LOW←TECH MAGAZINE English

The Compressed Book Edition

Thu, 20 Mar 2025 00:00:00 +0000

Image: The Compressed Book Edition. Image by Marie Verdeil and Hugo Lopez.

The Compressed Edition is available in our bookshop as a paperback and hardcover.

In 2018, Low-tech Magazine launched a low-energy website that runs on solar power. To reduce energy use and make the content accessible for readers with old computers and slow internet connections, we opted for a back-to-basics web design, optimising image and file sizes, as well as using a static site generator instead of a database-driven content management system. In 2019, we also launched a book edition of Low-tech Magazine, which consists of three volumes with articles and one volume with comments.

The Compressed Book Edition

While a book looks and feels more low-tech than a website, it has an environmental footprint as well. Industrial book publishing and distribution involves wood harvesting, pulp and paper production, printing, ink-making, and lots of shipping throughout the supply chain. Even if the wood for paper production is harvested sustainably, which is rarely the case, all these processes require energy and produce carbon emissions.

To address these issues, and to keep practicing what we preach, Low-tech Magazine has now made a “compressed edition” of the chronological book series. Inspired by the image compression on our website, we squeezed the article catalog of three volumes into just one book. Consequently, we reduced the paper consumption and carbon emissions by almost a factor of three. The compressed edition contains 84 articles and over 700 images on slightly more than 600 pages.

We did this by switching to a smaller font size (similar to the one used in the comments book), by downsizing most images, and by opting for a two-column layout. I rewrote some articles, especially older ones, resulting not only in fewer pages but also in better articles. Laia Comellas and Marie Verdeil collaborated on the design of the compressed edition.

Image: The original, "uncompressed" book series. Image by Marie Verdeil and Hugo Lopez.

Books or Website?

Ever since the launch of the book collection, readers have asked what is most sustainable: reading Low-tech Magazine online or on paper? While it’s a relevant question, comparing the carbon emissions of books and websites is complex and somewhat pointless. There are so many variables influencing this calculation that you could tilt the result toward your preferred answer.

Comparing the carbon emissions of books and websites is complex and somewhat pointless.

A crucial factor is the time spent reading. If you only read a few articles, reading online likely has a lower carbon footprint. But, if you read all articles, and maybe even go back to them regularly afterward, the difference between online and offline reading becomes smaller. For the books, all carbon emissions occur before the reading process starts. In contrast, the more time you spend on the website, the higher the carbon emissions. ¹

There’s another difference between online and offline reading: Unlike a website, a book can be read by more than one person without raising its carbon emissions — for example, when it’s available in a library. ² Books usually have very long lifetimes, between 25 and 500 years. ³ Therefore, they can be read by dozens of people. That is especially the case for hardcover books, which take a bit more resources to produce than paperbacks but are more resistant to abuse.

Carbon Emissions & Energy Use

To estimate the carbon emissions of Low-tech Magazine’s books, I used a study of a 320-page hardcover book weighing 0.75 kg. ³ According to the researchers, the complete production process of the book has a carbon footprint of between 2 and 3 kg of CO2-equivalents, depending on what happens when the book is discarded after 25 years (landfill or recycling).

The three “uncompressed” paperback books together weigh 2,531 grams, which corresponds to an estimated carbon footprint of between 6.7 and 10.1 kg CO2-equivalents. For the compressed edition, at 929 grams, the carbon footprint comes down to between 2.48 and 3.70 kg of CO2-equivalents. These numbers are surprisingly high compared to the carbon footprint of Low-tech Magazine’s web server, which we calculated to be 9 kg CO2-equivalents per year. ⁴ Although our web server runs on solar power, these carbon emissions result from producing the solar panel, the battery, the solar charge controller, and the printed circuit boards, measured across their estimated lifetime.

However, the book does not require any infrastructure to be read, while the carbon footprint of our web server is only one part of the total footprint of the website. Readers of the website need a computer to access it, and the resulting energy use and carbon emissions of powering and manufacturing that device should also be included. Assuming 60 hours to read all the articles, a laptop power use of 25-50 watts, and the average power grid carbon intensity in Europe (300g/kWh), the power use of the laptop would add between 0.45 and 0.90 kg of carbon emissions for reading content online.

Image: The Compressed Book Edition (hardcover). Image by Marie Verdeil and Hugo Lopez.

To this should be added the energy that was required to manufacture the laptop and the power grid (of which in both cases only a part can be attributed to reading Low-tech Magazine). Because life cycle analyses show that the energy used for manufacturing a laptop surpasses its operational energy use, we can — conservatively — double this result to between 0.90 kg and 1.8 kg of CO2-equivalents. ⁵ Because our website uses very little energy, almost the complete carbon footprint of Low-tech Magazine is due to the devices of our readers.

The book needs to be read by at least two to three people before its environmental footprint becomes smaller than the one caused by reading all Low-tech Magazine articles online.

