good size pool is about, oh, 25,000 gallons so that would be a little over 9 swimming pools.
That's a hell of a lot of weight and of water.

[rita mae]

No help here. 20 days and counting.

Rita Mae

[Curtis]

Here is some of the math that would be involved:

Horsepower = gallons per minute x head in feet / 3960

1 horsepower = 746 watts

To generate 746 watts continuously you would need 396 gallons per minute of water falling 10 feet

Of course this doesn't take into account the inefficiencies of the pump/turbine (~70%) and of the motor/generator (~90%)

[Curtis]

Here is some of the math that would be involved:

Horsepower = gallons per minute x head in feet / 3960

1 horsepower = 746 watts

To generate 746 watts continuously you would need 396 gallons per minute of water falling 10 feet

Of course this doesn't take into account the inefficiencies of the pump/turbine (~70%) and of the motor/generator (~90%)

[Craig]

I look forward to the answer. I suspect that way too much water would be needed. But I eagerly await the science.

But, it doesn't have to happen at the home level. We have lots of dams where we could recycle the water back into the dam.

[Brad K.]

Why at night? Why not wind power, with night-rate backup in periods of low wind? With a modest wind generator, you could even sell excess energy back to the utilities company.(They have to buy it from you, a federal program began the requirement some 20 years ago or more. Thank Mother Earth News.)

Leo Frankowski covered a wind-water system in his novel "Cross Time Engineer", about a modern engineer re-inventing the industrial revolution in 13th century Poland.

[Barry]

I did a very, very rough calculation.

Turns out that the head of the water determines the power generated of water flowing through a turbine.

I assumed a house requires 900W, and that the turbine is 60% efficient. Also, that the head was 5m. Don't want it too tall.

After all of that, I got a number of about 690,000 gallons daily (imagine a square box 42ft to a side). This is assuming you can keep the average height of the head at about 16 feet.

[Robert]

I did it back of the envelope a couple of years ago. You basically need a tall hill in your backyard with room for a small lake. Perfectly achievable for a billionaire with an estate, ludicrously unattainable for ordinary homeowners. Sorry. ;(

(I was bummed too.)

Now, it might be attainable if you built a really, really, really tall tower - like >150 feet - and had a merely-gigantic reservoir up there. But then you're talking about hundreds of thousands in construction costs - again, out of the realm of the ordinarily possible.

[jacob]

I guess a high, small water tower could store as much energy as a low, big one. The height difference is just as important as the volume.
The efficiency of your pumps and generators will probably be the most important factor.

[jake]

i don't think they're really the smartest. just the most talkative and handy with a Google search. no one can resist proving a famous person wrong.

[James]

I looked into this a while back out of curiousity. I don't remember the details but I know it has a lot to do with the height the water falls, not just the volume of water. And obviously how much depends on the house size, what they use the power for, and in general how much they use overnight.

If you had say a baseball park in your back yard and wanted to run the lights for some night ball, you might need a bit more water than to say sit in bed and read a book.

The same would be true of compressed air though... A side benefit I had never thought of before was that with compressed air, the decompression would create cold which you could possibly use for cooling in the house or a refrigerator. Just a thought.

Of course the biggest energy usage in homes (not necessarily electrical) is the heating/cooling of homes as well as the heating of water for use in the home. So to really reduce the energy needs you could simply use the thermal properties of water or other heat sinks to capture the sun's energy for use in that fashion. If you are looking for buck-to-bang ratios, that is where to start digging. You can also do a few things like recapturing the heat in waste water, designing the house to lose less heat or cold, and in general push towards less waste of energy.

The real problem is that wasting less and improving efficiency like putting in better insulation or sealing air gaps in your exterior are just not as sexy as cool new solar panels on your roof or other high tech gadgets that let you wear your eco-badge on your sleeve. For real reductions in energy use people could do just that, insulate pipes, seal gaps, boring boring boring...

[F]

Or if it helps 100000 liters is about the size of the average hotel room (all rooms and closets included) in the US.

