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How to Escape From the Iron Age?

Mon, 18 Mar 2024 00:00:00 +0000

Image: Steel rebar construction for the concrete foundation of a wind turbine in Gilliam County, US. Image by Goose Chap, Wikimedia Commons (CC BY-SA 4.0)

Trapped in the Iron Age

In 1836, Danish antiquarian and curator Christian Jürgensen Thomsen distinguished three prehistorical eras based on the dominant materials used for weapons and cutting implements: the Stone Age, the Bronze Age, and the Iron Age.¹ Thomsen’s classification refers to the past, but according to his criteria, we have never evolved beyond the Iron Age. Even in the 21st century, iron remains the dominant material, not just for weapons and cutting implements but for about every modern technology.

We now use most iron in the form of steel. However, according to Thomsen’s criteria, we cannot speak of a “Steel Age.” First, steel is merely an alloy of iron (>98%) and carbon (<2%). Second, humans have been producing steel since the beginning of the Iron Age. That is a little-known fact in the Western world, where steel production only took off in the nineteenth century with fossil fuels. However, Asian and African metallurgists developed high-quality steels much earlier, and this knowledge eventually allowed Europeans to do the same – on a much larger scale.²

By 2021, the global iron and steel output reached 1,950 million tonnes (Mt). That is 22 times larger than the combined aluminum and copper output (88 Mt). The global iron and steel output corresponds to five times the global plastics output (391 Mt) and dwarfs the worldwide production of silicon (8.5 Mt) and lithium (0.1 Mt).³⁴ Steel is the fundamental material of industrial societies. Without plastics, lithium, or silicon, we would still be in an industrial society. Without iron and steel, we would be thrown back 3,000 years into the Bronze Age.

Where is all that steel?

The massive presence of steel in industrial society is not so obvious.⁵ At home, we find several steel appliances such as the refrigerator, washing machine, water boiler, bathtub, and cooking, heating, and cooling appliances. However, only 2-3% of total steel production ends up in domestic appliances.⁶⁷⁸ Outdoors, there’s a lot of steel in the form of vehicles. These are especially passenger cars that use around 10% of all steel globally (20% in rich countries). Busses, trucks, trains, and ships add another 4-5%. Altogether that is still less than 20% of the global steel output.

Most steel is embedded in other materials, located underground, or far away from residential areas.

Most steel is embedded in other materials, located underground, or far away from residential areas. More than half of global steel production goes into construction, which includes buildings (residential, commercial, industrial) and infrastructures (bridges, tunnels, harbors, canals, runways, oil rigs, refineries, pipelines, power plants, transmission lines, railways, subways, and so on). Much of that steel is embedded in concrete. Reinforced concrete is the world’s primary building material, and concrete is the only material that can match the output of steel (1,819 Mt in 2021).

Roughly 15% of global steel production serves to make machinery, including machine tools, industrial equipment, electrical hardware, and construction, mining, and farming machines. Even products made of other materials – such as other metals, plastics, and wood – are shaped by steel tools.⁵ The final 15% of steel production ends up in a variety of objects, from screws over food packaging to furniture and shipping containers.⁶⁷⁸

Image: Reinforced concrete is the world’s primary building material. Hole on Interstate 84, US. Image by Tony George, Oregon Department of Transportation, Wikimedia Commons (CC BY 2.0).

The environmental footprint of the steel industry

Steel is often presented as one of the most sustainable materials. Unlike plastics, steel can be recycled without any loss in quality. The steel industry has made great advances in energy efficiency, more so than many other industries. Making one ton of crude steel now requires roughly 20 gigajoules (GJ) of primary energy on average – three times less than in 1950.⁹ This compares very favorably to other materials such as aluminum (175 GJ/t), plastics (80-120 GJ/t), or copper (45 GJ/t).⁷ Unlike plastics, steel is a biodegradable material.¹⁰ Finally, iron ore is not in short supply. It makes up 5 percent of the Earth’s crust and is fourth in abundance among the elements.¹¹ For comparison, copper only makes up 0.01%.⁵

However, despite all these advantages, the global iron and steel industry consumes more energy and produces more carbon emissions than any other industry. The total primary energy use of crude steel production was 39 exajoules (EJ) in 2021, which corresponds to 7% of all energy used worldwide in that year (595 EJ). The greenhouse gas emissions are even higher because around 75% of energy use comes from coal – the fuel with the highest carbon emissions. In 2021, the iron and steel industry produced 3.3 Gt of carbon emissions, roughly 9% of global emissions (36.3 Gt).¹² The concrete industry follows closely with 8%.

The iron and steel industry consumes more energy and produces more carbon emissions than any other industry.

The estimates above come from the World Steel Association and the International Energy Agency. These data are available for all metals and have been documented over a long period, allowing for historical comparisons. However, they only refer to the smelting of the metal. They do not include the energy use and carbon emissions for mining and transporting iron ore, coal, limestone, scrap, and steel products. Nor do they include the energy and emissions for coke production and ore preparation – all essential to the steel production process.⁷

Scientific studies that have set wider boundaries for the iron and steel industry conclude that the energy cost of steel production increases by 50% to 100%.¹³ One report concludes that the methane emissions from metallurgical coal mining alone could increase emissions by 27%. Another study estimates that seaborne transport of iron ore and steel adds 10-15% extra emissions.¹⁴¹⁵ Iron and steel production also create other environmental problems, such as high water use, solid waste production, and significant air and water pollution.

The carbon footprint of the iron and steel industry is incompatible with current ambitions to eliminate net carbon emissions by 2050, even less so because steel production is very likely to expand further. Steel production grew tenfold since 1950 and doubled between 2000 and 2020, growing faster than many researchers had predicted.¹⁶ Furthermore, efficiency gains have decreased, and there is a scientific consensus that current technologies have reached their thermodynamic limits.⁷⁹¹⁷ During the last two decades, the average energy use for the production of 1 ton of steel has remained around 20 GJ/t.⁹¹⁸

How to make steel without fossil fuels?

There are two ways to make steel, and one is much more sustainable than the other.¹⁹ On the one hand, there is the blast furnace or basic oxygen furnace, in which steel is made from iron ore and coal. This technology is – in its essential form – 2000 years old. On the other hand, there is the electric arc furnace, in which steel is made from steel scrap and electricity. The electric arc furnace, which is a relatively new technology, consumes much less energy than the blast furnace, makes use of a recycled resource (no need to mine iron ore), and works without the direct use of coal or other fossil fuels (the electricity can be supplied by solar, wind, or atomic power).

The most energy-efficient electric arc furnaces now consume less than 300 kilowatt-hours of electricity per ton of steel produced.⁹²⁰ Hypothetically, if we had produced all steel in 2021 (1,950 Mt) in such furnaces, the total power consumption of the global iron and steel industry would have been only 585 terawatt-hours (Twh). That corresponds to just one-third of all electricity generated by wind turbines worldwide in the same year (1,848 Twh). Unfortunately, more than 70% of global steel output was made in blast furnaces fed by coal and iron ore.⁹²⁰ A blast furnace consumes twenty times more energy and cannot be operated by electricity because coal is both the fuel source and the chemical reductant. The combustion of coal produces carbon monoxide that reduces the iron from its ore.⁷

Not enough scrap available

The solution seems obvious: let’s produce all that steel in electric arc furnaces. However, this is impossible. There’s not enough scrap available: the continuous growth of the global steel output makes a circular flow of resources impossible.²¹ It takes decades before most steel becomes available for recycling. For example, there is 543 Mt of steel stocked in ships.²² The scrap available for recycling in 2021 corresponds to the production level of 1965 when global steel production was less than one-quarter of what it is today (450 Mt).⁹¹⁰¹⁵²³ Consequently, the other three quarters need to be produced in blast furnaces using coal and freshly mined iron ore.

Image: Cars for scrapping at the Port of Cardiff. Gareth James via Wikimedia Commons (CC BY-SA 2.0).

Nowadays, China produces roughly half of the steel in the world and does that almost exclusively (+90%) in blast furnaces using coal and iron ore. Many other steelmaking nations have a higher share of electric arc furnaces. However, it makes little sense to point the finger at China. First, the US and Europe have outsourced many of their industries to China since the 2000s, a trend that corresponds neatly with the growing steel output in that country. Furthermore, twenty to forty years ago, China hardly used any steel. Consequently, there is almost no scrap available. China has no other choice than to use blast furnaces.²⁴

Ever higher grades of steel

A second obstacle is the continuous development of higher grades of steel. There are now over 2,500 different types of steel with a variety of properties, such as increased strength, tolerance to high temperatures, or corrosion resistance.⁷⁹²³²⁵ Although these higher quality steels can be produced in electric arc furnaces, they are not made from scrap, and they have much higher energy use.

Steel available for recycling forms a mix of steel grades. That mix is suitable for making plain carbon steel but not highly alloyed steels, which require scrap with similar qualities. However, that scrap is not available. For example, stainless steel, the most produced special steel grade, has a recycling rate of only 15%. Almost 60 Mt of stainless steel was produced in 2021, compared to only 4 Mt in 1980.²⁶ The traditional use of stainless steel was in cutlery, surgical tools, and medical and food processing equipment. However, it is now also used in the construction of tunnels and outdoor furniture, wastewater treatment, seawater desalination, nuclear engineering, and the production of biofuels.⁷

The low recycling rate and the need for the extraction of additional elements such as chrome and nickel make higher grades of steel more energy-intensive to produce. For example, stainless steel production requires almost 80 GJ per ton, four times more than the production of plain carbon steel.⁷²³ The continuous development of higher-grade steels is stimulated by environmental legislation (such as the use of lighter steel in cars) and by competition from other materials, mainly aluminum and plastic composites.⁷⁹²³²⁵ Ironically, the competition with these materials, which consume even more energy, makes steel less and less sustainable.

