Energy Vault: A mashup of two of my interests

Image Credit: Energy Vault

My occupation is as an executive in a firm that provides materials testing, inspection, consulting, and engineering in the construction space. We deal with all aspects of the built environment with the exception of single family housing. As such, concrete is a fundamental area of my firm's expertise. And, as is clear, energy in all its aspects is a personal interest, not to say obsession, of mine. Finally, as I've elaborated in multiple posts, to bring renewable energy to the level of being able to provide base load power, I contend that relatively inexpensive energy storage will be needed.

Energy comes, basically, in two forms: kinetic; and potential. And while storage via kinetic energy is possible (think flywheels and thermal storage), most forms of storage utilize potential energy. Batteries utilize chemical potential energy, compressed air energy storage utilizes mechanical energy, etc. And finally, gravitational potential energy is utilized in various systems. In fact, most grid scale storage currently in place uses pumped hydro storage.

But any system of raising a mass against the force of gravity has the potential (get it?) to be used for storage. And a Swiss firm called Energy Vault has constructed a prototype of a storage solution using concrete lifted by tower cranes. It's clear that the technology for producing concrete is not new, and tower cranes are ubiquitous in the developed world. The innovation claimed by Energy Vault lies in the software to efficiently determine crane movements to optimize storage of excess energy and to deliver energy when needed.

In a previous set of posts (the last one is here), I estimated that a 3 MW nameplate capacity wind turbine combined with 40 MWh of storage could reliably provide 725 kW of base load power. What would 40 MWh of storage look like with the Energy Vault system? Energy Vault's web site states that an operational plant would have the capacity to store "between 10 and 35 MWh" of electrical energy and be able to deliver that energy at a rate of from 2 to 5 MW. Based on this claim, perhaps two such plants would be sufficient to provide storage for our hypothetical 735 kW plant and would be able to deliver the energy at the needed rate.

So as not to subject my readers to endless calculations, suffice it to say that the energy stored by lifting a mass against gravity is simply the product of the mass of the object lifted, the height to which it is lifted, and the local gravitational acceleration constant. Let's say we'll settle for two storage plants, each with a capacity of 20 MWh. For calculating purposes, we need to convert 20 MWh to the 7.2*10^10 J (joules, the SI unit of energy). 

We have two "knobs" that we can control to determine how much energy is stored in a storage system of the nature of that of Energy Vault. We can control the height to which our masses are lifted and we can control the amount of mass. And (net of losses), energy stored by lifting a mass against gravity is E=mgh, where E is the energy, m is the mass, g is the acceleration of gravity, and h is height. However, for the purposes of the physical logistics of our plant, we're really concerned about the volume of concrete so we'll use m=ρ*v where ρ is density and v is volume. This yields E=ρvgh. To isolate the knobs we can control, a little algebra yields E/(ρg)=vh. Concrete is typically quoted as having a density of 2,400 kg/m^3, g is 9.8m/s^2 and we need 7.2*10^10 J. Plugging these in, we see that we need v*h=7.2*10^10/(9.8*2,300)=3.06*10^6. This is the required product of height in meters times volume in meters^3.

In order to determine the feasibility we need to understand what an actual installation might look like, and Energy Vault helpfully includes an animated video of a hypothetical production facility.

While there's not a lot of information on the Energy Vault site with respect to tower height, plant radius, etc., Quartz has a writeup on the system that states that a tower would be on the order of 120 meters tall and the diameter of the installation would be around 100 meters. For our 40 MWh system, we need two of these.

It's also stated that each concrete block weighs about 35 metric tons (35,000 kg) and so the volume of each block would be 35,000/2,400 = 14.6m^3. Judging from the video, the concrete height is about 100 meters, and clearly it's not possible to have each block raised from the ground to 100 meters and lowered back to the ground, the blocks have to be stacked. I'd assume that it would be possible to have the average of the lift and drop to be 50 meters.

Does all of this make sense in comparison to the numbers from the earlier paragraph? We need height times volume to be 3.06*10^6 m^4. This means that we need about 3.06*10^6/50 = 61,200 m^3 of concrete. The video shows the beginning configuration to be basically a cylinder of concrete about 100 meters tall and a radius of, I estimate, 13 meters. This yields a volume of about 53,000m^3. This is not too bad, given the accuracy of estimates for height, radius, etc.

Now, in the U.S., we typically measure concrete volume in cubic yards, where a cubic meter is 1.308 cubic yards, so we're talking about a little over 80,000 yd^3 of concrete. Right now, a cubic yard of generic concrete costs around $80/yd^3 so the concrete cost alone (not counting concrete for the foundation) would be on the order of $6.4MM. However, Energy Vault claims to have developed the capability to use discarded materials as aggregate. Further, the concrete really needs very little compressive strength and so the cement requirement could be very low. Let's generously cut the $6.4MM by two thirds and call it $2.13MM. So we see that the concrete cost might be on the order of $2.13MM/20MWh = $156,500/MWh or $156.50/kWh. This is similar to the current cost of a lithium ion battery storage but doesn't include the cranes, the foundation, the construction, the control system, or the power electronics. To be fair, Li ion storage costs also are higher than strictly the battery costs.

Among the advantages of the Energy Vault solution are: negligible degradation of capacity over time; no use of rare elements; no toxic chemicals; no danger of thermal runaway. Is this the answer for turning our 3MW wind turbine into a reliable 725kW base load energy system? Well... as in so many things, it boils down to economics. I'll cover that in a future post at some yet to be determined future time.