# Most efficient way to store excess energy **URL:** \ **Category:** Factual Questions\ **Created:** [June 9, 2014, 9:16pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434 "2014-06-09T21:16:48Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [June 9, 2014, 9:16pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/1 "2014-06-09T21:16:48Z")
Suppose I had a property in the desert that was basicaly a solar farm. For some reason there were periods that I could not sell my power back to the electric company so I decided to continue to generate but store the excesss. What would be the most efficient and practical way to store that energy. My first instinct tells me simply pumping a liquid to a higher level or possibly even lifting very heavy weights that could be slowly dropped to retrieve power. Winch a bunch of old cars up a steep hilll and let them go back down by gravity pulling a wheel. What would be the best strorage method? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [June 9, 2014, 9:29pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/2 "2014-06-09T21:29:35Z")
Magnetically levitated superconducting vacuum flywheel. ---
**Author:** ![Terr](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Terr](https://boards.straightdope.com/u/Terr)\ **Post date:** [June 9, 2014, 9:37pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/3 "2014-06-09T21:37:49Z")
> [@HoneyBadgerDC](#): > > Suppose I had a property in the desert that was basicaly a solar farm. For some reason there were periods that I could not sell my power back to the electric company so I decided to continue to generate but store the excesss. > > What would be the most efficient and practical way to store that energy. My first instinct tells me simply pumping a liquid to a higher level or possibly even lifting very heavy weights that could be slowly dropped to retrieve power. Winch a bunch of old cars up a steep hilll and let them go back down by gravity pulling a wheel. > > (1) What would be the best strorage method? Let’s say you have 10 acre solar farm. In the desert, it will generate around 7 megawatt-hours of electricity per day per acre using modern panels - so 70 MWh per day. That’s 2.52\*10^11 joules. (2) An average car weighs maybe 3,500 lb - that’s ~1600kg. To raise that one meter on Earth would cost you about 15.5KJ. Let’s say you have 1000 such cars. Lifting them all one meter is 1.55\*10^7 joules. So - divide (1) by (2) - to save the energy generated by your 10 acre solar farm in one day you will have to raise 1000 cars about 10 miles straight up (and of course to recover at least some of it, would have to lower them fairly carefully from there). That’s not accounting for reduced gravity up there (which would increase the distance). Not exactly launching them into space, but still quite a distance. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [June 9, 2014, 9:38pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/4 "2014-06-09T21:38:54Z")
For smaller scale, batteries are probably your best bet, though they are a bit pricey with current technology. On a larger scale, pumping water to a higher elevation isn’t all that uncommon. This may be less practical in a desert setting where you’ll lose a lot of water to evaporation. Another large scale option is to store the energy as heat, then use some type of heat engine (sterling engine maybe on a smaller scale, steam engine for something big) to convert the heat back to electricity. A lot of small energy plants are using molten salt for heat storage and seem to be doing quite well with that technology. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [June 9, 2014, 10:14pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/5 "2014-06-09T22:14:01Z")
you could store the energy as heat. heat up a building full of rocks. use the heat to run a Stirling engine to run a generator. depends on your scale, upfront investment, level of complexity. lower efficiency at a lower lifetime cost might work. ---
**Author:** ![filmstar-en](https://avatars.discourse-cdn.com/v4/letter/f/50afbb/32.png) [@filmstar-en](https://boards.straightdope.com/u/filmstar-en)\ **Post date:** [June 9, 2014, 10:22pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/6 "2014-06-09T22:22:51Z")
> **[Grid energy storage](https://en.wikipedia.org/wiki/Grid_energy_storage)** > > Grid energy storage (also called large-scale energy storage) is a collection of methods used for energy storage on a large scale within an electrical power grid. Electrical energy is stored during times when electricity is plentiful and inexpensive (especially from intermittent power sources such as renewable electricity from wind power, tidal power and solar power) or when demand is low, and later returned to the grid when demand is high, and electricity prices tend to be higher. > As of 2020\[u... There are lots of answers to this, none of them very satisfactory. They all have serious disadvantages. Stored water is one viable solution, provided you have a mountain and a lake and a lot of cash. We have no ‘big battery’ solution that is ready for widescale deployment. It is one of the great missing inventions of the modern era and a pressing concern for the renewable energy business. ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [June 9, 2014, 10:31pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/7 "2014-06-09T22:31:36Z")