Nevertheless, this carbon footprint is still lower than the 3 kg of CO2-equivalents for the compressed book edition. Although this is a very rough estimate, it seems to suggest that this book needs to be read by at least two to three people before its environmental footprint becomes smaller than the one caused by reading all Low-tech Magazine articles online. Of course, we have set the bar very high for ourselves because of our light-weight website. Compared to the old blog, which was much more carbon-intensive than the solar-powered website, the compressed book edition — and perhaps even the uncompressed book edition — would be the more sustainable option even if it is read only by one person.

How Many Trees Have We Cut Down?

The environmental footprint of books does not only show in carbon emissions. Books are made from paper, and paper is — nowadays — almost exclusively made from dead trees. With close to 10,000 Low-tech Magazine books sold, an uncomfortable question pops up: how many trees did I kill? That is not an easy question to answer, because the only reference I could find says that “one tree can produce 25 books” ⁶, without specifying what size of tree or type of book these numbers refer to.

Nevertheless, if this estimation is valid, the Low-tech Magazine books would be responsible for chopping down roughly 400 unspecified trees. Whether or not these trees were replaced by other trees, I have no way of knowing. However, by reducing the page number from 1,700 to a little over 600 pages, at least the Compressed Edition attempts to limit this resource use.

Compressing the content — an editorial and design choice — produces a larger reduction in resource use than printing on recycled paper could ever do.

Low-tech Magazine’s “tree consumption” could be further reduced by printing on recycled paper, and we would probably do so if our book distributor and printer — Lulu — would offer that option. However, printing on recycled paper is not a panacea. Paper can only be recycled a couple of times before it needs to be incinerated or landfilled.

Partly because of this, and partly because of economic growth, there is not enough recycled paper available to print the ever-increasing number of books that are published each year. If Low-tech Magazine prints on recycled paper, it means that someone else won’t. Moreover, printing on recycled paper often increases the carbon emissions of paper production. ³ Compressing the content — an editorial and design choice — produces a larger reduction in resource use than printing on recycled paper could ever do.

How Many Books are Thrown Away?

The carbon footprint and wood consumption of an individual book only tell a part of the story. Most environmental damage in the book industry is done by overproduction. A very large number of printed books are not sold but discarded before anyone can read them. Overproduction occurs in two ways. First, most books that come on the market fail commercially, which is a consequence of the business strategies of book publishers. ⁷ Large publishers invest in a massive number of titles in the hope that one will become a bestseller.

Image: The Compressed Book Edition (paperback). Image by Marie Verdeil and Hugo Lopez.

Second, higher print runs significantly lower the printing costs per copy, and thus encourage overproduction. To give an example, printing 100 copies of a 600-page book costs 7,3 euros per copy, while printing 1,000 copies costs only 4,30 euros per copy. ⁸ As a result, it can be more profitable to print more books and discard the unsold copies. Even books that are in demand can be thrown away. For example, unsold copies at events are usually destroyed rather than sent back to publishers because it’s cheaper. ⁹

A less wasteful approach is printing on demand, in which a copy only gets printed once it is bought. In this case, there is no waste unless a book is printed badly. However, the energy use and carbon emissions per printed copy are probably higher, with the printing equipment taking a larger share in the total resource use. The printing costs are much higher, too (around 15 euros per copy for a 600-page book). ¹⁰ Almost all Low-tech Magazine books are printed on demand, but we can only do this because we have our own direct sales channel (the website). If books are sold through Amazon or in bookstores, print-on-demand results in a very high sales price or a very low profit for the book publisher.

Other Low-tech Magazine Books

The launch of the compressed edition does not mean that the “uncompressed” books will no longer be for sale. They will be redesigned in the next months, reflecting the same changes in articles (shorter and better) but printed with a larger font, larger images, and a one-column layout. Their carbon footprint will decrease compared to the earlier volumes but in a less spectacular way. Nevertheless, while those uncompressed books remain the most comfortable option for reading Low-tech Magazine, we expect a significant part of potential buyers to opt for the more compact edition, as it provides the most bang for the buck.

Obviously, “compressing” the books also addresses printing costs, which have skyrocketed in the last few years. Higher printing costs result in a higher sales price and a lower profit, or both. The compressed edition allows us to roughly halve the sales price for the whole archive while maintaining two-thirds of the profit. For the redesigned uncompressed editions, a more modest reduction in the page number will allow us to keep the current sales price but restore the profit per copy to pre-pandemic levels — important for the survival of the magazine. After all, people buy books for the content they contain, not for their weight on paper.

Ebooks and Thematic Books

The Compressed Edition is part of an ongoing research project into the sustainability of Low-tech Magazine’s publishing operations. Apart from the solar-powered website, we have introduced ebooks (2024), and these are perhaps the most sustainable option to read Low-tech Magazine. We also started the publication of a thematic books series (2023), aimed at people who are only interested in certain topics and themes of Low-tech Magazine. The ebooks and thematic books are made in collaboration with Marie Verdeil.

Book Launch Event

We hold a book launch event in Barcelona on March 28.

Image: Book launch event in Barcelona. Poster by Hugo Lopez.