Steel and renewable energy

The steel industry is heavily dependent on the energy supply, but the energy supply is also heavily dependent on the steel industry. Almost 10% of the global steel output goes into building and maintaining energy supply infrastructure. That amount corresponds to the entire steel output in 1950. A great share of that steel goes to gas and oil infrastructure.²⁷. Oil and gas mining, production, and transportation require steel for offshore drilling platforms, pipelines, refineries, tankers, and storage tanks. Coal mining depends on steel for cutters, loaders, conveyors, excavators, and trucks.⁷

Unfortunately, the planned switch to low-carbon energy sources and the electrification of heating and transport technologies will not decrease our dependency on the steel industry – on the contrary. A low-carbon power grid requires much more steel (and other materials) than an infrastructure based on fossil fuels. Wind and solar power are very diffuse power sources compared to fossil fuels. Therefore, it takes much more materials (and land) to produce the same energy. In jargon, wind and solar have low “power density” or high “material intensity.”²⁸²⁹³⁰³¹³²

A low-carbon power grid requires much more steel than an infrastructure based on fossil fuels.

The “steel intensity” of thermal gas and coal power plants is between 50 and 60 tonnes of steel per megawatt of installed power.³³ Hydroelectric power plants have a lower steel intensity, with 20-30 tonnes of steel per MW.⁷³³ Atomic power’s steel intensity is also lower at between 20 and 40 tonnes of steel per installed MW.³³³⁴ On the other hand, solar PV requires between 40 and 170 tonnes of steel per installed MW.³³³⁵ Although there is little or no steel in the solar panels themselves, it’s the material of choice for the structures that support them.

Steel and wind power

The most steel-intensive power source – by far – is the modern wind turbine. The steel intensity of a wind turbine depends on its size. A single, large wind turbine requires significantly more steel per megawatt of installed power than two smaller wind turbines.³⁶ For example, a 3.6 MW wind turbine with a 100-meter tall tower requires 335 tons of steel (83 tons/MW), while a 5 MW wind turbine with a 150-meter tall tower needs 875 tons of steel (175 tons/MW).³⁷ The trend is towards taller wind turbines and a higher steel intensity.

Image: Steel towers for wind turbines in the port of Rotterdam. Image: Melle Smets.

Steel consumption further increases for offshore wind turbines. Onshore wind power plants rely on reinforced concrete for their foundations, but offshore wind turbines need massive steel structures such as monopiles and jackets.³⁸ The steel intensity for offshore wind turbines is calculated to be around 450 tonnes per MW for a 5 MW turbine – eight times higher than the steel intensity of a thermal power plant.³⁶. As these wind turbines get taller and move into deeper waters, their steel use further increases.

The most popular offshore wind turbine nowadays has a capacity of 7 MW, while the largest ones have a capacity of 14 MW.³⁶ If we make a conservative estimate based on the data above (the steel intensity doubles for every doubling of the power capacity), a 14 MW offshore wind turbine would require 1,300 tons of steel per MW or 18,200 tonnes in total. Such a wind turbine thus consumes 24 times more steel than a coal or gas power plant of the same power capacity.

Shorter life expectancy

The difference between renewable power sources and fossil fuels becomes even larger if the steel intensity is calculated per unit of energy rather than power (MWh instead of MW). In contrast to coal and gas power plants, the output of wind and solar power plants depends on the weather, and they do not always produce their maximum power capacity. Therefore, replacing 1 MW of fossil electricity generation capacity requires the installation of (on average) 4 MW of solar power or 2 MW of wind power.³⁹ A 14 MW offshore wind turbine thus has a steel intensity that is almost 50 times higher than a fossil fuel power plant for every kilowatt-hour of electricity produced.⁴⁰

A 14 MW offshore wind turbine has a steel intensity that is almost 50 times higher than a fossil fuel power plant for every kilowatt-hour of electricity produced.

Solar and wind power plants also have shorter lifetimes (20-30 years) compared to thermal power plants (30-60 years).³¹ While this does not affect the steel intensity per MW of power installed, it again increases the steel intensity per unit of energy produced over time. That does not always lead to a doubling of steel use because foundations for offshore wind turbines and structures for solar panels may have longer lifetimes than the power sources they support and could thus be reused.⁴¹

Power transmission infrastructure

The data above only include the steel used in the power plants themselves. For fossil fuel power plants, they do not include the steel used in the pipelines, oil rigs, coal excavators, and the like. However, the same goes for the low-carbon power sources. Because they need much more resources than thermal power plants (steel but also other metals and materials), they depend on a global mining and transport infrastructure that is just as steel-intensive as the supply chain for fossil fuels.

Furthermore, because they are more diffuse power sources with intermittent and unpredictable power production, often located far away from energy consumption centers, renewable power plants drive the expansion of transmission infrastructure. That infrastructure is also based on steel – from switchyard equipment over towers to conduction cables.²⁸²⁹³⁰³¹³²⁴²

Finally, low-carbon power sources also have a high need for special grades of steel, which are more energy-intensive to produce. Steel for off-shore wind turbines should resist corrosion, and stainless steel is increasingly used for solar panel support structures.⁴³ Electrical lamination steel (iron-silicon) is indispensable for transformers in the power network.⁷ Nuclear power plants may have a relatively low steel intensity but are completely built up of energy-intensive specialty steels. For example, cladding the fuel elements containing fissionable uranium requires zirconium steel, while all structural elements contain austenitic stainless steel.⁷⁴⁴

Low carbon grid cannot be made from recycled steel

The high steel intensity of low carbon power sources confronts us with a so-called “catch-22”, a situation in which there seems to be no escape from a problem no matter what we do. We need much more steel if we replace thermal power plants with renewable ones. Because there is not enough steel scrap available, we can only produce that extra steel from iron ore in blast furnaces burning fossil fuels. To address climate change, we need to build low-carbon sources quickly and in great numbers. However, to achieve circular material flows and build low-carbon power sources from scrap and renewable electricity, we would have to do the opposite: slow down the development of a low-carbon power grid.

Image: Steel foundations for off-shore wind turbines. Image by Glen Wallace, Wikimedia Commons (CC BY 2.0).

A well-cited study from 2013 concluded that if wind and solar power would supply 25,000 Twh of electricity – which corresponds to total global electricity demand in 2021 – we need about 3,200 Mt of steel to build the power plants alone.³³⁴⁵ Global electricity demand is projected to grow to between 52,000 and 71,000 terawatt-hours in 2050, which would increase the extra steel demand to between 6,400 and 8,960 Mt.⁴⁶ Spread out over the lifetime of solar panels and wind turbines (25 years), we would have to produce 256 to 358 Mt extra steel per year to make wind turbines and solar panel structures – comparable to the steel demand for passenger cars (195 Mt) and other transportation modes (98 Mt) combined.

That is still a very optimistic estimation. Electricity demand only makes up around 20% of total energy demand. If the total energy demand (177,000 Twh in 2021) would be supplied by wind and solar, we would need 22,400 Mt of steel. That’s an extra 896 Mt steel per year – as much as the global production in the early 2000s. You could argue that electricity can be used more efficiently than fossil fuels, for example, in cars and heating systems. However, at the same time, total energy demand is expected to rise further, countering the gains made by increased energy efficiency.

The high-tech solutions

The steel industry counts on technological solutions to make steel production carbon neutral. One option is to replace coal by gas, an approach that is already common in the Middle East and North America. Gas-based steelmaking results in somewhat lower carbon emissions, but they are still much higher than in the case of the electric arc furnace. Therefore, most attention goes to hydrogen, which can replace purified coal (coke) as a reducing agent in a direct reduction shaft furnace.⁴⁷ However, hydrogen-based steelmaking does not offer an escape from the catch-22 because it further increases the need for a steel-intensive infrastructure.

The production of hydrogen is energy-intensive. It takes 50-55 kilowatt-hour to make 1 kg of hydrogen and 60 kg of hydrogen to make 1 ton of steel.⁴⁷ The production of 1 ton of steel from hydrogen thus consumes 3,000 kWh of electricity, which is ten times higher than the electricity use of an electric arc furnace making steel from scrap. Consequently, hydrogen-based steelmaking requires roughly ten times more wind turbines and solar panels than scrap-based steel production – and thus ten times more steel. On top of this comes the steel for building the pipelines and storage tanks that are part of the hydrogen infrastructure.

Image: Worker in a blast furnace. Bundesarchiv, B 145 Bild-F079044-0020 / CC-BY-SA 3.0.

Carbon capture and storage, in which the carbon emissions of steelmaking plants are captured and then stored underground, faces the same problems. It requires a steel infrastructure and extra energy, thus indirectly raising the use of fossil fuels. Reverting to older, preindustrial steelmaking processes is not the answer either. Today’s blast furnace is essentially still the blast furnace from earlier centuries, only much more energy efficient.⁷

The low-tech solutions

The picture painted above seems to offer little hope for carbon-neutral steelmaking and power production. However, there is a low-tech solution that could achieve it. We could adjust steel production to the available scrap supply both in quantity and quality. That would allow us to produce all steel from scrap in electric arc furnaces, dramatically reducing energy consumption and eliminating almost all carbon emissions. Of course, the intent should not be to replace steel with plastic composites and aluminum because they are even more energy-intensive to produce. The only solution is to reduce material use overall.