> [@HoneyBadgerDC](#): > > Suppose I had a property in the desert that was basicaly a solar farm. For some reason there were periods that I could not sell my power back to the electric company so I decided to continue to generate but store the excesss. > > What would be the most efficient and practical way to store that energy. My first instinct tells me simply pumping a liquid to a higher level or possibly even lifting very heavy weights that could be slowly dropped to retrieve power. Winch a bunch of old cars up a steep hilll and let them go back down by gravity pulling a wheel. > > What would be the best strorage method? Well, aside from what’s already been noted, I suppose you could use the excess energy to produce hydrogen, then either burn it or use fuel cells to get the energy back when it’s dark. I doubt this is the most efficient way, but it would work (there would be some technical issue with storage and such). For a small solar farm, I’m guessing batteries in series would be the most efficient verse the engineering costs (it’s what I did with my own roof top solar…I always wanted to try the hydrogen thing, but I don’t have that level of expertise, and the wife wouldn’t go for storing hydrogen in our shed :p). ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [June 9, 2014, 10:46pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/8 "2014-06-09T22:46:02Z")
What if a company specialized in harnessing the energy of heavy loads harvested in mountains or highlands and then had to be transported downhill. Logging, mining, etc. They could go around to any large downhill operations and engineer out a way of recovering some energy at a minimum investment. It might be enough energy to warrant some degree of infrastructure in some places. ---
**Author:** ![j\_sum1](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@j\_sum1](https://boards.straightdope.com/u/j_sum1)\ **Post date:** [June 9, 2014, 10:48pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/9 "2014-06-09T22:48:49Z")
Similar to what is being discussed in this thread: [Offsetting Excess Wind and Solar Power Generation](http://boards.straightdope.com/sdmb/showthread.php?t=725471). I would so love there to be an easy solution. I will say that for both storage and transport, liquid hydrocarbons are hard to beat. If you are considering only energy solutions where electrical energy is to be fed into the grid, then there storage is problematic. I will repost the TED talk I linked in the other thread. [Donald Sadoway: The missing link to renewable energy | TED Talk](http://www.ted.com/talks/donald_sadoway_the_missing_link_to_renewable_energy). Given that the talk is a couple of years old and I have not heard anything further on it, I suspect it is a great idea but a dead end (for probably obvious reasons.) ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [June 9, 2014, 10:50pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/10 "2014-06-09T22:50:10Z")
Keep in mind that \*efficiency \*is not necessarily the most important parameter. As an example, an certain energy vessel may be 99% efficient, but is completely impractical for large-scale use because you can’t transfer more than 10 joules per second to/from it. In other words, _power_ is also important. ---
**Author:** ![Gangster\_Octopus](https://avatars.discourse-cdn.com/v4/letter/g/a8b319/32.png) [@Gangster\_Octopus](https://boards.straightdope.com/u/Gangster_Octopus)\ **Post date:** [June 9, 2014, 11:00pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/11 "2014-06-09T23:00:32Z")
> [@Terr](#): > > Let’s say you have 10 acre solar farm. In the desert, it will generate around 7 megawatt-hours of electricity per day per acre using modern panels - so 70 MWh per day. This is not even close to right. [10 acres will get you about 2 MW (page 20)](new zealand). ---
**Author:** ![Chessic\_Sense](https://avatars.discourse-cdn.com/v4/letter/c/7c8e57/32.png) [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)\ **Post date:** [June 9, 2014, 11:12pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/12 "2014-06-09T23:12:53Z")
Can I piggy-back on your question, or is that hijacking? I was wondering today what the most efficient way would be for me to store all this excess heat I’m pumping out of my house today for release in the winter months. Can I pump weights uphill to do that? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [June 9, 2014, 11:18pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/13 "2014-06-09T23:18:28Z")
