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How to Build a Low-tech Solar Panel?

Tue, 05 Oct 2021 00:00:00 +0000

George Cove stands next to his third solar array. Source: \"Generating electricity by the sun's rays\", Popular Electricity, Volume 2, nr. 12, April 1910, pp.793.

More efficient, less sustainable

Ever since Bell Labs presented the first practical solar PV panel in the 1950s, technological development has focused on reducing costs and increasing the efficiency of solar cells. According to these standards, researchers have made a lot of progress. The efficiency of solar panels increased from less than 5% in the 1950s to over 20% today, while the costs decreased from 30 dollars per watt-peak in 1980 to less than 0.2 dollars per watt-peak in 2020. Lower costs – to which higher efficiencies contribute – are considered of paramount importance because they allow solar PV panels to compete in the market with electricity generated by fossil fuels.

However, in terms of sustainability, very little progress has been made. To start with, ever since the 1950s, solar panels have been unfit for recycling, resulting in a waste stream that ends up in landfills. This waste stream will grow significantly during the coming years. Solar panels are discarded only after at least 25 to 30 years, and most have been installed only in recent years. By 2050, researchers expect that almost 80 million tonnes of solar panels will reach the end of their lives. ¹ ² ³ That is a significant waste of resources and a danger to the environment – discarded solar PV panels contain toxic elements and present a fire hazard.

The need for capital-intensive technology and long supply lines prevents the local production of solar panels by less affluent societies or DIY communities.

The manufacturing of solar PV panels is just as problematic. It produces toxic waste and requires a global supply chain, including capital-intensive factories, complex machinery, mined materials, and a steady input of fossil fuels. In life cycle analyses of solar panels, scientists calculate how much energy and materials are required to build a solar panel. However, they ignore the massive amount of energy and materials needed to set up and maintain the solar PV supply chain itself. ⁴ ⁵ ⁶ ⁷ ⁸ ⁹ ¹⁰ ¹¹ Consequently, these studies do not reveal the actual cost of solar panels in terms of fossil fuel dependence, emissions, and other environmental pollution. Furthermore, the need for capital-intensive technology and long supply lines prevents the local production of solar panels by less affluent societies or DIY communities.

Finding inspiration in the past

Are solar PV panels inherently unsustainable, unrecyclable, and dependent on high-tech and capital-intensive manufacturing processes? Or, is it possible to build them with local, recyclable and less energy-intensive materials and production methods? In other words, can we build low-tech solar panels? And, if so, what would that mean for costs and efficiency?

Before we try to answer this question, it’s important to note that the best low-tech alternative for a high-tech solar panel is often not a low-tech solar panel but direct use of solar energy. That is, putting solar energy to use without converting it to electricity first. For example, a clothesline and a solar thermal water boiler are much more efficient, sustainable, and economical than an electric tumble dryer and a water boiler powered by solar PV panels. Direct use of solar energy can happen with local materials, relatively simple manufacturing technologies, and short supply lines.

Nevertheless, in this article, I take the question literally: can we build low-tech photovoltaic devices, which convert sunlight into electricity? In a previous article, we have seen that history offers inspiration for building more sustainable wind turbines. Can history also inspire us to make more sustainable solar cells?

The prehistory of solar cells

Bell Labs’ solar PV panel, presented in 1954, came not out of nowhere. The silicon solar cell had its roots in less complex devices that could produce electricity from either light or heat.

In 1821, Thomas Seebeck found that an electrical current will flow in a circuit made from two dissimilar metals, with the junctions at different temperatures. This “thermoelectric effect” formed the basis for the “thermoelectric generator” – which converts heat (for example, from a wood stove) directly into electricity. In 1839, Antoine Becquerel discovered that light could also convert into electricity, and during the 1870s, several scientists proved this effect in solids, most notably in selenium. This “photoelectric effect” formed the basis for the “photoelectric generator” – which we now call a “photovoltaic” generator or solar PV cell. In 1883, Charles Fritts constructed the first photovoltaic module ever made, using selenium on a thin layer of gold. ¹² ¹³ ¹⁴

Throughout this period – and until the 1950s – the practical uses of thermoelectric and photoelectric devices were limited. Inventors built many experimental thermoelectric generators, usually powered by a gas flame, but their efficiency did not exceed 1%. Likewise, Charles Fritts’ solar panel, and the selenium solar cells made afterward, obtained just 1-2% efficiency in converting sunlight into electricity. ¹⁵ In short, the period before the 1950s doesn’t seem to offer much inspiration for building more sustainable solar PV panels.

A forgotten pioneer of solar power

However, the prehistory of the solar panel may be incomplete. In 2019, I received a mail from a reader of Low-tech Magazine, Philip Pesavento:

“I have been studying an early pioneer in solar cell technology from the pre-WWI era since the early 1990s. I am getting too old to continue doing anything with this, and even though there have been one or two scholarly articles about Mr. Cove, they have completely missed what he accomplished. I am enclosing a PDF of a PowerPoint that I put together back in 2015 and never presented to anyone. If you are interested in pursuing writing a paper yourself, I could mail you a thumb drive with all the background material that I have collected.”

If Philip Pesavento’s historical account and hypotheses are correct, George Cove set out to build a thermoelectric generator but accidentally made a photovoltaic generator – a PV solar cell. Although this happened in the early 1900s, Cove obtained a comparable power output and efficiency to the Bell Labs scientists in 1954. His design also showed much higher performance than the selenium solar cells built between the 1880s and the 1940s. ¹⁶ Philip Pesavento:

“It would be quite exciting to prove that relatively high-efficiency solar cells were invented 40 years before the development of silicon cells. More importantly, if it turns out there was a solar photovoltaic cell and panel system before World War I, it might also have some advantages concerning the cheapness of raw materials, low embodied energy to convert the ores into metallic materials, the efficiency of the final PV cells, and ease of fabrication.”

In other words, if Philip Pesavento’s historical account and hypotheses are correct, it may be possible to build low-tech solar panels.

George Cove’s solar electric generator

George Cove presented his first “solar electric generator” in 1905 in the Metropole Building in Halifax, Nova Scotia, Canada. Apart from an image, there are no data about this panel. ¹⁷ However, its power output and efficiency were remarkable enough for US investors to send an expert to Halifax. Based on this expert’s examination of the machine, they then brought Cove to the US (Sommerville, Mass.) to continue the development of his device.

Cove presented his second solar electric generator there in 1909. This 1.5m2 panel could produce 45 watts of power and was 2.75% efficient in converting solar energy into electricity. By mid-1909, Cove had moved to New York City, where he presented his third prototype, a solar array consisting of four solar panels of 60 watt-peak each, which charged a total of five lead-acid batteries. The total surface area was 4.5 m2, the maximum power output was 240 watts, and efficiency rose to 5% – similar to the first solar panel presented by Bell Labs. ¹⁸

Above: George Cove's first solar panel, demonstrated in 1905. Source: Technical World Magazine 11, nr.4, June 1909.

Above: Cove's second solar panel, with one section missing. Source: Technical World Magazine 11, nr.4, June 1909.

Above: George Cove's third solar panel. Source: \"Harnessing sunlight\", René Homer, Modern Electrics, Vol. II, No.6, September 1909.

Above: George Cove's third solar panel. The panels are now tilted at an angle as opposed to lying flat. Source: Literary Digest 1909, pp. 1153.

Above: One of the solar panels of Cove's third solar array, with the glass cover removed. Source: \"Harnessing sunlight\", René Homer, Modern Electrics, Vol. II, No.6, September 1909.

Although George Cove is absent from most historical accounts of solar power, his solar electric generator impressed some popular tech media of the day. For example, in 1909, Technical World Magazine wrote that “such a machine is cheap and indestructible as a kitchen range. Even in its present and somewhat crude and experimental state, given two days of sun, it will store sufficient electrical energy to light an ordinary house for a week. The inventor has proved this now for months in his establishment”. ¹⁹

Plugs set in asphalt

How did George Cove manage to build a solar panel that was 40 years ahead of its time? According to Philip Pesavento, who has a background in semiconductor engineering, Cove intended to build a better thermoelectric generator (TEG). He exposed his generator to the heat from a wood stove and direct solar energy – Edward Weston had made the first experimental solar thermoelectric generator (or STEG) in 1888. Cove’s intentions are also clear from how he described his device:

“The frame contains a number of panes of violet glass, behind which are set, through an asphalt compound backing, many little metal plugs. One end of the plugs is always exposed by sunlight, while the other end is cool and sheltered.”

Creating the largest possible temperature difference is key to thermoelectric power production, so Cove’s design makes sense. The problem is that when he measured the power output of his generator, it did not respond to heat like a thermo-electric generator was supposed to do. Initially, Cove observes that his invention uses both heat and light to produce electricity when exposed to solar energy:

“The principal part of my invention is the peculiar composition of the metallic plugs which are acted upon by the sun in such a way that the current is generated not only by heat rays but the violet rays as well”.

However, after further experiments with both the wood stove and solar energy, Cove states:

“When the machine is exposed to various sources of artificial heat it gives no electricity whatsoever. Other than the heat rays of the sun (short-wave infrared), perhaps the violet or ultraviolet rays are active in setting up the electrical current”.

The primary cell of Cove’s solar PV panel was a three-inch-long plug or rod of metallic composition, an alloy of several common metals. The 1.5 m2 panel had 976 rods, while the 4.5 m2 array had 4 x 1804 plugs. However, keeping the rods cool on one side and hot on another – separated by an asphalt layer – did not matter. What mattered is that Cove had unknowingly built a metal-semiconductor contact.

The semiconductor bandgap

George Cove did not understand how his solar generator worked, and neither did anyone else at the time. It was only with Einstein’s work on the photoelectric effect (in 1905) and later work in quantum mechanics (1930s and beyond) that the concept of a semiconductor bandgap was realized. Electrons orbit the nucleus of an atom in different “states”, which form regions that are called “bands”. These bands keep their electrons firmly in place. In between these bands are “bandgaps” – states in which no electron can be.