We could adjust steel production to the available scrap supply both in quantity and quality.

Reducing the steel output and using more common steel grades would not bring us back to the Bronze Age. As noted, global end-of-life ferrous scrap availability was approximately 450 Mt in 2021, which would allow us to produce roughly one-quarter of the current steel output. Furthermore, the scrap supply will continue to rise for the next 40 years, enabling us to produce more and more low-emission steel each year. By 2050, scrap availability is expected to rise to about 900 Mt, almost half of today’s global steel production.⁴⁸ All that extra steel could be invested in expanding the low-carbon power grid without raising emissions first.

There is a lot of room to reduce the steel intensity of modern society. All our basic needs – and more – could be supplied with much less steel involved. For example, we could make cars lighter by making them smaller. That would bring energy savings without the need for energy-intensive high-grade steel. We could replace cars with bicycles and public transportation so that more people share less steel. Such changes would also reduce the need for steel in the road network, the energy infrastructure, and the manufacturing industry. We would need fewer machine tools, shipping containers, and reinforced concrete buildings. Whenever steel intensity is reduced, the advantages cascade throughout the whole system. Preventing corrosion and producing steel more locally from local resources would also reduce energy use and emissions.¹⁰¹⁴

The continuous growth of the steel output – the increasing steel intensity of human society – makes sustainable steel production impossible. No technology can change that because it’s not a technological problem. Like forestry can only be sustainable if the wood demand does not exceed the wood supply, steel is sustainable or not depending on the balance between (scrap) supply and (steel) demand. We may not be able to escape the Iron Age, but we have an option to escape the catch-22 that inextricably links steel production with fossil fuels.⁴⁹

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Forthcoming article, Kris De Decker, Low-tech Magazine. Subscribe to Low-tech Magazine’s newsletter. ↩︎
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See, for example: Hatayama, Hiroki, et al. “Outlook of the world steel cycle based on the stock and flow dynamics.” Environmental science & technology 44.16 (2010): 6457-6463. This paper predicted steel demand to reach 1.8 billion tonnes only by around 2025. ↩︎
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About 5% of global steel is produced by a third method: gas-based direct iron reduction. These furnaces use gas instead of coal and have therefore lower carbon emissions. However, emissions are still much higher than in the case of the electric arc furnace. Gas-based steelmaking mainly happens in the Middle East and North America. ↩︎
He, Kun, and Li Wang. “A review of energy use and energy-efficient technologies for the iron and steel industry.” Renewable and Sustainable Energy Reviews 70 (2017): 1022-1039. This source gives a value of 1-1.5 GJ/ton of crude steel. ↩︎ ↩︎
This also holds true for many other materials. See: “How circular is the circular economy?”, Kris De Decker, Low-tech Magazine, November 2018. https://qelnixcor.cloud/2018/11/how-circular-is-the-circular-economy/ ↩︎
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In the West, the expansion of steel use happened over a period of 150 years, in tandem with technological evolution. In contrast, China compressed this technological evolution in just a few decades: shipping and railways, electrification, steel buildings, the car and the airplane, the internet, and renewable power technologies. There are still large parts of the world where the steel intensity of society is very low, such as India and Africa. There is thus still a lot of room for the growth of the steel output. Source: Smil, Vaclav. Still the iron age: iron and steel in the modern world. Butterworth-Heinemann, 2016. ↩︎
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Gervásio, Helena, et al. “Comparative life cycle assessment of tubular wind towers and foundations–Part 2: Life cycle analysis.” Engineering structures 74 (2014): 292-299. & Rebelo, Carlos, et al. “Comparative life cycle assessment of tubular wind towers and foundations–Part 1: Structural design.” Engineering structures 74 (2014): 283-291. ↩︎
Assessing the significance of steel to the global wind industry, S&P Global, Commodity Insights. December 2021. https://www.spglobal.com/commodityinsights/en/ci/research-analysis/assessing-the-significance-of-steel-to-the-global-wind-industry.html ↩︎
Bolson, Natanael, Pedro Prieto, and Tadeusz Patzek. “Capacity factors for electrical power generation from renewable and nonrenewable sources.” Proceedings of the National Academy of Sciences 119.52 (2022): e2205429119. https://www.pnas.org/doi/10.1073/pnas.2205429119 ↩︎
This result corresponds well with Vidal, Olivier, Bruno Goffé, and Nicholas Arndt. “Metals for a low-carbon society.” Nature Geoscience 6.11 (2013): 894-896. The data are in the supplementary info: https://www.nature.com/articles/ngeo1993#Sec5 ↩︎
For off-shore wind turbines, the lifetime of the foundations is estimated to be 100 years, so in principle they could serve for replacement wind turbines of the same size. On the other hand, it is not self-evident that these steel foundations will eventually be recycled. First, only around 10% of decommissioning costs can be recovered by recycling the metal, meaning that it is not economically and perhaps even energetically interesting to do it. Second, in some cass marine life has flourished around the foundations. The four offshore wind farms that had been decomissioned in 2019 lasted for 15, 18, 20 and 26 years. Source: Topham, Eva, et al. “Recycling offshore wind farms at decommissioning stage.” Energy policy 129 (2019): 698-709. ↩︎
See https://www.fedsteel.com/insights/steels-role-in-the-us-power-infrastructure/ ↩︎
See https://industry.arcelormittal.com/products-solutions/Products_in_the_spotlight/magnelis ↩︎
Maziasz, Philip J., and Jeremy T. Busby. Properties of austenitic stainless steels for nuclear reactor applications. Oak Ridge National Lab.(ORNL), Oak Ridge, TN (United States), 2012. ↩︎
Part of this has already been built. The researchers start from the solar and wind power production in 2013, which was 400 Twh, while both power sources produced 2,894 Twh in 2021. ↩︎
Electricity consumption worldwide from 2000 to 2022, with a forecast for 2030 and 2050, by scenario. Statista. https://www.statista.com/statistics/1426308/electricity-consumption-worldwide-forecast-by-scenario/#:~:text=According%20to%20a%20recent%20forecast,on%20the%20energy%20transition%20scenario ↩︎
Bhaskar, Abhinav, et al. “Decarbonizing primary steel production: Techno-economic assessment of a hydrogen based green steel production plant in Norway.” Journal of Cleaner Production 350 (2022): 131339. ↩︎ ↩︎
Scrap use in the steel industry, World Steel Association. May 2021. https://worldsteel.org/wp-content/uploads/Fact-sheet-on-scrap_2021.pdf ↩︎
Another motivation for reducing the steel intensity of modern society is to limit the consequences of geopolitical conflicts. The more steel we produce for peaceful purposes, the more steel becomes available for war and destruction. Remarkably, the production of military equipment is absent from modern steel statistics, and if mentioned, its share is very low. However, in times of war, steelmaking facilities switch to producing steel for military purposes. The steel industry can thus be converted into a weapons industry at any moment, and there is now a lot more steel production capacity available than there has ever been in history. ↩︎

Keeping Some of the Lights On: Redefining Energy Security

Sun, 09 Dec 2018 00:00:00 +0000

Maintaining a steady supply of something that’s finite is impossible. Image: [Camilla MP](https://www.flickr.com/photos/dieknochenblume/8454004839/in/photolist-nJrNa3-z9St6d-vicpX8-bjNYMa-CNWajb-PKUbFu-8TqWZX-qzaoch-r3Gb3J-28jYUV3-p3gMD1-snwVj-2chyArN-4ehCVH-cWuLz-dT3Z78-pnFKK9-5qGDSP-hxU2d7-24uoKVs-f7CoCe-93ZqZQ-jPMVaK-T4yoN-4HiX59-97Kq68-23hFdSw-jE59uD-9aFpr7-68DbEo-NvymKZ-335BtT-8RtT65-a6Jut4-nt2zNy-qrkSGP-HPM9ee-bcdyA2-5Fy731-FGSpvq-eqKSpH-8jGFmq-qcFSw4-6USSog-dJEYby-jk3JQ2-7BMzWV-jetX2F-hLnHJy-5SHzAW).

As a society depends more on energy sources for its daily functioning, it becomes more vulnerable if the supply of energy is interrupted. This obvious fact is ignored in current strategies to achieve energy security, making them counter-productive.

What is Energy Security?

What does it mean for a society to have “energy security”? Although there are more than forty different definitions of the concept, they all share the fundamental criterium that energy supply should always meet energy demand. This also implies that energy supply needs to be constant – there can be no interruptions in the service. ¹ ² ³ ⁴ For example, the International Energy Agency (IEA) defines energy security as “the uninterrupted availability of energy sources at an affordable price”, the US Department of Energy and Climate Change (DECC) defines the concept as meaning that “the risks of interruption to energy supply are low”, and the EU defines it as a “stable and abundant supply of energy”. ⁵ ⁶ ⁷

Historically, energy security was achieved by securing access to forests or peat bogs for thermal energy, and to human, animal, wind or water power sources for mechanical energy. With the arrival of the Industrial Revolution, energy security came to depend on the supply of fossil fuels. As a theoretical concept, energy security is most closely related to the oil crises from the 1970s, when embargoes and price manipulations limited oil supply to Western nations. As a result, most industrialised societies still stockpile oil reserves that are equivalent to several months of consumption.

Although oil remains as vital to industrial economies as it was in the 1970s, mainly for transportation and agriculture, it’s now recognised that energy security in modern societies also depends on other infrastructures, such as those supplying gas, electricity, and even data. Furthermore, these infrastructures increasingly interconnect and depend on each other. For example, gas is an important fuel for power production, while the power grid is now required to operate gas pipelines. Power grids are needed to run data networks, and data networks are now needed to run power grids.

