Super-capacitors and their uses

This thread belongs in several of the Energy Forums but since its relatively new technology.. sort of.. I'm posting it here.

I came across THIS article on Mouser when I was buying some bits to repair some gear/ finish another project. I was attracted to the shiny 'Energy Harvesting' Header.

The article discusses eventually replacing batteries with super-capacitors, though there are a limited number of situations in which this can actually work now and also talks about harvesting and storing very small amounts of energy in super capacitors which is useful but not the most interesting part.

At some point I saw a product on the market that was designed to sit between ones home and the grid and take the spike in draw when large appliances started up..anything with a big motor basically. The idea was that these devices would save you money on your electric bills but I only see that working for the first couple spikes of the more expensive part of the day. There's a similar concept mentioned in here that is being employed by Hybrid Car Manufacturers that it seems could be interesting for those generating their own electricity off-grid by whatever means and storing with a battery bank. Specifically installing super-capacitors across battery terminals, or I suppose across the output of a bank of batteries if that's the situation and like the product I saw advertised long ago take the 'hit' or spikes in the system when large loads are applied. Smoothing out the draw on the battery or bank and thus improving its life expectancy vastly...according to the article. Having, at this point very little practical experience with off-grid systems and the battery products in use in them I can't say if this would be as useful in this scenario as in a Hybrid Car which is starting and restarting itself constantly... but it could be. Opinions? Is this basic concept already applied in most off grid systems? It seems to me that in lots of the discussions I've seen and the books that I have the batteries are left to do the buffering.

The article also got my wheels turning again on an old idea. I've often thought how large capacitors, or banks of capacitors could be used to buffer energy flow from storage system toward a Load. As the Load changes, the caps take the hit. I've wondered if one could successfully use such a buffer to allow a small off grid system powered by potential mechanical energy to be regulated depending on load...but that's a big digression and another thread.

THANKS!
j

Here is a site with useful projects that use capacitors: http://laserhacker.com/?cat=6 , http://laserhacker.com/?p=219

Can't find anywhere that even comes close to his $6 price per. Finding $80 per and up for the caps. Great cold weather preformance but according to the maxell Data sheets(what was used) life expectency takes a real hit when used at over 60 degree C which the engine compartment in a car would easily reach. Also from the data sheets capacitance drops over time if the device is held in the peak voltage condition for longer periods of time. Using more caps so lower peak voltages apply would help here. The other flaw is the total number of watt hours stored in those vs a battery is way lower. So a car that was hard to start or someone leaving a draw on would more quickly deplete the capacitors than a battery. On the plus side the internal resistance of the capacitors is nearly 0 compared to a battery so their cold cranking amps should be really solid even if the voltage is low.\

C. Letellier wrote:Can't find anywhere that even comes close to his $6 price per. Finding $80 per and up for the caps. Great cold weather preformance but according to the maxell Data sheets(what was used) life expectency takes a real hit when used at over 60 degree C which the engine compartment in a car would easily reach. Also from the data sheets capacitance drops over time if the device is held in the peak voltage condition for longer periods of time. Using more caps so lower peak voltages apply would help here. The other flaw is the total number of watt hours stored in those vs a battery is way lower. So a car that was hard to start or someone leaving a draw on would more quickly deplete the capacitors than a battery. On the plus side the internal resistance of the capacitors is nearly 0 compared to a battery so their cold cranking amps should be really solid even if the voltage is low.\

The author states they bought those for $6 each on sale with normal cost at around $160, that makes $80 sound good.

The engine compartment may reach 60C, but the pack does not have to be mounted in the same place. Many cars have the battery in the trunk for example, or the unit could be put in a slightly finned box at the bottom of the engine compartment. The battery is mounted where it is to facilitate replacement and, in days gone by, maintenance.

I agree, I would want to run them at a lower voltage too. Caps are nice in that way as they have no intrinsic voltage and can be run anywhere under the max.

Battery sizing is based on the quirks and failures of the flooded lead acid battery:
- at half of their life, they have half the life... or half the charge
- they don't like being over charged
- each cell has an innate voltage
- they don't like being under charged, even deep cycle batteries don't do well if drained beyond 50%. So even brand new, a 100 amp hour battery can only supply 50 amps for an hour before needing to be recharged. Car batteries are worse 10% discharge is what they are designed for. That is why the author was able to start the car so many times on a much smaller pack.

Considering these things, I think super caps could be considered as only needing 1/4 the storage of a flooded lead acid battery to be equivalent. Perhaps even less with an inverter designed to work with a wider range of input voltage. The same inverter techniques could be used do provide various DC voltages as well.

So I think large Cap banks could replace much larger battery banks than the straight spec would indicate. The main problem is price. In a car the cost of two batteries in the life time of the car (few people even keep their car that long) is still a lot less than the cost of a set of caps. In the case of an off grid solar system, that may change pretty quick. The cap bank could last 30 years or more in which time the batteries may have been replaced 6 times. The cap bank could be sized at 50% the available charge of a 5 year old battery to fill the same roll. Possibly less with a purpose built inverter with the right a/c d/c outputs. The charge controllers already work with a wide voltage input, the inverters are not right now, but could be designed that way. The output voltage from a battery is about 11 to 15v. A cap bank designed to replace it could supply good usable power from 1v to 15v. The thing is, if the cap bank was a 24v replacement, it could give good use from 1v to 30v. (Note the low end voltage would be entirely dependent on the inverter circuit design) We could use pretty much the whole charge on the caps... there may need to be circuitry with series caps to keep any of them from trying to reverse charge as well as for balancing.