George Cove did not understand how his solar generator worked, and neither did anyone else at the time.

Conductors have no bandgaps, and so electrons flow through them. That is why a copper wire conducts electricity, for example. In insulators (like wood, glass, plastics, or ceramics), there is a very wide bandgap, which blocks the flow of electricity. Finally, in semiconductors, there’s a relatively narrow bandgap. That allows them to either act as an insulator or a conductor. Semiconductors can become conductors when they absorb a “photon” (an elementary particle of light) with an energy potential equal to or greater than the bandgap of the semiconductor material. ²⁰

The understanding of semiconductors led to the birth of the modern solar PV cell in the 1950s. It also improved the performance of thermoelectric generators – be it for different reasons. Thermoelectric generators do not take advantage of the semiconductor bandgap. However, semiconductors have higher thermo-voltages and lower thermal conductivities than metal and metal alloys with no bandgap, making thermoelectric generators more efficient.

The Schottky Junction

For a photovoltaic effect to exist, there must be some inhomogeneity in the system. In the 1950s, Bell Labs scientists managed to do this with the so-called p-n junction, which forms a boundary between a positively charged and a negatively charged semiconductor. P-type semiconductors have electron vacancies called “holes” (which attract electrons), while N-type semiconductors have extra electrons. At the junction between both is an electric potential.

However, it’s also possible to create a PV cell from a so-called Schottky junction, which connects a semiconductor with a metal. In this case, the metal functions as the n-type semiconductor. Philip Pesavento:

“My hypothesis is that George Cove stumbled upon a Schottky contact photovoltaic cell, decades before it was described by Walter Schottky. ²¹ There is the possibility of both photovoltaic (predominantly) and thermoelectric responses from these devices. The plug was an alloy of zinc and antimony – which we now know is a semiconductor. It was alternately capped by German silver (a nickel, copper, and zinc alloy) and copper on opposite ends. This formed an ohmic contact and Schottky contact, respectively. This is a photovoltaic device.”

Accidental discovery

According to Philip Pesavento, George Cove probably started with “German silver” as the negative material on both ends of the plugs, and an antimony-zinc alloy (ZnSb) as the positive material. These were the best available thermoelectric materials at the time:

“He probably ran out of German silver and substituted copper to finish making up a bunch of plugs since the difference in thermoelectric voltage between using copper and German silver was small. Then, during testing, Cove noted that these plugs (with a German silver cap at one end and a copper cap at the other end) gave a much greater voltage: 100s of mV’s versus the usual 10s of mV for a thermoelectric generator.”

What happened? By using copper, Cove had unknowingly built a Schottky junction. That converted his thermoelectric generator into a “thermophotovoltaic generator.” Such a device works the same as a photovoltaic solar cell but on a different wavelength. The solar spectrum covers a range of approximately 0.5 to 2.9 electron-Volts (eV), from infrared to ultraviolet. A semiconductor with a bandgap between 1 and 1.7 eV efficiently converts visible light into electricity (a photovoltaic generator) – while a semiconductor with a bandgap between 0.4 and 0.7 eV efficiently converts short-wave infrared solar energy into electricity (a thermophotovoltaic generator).

Above: This drawing from [Cove's 1906 patent](https://patentimages.storage.googleapis.com/bc/bb/50/6683e8b44edd4c/US824684.pdf) shows the zinc-antimony alloy “b”; the german silver (ohmic) end cap “c”; and the copper or tin (Schottky) end cap “f”. All these are press-fit because soldering the connections lowered the efficiency.

We now know that ZnSb – the negative material in Cove’s plugs – is a semiconductor with a bandgap of 0.5 eV. That largely explains why the inventor initially observed that his solar generator converted both heat and light into electricity. A thermophotovoltaic generator matches not only the infrared tail of the solar spectrum – it also matches the direct spectrum of a burning flame or a red hot emitting surface which is heated by burning wood or natural gas. It also converts the lower portion of the visible spectrum into electricity, be it very inefficiently.

According to Philip Pesavento, Cove then managed to refine the composition of the alloy close to Zn4Sb3 – a zinc-antimony alloy with proportions of 4 parts zinc to 6 parts antimony. That, we now know, is also a semiconductor. However, it has a bandgap of 1.2 eV – very close to the bandgap of silicon (1.1 eV). Consequently, it turned his thermophotovoltaic generator into a photovoltaic generator:

“In his enthusiasm, Cove probably made up a larger number of plugs and somehow got the proportions “wrong” on one batch. He then measured an even larger voltage. Finally, he made a careful study of zinc-antimony alloys and found that the 40-42% range zinc alloy gave the highest voltage (compared to 35% zinc in ZnSb). Having – accidentally – discovered Zn4Sb3, the higher bandgap of this semiconductor meant that it no longer worked when it was exposed to the heat from a wood stove. However, it worked even better when it was exposed to solar energy – because it was now converting far more of the visible spectrum of sunlight efficiently into electricity.”

Using colored glass filters, George Cove determined that most of the response was from the violet end of the spectrum and only a little from the so-called heat rays. His earlier PV plugs had responded equally well to heat rays and violet rays, while the older thermoelectric generators (German silver at both sides) did not respond to the violet rays at all.

Bring back the Schottky solar cell?

Schottky junction solar cells have commanded only a small amount of attention from researchers and corporations – few solar cell designs use metals in the active region, other than for contacts. ²² Nevertheless, Philip Pesavento believes that it would be worthwhile to attempt to fabricate some Schottky solar cells according to Cove’s design:

“If it could be demonstrated that Zn4Sb3 (bandgap 1.2 eV) can be used in a photovoltaic cell, there is a good chance that such a solar cell design will be sustainable. It would be a good candidate for a quick EROI and have an acceptably long operational life with a surplus energy output over several decades. It’s astounding that everyone seems to have missed this material and its application to photovoltaic cells and that no development has been done – even after researchers briefly recognized it as being a possible option in the early to mid-1980s. It fits in the category of a premature discovery which should mean it could be developed very quickly in this day and age.”

Apart from solar PV, Philip Pesavento sees potential in thermophotovoltaics for a wood stove, solar thermal, or dual junction tandem applications, using ZnSb instead of Zn4Sb3. Furthermore, if the plug-type solar cells prove to be effective, he believes that they would allow line concentrator solar collectors – such as parabolic troughs or non-imagining CPC concentrators – to be built at greatly reduced costs.

Low-tech manufacturing

The primary advantage of Cove’s design would be its low-tech fabrication method. In the 1970s and 1980s, scientists investigated Zn4Sb3 for use in photovoltaics and concluded that the material’s “obvious advantages are apparent simplicity and relatively low temperature of the preparation procedure.” ²³ The melting point for Zn4Sb3 is 570 degrees Celsius, while it’s 1,400 degrees for silicon.

Researchers studied metal-semiconductor junction solar cells based on other types of semiconductors than Zn4Sb3 in the 1970s. Again, their motivation was the simple and cost-effective fabrication procedure compared to silicon p-n junction solar cells at the time. ²⁴ ²⁵ Schottky cells do not require a high-temperature phosphorus-diffusion step, which ordinarily creates the n-layer of the p-n junction in silicon today. This alone reduces the energy input into the solar cell production process by 35%. ²²

During the 1980s, researchers made important advances in silicon p-n junctions, and interest in alternative configurations waned. However, there has been renewed interest in recent years. For example, research into graphene/silicon Schottky solar cells concludes that “simple and cost-effective device fabrication that does not require high temperatures is one of the advantages.” ²⁶ In other recent studies, scientists conclude that Schottky-type “selenium devices are… extremely simple and cheap to fabricate”. ²⁷ ²⁸ ²⁹ ³⁰

Easier recycling

Another advantage of Schottky solar cells may be easier recycling. Silicon modules are sandwiched between two laminate encapsulant layers (usually EVA, an ethylene/vinyl acetate copolymer). These layers are essential to ensure module service lifetime. ¹ ² ³ To recycle the silicon – the most valuable component of a solar panel – these layers need to be separated, but burning them also destroys the modules. Silicon cells can only be recycled by a combination of thermal, chemical, and metallurgical steps. That is an expensive process with an impact on the environment. Although you can find statements claiming that around 10% of solar panels are “recycled", they are more likely to be “downcycled”. The modules are shredded, and the resulting material is used as a filler material in asphalt and cement industries.

In contrast, the solar cells built by George Cove were entirely recyclable. They required no protective layer and did not even contain solder. Philip Pesavento:

“If you were to build the cells exactly the way Cove did by press-fitting the caps and then overwrapping them with wire to try to keep them tight, they would also be easier to recycle, being strictly a mechanical operation, no chemicals need to be involved. It would be labor-intensive to put them together and take them apart again, but it could be automated, too.”

Pesavento believes that it’s also possible to build flat solar cells from Cove’s material. However, whether or not those would need a protective layer that interferes with recycling remains to be seen. In the 1970s, Schottky solar cells based on other materials did not always need protective layers to reach more than 20 years of life expectancy. ²⁴

Efficiency

If we could build more low-tech solar panels, how efficient could we make them? According to Philip Pesavento, Schottky cells are slightly less efficient for the same materials than p-n junctions because p-n junctions generate a higher voltage – they get more of the energy in the photons they absorb.

“When every bit of efficiency counts, you do that. If making solar cells easier to manufacture using manual or artisan methods is your goal, the Schottky diode would be a more logical choice.”

On the other hand, it may be possible to build Schottky cells thinner than silicon solar cells – and that would increase their efficiency. Philip Pesavento:

“I have not found the specific numbers for the parameters – carrier velocity, recombination lifetime, absorption coefficient – to say this unequivocally. But the fact that Cove made such long skinny cells and got as high efficiencies as he did bodes well for making them thinner.”