That is mainly because of the energy use of the end-use devices that people use to access the website. Surprisingly, the number of website visitors has no influence on the power use of our web server. That is likely due to the fact that it is a very lightweight, static website. For “normal”, dynamic websites, the energy use is closely related to the number of visitors. ↩︎
Low-tech Magazine’s books are available in several libraries. See: https://search.worldcat.org. If it is not available in your local library, you can file an acquisition request there. ↩︎
Wells, Jean‐Robert, et al. “Carbon footprint assessment of a paperback book: Can planned integration of deinked market pulp be detrimental to climate?.” Journal of Industrial Ecology 16.2 (2012): 212-222. ↩︎ ↩︎ ↩︎
See: “How sustainable is a solar powered website?”: https://qelnixcor.cloud/2020/01/how-sustainable-is-a-solar-powered-website/ ↩︎
There’s also the power use of the router. However, while the grid-powered router uses 240 Wh of electricity per day, it is shared by all website visitors (and by the author). Even on a calm day (around 2,000 unique visitors), extra energy use due to the router is only 0.12 watt-hour per visitor. That is much lower than the energy use of the laptop, even during a relatively short visit. ↩︎
https://publishyourpurpose.com/blog/environmental-impact-book-publishing/ ↩︎
Biel, Joe. People’s Guide to Publishing: Building a Successful, Sustainable, Meaningful Book Business From the Ground Up. Microcosm Publishing, 2018. ↩︎
Based on the prices of expresta.eu ↩︎
Bookstores need to prove that they destroyed the books by ripping off the covers and sending them to the publisher. Source: personal communication with book sellers at events. ↩︎
Book cost calculator, Lulu. https://www.lulu.com/pricing ↩︎

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.

Chen, Haisheng, et al. “Compressed air energy storage.” Energy Storage-Technologies and Applications. InTech, 2013. https://www.intechopen.com/books/energy-storage-technologies-and-applications/compressed-air-energy-storage ↩︎ ↩︎
Luo, Xing, et al. “Overview of current development in electrical energy storage technologies and the application potential in power system operation.” Applied Energy 137 (2015): 511-536. https://www.sciencedirect.com/science/article/pii/S0306261914010290 ↩︎ ↩︎
Barnhart, Charles J., and Sally M. Benson. “On the importance of reducing the energetic and material demands of electrical energy storage.” Energy & Environmental Science 6.4 (2013): 1083-1092. https://gcep.stanford.edu/pdfs/EES_reducingdemandsonenergystorage.pdf ↩︎ ↩︎
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. ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎
“Compressed air: experiments upon the transmission of power by compressed air in Paris (Popp’s system)”, F.E. Idell, 1892 ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎
“The transmission and distribution of power from central stations by compressed air”. William Cawthorne Unwin, B. 1891. ↩︎
“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”, Gardner D. Hiscox, 1909 ↩︎
“La SUDAC, un siècle d’air comprimé au bord de la Seine”, Denis Cosnard, Des usines à Paris, 2011. ↩︎
“Histoire de la SUDAC (1877-1996)” (PDF), Tristan de la Broise & Florence Meffre, 1996 ↩︎
“The transmission of power by compressed air”, Robert Zahner, 1890 ↩︎
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 ↩︎
Zakeri, Behnam, and Sanna Syri. “Electrical energy storage systems: A comparative life cycle cost analysis.” Renewable and Sustainable Energy Reviews 42 (2015): 569-596. https://www.researchgate.net/profile/Behnam_Zakeri/publication/281277805_Electrical_energy_storage_systems_A_comparative_life_cycle_cost_analysis_2015/links/55deac0008ae79830bb58ede.pdf ↩︎ ↩︎
Bagdanavicius, Audrius, and Nick Jenkins. “Exergy and exergoeconomic analysis of a Compressed Air Energy Storage combined with a district energy system.” Energy Conversion and Management 77 (2014): 432-440. https://lra.le.ac.uk/bitstream/2381/37140/2/ECM_CAESpaper_final.pdf ↩︎ ↩︎ ↩︎
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. ↩︎ ↩︎

Power Water Networks

Wed, 30 Mar 2016 00:00:00 +0000

Image: A hydraulic accumulator. Picture: [Les Chatfield](https://www.flickr.com/photos/61132483@N00/7184633723)

During the second half of the nineteenth century, water motors were widely used in Europe and America. These small water turbines were connected to the tap and could power any machine that is now driven by electricity. As we have seen in a previous article, operating motors with tap water was not very sustainable. Because of the low and irregular water pressure of the town mains, these motors used unacceptably high amounts of drinking water.

While the use of water motors in the US came to an end early in the twentieth century, the Europeans found a solution for the high water use of water motors and took hydraulic power transmission one step further. They set up special “power water” networks, which distributed water under pressure for motive power purposes only, and switched to a much higher and more regular water pressure, made possible by the invention of the hydraulic accumulator.

Almost all these power water networks remained in service until the 1960s and 1970s. Hydraulic power transmission is very efficient compared to electricity when it is used to operate powerful but infrequently used machines, which can be distributed over a geographical area the size of a city.