Power grids are needed to run data networks, and data networks are needed to operate power grids.

This article investigates the concept of energy security by focusing on the power grid, which has become just as vital to industrial societies as oil. Moreover, electrification is seen as a way to decrease dependency on fossil fuels – think electric vehicles, heat pumps, and wind turbines.

The “security” or “reliability” of a power grid can be measured precisely by indicators of continuity such as the “Loss-of-Load Probability” (LOLP), and the “System Average Interruption Duration Index” (SAIDI). Using these indicators, one can only conclude that power grids in industrial societies are very secure. For example, in Germany, power is available for 99.996% of the time, which corresponds to an interruption in service of less than half an hour per customer per year. ⁸ Even the worst performing countries in Europe (Latvia, Poland, Lithuania) have supply shortages of only eight hours per customer per year, which corresponds to a reliability of 99.90%. ⁸ The US power grid is in between these values, with supply interruptions of less than four hours per customer per year (99.96% reliability). ⁹

How Secure is a Renewable Power Grid?

In the current operation of infrastructures, the paradigm is that consumers could and should have access to as much electricity, gas, oil, data or water as they want, anytime they want it, for as long as they want it. The only requirement is that they pay the bill. Looking at the power sector, this vision of energy security is quite problematic, for several reasons. First of all, most energy sources from which electricity is made are finite – and maintaining a steady supply of something that’s finite is of course impossible. In the long run, the strategy to maintain energy security is certainly doomed to fail. In the shorter term, it may disrupt the climate and provoke armed conflicts.

The International Energy Agency (IEA), which was set up following the first oil crisis in the early 1970s, encourages the use of renewable energy sources in order to diversify the energy supply and improve energy security in the long term. A renewable power system is not dependent on foreign energy imports nor vulnerable to fuel price manipulations – which are the main worries in an energy infrastructure that is largely based on fossil fuels. Of course, solar panels and wind turbines have limited lifetimes and need to be manufactured, which also requires resources that could come from abroad or which can become depleted. But, once they are installed, renewable power systems are “secure” in a way and for a period of time that fossil fuels (and atomic energy) are not.

Renewable energy sources pose fundamental challenges to the current understanding of energy security

Furthermore, solar and wind power provide more security concerning physical failure or sabotage, even more so when renewable power production is decentralised. Renewable power plants also have lower CO2-emissions, and the extreme weather events caused by climate change are a risk to energy security as well. However, in spite of all these advantages, renewable energy sources pose fundamental challenges to the current understanding of energy security. Most importantly, the renewable energy sources with the largest potential – sun and wind – are only intermittently available, depending on the weather and the seasons. This means that solar and wind power don’t match the criterium that all definitions of energy security consider to be essential: the need for an uninterrupted, unlimited supply of power.

Image: [Eduard Bezembinder](https://www.flickr.com/photos/bezembinder/3560945758/in/photolist-6qEM7w-7urQui-iSeKZ-8VjqeD-dUgKQ-e4ybCy-eke2Zk-ekeCdc-eke4NV-qBE1z-6Dfw5n-68EJKh-ekk6Rs-qBE2V-NqkS-oWp8Du-psYQc1-pCDop-5JSFFH-9fr321-oguPbE-6pZ6MT-dZ9YLx-vhpHJb-3oeLdu-69J2h1-7hatWp-d26CpQ-27dVzAC-5BEpZz-sUBfz-7B8zeq-HkygG-bHhG5R-2UoYjD-bRCZnx-o1e2oL-4LcBmy-69vhwD-ekz9ec-bLqreV-5jtvAp-2GUCLK-GpCny7-s36gn-dy6aBU-8moRHP-8rrRxd-5BJJyC-8KdmGR).

The reliability of a power grid with a high share of solar and wind power would be significantly below today’s standards for continuity of service. ¹⁰ ¹¹ ¹² ¹³ ¹⁴ In such a renewable power grid, a 24/7 power supply can only be maintained at very high costs, because it requires an extensive infrastructure for energy storage, power transmission, and excess generation capacity. This additional infrastructure risks making a renewable power grid unsustainable, because above a certain threshold, the fossil fuel energy used for building, installing and maintaining this infrastructure becomes higher than the fossil fuel energy saved by the solar panels and the wind turbines.

Renewable energy sources like wind and sun have advantages that current definitions of energy security don’t capture

Intermittency is not the only disadvantage of renewable energy sources. Although many media and environmental organisations have painted a picture of solar and wind power as abundant sources of energy (“The sun delivers more energy to Earth in an hour than the world consumes in a year”), reality is more complex. The “raw” supply of solar (and wind) energy is enormous indeed. However, because of their very low power density, to convert this energy supply into a useful form solar panels and wind turbines require magnitudes of order more space and materials compared to thermal power plants – even if the mining and distribution of fuels is included. ¹⁵ Therefore, a renewable power grid cannot guarantee that consumers have access to as much electricity as they want, even if the weather conditions are optimal.

How Secure is an Off-the-Grid Power System?

Today’s energy policies related to electricity try to reconcile three aims: an uninterrupted and limitless supply of power, affordability of electricity prices, and environmental sustainability. A power grid that is mainly based on fossil fuels and atomic energy cannot achieve the aim of environmental sustainability, and it can only achieve the other goals as long as foreign suppliers do not cut off supplies or raise energy prices (or as long as national or international reserves are not depleted).

However, a renewable power grid cannot reconcile these three goals either. To achieve an unlimited 24/7 supply of power, the infrastructure needs to be oversized, which makes it expensive and unsustainable. Without that infrastructure, a renewable power grid could be affordable and sustainable, but it could never offer an unlimited 24/7 supply of power. Consequently, if we want a power infrastructure that is affordable and sustainable, we need to redefine the concept of energy security – and question the criterium of an unlimited and uninterrupted power supply.

If we look beyond the typical large-scale central infrastructures in industrial societies, it becomes clear that not all provisioning systems offer a limitless supply of resources. Off-the-grid microgeneration – the local production and storage of electricity using batteries and solar PV panels or wind turbines – is one example. In principle, off-the-grid systems can be sized in such a way that they are “always on”. This can be done by following the “worst-month method”, which oversizes generation and storage capacity so that supply can meet demand even during the shortest and darkest days of the year.

Matching supply to demand at all times makes an off-the-grid system very costly and unsustainable, especially in high seasonality climates

However, just like in an imaginary large-scale renewable power grid, matching supply to demand at all times makes an off-the-grid system very costly and unsustainable, especially in high seasonality climates. ¹⁶ ¹⁷ ¹⁸ Therefore, most off-the-grid systems are sized according to a method that aims for a compromise between reliability, economic cost and sustainability. The “loss-of-load probability sizing method” specifies a number of days per year that supply does not match demand. ¹⁹ ²⁰ ²¹ In other words, the system is sized, not only according to a projected energy demand, but also according to the available budget and/or the available space.

Image: [Stephen Yang / The Solutions Project](https://www.flickr.com/photos/149368236@N06/33068752693/in/photolist-Sob15v-bBnpyx-keyKG-cuaVX3-nuP1zk-U2eVh7-cuaWEf-pskKMf-cuaswE-p27cJW-cu9SQu-cuaMky-mCLFCt-ajiCfB-4AFrsp-943usV-TyoqrN-pu9HK-erKVcJ-aYHgDT-7zrUXc-tQv77b-6xot6g-baF4gg-Xjymka-qHgAkg-ii2jys-9eD7tj-9fJDFi-Ge2Mn-guUowg-amvdKB-cvDZ15-79wfLn-c6XjSS-ddFjjF-9KYuQV-8Zp8z6-guV3wK-9P1nHp-q5c2cz-9RCRVu-cD8w4d-9YDNzC-7ehy1e-4obYkG-8tkNMS-cvDZru-4obYtN-23Aqhr).

Sizing an off-the-grid power system in this way generates significant cost reductions, even if “reliability” is reduced just a little bit. For example, a calculation for an off-the-grid house in Spain shows that decreasing the reliability from 99.75% to 99.00% produces a 60% cost reduction, with similar benefits for sustainability. Supply would be interrupted for 87.6 hours per year, compared to 22 hours in the higher reliability system. ¹⁶

According to the current understanding of energy security, off-the-grid power systems that are sized in this way are a failure: energy supply doesn’t always meet energy demand. However, off-gridders don’t seem to complain about a lack of energy security, on the contrary. There’s a simple reason for this: they adapt their energy demand to a limited and intermittent power supply.

In their 2015 book Off-the-Grid: Re-Assembling Domestic Life, Phillip Vannini and Jonathan Taggart document their travels across Canada to interview about 100 off-the-grid households. ²² Among their most important observations is that voluntary off-gridders use less electricity overall and routinely adapt their energy demand to the weather and the seasons.

Voluntary off-gridders use less electricity overall and routinely adapt their energy demand to the weather and the seasons.