Again, recent research into Schottky cells based on other materials seems to confirm this. For example, recent experiments with Schottky selenium cells brought layer thickness back to only 100 µm, compared to between 200 and 500 µm for silicon cells. ²⁷ ³¹ Scientists also reached 17% experimental efficiency for a graphene/silicon Schottky cell, up from 1.5% ten years earlier. ²⁶

We can also question the current obsession with higher efficiencies. Many people will argue that if low-tech solar panels are less efficient, we would need more solar panels to produce the same power output. Consequently, the resources saved by low-tech production methods would be compensated by the extra resources to build more solar panels. However, efficiency is only crucial when we take energy demand for granted, sacrificing some efficiency may gain us a lot in sustainability.

What happened to George Cove?

If Cove’s solar panel was so revolutionary, why is it forgotten? On this question, Philip Pesavento’s research material reads like a crime novel. Cove’s attempt to produce and market his solar energy device failed in mysterious ways.

The inventor became involved with a stock manipulator – Elmer Burlingame – who in 1909 and 1910 issued stock from businesses that were not his, including Cove’s start-up the Sun Electric Generator Company. In October 1909, Cove was allegedly kidnapped, and his life was threatened if he did not cease the development of his solar invention. However, the police dismissed Cove’s kidnapping as a hoax. In 1911, both Cove and Burlingame were arrested for stock fraud and spent a year in jail. Although Cove worked on other inventions after that, none of those were related to solar energy. ³²

In October 1909, Cove was allegedly kidnapped, and his life was threatened if he did not cease the development of his solar invention.

Was George Cove a charlatan? Was he the victim of one? Or was his reputation destroyed because the solar electric generator threatened other companies’ interests? There are many historical examples of suppression of technological innovations by large US corporations. George Cove was active in the same period as the Edison Electric Illuminating Company of New York, whose unscrupulous practices against competitors are well-documented. If Cove’s solar electric generator worked, it could have reduced the growing demand for Edison’s coal and oil-fired power stations. ³² Earlier, in the 1880s, Edison had bought the company that produced the best thermoelectric generator at the time – Clamonds’s Improved Thermopile – and subsequently stopped the development of the machines. ³³

More Mysteries

However, while it’s tempting to see George Cove as a victim, we can only speculate. Philip Pesavento’s archive material contains more mysteries, such as Cove’s patent – applied for in 1905, granted in 1906. In his patent, the inventor describes the making of his Zn4Sb3 plugs in detail, which helped Pesavento to calculate the power output and efficiency of the solar arrays. However, Cove describes these plugs for converting heat from a wood stove into electricity, which is not compatible with his choice of material. To make the stove generator work, it required ZnSb plugs with a bandgap of 0.5 eV. Philip Pesavento:

“Was this misdirection on the part of Cove to prevent folks from copying his stove patent and getting it to work? I don’t know.”

Even more surprisingly, an image that shows Cove standing beside one of his solar panels also appears in John Perlin’s 2013 historical overview of solar power Let It Shine: The 6,000-Year Story of Solar Energy. However, the solar panel in the image is attributed to Charles Fritts, the inventor of the selenium solar cell. Furthermore, George Cove himself has disappeared from the image. Excerpts from the book, as well as the photo, have appeared on several websites. Philip Pesavento was not surprised when I got back in touch:

“I made this discovery several years ago. I guess that somebody badly needed an image of Fritts’ solar panels, found this image, and then photoshopped George Cove out of it. After all, Cove is totally unknown and when known is thought to have invented a solar thermoelectric generator, not a solar PV panel. If you look closely at the two photos, you can see that the top of the right column portico behind him was cut and pasted to where Cove had been standing, it’s not quite right in its perspective.”

Update: Bellingcat untangled the mystery of the image.

Weckend, Stephanie, Andreas Wade, and Garvin A. Heath. End of life management: solar photovoltaic panels. No. NREL/TP-6A20-73852. National Renewable Energy Lab.(NREL), Golden, CO (United States), 2016. ↩︎ ↩︎
Xu, Yan, et al. “Global status of recycling waste solar panels: A review.” Waste Management 75 (2018): 450-458. ↩︎ ↩︎
Sica, Daniela, et al. “Management of end-of-life photovoltaic panels as a step towards a circular economy.” Renewable and Sustainable Energy Reviews 82 (2018): 2934-2945. ↩︎ ↩︎
Hornborg, Alf, Gustav Cederlöf, and Andreas Roos. “Has Cuba exposed the myth of “free” solar power? Energy, space, and justice.” Environment and planning E: Nature and space 2.4 (2019): 989-1008. ↩︎
Cederlof, Gustav, and Alf Hornborg. “System boundaries as epistemological and ethnographic problems: Assessing energy technology and socio-environmental impact.” Journal of Political Ecology 28.1 (2021): 111-123. ↩︎
Bartie, N. J., et al. “The resources, exergetic and environmental footprint of the silicon photovoltaic circular economy: Assessment and opportunities.” Resources, Conservation and Recycling 169 (2021): 105516. ↩︎
Powell, Douglas M., et al. “The capital intensity of photovoltaics manufacturing: barrier to scale and opportunity for innovation.” Energy & Environmental Science 8.12 (2015): 3395-3408. ↩︎
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Carvalho, Maria, Antoine Dechezleprêtre, and Matthieu Glachant. Understanding the dynamics of global value chains for solar photovoltaic technologies. Vol. 40. WIPO, 2017. ↩︎
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Fritts, Charles E. “On a new form of selenium cell, and some electrical discoveries made by its use.” American Journal of Science 3.156 (1883): 465-472. ↩︎
Effect of Light on Selenium During the Passage of An Electric Current*. Nature 7, 303 (1873). ↩︎
Green, Martin A. “Silicon photovoltaic modules: a brief history of the first 50 years.” Progress in Photovoltaics: Research and applications 13.5 (2005): 447-455. ↩︎
Perlin, John. Let it shine: the 6,000-year story of solar energy. New World Library, 2013. ↩︎
Selenium Cells, Thomas William Benson, 1919. ↩︎
Extrapolating from the performance of the next panel, we can guess that this one had a power output of about 25W and just under 3% efficiency. ↩︎
Cove claimed to have built an even larger panel of 9 m2, but no image has survived. It was said to have had a power output of 768 watt at 8% efficiency assuming 100 W/ft2 solar insolation. This array consisted of 8 panels with a total of 14,432 plugs. ↩︎
Winthrop Packard, Technical World Magazine 11, nr.4, June 1909. ↩︎
Why don’t we use conductors for solar panels? When light hits a conductor surface it mostly reflects, and little or no energy is absorbed. Furthermore, in conductors, the free electrons move randomly, there is no flow of current, no directional capacity. ↩︎
Cove was not the first, though. Charles Fritts’ solar cell was also based on a Schottky junction. ↩︎
Byrnes, Steve. “Schottky junction solar cells.” (2008). ↩︎ ↩︎
Tapiero, M., et al. “Preparation and characterization of Zn4Sb4.” Solar Energy Materials 12.4 (1985): 257-274. https://www.sciencedirect.com/science/article/abs/pii/0165163385900516. See also: Mozharivskyj, Yurij, et al. “A promising thermoelectric material: Zn4Sb3 or Zn6-δSb5. Its composition, structure, stability, and polymorphs. Structure and stability of Zn1-δSb.” Chemistry of Materials 16.8 (2004): 1580-1589. https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1787&context=chem_pubs ↩︎
Rothwarf, A., and K. W. Böer. “Direct conversion of solar energy through photovoltaic cells.” Progress in Solid State Chemistry 10 (1975): 71-102.. ↩︎ ↩︎
Anderson, W. A., A. E. Delahoy, and R. A. Milano. “An 8% efficient layered Schottky‐barrier solar cell.” Journal of Applied Physics 45.9 (1974): 3913-3915. ↩︎
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Selenium can be deposited by thermal evaporation at only 200°C. This temperature is within easy reach of solar thermal technologies, which means that in principle these processes could be run by direct use of solar energy. ↩︎
Hadar, Ido, et al. “Modern processing and insights on selenium solar cells: the world’s first photovoltaic device.” Advanced Energy Materials 9.16 (2019): 1802766. ↩︎
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More details in “George Cove’s solar energy device”, Dennis Bartels, 1997. ↩︎ ↩︎
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How to Design a Sailing Ship for the 21st Century?

Tue, 11 May 2021 00:00:00 +0000

On board the ship *Garthsnaid* at sea. A view from high up in the rigging. Image by Allan C. Green, circa 1920.

The sailing ship is a textbook example of sustainability. For at least 4,000 years, sailing ships have transported passengers and cargo across the world’s seas and oceans without using a single drop of fossil fuels. If we want to keep travelling and trading globally in a low carbon society, sailing ships are the obvious alternative to container ships, bulk carriers, and airplanes.

However, by definition, the sailing ship is not a carbon neutral technology. For most of history, sailing ships were built from wood, but back then whole forests were felled for ships, and those trees often did not grow back. In the late nineteenth and early twentieth century, sailing ships were increasingly made from steel, which also has a significant carbon footprint.

The carbon neutrality of sailing in the 21st century is even more elusive. That’s because we have changed profoundly since the Age of Sail. Compared to our forebears, we have higher demands in terms of safety, comfort, convenience, and cleanliness. These higher standards are difficult to achieve unless the ship also has a diesel engine and generator on-board.

The revival of the sailing ship

The sailing ship has seen a modest revival in the last decade, especially for the transportation of cargo. In 2009, Dutch company Fairtransport started shipping freight between Europe and the Americas with the Tres Hombres, a sailing ship built in 1943. The company remains active today and has a second ship in service since 2015, the Nordlys (built in 1873).

Since then, others have joined the sail cargo business. In 2016, the German company Timbercoast started shipping cargo with the Avontuur, a ship built in 1920. ¹ In 2017, the French Blue Schooner Company started transporting cargo between Europe and the Americas with the Gallant, a sailing ship that was built in 1916. ² All these sailing ships were constructed in the twentieth or nineteenth century, and were restored at a later date. However, a revival of sail cannot rely on historical ships alone, because there’s not enough of them. ³

The Noach, built in 1857.

