5 ms·

4 meter radius bore, go 2km down, then bore a 10 km tunnel. fill it with water to generate power, pump water out to store power losing 20% roundtrip. (pi * ((4

4 meter radius bore, go 2km down, then bore a 10 km tunnel. fill it with water to generate power, pump water out to store power losing 20% roundtrip. (pi * ((4 meters)^2) * (10 km)) * (1 (kg / liter)) * (9.81 (m / (s^2))) * (2 km) = 2.73946879 gigawatt hours

ladberg 7y ago
Do you know how hard/expensive it would be to even dig out a hole that wide and deep?
- amirhirsch 7y ago
  nope, but I know that someone at Tesla knows (who also works in the boring field). Probably costs less than one rated for humans, where they quoted $10M per mile. So 10km ($62M) we're at $22 per kwh. Losing 20% of the energy every time will add up to about $40K per cycle so after like 6000 cycles it will be roughly the same price as $100/kwh lithium. I'm not sure about the technical issues regarding narrowness and depth, though a quick search indicates the deepest hole was a Russian made a 9 inch diameter hole 40,230 feet deep and had to stop because it was too hot. Anyway, I bet that offshore gravity batteries can also scale up very cheaply once you get over the HVDC cable fixed costs (this almost certainly means building 10GWh level capacity to justify a $100M cable to a place where ocean depths are 2-4km)
  - Retric 7y ago
    Costs to dig go way up 2km underground. This might be less so with your example as you don’t need access tunnels, but it’s still significant.
    - amirhirsch 7y ago
      I guess I'm just trying to introduce the idea that cheaper boring will reduce capital costs for pumped hydrostorage (see https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit... ) I don't think lithium ion batteries, which are optimized for their energy density (mass per joule), belong in grid storage situations where we should care mostly about lowest cost total capacity (regardless of round-trip losses) and lifetime.
      - Retric 7y ago
        Due to economies of scale resulting in continuous price declines they are the cheapest long term solution. They also last much longer, can regularly use a wider range of their capacity, and take less maintenance than lead acid while being easier to transport and install. Part of this is better power management, but it’s also because they are useful even after significant reductions in capacity.
        kragen 7y ago
        No, pumped hydrostorage is enormously cheaper, if you already have the dam. I also think some of your assertions about lead–acid batteries are wrong, but they're plausible.
        Retric 7y ago
        Unfortunately, only finite amounts of existing dams with huge changes in elevation exist. Beyond that “The round-trip energy efficiency of PSH varies between 70%–80%.” That’s a major ongoing cost, at say 6c / kWh * .25 * 365 * 15 years = 82$/kWh which is a large fraction of the Lithium Ion batteries costs. Though this increases if it’s used more than once a day, and decreases with unused reserve capacity.
        kragen 7y ago
        Indeed, I think the finite amounts of existing dams are the reason people are looking to batteries. Your point about the efficiency is interesting, although I didn’t understand it at first. I think you’re saying that the capital cost of the lithium-ion battery storage is partly defrayed by the higher efficiency of the storage system? Like, for each kilowatt-hour of Li-ion storage (with, let's say, a round-trip efficiency of 95%, although I think that's too high), you “get back”, say, 0.25 kWh every time you use it, that you would have lost if you'd stored that energy in pumped storage instead? So over, say, 15 years, you “get back” US$82 or so, at US$5.48 per year? https://electrek.co/2018/11/20/tesla-gigafactory-battery-cells-made-cost-advantage-panasonic-lg-report/ https://electrek.co/2018/11/20/tesla-gigafactory-battery-cel... claims that the battery cell cost is US$111 per kWh at the moment (though other manufacturers are still stuck around US$140), so that would work out to about a 5% annual IRR if the cells were the only cost; I think that in fact they are on the order of half the cost (though Tesla's blog post here doesn't actually list prices!) and so that would be a 2.5% or so IRR. Not enough to justify the battery investment on its own, but it would definitely be a significant boost to the project's ROI. I have a couple of objections to that line of reasoning, one trivial and one serious. The trivial objection is that the wholesale cost of electrical power, although it varies a lot, averages about half of the 6¢/kWh you're imputing. https://www.zmescience.com/ecology/climate/cheapest-solar-power/ https://www.zmescience.com/ecology/climate/cheapest-solar-po... talks about the just-signed Atacama project at 2.9¢/kWh, which I think includes the cost of some storage. So the numbers are more like 1.25% IRR rather than the 2.5% I suggested above or the 5% you suggest. The more serious objection is that, when you're filling up your utility-scale storage during hours of excess power production, you're not paying 6¢/kWh or 2.9¢/kWh. In fact, due to non-dispatchable “baseload” plants like coal and nuclear, it's common right now for the power plant to pay you to take the power, with the price typically around -4¢/kWh, which is the cost of burning it up in giant resistors. When instantly-dispatchable solar plants come to dominate power production, we can expect to see a price floor of 0¢/kWh. Maybe if a storage-plant operator is paying a solar-plant operator to leave their PV plants running, they'll have to pay 0.01¢/kWh or 0.1¢/kWh. But they won't be paying anywhere close to the average price of electrical energy. They'll be paying the marginal price of generating electrical energy when it is cheapest. So that means that the amount of money you make from a utility-scale energy storage plant isn't going to be determined by how much energy you need to charge it up. Your round-trip energy efficiency could be 10% or 5% and you still wouldn't pay a significant percentage of your revenues to obtain that energy. What determines your revenues is how much energy you can release once you are selling energy rather than buying it. (And the quality of your trading strategy, of course; if you decide to wait to sell your energy until your LMPs go above US$45/MWh, and they sit at US$42/MWh all night long, you don't make any money.) Round-trip energy efficiency only matters at all in the sense that it diminishes your effective storage capacity — if theoretically you have “1 MWh” stored, but when you turn it on, only 0.9 MWh flows to the grid, you only get paid for that 0.9 MWh, and that's what you need to pay your capex and opex with. But it only matters very marginally whether you had to buy 1.1 MWh or 2 MWh or 5 MWh or 10 MWh to charge up your storage facility.
  - dharma1 7y ago
    Deep drilling is actually a thing for deep geothermal. Still early days but can produce a lot of energy on its own, no solar required. Expensive to go that deep though http://www.thinkgeoenergy.com/drilling-finlands-deepest-well-on-record-the-geothermal-well-at-otaniemi/ http://www.thinkgeoenergy.com/drilling-finlands-deepest-well...
abdullahkhalids 7y ago
Pretty sure you can't make a bore that narrow and that deep without it collapsing pretty soon.
deleted 7y ago
[deleted]
nisten 7y ago
Hang on a sec, you're assuming that the water is smoothly accelerating at 9.81ms^2 for the entire 2km way down. You forgot to subtract energy losses due to drag which cancel out your exponential advantage.
- Majromax 7y ago
  > Hang on a sec, you're assuming that the water is smoothly accelerating at 9.81ms^2 for the entire 2km way down. No, that's the raw potential energy calculation. For power, you use the basic relationship that P=FV (force times velocity). The system would operate in a steady state for any given rate of power input/output, but that potential energy calculation governs how long it would take to charge/discharge. The acceleration due to gravity only matters if you're trying to discharge all of the energy of a parcel of water on impact at the bottom (via smashing it into something). A potential energy storage system would instead want to charge and discharge close to steady state, with very little acceleration throughout.
osamagirl69 7y ago
Another thing to consider with the 'dig a hole' based pumped storage approach is that you can only 'suck' water about 15ft vertically (well technically you can't really suck water any appreciable distance, but the atmosphere can push it up about 15ft) so it means that your pumps need to be at the bottom of the hole. You might be able to get away with some sort of remote actuated pump head like that used in an oil well (energy for pumping provided by a reciprocating metal rod) but the efficiency of such schemes is never going to get anywhere near the 80% you were targeting. Better hope that the pumps don't fail while the hole is full!
- amirhirsch 7y ago
  Yea, I’ve been thinking about the requirement that the pump is in the bottom is probably the Achilles heel. I’m more keen on dropping heavy blocks of metal now :)
jaakl 7y ago
This has been tried. So far unsuccessfully, for practical challenges see http://www.ipsnews.net/2013/08/geothermal-energy-stuck-in-a-hole-in-switzerland/ http://www.ipsnews.net/2013/08/geothermal-energy-stuck-in-a-...