“The use of water is a curiously neglected subject in the literature of engineering. As a romantic or popular facet of engineering, hydraulic power has never caught the public eye like the steam engine, the locomotive or even the internal combustion engine.” Ian McNeil, Hydraulic Power, 1972

The theoretical basis for hydraulic power transmission was laid in 1647 by French whizz-kid Blaise Pascal. By means of experiments, he discovered that water — unlike air — is virtually incompressible and transmits pressure equally in all directions.

The implications of the “hydrostatic paradox” were demonstrated in Pascal’s “machine for multiplying forces”, illustrated below. It consists of two upright cylinders, connected together by a pipe. The whole system is filled with water and sealed water-tight. One cylinder contains a small diameter plunger, while the other cylinder contains a plunger that has a cross-sectional area 100 times larger.

Machine for multiplying forces.

Pascal demonstrated that if a weight is placed on top of the small piston, it will be able to raise a weight placed on top of the larger piston that is 100 times heavier. Pascal’s machine thus allowed forces to be multiplied — in the example above, the ratio of force output to force input is 100 to 1. In other words, you can produce an output force of 100 kg for an input force of only 1 kg.

A Machine for Multiplying Forces

Force multiplication was anything but new in the 1600s. More simple devices such as pulleys, gear trains, capstans, winches and treadwheels — all variations on the 7,000 year old lever — could also derive a high output force output from a small input force. For example, the Romans built cranes with a mechanical advantage of up to 70 to one, meaning that one man exerting a force of only 25 kg could raise a weight of 1.75 tonnes.

However, the hydraulic version of the lever has one outstanding advantage over earlier mechanisms: the friction loss is very small and independent of the mechanical advantage. Therefore, the possible multiplication ratio is almost infintely greater and both pistons may be a considerable distance apart — up to about 25 km, as we shall see.

In hydraulics, friction loss is independent of the mechanical advantage, therefore the possibile force multiplication ratio is almost infinite

Increasing force multiplication could be done by either extending the proportion between the diameter of both plungers, or by applying greater power to the smaller piston. In common with the earlier mechanisms, what is gained in mechanical advantage is lost in velocity ratio.

If a small hydraulic force is converted into a larger force, its speed of operation will be reduced in exactly the inverse proportion, because the distance traversed increases in the same proportion as the force. For example, a person pressing down the small piston 10 centimetres would move the other piston up only 1/100th of that distance.

Illustration: Pascal's Barrel experiment. Source unknown.

Consequently, in a closed system, the heavier weight could be lifted only over a very limited distance, depending on the length of the plunger. However, this limit is removed when more water is added to the system and the smaller piston, instead of coming down just once, makes a number of strokes — in other words, when it functions as a pump. In this case, the larger piston will keep rising.

The Hydraulic Press

Pascal could only prove his point indirectly, as the available materials at the time were not strong enough to withstand the pressure. It would take another century and a half before hydraulic force multiplication was put into practice. Its first use was not a lifting device, but rather the opposite: the hydraulic press, which generates a compressive force.

The conventional screw press of the time, little developed since the Romans had used it for pressing olives and grapes, required a great effort to operate, had large frictional energy loss (+80%), and could not have exerted more than 25 tonnes load. (The screw, which converts rotational motion into linear motion, is basically an inclined plane wrapped around a cylinder).

Left: The screw press. Picture credit: [Bruce K. Satterfield](http://emp.byui.edu/SATTERFIELDB/Olive%20Tree/olive%20tree%20horticulture.htm) Right: The hydraulic press.

The hydraulic press was invented in 1796 by English locksmith and carpenter Joseph Bramah. It was entirely based on the theoretical work of Pascal. Bramah’s hydraulic press, which was driven by a hand-operated pump, brought a large increase in the load that could be exerted by a human.

With the available materials at the time, Bramah achieved an overall ratio of 1,000 to 1, which means that an effective load of 60 tonnes on the lifting piston could be balanced by a mere 60 kg on the pump handle. The efficiency of the hydraulic press was over 90%.

Harbours and Dockyards

In spite of its eminent suitability for crane operation, hydraulics made little progress in this field during the first half of the nineteenth century. This was largely due to the problem of reliably and efficiently translating the linear motion of a ram to rotary motion of the crane barrel or drum. During the first half of the nineteenth century, cargo handling in harbours, dockyards and railway yards was still done by means of human powered cranes, but the need for taller and stronger cranes was great.

Starting in the 1830s, iron began to be used as a material for ship building, with a parallel growth in the dimensions of ships. Conventional lifting systems were no longer adequate. In most countries, the solution was found in the steam powered crane, which appeared in the 1850s. However, in harbours and dockyards in Britain, a worthy alternative appeared: the water powered crane.

During the first half of the nineteenth century, cargo handling in harbours, dockyards and railway yards was still done by means of human powered cranes

British engineer William Armstrong started designing and operating powerful hydraulic cranes in the 1840s. Being fully aware that hydraulics was best adapted for giving a slow, steady motion, Armstrong deviced a method of lifting the load at one stroke of a ram or piston, multiplying the motion sufficiently by means of pulleys.

Image: hydraulic crane

However, his efforts were complicated by the low and irregular pressure of the town mains, which was the power source for these machines. The maximum power output of a water powered machine is determined by water pressure and water flow. In the town mains, water pressure was (and often still is) supplied by a water tower. Because the practical height of a water tower is limited, so is the water pressure. A 50 m (165 ft.) tall water tower can produce a water pressure of 70 pound-per-square-inch (psi).

