I'm in the (long) process of building a house. The house will have solar panels, but it bugs me that I can't be off the electric grid entirely. There's no convenient and economical way to store energy at your own house while the sun is shining. But is that technology imminent?

Some car companies are allegedly coming out with vehicles that operate on compressed air. Here's one.


How hard would it be to convert that compressed air technology to a home generator? My solar cells could compress air during the day and the compressed air engine would produce electricity at night. There would be plenty of waste in the process, I assume, but it sounds feasible to me.

I'd also like to have a house with two elevators that are balanced so that when one goes down, the other is pulled up. And I would only use the elevators for going down, so my weight causes one side to be heavier than the other. To slow the descent, I'd be compressing air into my home air battery. If you need to go up, you use the stairs. It's healthier. I'd have a full-power elevator option for the elderly and handicapped, but everyone else would be an energy producer.

Then I'd put the guest bathroom on the second floor so I gain some electricity every time a guest goes to take a whiz. It wouldn't balance out the water use, but it would make me feel better. And every time my wife or kids asked me where some lost item or other was, I'd say, "I saw it upstairs."

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Sep 8, 2008
Before I answer let me first state a disclaimer: I am the business development director for a compressed air battery company but also an electrical engineer with a background in applications with prior experience in fuel cell.
The issue you will run into is size of the energy storage (footprint). In order to supply uninterrupted power the issue for most alternative energy schemes is bridging from the primary source to the backup.
This can be done by flywheels, ultra-caps or as most fuel cells have employ a good old lead acid battery.
The old technology stays around for two main reasons, size and cost.
Most compressed air systems use a turbine which do not do well with variations in load but are efficient at the tuned power level which also is usually much higher than you would require (100KW). The down side is they require a longer bridging time.
Any compressed air system needed to run for 8-12 hours will be limited to < 10KW just due to the amount of air required regardless of what is being used to generate the rotations.
IF you look at a company called Energetix PNU Power (www.energetixgroup.com)
You will see we use a scroll compressor, a generator, rectifier and inverter. All are commonly available parts that have never been assembled in this manner for this purpose. The scroll compressor is used in the airline industry to compress water as well as automotive for air conditioning. The trick is Energetix runs the scroll in reverse as an expander. The air turns the scroll the scroll turns the generator and you have power! The power levels are more in line with what you would need. The advantage of the scroll is the start time (milliseconds) so a simple low tech capacitor can be used to bridge the device between charge and discharge. This also lowers the system cost. The unit has distinct advantages over batteries.
The unit works off of low pressure but uses higher pressure tanks just to save storage room. That may pose some issues in trying to run a compressor from a solar panel. There is NO FREE LUNCH. Lower pressure devices that are being looked at by the utility companies require a massive storage area.
A 5KW device for 8 hours would require >150 cylinders at high pressure or about 85 thousand cubic feet of air. So the limiting factor for your design would be the ability to compress the air versus charge a battery. Otherwise, if you do not mind a big white 2 story low pressure storage tank in your yard you may have a shot. You can bury the tank too.
It can be done and has advantages as the air system doesn’t mind the repetitive cycles of charge and discharge and I did not mention but the escaping compressed air of course is very cold and can provide free air conditioning. I hope you live in a warm climate!
Since you seem interested is saving energy check out the other Energetix devices there is one that is a micro-CHP using heat from the boiler to net meter and another that regulate the voltage going into a home to increase efficiency.

+2 Rank Up Rank Down
Aug 20, 2008
Might be too big for your house, but these guys http://www.activepower.com/ do a compressed air/thermal/flywheel UPS. Good luck.
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May 29, 2008
A big flywheel -- NASA are developing them for satellites and there are some for buses etc, of course you'd probably want to stick it in the ground in case it explodes... hmm, wonder how stable it needs to be to not explode during an earthquake... none of them near you, are there? Thought not.
May 28, 2008

why use solar and stress about what happens when the sun goes down? try compost energy instead


+2 Rank Up Rank Down
May 27, 2008

once i saw something very clever in a japanese hotel. To save water, they attached a special sink on top of the toilet tank, so if you flush it, the water to refill the tank would flow out of the sinks armature into the sink and then into the toilets tank. this way you can wash your hands with the water needed to fill the tank.
Also this reminds your kids and guests to wash their hands after they use the toilet
May 27, 2008
Hello Scott,

at first i thought about hydrogen and fuel cells to be a good energy storage, but available systems for home use with solar/windpower have a overall efficiency (electricity to hydrogen and back to electricity, known as round-trip efficiency) of between 30 and 50%, depending on conditions.
A cheaper standard lead acid battery will deliver up to 90%.

In general, if you want to have more than 90% efficiency, and want to achieve this with mechanic movements, like compress air or pump water etc. you will have a hard time due to the losses to heat and friction.

BTW. Cars that run on air pressure do not really have a mileage of 100mpg as it is advertized on this hompage. They state that the energy which is used to heat up the compressed air is calculated but not the energy which is used to actually compress the air. There are hughe losses, and i do not think that this engines will have a better efficiency than a diesel engine with a plant to road efficiency of about 20%.
Much better are full EV's with Li/ion batteries, as long they do not find anything with more capacity and smaller size and weight.

May 26, 2008
In theory, there is always air flow... if there is clouds, there is a breeze. If there are no clouds, your solar panels should work off of the moon.. at a minimum.. but none the less. Have you thought of putting up wind generators as a "backup" of sorts to the solar. I have heard they make wind generators efficient enough to work off of a short breeze.
May 21, 2008
Ben Franklin had a turnstile in the doorway that put a gallon of water on the roof.

