Compressed air as residential energy storage

[Manu Sharma]

Thinking about the previous post (air car) got me thinking about using compressed air as energy storage. Compressed air is already used in a few places around the world to store cheap electricity in off-peak hours to feed demand during peak hours when it's much more expensive. There are plants in U.S. and Germany that store compressed air underground in caves. This is later used to run a turbine producing electricity. Although there are losses involved but the theoretical efficiency approaches 100%.
http://en.wikipedia.org/wiki/Compressed_air_energy_storage

It's possible that the same method can be used in homes using the air car engine. Here's how it will work...your rooftop solar panels or solar PV concentrators or micro turbines will produce electricity to power the house during the day. The additional power produced will run the compressor that will compress air in underground tanks. This will be utilised later when there's no sun / wind to run the compressed air engine producing electricity. As far as I know, there are no micro applications of compressed air energy storage (CAES).

I have no idea about the efficiency of micro CAES but its performance in the air car (cost per km and range) suggests it's highly efficient. Perhaps the cost of an independent system might not be justified but if you buy the air car you can use it in combination with rooftop generation and some extra storage tanks. In fact, someone in US is already using a hybrid car as a home UPS ( see http://www.priups.com/ ).

I just looked up the French company's site again which suggests that they have thought of similar idea. Under "Further Applications" they mention that their engine, if used as a generator, can produce "15 - 20 kW at a very attractive price." Sounds perfect for a bunch of houses in an apartment with rooftop power generation.
http://www.theaircar.com/further_applications.html

Thanks,
Manu

[mah...@iaindia.com]

Excellent thought sir!

I have one idea! The efficieny of the Petrol car or Diesel car (IC engines) is only 33% remaining heat just goes as waste thru radiator's. Cant we think of using the waste heat to produce air conditioing for the car thru absorption chiller or adsorption chiller. I know the difficulty in manufacturing 1 or 2 TR absorption machine! Also the absorption mahcine requieres water cooled condensor (it is an water cooled chiller not air cooled one).

Bigger size CHP's are already available in the market! but can we make this small and fit into car.

Thanks

Mahesh

[ www.iaindia.com  / www.climaticdesign.com ]

[Manu Sharma]

On 6/5/07, mah...@iaindia.com <mah...@iaindia.com> wrote:

Cant we think of using the waste heat to produce air conditioing for the car

Hi Mahesh,

As far as I know, thermal powered air conditioning hasn't been very successful in the past, certainly not at small scale. Otherwise we would all have solar powered A/Cs in our homes. In a car, it's even more challenging since waste heat is produced at a variety of places and you can't add too much to the weight of the car or you will reduce its efficiency even further.

Thanks,
Manu

[Manu Sharma]

Coming back to compressed air energy storage for homes. Micro CAES is attractive for three main reasons:

Low emissions: Solar power has been criticized by some because, when not connected with the grid, it uses lead-acid batteries for storage which have large carbon footprint. I've said from the start that this is falsely implicating solar power which is clean by itself. Energy storage is a separate component from energy generation. Nevertheless, unlike lead acid batteries, Micro CAES would be completely clean as there are fairly little emissions involved in producing the tanks used for storing compressed air.

Long term storage: Lead acid batteries are good for only a few hours of energy storage as there are big losses involved in storing energy any longer. What I've read on CAES (as used traditionally) suggests that it is the only energy storage mechanism that can go for as long as you want.

Low cost storage: Over the lifetime of a system (20-25 years), the cost of storage should be quite low compared to batteries which need to be replaced every few years.

Another big advantage that is assumed here is that unlike capacitors or flywheels, micro CAES can store very large amounts of energy. In traditional CAES, that's in the range of 50-300 MW. The capacity is only limited by how much storage space you have. It also has the highest energy to size ratio amongst all energy storage technologies. Certainly looks worth looking into.

Manu

[Ing Walter Hartmann]

With a car powered by non-oiled compressed air, which is usually filtered btw., all you have to do is to have the 'exhaust' go into the cabin and the faster you go the cooler it gets there - so traffic jams are really nuisance and might become a 'hot subject'!
... all the best !!!
Walter

[Manu Sharma]

On 6/5/07, Ing Walter Hartmann <ing.walte...@gmail.com> wrote:

With a car powered by non-oiled compressed air, which is usually filtered btw., all you have to do is to have the 'exhaust' go into the cabin and the faster you go the cooler it gets there

That's a good point, Walter. I forgot to add this in my main post. The "exhaust" in the case of an air car is cold air! You can indeed point the exhaust pipe into the car and turn it into an A/C.

