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Over at Hackernews is a link to a WindPower Engineering article from September on Electricity stored as a temperature difference.
This is basically a write up of the Isentropic energy storage system which uses a gravel storage medium and argon gas as a thermal transfer medium:
James Macnaghten and Mark Wagner, co-founded Isentropic, Ltd., a company that is currently developing a new storage technology called pumped-heat electricity storage (PHES), which stores electricity as heat and cold. PHES, Isentropic claims, is cheaper than pumped hydro, is deployable anywhere in the world, and is comparable—and in some cases superior—to pumped hydro with a round-trip storage efficiency of 72 to 80%.
...
An independent study by Parsons Brinckerhoff reports that PHES costs 30% less than pumped hydropower with a per hour storage cost of $103/kWh. Currently the technology is scaled to support up to a 2,000-home town. For this scale, the building housing the system is estimated to be 20 to 40m tall.
Note: as highlighted in the HN threads the storage cost quoted above appears to be capital cost of building the store, not the operating cost.
The Isentropic homepage has an informative Youtube video describing the PHES process, and an Isentropic process is formally defined as one where "the entropy of the system remains constant throughout".
Original HackerNews link. Although relatively old this is an interesting storage technology I haven't seen covered previously on SN.
(Score: 2) by Immerman on Monday November 24 2014, @05:50AM
I think the issue with welding is reactivity - at the temperatures involved you're likely going to be breaking the molecular bonds in the gas, resulting in a bunch of free nitrogen (or carbon and oxygen) which will then react with your metal. Argon though is a noble gas - it's extremely difficult to get it to react with anything, even intentionally.
For thermal systems the only thing that jumps to mind is that argon is monatomic - it's normal molecular structure is just a single unbound atom, which makes it behave almost exactly like an ideal gas, with thermal energy being stored only in the atom motion, and not also as vibration of the molecular bonds.
(Score: 1) by anubi on Monday November 24 2014, @07:41AM
Thanks for bringing up the monoatomic angle.
Thanks for the observations about the gases breaking down during the heat and metallic chemical reactions during welding. I considered N2 and CO2 relatively inert, but then I was not evaluating them in the presence of welding metals.
I do not know how much being monoatomic would affect the thermodynamic aspects, as all the gases I have worked with by and large came pretty close to ideal gas. I know there are correction factors to throw in if one wants to get within a couple of percent. I never have compressed anything but air just for compression, as most of the time I have compressed a gas, I was trying to get it to reject heat and change to a liquid, so none of my pressure changes were very extreme.
I have known I could make a stream of compressed air cool a chamber, but the amount of energy I had to burn in the compressor made this impractical for most things.
I know there is a lot of heat storage and transfer possible with phase changes - and I was not seeing that in this device.
Its not the electricity to heat conversion that I find tricky, rather its the other way around... I have found it hard as all getout to extract usable shaft work from differential temperatures. I can do it, but I run up against the Carnot limit pretty damn fast, and as of yet, I have found no way of getting around it.
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 2) by Immerman on Monday November 24 2014, @08:11PM
As I recall gasses mostly start departing dramatically from the ideal behavior near near the extremes - very low temperatures, high pressures, anywhere the molecules start getting jammed closer together or the chemical bonds start getting excited. Couldn't swear one way or the other about the cold-vacuum extreme. As for his system, at 12bar I'm not sure if he's pushing those extremes or not - but any deflection from the ideal gas law is a loss to entropy, and if he's trying to run an isentropic process even only moderate losses might be worth eliminating up front.
Something about the system bothers me as well, though I can't put my finger on it. Keep in mind though that this system has both a heat and pressure differential to work with - in fact it has to keep them perfectly balanced to avoid altering the entropy levels. When charging he's converting mild low-pressure air to hot, high-pressure air, while simultaneously converting mild high-pressure air to cold, low-pressure air. And vice-versa when discharging. Almost like he's managed to replace the "vertical" entropy-altering legs of the Carnot-cycle with a mechanical interlink, allowing the process to be made reversible.