ladberg 7y ago

Do you know how hard/expensive it would be to even dig out a hole that wide and deep?

amirhirsch 7y ago
nope, but I know that someone at Tesla knows (who also works in the boring field). Probably costs less than one rated for humans, where they quoted $10M per mile. So 10km ($62M) we're at $22 per kwh. Losing 20% of the energy every time will add up to about $40K per cycle so after like 6000 cycles it will be roughly the same price as $100/kwh lithium. I'm not sure about the technical issues regarding narrowness and depth, though a quick search indicates the deepest hole was a Russian made a 9 inch diameter hole 40,230 feet deep and had to stop because it was too hot. Anyway, I bet that offshore gravity batteries can also scale up very cheaply once you get over the HVDC cable fixed costs (this almost certainly means building 10GWh level capacity to justify a $100M cable to a place where ocean depths are 2-4km)
- Retric 7y ago
  Costs to dig go way up 2km underground. This might be less so with your example as you don’t need access tunnels, but it’s still significant.
  - amirhirsch 7y ago
    I guess I'm just trying to introduce the idea that cheaper boring will reduce capital costs for pumped hydrostorage (see https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit... ) I don't think lithium ion batteries, which are optimized for their energy density (mass per joule), belong in grid storage situations where we should care mostly about lowest cost total capacity (regardless of round-trip losses) and lifetime.
    - Retric 7y ago
      Due to economies of scale resulting in continuous price declines they are the cheapest long term solution. They also last much longer, can regularly use a wider range of their capacity, and take less maintenance than lead acid while being easier to transport and install. Part of this is better power management, but it’s also because they are useful even after significant reductions in capacity.
      - kragen 7y ago
        No, pumped hydrostorage is enormously cheaper, if you already have the dam. I also think some of your assertions about lead–acid batteries are wrong, but they're plausible.
        Retric 7y ago
        Unfortunately, only finite amounts of existing dams with huge changes in elevation exist. Beyond that “The round-trip energy efficiency of PSH varies between 70%–80%.” That’s a major ongoing cost, at say 6c / kWh * .25 * 365 * 15 years = 82$/kWh which is a large fraction of the Lithium Ion batteries costs. Though this increases if it’s used more than once a day, and decreases with unused reserve capacity.
        kragen 7y ago
        Indeed, I think the finite amounts of existing dams are the reason people are looking to batteries. Your point about the efficiency is interesting, although I didn’t understand it at first. I think you’re saying that the capital cost of the lithium-ion battery storage is partly defrayed by the higher efficiency of the storage system? Like, for each kilowatt-hour of Li-ion storage (with, let's say, a round-trip efficiency of 95%, although I think that's too high), you “get back”, say, 0.25 kWh every time you use it, that you would have lost if you'd stored that energy in pumped storage instead? So over, say, 15 years, you “get back” US$82 or so, at US$5.48 per year? https://electrek.co/2018/11/20/tesla-gigafactory-battery-cells-made-cost-advantage-panasonic-lg-report/ https://electrek.co/2018/11/20/tesla-gigafactory-battery-cel... claims that the battery cell cost is US$111 per kWh at the moment (though other manufacturers are still stuck around US$140), so that would work out to about a 5% annual IRR if the cells were the only cost; I think that in fact they are on the order of half the cost (though Tesla's blog post here doesn't actually list prices!) and so that would be a 2.5% or so IRR. Not enough to justify the battery investment on its own, but it would definitely be a significant boost to the project's ROI. I have a couple of objections to that line of reasoning, one trivial and one serious. The trivial objection is that the wholesale cost of electrical power, although it varies a lot, averages about half of the 6¢/kWh you're imputing. https://www.zmescience.com/ecology/climate/cheapest-solar-power/ https://www.zmescience.com/ecology/climate/cheapest-solar-po... talks about the just-signed Atacama project at 2.9¢/kWh, which I think includes the cost of some storage. So the numbers are more like 1.25% IRR rather than the 2.5% I suggested above or the 5% you suggest. The more serious objection is that, when you're filling up your utility-scale storage during hours of excess power production, you're not paying 6¢/kWh or 2.9¢/kWh. In fact, due to non-dispatchable “baseload” plants like coal and nuclear, it's common right now for the power plant to pay you to take the power, with the price typically around -4¢/kWh, which is the cost of burning it up in giant resistors. When instantly-dispatchable solar plants come to dominate power production, we can expect to see a price floor of 0¢/kWh. Maybe if a storage-plant operator is paying a solar-plant operator to leave their PV plants running, they'll have to pay 0.01¢/kWh or 0.1¢/kWh. But they won't be paying anywhere close to the average price of electrical energy. They'll be paying the marginal price of generating electrical energy when it is cheapest. So that means that the amount of money you make from a utility-scale energy storage plant isn't going to be determined by how much energy you need to charge it up. Your round-trip energy efficiency could be 10% or 5% and you still wouldn't pay a significant percentage of your revenues to obtain that energy. What determines your revenues is how much energy you can release once you are selling energy rather than buying it. (And the quality of your trading strategy, of course; if you decide to wait to sell your energy until your LMPs go above US$45/MWh, and they sit at US$42/MWh all night long, you don't make any money.) Round-trip energy efficiency only matters at all in the sense that it diminishes your effective storage capacity — if theoretically you have “1 MWh” stored, but when you turn it on, only 0.9 MWh flows to the grid, you only get paid for that 0.9 MWh, and that's what you need to pay your capex and opex with. But it only matters very marginally whether you had to buy 1.1 MWh or 2 MWh or 5 MWh or 10 MWh to charge up your storage facility.
- dharma1 7y ago
  Deep drilling is actually a thing for deep geothermal. Still early days but can produce a lot of energy on its own, no solar required. Expensive to go that deep though http://www.thinkgeoenergy.com/drilling-finlands-deepest-well-on-record-the-geothermal-well-at-otaniemi/ http://www.thinkgeoenergy.com/drilling-finlands-deepest-well...