At the moment, there are at least two sailing ships in development that are being built from scratch: the Ceiba and the EcoClipper500. The first ship is being constructed in Costa Rica by a company named Sailcargo. She is built from wood and inspired by a Finnish ship from the twentieth century. The second ship is designed by a company called EcoClipper, which is led by one of the founders of the Dutch FairTransport, Jorne Langelaan. Their EcoClipper500 is a steel replica of a Dutch clipper ship from 1857: the Noach.

“Old designs are not necessarily the best", says Jorne Langelaan, “but whenever proven design is used, one can be sure of its performance. A new design is more of a gamble. Furthermore, in the 20th and 21st century, sailing technology developed for fast sailing yachts, which is an entirely different story compared to ships which need to be able to carry cargo.”

More economical sailing ships

These two ships – one under construction and one in the design phase – have the potential to make sail cargo a lot more economical than it is today. That’s because they have a much larger cargo capacity than the sailing ships currently in operation. As a ship becomes longer, her cargo capacity increases more than proportionally.

The EcoClipper500 is a full-scale replica of the Noach.

The 46 metre long Ceiba is powered by 580 m2 of sails and carries 250 tonnes of cargo. The 60 metre long EcoClipper500 is powered by almost 1,000 m2 of sails and takes 500 tonnes of cargo. For comparison, the Tres Hombres is not that much shorter at 32 metres, but she takes only 40 tonnes of cargo – twelve times less than the EcoClipper500. A larger ship is also faster and saves labour. The Tres Hombres requires a crew of seven, while the EcoClipper500 only has a slightly larger crew of twelve.

Life cycle analysis of a sailing ship

Although the EcoClipper500 is still in the design phase, she will be the focus of this article. This is because the company conducted a life cycle analysis of the ship prior to building it. ⁴ As far as I know, this is the first life cycle analysis of a sailing ship ever made. The study reveals that it takes around 1,200 tonnes of carbon to build the ship.

Half of those emissions are generated during steel production, and roughly one third is generated by steel working processes and other shipyard activities. Solvent-based paints as well as electric and electronic systems each account for roughly 5% of emissions. The emissions produced during the manufacturing of the sails are not included because there are no scientific data available, but a quick back-of-the-envelope calculation (for sails based on aramid fibres) signals that their contribution to the total carbon footprint is very small. ⁵

The EcoClipper500 has a carbon footprint of 2 grammes of CO2 per tonne-kilometre, which is five times less than the carbon footprint of a container ship.

If these 1,200 tonnes of emissions are spread out over an estimated lifetime of 50 years, then the EcoClipper500 would have a carbon footprint of about 2 grammes of CO2 per tonne-kilometre of cargo, concludes researcher Andrew Simons, who made the life cycle analysis for the ship. This is roughly five times less than the carbon footprint of a container ship (10 grammes CO2/tonne-km) and three times less than the carbon footprint of a bulk-carrier (6 grammes CO2/tonne-km). ⁶

Looking aft from aloft on the 'Parma' while at anchor. Alan Villiers, 1932-33. Villiers's work vividly records the period of early 20th century maritime history when merchant sailing vessels or ‘tall ships’ were in rapid decline.

Transporting one ton of cargo over a distance of 8,000 km (roughly the distance between the Caribbean and the Netherlands) would thus produce 16 kg of carbon with the EcoClipper500, compared to 80 kg on a container ship and 48 kg on a bulk carrier. The proportions are similar for other environmental factors, such as ozone depletion, ecotoxicity, air pollution, and so on.

Although the sailing ship boasts a convincing advantage, it may not be as big as you might have expected. First, as Simons explains, there’s scale. A container ship or bulk carrier enjoys the same benefits over the EcoClipper500 as the EcoClipper500 enjoys over the Tres Hombres. It can take a lot more cargo – on average 50,000 tonnes instead of 500 tonnes – and it needs only a slightly larger crew of 20-25 people. ⁷

Second, fossil fuel powered ships are faster than sailing ships, meaning that fewer ships are needed to transport a given amount of cargo over a given period of time. The original ship on which the EcoClipper500 is based, sailed between the Netherlands and Indonesia in 65 to 78 days, while a container ship does it in about half the time (taking the short cut through the Suez canal).

Building a fleet of sailing ships

There’s two ways to further lower the carbon emissions of sailing ships in comparison to container ships and bulk carriers. One is to build ships from wood instead of steel, such as the Ceiba. If the harvested trees are allowed to grow back (which the makers of the Ceiba have promised), such a ship may even be considered a carbon sink.

However, there’s a good reason why the EcoClipper500 will be made from steel: the company’s aim is to build not just one ship, but a fleet of them. Jorne Langelaan: “There are few shipyards who can deliver wooden ships nowadays. Steel makes it easier to build a fleet in a shorter period.”

A possible compromise would be a composite construction, in which a steel skeleton is clad with timber keel, planks, and deck. Andrew Simons: “This would reduce the carbon footprint of construction by half. It could also be feasible to make superstructures and some of the mast sections and spars from timber instead of steel.”

Driving sprays over the main deck of the 'Parma'. Alan Villiers, 1932-33.

Towards the future, another possibility to further decrease a sailings ship’s emissions per tonne-km is to build it even larger. While the EcoClipper500 has much more cargo capacity than the cargo sailing ships now in operation, she is far from the largest sailing ship ever built.

Historical ships such as the Great Republic (5,000 tonnes), the Parma (5,300 tonnes), the France II (7,300 tonnes), and the Preussen (7,800 tonnes), were more than 100 metres long and could take more than ten times the freight capacity of the EcoClipper500. Langelaan already dreams of a EcoClipper3000.

Passengers

Most cargo sailing ships travelling across the oceans today can also take some passengers. Fully loaded with cargo, the EcoClipper500 takes 12 crew members, 12 passengers, and 8 trainees (passengers who learn how to sail). If the upper hold deck is not used for cargo, another 28 trainees can join, so that the ship can take up to 60 people on board (with a smaller cargo volume: 480 m3 instead of 880 m3).

The carbon footprint for passengers amounts to 10 g per passenger-km, compared to roughly 100 g per passenger-km on an airplane.

Consequently, and since ocean liners have disappeared, the EcoClipper500 also becomes an alternative to the airplane. According to the results of the life cycle analysis, the carbon footprint for passengers on the EcoClipper500 amounts to 10 grammes per passenger-kilometre, compared to roughly 100 grammes per passenger-kilometre on an airplane. Transporting one passenger thus produces as much carbon emissions as transporting 1 tonne of freight.

Engine or not?

Importantly, the life cycle analysis of the EcoClipper500 assumes that there is no diesel engine on-board. On a sailing ship, a diesel engine can serve two purposes, which can be combined. First, it allows to propel the ship when there is no wind or when sails cannot be used, for example when leaving or entering a harbour. Second, combined with a generator, a diesel engine can produce electricity for daily life on board of the ship.

For most of history, energy use on-board of a sailing ship was not too problematic. There was firewood for cooking and heating, and there were candles and oil lamps for lighting. There were no refrigerators for food storage, no showers or laundry machines for washing and cleaning, no electronic instruments for navigation and communication, no electric pumps in case of leaks or fire.

However, we now have higher standards in terms of safety, health, hygiene, thermal comfort, and convenience. The problem is that these higher standards are difficult to achieve when the ship does not have an engine that runs on fossil fuels. Modern heating systems, cooking devices, hot water boilers, refrigerators, freezers, lighting, safety equipment, and electronic instruments all need energy to work.

Crewman of the 'Parma' with a model of his ship. Alan Villiers, 1932-33.

Modern sailing ships often use a diesel engine to provide that energy (and to propel the ship if necessary). An example is the Avontuur from Timbercoast, who has an engine of 300 HP, a 20 kW generator, and a fuel tank of 2,330 litres. Large sail training vessels and cruising ships have several engines and generators on-board. For example, the 48m long Brig Morningster has a 450 HP engine and three generators with a total capacity of 100 kW, while the 56m long Bark Europa has two 365 HP engines with three generators – and burns hundreds of litres of oil per day.

Depending on the lifestyle of the people on board, the emissions per passenger-km may rise to, or surpass, the levels of those of an airplane.

Obviously, the emissions and other pollutants of these engines need to be taken into account when the environmental footprint of a sail trip is calculated. Depending on the lifestyle of the people on board, the emissions per passenger-km may rise to, or surpass, the levels of those of an airplane. To a lesser extent, electricity use on-board also increases the emissions of cargo transportation.

Energy use on board a sailing ship

The EcoClipper500 has no diesel engine on board, which is a second reason to focus on this ship. Obviously, a sailing ship without an engine cannot proceed her voyage when there’s no wind. This is easily solved in the old-fashioned way: the EcoClipper500 stays where she is until the wind returns. A ship without an engine also needs tug boats – which usually burn fossil fuels – to get in and out of ports. For the EcoClipper500, these tug services account for 0.3 g/tkm of the total carbon footprint of 2 g/tkm.

Without a diesel engine, the ship also needs to generate all energy for use on board from local energy sources, and this is the hard part. Renewable energy is intermittent and has low power density compared to fossil fuels, meaning that more space is needed to generate a given amount of power – which is more problematic at sea than it is on land.

Renewing caulking on the poop of the 'Parma'. Alan Villiers, 1932-33.

To make the EcoClipper500 self-sufficient in terms of energy use, a first design decision was to shift energy use away from electricity whenever possible. This is especially important for high temperature heat, which cannot be supplied by electric heat pumps. The ship will have a pellet-stove on board to provide space heating, as well as a biodigester – never before used on a ship – to convert human and kitchen waste into gas for cooking. Thermal insulation of the ship is another priority.

