Climate Change Mitigation & Energy Carriers

The Role of Carriers in the Energy Transition

As part of our ongoing analysis of energy’s role in mitigating climate change as part of the food-energy-water nexus, our focus today falls on energy carriers. These are not sources of energy. Instead, they are substances (or sometimes phenomena such as electricity) that can be used to produce mechanical work or heat. They can include springs, electrical batteries, capacitors, pressurised air, dammed water, hydrogen, petroleum and other fuels, processed coal and wood pellets, among others. These are all substances that store energy ready to operate a chemical or physical process.

Energy sources, on the other hand, are the original resource from which an energy carrier is produced. Take hydrogen, for example: It can be made using various energy-conversion processes, such as water electrolysis, but it’s not a source of energy itself. Energy sources can include renewable resources such as biomass, sunlight, wind and falling water, or non-renewable energy sources such as oil, unprocessed coal or natural gas.  

Why bring attention to this distinction? 

Because it partly helps to explain both the hype and skepticism of developing the global hydrogen economy as an ecologically friendly form of energy dispersal. The hype stems from the fact that if the hydrogen (H2) is produced via electrolysis to make so-called ‘green hydrogen’, it becomes a renewable energy option, since solar or wind can be the source that powers the electrolysis. When the hydrogen is later converted to energy, the only byproduct comes in the very environmentally friendly form of H20 or water. 

That sustainable end-to-end process is still a bit of a holy grail, however, as making green hydrogen is currently a very expensive and inefficient process. Today, estimates suggest that between 95% to 99.9% of the world’s hydrogen is produced using steam reformation, where fossil fuels, mostly coal and natural gas, are used to separate the oxygen from the hydrogen. This process produces a lot of CO2, making it an unsustainable solution. And the path to achieving affordable and efficient green hydrogen is still, even by optimistic estimates, still around a decade away

That time span as well as hydrogen’s highly flammable nature and difficult transportability help explain some of the scepticism. So in today’s Insight Article, we examine why many governments around the world are nevertheless factoring hydrogen into their plans for a carbon-neutral 2050 and beyond. Part of the explanation lies with what are called VREs, or variable renewable energy sources such as wind and solar. 

Wind and solar are predicted to supply 50% of the world's energy demand by 2050.

Both of these renewable resources are “intermittent” and “non-despatchable”, meaning they can’t be shipped around and deployed at off-peak times in the way we do now with fossil fuels. The use of small amounts of intermittent power has little effect on national grid operations but large amounts require upgrades or even a redesign of the grid infrastructure. This explains why grids around the world are powered predominantly by despatchable resources: They are simply designed for the regularity fossil fuels have provided.

The collective shift toward cleaner sources of power now requires societies to rethink how they can best capture and store the intermittent power of renewables and make it despatchable. There are different ways to do this. Some options include absorbing large shares of variable energy into the grid using: 

  1. storage options such as via batteries and other energy carriers, 
  2. improved interconnection between different variable sources to smooth out supply, 
  3. using dispatchable energy sources such as hydroelectricity and having overcapacity, so that sufficient energy is produced even when weather is less favourable. 

If we are only to look at storage, the following graphic gives a limited overview of some energy storage options and how they rank in terms of storage capacity and discharge time (the amount of time taken to fully discharge energy at its rated power by the storage system). Note that there are many other storage options not depicted here.

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.

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