> [@Chessic\_Sense](#): > > Can I piggy-back on your question, or is that hijacking? > > I was wondering today what the most efficient way would be for me to store all this excess heat I’m pumping out of my house today for release in the winter months. Can I pump weights uphill to do that? A huge pool of water. You freeze it in the winter, and use it as a heat sink for a heat pump in the summer. Then, when it’s nice and warm, you reverse the process to heat the house in the winter. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [June 9, 2014, 11:24pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/14 "2014-06-09T23:24:15Z")
though a heat pump using the earth as a sink or source is really efficient. ---
**Author:** ![filmstar-en](https://avatars.discourse-cdn.com/v4/letter/f/50afbb/32.png) [@filmstar-en](https://boards.straightdope.com/u/filmstar-en)\ **Post date:** [June 9, 2014, 11:28pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/15 "2014-06-09T23:28:34Z")
That would be a STES system. > **[Seasonal thermal energy storage](https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage)** > > Seasonal thermal energy storage (STES), also known as inter-seasonal thermal energy storage, > is the storage of heat or cold for periods of up to several months. The thermal energy can be collected whenever it is available and be used whenever needed, such as in the opposing season. For example, heat from solar collectors or waste heat from air conditioning equipment can be gathered in hot months for space heating use when needed, including during winter months. Waste heat from industrial proce It kind of helps if you have some suitable bedrock and a large drill. ---
**Author:** ![Terr](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Terr](https://boards.straightdope.com/u/Terr)\ **Post date:** [June 9, 2014, 11:28pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/16 "2014-06-09T23:28:54Z")
> [@Gangster\_Octopus](#): > > This is not even close to right. [10 acres will get you about 2 MW (page 20)](new zealand). Don’t remember where I saw that figure. But your number is incorrect as well. It’s about [1MWh per day per acre](http://www.energymanagertoday.com/it-takes-2-8-acres-of-land-to-generate-1gwh-of-solar-energy-per-year-says-nrel-094185/) - 1GWh divided by 365 days divided by 2.8 acres. So - off by a factor of 7. Divide my result accordingly. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [June 9, 2014, 11:30pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/17 "2014-06-09T23:30:53Z")
Details for an answer to the op would vary on specifics of location and needs The linked wiki article is not bad. Deserts can get cold at night. Storing as heat and using it for your town’s heating needs makes the most sense. Have a fleet of hydrogen cars? Producing hydrogen then. [Flywheels](http://energy.gov/sites/prod/files/Beacon.pdf) are also an option. Space no problem (it is a desert), [new liquid flow batteries](http://www.forbes.com/sites/uciliawang/2014/02/04/here-comes-a-flow-battery-technology-and-a-20m-plan-to-bring-it-to-market/) in development might be a good bet. Near a large salt water body? Use the excess to desal instead. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [June 9, 2014, 11:40pm UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/18 "2014-06-09T23:40:54Z")
> [@DSeid](#): > > Details for an answer to the op would vary on specifics of location and needs > > The linked wiki article is not bad. > > Deserts can get cold at night. Storing as heat and using it for your town’s heating needs makes the most sense. Have a fleet of hydrogen cars? Producing hydrogen then. [Flywheels](http://energy.gov/sites/prod/files/Beacon.pdf) are also an option. Space no problem (it is a desert), [new liquid flow batteries](http://www.forbes.com/sites/uciliawang/2014/02/04/here-comes-a-flow-battery-technology-and-a-20m-plan-to-bring-it-to-market/) in development might be a good bet. Near a large salt water body? Use the excess to desal instead. This is a fanatastic method, simply storing charged electrolite that can be cycled back into batteries as needed. My favorite so far and so simple ( I think) ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [June 10, 2014, 2:15am UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/19 "2014-06-10T02:15:10Z")
