5 ms·

Interesting option. It's all about the cost and efficiencies in the end. Converting electricity into heat and then back into electricity is a lossy process tha

Interesting option. It's all about the cost and efficiencies in the end. Converting electricity into heat and then back into electricity is a lossy process that is constrained by the second law of thermodynamics. Basically generating heat is easy and efficient. But converting it back to electricity is typically less than 50% efficient. Basically, it's several energy conversions in one process: you use electricity to produce heat, heat to produce mechanical energy, and then mechanical energy to produce electricity. All of those steps are lossy, some more than others. And it multiplies. It adds up to less than 50% efficiency, at best. Lithium ion batteries are much better at over 90% efficiency. But they are relatively expensive for large amounts of storage. So they don't scale. But they have no moving parts and you can scale the capacity at which they absorb and release energy pretty much linearly. Which is why they are popular for balancing the grid. Pumped hydro, which is a type of battery that has a relatively large amount of deployed capacity in some countries (many gwh of power stored) is both efficient and cost effective. But it's highly dependent on terrain. Yet parts of Canada run almost exclusively on hydro and pumped hydro.

moffkalast 4y ago
I wonder if it would be possible to scale down pumped hydro to something the size of a tall water tower. Probably not worth it due to the small height difference and volume, and you'd also lose efficiency due to the smaller turbine, but I still do wonder what the actual figures are like for something like that. Especially in cost per kWh stored compared to lithium.
- martin_a 4y ago
  I saw a video like a month ago on YouTube where somebody pumped something like 250 liters of water on his roof (7 meters elevation, if I remember correctly) and let it run through a homebuild generator. The power capacity of that setup was like that of an AA battery, if I remember correctly. You need huge amounts of water and lots of height difference.
  - ed312 4y ago
    Based on some extremely back of napkin math (e.g. doing one of those activities where you cycle to power a light bulb) that seems pretty far off. Was the flow rate extremely low? Have a link?
    - rootusrootus 4y ago
      Sounds like this one: https://www.youtube.com/watch?v=CMR9z9Xr8GM https://www.youtube.com/watch?v=CMR9z9Xr8GM They do say it is about equivalent to a single AA alkaline. It is not exactly an optimized efficient setup, but still, it gets the point across. Pumped hydro is a grid scale option, in places that have the ability to store a lot of water.
      - martin_a 4y ago
        That's the one!
    - dmurray 4y ago
      250 litres x 7 metres is 1750 kilogram-force-metres. Wolfram Alpha [0] says this is about 5 Watt-hours, and gives some other handy comparisons: > ≈ 0.45 × metabolic energy of one gram of fat ( ≈ 38000 J ) > ≈ 0.63 × energy released by burning 1 gram of ethanol ( ≈ 27000 J ) > ≈ metabolic energy of one gram of sugar or protein ( ≈ 17000 J ) > ≈ (0.02 to 0.09) × typical kinetic energy of a car at highway speeds ( 200000 to 900000 J ) > ≈ 0.5 × typical battery energy content of an alkaline long-life C battery ( ≈ 9.6 W h ) > ≈ 0.55 × typical battery energy content of a nonrechargeable Lithium-Thionyl Chloride AA battery ( ≈ 8.6 W h ) > ≈ 0.92 × typical battery energy content of a carbon-zinc D battery ( ≈ 5.2 W h ) Gravitational potential energy is just really, really low energy density. Fortunately it's reasonable to have pumped-storage reservoirs that contain trillions of litres of water. [0] https://www.wolframalpha.com/input?i=1750+kg+metres+in+watt-hours https://www.wolframalpha.com/input?i=1750+kg+metres+in+watt-...
      - RosanaAnaDana 4y ago
        Would you be able to help me figure out the gravitational potential energy of the red hill tanks? https://www.epa.gov/red-hill/what-red-hill-bulk-fuel-storage-facility https://www.epa.gov/red-hill/what-red-hill-bulk-fuel-storage... They were recently decommissioned due to leaks, but I've often wondered if they could be repurposed as gravity battery storage. edit: if you want to check my math. I calculated the tanks at ~567 million liters, and am saying there is 120 meters of vertical distance. They would be equivalent to a 0.1856 GW h (gigawatt hours) battery if repurposed in this way. https://www.wolframalpha.com/input?i=567810000*120+kg+metres+in+watt-hours https://www.wolframalpha.com/input?i=567810000*120+kg+metres...
- prox 4y ago
