Metal-based energy

Oil is the most consumed energy resource worldwide, followed by coal and natural gas. However, the need for environmentally friendly energy sources drives the search for new, sustainable energy options. Can metals serve as fuel, and what are the prospects for metal-based energy?

Oil as an Energy Source

Today, oil is the most consumed primary energy source, accounting for one-third of global energy use. More than 60% of oil is dedicated to motor fuel production for transportation, forming the backbone of non-stationary energy, which powers nearly all forms of transportation: automobiles and air and marine vessels. According to recent data from major oil companies, the world’s proven oil reserves are estimated at 1,700 billion barrels. This supply would last approximately 50 years at current consumption levels—a similar forecast to what was projected half a century ago.

The era of oil is unlikely to end due to resource depletion. According to the International Energy Agency, the energy sector accounts for 70% of all anthropogenic emissions. Thus, humanity is more likely to face crises related to other resource limitations rather than running out of oil. For example, oxygen may become a limiting factor, as it is required to combust hydrocarbon fuel. More precisely, the atmospheric oxygen concentration may decrease, carbon levels will rise, and the greenhouse effect will intensify. This leaves us with two possible scenarios: the pessimistic one just described and an optimistic scenario, which we will discuss further.

The energy sector, specifically transportation energy, has urgently needed solutions supporting sustainable human development for decades. Air pollution remains one of the leading environmental challenges today, with transportation being the primary source of pollution in urban areas. Two major solutions have been proposed to address this issue: electric motors powered by batteries or engines running on hydrogen fuel. However, both approaches present particular challenges.

Battery Storage and Hydrogen Fuel

When considering electric vehicles, the increase in battery storage inevitably places a strain on urban power grids. Let’s take a 75-kilowatt-hour (kWh) battery, commonly used in electric cars, as an example. A 75 kilowatt (kW) power supply is required to charge such a battery within one hour. If charged over five hours, it needs 15 kW. Now, assume a big city has about 3.5 million cars on the road daily, and imagine all of them suddenly switching to electric. Charging them in just one hour would require around 260 million kW, or 260 gigawatts (GW). The total installed power capacity of the country’s power plants would fall short of meeting this demand—considering the installed capacity, not the actual capacity in use. Even if every power station in the country ran at total capacity, it still wouldn’t suffice to charge all-electric cars.

What does 75 kWh mean in practical terms? Let’s compare it to a fuel tank to put it into perspective for the average driver. Converting kWh to megajoules (MJ) by multiplying by 3.6 gives us 270 MJ. Now, accounting for the internal combustion engine’s efficiency of 25%, we would need an energy reserve of 1080 MJ. Dividing this by gasoline’s specific energy density of 33 MJ per liter gives us 32 liters. Thus, a 75 kWh battery equates to a gasoline reserve of 32 liters, with the battery itself weighing nearly half a ton. In the case of a truck, the battery weight alone could rival the weight of the vehicle itself. This is not a significant energy reserve; one cannot travel far.

Suppose we take an electric vehicle out of town for a nature trip or another city. Instead of traditional fuel stations along the highway, there would need to be electric charging stations. To charge a car quickly, ideally within an hour, these stations would need to provide a power supply of around 100 kW per vehicle, equivalent to the power consumption of several private homes. Charging five cars simultaneously would then require 0.5 MW. Electrifying fuel stations on such a scale is a challenge, particularly in our country. Large-capacity energy storage batteries are needed to support these charging stations, but we also face issues. Such storage systems are still under development. One effective method of storing electrical energy is hydroelectric storage, yet these facilities can only be built in some places and are costly.

A potential increase in battery usage would also result in shortages of the rare metals used in them. Lithium, for instance, is far less abundant in the Earth’s crust than carbon, which is concerning for lithium-ion batteries. Additionally, there are supply issues with cobalt, which is used in battery electrodes, which could limit the growth of this sector.

Before the surge of interest in electric motors, hydrogen fuel engines were actively discussed. Experimental models were developed and are still in operation today. However, hydrogen has not gained widespread use in energy sectors because storage and transportation issues still need to be solved: it is one of the most explosive gases and has low density under standard conditions. Hydrocarbon-based gaseous fuels, such as methane, propane, or butane, are less explosive than hydrogen and more eco-friendly and cost-effective than gasoline and diesel. However, they cannot replace traditional liquid hydrocarbon fuels for similar reasons: they are challenging to store and transport. Hydrogen is even more cumbersome in this respect.