For example, washing machines, vacuum cleaners, power tools, toasters or videogame consoles are not used at all, or they are only used during periods of abundant energy, when batteries can accommodate no further charge. If the sky is overcast, off-gridders act differently to draw less power and have some more left over for the day after. Vannini and Taggart also observe that voluntary off-gridders seem to feel perfectly happy with levels of lighting or heating that are different from the standards that many in the western world have come to expect. Often, this shows itself in concentrating activities around more localised sources of heat and light. ²²

Similar observations can be made in places where people – involuntarily – depend on infrastructures that are not always on. If centralised water, electricity and data networks are present in less industrialised countries, they are often characterised by regular and irregular interruptions in the supply. ²³ ²⁴ ²⁵ However, in spite of the very low reliability of these infrastructures – according to common indicators of continuity – life goes on. Daily household routines are shaped around disruptions of supply systems, which are viewed as normal and a largely accepted part of life. For example, if electricity, water or Internet are only available during certain times of the day, household taks or other activities are planned accordingly. People also use less energy overall: the infrastructure simply doesn’t allow for a resource-intensive lifestyle. ²³

More Reliable, Less Secure?

The very high “reliability” of power grids in industrial societies is justified by calculating the “value of lost load” (VOLL), which compares the financial loss due to power shortages to the extra investment costs to avoid these shortages. ¹¹⁰ ²⁶ ²⁷ ²⁸ ²⁹ However, the value of lost load is highly dependent on how society is organised. The more it depends on electricity, the higher the financial losses due to power shortages will be.

Current definitions of energy security consider supply and demand to be unrelated, and focus almost entirely on securing energy supply. However, alternative forms of power infrastructures like those described above show that people adapt and match their expectations to a power supply that is limited and not always on. In other words, energy security can be improved, not just by increasing reliability, but also by reducing dependency on energy.

Image: Natural gas storage terminal. [Jason Woodhead](https://www.flickr.com/photos/woodhead/7150825737/in/photolist-bTTRmV-85JomL-jysSQn-fw7gTZ-5Jkm2T-eDueWy-ohYc4x-fFxZCm-eD8VG8-eDfhqy-8pCnxZ-qPTdqx-22WNtVf-fFybmb-fFxRVG-fFyhCf-mGNU1p-24mDPG2-8efS2s-fFguSX-nN4pMi-fFgpjT-6br69i-hVGdgU-9DSQQ5-cDwVt-EqVP-dp7vJX-fwmwQh-oHAfHH-fFy6QS-fFgvS8-aaCofJ-fFxW5L-agEkAL-eDfonE-fFgrrn-eD9m9a-PLLffy-fFggcX-fFgka6-nRdzs-fFgwFH-88JrU8-nN4epz-2atchc9-nN523B-24mDNL4-2atciAb-GFzRM).

Demand and supply are also interlinked, and mutually influence each other, in 24/7 power systems – but with the opposite effect. Just like “unreliable” off-the-grid power infrastructures foster lifestyles that are less dependent on electricity, “reliable” infrastructures foster lifestyles that are increasingly dependent on electricity.

Industrial societies with “reliable” power grids are in fact the weakest and most fragile in the face of supply interruptions

In their 2018 book Infrastructures and Practices: the Dynamics of Demand in Networked Societies, Olivier Coutard and Elizabeth Shove argue that an unlimited and uninterrupted power supply has enabled people in industrial societies to adopt a multitude of power dependent technologies – such as washing machines, air conditioners, refrigerators, automatic doors, or 24/7 mobile internet access – which become “normal” and central to everyday life. At the same time, alternative ways of doing things – such as washing clothes by hand, storing food without electricity, keeping cool without air-conditioning, or navigating and communicating without mobile phones – have withered away, or are withering away. ³⁰

As a result, energy security is in fact higher in off-the-grid power systems and “unreliable” central power infrastructures, while industrial societies are the weakest and most fragile in the face of supply interruptions. What is generally assumed to be a proof of energy security – an unlimited and uninterrupted power supply – is actually making industrial societies ever more vulnerable to supply interruptions: people increasingly lack the skills and the technology to function without a continuous power supply.

Redefining Energy Security

To arrive to a more accurate definition of energy security requires the concept to be defined, not in terms of commodities like kilowatt-hours of electricity, but in terms of energy services, social practices, or basic needs. ¹ People don’t need electricity in itself. What they need, is to store food, wash clothes, open and close doors, communicate with each other, move from one place to another, see in the dark, and so on. All these things can be achieved either with or without electricity, and in the first case, with more or less electricity.

Defined in this way, energy security is not just about securing the supply of electricity, but also about improving the resilience of the society, so that it becomes less dependent on a continuous supply of power. This includes the resilience of people (do they have the skills to do things without electricity?), the resilience of devices and technological systems (can they handle an intermittent power supply?), and the resilience of institutions (is it legal to operate a power grid that is not always on?). Depending on the resilience of the society, a disruption of the power supply may or may not lead to a disruption of energy services or social practices.

For example, although our food distribution system is dependent on a cold chain that requires a continuous power supply, there are many alternatives. We could adapt refrigerators to an irregular power supply by insulating them much better, we could reintroduce cold cellars (which keep food fresh without electricity), or we could relearn older methods of food storage, like fermentation. We could also improve people’s skills in terms of fresh cooking, switch to diets based on ingredients that don’t need cold storage, and encourage local daily shopping over weekly trips to large supermarkets.

To improve energy security, we need to make infrastructures less reliable.

If we look at energy security in a more holistic way, taking into account both supply and demand, it quickly becomes clear that energy security in industrial societies continues to deteriorate. We keep delegating more and more tasks to machines, computers and large-scale infrastructures, thus increasing our dependency on electricity. Furthermore, the Internet is becoming just as essential as the power grid, and trends like cloud computing, the Internet of Things, and self-driving cars are all based on several interconnected layers of continuously operating infrastructures.

Image: An abandoned power line. [Miura Paulison](https://www.flickr.com/photos/paulisson_miura/10318768955/in/photolist-gHQovz-kCLi9r-82pqq6-f4539G-6i3Aih-5m5G9b-6RkZvr-6V6k85-2b9wdNP-4DvxJx-WfvmJT-5CGLgF-5C1ojh-eANWrM-kjDG4Z-9QKWz-DnnTH9-ntvKWL-82sxbf-UssMS3-deJRBD-d6qh1S-5C1ooU-tkcYLj-MpbqCB-84zF9u-5CM5d7-5CM51J-82ppX6-a1H2sr-Rd9o59-a1LEed-6W3He9-VCD56X-bg3vgT-5BW5CT-82sxDb-2b1hTxi-6hpZ1g-8d19tj-qm9Cy-cgpx3-gszM15-eANtbt-MpbCWK-98h2dj-7HyrGe-5md8aD-d9fLdq-2cyGoSv).

Because demand and supply influence each other, we come to a counter-intuitive conclusion: to improve energy security, we need to make the power grid less reliable. This would encourage resilience and substitution, and thus make industrial societies less vulnerable to supply interruptions. Coutard and Shove argue that “it would make sense to pay more attention to opportunities for innovation that are opened when large network systems are weakened and abandoned, or when they become less reliable”. They add that the experiences of voluntary off-gridders “provide some insights into the types of configuration at stake”. ³⁰

Arguing for a less reliable power supply is sure to be controversial. In fact, “Keeping the lights on” is a phrase that is often used to justify energy reforms such as building more atomic plants, or keeping them in operation past their planned lifetimes. To achieve real energy security, “keeping the lights on” should be replaced by phrases like “keeping some of the lights on”, “which lights should we turn off next?”, or “what’s wrong with a bit more dark?”. ³¹ Obviously, a less reliable energy supply would bring fundamental changes to routines and technologies, whether it is in households, factories, transport systems, or communications networks – but that’s exactly the point. Present ways of life in industrial societies are simply not sustainable.

This article was originally written for the UK Demand Centre.

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Posadillo, R., and R. López Luque. “A sizing method for stand-alone PV installations with variable demand.” Renewable Energy33.5 (2008): 1049-1055. https://www.sciencedirect.com/science/article/pii/S096014810700184X ↩︎
Khatib, Tamer, Ibrahim A. Ibrahim, and Azah Mohamed. “A review on sizing methodologies of photovoltaic array and storage battery in a standalone photovoltaic system.” Energy Conversion and Management 120 (2016): 430-448. https://staff.najah.edu/media/published_research/2017/01/19/A_review_on_sizing_methodologies_of_photovoltaic_array_and_storage_battery_in_a_standalone_photovoltaic_system.pdf ↩︎
Vannini, Phillip, and Jonathan Taggart. Off the grid: re-assembling domestic life. Routledge, 2014. http://lifeoffgrid.ca/off-grid-living-the-book/ ↩︎ ↩︎
“Materialising energy and water resources in everyday practices: insights for securing supply systems”, Yolande Strengers, Cecily Maller, in “Global Environmental Change 22 (2012), pp. 754-763. http://researchbank.rmit.edu.au/view/rmit%3A17990/n2006038376.pdf ↩︎ ↩︎
Pillai, N. “Loss of Load Probability of a Power System.” (2008). https://mpra.ub.uni-muenchen.de/6953/1/MPRA_paper_6953.pdf ↩︎
Al-Rubaye, Mohannad Jabbar Mnati, and Alex Van den Bossche. “Decades without a real grid: a living experience in Iraq.” International Conference on Sustainable Energy and Environment Sensing (SEES 2018). 2018. https://biblio.ugent.be/publication/8566224 ↩︎
Telson, Michael L. “The economics of alternative levels of reliability for electric power generation systems.” The Bell Journal of Economics (1975): 679-694. https://www.jstor.org/stable/3003250?seq=1#page_scan_tab_contents ↩︎
Schröder, Thomas, and Wilhelm Kuckshinrichs. “Value of lost load: an efficient economic indicator for power supply security? A literature review.” Frontiers in energy research 3 (2015): 55. https://www.frontiersin.org/articles/10.3389/fenrg.2015.00055/full ↩︎
Ratha, Anubhav, Emil Iggland, and Goran Andersson. “Value of Lost Load: How much is supply security worth?.” Power and Energy Society General Meeting (PES), 2013 IEEE. IEEE, 2013. https://www.ethz.ch/content/dam/ethz/special-interest/itet/institute-eeh/power-systems-dam/documents/SAMA/2012/Ratha-SA-2012.pdf ↩︎
De Nooij, Michiel, Carl Koopmans, and Carlijn Bijvoet. “The value of supply security: The costs of power interruptions: Economic input for damage reduction and investment in networks.” Energy Economics 29.2 (2007): 277-295. https://s3.amazonaws.com/academia.edu.documents/40102922/The_Value_of_Supply_Security_The_Costs_o20151117-24458-1eo081r.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1544213977&Signature=d01qoyIcopj1rE5HpSWkCGcQzRk%3D&response-content-disposition=inline%3B%20filename%3DThe_value_of_supply_security.pdf ↩︎
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History and Future of the Compressed Air Economy

Tue, 15 May 2018 00:00:00 +0000

Hiscox straight line air compressor

Compressed air energy storage (CAES) is considered to be an important component of a renewable power grid, because it could store surplus power from wind turbines and solar panels on a large scale. However, in its present form, the technology suffers from large energy losses and depends on natural gas to operate.