  Big fan of gravity well storage, it seems simple and elegant. https://energyskeptic.com/2019/gravity-energy-storage/ https://energyskeptic.com/2019/gravity-energy-storage/ The above link has some numbers (2019 had it bookmarked so maybe it has improved)
  - Schroedingersat 4y ago
    That website is full of deranged ramblings which are largely disconnected from reality. Case in point: > MY NOTE: well whoop-dee-doo. China generates 16.2 trillion terawatt-hours (TWh) a day. That’s 64 billion times more than all of the open-cast mines can provide. Better start digging more holes! 16.2 trillion terawatt hours a day is ~4 million times the total global insolation or 35 billion times total global energy usage. This is more wrong than asserting that a single man on a bicycle generator could produce our entire primary energy worldwide.
    - prox 4y ago
      Thank you for letting me know! Did a bit of checking and deleting this from my bookmarks!
      - Schroedingersat 4y ago
        I find it's a useful barometer of what crazy disinformation is going to be spread next because it has been SEO'd so well. It also often covers interesting topics, and many of the stories are actually about something important or useful. Additionally many of the things it 'debunks' are actually terrible ideas, and some of the arguments it uses are valid. For gravity storage the costs and inefficiencies are a bit prohibitive for the use case of an unsubsidized high capacity (more than 1 day) option, and it's not really competitive with batteries for less (and is highly power limited).
- empyrrhicist 4y ago
  Water is just too heavy. Pressurized air would be easier.
  - killjoywashere 4y ago
    Joule for joule, pretty sure I'd rather be standing 10 feet from a failed water tower than a failed pressure vessel. 1/2 m*v^2. That squared gets you every time.
  - moffkalast 4y ago
    I think there's a start-up in Italy working on pressurized CO2 which apparently has some major benefits over using air.
    - VaxWithSex 4y ago
      Yes, higher liquifying point.
- jillesvangurp 4y ago
  It can work, it's just a cost equation. You need a large amount of mass to get meaningful amounts of energy stored. So pumped hydro is nice because you are basically using natural reservoirs and some cheap pumps and pipes. A water tower needs to be built first and only stores a limited amount of potential energy. The problem is not the efficiencies but the cost of building a big enough tower. A tower with the same capacity of a pumped hydro reservoir would be insanely expensive. Or alternatively building many thousands of smaller towers that add up to the same capacity would also not be cheap. There are a few alternative gravity systems being tested out in the form of e.g. big cranes with weights that drive generators, mine shafts with similar systems, etc. Proving they work efficiently is not the issue. It's just scaling their deployment cost effectively.
  - killjoywashere 4y ago
    Why would you need a water tower? If you're in flat geography at sea level, you could probably access tidal energy to drive generators. If you're flat and far from water, couldn't you dig a shallow hole and a deeper hole, and put the generator and pumps in between? I mean, even in Kansas it's not that hard to find an elevation difference of 50 feet.
    - Sakos 4y ago
      > If you're in flat geography at sea level, you could probably access tidal energy to drive generators There are projects testing this, but ocean water is highly corrosive and maintenance is a nightmare.
    - jillesvangurp 4y ago
      It's just an example from the comment I replied to was using; I agree it's not a very feasible thing to do. But I used it to illustrate the difference in cost between that and pumped hydro using natural reservoirs. The point being here that you'd need a stupidly large amount of water towers to come close to the amount of energy you can store in a pumped hydro reservoir Whether it's a water tower, a big crane, or whatever that holds the mass that you then use to drive a generator. You have to build it or use nature to have some natural height difference. Building infrastructure like that is expensive. So nature is preferable and way cheaper. I come from a relatively flat part of the world; so that's not really an option there. Gravity based batteries are nice where they can be done cheaply. But otherwise probably not ideal.
- elif 4y ago
  That's the problem with the US capitalist approach to renewable energy. "Can I downsize this to a scale that makes sense in our economic reality?" Dang, there dies another viable idea. Instead of changing the status quo we have some warm libraries and city halls.
