Mechanical Energy Storage

There are growing questions about the rising electricity demands from artificial intelligence, high-performance computing, quantum computing, robotics, digital assets and a wave of other technological advancements that gobble electrons.

Elon Musk and others point out that capturing a fraction of the sunlight that approaches earth provides ample energy. Similar statements have been made about geothermal energy, harvesting energy from the earth’s heat… but all these power generation solutions require storage if they are to fulfill the rapidly expanding base load.

We need better batteries.

That’s the key to the power lock.

The Wonder of Mechanical Batteries

Mechanical Energy Storage (MES) encompasses a range of technologies designed to convert electrical energy into potential or kinetic energy, storing it for later reconversion back into electricity.

This approach fundamentally differs from electrochemical methods aka the 🔋 batteries we are are used to dealing with traditionally.

Mechanical batteries rely on physical principles rather than chemical reactions. Key MES technologies include the mature and widely deployed Pumped Hydro Storage (PHS), which utilizes gravitational potential energy of water; Compressed Air Energy Storage (CAES), storing energy in pressurized air often within geological formations; Flywheel Energy Storage (FES), which stores kinetic energy in a rotating mass; and Gravity Energy Storage (GES), using the potential energy of solid masses.  

These technologies exhibit diverse characteristics. They have different tradeoffs that system designers must weigh and measure.

PHS and CAES are typically suited for large-scale, grid-level applications requiring long storage durations (hours to days). GES is being developed primarily for long-duration storage, aiming to overcome the geographical constraints of PHS. FES systems excel in high-power, short-duration applications (seconds to minutes), offering rapid response times ideal for grid stability services. There are several companies developing grid-scale FES solutions. In the spirit of full disclosure: I invested into one earlier this year. They have a breakthrough that extends the discharge duration to hours. This is what prompted my study of these potential solutions to the energy problem.

Compared to dominant electrochemical technologies like Lithium-ion (Li-ion) batteries, MES generally offers significantly longer operational lifespans (decades vs. 5-15 years), higher cycle lives, and utilizes more common, less environmentally problematic materials (steel, concrete, water, air), avoiding reliance on mined resources like lithium and cobalt.

MES typically suffers from lower energy density, requiring larger physical footprints, and technologies like PHS and CAES face significant geographical siting constraints. Key challenges hindering broader adoption include high upfront capital costs, the need for specific site characteristics for PHS and CAES, difficulties in monetizing the full value stack of services in current market structures, and competition from rapidly falling Li-ion battery prices.

Despite these hurdles, the market potential for MES is growing, driven by the increasing penetration of variable renewable energy sources, the critical need for grid flexibility and resilience, and the specific demand for long-duration energy storage solutions where MES technologies hold inherent advantages.

That was very high-level summary information but to really appreciate the enormity of these breakthroughs you need a primer on how to evaluate batteries.

The Science of Mechanical Energy Storage

Mechanical Energy Storage, sometimes referred to as Electromechanical Energy Storage, encompasses a class of technologies where electrical energy is converted into either potential energy (stored by virtue of position or state) or kinetic energy (stored by virtue of motion).

This stored mechanical energy can subsequently be reconverted back into electrical energy upon demand. This fundamental mechanism distinguishes MES from other major energy storage categories.

Unlike electrochemical storage (e.g., batteries) that rely on reversible chemical reactions , or thermal energy storage that involves storing heat or cold, MES systems manipulate physical states or motion.  

Evaluating Batteries

I looked more than 35 different battery technology companies during due diligence for my recent investment.

Evaluating the performance of a battery requires looking beyond a single number and assessing a range of interconnected characteristics that determine its suitability for a specific task. Key among these are energy capacity, measured in kilowatt-hours (kWh), which tells you the total amount of energy stored, and power capacity, measured in kilowatts (kW), indicating how quickly that energy can be delivered or absorbed. For instance, a large pumped hydro storage system might have enormous energy capacity determined by reservoir size and height difference, while a flywheel system might prioritize high power output derived from its rotational speed and mass, even with lower total energy stored.

