Covering all of physics and astronomy, from the history and the people to the latest hot topics, in 200 pages is a big challenge. That, however, is what the theoretical physicist Nicholas Mee set out to do in his new bookThe Cosmic Mystery Tour: a High-speed Journey Through Space and Time. In short chapters, Mee briefly sketches the main topics that make up modern physics, combining historical and cultural background with clear, concise explanations of the science itself. The history adds a splash of colour, and for the most part I enjoyed Mee’s decisions on who to include, not always going for the obvious choice.
Brevity is a style that works for Mee, though it did mean that some of the cultural asides felt out of place. I love Greek mythology, but a two-page explanation of the myths surrounding Perseus seems indulgent in a book where the Standard Model of particle physics gets just 1.5 pages. From Newtonian gravity to electromagnetic fields to fundamental particles, the early chapters feel a little lacking in any sense of order, jumping around in history and topic. By the halfway point it becomes clear that Mee’s main preoccupation is astrophysics, and all the scientific and historical background is leading in that direction. The book begins to flow better here and the non-sequiturs disappear.
If you like your science peppered with references to culture, from Arthur C Clarke and Douglas Adams to Joy Division and Van Gogh, give this book a try.
2019 Oxford University Press £16.99/$24.95hb 224pp
A simple body-integrated self-powered system (BISS) can convert mechanical motions of the human body into electrical energy by exploiting the triboelectric effect. The device works without the need for complicated structures or high-cost production and maintenance thanks to research by a team in China, led by Zhou Li and Zhong Lin Wang at Beijing Institute of Nanoenergy and Nanosystems.
In the paper published in ACS Nano, they show how the BISS can power wearable and implantable devices. Flexibility in the choice of material, size and dimension offers a plethora of possible applications. Among them, the BISS could be integrated into wearable electronic devices, such as smart glasses or wristbands or implanted devices like cardiac pacemakers.
How does it work?
The triboelectric effect, at the base of this system, is a contact electrification mechanism that causes two materials in contact to acquire electric charges upon separation. A common example is a plastic pen that is rubbed on a sleeve: once electrified it picks up small pieces of paper. Triboelectric nanogenerators (TENGs), first invented by Wang, can collect this energy.
In a similar way, movement such as stepping, walking, running and jumping, generates an electric charge between the soles of shoes and the floor. As a consequence, the body acquires electric potential (body electric potential, BEP) that is enough to produce an electric flow if a load is connected between the body and the floor. An electrode attached to the skin can harvest the charge created via BEP. Thus with just the human body and an electrode the BISS can power an electrical appliance from motion.
In their theoretical model, the human body circuit is simplified as a capacitance and a resistance in series, thus the BEP is inversely proportional to the capacitance of the body. The authors demonstrate how the BISS output depends on the people wearing the device, the type of movement, the characteristic of shoes and environment and the distance between the foot and the floor during the movements. On the contrary, it is not affected by the electrode dimensions.
Human power banks
BISS works both in vitro and in vivo. In the first case, the authors manufacture a wearable prototype, with the electrodes in carbon cloth and in the second case, they verify the activity in vivo, by implanting the electrode in laboratory animals (a rabbit and rats). Furthermore, they test the device with a human that holds the electrode in his mouth (to simulate in vivo conditions), powering up LED bulbs by stepping. In a simplistic way, in the future we may be our own power banks.
Scientists at Google and several research institutes in North America have reopened what they call the “cold case” of cold fusion. The researchers argue that those dismissing work on cold fusion – for which there is no reproducible experimental evidence – may have been “premature” in their assessment of its worth. The team has also carried out experiments of its own, which did not provide any evidence for cold fusion. Despite the many failures to observe cold fusion, the scientists maintain that the case is not yet closed. They also argue that cold-fusion research benefits other areas of science and technology.
Cold fusion hit the headlines 30 years ago in 1989, when chemists Martin Fleischmann of the University of Southampton in the UK and Stanley Pons of the University of Utah in the US announced at a press conference in Utah that they had observed nuclear fusion in a remarkably simple experiment. When passing electricity through samples of heavy water (deuterium oxide) containing palladium electrodes, they reported the net production of energy as well as the detection of neutrons and tritium. The reactions responsible, they claimed, must have been nuclear rather than chemical. This meant that some unknown mechanism had caused the deuterium nuclei in the electrodes to overcome their mutual electrostatic repulsion and get close enough to fuse.
Hailed by some at the time as a virtually inexhaustible source of clean nuclear energy, cold fusion also generated huge controversy. Many physicists insisted that fusion could not occur to any measurable degree at room temperature. Then and now, conventional fusion experiments rely on very powerful magnets or lasers to heat nuclei to millions of degrees to overcome their mutual repulsion. The biggest fusion reactor today, known as ITER, is being built in the south of France to the tune of €20bn.