- lotharcable 4y ago
  It is 100% possible. The problem is that it's monumentally expensive. You will need to have a pretty big water tower to handle enough energy to run a microwave for a hour. P.E. = mg(h) So say we have 2,000 liters of water that is 50 meters high. 2000kg * 9.81m/s^2 * 50 * 80% efficiency = 784,800.0 joules. Say you have a 1100 watt microwave. A watt is 1 joule per second. That gives you enough energy to run that microwave for about 11 minutes. A better example would be existing municipal water towers. A 500,000 gallon water tower that is 129 feet tall costs about 2 million dollars. This is based on a public notice I found for a city in the USA. That is 1892700 liters and 39 meters. I am guessing that the average height the water actually falls is 25 meters. The average American household uses about 30Kwh per day. Which is 108,000,000 joules. 1892700 * 9.81 * 25 * .80 / 108,000,000 = 3.4 So 2 million dollars for 3.5 households-days worth of energy. I don't know if having 1 days worth of energy is too much or too little per household. And beyond the cost of the tower itself you need the generators and lines and holding pond for the water. Or underground tank or whatever you want to use. It's very expensive. However it is probably the cheapest option right now as far as current technology goes. It would probably be cost effective for a off-grid shack in the woods, though.
  - moffkalast 4y ago
    > Or underground tank Hmm that is an interesting idea, but it would be even more expensive. If you take that water tower, place it on top of a 80 m hill, then dig down for say another 40 m for the second tank you now have a total of around 160 m of height difference. That would bring it up to 22 households-days or if my conversions are correct, 660kWh, which is not insignificant. Then again if google is correct the price for an equivalent lithium bank would be only like 100k so it's not even close.
Someone 4y ago
If you then use the lithium ion to produce heat, things start looking less favorable for it. Also, I think the “geothermal” part of this means your ‘battery’ gets partly charged for free.
- Schroedingersat 4y ago
  Counter point is if you have work (or high grade heat) you can use it to move heat rather than just thermalising it.
- jillesvangurp 4y ago
  This article is about storing energy generated using renewables in reservoirs; not about tapping into geothermal energy. The issue with geothermal is not that the energy isn't there but that getting to it involves a lot of expensive infrastructure. It's cost effective in some parts of the world and not really cost effective (relative to cheaper solutions) in other parts of the world. Using this instead would mean having to drill much less deep (so it's cheaper). You are sill throwing away more than half of the energy you put in though. Geothermal has similar inefficiencies except you get the energy that needs to put in for free. Not so with this solution: you have to generate the energy yourself.
  - Someone 4y ago
    > This article is about storing energy generated using renewables in reservoirs; not about tapping into geothermal energy. I think it’s about both. They plan to create reservoirs that heat water _and_ use them to store warm water. Also for the latter the outside heat source will provide energy. FTA: Enhanced geothermal systems (EGS) get around this geographical limitation by creating artificial reservoirs. Developers create fractures in hot, dry rock formations by drilling into or melting the rock, and then injecting water into the fissures. Production wells bring the heated water up for producing electricity. “For scales necessary to contribute to national or global electricity decarbonization, we need to be able to extract geothermal heat outside of conventional formations,” Ricks says. […] Once these EGS systems are in place, they would be ideal for storing energy as well as producing electricity. Excess wind or solar energy could be used to inject water into the artificial reservoirs
Retric 4y ago
Using batteries for the grid requires roughly the same amount of batteries as EV’s do. A 100kWh battery pack for a car * 1,000 charge cycles / 25 years = 4,000 kWh of battery per year * ~290 million cars. That’s enough to store almost 1/3 of all current electricity used in the US while ignoring busses and farm equipment etc. Of course actual cost depends on how much storage we need and future battery prices but adding something like 20-50$/month on future electricity bills gets offset by both cheaper electricity from solar and wind and less excess generating capacity. In that context large scale batteries seem like a perfectly reasonable solution. Which is supported by many current grid scale PV instillations including them so they can avoid a AC>DC step when charging the batteries and then sell during peak demand at a premium.