Consequently, the only way to further increase the power output of a crane running on water from the town mains is to increase the water flow. However, this raises potable water consumption and increases the size and costs of pipes, valves, cylinders, and other parts of the system. Moreover, if there is a higher than average demand for potable water from other users, the water level in a water tower will fall, and so will the water pressure and the power output of the machine.

The Hydraulic Accumulator

In 1851, Armstrong came up with an alternative solution that solved these issues: the hydraulic accumulator. Although much more compact than a water tower, it could produce a regular water pressure of 700 psi or higher — at least 10 times the water pressure in the town mains. This allowed to produce an order of magnitude more power without raising water consumption or increasing the size of system components.

Armstrong’s hydraulic accumulator was a contraption in which a ram or piston exerted pressure on the water in a vertical cylinder. The piston was loaded by dead weight ballast, which generally took on the form of a cylindrical ballast container surrounding the central cylinder (image below, on the left). The container was filled with crushed rock, scrap iron or other ballast material.

Left: Hydarulic Accumulator in Bristol Harbour. Wikipedia Commons. Right: Hydraulic Accumulator, Walsh Bay, Sydney. Source: [NSW HSC Online](http://hsc.csu.edu.au/engineering_studies/application/lift/3377/hydraulics.htm)

For a water pressure of 700 psi the ballast was about 100 tonnes, acting on a ram of about 45 cm in diameter with a vertical stroke of 6 to 7 meters. Another type of accumulator utilised a rectangular platen to support a brickwork ballast (image above, on the right) or steel slabs. Hydraulic accumulators could be set up outdoors, or housed in a purpose designed building.

In comparison with a water tower, a hydraulic accumulator could deliver ten times more power, and maintain an even pressure all over the network

The workings of the hydraulic accumulator are somewhat similar to those of a water tower. The central cylinder has a water inlet and outlet at the bottom. Water from the docks could be pumped in through the inlet by a steam powered pump, raising the piston, while it could be pushed out through the outlet into the mains for distribution, lowering the piston.

Energy was stored by upward movement of the ram and recovered upon its descent. The pumping rate of the steam engine was regulated in function of the water level in the accumulator, either automatically via mechanical linkages or via the aid of a human being.

Fielding and Platt hydraulic accumulator

Contrary to a water tower, however, the accumulator could maintain an even pressure all over the system regardless of the volume of water in the cylinder, because it’s the weight of the ballast and not the weight of the water that creates the pressure — in other words, the hydraulic accumulator gives pressure by load instead of by elevation.

With a charging/discharging efficiency above 98%, and no self-discharge, the hydraulic accumulator was an extremely energy efficient device.

Water Powered Factory Machinery

The introduction of the hydraulic accumulator had two important effects. First, it greatly expanded the range of hydraulically operated machines. The water motors connected to the town mains were household devices and workshop tools. But Armstrong and other engineers adapted high pressure water to a variety of industrial applications that required great power such as forging, punching, stamping, flanging, shearing and riveting (the predecessor of welding).

Hydraulically powered riveting machine.

In harbours, high pressure water not only operated cranes and hoisting machines handling cargo on docks and in warehouses, but also lock gates, swing bridges, boat lifts, and graving docks. At railway yards, hydraulic power transmission was used for freight handling and for moving railway cars (using hydraulic capstans), as well as for operating turntables, elevators and traversing mechanisms. All these applications of hydraulic power would have been impossible with the low and irregular pressure prevailing on the town mains.

To give an idea of the importance of hydraulic power, it suffices to look once more at the evolution of lifting devices. In 1586, a 344 ton obelisk was moved between squares in Rome. Domenic Fontana, master builder of the Vatican, raised the obelisk with the help of 40 capstans worked by 400 men and 75 horses. In 1878, John Dixon raised another obelisk — Cleopatra’s needle, weighing 209 tons — using four hydraulic lifting jacks, worked by four men.

Power Water Networks

Secondly, the hydraulic accumulator made it possible to transmit power efficiently over large distances. For a 30 cm diameter pipeline, the pressure drop in water distribution amounts to about 10 psi per mile, a figure that is independent of water pressure. Thus, if you transmit water with a pressure of 70 psi over a distance of 7 miles (12 km), all energy is lost. But if you transmit water over the same distance with a pressure of 700 psi, a water pressure of 630 psi remains, which comes down to a transmission efficiency of 90%.

The high transmission efficiency of high-pressure water led to the construction of at least a dozen public power water networks with accumulator storage, half of them in Britain, in which centrally located steam engines pumped water into hydraulic accumulators that distributed high pressure water over a large geographical area. One or more accumulators would be installed at each hydraulic power station and others could be sited at strategic points along the supply main as sub-stations.

The idea of a truly hydraulic power network — analogous to the electric grid that came a bit later — was already outlined in a 1812 patent by Joseph Bramah, the inventor of the hydraulic press.