Compressing air gives you a heat problem. Most of the energy heats up the air while compressing it (dW=PdV), and this energy is lost unless you can contrive a way to keep the air hot over the period you want to store the energy.

You're much better off raising a weight, like Franklin.
May 20, 2008
Chasing the Holy Grail.

Efficient, safe and inexpensive energy storage systems are far off and elusive.

The most common off-grid electric system uses solar panels with a standard lead-acid battery pack. There are thousands of these solar off-grid conventional battery systems currently working just fine for residential and commercial users.

The average home can get by quite well with 16 heavy-duty six volt deep cycle batteries wired for 24 volts DC. They take up less footprint than a small rider-mower. The batteries are readily available and add about $5, 000 to the cost of the system with a little more for a charger/inverter, charge-controller, installation (need a battery rack) and other electronics and wiring.

The solar panels are usually 24 volt and are very efficient at charging the battery pack. The batteries slowly up charge from solar panels. They then down charge slowly as the power is used (ideal for deep cycle batteries). The batteries give off minimal gases so should be vented properly.

The batteries require little maintenance (best if put out of the weather in a garage or close-by shed). There are very efficient self water-filling systems and remote monitoring. Properly installed and well maintained this battery pack should last 10-12 years (about the same as the inverter). Need more power? Add solar panels and batteries.

If there are occasional extreme cases of energy use (many no-sun days, hot days etc.) a stationary standby natural gas powered generator is plenty of backup and environmentally sound. Many of these backup generators are never or rarely used once the home or business owner becomes savvy about adopting a realistic energy lifestyle.

Yours will be the only house on the block with the lights on during a power outage. How cool is that?

May 20, 2008
What the frickety frick is the motherdiddly point of ticking the 'remember me' box, if the site still insists that I must login every time I want to make acomment/vote for a strip/rank a comment? What does ticking that box actually do, apart from raising a person's blood pressure?
(Oh, how motherhood has changed so much, and not just my vocabulary...)
May 20, 2008
I don't know about that, but I could see God's Debris doing well at Sundance.
May 20, 2008
Why are batteries ruled out? This is completely doable and you eliminate the inefficiencies of conversion electricity -> airpressure (heat losses of various kinds) -> electricity (less but still some losses). Another option to consider is not storing excess power as electricity. If are on a well, then use excess power to pump water and store it high enough to give you the pressure you desire or you could heat/cool a thermal mass to be recovered later. Maybe the most interesting idea would be to get yourself a Tesla (http://www.teslamotors.com/) or a plug in conversion Prius and dump your excess there and see if there is a way to use that battery pack in both directions.
May 20, 2008
Registering to comment sucks so bad that I felt like registering just to leave my rant.
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May 20, 2008
seeing the number of comments fall into the the 30's made me feel kind of sorry for you, so I went through the hassle of registering. Don't make me regret it by sending a bunch of garbage to the email address I used, please.

Anyway, my kids would just use the "full power handicapped option" even if I yelled at them for doing so. I'd need to incorporate some kind of manual hand crank to raise an additional counter weight for that option so using it was just as much effort (or more) than walking up the stairs.
May 20, 2008
hmm...i would have 3 kids and have a buddy build a couple of those hamster wheel things. ( i would pay him with beer) once the kids where old enough to run, i would raise them to think running in the wheel after the sun goes down is fun. That way, the kids power the house and they stay out of my way.
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May 20, 2008
cant help with the elevators but we have a battery that does store all the electric power you need. not cheap, but at least effective. high efficiency too... they come in 5kw size able to store 80kwh or more. 5 year warranty, 10 year life. two currently in Kenya, about to be installed at safaricom sites (local cell phone operator)
Also can finance over 7 years.
May 19, 2008
Gah! I wanted to mark the post up, but clicked on down :> Sorry :>
+2 Rank Up Rank Down
May 19, 2008
You can install a simple water based energy storgae device, by pumping water up to a holding tank during the day, then let it power a generator as it runs back during the night - or you could make the whole house a battery, just install hydraulic rams under the ground floor. During the day use the excess energy to jack the house up, then let the force of your entire house generate electricity as it slowly falls back during the night.
May 19, 2008
Since your building your house slowly you might consider some old school energy storage, The Flywheel.

Get yourself a truck wheel and axle ensemble (probably for little or no money just effort)

Make a very heavy outer wheel, maybe reinforced concrete. Steel plates, something big heavy and round.
Balance the wheel.

Connect the axle to a generator on one side and an electrical motor on the other with clutches.

Use surplus energy to spin the wheel with the electric motor. Get it back by clutching out the motor and in the generator.

The motor and generator could be the same device but probably easier to not deal with the differential gearing....

You can build this gizmo from an automobile junkyard.

Also you might set up a parallel DC and micro AC electrical system for lighting and device powering. Base it on 12 volt car batteries. Maybe settle on micro USB wall plates and/or a couple popular pin plugs.

May 19, 2008
I don't think that 'getting off of the grid' is such a good idea. You should just be happy to give your excess power to the grid during the high dollar peak of the day, and get back what you need during the off-peak night time hours.
With two meters you will possibly end up with a credit on your power bill (at least in the summer), and you will have helped everyone keep their bill down.
The world currently uses between 12 and 13 TW (terawatts) of power, by mid-century that may be 30 TW. We need to find the other 17 TW of power without burning anything! We will need a lot of roofs, and a lot of other things for that to happen. My favorite is Space solar power (SSP), but we'd need 300 SSP stations the size of Manhattan to get enough power.
See Hoffert etal "Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet" in Science Nov. 2002, for more details, and several more interesting ideas of where that power can come from.
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