- chriswarbo 4y ago
  > Using batteries for the grid requires roughly the same amount of batteries as EV’s do EVs have much harder constraints. The most obvious is that they need to move around (requiring high energy density, for both mass and volume); they also need to cope with sporadic charging times, and be reasonably fast to charge. It's very hard to compete with the leading Lithium-ion batteries in this space. Grid storage isn't as constrained. Larger, heavier batteries are fine since they're just going to sit in one place. It's also easier to accomodate awkward/slow charging requirements, since they're always plugged in to the grid, and can be coordinated with other electricity sources/sinks if needed. This allows different chemistries to compete, based on e.g. price, longevity, safety, etc.
  - Retric 4y ago
    Yep, it’s very possible that wildly different chemistry or even some other method wins. However, using the same battery chemistry in a cheaper form factor is the worst case. Aka if 2.3 Trillion on EV batteries works then the winner must cost less than 2.3 trillion.
    - macintux 4y ago
      Or have fewer externalities, since that’s never properly priced into anything.
ksidudwbw 4y ago
Not used for electricity just cooling and heating with heat pumps
rnhmjoj 4y ago
> But converting it back to electricity is typically less than 50% efficient. It's generally much much worse than 50%. Say that you used a resistor to heat the water in a (perfectly insulated) boiler and want to know how much useful work you can extract from it, ideally. The answer is given by the exergy efficiency, which can be computed by attaching an imaginary heat engine to it, integrating the infinitesimal works δW = δQ⋅η_Carnot while the temperature drops from the initial T_boiler to T_ambient and dividing by the internal energy of the boiler. The result is η_exergy = 1 - T_ambient/T_lmtd where T_lmtd = (T_boiler - T_ambient)/log(T_boiler/T_ambient) is the logarithmic mean temperature difference. Even heating water to 90°C gives an efficiency of only 9%.
- jbay808 4y ago
  > Say that you used a resistor to heat the water If you're looking to maximize round trip efficiency, this is probably not a good starting point. Better to use a heat pump, as mentioned elsewhere.
  - rnhmjoj 4y ago
    Yes, if you're counting the heat output as part of the useful energy, but parent was specifically talking about converting electricity into heat and back again.
    - jbay808 4y ago
      So am I. If you're going from electricity to heat for storage and back to electricity, you'll get better efficiency by using a heat pump to generate that stored heat. In general, sticking to processes that are as close to reversible as possible will be the most efficient.
  - rnhmjoj 4y ago
    Ok, I see your point now.
socialdemocrat 4y ago
They claim 90% efficiency, so I doubt you can compare this to heating a container and them driving a stream engine afterwards. I suspect they are here also utilizing geothermal heat and heatpumps which allows you to increase efficiency. If you heat water that is already quite hot, then you can get more energy back than you put in. Higher temperature differentials give better efficiency.
pfdietz 4y ago
Pumped thermal storage, where a reversible device is used to generate heat (and cold), then generate power from the heat (and the cold), can have efficiencies of up to 75%. https://aip.scitation.org/doi/10.1063/1.4994054 https://aip.scitation.org/doi/10.1063/1.4994054 "Insofar as the numbers I have presented in this paper are correct, they demonstrate that energy storage is a problem of 19th century science. No future laboratory breakthroughs or discoveries are required for solving it. All that is needed is fine engineering and assiduous attention to detail. Said poetically, this is 21st century rocket science." Even resistive heat can give efficiencies > 50% if the storage temperature is sufficiently high. For example, the ENDURING system from NREL (now being commercialized by Babcock & Wilcox) has a projected round trip efficiency of 53%, using sand heated to 1200 C to store the heat. Heat transfer from sand to the working gas is via a fluidized bed heat exchanger, a nicely compact system with very large heat transfer area. https://arpa-e.energy.gov/sites/default/files/2021-03/07%20Day1-Zhiwen%20Ma_NREL.pdf?msclkid=2566b8e0bdaf11ecbb822211b5b178f3 https://arpa-e.energy.gov/sites/default/files/2021-03/07%20D...