Nevertheless, even with pellet-stove and biodigester (which themselves require electricity to operate), and with thermal insulation, energy demand on the ship can be as high as 50 kilowatt-hours of electricity per day (2 kW average power use). This concerns a “worst-case normal operation” scenario, when the ship is sailing in cold weather with 60 people on board. Power use will be lower in warmer weather and/or when less people are taken. During an emergency, the power requirements can amount to 8 kW, while more than 24 kWh of energy can be needed in just three hours.

Hydrogenerators

How to produce this power? Solar panels and wind turbines are only a small part of the solution. Producing 50 kWh of energy per day would require at least 100 square metres of solar panels, for which there is little space on a 60 m long sailing ship. Vulnerability and shading by the sails make for further problems. Wind turbines can be attached in the rigging, but their power output is also limited. The low potential of solar and wind power are demonstrated by the earlier mentioned sailing ship Avontuur. She has a 20 kW generator, powered by the diesel engine, but only 2.1 kW of solar panels and 0.8 kW of wind turbines.

The hydrogenerator is the only renewable power source that can provide a large sailing ship with enough energy for the use of modern technology on board.

The hydrogenerator is the only renewable power source that can provide a large sailing ship with enough energy for the use of modern technology on board. Hydrogenerators are attached underneath the hull and work in the opposite way as a ship’s propeller. Instead of the propeller powering the ship, the ship powers the propeller, which turns a generator that produces electricity. In spite of its name and appearance, the hydrogenerator is actually a form of wind energy: the sails power the propellers. Obviously, this only works when the ship is sailing fast enough.

Furling sail on the main yard of the Parma. Alan Villiers, 1932-33.

The EcoClipper500 will be equipped with two large hydrogenerators, for which Simons calculated the power output at different speeds, taking into account the fact that the extra drag they produce slows down the ship somewhat. He concludes that the EcoClipper500 needs to sail at a speed of at least 7.5 knots to generate enough electricity. At that speed, the hydrogenerators produce an estimated 2,000 watts of power, which converts to roughly 50 kWh of electricity per day (24 hours of sailing).

At a lower speed of 4.75 knots, the generators produce 350 watts, which comes down to 8.4 kWh of energy over a period of 24 hours – only 1/6th of the maximum required energy. On the other hand, at higher speeds, the hydrogenerators produce more energy than necessary. At a speed of almost 10 knots they provide 120 kWh/day, at a speed of 12 knots this becomes 182 kWh/day – 3.5 times more than needed.

Saltwater batteries

According to her hull speed, the EcoClipper500 will be able to sail a little over 16 knots at absolute top speed – this is double the minimum speed required to generate enough power. Achieving this speed will be rare, because it needs calm seas and strong winds from the right direction. Nevertheless, in good wind conditions, the ship easily sails fast enough to produce all electricity for use on board.

Good wind conditions can last for days, especially on the oceans, where winds are more powerful and predictable than on land. However, they are not guaranteed, and the ship will also sail at lower speeds, or find herself in becalmed conditions – when hydrogenerators are as useless as solar panels in the middle of the night.

Because she has no engine, the EcoClipper500 faces a double problem when there’s no wind: she cannot continue her voyage, and she has no energy to maintain life on board.

Because she has no engine, the EcoClipper500 faces a double problem when there’s no wind: she cannot continue her voyage, and she has no energy to maintain life on board. The first problem is easily solved but the second is not. Life on board goes on, and so there is a continued need for power. To provide this, the ship needs energy storage.

To cover the needs for three days drifting in cold weather, an energy storage of 150 kWh would be required, not taking into account charge and discharge losses. Five or seven days of energy use on-board would require 250 to 350 kWh of storage. For emergency use, another 25 kWh of energy storage is needed.

Scraping the deck onboard the 'Parma'. Alan Villiers, 1932-33.

Not having an engine, generator and fuel tank saves space on board, but this advantage can be quickly lost again when one starts to add batteries for the hydrogenerators. Lithium-ion batteries are very compact, but they cannot be considered sustainable and bring safety risks. That’s why Jorne Langelaan and Andrew Simons see more potential in – very aptly – saltwater batteries, which are non-flammable, non-toxic, easy to recycle, have wide temperature-tolerance, and can last for more than 15 years. Like the biodigester, they have never been used on a sailing ship before.

Unlike lithium-ion batteries, saltwater batteries are large and heavy. At 60 kg per kWh of storage capacity, a 150 kWh battery storage would add a weight of 9 tonnes, while a 350 kWh storage capacity would add 21 tonnes. Still, this compares favourably to the total cargo capacity (500 tonnes), and the batteries can serve as ballast if they are placed in the lower part of the ship’s hull. The space requirements are not too problematic, either. Even a 350 kWh energy storage only requires 14 to 29m3 of space, which is small compared to the 880m3 of cargo volume.

The emissions that are produced by the manufacturing of the hydrogenerators, biodigester, and batteries are not included in the life cycle analysis of the ship, because there are no data available. However, these emissions must be relatively small. Hydrogenerators have much higher power density than wind turbines, and thus a relatively low embodied energy. A quick back-of-the-envelope calculation learns that the carbon footprint of 350 kWh saltwater batteries is around 70 tonnes of CO2. ⁸

Human Power

There’s another renewable power source and energy storage on board of the EcoClipper, and that’s the humans themselves. Like the pellet stove and the biodigester, the use of human power could reduce the need for electricity. Nowadays, cargo ships and most large sailing ships have electric or hydraulic winches, pumps, and steering gear, saving manual labour at the expense of higher energy use. In contrast, EcoClipper sticks to manual handling of the ship as much as possible.

Crew at the capstan of the Parma, weighing anchor. Alan Villiers, 1932-33.

Simons and Langelaan are also considering the addition of a few rowing machines, coupled to generators, to produce emergency power. Two rowing machines could provide roughly 400 watts of power. If they are operated around the clock in shifts, they could supply the ship with an extra 9.6 kWh of energy per day (ignoring energy losses) – one fifth of the total maximum electricity use.

In fact, as I tell Simons and Langelaan ten rowing machines operated continually in shifts would provide as much power as the hydrogenerators at a speed of 7.5 knots. If there are 60 people on board, and everybody would generate power for less than one hour per day, no hydrogenerators and batteries would be needed at all. “A very interesting thought”, answers Simons, “but what impression would we be painted with?”

Hot Showers?

Even with a biodigester, hydrogenerators, batteries, and rowing machines, the passengers and crew on board the EcoClipper500 would be far short of luxurious, and perhaps too short of comfortable for some. For example, if 60 people on board the ship would take a daily hot shower – which requires on average 2.1 kilowatt-hours of energy and 76.5 litres of water on land – total electricity use per day would be 126 kWh, more than double the energy the ship produces at a speed of 7.5 knots.

The ship could supply this energy at a higher sailing speed, but there would also be a need for 4,590 liters of water per day, a quantity that could only be produced from seawater – a process that requires a lot of energy. Even a crew of 12 taking a daily hot shower would require 25.2 kWh of energy per day, half of what the hydrogenerators produce at a sailing speed of 7.5 knots. The Bark Europa is the only sailing ship mentioned in this article that has hot showers in every (shared) cabin, but it is also the ship with the biggest generators and the highest fuel use.

On the forecastle head of the Parma in fine weather. Image by Alan Villiers, 1932.

Andrew Simons: “On the EcoClipper500 there needs to be a manageable compromise between energy use and comfort. Energy use on board will have to be actively managed. Resources are finite, just like for the planet. In many ways the ship is a microcosm of challenges that the wider world has to face and find solutions to.”

Jorne Langelaan: “At sea you are in a different world. It doesn’t matter anymore if you can take a daily shower or not. What matters are the people, the movements of the ship, and the vast wilderness of ocean around you”.

Measuring the right things

This article has compared the EcoClipper500 sailing ship with the average container ship, bulk carrier, and airplane in terms of emissions per tonne- or passenger-kilometer. However, these values are abstractions that obscure much more important information: the total emissions that are produced by all passengers and all cargo, over all kilometres.

The international ocean freight trade increased from 4 billion tonnes of cargo in 1990 to 11.2 billion tonnes in 2019, resulting in more than 1 billion tonnes of emissions. International air passenger numbers grew from 1 billion in 1990 to 4.5 billion in 2019, resulting in 915 million tonnes of emissions. Consequently, lowering the emissions per tonne- and passenger-kilometre is neither a necessity nor a guarantee for a reduction in emissions.

If we cut international cargo traffic more than fivefold, and passenger traffic more than tenfold, then the emissions of all container ships and airplanes would be lower than the emissions of all sailing ships carrying 11.2 billion tonnes of cargo and 4.5 billion of passengers. Vice versa, if we switch to sailing ships, but keep on transporting more and more cargo and passengers across the planet, we will eventually produce just as much in emissions as we do today with fossil fuel powered transportation.

The mizzen of the 'Grace Harwar'; view aft from the main crosstrees. Alan Villiers, 1932-33.

Of course, none of this would ever happen. The amount of cargo that was traded across the oceans in 2019 equals the freight capacity of 22.4 million EcoClippers. Assuming the EcoClipper500 can make 2-3 trips per year, we would need to build and operate at least 7.5 million ships, with a total crew of at least 90 million people. Those ships could only take 0.5 billion passengers (12 passengers and 8 trainees per ship), so we would need millions of ships and crew members more to replace international air traffic.

We should not be fooled by abstract relative measurements, which only serve to keep the focus on growth and efficiency.

All of this is technically possible, and as we have seen, it would produce less in emissions than the present alternatives. However, it’s more likely that a switch to sailing ships is accompanied by a decrease in cargo and passenger traffic, and this has everything to do with scale and speed. A lot of freight and passengers would not be travelling if it were not for the high speeds and low costs of today’s airplanes and container ships.

It would make little sense to transport iPhones parts, Amazon wares, sweatshop clothes, or citytrippers with sailing ships. A sailing ship is more than a technical means of transportation: it implies another view on consumption, production, time, space, leisure, and travel. For example, a lot of freight now travels in different directions for each next processing stage before it is delivered as a final product. In contrast, all sail cargo companies mentioned in this article only take cargo that cannot be produced locally, and which is one trip from producer to consumer. ⁹

This also means that even if sailing ships have diesel engines on board, they would still bring a significant decrease in the total emissions for freight and passenger traffic, simply because they would reduce the absolute number of passengers, cargo, and kilometers. We should not be fooled by abstract relative measurements, which only serve to keep the focus on growth and efficiency.