From the 1870s to the 1890s, hydraulic power networks were established in the leading industrial cities of Britain: Kingston upon Hull, London, Liverpool, Birmingham, Grimsby, Manchester and Glasgow. Dock and railway companies pioneered the technology, and remained the most important users for decades.

Illustrations of a hydraulic accumulator, a hydraulic crane, and a hydraulic lift.

However, power water was also running manufacturing processes in factories, operating elevators in public, private and commercial buildings, and activating household devices and workshop tools. Anybody who was lucky enough to have a mains running through the street could connect to the public network. Power water consumption was metered, as it happens today with potable water and electricity.

The idea of a truly hydraulic power network — analogous to the electric grid that came a bit later — was already outlined in a 1812 patent by Joseph Bramah, the inventor of the hydraulic press. But Bramah, who also conceived the hydraulic accumulator and the hydraulic crane, was ahead of his time. It took another sixty years before his ideas were brought into practice by Armstrong and his contemporaries.

London Hydraulic Power Company

The most extensive hydraulic power network was built in London, operated by the “London Hydraulic Company”. At the company’s peak in 1917, five interconnected central power stations pumped high pressure water in about a dozen hydraulic accumulators and almost 300 km of supply mains, powering more than 8,000 machines and serving most of the city. In London theatres and other cultural buildings, power water was moving floors, organ consoles, fire curtains and stages. Water under pressure worked water pumps and lifted the bascules of the Tower Bridge.

Illustration: layout of London Hydraulic Power Co. mains and pumping stations, 1895.

Fire hydrants were also advantageously served by the high pressure system and several hundreds of them were connected to the London Hydraulic Power Company’s mains. These fire-fighting systems increased the pressure of the domestic water mains by injecting a small amount of high pressure water in them, using a jet pump. By itself, water at high pressure from the hydraulic power mains could not be supplied in adequate quantity to have an effect on a large fire, while the domestic supply mains had enough quantity but not enough pressure to reach the top floors of buildings.

In London, five interconnected central power stations pumped high pressure water in a dozen hydraulic accumulators and almost 300 km of supply mains, powering more than 8,000 machines and serving most of the city.

Another remarkable application of high pressure water in London was the Silent Dustman, a water powered vacuum cleaning system that came on the market in 1910. Several large hotels were completely “wired” for this system: water from the town mains was used in a jet pump to produce a vacuum in a pipe to which the system was to be fitted. Along these pipes were a number of nozzles to which flexible hoses could be fixed. Thus the dirt from the sweepers was drawn into the hydraulic pipe and carried away into the drains. The system, which operated silently and efficiently, remained in operation until 1937.

One of the London power stations. Note the tower on the right, which houses the hydraulic accumulators.

In London, however, hydraulic power does not seem to have made a great impact on the domestic scene. In The Hydraulic Age (1980), B. Pugh notes that this was “possibly due to the fact that in its day domestic labour was cheap and in plentiful supply. Had present-day conditions operated then possibly the story would have been different since the potentialities of hydraulic power were not less than those of electricity today.”

Most public power water networks supplied water under a pressure of 700 to 800 psi (48 to 55 bar), with the exception of Manchester and Glasgow, where water was pressurized to 1120 psi. In these cities, there was a heavy demand for power for hydraulic presses used for baling, an application that required a higher pressure.

Power Networks Outside Britain

The British power systems inspired similar networks elsewhere: Antwerp in Belgium, Buenos Aires in Argentina, and Melbourne and Sydney in Australia. While the Australian systems were reminiscent of those in Britain (with 80 km of mains, the one in Melbourne was the second largest ever built), the Argentinian system was used to pump sewage, and the network in Antwerp was aimed at the combined production of mechanical power and electricity. The latter was an attempt to overcome the very high transmission losses of electricity at the time.

\"Zuiderpershuis\": a former hydraulic pumping plant in Antwerp. The towers housed the hydraulic accumulators.

In The Hydraulic Age, B. Pugh writes that:

“For power transmission, the early electric stations were faced with the same difficulties as the hydraulic power stations, their voltage being analogous to working pressure, and voltage drop due to mains resistance analogous to pressure drop due to pipe friction. The early electric public power stations were direct or continuous current stations, the voltage of generation essentially being only slightly higher (by the voltage drop in the cables) than at the consumer’s premises which for safety reasons had to be less than 250 volts. Due to voltage limitation, the area of supply as well as the amount of power that could be transmitted was limited.”

The network in Antwerp was aimed at the combined production of mechanical power and electricity.

Since 1865, Antwerp had been using a high pressure hydraulic network for powering cranes, bridges and sluices in the harbour. To this was added a second network in 1893, which distributed high pressure water to electric substations scattered across the city (twelve according to the plan, but only three were built). There, water turbines generated electricity which was distributed in a radius of 500 m via underground electric conduits — this was about the distance at which low voltage could be distributed efficiently.

Hydraulic cranes in Antwerp harbour. Picture by Low-tech Magazine.

The Antwerp system, which was used for operating street lighting, thus did on a large scale what water motors connected to dynamos did on a small scale with water from the town mains (see the previous article. About 66% of the hydraulic energy was converted to electricity. At its peak, the network reached a length of 23 km with an output of 1200 hp. There were also a number of places in London where consumers ran small electric generators from the hydraulic supply.