moffkalast 4y ago

I wonder if it would be possible to scale down pumped hydro to something the size of a tall water tower. Probably not worth it due to the small height difference and volume, and you'd also lose efficiency due to the smaller turbine, but I still do wonder what the actual figures are like for something like that. Especially in cost per kWh stored compared to lithium.

martin_a 4y ago
I saw a video like a month ago on YouTube where somebody pumped something like 250 liters of water on his roof (7 meters elevation, if I remember correctly) and let it run through a homebuild generator. The power capacity of that setup was like that of an AA battery, if I remember correctly. You need huge amounts of water and lots of height difference.
- ed312 4y ago
  Based on some extremely back of napkin math (e.g. doing one of those activities where you cycle to power a light bulb) that seems pretty far off. Was the flow rate extremely low? Have a link?
  - rootusrootus 4y ago
    Sounds like this one: https://www.youtube.com/watch?v=CMR9z9Xr8GM https://www.youtube.com/watch?v=CMR9z9Xr8GM They do say it is about equivalent to a single AA alkaline. It is not exactly an optimized efficient setup, but still, it gets the point across. Pumped hydro is a grid scale option, in places that have the ability to store a lot of water.
    - martin_a 4y ago
      That's the one!
  - dmurray 4y ago
    250 litres x 7 metres is 1750 kilogram-force-metres. Wolfram Alpha [0] says this is about 5 Watt-hours, and gives some other handy comparisons: > ≈ 0.45 × metabolic energy of one gram of fat ( ≈ 38000 J ) > ≈ 0.63 × energy released by burning 1 gram of ethanol ( ≈ 27000 J ) > ≈ metabolic energy of one gram of sugar or protein ( ≈ 17000 J ) > ≈ (0.02 to 0.09) × typical kinetic energy of a car at highway speeds ( 200000 to 900000 J ) > ≈ 0.5 × typical battery energy content of an alkaline long-life C battery ( ≈ 9.6 W h ) > ≈ 0.55 × typical battery energy content of a nonrechargeable Lithium-Thionyl Chloride AA battery ( ≈ 8.6 W h ) > ≈ 0.92 × typical battery energy content of a carbon-zinc D battery ( ≈ 5.2 W h ) Gravitational potential energy is just really, really low energy density. Fortunately it's reasonable to have pumped-storage reservoirs that contain trillions of litres of water. [0] https://www.wolframalpha.com/input?i=1750+kg+metres+in+watt-hours https://www.wolframalpha.com/input?i=1750+kg+metres+in+watt-...
    - RosanaAnaDana 4y ago
      Would you be able to help me figure out the gravitational potential energy of the red hill tanks? https://www.epa.gov/red-hill/what-red-hill-bulk-fuel-storage-facility https://www.epa.gov/red-hill/what-red-hill-bulk-fuel-storage... They were recently decommissioned due to leaks, but I've often wondered if they could be repurposed as gravity battery storage. edit: if you want to check my math. I calculated the tanks at ~567 million liters, and am saying there is 120 meters of vertical distance. They would be equivalent to a 0.1856 GW h (gigawatt hours) battery if repurposed in this way. https://www.wolframalpha.com/input?i=567810000*120+kg+metres+in+watt-hours https://www.wolframalpha.com/input?i=567810000*120+kg+metres...
prox 4y ago
Big fan of gravity well storage, it seems simple and elegant. https://energyskeptic.com/2019/gravity-energy-storage/ https://energyskeptic.com/2019/gravity-energy-storage/ The above link has some numbers (2019 had it bookmarked so maybe it has improved)
- Schroedingersat 4y ago
  That website is full of deranged ramblings which are largely disconnected from reality. Case in point: > MY NOTE: well whoop-dee-doo. China generates 16.2 trillion terawatt-hours (TWh) a day. That’s 64 billion times more than all of the open-cast mines can provide. Better start digging more holes! 16.2 trillion terawatt hours a day is ~4 million times the total global insolation or 35 billion times total global energy usage. This is more wrong than asserting that a single man on a bicycle generator could produce our entire primary energy worldwide.
  - prox 4y ago
    Thank you for letting me know! Did a bit of checking and deleting this from my bookmarks!
    - Schroedingersat 4y ago
      I find it's a useful barometer of what crazy disinformation is going to be spread next because it has been SEO'd so well. It also often covers interesting topics, and many of the stories are actually about something important or useful. Additionally many of the things it 'debunks' are actually terrible ideas, and some of the arguments it uses are valid. For gravity storage the costs and inefficiencies are a bit prohibitive for the use case of an unsubsidized high capacity (more than 1 day) option, and it's not really competitive with batteries for less (and is highly power limited).
empyrrhicist 4y ago
Water is just too heavy. Pressurized air would be easier.
- killjoywashere 4y ago
