The reason hydrogen has made headway around the world is its perceived cleanliness–provided sufficient investment is made now to make it cost effective in the future, similar to the way governments invested in VRE development in the past. This naturally comes at a risk, because it also requires infrastructure investments all along the supply chain to make hydrogen ‘work’ as a solution. H2 is also highly flammable, and transporting it around requires additional care. Related to this, as explained in the previous Insight Article, is the risk of leaks that present yet another possible greenhouse gas when it escapes from pipes.

Hydrogen is therefore not a panacea, but when compared to other storage alternatives, it becomes more a matter of which dispersal option creates the least possible harm at the most efficiency. Hydrogen doesn’t fare poorly when viewed in this light. Consider lithium-ion batteries, for example: They allow for rechargeability, have become much cheaper to produce over the past 30 years (during which time their energy density has more than tripled) and they can store the solar and wind energy for transportation. But the mining of lithium poses significant environmental and sociological issues, not only because extracting it requires large amounts of water and energy.

Another cleaner alternative is gravity batteries. In this system, excess power, which can come from renewable energy sources, is used to raise weights that then store the energy until needed. When the weights drop, for example within a tower or into a former mine shaft, electricity gets produced. The main drawback of this option is the space required to house and perform the process; an issue that faces VREs more generally. 

Like the gravity battery, pumped-storage hydroelectricity is a kind of water-based gravity battery. It involves pumping water uphill and releasing it back to rotate the turbines of a hydroelectric generator. According to the International Hydropower Association, pumped hydro already provides more than 90% of the world’s high capacity energy storage. There are various location options for this technology such as disused mines, non-powered dams or caves. Pump-hydro facilities are, however, costly to build, have significant ecological impacts and again require large amounts of water, rendering them impractical in many parts of the world.

Compressed air energy storage (CAES) plants work in a similar way to pumped hydro. Instead of pumping water uphill during periods of excess power, ambient air or another gas is compressed and stored under pressure in an underground cavern or container. When electricity is needed, the pressurised air gets heated and expanded in an expansion turbine driving a generator for power production. The difficulty arises when the compression of air leads to an unwanted temperature increase that can then reduce operational efficiency and lead to damage.

Molten salt energy storage, on the other hand, uses heat generated via renewable energy instead of gravity or pressure. Here the medium is salt, a non-flammable, non-toxic substance with a high melting point. The molten salt sits inside an insulating container at a temperature of at least 250°C during off-peak hours. When energy is needed, the salt gets pumped into a steam generator where water is boiled to spin turbines to generate electricity. The downside of this process is the cost involved keeping the salt molten, particularly during winter months.

These are only some of the numerous energy storage options available, and don’t include some of the newest developments in the field. Seen holistically, however, they should at least demonstrate that there is no one clear carrier or storage solution defining the energy sector’s role in mitigating climate change. Each approach has its benefits and drawbacks related to portability, environmental impact, and space consumption. This includes hydrogen, an option that nevertheless remains the focus of many progressive nation’s renewable energy transition goals due to its theoretical production-to-consumption cleanliness, however distant that goal may be.