The problem of energy storage, especially with regard to renewables, remains unsolved. Or at least there do not seem to be any popular solutions.

Why not gravity batteries?

I love the idea, and they look like a beautiful and effective approach to the problem. The low-tech, simple principles grant it so many different region-taylored forms. Many of these designs could easily be low-cost. I could go on.

Are there some shortcomings that I am missing here?
I asked this same question to the physics stack exchange, but I don't feel satisfied as to why they aren't more commonplace in renewable energy storage applications.

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    $\begingroup$ Are you missing this? deciwatt.global Or check out Dinorwig... $\endgroup$ – Solar Mike Sep 4 '19 at 17:44
  • $\begingroup$ Okay, so I see an example with Dinorwig. (Although, do they pump water up to store energy from renewables? Idk...) I'm wondering though, if there is a reason these aren't more popular in other electric-grids. $\endgroup$ – SuaveSouris Sep 4 '19 at 17:54
  • $\begingroup$ Dinorwig uses the spare energy at night to pump the water back up... $\endgroup$ – Solar Mike Sep 4 '19 at 18:05
  • $\begingroup$ So, I'm specifically asking if there is some undesirable feature about gravity batteries and storing renewable-produced energy for the electrical-grid. $\endgroup$ – SuaveSouris Sep 4 '19 at 18:13
  • $\begingroup$ So, the title is misleading then. Any research on your part? What did Canada look at for storing wind energy? $\endgroup$ – Solar Mike Sep 4 '19 at 18:15

So, this battery costs US$70, weighs 406 grams, and holds around 200,000 Joules of energy.

To get the same energy from that weight as a "gravity battery" you'd have to pick it up over 49 kilometers into the air.

There are applications where picking stuff up and powering things as it falls is useful -- the gravity-powered lighting for 3rd-world countries is a great example, and there are some hydropower storage schemes that pump water uphill to a lake and then generate power as it drops. But if you just take the trouble to do the math you'll see that "big" and "unwieldy" are definitely words that go with "gravity energy storage".

  • $\begingroup$ Comparing the energy potential of a lifted battery seems moot. Hmm I wonder how much "big" and "unwieldy" are the Achilles heel. Thanks, though $\endgroup$ – SuaveSouris Sep 4 '19 at 20:20
  • $\begingroup$ "I wonder how much "big" and "unwieldy" are the Achilles heel". All you need is high-school physics to do the math and decide for yourself. $\endgroup$ – TimWescott Sep 4 '19 at 20:53
  • $\begingroup$ @TimWescott methinks this might be homework, OP wants a lot more explanation but we are just giving examples... $\endgroup$ – Solar Mike Sep 5 '19 at 5:38
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    $\begingroup$ Gravity batteries exist and are common (~ 5% of EUs power generating capacity; IIRC typically a few hours capacity), they are called "pumped hydro" and are about as big and unwieldy as a lake. Read through other proposals for grid-scale energy storage (like ADELE or the crazy scheme with the plug) and you'll see that they mostly adress land use. $\endgroup$ – mart Sep 5 '19 at 11:38

Consider 1 kWh = 1000 W * 3600 seconds = 3,600,000 J.

1 kg lifted 1 m = ~10J.

Let's use 1 metric tonne. 1000Kg. So we get 10,000 J per meter of elevation.

3,600,000 / 10,000 = 360 meters. Around a thousand feet.

Ok. Getting a 15 cm diameter well dug and cased is on the order of \$20/foot. So \$20,000 buys you a 1000 foot deep hole. Use whatever length of 10 cm bar stock gets you a ton. (In actual fact, using drill collar would be a prefab way to do this) Now all you need is some long cable, and a motor/generator.

But really: Add a few more chunks of drill collar increase your weight up to, say 50T.

Now you have a battery that will store 50 kWh -- about 2 days use by a modern home -- and it cost you only somewhere between 20 and 40 grand. Ball park a thousand bucks per kWh.

50 kWh is worth about \$4 at residential prices, about \$1.50 at wholesale prices.