A look at the 4,000 year long history of compressed air makes clear that this is not unavoidable. Although our ancestors were dependent on less energy efficient technology, they used compressed air in more intelligent configurations that had fewer energy conversion losses and were independent of fossil fuels.

These historical systems hold the key to the design of a low-tech, low-cost, robust, sustainable and relatively energy efficient energy storage medium. The compressed air economy could be the practical and realistic alternative to the hydrogen or all-electric utopias.

The Promise of Compressed Air

While the potential of wind and solar energy is more than sufficient to supply the electricity demand of industrial societies, these resources are only available intermittently – is one way to deal with the variability and uncertainty of renewable power, but it has its limits. Therefore, a renewable power grid needs at least some energy storage, and the same goes for an off-the-grid system based on solar or wind power.

Today, more than 99% of worldwide electrical storage capacity consists of pumped hydropower energy storage plants, where surplus electrical energy from solar or wind power plants is stored for later use by pumping water from a lower to a higher reservoir. Pumped hydropower energy storage is pretty efficient and low-tech, but it requires a suitable geography for two large water bodies, separated vertically, and one or two dams. It also floods large areas of land. Most suitable sites are already in use, which means that there is little potential for further growth. ¹²

That’s why many people are seeing a promising alternative in Compressed Air Energy Storage (CAES), another form of mechanical energy storage. In these systems, electricity is used to compress air, which is stored in an underground cavern. To make use of the stored energy, the air is decompressed and converted back to electricity.

Although CAES also requires favourable geography to provide the underground air storage caverns, it is believed that there are many more suitable sites worldwide than for pumped hydropower energy storage. ³

If the energy stored over the lifetime of a storage device is compared to the amount of primary energy required to build the device, CAES is vastly superior to electrochemical batteries

Importantly, CAES is the most sustainable energy storage around. Unlike pumped hydropower energy storage, compressed air energy storage presents no environmental issues caused by the flooding of land and the damming of rivers.

Furthermore, if the energy stored over the lifetime of a storage device is compared to the amount of primary energy required to build the device, CAES surpasses pumped hydropower energy storage and is vastly superior to electrochemical batteries, which require 10 to 100 times more embodied energy for a given storage capacity. ³

This is a crucial advantage, because high energy use for the production of the energy storage can greatly decrease the sustainability of a renewable power grid.

The Problem with Compressed Air

In spite of these advantages, there are currently only two large-scale CAES plants in operation worldwide: one in Germany, built in 1979, and one in the USA, built in 1991. ⁴ This limited uptake is mainly attributed to the fact that more than half of the energy is lost when charging and discharging a compressed air “battery”.

While pumped hydropower storage has a charge/discharge efficiency of 70-85%, and chemical batteries reach 65-90%, the CAES plants in operation in Germany and the US have an electric-to-electric efficiency of only 40-42% and 51-54%, respectively. ² ⁵ ⁶

The low energy conversion efficiency is mainly due to the fact that air increases in temperature when being compressed to high pressures (both CAES plants operate at 50-70 bar, which is 10 to 20 times the air pressure in a bicycle tyre). Because the energy density of air decreases with rising temperature, both CAES plants remove the heat prior to storage and dump it into the atmosphere. This implies a significant source of energy loss. ⁷ ⁸

Vintage three-stage compressor

Furthermore, when air is decompressed from a high pressure, the temperature decreases to such an extent that the water vapour in the air can freeze, thereby damaging the valves and the expander of the storage system. To prevent this, and to increase power output, both CAES plants heat the air in combusters using natural gas fuel prior to expansion. Obviously, this further decreases the energy efficiency of the overall process, rendering the present CAES systems entirely dependent on fossil fuels for their operation. ¹ ⁷ ⁹

A conversion efficiency of 40-50% means that wind or solar power generation capacity must be doubled to make up for that loss. Consequently, we need more energy, more materials, and more space for the same energy output. The environmental friendliness of CAES is thus at least partly negated by its low efficiency.

Moreover, CAES’s low energy conversion efficiency is inherently linked to its low energy density, which means it relies on very large storage reservoirs. In principle, the energy density of compressed air can be greatly improved by using higher air pressures, but as the air pressure increases, more energy is turned into waste heat and the efficiency of the whole process further deteriorates. Consequently, a CAES system – in its current configuration – is always a compromise between efficiency and energy density.

4,000 Years of History

The very low energy efficiency of today’s compressed air energy storage systems is remarkable in a historical context. The use of compressed air dates back more than 4,000 years and has always been an important driver of technological progress. Although these historical applications were not aimed at energy storage, they offer inspiration to improve both the energy efficiency and energy density of today’s CAES systems.

The earliest and arguably most important use of compressed air throughout history has been fueling the fire. This happened in the kitchen and in all heat-based production processes, but it was especially important in metal making processes. An unaided charcoal fire could reach 900°C, but a powerful forced air supply could raise its temperature to nearly 2000°C. ¹⁰

The earliest use of compressed air in history has been fueling the fire.

Although there were important regional differences, the history of metal smelting shows an evolution from metals with relatively low melting points, such as tin (230°C), to metals with higher melting points, first copper (1050°C) and then iron (1500°C).

This progress was in part driven by the improvements in air compressor technology, which evolved from air treading bags, wooden cylinders and pistons, and various forms of bellows, all human powered, to much larger and more powerful accordion bellows made of wood and bull hides, which were double-acting and operated by water power. ¹¹

Progress in metal smelting was in large part driven by improvements in air compressor technology

Starting in the 1860s and continuing into the 1900s, compressed air (or “pneumatics”) was at the centre of another technological revolution. This time, pneumatics established itself as the most versatile and widely used power transmission technology before the introduction of electricity. ¹²

Because electric power was still distributed at low voltages (“hydraulics”) had better transmission efficiencies over longer distances. However, compressed air has a very practical advantage over water under pressure: air is available anywhere and its exhaust poses no problems, while hydraulic systems require a sufficient water supply as well as a means to drain the fluid after use.

As a power transmission technology, compressed air was first applied in tunneling and mining. It provided an answer to the pressing need for a mechanical rock drill in the building of canals and railways, where tunnel construction formed a major bottleneck. Under severe hard-rock conditions, tunnel advance with hand drilling – using a pickaxe and explosives – was measured in inches per day, and tunnels of as little as half a mile in length could take years to complete. ¹²

In the new configuration, steam engines above-ground produced compressed air that was piped into the shafts or tunnels. The breakthrough of compressed air power transmission and pneumatic drilling tools happened with the digging of the 13.7 km long Mont Cenis tunnel in the Alps, which was completed in just 14 years (1857-1871). The technology quickly spread to the mining industry, especially in the US, where compressed air not only powered rock drills but also other machinery, such as hauling, pumping and stamping machines. ¹²¹³

The Paris Compressed Air Network

With its effectiveness demonstrated so dramatically in power drilling, compressed air was adapted to a widening range of industrial operations: hammering, riveting, painting and spraying, pressure handling of fluids in processing, and a host of other uses. In the US, pneumatics came to be widely introduced as an auxiliary power system in manufacturing from the 1880s. The Census of 1900 referred to the widespread introduction of small pneumatic tools as possibly “the most important single tool development of the decade”. ¹²

Around the same time in Europe, the French took pneumatic power transmission one step further by setting up a city-wide power distribution network in Paris. It would remain in use for more than 100 years (from 1881 to 1994), distributing compressed air at a relatively low pressure of 5-6 bar over a network of (eventually) more than 900 km of mains, serving more than 10,000 customers. ¹²¹³

Distribution room for the pneumatic clock network in Paris.