Efficiency and energy losses are also critical. Round-trip efficiency, expressed as a percentage, reveals how much energy is returned during discharge compared to what was put in during charge, accounting for losses typically to heat or friction. Mechanical systems vary here: pumped hydro might achieve 70-85% efficiency, while flywheels can exceed 90% if friction and air resistance are minimized, whereas compressed air systems often have lower efficiencies unless waste heat is effectively managed. Another factor is self-discharge, or how quickly the battery loses energy while idle; this is negligible for gravity or pumped hydro storage but can be noticeable in flywheels due to friction or in compressed air systems due to leakage.

Longevity is assessed through cycle life – the number of charge-discharge cycles before significant capacity degradation – and calendar life, its total operational time regardless of cycles. Mechanical batteries often excel here, boasting very long lifespans (decades for pumped hydro or compressed air caverns) and extremely high cycle counts, typically limited by the durability of mechanical components like turbines, bearings, or structures rather than the energy storage medium itself.

Physical attributes like energy density (energy per volume or mass) and power density (power per volume or mass) are vital, particularly for mobile or space-constrained applications. Mechanical batteries generally have lower energy densities compared to chemical batteries, requiring significant space or large structures, making them better suited for stationary applications. However, their power density can vary significantly; flywheels, for example, offer high power density and very fast response times (milliseconds), making them ideal for tasks like frequency regulation, while systems like pumped hydro respond more slowly (seconds to minutes).

Costs are ultimately king. While upfront capital costs ($/kWh or $/kW) for some mechanical systems like pumped hydro can be high, their long lifespan and low operating costs can lead to a competitive Levelized Cost of Storage (LCOS, $/MWh) over their lifetime. Safety considerations differ from chemical batteries; mechanical systems avoid thermal runaway fire risks but introduce potential mechanical failure modes and often have larger environmental footprints related to land use or construction.

Choosing the best battery involves weighing these diverse performance metrics against the specific demands and constraints of the intended application.

Fundamental Operating Principles

The core principle underlying all MES technologies involves a two-stage energy conversion process facilitated by mechanical devices.

Sounds complicated, but if you break it down into small parts the entire concept becomes easy to grasp.

During the charging phase, electrical energy, often sourced from the grid during periods of surplus generation or low demand (off-peak), powers machinery such as motors, pumps, or compressors. This machinery performs mechanical work, resulting in the storage of energy: water is pumped to a higher elevation (potential energy in PHS), air is compressed into a storage volume (potential energy in CAES), or a massive rotor is accelerated to high rotational speeds (kinetic energy in FES). During the discharging phase, the process is reversed. The stored potential or kinetic energy is released, driving turbines or allowing a flywheel to decelerate, which in turn drives an electrical generator to produce electricity that is fed back into the grid. This cycle enables the temporal shifting of energy, storing it when abundant or cheap and releasing it when needed.  

The reliance on physical energy state changes, rather than chemical reactions, inherently links the performance characteristics of MES systems—such as scalability, lifespan, and energy density—to fundamental physical laws and the properties of the materials used. This contrasts sharply with electrochemical systems, where degradation pathways involving chemical changes within the battery materials often limit lifespan and cycle count. Consequently, MES technologies often exhibit different failure modes and possess the potential for significantly longer operational lives compared to their chemical counterparts, influencing long-term economic calculations like the Levelized Cost of Storage (LCOS).  

MES technologies play a crucial and historically significant role in power systems. PHS, in particular, has long been the dominant form of large-scale energy storage globally, accounting for the vast majority of installed capacity for decades. The primary functions of MES include grid balancing, such as peak shaving (reducing demand during peak hours by discharging stored energy) and load shifting (moving energy consumption from peak to off-peak periods). They are also vital for providing ancillary services essential for grid stability, including frequency control (maintaining grid frequency at its nominal value, e.g., 50 or 60 Hz) and voltage control (maintaining grid voltage within acceptable limits).  