More on flow batteries from a recent Science article (April 25 2014 but behind the wall). > [@](#): > > … Unlike traditional batteries, which pack their chemical power supply and the electrodes needed to tap it into one package, flow batteries separate those two jobs, storing energy in tanks of liquid electrolyte that can be scaled up to industrial dimensions. That strategy could help them avoid an unwelcome trade-off: Normal batteries are good at producing either large amounts of power for short periods or small amounts of power for days. “The key challenge is to find a battery system that can span these two,” … > > … The vagaries of sun and wind will create peaks and valleys of renewable generation, which will have to be smoothed out to provide sustained, reliable power. “Most people believe that when renewables make up 15% to 20% of [electricity generating] capacity, the intermittency issue will become a problem,” Fisher says. > > Not everybody agrees with Fisher’s numbers, and there are alternatives to energy storage for coping with the fluctuating power from renewables—for example by shuttling power between different regions of the grid. But many energy researchers see storage as key. … Today, many communities store electricity by using it to pump water to hilltop reservoirs, and then letting the water run back down through turbines when the power is needed. Such pumped hydro setups typically store power for about $100 per kilowatt-hour of capacity, now the cheapest option. Other companies and communities use conventional lead-acid, lithium-ion, and sodium-sulfur batteries for smaller scale energy storage (see figure). But not all communities have the topography and water needed for pumped hydro, and conventional batteries are prone to fires and typically have trouble producing large power loads for many hours or days at a time. > > That’s where flow batteries could find their niche. … > > … Aziz’s team at Harvard recently offered a potentially even cheaper option. They replaced the metal ions with far cheaper organic compounds called quinones, abundant in both plants and petroleum, which consist of at least one ring of carbon atoms, plus a tail of carbons, hydrogens, and oxygens. They have an electronic advantage as well as a cost one, Aziz says. In most flow battery setups, metal ions typically shuttle single electrons at a time. Each quinone molecule, by contrast, can ferry pairs of electrons—potentially giving quinone flow batteries a higher energy density than most metal-based flow batteries. Going organic carries another bonus: getting rid of water. Redox flow batteries have to keep their output voltage low or they run the risk of destroying their electrolyte by splitting water into hydrogen and oxygen gas. Organics without water can run at higher voltages and thus produce more power with fewer electrodes in the stack, Gyuk says. > > Quinones readily snag and give up electrons. So several years ago, Aziz began experimenting with batteries using hydrogen-containing quinones, known as hydroquinones, as the charge carriers. After some mediocre results, he and his students teamed up with theoretical chemists at Harvard led by Alán Aspuru-Guzik. Guzik and his colleagues calculated properties of more than 10,000 quinones and hit on a likely top performer, abbreviated AQDSH2—a compound nearly identical to one naturally found in rhubarb. … Lots more in the article if you can get to a library to read it. One other battery option might emerge as EV batteries age out of their useful lives. An EV battery becomes inadequate for the task when it gets down to something like 70 to 80% of original capacity (range gets too limited) - but [repurposing these used batteries](http://www.autonews.com/article/20130506/OEM05/305069985/2nd-lives-for-ev-batteries-could-cut-prices) into large arrays (when they become available in another decade or two) may find utility in utility applications. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 10, 2014, 2:24am UTC](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434/20 "2014-06-10T02:24:50Z")
> [@](#): > > Quoth **Crafter\_Man** : > > Keep in mind that efficiency is not necessarily the most important parameter. Indeed. Capacitors are as near as no difference to 100% efficient, but with current tech it would take way too many of them to store a useful amount of energy. This may change in the not-too-distant future, since capacitor technology is advancing at a ludicrously fast rate, but it’s not there yet. Based on what power companies are actually doing now, it looks like in most circumstances pumped water storage is the most practical, though as stated this depends on your precise situation. Ideally, you’d find some customer that can tolerate the intermittent supply, and sell to them. [Next page](https://boards.straightdope.com/t/most-efficient-way-to-store-excess-energy/690434.md?page=2)