Metals as Energy Carriers and Fuel

Modern transportation is constructed from metals and runs on hydrocarbon fuels. In the future, however, it may be built from hydrocarbons and powered by metals. What are the advantages of using metals as fuel and energy carriers? When we use metal as fuel, it combusts in oxygen or another oxidizer, generating energy that we can convert into useful forms. The process is similar to burning gasoline or diesel, but metal oxidation produces metal oxide, unlike hydrocarbon fuels, which emit harmful greenhouse gases. For most metals, metal oxide is a solid under standard conditions, which means it doesn’t disperse into the air; instead, it must be collected and returned to the metal production cycle.

Which Metals are Suitable?

For energy purposes, it’s practical to focus on elements that are abundant in nature, such as oxygen, silicon (though not a metal, it’s a promising inorganic energy-storing material that can burn with significant heat release), aluminium, iron, calcium, sodium, potassium, magnesium, and others. Alkali and alkaline earth metals react readily with oxygen, so they must be stored in oxygen-free environments. In contrast, iron and aluminium develop a dense oxide layer on their surfaces when exposed to air, which prevents further oxidation.

The primary challenges in developing metal-based energy systems lie in metal oxidation and converting the heat from oxidation into useful energy. Two fundamentally different approaches exist to transforming metals’ chemical energy into usable energy. 

The first approach directly converts chemical energy into electricity using metal-air fuel cells. Here, the metal acts as the anode, and an air electrode is the cathode, where oxygen reduction occurs. During oxidation, positive metal ions move into the electrolyte to meet with the electrolyte’s negative ions. Electrons from the metal travel through an external circuit to the cathode, generating useful electrical energy. The theoretical maximum efficiency of metal-air fuel cells exceeds 90%, while actual efficiency, which depends on device power and current density, can range from 50–60%.

The second approach to converting the chemical energy of metals into usable energy or electricity is to oxidize them in an oxidizer and convert the resulting heat via thermal machines. Using an internal combustion engine is impractical for metals compared to hydrocarbon fuels: metal oxidation produces solid particles that would quickly render an internal combustion engine unusable if introduced. The oxidizer can be either oxygen or water. When metal oxidizes in water, it displaces hydrogen, which can then be directed into a fuel cell or an internal combustion engine. The advantage here is that hydrogen is produced on-site, eliminating the need for transportation.

Metal Energy Density and Regeneration

Metals have some of the highest energy densities. For example, aluminium has an energy density of around 31 megajoules per kilogram, magnesium around 25 megajoules per kilogram, and iron nearly 7.5 megajoules per kilogram. High-quality coal has a calorific value of about 25 megajoules per kilogram, while oil and petroleum products range from 40 to 45 megajoules per kilogram. By mass, liquid hydrocarbons are somewhat higher in energy density than metals, but in terms of energy per volume, some metals, like aluminium, are almost twice as dense as gasoline.

When metals oxidize, we obtain energy and metal oxide. To retrieve the metal from its oxide, energy must be expended. Depending on the metal, the regeneration process can vary. Aluminum, for example, is obtained by electrolysis. The industrial Hall–Héroult process extracts aluminium from alumina in the presence of carbon electrodes, consuming 14–15 kWh of electricity and releasing carbon dioxide and carbon monoxide. However, the aluminium production process can be made environmentally friendly by replacing carbon electrodes with inert anodes, increasing energy consumption by 1.4 times but reducing the process emissions. This increase in energy use is manageable if you have access to cheap electricity, such as from hydroelectric or nuclear power plants operating in base-load mode.

Iron, traditionally produced by carbon reduction in blast furnaces, can be generated through direct hydrogen reduction. For instance, surplus electricity from a renewable source or nuclear power can produce hydrogen via electrolysis, reducing iron oxide. The advantage here is that while hydrogen is difficult to transport, metal is not; it’s much easier to ship metal, even between continents, than liquefied natural gas.

However, metal production is energy-intensive, so the cost of energy derived from burning metal remains higher today than that from burning hydrocarbon fuel. Nevertheless, many research groups are working in this field. Though it’s unlikely that metal-based energy will surpass the cost-effectiveness of hydrocarbons, this doesn’t preclude metals from becoming viable energy carriers.

We can recall Thomas Edison’s words when he was told that electric lighting was prohibitively expensive: “We will make electricity so cheap that only the wealthy can afford to burn candles.” Perhaps one day, at a fueling station, you’ll fill up not with gasoline or diesel but with some form of metal or alloy.