In the context of the modern energy transition, MES is increasingly important for enabling the large-scale integration of variable renewable energy (VRE) sources like solar and wind power. By storing excess energy generated during sunny or windy periods when production may exceed demand, MES helps smooth out the intermittency of VRE, making the overall power supply more reliable and predictable. This capability enhances overall grid stability and resilience and can potentially defer or avoid costly upgrades to transmission and distribution infrastructure by managing power flows more effectively.  

Major Mechanical Energy Storage Technologies

Pumped Hydro Storage (PHS)

PHS operates by converting electrical energy into gravitational potential energy and back again. During periods of low electricity demand or surplus renewable generation, electricity is used to pump water from a lower reservoir to an upper reservoir situated at a higher elevation. This process stores energy in the elevated water mass. When electricity demand increases, the water is released from the upper reservoir, flowing downhill through pipes or tunnels to drive turbines connected to generators, producing electricity. PHS is the most mature and widely deployed large-scale energy storage technology globally, representing over 90% of the world's grid storage capacity for many years. Many modern PHS plants utilize reversible pump-turbines, often of the Francis design, which can operate efficiently in both pumping and generating modes.

PHS systems are broadly categorized based on their connection to natural water bodies. Open-loop systems have one or both reservoirs continuously connected to a naturally flowing river or lake. Closed-loop systems, often referred to as 'off-river' PHS, involve two reservoirs that are not naturally connected to a flowing water source, minimizing impacts on existing river systems. Variations on the basic concept are also being explored or have been demonstrated, including the use of seawater instead of freshwater (e.g., Yanbaru project in Okinawa, Rance tidal plant in France) , the development of underground reservoirs (using existing mines) , or the construction of coastal reservoirs.

A key characteristic of PHS is its strong dependence on specific geographical and topographical features. Suitable sites require significant elevation differences between the two reservoirs over a relatively short distance, the availability of sufficient water, and geological conditions capable of supporting large dams and reservoirs. Mountainous regions are typically favorable. This inherent reliance on specific natural landscapes significantly restricts the potential locations for PHS deployment, making it a resource-constrained technology despite its proven scale and performance. This limitation is a primary motivation for the research and development of alternative large-scale storage technologies, such as advanced CAES and GES, which aim to offer greater siting flexibility.

Compressed Air Energy Storage (CAES)

CAES technology stores energy by compressing air and storing it under high pressure. Surplus electricity, typically available during off-peak hours or periods of high renewable generation, drives compressors to pressurize ambient air. This high-pressure air is then stored, most commonly in large underground geological formations like salt caverns, depleted mines, or porous rock aquifers, due to the large volumes required for utility-scale storage. Above-ground tanks or pressure vessels are also feasible, particularly for smaller-scale systems. When electricity is needed, the pressurized air is released, heated (depending on the CAES type), and expanded through a turbine connected to a generator. In terms of grid applications, CAES is often considered analogous to PHS, providing large-scale storage capabilities.

Gravity Energy Storage (GES) or Gravity Energy Technology (GET)

GES technologies harness the fundamental principle of gravitational potential energy, similar to PHS, but typically utilize solid masses instead of large bodies of free-flowing water. Electrical energy is used to lift heavy weights (composed of materials like concrete, rocks, soil, sand, or composite blocks often utilizing waste materials) to a higher elevation, storing potential energy. When electricity is required, these masses are lowered under controlled conditions, and the gravitational force drives a generator (via winches, motors operating in reverse, or other mechanisms) to produce power. The amount of energy stored is directly proportional to the mass being lifted and the vertical height difference it traverses (Energy = mass × gravity × height, E=mgh).

GES is an emerging field with several distinct configurations being developed and demonstrated. They each hold charge in different ways.