More about the EcoClipper500. Most images: Alan Villiers collection.

Between 1978 and 2004, the Avontuur was operated as sail cargo vessel under Captain Paul Wahlen. The Apollonia, originally built in 1946, is another cargo sailing ship in operation since 2014. It is 19.5 metres long and carries 10 tonnes of cargo. ↩︎
Very recently, Grain de Sail was built and launched for Trans-Atlantic shipping of wine and cocoa. She is a modern sailing ship without an engine, built from aluminium, and can take 35 tonnes of cargo. ↩︎
Andrew Simons: “There are plenty historical sailing ships, but either very costly to get into service as a regulatory compliant cargo vessel, because they are still used for other purposes, or not suitable.” ↩︎
The study can be downloaded when you subscribe to EcoClipper’s newsletter. The research is based on a typical life cycle analysis, but note that this is not a peer reviewed study. ↩︎
Unfortunately the envelope got lost. ↩︎
In the case of the EcoClipper, most of the emissions are produced during the construction of the ship, while in the case of bulk carriers and container ships, they are mainly produced during operation and fuel production. ↩︎
The largest container ships now take 190,000 tonnes of cargo. ↩︎
There is not much data available on saltwater batteries, but they are less energy-intensive to build than many other types of batteries. The calculation is based on an estimate of 66 kg CO2/kWh of storage capacity and three generations of batteries over a period of 50 years. ↩︎
Almost one third of all cargo transported are fossil fuels themselves. ↩︎

How to Make Biomass Energy Sustainable Again

Sun, 20 Sep 2020 00:00:00 +0000

Image: Pollarded trees in Germany. Image: René Schröder (CC BY-SA 4.0).

How is Cutting Down Trees Sustainable?

Advocating for the use of biomass as a renewable source of energy – replacing fossil fuels – has become controversial among environmentalists. The comments on the previous article, which discussed thermoelectric stoves, illustrate this:

“As the recent film Planet of the Humans points out, biomass a.k.a. dead trees is not a renewable resource by any means, even though the EU classifies it as such.”
“How is cutting down trees sustainable?”
“Article fails to mention that a wood stove produces more CO2 than a coal power plant for every ton of wood/coal that is burned.”
“This is pure insanity. Burning trees to reduce our carbon footprint is oxymoronic.”
“The carbon footprint alone is just horrifying.”
“The biggest problem with burning anything is once it’s burned, it’s gone forever.”
“The only silly question I can add to to the silliness of this piece, is where is all the wood coming from?”

In contrast to what the comments suggest, the article does not advocate the expansion of biomass as an energy source. Instead, it argues that already burning biomass fires – used by roughly 40% of today’s global population – could also produce electricity as a by-product, if they are outfitted with thermoelectric modules. Nevertheless, several commenters maintained their criticism after they read the article more carefully. One of them wrote: “We should aim to eliminate the burning of biomass globally, not make it more attractive.”

Apparently, high-tech thinking has permeated the minds of (urban) environmentalists to such an extent that they view biomass as an inherently troublesome energy source – similar to fossil fuels. To be clear, critics are right to call out unsustainable practices in biomass production. However, these are the consequences of a relatively recent, “industrial” approach to forestry. When we look at historical forest management practices, it becomes clear that biomass is potentially one of the most sustainable energy sources on this planet.

Coppicing: Harvesting Wood Without Killing Trees

Nowadays, most wood is harvested by killing trees. Before the Industrial Revolution, a lot of wood was harvested from living trees, which were coppiced. The principle of coppicing is based on the natural ability of many broad-leaved species to regrow from damaged stems or roots – damage caused by fire, wind, snow, animals, pathogens, or (on slopes) falling rocks. Coppice management involves the cutting down of trees close to ground level, after which the base – called the “stool” – develops several new shoots, resulting in a multi-stemmed tree.

Image: A coppice stool. Credit: Geert Van der Linden.

Image: A recently coppiced patch of oak forest. Credit: Henk vD. (CC BY-SA 3.0)

Image: Coppice stools in Surrey, England. Credit: Martinvl (CC BY-SA 4.0)

When we think of a forest or a tree plantation, we imagine it as a landscape stacked with tall trees. However, until the beginning of the twentieth century, at least half of the forests in Europe were coppiced, giving them a more bush-like appearance. ¹ The coppicing of trees can be dated back to the stone age, when people built pile dwellings and trackways crossing prehistoric fenlands using thousands of branches of equal size – a feat that can only be accomplished by coppicing. ²

Maps: The approximate historical range of coppice forests in the Czech Republic (above) and in Spain (below). Source: \"Coppice forests in Europe\", see [^1]

Ever since then, the technique formed the standard approach to wood production – not just in Europe but almost all over the world. Coppicing expanded greatly during the eighteenth and nineteenth centuries, when population growth and the rise of industrial activity (glass, iron, tile and lime manufacturing) put increasing pressure on wood reserves.

Short Rotation Cycles

Because the young shoots of a coppiced tree can exploit an already well-developed root system, a coppiced tree produces wood faster than a tall tree. Or, to be more precise: although its photosynthetic efficiency is the same, a tall tree provides more biomass below ground (in the roots) while a coppiced tree produces more biomass above ground (in the shoots) – which is clearly more practical for harvesting. ³ Partly because of this, coppicing was based on short rotation cycles, often of around two to four years, although both yearly rotations and rotations up to 12 years or longer also occurred.

Images: Coppice stools with different rotation cycles. Credit: Geert Van der Linden.

Because of the short rotation cycles, a coppice forest was a very quick, regular and reliable supplier of firewood. Often, it was cut up into a number of equal compartments that corresponded to the number of years in the planned rotation. For example, if the shoots were harvested every three years, the forest was divided into three parts, and one of these was coppiced each year. Short rotation cycles also meant that it took only a few years before the carbon released by the burning of the wood was compensated by the carbon that was absorbed by new growth, making a coppice forest truly carbon neutral. In very short rotation cycles, new growth could even be ready for harvest by the time the old growth wood had dried enough to be burned.

In some tree species, the stump sprouting ability decreases with age. After several rotations, these trees were either harvested in their entirety and replaced by new trees, or converted into a coppice with a longer rotation. Other tree species resprout well from stumps of all ages, and can provide shoots for centuries, especially on rich soils with a good water supply. Surviving coppice stools can be more than 1,000 years old.

Biodiversity

A coppice can be called a “coppice forest” or a “coppice plantation”, but in reality it was neither a forest nor a plantation – perhaps something in between. Although managed by humans, coppice forests were not environmentally destructive, on the contrary. Harvesting wood from living trees instead of killing them is beneficial for the life forms that depend on them. Coppice forests can have a richer biodiversity than unmanaged forests, because they always contain areas with different stages of light and growth. None of this is true in industrial wood plantations, which support little or no plant and animal life, and which have longer rotation cycles (of at least twenty years).

Image: Coppice stools in the Netherlands. Credit: K. Vliet (CC BY-SA 4.0)

Image: Sweet chestnut coppice at Flexham Park, Sussex, England. Credit: Charlesdrakew, public domain.

Our forebears also cut down tall, standing trees with large-diameter stems – just not for firewood. Large trees were only “killed” when large timber was required, for example for the construction of ships, buildings, bridges, and windmills. ⁴ Coppice forests could contain tall trees (a “coppice-with-standards”), which were left to grow for decades while the surrounding trees were regularly pruned. However, even these standing trees could be partly coppiced, for example by harvesting their side branches while they were alive (shredding).

Multipurpose Trees

The archetypical wood plantation promoted by the industrial world involves regularly spaced rows of trees in even-aged, monocultural stands, providing a single output – timber for construction, pulpwood for paper production, or fuelwood for power plants. In contrast, trees in pre-industrial coppice forests had multiple purposes. They provided firewood, but also construction materials and animal fodder.

The targeted wood dimensions, determined by the use of the shoots, set the rotation period of the coppice. Because not every type of wood was suited for every type of use, coppiced forests often consisted of a variety of tree species at different ages. Several age classes of stems could even be rotated on the same coppice stool (“selection coppice”), and the rotations could evolve over time according to the needs and priorities of the economic activities.

Image: A small woodland with a diverse mix of coppiced, pollarded and standard trees. Credit: Geert Van der Linden.

Coppiced wood was used to build almost anything that was needed in a community. ⁵ For example, young willow shoots, which are very flexible, were braided into baskets and crates, while sweet chestnut prunings, which do not expand or shrink after drying, were used to make all kinds of barrels. Ash and goat willow, which yield straight and sturdy wood, provided the material for making the handles of brooms, axes, shovels, rakes and other tools.

Young hazel shoots were split along the entire length, braided between the wooden beams of buildings, and then sealed with loam and cow manure – the so-called wattle-and-daub construction. Hazel shoots also kept thatched roofs together. Alder and willow, which have almost limitless life expectancy under water, were used as foundation piles and river bank reinforcements. The construction wood that was taken out of a coppice forest did not diminish its energy supply: because the artefacts were often used locally, at the end of their lives they could still be burned as firewood.

Image: Harvesting leaf fodder in Leikanger kommune, Norway. Credit: Leif Hauge. Source: [^19]

Coppice forests also supplied food. On the one hand, they provided people with fruits, berries, truffles, nuts, mushrooms, herbs, honey, and game. On the other hand, they were an important source of winter fodder for farm animals. Before the Industrial Revolution, many sheep and goats were fed with so-called “leaf fodder” or “leaf hay” – leaves with or without twigs. ⁶

Elm and ash were among the most nutritious species, but sheep also got birch, hazel, linden, bird cherry and even oak, while goats were also fed with alder. In mountainous regions, horses, cattle, pigs and silk worms could be given leaf hay too. Leaf fodder was grown in rotations of three to six years, when the branches provided the highest ratio of leaves to wood. When the leaves were eaten by the animals, the wood could still be burned.