But don't try this with your internal combustion engine car!!

Manu

[Ajay Chandak]

The key issue is missed out. From where you get compressed air? 

Compressing air is a very inefficient exercise and does require prime mover like engine or motor to run compressors. As we use engine or motor as prime movers, we finally come back to conventional energy sources like oils and electricity. There is huge power loss in the exercise of compressing air and then trying to get back power/energy from this compressed air.

 

Prof. Ajay Chandak.

----- Original Message -----

From: Ing Walter Hartmann

To: green...@googlegroups.com

Sent: 05 June, 2007 12:26 PM

Subject: [Green-India] Re: Compressed air as residential energy storage

With a car powered by non-oiled compressed air, which is usually filtered btw., all you have to do is to have the 'exhaust' go into the cabin and the faster you go the cooler it gets there - so traffic jams are really nuisance and might become a 'hot subject'!
... all the best !!!
Walter


Manu Sharma wrote:

[Manu Sharma]

Ajay Chandak wrote:

The key issue is missed out. From where you get compressed air? 

I'm not sure if you read all the posts but I did mention in the air car message that the primary source is electricity.

What we are discussing in this thread is using renewable sources to compress air for storage and use it later for generation. Please refer to the first email of this thread.

There is huge power loss in the exercise of compressing air and then trying to get back power/energy from this compressed air.

Not in conventional CAES plants. I'm told that the loss of air is as low as 0.1 percent and that the generation efficiencies are around 80%.

Also, as mentioned previously in this thread, the figures from air car suggest that the process is quite efficient.

Manu

[Kwality Electricals]

Prf Chandak is so very right.

But somebody mentioned , in this message string that the process is nearly 100% efficient ! This contrary to experience.All mechanical rotary systems are so very inefficient and difficult to maintain at peak efficiency.

In this case storage of energy is the sole purpose of Compressed air. , suitable for Solar PV, in places where storage area is available..

Vijay Gupta

[Manu Sharma]

On 6/5/07, Kwality Electricals <gup...@kwalityindia.com> wrote:

But somebody mentioned , in this message string that the process is nearly 100% efficient ! This contrary to experience.

Please pay attention when you read if you're going to pick holes in something.

I gave that figure for the theoretical efficiency of CAES. I even cited the source right in the next line. The link perhaps went unnoticed too so here is the relevant quote from that page

As the isothermal process is reversible, the efficiency of compressed air storage approaches 100% and the equation above represents the maximum energy storable. In practice the process will not be isothermal and the compressors and motors have losses.

Source: http://en.wikipedia.org/wiki/Compressed_air_energy_storage

Manu

[Ajay Chandak]

Dear Manu,

What you mentioned in the article was efficiency of storage. What matters is the efficiency of compressor and efficiency of compressed air motors/generators. Compressor efficiencies are extremely low and it reduces with increase in storage pressure. Net energy output from source energy to power delivered by air engine is the key, and nobody talks about it, because concerned people know what is the catch.

Ajay Chandak.

----- Original Message -----

From: Manu Sharma

To: green...@googlegroups.com

Sent: 05 June, 2007 4:19 PM

Subject: [Green-India] Re: Compressed air as residential energy storage

[Kwality Electricals]

Thank you. yes you did say compressors and motors have losses. But the figure "100%" some how got stuck in memory.

Picking holes ? In discussions some point  could go against one's stand, quite often. One should not be so sensitive.

Vijay Gupta

[Manu Sharma]

On 6/5/07, Ajay Chandak <chan...@sancharnet.in> wrote:

Compressor efficiencies are extremely low and it reduces with increase in storage pressure.

Dear Prof Chandak,

This doesn't seem to be true from the figures of the air car. It can be refueled / recharged from home in under 4 hours for a 200 km range. Also note that the compressed air is stored at a very high pressure of 4500 psi. It's possible that they have come up with a more efficient compressor design.

This also doesn't explain why CAES has been successfully employed in plants for over 30 years and with storage capacities reaching 300 MW.

(source: http://www.doc.ic.ac.uk/~matti/ise2grp/energystorage_report/node7.html )

If it was as inefficient as you claim, it would have been a financial disaster for the power companies at such large capacities.

Net energy output from source energy to power delivered by air engine is the key

I completely agree and look forward to getting the figures for the air car engine when it's released.

Manu