The Paris compressed air network started as a system designed exclusively for regulating clocks by impulses of compressed air sent through subterranean pipes. By 1889, the network in Paris was regulating 8,000 clocks through 65 km of mains. The clock regulating service was retired in 1927, after it became clear that electricity was better suited for the job. However, by that time, the compressed air network in Paris had proved highly successful in small industrial and service establishments. ¹²¹⁴¹⁵¹⁶¹⁷¹⁸¹³

The French set up a city-wide power distribution network in Paris, which served more than 10,000 customers and remained in use for 100 years

Already in 1892, F.E. Idell wrote that “among the smaller industrial purposes for which the air motors are used in Paris, I find the driving of lathes for metal and wood, of circular saws, drills, polishing machines, and many others. They are also used in the workshops of carpenters, joiners and cabinet-makers, of smiths, of umbrella makers, of collar-makers, of bookbinders, and naturally in a great many places where sewing machines are used, both by dressmakers, tailors, and shoemakers, from the smallest to the largest scale.” ¹³

Power station of the compressed air network in Paris. Source: Tom Bates, The Manufacturer and Builder, 1889. Image found online at the [Museum of Retrotechnology](http://www.douglas-self.com/MUSEUM/POWER/airnetwork/airnetwork.htm)

Over the years, the share of commercial and domestic use of compressed air decreased, as electricity became more important. However, industrial consumption of compressed air kept growing, and many large factories in Paris – from car producers to glass manufacturers – were connected to the unique power distribution network until the very end. Dentists became new users during the 1970s and 1980s. ¹²¹³

First Lesson: Avoid Energy Conversions

What can be learned from comparing historical and current technologies based on compressed air? A first and crucial difference is the number of energy conversions involved. In historical systems, mechanical energy (for example, from a waterwheel or a steam engine) was directly converted to compressed air (using an air compressor), and then – most often – converted back to mechanical energy (for example, moving a pneumatic hammer). Consequently, there were only two sources of energy conversion loss: in the air compressor, and in the air expander.

Compressed air is still vital to the productivity of many industries and services around the globe, being used in thousands of applications – from food packaging and metal smelting to the manufacturing of microchips and plastics. However, compressed air is now produced by air compressors that run on electricity. This introduces two additional sources of energy loss: the electric generator (which converts mechanical energy from an energy source into electricity) and the electric motor (which converts electric energy back into mechanical energy to run the air compressor).

As a result, today’s industrial use of compressed air is very wasteful: assuming each converter is 75% efficient, and assuming no other energy losses, only 30% of the energy input is converted into useful output. ¹⁹

In Paris, compressed air was piped through the sewer system.

The overall system efficiency of the two existing CAES plants is even worse than that: not only is there the extra conversion step at the beginning of the chain (the energy loss in the windmill generator and in the electric motor running the compressor), but also at the end of the chain. This contrasts with industrial applications, where the end product is compressed air – a CAES plant converts the compressed air back into electricity.

When the efficiency of a CAES plant is said to be 40-50%, this only refers to the losses in the air compressor and the air expander (electric-to-electric efficiency). However, if we include the conversions to and from electricity, the overall system efficiency decreases to less than 20%, again assuming that each converter has an efficiency of 75%.

Image: Stone dressing using pneumatic hammers.

Now, imagine that a factory uses electricity from a CAES plant to power its industrial air compressors – a perfectly possible scenario. We then get the following energy conversion chain: mechanical energy is converted into electricity, electricity is converted into compressed air, compressed air is converted into electricity, electricity is converted into compressed air, and compressed air is converted in mechanical energy. That’s not two, or four, but six sources of energy conversion losses. Assuming each converter is 75% efficient, overall system efficiency now drops below 10%.

If we would connect a CAES plant directly to a factory that uses pneumatic tools, by piping compressed air from one to the other, there would be no need to convert compressed air into electricity and back.

On the other hand, if we would connect a CAES plant directly to a factory that uses pneumatic tools, by piping compressed air from one to the other, we would suffer just four sources of energy loss (generator, motor, compressor, expander). In the CAES plant, there is no longer a need to convert the stored compressed air back to electricity, while in the factory there is no need to compress the air a second time, using electricity. CAES and a factory could be up to 25 km apart – the distance up to which compressed air can be distributed efficiently.

Map via [Museum of Retrotechnology](http://www.douglas-self.com/MUSEUM/POWER/airnetwork/airnetwork.htm)

The obvious next step is to compress the air in a CAES plant using a direct mechanical link between the wind mill and the air compressor, thus skipping the conversion from direct mechanical energy to electricity and back. Such an approach – which has been demonstrated on a small scale, in slightly different configurations ⁸²⁰²¹ – would make CAES entirely independent of electricity and would bring the energy conversion steps back to two, as in all historical systems. The only remaining energy conversion losses would be in the air compressor and in the air expander.

A rigid connection between windmill shaft and air compressor would also improve the efficiency of a CAES plant that is not connected to a factory but supplies electricity for general purposes, although the efficiency gain will be smaller. Obviously, compressing the air mechanically only works with windmills and not with solar PV panels, which do not produce mechanical energy.

Second Lesson: Use Heat and Cold for Other Purposes

A second, related difference between present and historical uses of compressed air is how to deal with the temperature differences caused by compression and expansion of air. To improve efficiency, both CAES plants in operation use multiple air compressors. Multi-stage compression progressively increases the pressure and cools the air after each compression stage, using circulating water that is pumped to a cooling tower and released into the atmosphere. ²² ²³

Today, most CAES engineers are focused on further improving efficiency by using the waste heat of compression to reheat the compressed air upon expansion. This method is called “Advanced Adiabatic CAES” (AA-CAES) or “fuelless CAES” and removes the need to reheat with natural gas as in the standard “diabatic” CAES. The technology is expected to reach an overall efficiency of roughly 70%, bringing it closer to the efficiency of chemical batteries and pumped hydropower storage plants. ⁷

In Paris, compressed air was usually heated by a coke fire before it was used by an air motor, increasing the power output in a way that is very similar to the use of natural gas in present-day CAES systems. Source: Hiscox, Gardner D. Compressed air, its production, uses, and applications; comprising the physical properties of air from a vacuum to its highest pressure, its thermodynamics, compression, transmission and uses as a motive power… New York: N. W. Henley (1909): 271.

However, AA-CAES remains an unproven technology so far: a number of plants have been proposed, but none have yet made it past the design stage. ²²²³ The problem is twofold: first, the process enhancement increases the costs of a CAES plant by 20 to 40%; second, re-using the waste heat of the compression process is technologically challenging. To transfer heat at a high rate with a minimal temperature difference requires a very large surface area of contact. ⁷

In the Paris compressed air power network, the cooling provided by the expansion of air was used for refrigeration, freezing, cooling and ventilation

If we look at older pneumatic systems, we see that there are other, easier ways to take advantage of temperature differences due to compression and expansion. In the Paris compressed air power network, engineers took advantage of the cooling that is provided by the expansion of air. In Paris, compressed air was usually heated by a coke fire before it was used by an air motor, increasing the power output in a way that is very similar to the use of natural gas in present-day CAES systems.

However, in bars and restaurants, these reheaters were not used. Instead, the cold air was used for refrigeration, freezing, cooling or ventilation purposes. In 1892, F.E. Idell described a Paris restaurant where “the exhaust was carried through a brick flue into the beer cellar. In this flue the carafes were set to freeze, and large moulds of block ice were also being made for table use, while the air was still cold enough in passing away through the beer cellar to render the use of ice for cooling quite unnecessary, even in the hottest weather.” ¹³

The use of compressed air for cooling or freezing sometimes went together with the production of electricity for lighting, driving a dynamo. In these cases, the air motors were basically worked for their exhaust, with electric light being the by-product. Taking advantage of temperature differences also happened in the earlier mining applications, where the exhaust of the rock drills helped to cool (and ventilate) the mines.

A similar and promising idea today, is compressed air energy storage combined with thermal storage to provide electricity, heating, cooling, refrigeration and/or ventilation at the same time. In fact, this approach also avoids several energy conversions, as it could replace refrigerators, freezers, air-conditioners and heating systems running on electricity. The method could work at the level of a city district or an industrial area ²³, but it is especially interesting for decentralised energy storage using aboveground storage containers

As we have seen, a higher air pressure can greatly reduce the size of a compressed air storage vessel, but only at the expense of increased waste heat. In individual buildings, space for storage vessels is limited, while there is a large demand for heat and cold, as well as electricity. Increasing the air pressure makes the storage vessel smaller and increases the production of heat and cold, meeting all energy needs of a household.

Some proposed designs follow other approaches to deal with the heat of compression, and these could work for both large-scale and small-scale CAES systems. One interesting idea is a compressed air energy storage system that runs on wind energy as well as solar energy. ²⁴ Wind energy is stored in the form of compressed air by compressor chain, as in the other CAES plants. However, solar energy from a parabolic dish is stored in an insulated solar thermal tank and used to reheat the compressed air prior to expansion. Because the heat from the compression process is no longer needed to warm the air upon expansion, it is used to produce hot water.

A similar concept for a hybrid thermal and compressed air energy storage design uses electric heating instead of solar thermal power. ²⁵ Because the workload in these systems is shifted from pure conversion to investing partially in thermal storage, energy densities well in excess of traditional CAES can be achieved, and the size of the air storage can be reduced.

Third Lesson: Improve the Air Compressor

A third way to improve the efficiency of compressed air energy storage is by using more energy efficient air compressors and expanders. This strategy is opposite to the one we explained before. Instead of taking advantage of heat and cold to make the system more efficient, it tries to minimize waste heat production during compression (and, consequently, to limit cooling during expansion).

Once again, it pays to look to the past for inspiration. Surprisingly, the holy grail of “isothermal” air compression – in which no waste heat is produced at all – was found at least 400 years ago. The hydraulic air compressor – or “trompe”, as it was originally known – was an Italian invention first mentioned by name in 1588, but possibly already known in Antiquity.