  Joule for joule, pretty sure I'd rather be standing 10 feet from a failed water tower than a failed pressure vessel. 1/2 m*v^2. That squared gets you every time.
- moffkalast 4y ago
  I think there's a start-up in Italy working on pressurized CO2 which apparently has some major benefits over using air.
  - VaxWithSex 4y ago
    Yes, higher liquifying point.
jillesvangurp 4y ago
It can work, it's just a cost equation. You need a large amount of mass to get meaningful amounts of energy stored. So pumped hydro is nice because you are basically using natural reservoirs and some cheap pumps and pipes. A water tower needs to be built first and only stores a limited amount of potential energy. The problem is not the efficiencies but the cost of building a big enough tower. A tower with the same capacity of a pumped hydro reservoir would be insanely expensive. Or alternatively building many thousands of smaller towers that add up to the same capacity would also not be cheap. There are a few alternative gravity systems being tested out in the form of e.g. big cranes with weights that drive generators, mine shafts with similar systems, etc. Proving they work efficiently is not the issue. It's just scaling their deployment cost effectively.
- killjoywashere 4y ago
  Why would you need a water tower? If you're in flat geography at sea level, you could probably access tidal energy to drive generators. If you're flat and far from water, couldn't you dig a shallow hole and a deeper hole, and put the generator and pumps in between? I mean, even in Kansas it's not that hard to find an elevation difference of 50 feet.
  - Sakos 4y ago
    > If you're in flat geography at sea level, you could probably access tidal energy to drive generators There are projects testing this, but ocean water is highly corrosive and maintenance is a nightmare.
  - jillesvangurp 4y ago
    It's just an example from the comment I replied to was using; I agree it's not a very feasible thing to do. But I used it to illustrate the difference in cost between that and pumped hydro using natural reservoirs. The point being here that you'd need a stupidly large amount of water towers to come close to the amount of energy you can store in a pumped hydro reservoir Whether it's a water tower, a big crane, or whatever that holds the mass that you then use to drive a generator. You have to build it or use nature to have some natural height difference. Building infrastructure like that is expensive. So nature is preferable and way cheaper. I come from a relatively flat part of the world; so that's not really an option there. Gravity based batteries are nice where they can be done cheaply. But otherwise probably not ideal.
elif 4y ago
That's the problem with the US capitalist approach to renewable energy. "Can I downsize this to a scale that makes sense in our economic reality?" Dang, there dies another viable idea. Instead of changing the status quo we have some warm libraries and city halls.
lotharcable 4y ago
It is 100% possible. The problem is that it's monumentally expensive. You will need to have a pretty big water tower to handle enough energy to run a microwave for a hour. P.E. = mg(h) So say we have 2,000 liters of water that is 50 meters high. 2000kg * 9.81m/s^2 * 50 * 80% efficiency = 784,800.0 joules. Say you have a 1100 watt microwave. A watt is 1 joule per second. That gives you enough energy to run that microwave for about 11 minutes. A better example would be existing municipal water towers. A 500,000 gallon water tower that is 129 feet tall costs about 2 million dollars. This is based on a public notice I found for a city in the USA. That is 1892700 liters and 39 meters. I am guessing that the average height the water actually falls is 25 meters. The average American household uses about 30Kwh per day. Which is 108,000,000 joules. 1892700 * 9.81 * 25 * .80 / 108,000,000 = 3.4 So 2 million dollars for 3.5 households-days worth of energy. I don't know if having 1 days worth of energy is too much or too little per household. And beyond the cost of the tower itself you need the generators and lines and holding pond for the water. Or underground tank or whatever you want to use. It's very expensive. However it is probably the cheapest option right now as far as current technology goes. It would probably be cost effective for a off-grid shack in the woods, though.
- moffkalast 4y ago
  > Or underground tank Hmm that is an interesting idea, but it would be even more expensive. If you take that water tower, place it on top of a 80 m hill, then dig down for say another 40 m for the second tank you now have a total of around 160 m of height difference. That would bring it up to 22 households-days or if my conversions are correct, 660kWh, which is not insignificant. Then again if google is correct the price for an equivalent lithium bank would be only like 100k so it's not even close.