abdullahkhalids 7y ago

Pretty sure you can't make a bore that narrow and that deep without it collapsing pretty soon.

deleted 7y ago

[deleted]

nisten 7y ago

Hang on a sec, you're assuming that the water is smoothly accelerating at 9.81ms^2 for the entire 2km way down. You forgot to subtract energy losses due to drag which cancel out your exponential advantage.

Majromax 7y ago
> Hang on a sec, you're assuming that the water is smoothly accelerating at 9.81ms^2 for the entire 2km way down. No, that's the raw potential energy calculation. For power, you use the basic relationship that P=FV (force times velocity). The system would operate in a steady state for any given rate of power input/output, but that potential energy calculation governs how long it would take to charge/discharge. The acceleration due to gravity only matters if you're trying to discharge all of the energy of a parcel of water on impact at the bottom (via smashing it into something). A potential energy storage system would instead want to charge and discharge close to steady state, with very little acceleration throughout.

osamagirl69 7y ago

Another thing to consider with the 'dig a hole' based pumped storage approach is that you can only 'suck' water about 15ft vertically (well technically you can't really suck water any appreciable distance, but the atmosphere can push it up about 15ft) so it means that your pumps need to be at the bottom of the hole. You might be able to get away with some sort of remote actuated pump head like that used in an oil well (energy for pumping provided by a reciprocating metal rod) but the efficiency of such schemes is never going to get anywhere near the 80% you were targeting. Better hope that the pumps don't fail while the hole is full!

amirhirsch 7y ago
Yea, I’ve been thinking about the requirement that the pump is in the bottom is probably the Achilles heel. I’m more keen on dropping heavy blocks of metal now :)

jaakl 7y ago

This has been tried. So far unsuccessfully, for practical challenges see http://www.ipsnews.net/2013/08/geothermal-energy-stuck-in-a-hole-in-switzerland/ http://www.ipsnews.net/2013/08/geothermal-energy-stuck-in-a-...