From the 1600s onwards, dozens of “trompes” furnished a continuous air blast to early iron and brass-smelting furnaces in the French/Spanish Pyrenees. ²⁶²⁷ Compared to a waterwheel running a wooden piston compressor, it was roughly three times more efficient, allowing higher iron production with less water power resources.

Image: The trompe compressed air without any moving parts, other than valve gates to shut off incoming water flow. De Re Metallica, Georgius Agricola, 1556.

The trompe consisted of one or more vertical wooden tubes through which water was channeled by gravity. Upon its descent, the water absorbed air through holes in the tube and acted as a continuous piston in compressing the air. At the bottom of the tube, the air was separated from the water in a receptacle, after which it was sent to the furnace nozzle by adjustable pressure. Remarkably, the hydraulic air compressor produced compressed air without any moving parts, other than gate valves to shut off incoming water flow. This made it an extremely reliable device. ²⁶²⁸

The hydraulic air compressor produced compressed air without any moving parts, which made it an extremely reliable and efficient device

In the 19th century, the design of the hydraulic air compressor was further improved, making it more efficient and practical. In 1861, a hydraulic air compressor was built to power the rock drills for the construction of the Mont Cenis tunnel in the Alps, but the technology reached its heydays only at the end of the nineteenth century, this time in the mining industry.

Over a 33-year period starting in 1896, eighteen gigantic hydraulic air compressors were built, mostly in the US, Canada, Germany and Sweden. In the largest of these installations, which were partly or completely built underground, water and air fell through pipes and shafts – hewn out of the rocks – which could be more than 100 metres deep and up to 4 metres wide. The delivery pressure amounted to 8 bar and the power output could reach 3,000 kilowatts. ²⁹³⁰

Image: Scientists gather to study the Taylor air compression system. Image: Canadian Journal of Fabrics, septembre 1897.

The first installations used a multitude of small downward air pipes, as in the original trompe, while later installations would use only two shafts. Leets and penstocks delivered water to air-water ‘mixing heads’ of various designs, and the compressed air was often subdivided to reach different mines and piped over distances of many kilometres. Most hydraulic air compressors operated for decades, the last one until 1981. ²⁹ ³⁰

Performance tests, periodically carried out between the 1890s and 1950s, report that the hydropower-to-pneumatic power conversion efficiency ranged between 53% and 88%. More recent research has lowered these numbers to account for gas solubility effects, reporting efficiencies of 40 to 78%. ²⁹²⁸ Although hydraulic air compression produces little waste heat, a new type of energy loss is introduced: some of the air dissolves in the water and thus bypasses the air-water separation process, reducing the mass flow of air at outlet. ²⁹

The hydraulic air compressor has seen renewed interest lately. A Canadian research team developed a 30-m tall hydraulic air compressor demonstrator rig in a former mine elevator shaft. ²⁹³¹ The “HAC Demonstrator Project” measures and verifies the energy savings potential of the technology primarily for deep mining applications. However, it could also be an alternative for multi-stage compressors used in industry and in CAES systems. This is because the new design can also be set up in closed-loop configuration, using a pump instead of a natural head of water.

Image: A newly developed hydraulic air compressor in Canada. Source: HAC Demonstrator Project (https://electrale.com).

Although the pump introduces extra energy use, a closed-loop configuration has two important advantages. First, it could be applied anywhere, rather than just in proximity to an exploitable water source and a large height difference. Second, it offers the opportunity to suppress the undesirable effects of solubility physics, for example through the addition of salt to the circulating water.

According to the researchers, a closed-loop hydraulic air compressor could have an efficiency of 75%, taking into account the extra energy use from the pump. This is 13% more efficient than a three-stage centrifugal compressor, and cost advantages will be larger because of lower maintenance requirements. ²⁹³¹

The hydraulic air compressor seems like a perfect match for large-scale CAES systems with underground reservoirs. In fact, many of the 19th and 20th century hydraulic air compressors used the lower air separator chamber also for compressed air energy storage, in what could be considered the first large-scale use of CAES. The storage – which could be as large as 5,600 m3 – was used to meet a short-time excess demand for air, meaning that the hydraulic air compressor did not have to be designed for the largest loads. ²⁸

The Future of Compressed Air

None of these ideas will make CAES plants 100% energy efficient. However, they could help make them reach similar efficiencies to batteries, but with much lower environmental issues and much less energy invested. In the next article, we focus in more detail on small-scale CAES systems, which promise to be a sustainable alternative to chemical batteries in off-the-grid systems

Thanks to George Fleming.

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Only one of these CAES plants is (partially) used to store surplus wind energy. Both were designed as peaker plants based on economic motives. ↩︎
Kaiser, Friederike. “Steady State Analyse of existing Compressed Air Energy Storage Plants.” Power and Energy Student Summit (PESS). Dortmund, Germany (2015). https://www.efzn.de/uploads/tx_wiwimitarbeiter/S02.2.pdf ↩︎
The highest efficiencies are reached under optimal operation conditions. Additional efficiency loss is caused by the fact that during expansion the storage reservoir is being discharged and the pressure drops. Meanwhile, the input pressure for the expander is required to vary only in a minimal range to assure high efficiency during expansion. To bring together both requirements, air can be stored in a tank with surplus pressure and throttled down to the required expander input pressure – which is obviously linked to efficiency loss. [5] ↩︎
Advanced Adiabatic Compressed Air Energy Storage (AA-CAES), Energy Storage Association. Retrieved May 2018. http://energystorage.org/advanced-adiabatic-compressed-air-energy-storage-aa-caes ↩︎ ↩︎ ↩︎ ↩︎
Sun, Hao, Xing Luo, and Jihong Wang. “Feasibility study of a hybrid wind turbine system–Integration with compressed air energy storage.” Applied Energy 137 (2015): 617-628. https://www.sciencedirect.com/science/article/pii/S0306261914006680 ↩︎ ↩︎
In fact, today’s CAES plants are essentially conventional gas turbines in which the compression of the combustion air is separated from the actual gas turbine process. Unlike conventional gas turbines, which consume about two-thirds of their input fuel to compress the air at the time of power generation, CAES precompresses the air using low cost electricity from the power grid at off-peak times, and utilizes it with some gas fuel to generate electricity when required. ↩︎
Smil, Vaclav. “Energy in world history.” (1994). ↩︎
Ewbank, Thomas. A Descriptive and Historical Account of Hydraulic and Other Machines for Raising Water, Ancient and Modern: Including the Progressive Development of the Steam Engine. No. 32707. Tilt and Bogue, 1842. ↩︎
Nye, David E. “Hunter Louis C. and Bryant Lynwood. A History of Industrial Power in the United States, 1780–1930. Volume 3: The Transmission of Power. Cambridge, Mass, and London: MIT Press, 1991. Pp. xxv+ 596 ISBN 0-262-08198-9.” The British Journal for the History of Science 25.4 (1992): 476-477. ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎
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Energy conversions are not necessarily a bad thing. Mechanical power transmission involves no energy conversions, but it has very high energy losses when transported over longer distances and when subdivided to a large number of machines. This is why so-called “fluid powers” – pneumatics, hydraulics and electricity – came onto the scene in the 19th century. Although their conversion to another form of energy involves energy loss, this loss is compensated for by their much higher efficiency in transmission and subdivision. However, combining two fluid powers – such as compressed air and electricity – is wasteful by definition. ↩︎
Ibrahim, Hussein, et al. “Study and design of a hybrid wind–diesel-compressed air energy storage system for remote areas.” Applied Energy 87.5 (2010): 1749-1762. http://www.academia.edu/download/42460658/Study_and_design_of_a_hybrid_winddiesel-20160209-23813-kip9us.pdf ↩︎
Cheng, Jie. Configuration and optimization of a novel compressed-air-assisted wind energy conversion system. The University of Nebraska-Lincoln, 2016. https://digitalcommons.unl.edu/cgi/viewcontent.cgi?referer=https://www.google.es/&httpsredir=1&article=1081&context=elecengtheses ↩︎
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Ji, Wei, et al. “Thermodynamic analysis of a novel hybrid wind-solar-compressed air energy storage system.” Energy Conversion and Management 142 (2017): 176-187. ↩︎
Houssainy, Sammy, et al. “Thermodynamic analysis of a high temperature hybrid compressed air energy storage (HTH-CAES) system.” Renewable Energy 115 (2018): 1043-1054. ↩︎
Torrence, Euart Carl. “Hydraulic air compressors.” (1898). http://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=1385&context=bachelors_theses ↩︎ ↩︎
Tomàs, Estanislau. “The Catalan process for the direct production of malleable iron and its spread to Europe and the Americas.” Contributions to science (2000): 225-232. https://www.raco.cat/index.php/Contributions/article/viewFile/157654/209545 ↩︎
Schulze, Leroy E. Hydraulic air compressors. Vol. 7683. Dept. of the Interior, Bureau of Mines, 1954. https://babel.hathitrust.org/cgi/pt?id=mdp.39015078460238;view=1up;seq=11 ↩︎ ↩︎ ↩︎
Hydraulic Air Compressor (HAC) Demonstrator Project, Dean Millar, 2017. https://aceee.org/files/proceedings/2017/data/polopoly_fs/1.3687890.1501159068!/fileserver/file/790271/filename/0036_0053_000034.pdf ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎
Hartenberg, R. S., and J. Denavit. “The fabulous air compressor.” Mach. Des 21 (1960): 168-170. ↩︎ ↩︎
Millar, Dean L. “A review of the case for modern-day adoption of hydraulic air compressors.” Applied Thermal Engineering 69.1-2 (2014): 55-77. ↩︎ ↩︎