Someone 4y ago

If you then use the lithium ion to produce heat, things start looking less favorable for it. Also, I think the “geothermal” part of this means your ‘battery’ gets partly charged for free.

Schroedingersat 4y ago
Counter point is if you have work (or high grade heat) you can use it to move heat rather than just thermalising it.
jillesvangurp 4y ago
This article is about storing energy generated using renewables in reservoirs; not about tapping into geothermal energy. The issue with geothermal is not that the energy isn't there but that getting to it involves a lot of expensive infrastructure. It's cost effective in some parts of the world and not really cost effective (relative to cheaper solutions) in other parts of the world. Using this instead would mean having to drill much less deep (so it's cheaper). You are sill throwing away more than half of the energy you put in though. Geothermal has similar inefficiencies except you get the energy that needs to put in for free. Not so with this solution: you have to generate the energy yourself.
- Someone 4y ago
  > This article is about storing energy generated using renewables in reservoirs; not about tapping into geothermal energy. I think it’s about both. They plan to create reservoirs that heat water _and_ use them to store warm water. Also for the latter the outside heat source will provide energy. FTA: Enhanced geothermal systems (EGS) get around this geographical limitation by creating artificial reservoirs. Developers create fractures in hot, dry rock formations by drilling into or melting the rock, and then injecting water into the fissures. Production wells bring the heated water up for producing electricity. “For scales necessary to contribute to national or global electricity decarbonization, we need to be able to extract geothermal heat outside of conventional formations,” Ricks says. […] Once these EGS systems are in place, they would be ideal for storing energy as well as producing electricity. Excess wind or solar energy could be used to inject water into the artificial reservoirs

Retric 4y ago

Using batteries for the grid requires roughly the same amount of batteries as EV’s do. A 100kWh battery pack for a car * 1,000 charge cycles / 25 years = 4,000 kWh of battery per year * ~290 million cars. That’s enough to store almost 1/3 of all current electricity used in the US while ignoring busses and farm equipment etc. Of course actual cost depends on how much storage we need and future battery prices but adding something like 20-50$/month on future electricity bills gets offset by both cheaper electricity from solar and wind and less excess generating capacity. In that context large scale batteries seem like a perfectly reasonable solution. Which is supported by many current grid scale PV instillations including them so they can avoid a AC>DC step when charging the batteries and then sell during peak demand at a premium.