Power Water Versus Electricity

The breakthrough in high voltage electric transmission at the turn of the century made systems like those in Antwerp immediately obsolete. The electricity generating part of the network disappeared in 1900. Producing water under pressure in order to produce electricity involves a fourfold energy conversion, which is needlessly wasteful if you can just produce electricity and transport it efficiently.

The expansion of efficient electrical transmission also stopped the construction of other large-scale power water networks before the century was over. “Had these systems been started some years earlier, they might have become vastly more popular”, writes Ian McNeil in Hydraulic Power (1972). “A few years later, and they would probably never have been built at all.”

However, almost all public power water systems that were built between the 1870s and 1890s remained in service until the 1960s and 1970s, eventually using electric motors instead of steam engines for pumping. The power water network operated by the London Hydraulic Company, the last to survive, worked until 1977. Most of the public power water networks kept growing during the first decades of the twentieth century, reaching their heydays at the end of the 1920s. The fatal decline came only when factories started leaving the cities in the 1960s and 1970s.

If electricity is the most efficient and practical way of transmitting and distributing power, then why did almost all power water networks remain in service for almost a century?

This raises two questions. First, why didn’t power water become the universal method of power distribution that Joseph Bramah and William Armstrong had envisioned? And second, if electricity is the most efficient and practical way of transmitting and distributing power, then why did almost all power water networks remain in service for almost a century?

Advantages of Electric Power

As a power transmission technology, power water has three important disadvantages in comparison to electricity. First of all, electricity can be transported efficiently over much longer distances. Hydraulic power transmission was (and still is) at least as efficient as electric power transmission up to distances of 15 to 25 km. Beyond those distances, however, electric transmission is a clear winner.

Greenland dock hydraulic lock gates in London, built in the 1880s. Picture credit: [Chris Allen](http://www.geograph.org.uk/photo/2569524)

A second shortcoming of hydraulic transmission is that a complex distribution network introduces additional energy loss. Every curve or bend in the mains increases friction losses. The more intricate the network, the less efficient it becomes. Electric transmission doesn’t have this problem, at least not in a significant way. The friction losses in the water mains limit the amount of machines that can be attached to a power water network, while electricity can be subdivided almost infinitely.

The third limitation of power water is the limited capacity of a hydraulic transmission line. Water under pressure can only be moved through thin pipes at walking speeds in order to avoid excessive friction losses. At higher speeds, the loss of friction increases as the square of the velocity and efficiency goes down fast, even over relatively short distances. This limits the flow rate and thus the power that could be delivered by a hydraulic transmission line.

Using a 10 to 12 cm diameter pipe — a common size in most high pressure system at the time — a hydraulic transmission line could produce a maximum continuous power of 115 to 205 horse power (85 to 150 kW). High voltage electric transmission lines of similar size can carry an amount of power that was orders of magnitude greater than that.

Advantages of Power Water

However, none of these disadvantages mattered for the power water networks that we have discussed. These were all decentralized systems, with machines no more than 15-25 km away from the power source. Secondly, because the hydraulically operated machinery in harbours, railway yards, factories and buildings was characterized by slow motion and infrequent use, the slow transmission speed of power water presented no obstacle.

With the exception of the short-lived electricity generating system in Antwerp, none of the Armstrong-type power water networks supplied power to a large amount of continuously operating machines. (But note the medium pressure power water networks in Switzerland. Lastly, because a power water network operated relatively few (but very powerful) machines, friction loss through bends and curves in the network was limited.

Hydraulic pump, accumulator and press. Source: Portefeuille économique des machines, de l'outillage et du matériel, December 1864, [Bibliothèque nationale de France](http://gallica.bnf.fr/ark:/12148/bpt6k5539152w/f79.pleinepage.langFR)

The limitations of hydraulic transmission were very well understood at the end of the nineteenth century. However, engineers also grasped the unique benefits of the technology, which still hold today. For example, Robert Zahner, an advocate of yet another alternative to electricity, compressed air, wrote in The Transmission of Power by Compressed Air (1890) that:

“The practical incompressibility of water renders the hydraulic method unfit for transmitting regularly a constant amount of power. It can be used to advantage only where motive power is to be accumulated and applied at intervals, such as raising weights, operating punches, compressive forging and other work of intermittent character, requiring a great force through a small distance.”

Hydraulic transmission is “admirably adapted for use with heavy machinery and equipment in operations requiring marked concentration of power, reciprocating straight-line motion, and intermittent action”, wrote Louis Hunter in The Transmission of Power (1991). The main excellence of the hydraulic accumulator is that it allows to operate machines that require much more power than the energy source can supply — Pascal’s “force multiplication”.

The limitations of hydraulic transmission were very well understood at the end of the nineteenth century. However, engineers also grasped the unique benefits of the technology, which still hold today.

When high force or torque are needed, hydraulic power systems are a much more compact and energy efficient solution than mechanical or electric drives. Both electric motors and combustion engines often need mechanical power transmission (gears, chains, belts) to convert their high rotational speed to a slower speed with higher torque.