Pollards & Hedgerows

Coppice stools are vulnerable to grazing animals, especially when the shoots are young. Therefore, coppice forests were usually protected against animals by building a ditch, fence or hedge around them. In contrast, pollarding allowed animals and trees to be mixed on the same land. Pollarded trees were pruned like coppices, but to a height of at least two metres to keep the young shoots out of reach of grazing animals.

Illustration: Different ways of lopping trees. Credit: Helen J. Read, see [^1]

Image: Pollarded trees in Segovia, Spain. Credit: [Ecologistas en Acción](https://www.ecologistasenaccion.org/35724/).

Wooded meadows and wood pastures – mosaics of pasture and forest – combined the grazing of animals with the production of fodder, firewood and/or construction wood from pollarded trees. “Pannage” or “mast feeding” was the method of sending pigs into pollarded oak forests during autumn, where they could feed on fallen acorns. The system formed the mainstay of pork production in Europe for centuries. ⁷ The “meadow orchard” or “grazed orchard” combined fruit cultivation and grazing – pollarded fruit trees offered shade to the animals, while the animals could not reach the fruit but fertilised the trees.

Image: Forest or pasture? Something in between. A \"dehesa\" (pig forest farm) in Spain. Credit: Basotxerri (CC BY-SA 4.0).

Image: Cattle grazes among pollarded trees in Huelva, Spain. (CC BY-SA 2.5)

Image: A meadow orchard surrounded by a living hedge in Rijkhoven, Belgium. Credit: Geert Van der Linden.

While agriculture and forestry are now strictly separated activities, in earlier times the farm was the forest and vice versa. It would make a lot of sense to bring them back together, because agriculture and livestock production – not wood production – are the main drivers of deforestation. If trees provide animal fodder, meat and dairy production should not lead to deforestation. If crops can be grown in fields with trees, agriculture should not lead to deforestation. Forest farms would also improve animal welfare, soil fertility and erosion control.

Line Plantings

Extensive plantations could consist of coppiced or pollarded trees, and were often managed as a commons. However, coppicing and pollarding were not techniques seen only in large-scale forest management. Small woodlands in between fields or next to a rural house and managed by an individual household would be coppiced or pollarded. A lot of wood was also grown as line plantings around farmyards, fields and meadows, near buildings, and along paths, roads and waterways. Here, lopped trees and shrubs could also appear in the form of hedgerows, thickly planted hedges. ⁸

Image: Hedge landscape in Normandy, France, around 1940. Credit: W Wolny, public domain.

Image: Line plantings in Flanders, Belgium. Detail from the Ferraris map, 1771-78.

Although line plantings are usually associated with the use of hedgerows in England, they were common in large parts of Europe. In 1804, English historian Abbé Mann expressed his surprise when he wrote about his trip to Flanders (today part of Belgium): “All fields are enclosed with hedges, and thick set with trees, insomuch that the whole face of the country, seen from a little height, seems one continued wood”. Typical for the region was the large number of pollarded trees. ⁸

Like coppice forests, line plantings were diverse and provided people with firewood, construction materials and leaf fodder. However, unlike coppice forests, they had extra functions because of their specific location. ⁹ One of these was plot separation: keeping farm animals in, and keeping wild animals or cattle grazing on common lands out. Various techniques existed to make hedgerows impenetrable, even for small animals such as rabbits. Around meadows, hedgerows or rows of very closely planted pollarded trees (“pollarded tree hedges”) could stop large animals such as cows. If willow wicker was braided between them, such a line planting could also keep small animals out. ⁸

Image: Detail of a yew hedge. Credit: Geert Van der Linden.

Image: A hedgerow. Credit: Geert Van der Linden.

Image: Pollarded tree hedge in Nieuwekerken, Belgium. Credit: Geert Van der Linden.

Image: Coppice stools in a pasture. Credit: Jan Bastiaens.

Trees and line plantings also offered protection against the weather. Line plantings protected fields, orchards and vegetable gardens against the wind, which could erode the soil and damage the crops. In warmer climates, trees could shield crops from the sun and fertilize the soil. Pollarded lime trees, which have very dense foliage, were often planted right next to wattle-and-daub buildings in order to protect them from wind, rain and sun. ¹⁰

Dunghills were protected by one or more trees, preventing the valuable resource from evaporating due to sun or wind. In the yard of a watermill, the wooden water wheel was shielded by a tree to prevent the wood from shrinking or expanding in times of drought or inactivity. ⁸

Image: A pollarded tree protects a water wheel. Credit: Geert Van der Linden.

Image: Pollarded lime trees protect a farm building in Nederbrakel, Belgium. Credit: Geert Van der Linden.

Location Matters

Along paths, roads and waterways, line plantings had many of the same location-specific functions as on farms. Cattle and pigs were hoarded over dedicated droveways lined with hedgerows, coppices and/or pollards. When the railroads appeared, line plantings prevented collisions with animals. They protected road travellers from the weather, and marked the route so that people and animals would not get off the road in a snowy landscape. They prevented soil erosion at riverbanks and hollow roads.

All functions of line plantings could be managed by dead wood fences, which can be moved more easily than hedgerows, take up less space, don’t compete for light and food with crops, and can be ready in a short time. ¹¹ However, in times and places were wood was scarce a living hedge was often preferred (and sometimes obliged) because it was a continuous wood producer, while a dead wood fence was a continuous wood consumer. A dead wood fence may save space and time on the spot, but it implies that the wood for its construction and maintenance is grown and harvested elsewhere in the surroundings.

Image: Pollarded tree hedge in Belgium. Credit: Geert Van der Linden.

Local use of wood resources was maximised. For example, the tree that was planted next to the waterwheel, was not just any tree. It was red dogwood or elm, the wood that was best suited for constructing the interior gearwork of the mill. When a new part was needed for repairs, the wood could be harvested right next to the mill. Likewise, line plantings along dirt roads were used for the maintenance of those roads. The shoots were tied together in bundles and used as a foundation or to fill up holes. Because the trees were coppiced or pollarded and not cut down, no function was ever at the expense of another.

Nowadays, when people advocate for the planting of trees, targets are set in terms of forested area or the number of trees, and little attention is given to their location – which could even be on the other side of the world. However, as these examples show, planting trees closeby and in the right location can significantly optimise their potential.

Shaped by Limits

Coppicing has largely disappeared in industrial societies, although pollarded trees can still be found along streets and in parks. Their prunings, which once sustained entire communities, are now considered waste products. If it worked so well, why was coppicing abandoned as a source of energy, materials and food? The answer is short: fossil fuels. Our forebears relied on coppice because they had no access to fossil fuels, and we don’t rely on coppice because we have.

Our forebears relied on coppice because they had no access to fossil fuels, and we don’t rely on coppice because we have

Most obviously, fossil fuels have replaced wood as a source of energy and materials. Coal, gas and oil took the place of firewood for cooking, space heating, water heating and industrial processes based on thermal energy. Metal, concrete and brick – materials that had been around for many centuries – only became widespread alternatives to wood after they could be made with fossil fuels, which also brought us plastics. Artificial fertilizers – products of fossil fuels – boosted the supply and the global trade of animal fodder, making leaf fodder obsolete. The mechanisation of agriculture – driven by fossil fuels – led to farming on much larger plots along with the elimination of trees and line plantings on farms.

Less obvious, but at least as important, is that fossil fuels have transformed forestry itself. Nowadays, the harvesting, processing and transporting of wood is heavily supported by the use of fossil fuels, while in earlier times they were entirely based on human and animal power – which themselves get their fuel from biomass. It was the limitations of these power sources that created and shaped coppice management all over the world.

Image: Harvesting wood from pollarded trees in Belgium, 1947. Credit : Zeylemaker, Co., Nationaal Archief (CCO)

Image: Transporting firewood in the Basque Country. Source: Notes on pollards: best practices' guide for pollarding. Gipuzkoaka Foru Aldundía-Diputación Foral de Giuzkoa, 2014.

Wood was harvested and processed by hand, using simple tools such as knives, machetes, billhooks, axes and (later) saws. Because the labour requirements of harvesting trees by hand increase with stem diameter, it was cheaper and more convenient to harvest many small branches instead of cutting down a few large trees. Furthermore, there was no need to split coppiced wood after it was harvested. Shoots were cut to a length of around one metre, and tied together in “faggots”, which were an easy size to handle manually.

It was the limitations of human and animal power that created and shaped coppice management all over the world

To transport firewood, our forebears relied on animal drawn carts over often very bad roads. This meant that, unless it could be transported over water, firewood had to be harvested within a radius of at most 15-30 km from the place where it was used. ¹² Beyond those distances, the animal power required for transporting the firewood was larger than its energy content, and it would have made more sense to grow firewood on the pasture that fed the draft animal. ¹³ There were some exceptions to this rule. Some industrial activities, like iron and potash production, could be moved to more distant forests – transporting iron or potash was more economical than transporting the firewood required for their production. However, in general, coppice forests (and of course also line plantings) were located in the immediate vicinity of the settlement where the wood was used.

In short, coppicing appeared in a context of limits. Because of its faster growth and versatile use of space, it maximised the local wood supply of a given area. Because of its use of small branches, it made manual harvesting and transporting as economical and convenient as possible.

Can Coppicing be Mechanised?

From the twentieth century onwards, harvesting was done by motor saw, and since the 1980s, wood is increasingly harvested by powerful vehicles that can fell entire trees and cut them on the spot in a matter of minutes. Fossil fuels have also brought better transportation infrastructures, which have unlocked wood reserves that were inaccessible in earlier times. Consequently, firewood can now be grown on one side of the planet and consumed at the other.