chriswarbo 4y ago
> Using batteries for the grid requires roughly the same amount of batteries as EV’s do EVs have much harder constraints. The most obvious is that they need to move around (requiring high energy density, for both mass and volume); they also need to cope with sporadic charging times, and be reasonably fast to charge. It's very hard to compete with the leading Lithium-ion batteries in this space. Grid storage isn't as constrained. Larger, heavier batteries are fine since they're just going to sit in one place. It's also easier to accomodate awkward/slow charging requirements, since they're always plugged in to the grid, and can be coordinated with other electricity sources/sinks if needed. This allows different chemistries to compete, based on e.g. price, longevity, safety, etc.
- Retric 4y ago
  Yep, it’s very possible that wildly different chemistry or even some other method wins. However, using the same battery chemistry in a cheaper form factor is the worst case. Aka if 2.3 Trillion on EV batteries works then the winner must cost less than 2.3 trillion.
  - macintux 4y ago
    Or have fewer externalities, since that’s never properly priced into anything.

ksidudwbw 4y ago

Not used for electricity just cooling and heating with heat pumps

rnhmjoj 4y ago

> But converting it back to electricity is typically less than 50% efficient. It's generally much much worse than 50%. Say that you used a resistor to heat the water in a (perfectly insulated) boiler and want to know how much useful work you can extract from it, ideally. The answer is given by the exergy efficiency, which can be computed by attaching an imaginary heat engine to it, integrating the infinitesimal works δW = δQ⋅η_Carnot while the temperature drops from the initial T_boiler to T_ambient and dividing by the internal energy of the boiler. The result is η_exergy = 1 - T_ambient/T_lmtd where T_lmtd = (T_boiler - T_ambient)/log(T_boiler/T_ambient) is the logarithmic mean temperature difference. Even heating water to 90°C gives an efficiency of only 9%.

jbay808 4y ago
> Say that you used a resistor to heat the water If you're looking to maximize round trip efficiency, this is probably not a good starting point. Better to use a heat pump, as mentioned elsewhere.
- rnhmjoj 4y ago
  Yes, if you're counting the heat output as part of the useful energy, but parent was specifically talking about converting electricity into heat and back again.
  - jbay808 4y ago
    So am I. If you're going from electricity to heat for storage and back to electricity, you'll get better efficiency by using a heat pump to generate that stored heat. In general, sticking to processes that are as close to reversible as possible will be the most efficient.
- rnhmjoj 4y ago
  Ok, I see your point now.

socialdemocrat 4y ago

They claim 90% efficiency, so I doubt you can compare this to heating a container and them driving a stream engine afterwards. I suspect they are here also utilizing geothermal heat and heatpumps which allows you to increase efficiency. If you heat water that is already quite hot, then you can get more energy back than you put in. Higher temperature differentials give better efficiency.

pfdietz 4y ago

Pumped thermal storage, where a reversible device is used to generate heat (and cold), then generate power from the heat (and the cold), can have efficiencies of up to 75%. https://aip.scitation.org/doi/10.1063/1.4994054 https://aip.scitation.org/doi/10.1063/1.4994054 "Insofar as the numbers I have presented in this paper are correct, they demonstrate that energy storage is a problem of 19th century science. No future laboratory breakthroughs or discoveries are required for solving it. All that is needed is fine engineering and assiduous attention to detail. Said poetically, this is 21st century rocket science." Even resistive heat can give efficiencies > 50% if the storage temperature is sufficiently high. For example, the ENDURING system from NREL (now being commercialized by Babcock & Wilcox) has a projected round trip efficiency of 53%, using sand heated to 1200 C to store the heat. Heat transfer from sand to the working gas is via a fluidized bed heat exchanger, a nicely compact system with very large heat transfer area. https://arpa-e.energy.gov/sites/default/files/2021-03/07%20Day1-Zhiwen%20Ma_NREL.pdf?msclkid=2566b8e0bdaf11ecbb822211b5b178f3 https://arpa-e.energy.gov/sites/default/files/2021-03/07%20D...