[Chris Rovers]

Your best bet is probably hydrogen - produce it (and oxygen) from water and the excess electricity and store it, then produce electricity from a hydrogen fuel cell, with water as the waste product.

[Jay]

I don't have the answer, but here are some numbers someone else might be able use to find the answer:

938 kilowatt hours was the average American household monthly energy use in 2005.

(Source, some random web page google gave me: http://wiki.answers.com/Q/How_much_electricity_does_the_average_American_household_use )

divide that by 30 and again by 2 to get a very rough nightly energy use of about 15.6 kilowatt hours.

now hopefully a pump/generator engineer can tell us the volume of water or air needed to generate that much power.

[Rodrigo]

I do not know what to say to your question but I want a pool for me - I'm just reading the brazil practicing the English - beautiful your blog

[DuggleBogey]

I think the height of the water makes a difference too.

[F]

BTW, that's the minimum amount of water. If the distance were less, you would need proportionally more water.

[Scotty]

The A123 battery is a better viable option (smaller, less moving parts, long life span), it's currently in production and in investigation by GM for future hybrid cars. A great spread about it in this month's copy of Technology Review (Magazine).

[letra]

it's not the amount of water. it's the height of the fall.

the flow of water just has to be continuous.
but i think you'd need a pretty big fall. several stories high. i could be wrong though.

[F]

A typical home consumes 1 kW of power. This could be generated by water falling a distance of 200 meters at 1 liter/sec. At 86400 sec/day, each day you would need about 100000 liters of water. This would be a cube of water about 5 meters on a side, or about the size of 1-2 average sized rooms in your house.

[Frank]

The formula for potential energy is mgh, where m is the mass, g is the acceleration due to gravity (~9.8 m/s at sea level), and h is the height.

If you pump a tonne (cubic meter) of water up 100 meters, that will store a little over a quarter of a kilowatt-hour (or about 2 cents worth of electricity).

[tillerman]

Great questions. They are just the kind of questions that were on the scholarship exam for entrance to Cambridge University when I applied back in the 60's. I did win a scholarship... but since then I've lost all my envelopes with backs on, so I can't help you.

[Carl M]

It isn't just a function of how much water. It's a function of how high the water is lifted. Half the water lifter twice the height would (assuming no losses) store the same amount of potential energy.

[friskybeaver]

Based on a quick google, a 4 bedroom house uses 6000kW-hours a year.
(http://www.simplyswitch.com/energy/switchingguide/averageenergyuse.aspx)
Thats about 16 kW-hours, or 57 600 000 joules per day.

Potential energy is mass*g*height, lets say 10m for easy math, you would need almost 600 000 litres of water for all day.

Granted, overnight you would only need a fraction of that, it's still ridiculously impractical.

[mfrise]

Scott,

So the idea is 'as each house has moving water, harness the energy', yes? How about this then squire?: most homes drain from a central outlet to the city sewer, so place a wee turbine generator in between and voila, free energy with every flush. Well, until it clogs anyway, but we'll let the guys in white coats fiddle with that.

mfrise

[Scott]

I don't know about a single house, but I know there is something like this going on in Michigan.

http://en.wikipedia.org/wiki/Ludington_Pumped_Storage_Power_Plant

The difference is, they're pumping water at night when rates are low and selling the electricity during the day when rates are high.

[Kedron]

Say at night one kilowatt (would vary an awful lot between households - do you run an air-conditioner at night...leave your computer on, stay up late using lights, how efficient is your fridge, etc) * 12 hours = 43 million joules. Assuming 50% efficiency in your storage (a number obviously pulled from my ass) this would be equivalent to 880 cubic meters raised up 10 meters, or roughly a swimming pool 50 feet by 80 feet by 8 feet, above a 30 foot dam. As compressed air, lets assume a tank at 1000 psi 6.7 million pascals. Work is the integral of -PdV, so integrating 6.6*10^6*V0/V from volume V= V0 to 66V0 (where V0 is your storage volume, and assuming an isothermal expansion) gives energy = 6.7*10^6ln(67)= 28 million joules per cubic meter, so you'd need about 3 cubic meters of air at 1000 psi.