As the world looks to a renewable-energy future, storage becomes a concern because with renewables, supply and demand aren’t always in balance.

Renewable energy sources such as wind and the sun aren’t always “on” when consumers need energy, and excess power that can’t be used immediately is wasted unless it’s stored.

Storing energy can be expensive, however, so some utilities use plants that burn fossil fuels to make up the difference during times of peak demand. Those plants operate most efficiently when at full power, however, and using these plants to redistribute power can lead to more pollution.

Chemical batteries are useful for electric vehicles but they may not be the best option for utility companies. Chemical batteries’ life cycles can also be short. Lithium ion batteries, for example, last about five to 10 years. They’re expensive. And the metals used to make them raise issues of geopolitics and human rights.

Looking at other materials seems to be a good idea.

Here are six innovative materials and methods we might use instead:

PUMP STORAGE WITH WATER

This isn’t a new idea: People have been using pump storage since the early 20^th century. Early pump storage used fossil fuels to move water from a lower reservoir to a higher one during off-peak hours, when that energy was cheapest. Then when the energy was needed, gravity returned the water to the lower reservoir, turning turbines as it flowed. Such systems today can substitute renewable energy for power from fossil fuels. This is the most popular method of storing electricity today and accounts for 93 percent of utility-scale energy storage in the United States.

GRAVITY BATTERIES

As with the pump-storage system, this uses renewable energy to raise an object from a lower level to a higher one. But instead of water, it’s a heavy mass that generates gravitational potential energy. When the energy is needed, the mass is slowly dropped. The motor that raised it in the first place switches to generator mode and energy is sent off to the consumer. How much energy is produced and how long it is generated depends on the height and weight of the lift. One company working with the technology, Gravitricity in Scotland, is investigating the use of deep decommissioned mines for gravity energy storage. The company estimates that some 14,000 mines around the world could be repurposed for energy storage.

FLYWHEELS

A flywheel can be as simple as the power system in a child’s friction toy or as complex as NASA’s G2 system for energy storage in a spacecraft. The flywheel is essentially a mechanical battery with a heavy weight that rotates around an axis. Energy gets the wheel spinning. And if it spins fast enough, it can store energy. The limiting factors are friction and how much force the wheel can take before it breaks.

SAND BATTERIES

The sand battery uses sand or a sandlike substance heated to temperatures well above the boiling point of water – about 500 degrees C. Cool air blown through pipes in the storage facility picks up the heat and can be used, for example, to convert water into process steam. The first commercial sand battery in Finland uses about 100 tons of low-grade sand to warm homes, offices and a municipal swimming pool year-round, and its developers say the sand can hold its heat for months.

THERMODYNAMIC STORAGE USING COMPRESSED AIR

This system uses electrical energy to create high-pressure compressed air, which can be released later to drive a turbine generator. Utility-scale versions of these systems are generally located in caverns. A variant of this storage system is underwater compressed air energy storage, which benefits from the constant water pressure and could be useful for coastal locations.

WOOD BATTERIES

About 30 percent of a tree – depending on species – is lignin, the glue that holds its cellulose fibers together. The polymer lignin also contains carbon, which as it turns out is a great material for a battery part called an anode.

Finland’s Stora Enso happens to have lots of trees: It calls itself the one of the largest owners of private forest in the world. And according to the BBC, the company’s engineers say they can extract the lignin they need from waste pulp the company is already producing.

Stora Enso has entered into a partnership with Swedish company Northvolt to create batteries sourced sustainably in Nordic countries. They expect to be in production as early as 2025.