Now we sort of do this with pumped hydro storage. Water is a lot easier to work with than long slugs of steel pipe suspended by cables. It also scales better. Raise the rim of the reservoirs a few feet, and you can store a lot of extra tonnes of water. (A 1 km2 lake 1 meter deep is a million tonnes of water. The cycle efficiency is only about 70% or so, depending on how much you spend on big plumbing. (You put 10 kWh in and get 7 back next Tuesday)

Pumped hydro is very workable. If you have a mountain nearby.

Big LiON batteries are now getting down between 200 and 300 bucks per kWh. (https://www.nrel.gov/docs/fy19osti/71714.pdf 2018 figures) There are other technologies in the wings that should cut this by another factor of 4 for stationary batteries.

Even those mongo batteries such as Tesla made for Australia pay off only when used several cycles per day. They have addition payoff because they can prevent firing up gas generators to fill the gap as often -- takes about half an hour to bring a gas turbine online.

TL:DR; Gravity batteries are too big and bulky, and are too complicated to scale up easily.

  • $\begingroup$ Thanks for the illustrative response. I know I'm being particular, but most of the grav-batt cost is in digging the hole, which could be avoided. Indeed, the folks at Energy Vault lift blocks into the air and boast an LCOED of ¢6/kWh. $\endgroup$ – SuaveSouris Sep 5 '19 at 17:59
  • $\begingroup$ I'd have to look at their model for the LCOED. Above ground will have long term maintenance costs. 6c/kWh is still tripling the cost of wholesale power, plus the energy cost of imperfect cycle. If you use my 70% figure, you have to buy 10 kWh at 3c, store it, and release 7 kWh at 6c. Those 7 kWh cost 72c or just over 10c/kWh. $\endgroup$ – Sherwood Botsford Sep 7 '19 at 13:14
  • $\begingroup$ @SuaveSouris Some people do reckon that when you have an existing hole (eg a disused mine shaft), then a big weight on an electric winch might make economic sense. See gravitricity.com $\endgroup$ – Flyto Oct 1 '19 at 21:15
  • $\begingroup$ It may make sense for very short term surges. Do the math for a 100 MWh battery. The Tesla one in Australia is 129 MWh, has a peak delivery of 100 MW. They cycle it several times a day for peak shaving. $\endgroup$ – Sherwood Botsford Oct 2 '19 at 17:10

Are there some shortcomings that I am missing here?

Yes there are a couple of things you have missed.

Firstly I will deviate slightly from your question but I believe this is relevant.

The need for localised storage has only relatively recently become desirable. With the increase of large scale Solar and in particular wind farm projects the need for localised energy storage has become a priority. Despite what the Sales people wish to tell you or what propaganda environmental groups or Governments wish to spout Renewable energy IS NOT A SUCCESS, because a large number of fossil fuel plants have to remain running 24/7 to take up the load on the GRID when the wind drops or the sunshine disappears.

Hydro storage solutions cannot be compared as they are an older technology originally developed to solve the problem of water consumption in areas where the terrain was ideal but the flow of water was insufficient for hydro power generation. They also seldom meet the need for localised storage where the power is wind or solar generated.

So to repeat localised power storage is a relatively new requirement. Hence the lack of solutions.

The main points you have missed are the

Time The Big one of course is time. Gravity batteries are time limited. The length of time a gravity storage unit can resupply the grid on loss of renewable energy supply is quoted as 8-16 hours by Energy Vault. This is a relatively short period of time compared with how long the wind could crease to blow and 16 hours of relatively low sunshine in winter months is not exceptional. Therefore Fossil powered generation would still be needed 24/7.

Capital costs associated with building both the renewable energy sources and the localised storage. Whilst running costs for both are negligible (Charging of localised storage being affected by energy that would have "gone to waste"). However the interest payments on loans secured to build the facilities will add to the running costs.

Environmental costs The effect of wind farms on the scenic environment has already become the focus of fierce opposition in many locations and Gravity storage solutions will be even more detrimental to the landscape.