Tower-based GES: These systems typically involve a tall structure (often a frame or tower) equipped with cranes or lifting mechanisms that raise and lower large, heavy blocks (e.g., 35-ton concrete or composite blocks used by Energy Vault). Advanced software, often incorporating machine vision, is used to coordinate the precise stacking and lowering of these blocks to manage energy storage and dispatch. Energy Vault's EVx™ is a prime example currently being deployed commercially.

Shaft-based / Mine-based GES: This approach utilizes deep vertical shafts, either purpose-built or, significantly, existing abandoned mine shafts, to raise and lower one or more massive weights (potentially thousands of tonnes) suspended by cables and winches. Gravitricity's GraviStore technology focuses on this concept, highlighting the potential for repurposing mining infrastructure. Energy Vault's EV0™ concept also leverages underground spaces.

Rail-based GES: Systems like those developed by Advanced Rail Energy Storage (ARES) use electric locomotives or heavy rail cars on a sloped track. Energy is stored by moving the masses uphill using grid power and generated when they controllably descend the slope. This often targets areas with natural inclines or potentially repurposed mining sites.

Mountain-based / Terrain-based GES: Conceptualized by organizations like IIASA, this approach proposes using natural mountain topography, moving large quantities of bulk materials (like sand or gravel) between storage sites at different elevations using conveyor systems or other transport methods.

Modular Underground GES: Startups like Terrament are exploring concepts involving autonomous, modular weights stacked vertically within underground shafts, potentially offering scalability and redundancy.

A primary driver for GES development is to overcome the significant geographical and topographical constraints associated with traditional PHS. By using dense solid materials or contained water, GES aims to achieve large-scale energy storage with greater siting flexibility. Tower and rail systems can potentially be built on relatively flat land, while shaft-based systems can utilize existing mines or require less surface area than large reservoirs. This pursuit of location independence, while retaining the core benefits of gravity storage (long lifespan, potential for large scale, no chemical degradation), makes GES a promising area of innovation for LDES.

Repurposing Infrastructure: A particularly compelling aspect of shaft-based and some rail-based GES concepts is the potential to repurpose existing infrastructure, notably abandoned mine shafts. This approach could significantly reduce civil construction costs, which are a major component of new PHS or purpose-built shaft projects. One of the great things about this tech is it offers a potential pathway for economic revitalization in former mining communities by transforming liabilities (abandoned mines) into valuable energy assets. Projects by Gravitricity in Europe and Energy Vault in Italy are examples of Gravity Battery Tech. These areas have been hit hard by current economic trends and injecting energy (literally) into these areas would be wise.

Flywheel Energy Storage (FES)

FES systems store energy mechanically in the form of rotational kinetic energy. An electric motor, often integrated with a generator into a single unit (motor-generator), uses input electricity to accelerate a massive rotating cylinder or disc, known as a rotor or flywheel, to very high speeds. The energy stored is proportional to the rotor's inertia and, critically, the square of its angular velocity (rotational speed, RPM). To discharge the stored energy, the kinetic energy of the spinning flywheel drives the motor-generator, which now acts as a generator, producing electricity and causing the flywheel to decelerate.

A typical FES system consists of the rotor, the motor-generator, bearings to support the rotor, and a housing. Advanced FES designs incorporate several key features to maximize energy storage and efficiency.

While traditional flywheels used steel, modern high-speed systems utilize materials with high tensile strength-to-weight ratios, such as carbon fiber composites, fiberglass resins, or advanced polymers. These materials can withstand the immense centrifugal forces generated at high speeds, allowing for significantly higher RPMs and thus greater energy storage for a given mass. The tensile strength of the material is often more critical than its density in high-speed designs.

Conventional mechanical bearings introduce friction, limiting speed and efficiency and requiring maintenance. Advanced FES systems employ magnetic levitation bearings, which suspend the rotor using magnetic fields, virtually eliminating contact friction. Superconducting magnetic bearings (SMBs), utilizing high-temperature superconductors (HTSC), represent a further technological advancement.