Likewise, hydraulic power systems easily produce linear motion using hydraulic cylinders, while electric power requires costly linear motors or mechanical power transmissions such as rack-and-pinion assemblies. Hydraulic and electric power are complementary in this sense: one of the limitations of power water transmission was the relative difficulty of converting linear motion to rotary motion.

Illustration: hydraulic lift

Pelton wheels were the most obvious choice, but their high rotational speed involved the use of gearing for the operation of slow speed machinery. A number of hydraulic engines of the ram type was available to supply rotative power involving variable or slow speed operation, but these engines had few advantages compared to electric or mechanical drives.

A third important advantage of hydraulics is that the power is always readily available in the pipes and in the accumulator, but when there is no demand there is no waste. When none of the machines in a power water network was in operation, the hydraulic accumulators kept the lines pressurized without using any energy. This advantage is especially relevant when machines are used intermittently.

Hydraulics Today

Hydraulic power is still in use today, especially in heavy industrial equipment that requires a slow but powerful linear motion, and in mobile construction machinery such as excavators. However, the raised-weight hydraulic accumulator and the power water networks have disappeared.

The pressurized fluid is no longer water but oil, mixed with additives. (Vegetable oil had been used as a hydraulic medium in the 19th century). Unlike water, oil doesn’t freeze and is not corrosive. However, it makes hydraulic power more expensive and it obviously doesn’t permit the exhaust fluid to end up in the sewer network, the docks or the sea.

Partly as a consequence of the use of oil, there evolved the self-contained hydraulic power pack consisting of pump, hydraulic accumulator, and return flow systems, ready to be coupled to an electric motor or a diesel engine. The hydraulic accumulators in these systems are much smaller, they use a gas to compress the fluid, and they do not maintain a steady pressure.

Today's hydraulic accumulators (usually compressed gas types) have little in common with the raised-weight accumulators in power water networks. Picture: [HYD](http://www.hyd.com/tejas/products/products_frame.htm).

While the practical benefits of hydraulics remain — a large amount of power can be transferred and controlled precisely using very compact components — the modern approach erases an important efficiency advantage specific to the more centralized power water networks of the nineteenth and twentieth century. In a city-wide power water network, a comparably small central power source — a handful of hydraulic accumulators — could operate a large number of very powerful machines. The pumping engines didn’t have to be dimensioned for peak loads.

A great advantage of power water networks was that comparatively little power capacity was required to operate a large number of powerful machines over a wide area.

B. Pugh laments this evolution in The Hydraulic Age (1980):

“One century ago, only a few very large machines — swing bridges and an occasional hydraulic press — had their own individual pumping equipment. More recently, this trend spread throughout hydraulically operated machinery of all types and sizes, and is accepted practice today. With unit hydraulic power packs each piece of equipment will be driven by its own motor and will have its own instrumentation, filters, etcetera, which will call for periodic inspection and maintenance.”

“The motor will run continuously while the unit is in use regardless of the load on the pump it drives. In the case of a number of such units not all will be working to capacity all the time. Appreciable economy could be effected by having a central pumping plant to supply a number of units and due to the diversification of the load the maximum load at any one time will be less than the sum of the individual maximum loads.”

“An advantage of a large station over a number of smaller ones lies in the ability to meet diversity of demand. A number of small, independent power stations must each have sufficient capacity to meet the peak demand of its own area of supply and the peaks will not occur at the same time. A large station, embracing the total area of a number of small stations, will need only to meet the maximum simultaneous demand and this will normally be less than the sum total of the local peaks.”

Alternatives to Electricity

Just like mechanical power transmission technologies — such as jerker line systems and endless rope drives — power water networks have disappeared largely because electric transmission has superior efficiency over long distances. However, in a more decentralized energy system based on renewable energy, all these forgotten alternatives for electricity deserve to be reconsidered for specific purposes. Raised-weight hydraulic accumulators could be solar, wind or even pedal powered.

Picture: J.W. Gibson

Around 1900, the superiority of electricity for transmitting power over very long distances was not disputed. For moderate distances, however, quite a few authors doubted its usefulness. For example, R. Kennedy wrote in Modern Engines and Power Generators (1905):

“Electricity offers paramount advantages for power transmission to a distance in most cases. Electrical engineers, however, claim far too much for it. They are apt to forget other means for transmitting power, which means have paramount advantages over electricity in a good many cases.”

W.C. Unwin, the author of the most complete nineteenth-century book on power transmission (On the Development and Transmission of Power from Central Stations), expressed a similar concern in 1894:

“Granting that electrical distribution will play an important part before long in the development of systems of power distribution, there is a popular tendency at the moment to regard too exclusively electrical methods, and to overlook other means of power distribution which have been usefully applied in the past, and will, in suitable conditions, be still employed in the future… For transmission to moderate distances there is a choice of several means of transmission, and electrical distribution has not in such cases and up to the present established any universal superiority.”

In the next installment of our power transmission series, we will discuss compressed air, which is probably the most usable alternative for electricity.

This article is dedicated to Charles Steele. RIP.