Burj Khalifa Designer SOM Gravity Batteries in Skyscrapers

Yes, the architecture firm Skidmore, Owings & Merrill (SOM), designer of the Burj Khalifa, is actively developing a system that turns tall buildings into gravity batteries by using their elevator systems to lift and drop heavy weights, storing and releasing energy. This concept, known as Lift Energy Storage Technology (LEST), is being researched in collaboration with the International Institute for Applied Systems Analysis (IIASA) and is a serious proposal for making future skyscrapers dual-purpose energy storage facilities.

Background: Why Gravity Batteries in Skyscrapers?

The global shift to renewable energy sources like solar and wind has created a critical need for large-scale energy storage. These sources are intermittent—the sun doesn’t always shine, and the wind doesn’t always blow. Traditional batteries, like lithium-ion, are effective but have limitations: they can be expensive, have a finite lifespan, and require significant land and resources. Gravity-based energy storage offers a mechanical alternative. The principle is simple: lift a heavy mass to store potential energy, then let it descend to release that energy as electricity through a generator. Pumped-hydro storage is the most common form, but it requires specific geography (two reservoirs at different elevations). SOM’s proposal aims to use the vertical height of existing and future skyscrapers, turning them into massive mechanical batteries without needing extra land.

How SOM’s Gravity Battery Concept Works

The system, detailed in a 2022 research paper by IIASA and SOM, is elegantly simple. It leverages the building’s existing elevator infrastructure, which already includes motors, cables, and control systems. Here’s the step-by-step process:

• Charging (Storing Energy): When there is excess electricity on the grid (e.g., from a windy night), the elevator is used to lift a heavy weight—such as a specialized container filled with sand, gravel, or recycled materials—from a lower floor to a higher floor. This consumes electricity but stores it as gravitational potential energy.
• Discharging (Releasing Energy): When electricity is needed (e.g., during peak evening demand), the heavy weight is allowed to descend. The elevator’s motor acts as a generator, converting the downward motion into electricity that is fed back into the building’s power system or the grid.
• Management: A fleet of automated robotic tugs or the elevator itself moves the weights between a storage area (like a parking garage level) and the elevator car. The system can be scaled by using multiple elevators and multiple weights.

The key innovation is not the elevator itself, but the integration and control system that allows it to operate as a power plant while still performing its normal passenger and freight duties. The system can prioritize passenger service, using idle time for energy storage.

Real-World Applications and Pilot Projects

While still in development, the concept has moved beyond theory. SOM and IIASA have published detailed feasibility studies, and there are active discussions about retrofitting existing buildings. A notable example is the proposed use in a building in Vienna, Austria, where a pilot project is being planned. The firm has also designed a conceptual skyscraper for New York that incorporates the technology. The idea is particularly attractive for supertall buildings (over 300 meters), like the Burj Khalifa, because the potential energy stored (E = mgh) increases linearly with height (h). A taller building can store more energy for the same weight. For a building like the Burj Khalifa (828 meters), the theoretical storage capacity is substantial, though the practical capacity is limited by elevator speed, weight limits, and the need to maintain passenger service.

Advantages Over Conventional Batteries

Compared to lithium-ion battery storage, SOM’s gravity system offers several compelling benefits:

• Longevity: Mechanical systems like elevators and weights have a much longer operational life (30-50 years) than chemical batteries (10-15 years), with no degradation in storage capacity over time.
• Cost: The marginal cost of adding a gravity battery to a new building is low, as the elevator system is already required. The main costs are the weights, control software, and any structural reinforcements. The cost per kilowatt-hour of storage is projected to be competitive with, or even lower than, lithium-ion batteries, especially when considering the avoided cost of a separate battery installation.
• Safety and Sustainability: There is no risk of thermal runaway or fire, a known hazard with lithium-ion batteries. The system uses inert materials (like sand or concrete blocks) that can be sourced locally and are fully recyclable.
• Space Efficiency: It uses vertical space that is otherwise occupied by elevator shafts, not horizontal land. This is a huge advantage in dense urban environments where land is scarce and expensive.

Challenges and Limitations

Despite the promise, there are significant hurdles. The most obvious is the conflict between energy storage and passenger use. During peak hours, elevators are busy moving people, limiting the time available for lifting weights. However, the system can be designed to use only one elevator out of a bank, or to operate during off-peak hours. Another challenge is weight. To store a meaningful amount of energy (say, 20 megawatt-hours, which is a typical utility-scale battery size), you would need to lift a massive weight. For example, to store 20 MWh, you would need to lift a 10,000-tonne weight to a height of 72 meters. That is an enormous load that would require significant structural reinforcement of the building’s core and foundation. The elevator system itself would need to be upgraded to handle such loads, which is not trivial. Finally, the round-trip efficiency (the percentage of electricity you get back versus what you put in) is estimated at around 70-80%, which is lower than lithium-ion batteries (85-95%). This is due to friction in the elevator system and losses in the motor/generator.