To minimize aerodynamic drag, which becomes significant at high speeds, the rotor is typically housed in a sealed enclosure maintained at a near-vacuum.

The performance and economic viability of FES are heavily reliant on advancements in enabling technologies, particularly material science and bearing systems. The quest for higher energy storage capacity directly translates to achieving higher rotational speeds, as energy scales with the square of RPM. This necessitates the development and cost-effective manufacturing of high-strength, lightweight composite materials capable of handling extreme centrifugal stresses, alongside sophisticated, low-loss magnetic bearing systems (potentially including superconductors) and robust vacuum containment.

The inherent physical characteristics of legacy FES systems dictate their primary applications. Their ability to charge and discharge very rapidly (milliseconds to seconds), handle frequent cycling with minimal degradation, and deliver high power output makes them exceptionally well-suited for power-based grid applications. These include frequency regulation, voltage support, power quality improvement, and providing short-term uninterruptible power supply (UPS). However, their relatively short discharge duration (typically seconds to minutes) and lower energy density compared to PHS or CAES make them less suitable for energy-based applications like bulk energy shifting over multiple hours. This specialization positions FES as a valuable tool for specific grid stability functions rather than as a direct competitor to long-duration storage solutions.

Unless, of course.. if someone uses the power of engineering to evolve flywheel energy storage to the next level….

QNETIC Mechanical Battery

What if humanity had a way to scale the size of FES systems to increase their power generation capacity?

The company I teased earlier, Qnetic Corporation, have introduced a truly game changing evolution to FES systems.

Qnetic smashes the record on capacity at 1,000kWh. As mentioned, traditional FES are built for high power, short duration and low energy capacity. Qnetic is the opposite: long-duration, high energy capacity and moderate power. Traditional flywheels cannot be scaled-up for the grid: it would be like using a sports car to move freight.

Qnetic’s rotor is made from proprietary lightweight and high strength fibers. Due to centrifugal force, the heavier it is, the more the material wants to move outwards from the axis, raising the stresses as it spins and limiting the amount of energy that can be stored.

TLDR; Counter-intuitively, the rotor should be lightweight.

Qnetic units are installed below-ground making them intrinsically safe. Qnetic’s low height easily disappears from sight, preserving the natural beauty around us.

This image depicts a typical battery having 80 Qnetic units connected to provide 40MWh of energy and 10MW of constant power—enough to support the average requirements of 10,000 US households for four hours.

From the company’s website:

“Due to the energy equation, if we double the weight, we double the energy but if we double the speed, we quadruple the energy—so it’s much better to be light and go fast than be heavy.”

Oh, and it floats in the air. Completely suspended by powerful electromagnets.

With the rotor fully suspended in free space and guided by electromagnets there is zero contact, zero friction and zero wear. Qnetic will gradually self-discharge entirely but it will take about two weeks. Compare that to the minutes/hours discharge time for traditional flywheels and the grid scale capabilities of Qnetic become obvious.

The world needs 100x more grid energy storage than exists today - and our needs our rising at rising rates. Storage solutions need to be safe, non-toxic and long-duration, but the most critical technical problems to be solved are scalability and cost.

Technology will deliver the right solution and technologists and capitalists will bring them to market as efficiently as possible. Because of this we will enjoy constantly lowering cost of energy, cost of storage and in general, cost of living. Beyond our cost of living this will make developing new technologies and experiences even more feasible.

The forward progress we are witnessing right now is remarkable.

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I started Life in the Singularity in May 2023 to track all the accelerating changes in AI/ML, robotics, quantum computing and the rest of the technologies accelerating humanity forward into the future. I’m an investor in over a dozen technology companies and I needed a canvas to unfold and examine all the acceleration and breakthroughs across science and technology.

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