Location Again linked with environmental but also with the renewable energy sources. Energy Vault depicts a gravity storage tower in the mist of a group of wind turbines. The siting of which would play havoc with the wind required to generate the power to be stored.

Reliability of Power Supplies. The increase in use of renewable energy will eventually bring the whole Power grid system into a state of Chaos. Traditional generation techniques cannot match the running costs of renewable energy sources. However renewable energy sources are not reliable. So in Countries where the electricity supply is not state owned how will corporate bodies manage these discrepancies? How will fossil fuel generation remain viable whilst most days of the year consumers are buying low cost, locally generated, renewable energy?

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    $\begingroup$ What happens when coal plants run out of coal? An event not easily predicted, at least we know exactly when the sun disappears and reappears... And you should check out hydro storage more thoroughly as not all were created with the main purpose of supplying water... So they should be compared. $\endgroup$ – Solar Mike Sep 6 '19 at 5:48
  • $\begingroup$ Agree with Solar Mike. I daresay most Pumped Hydro where built for peak load demands (which fossile plants handle poorly (exception: gas)) and actually stabilize the grid. $\endgroup$ – mart Sep 6 '19 at 6:09
  • $\begingroup$ I stated the original reason. We are both correct, and incorrect Technically the original pumped water storage project was because of water availability, perfect terrain no water. I remember a project on this during my apprenticeship. Hydro handles peak load demands extremely well, pumped or otherwise. And they were built as you correctly state to hand peak loads. That is why I am not comparing them. This discussion is "not peak load" but no load at any time because the sun or wind has failed. $\endgroup$ – Brad Sep 6 '19 at 6:26
  • $\begingroup$ @Solar Mike It is pointless to compare them for the reasons stated above and because if we had enough Hydro power we would not be using Solar or Wind or fossil fuel. Hydro is at the moment the optimum source for electricity generation. Easy to increase/decrease generation according to demand and the power source is cheap even if the capital costs are not. However we realised along time ago that we did not have enough Hydro potential. $\endgroup$ – Brad Sep 6 '19 at 6:41
  • $\begingroup$ A combination of generation systems helps: Because of air conditioning, and increased industrial use in the working day, solar matches to demand quite well. Natural gas turbines while more expensive to run than coal takes about half an hour to ramp up. Batteries with storage capacities ranging from 1 to 4 hours can bridge ramp up times for fossil power. Add long distance DC grid connections, and a reduction to 10-20% supplied by fossil fuel becomes realiszable. $\endgroup$ – Sherwood Botsford Sep 7 '19 at 13:09

You are right, these seem like very simple storage systems; it does seem strange that we aren't using more of them. Until you look at the numbers.

Obviously, we do already use gravity storage, pumped-storage hydroelectricity, but let us ignore that for now because it is only possible in the rare circumstances where geography complies.

Lets instead take the example of a house-sized battery system, which can be installed virtually anywhere. By far the most well known current solution is the Tesla Powerwall 2, which can store 13.5kWh and costs on the order of $10k to purchase and install.

Let's imagine a gravity system you could fit in your back yard, say, a 1m shaft (you could equally say tower, but you'll soon see why that is impractical) which you could throw a 3m shed over for safety and to house the winch system. What's the biggest weight you could practically get down that hole? Well, let's imagine its a 4m long, 1m wide cylinder of nice and cheap reinforced concrete, weighing in at 10T.

So we want to store 13.5kWh using a 10T weight. Do the maths and assume something like 80% efficiency and you end up needing a shaft 620m long (and now you see why we ruled out a backyard tower).

Add to the price of you nice cheap weight; digging a huge hole, a winch capable of reeling 620m of cable, control electronics, generator, reinforcement and waterproofing for your hole.

Now, clearly this system might work better if you had a really big winch, a really big weight and a really big, well-drained hole that's already there. Disused mineshafts come to mind. But even at scale, the cost is going to look rather silly against chemical batteries.


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