The use of fossil fuels adds carbon emissions to what used to be a completely carbon neutral activity, but much more important is that it has pushed wood production to a larger – unsustainable – scale. [14] Fossil fueled transportation has destroyed the connection between supply and demand that governed local forestry. If the wood supply is limited, a community has no other choice than to make sure that the wood harvest rate and the wood renewal rate are in balance. Otherwise, it risks running out of fuelwood, craft wood and animal fodder, and it would be abandoned.

Image: Mechanically harvested willow coppice plantation. Shortly after coppicing (right), 3-years old growth (left). Credit: Lignovis GmbH (CC BY-SA 4.0).

Likewise, fully mechanised harvesting has pushed forestry to a scale that is incompatible with sustainable forest management. Our forebears did not cut down large trees for firewood, because it was not economical. Today, the forest industry does exactly that because mechanisation makes it the most profitable thing to do. Compared to industrial forestry, where one worker can harvest up to 60 m3 of wood per hour, coppicing is extremely labour-intensive. Consequently, it cannot compete in an economic system that fosters the replacement of human labour with machines powered by fossil fuels.

Coppicing cannot compete in an economic system that fosters the replacement of human labour with machines powered by fossil fuels

Some scientists and engineers have tried to solve this by demonstrating coppice harvesting machines. ¹⁴ However, mechanisation is a slippery slope. The machines are only practical and economical on somewhat larger tracts of woodland (>1 ha) which contain coppiced trees of the same species and the same age, with only one purpose (often fuelwood for power generation). As we have seen, this excludes many older forms of coppice management, such as the use of multipurpose trees and line plantings. Add fossil fueled transportation to the mix, and the result is a type of industrial coppice management that brings few improvements.

Image: Coppiced trees along a brook in 's Gravenvoeren, Belgium. Credits: Geert Van der Linden.

Sustainable forest management is essentially local and manual. This doesn’t mean that we need to copy the past to make biomass energy sustainable again. For example, the radius of the wood supply could be increased by low energy transport options, such as cargo bikes and aerial ropeways, which are much more efficient than horse or ox drawn carts over bad roads, and which could be operated without fossil fuels. Hand tools have also improved in terms of efficiency and ergonomics. We could even use motor saws that run on biofuels – a much more realistic application than their use in car engines. ¹⁵

The Past Lives On

This article has compared industrial biomass production with historical forms of forest management in Europe, but in fact there was no need to look to the past for inspiration. The 40% of the global population consisting of people in poor societies that still burn wood for cooking and water and/or space heating, are no clients of industrial forestry. Instead, they obtain firewood in much of the same ways that we did in earlier times, although the tree species and the environmental conditions can be very different. ¹⁶

A 2017 study calculated that the wood consumption by people in “developing” societies – good for 55% of the global wood harvest and 9-15% of total global energy consumption – only causes 2-8% of anthropogenic climate impacts. ¹⁷ Why so little? Because around two-thirds of the wood that is harvested in developing societies is harvested sustainably, write the scientists. People collect mainly dead wood, they grow a lot of wood outside the forest, they coppice and pollard trees, and they prefer the use of multipurpose trees, which are too valuable to cut down. The motives are the same as those of our ancestors: people have no access to fossil fuels and are thus tied to a local wood supply, which needs to be harvested and transported manually.

Image: African women carrying firewood. (CC BY-SA 4.0)

These numbers confirm that it is not biomass energy that’s unsustainable. If the whole of humanity would live as the 40% that still burns biomass regularly, climate change would not be an issue. What is really unsustainable is a high energy lifestyle. We can obviously not sustain a high-tech industrial society on coppice forests and line plantings alone. But the same is true for any other energy source, including uranium and fossil fuels.

Multiple references: Unrau, Alicia, et al. Coppice forests in Europe. University of Freiburg, 2018. // Notes on pollards: best practices’ guide for pollarding. Gipuzkoako Foru Aldundia-Diputación Foral de Gipuzkoa, 2014. // A study of practical pollarding techniques in Northern Europe. Report of a three month study tour August to November 2003, Helen J. Read. // Aarden wallen in Europa, in “Tot hier en niet verder: historische wallen in het Nederlandse landschap”, Henk Baas, Bert Groenewoudt, Pim Jungerius and Hans Renes, Rijksdienst voor het Cultureel Erfgoed, 2012. ↩︎
Logan, William Bryant. Sprout lands: tending the endless gift of trees. WW Norton & Company, 2019. ↩︎
Holišová, Petra, et al. “Comparison of assimilation parameters of coppiced and non-coppiced sessile oaks”. Forest-Biogeosciences and Forestry 9.4 (2016): 553. ↩︎
Perlin, John. A forest journey: the story of wood and civilization. The Countryman Press, 2005. ↩︎
Most of this information comes from a Belgian publication (in Dutch language): Handleiding voor het inventariseren van houten beplantingen met erfgoedwaarde. Geert Van der Linden, Nele Vanmaele, Koen Smets en Annelies Schepens, Agentschap Onroerend Erfgoed, 2020. For a good (but concise) reference in English, see Rotherham, Ian. Ancient Woodland: history, industry and crafts. Bloomsbury Publishing, 2013. ↩︎ ↩︎ ↩︎ ↩︎
While leaf fodder was used all over Europe, it was especially widespread in mountainous regions, such as Scandinavia, the Alps and the Pyrenees. For example, in Sweden in 1850, 1.3 million sheep and goats consumed a total of 190 million sheaves annually, for which at least 1 million hectares deciduous woodland was exploited, often in the form of pollards. The harvest of leaf fodder predates the use of hay as winter fodder. Branches could be cut with stone tools, while cutting grass requires bronze or iron tools. While most coppicing and pollarding was done in winter, harvesting leaf fodder logically happened in summer. Bundles of leaf fodder were often put in the pollarded trees to dry. References: Logan, William Bryant. Sprout lands: tending the endless gift of trees. WW Norton & Company, 2019. // A study of practical pollarding techniques in Northern Europe. Report of a three month study tour August to November 2003, Helen J. Read. // Slotte H., “Harvesting of leaf hay shaped the Swedish landscape”, Landscape Ecology 16.8 (2001): 691-702. ↩︎
Wealleans, Alexandra L. “Such as pigs eat: the rise and fall of the pannage pig in the UK”. Journal of the Science of Food and Agriculture 93.9 (2013): 2076-2083. ↩︎
This information is based on several Dutch language publications: Handleiding voor het inventariseren van houten beplantingen met erfgoedwaarde. Geert Van der Linden, Nele Vanmaele, Koen Smets en Annelies Schepens, Agentschap Onroerend Erfgoed, 2020. // Handleiding voor het beheer van hagen en houtkanten met erfgoedwaarde. Thomas Van Driessche, Agentschap Onroerend Erfgoed, 2019 // Knotbomen, knoestige knapen: een praktische gids. Geert Van der Linden, Jos Schenk, Bert Geeraerts, Provincie Vlaams-Brabant, 2017. // Handleiding: Het beheer van historische dreven en wegbeplantingen. Thomas Van Driessche, Paul Van den Bremt and Koen Smets. Agentschap Onroerend Erfgoed, 2017. // Dirkmaat, Jaap. Nederland weer mooi: op weg naar een natuurlijk en idyllisch landschap. ANWB Media-Boeken & Gidsen, 2006. // For a good source in English, see: Müller, Georg. Europe’s Field Boundaries: Hedged banks, hedgerows, field walls (stone walls, dry stone walls), dead brushwood hedges, bent hedges, woven hedges, wattle fences and traditional wooden fences. Neuer Kunstverlag, 2013. // If line plantings were mainly used for wood production, they were planted at some distance from each other, allowing more light and thus a higher wood production. If they were mainly used as plot boundaries, they were planted more closely together. This diminished the wood harvest but allowed for a thicker growth. ↩︎ ↩︎ ↩︎ ↩︎
In fact, coppice forests could also have a location-specific function: they could be placed around a city or settlement to form an impenetrable obstacle for attackers, either by foot or by horse. They could not easily be destroyed by shooting, in contrast to a wall. Source: ⁵ ↩︎
Lime trees were even used for fire prevention. They were planted right next to the baking house in order to stop the spread of sparks to wood piles, haystacks and thatched roofs. Source: ⁵ ↩︎
The fact that living hedges and trees are harder to move than dead wood fences and posts also has practical advantages. In Europe until the French era, there was no land register and boundaries where physically indicated in the landscape. The surveyor’s work was sealed with the planting of a tree, which is much harder to move on the sly than a pole or a fence. Source: ⁵ ↩︎
And, if it could be brought in over water from longer distances, the wood had to be harvested within 15-30 km of the river or coast. ↩︎
Sieferle, Rolf Pieter. The Subterranean Forest: energy systems and the industrial revolution. White Horse Press, 2001. ↩︎
Vanbeveren, S.P.P., et al. “Operational short rotation woody crop plantations: manual or mechanised harvesting?” Biomass and Bioenergy 72 (2015): 8-18. ↩︎
However, chainsaws can have adverse effects on some tree species, such as reduced growth or greater ability to transfer disease. ↩︎
Multiple sources that refer to traditional forestry practices in Africa: Leach, Gerald, and Robin Mearns. Beyond the woodfuel crisis: people, land and trees in Africa. Earthscan, 1988. // Leach, Melissa, and Robin Mearns. “The lie of the land: challenging received wisdom on the African environment.” (1998) // Cline-Cole, Reginald A. “Political economy, fuelwood relations, and vegetation conservation: Kasar Kano, Northerm Nigeria, 1850-1915.” Forest & Conservation History 38.2 (1994): 67-78. ↩︎
Multiple references: Bailis, Rob, et al. “Getting the number right: revisiting woodfuel sustainability in the developing world.” Environmental Research Letters 12.11 (2017): 115002 // Masera, Omar R., et al. “Environmental burden of traditional bioenergy use.” Annual Review of Environment and Resources 40 (2015): 121-150. // Study downgrades climate impact of wood burning, John Upton, Climate Central, 2015. ↩︎