Comparisons: Gravity Batteries vs. Other Storage

It’s helpful to see how this concept stacks up against other storage technologies:

• Pumped Hydro: The most mature and largest-scale storage, but requires specific geography and massive upfront investment. Skyscraper gravity batteries can be built anywhere, but on a much smaller scale per building.
• Compressed Air Energy Storage (CAES): Requires large underground caverns for air storage. Skyscraper batteries are more flexible in location.
• Lithium-ion Batteries: High energy density and fast response, but degrade over time and have supply chain and safety concerns. The building-based system has a longer life and is safer, but lower efficiency and energy density.
• Other Gravity Systems (e.g., Energy Vault): These use cranes to stack concrete blocks on the ground. The skyscraper approach leverages existing infrastructure, potentially reducing costs, but is limited to where tall buildings exist.

For a single building, the storage capacity is modest—enough to shave peak demand or provide backup power for a few hours. But if implemented across a city’s many tall buildings, the aggregate capacity could be significant, helping to stabilize the grid.

Costs and Economic Viability

While no official price list exists, the economic case is based on avoided costs. The primary cost of a skyscraper gravity battery is the elevator system itself, which is already a mandatory expense. The additional costs are the weights (e.g., concrete blocks), the control software, and any structural reinforcements. The IIASA study estimates that the levelized cost of storage (LCOS) could be in the range of $150-$250 per megawatt-hour, which is competitive with current lithium-ion battery storage costs (which are around $200-$300 per MWh, depending on the market). The long lifespan of the mechanical system further improves the economics, as the cost is amortized over decades. However, these estimates are based on simulations and have not been validated in a real-world, full-scale installation. The first pilot projects will be critical to proving the cost and performance.

Safety and Structural Considerations

Safety is paramount in any building system. The gravity battery concept introduces new risks that must be managed. The most significant is the potential for the weight to fall uncontrolled. To mitigate this, the system would use the elevator’s existing safety brakes, which are designed to stop a fully loaded car in an emergency. Additionally, the weights would be secured in containers that are designed to withstand impact. The building’s structure must be reinforced to handle the additional dynamic loads of lifting and lowering heavy weights. This is a well-understood engineering challenge, as buildings already handle heavy loads from HVAC systems, water tanks, and people. However, the repeated cycling of large weights could cause fatigue in the structure, so the design must account for this. Fire safety is less of a concern than with batteries, as the materials are non-flammable. Overall, the system can be made safe, but it requires careful engineering and rigorous testing.

Frequently Asked Questions

Is the Burj Khalifa itself being converted into a gravity battery?

No, not currently. The Burj Khalifa was designed before this concept was developed. However, SOM is using the experience and knowledge gained from designing the Burj Khalifa to inform the design of new buildings that could incorporate this technology. The Burj Khalifa serves as a proof-of-concept for the structural and elevator capabilities needed.

How much energy can a skyscraper gravity battery store?

It varies widely based on the building’s height, elevator capacity, and the weight used. A typical 50-story building (about 200 meters) with a 2,000 kg elevator weight could store about 1.1 kWh per trip. To store 10 MWh, you would need to make thousands of trips. In practice, a single building might store 10-20 MWh, which is enough to power the building itself for a few hours, not the whole grid.

What happens if the power goes out while a weight is being lifted?

The system is designed to be fail-safe. If power is lost, the elevator’s braking system engages automatically, holding the weight in place. It will not fall. Once power is restored, the system can resume its operation. There is no risk of a runaway weight.

Does this technology make sense for all new skyscrapers?

Not necessarily. It makes the most sense for very tall buildings (over 200 meters) in locations with high electricity prices or grid instability. For shorter buildings, the energy storage capacity is too small to justify the complexity. It also makes sense in regions where land is scarce and where the building is already being constructed with a robust elevator system.

Can existing buildings be retrofitted with this technology?

Yes, in principle, but it is more challenging and expensive than designing it in from the start. Retrofitting requires reinforcing the structure, possibly upgrading the elevator motor and control system, and finding space for the weights. The IIASA study suggests that retrofitting is feasible for many modern high-rises, but it is likely to be more cost-effective to include it in new construction.

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