The US Navy has a mechanical battery based on spinning flywheels, which store energy as they spin and can discharge it in a quick burst when stopped. It could be used to power laser weapons or railguns, or even to store energy for homes.

Generators provide sustained power, but can’t be cranked up for the short bursts of high power that are needed for directed energy weapons and railguns. For these, the US Navy currently uses banks of lithium-ion batteries. These can discharge rapidly, but pose risks to warships: they contain hazardous materials and are prone to thermal runaway and catching fire. Also batteries don’t work well at high and low temperatures.

To address these problems, researchers at Vishwa Robotics in Massachusetts and the Massachusetts Institute of Technology have designed a mechanical battery that uses an array of flywheels set inside a box. Flywheels can’t generally compete with chemical batteries when it comes to energy storage, but the mechanical battery has some innovative features. For a start, it is a collection of smaller units rather than a single large flywheel.

“By making the dimensions smaller each cell can be spun much faster,” says Bhargav Gajjar, president of Vishwa Robotics.

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Specially created bearings make the unit more efficient and economical. Gajjar says the design stores more energy than a lithium-ion battery of the same weight, and can release it faster with no thermal risk.

A 5-kilowatt diesel generator is typically the size of a washing machine and weighs more than 100 kilograms. The prototype 5-kilowatt mechanical battery is a disc just 25 centimetres across, and the next version will be less than 20 centimetres wide. And many can be stacked to power more energy-intensive weapons, such a lasers to counter drones, says Gajjar.

“Currently available energy conversion and storage devices that can power such long-range drone killer weapon systems have two problems. They are made with explosive chemicals, and they are very bulky. These two problems make it very difficult to integrate such weapons on ships, submarines, vehicles and unmanned systems,” says Gajjar.

Software manages the flywheel array, monitoring and drawing power from different wheels to match demand. Gajjar says the mechanical battery is also suitable for domestic use.

“A smaller 10 kilowatt-hour prototype is already running in my garage and powers my whole house at night,” says Gajjar. Rooftop solar panels recharge the unit during the day.

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Gajjar says the flywheel is a simple mechanical device and no rare materials are needed to make it, so it could be mass produced at low cost. Chemical batteries become less efficient after a few hundred charge/discharge cycles, but the mechanical battery shows no effect after tens of thousands of cycles and should last for decades, he says.

Keith Pullen at City University of London says the mechanical battery seems a good fit for applications requiring sudden bursts of power. He is doubtful about them producing greater energy that lithium-ion batteries by weight.

The Navy’s primary goal is better safety than chemical batteries, although there have been notable flywheel accidents in the past, when these large wheels flung debris or came loose while spinning, injuring people.

“Flywheels have a reputation for being dangerous,” says Pullen, “But they are safer than chemical batteries if they are properly engineered.”

The US Navy awarded a two-year development contract for the mechanical battery last month, which will include testing performance and safety under various conditions. The device will be evaluated for supplying power not just for weapons but for sensors and propulsion, for example in uncrewed submarines, and backup power.