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Energy Vault. Storing renewable energy using concrete blocks

Politics in the Zeros

Energy Vault plans to use excess solar and wind energy to construct a tower of huge concrete blocks. When electricity is needed, the blocks are lowered and the resultant kinetic energy creates electricity. One tower can create energy for hours, and it can store it indefinitely, which is a huge plus....


Bob Meinetz's picture
Bob Meinetz on Oct 21, 2019 7:02 pm GMT

"'We at the Vision Fund want to come in when a technology is proven and it’s ready to scale. That’s what’s so exciting about this technology. It’s not a science problem. It’s fifth-grade physics.'”

If whoever is so excited about "Energy Vault" is a fifth-grader, his/her excitement would be excusable. Certainly no adult who has taken one semester of high school physics could be excited about such an exceptionally stupid idea. Here's why (some high school physics required):

To raise an object weighing 1 kg a height of 1 meter requires 1 Newton-meter of energy, or 10 joules. Let's assume weights will be lifted to a height of 350 ft (107 meters), or 50 ft short of the height of the tower, to accommodate space for the weight itself. To raise six weights, each weighing 35 metric tonnes (35,000 kg), to a height of 107 meters, would require

35,000 kg x 6 weights x 107m x 10j = 224,700,000 joules = 62.4 kWh of energy

Weights raised to this height will return the same amount of energy required to lift them, or 62.4 kWh (we're ignoring friction and other losses, but we'll give this Exceptionally Stupid Idea every benefit of the doubt).

A medium-sized city of 400,000 residents might use an average of  ~7 MW of power at any given moment. At this rate, the 400-foot tower filled with concrete blocks would be capable of powering the city for 32 seconds.

How much would it cost for high-quality, Li-Ion batteries to store the electricity instead? With medium-duration batteries going for $1,500/kWh (May 2018): $1,500 x 62.4 kWh = $93,600.

Maybe there's an alternative universe where a 400-foot tower with generators, pulleys, and cables can be sited and built for $93,600, but building such a tower in this universe would be more expensive by two orders of magnitude.

Matt Chester's picture
Matt Chester on Oct 21, 2019 11:43 am GMT

Obviously claims made by the company at this point are due the right about of scrutiny and skepticism, but it's an interesting idea that evokes pumped hydropower storage but in a new way-- they claim some noteworthy costs:

Each tower can store between 20 and 80 megawatt hours at a cost of 6 cents per kilowatt hour (on a levelized cost basis), according to Piconi.

I suppose if they can vet it to show that maintenance and operation costs are significantly lower than pumped hydropower (these lower maintenance needs being one of the things they advertise) than it'll be interesting. At the very least, they've gotten over $100M in investment-- and that could very well be poorly invested money, but what they're showing is impressing somebody enough to fund it to the next level. Guess we'll have to stay tuned!

Bob Meinetz's picture
Bob Meinetz on Oct 21, 2019 7:01 pm GMT

Matt, they're not even close, and if someone has truly invested $100M in this nonsense they deserve to have their money taken. But maybe someone invested $100, and they borrowed some zeros for both numbers for the press release.

John Lawler's picture
John Lawler on Oct 23, 2019 11:57 am GMT

With the dispatch of Renewables being first in line in most markets, exactly when do you end up with excess Wind and solar energy?

Matt Chester's picture
Matt Chester on Oct 23, 2019 12:45 pm GMT

Curtailment of renewables is not uncommon in CAISO

There are two main reasons behind renewable energy curtailment: system-wide oversupply and local transmission constraints.

System-wide oversupply is what most people think of when explaining renewable curtailment. This kind of curtailment occurs when, on a large scale, there is simply not enough demand for all the renewable electricity that is available. Examples of this occur frequently in California during the spring months when renewable energy production can exceed electricity demand.

Local transmission constraints are an oft forgotten reason for renewable curtailment. This kind of curtailment occurs when there is so much renewable electricity in a local area that there is insufficient transmission infrastructure to deliver that electricity to a place where it could be used. A great example of this is in Texas, where wind energy curtailment fell from 17% in 2009 to 0.5% in 2014 mostly due to construction of additional transmission lines to move that wind energy out of local pockets to places where it could be used.

The CAISO, which operates the grid in most of California, keeps track of how much curtailment happens due to these two reasons. Surprisingly, in the first five months of 2019, just over half of all curtailment occurred due to local transmission constraints. And this isn’t an anomaly – roughly three-fifths of all curtailment in 2018 was due to local transmission constraints as well.

Bob Meinetz's picture
Bob Meinetz on Oct 23, 2019 5:33 pm GMT

Matt, many in California don't realize curtailments don't stop the so-called "negative pricing" costs faced by generators - and ultimately customers - required to dump electricity on neighboring states. With curtailments, over-generating solar and wind farms are literally paid to not generate electricity:

"Virtually all curtailments in the past three years have been where generators respond to CAISO’s call for less generation and get paid to do so. These "decremental" bids can be worth as much as generation, though they can have other revenue implications for renewable generators.

In economic terms, the decremental bid needs to be more than the opportunity cost of the producer to make it worth their while to turn down. Generators do get paid not to generate, but it is more economically efficient than the alternatives."

Though customers get nothing of value for decremental bids but "grid reliability", these costs are never accounted for in LCOEs for solar and wind - advocates pretend they don't exist.

Infuriating, that the reliability of nuclear plants not only goes unrewarded, but customers must pay more to both gas plants and solar/wind farms for nothing of value in return. A scam, by any measure.

John Lawler's picture
John Lawler on Oct 30, 2019 1:51 pm GMT

So the answer is to build these energy vaults in areas of greatest curtailment? Between the wind turbines and energy vaults it will look like 6 flags Superman Tower of Power rides all over the landscape.

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