Technical failures
Now, a study funded by the US-based technology giant Google has re-investigated the original cold-fusion result as well as some of those from the small number of groups that have continued to study the hypothetical phenomenon. Many scientists consider such research a waste of time, pointing to the many failed attempts at reproducing cold-fusion observations. But Matthew Trevithick and colleagues at Google reckoned that those failures might have been due to technical problems rather than the impossibility of cold fusion. “There were very few data available to assess whether cold fusion is correct or not,” says Trevithick, who takes a special interest in alternative energy sources.
In 2015, Trevithick approached Curtis Berlinguette, a chemist at the University of British Columbia in Canada, and then put together a roughly 30-strong research team using $10m of funding from Google – a small fraction of the company’s (undeclared) total research budget. The team has managed to keep its work under wraps until now – to the extent that even some of Berlinguette’s own research group were not aware of it. Now, the team leaders describe their results in a “Perspective” article in Nature.
Among its objectives, the team wanted to find out just how much deuterium they could pack into an electrode. Normally palladium absorbs about 0.7 hydrogen atoms for each atom in the metal lattice. According to a long-standing claim, however, fusion could kick-in if this reaches 0.875. Although the researchers exceeded this threshold just twice over the course of several dozen tests, Berlinguette says they now better understand how to boost absorption and limit leaks. “Our data confirm that hydrogen likes to enter palladium nanoparticles at vertices and not faces,” he explains. “That gives us some clues about how to design materials to get to higher sustained hydrogen concentrations”.
Potential pitfalls
The team also spent a lot of time trying to understand the potential pitfalls involved in measuring heat fluxes under extreme conditions. For many years, researchers in Italy have claimed that certain metallic powders yield excess energy when heated within hydrogen gas. So Berlinguette and colleagues modelled, built and tested several calorimeters (see figure) to try and pin down all the different ways that heat could flow into and out of a cold fusion experiment. Testing 420 samples of nickel powder, they found that none “provided evidence of excess heat”.
The team also looked at work done in the mid-1990s at the Los Alamos National Laboratory in New Mexico. The Los Alamos team did low-energy table top experiments that used a pulsed plasma discharge to drive deuterons into a palladium cathode. They reported detecting tritium, which is a by-product of fusion. In research described in the Nature article, Thomas Schenkel and colleagues at the Lawrence Berkeley National Laboratory in California carried-out similar experiments for weeks at a time but saw no enhanced production of tritium.
Overall, the researchers conclude that they “have found no evidence of anomalous effects claimed by proponents of cold fusion that cannot otherwise be explained prosaically”. Nevertheless, they maintain that their research “leaves open the possibility” that cold fusion is real. Indeed, Trevithick says that realizing what is known as a “reference experiment” – providing evidence of cold fusion that is unambiguous and independently verifiable – “is our goal and remains our goal”.
Plus, he adds, the research could prove useful in other areas. One example is the analysis of how hydrogen is absorbed by palladium – something, he and his colleagues write, which is relevant to energy storage, catalysis and sensing.
James Robson grows grapes in the Orange region of New South Wales, Australia. Ten years ago almost all his grapes went into wine for Australians; today 80% of his wine ends up in China. Australia exports beef, dairy products, fruit, honey and chocolate to China too; the nation is Australia’s number one export market. But this boost to the Australian economy has not been good news for Australia’s river systems.
Irene Soligno from the Politecnico di Torino in Italy and colleagues estimated the irrigation water used to produce 270 food and agricultural goods from 1986 to 2013. Then they estimated the environmental impact of this water withdrawal on different regions of the world.
Over this 28-year period, globalisation of the food trade doubled the impact on foreign rivers, the results showed. In some countries the changes have been dramatic.
“It was remarkable to see China change from being a net exporter to a major net importer of riverine resources over this period,” says Soligno, whose findings are published in Environmental Research Letters (ERL).
Today around 25% of the food produced for human consumption is traded internationally, compared to 15% in 1986. Yet there is little information about exactly where its virtually traded water comes from, and where it ends up.
Food production is the single biggest drain on Earth’s freshwater. A rising human population, combined with climate change and improving living standards, means that agriculture’s thirst is likely to increase in future years.
Hotspots of food-related river degradation were identified in Australia, Pakistan, South Africa and Spain. Often there was a significant imbalance in the environmental effects between major exporters and importers of river water. For example, food exports from Australia, India, Thailand and South Africa result in huge environmental impact to river systems in these countries, but food consumption in these countries has little impact on foreign river systems. Meanwhile, some countries, including China and Germany, are major importers of riverine resources.
For countries with wetter climates, such as Canada, the UK and much of northern Europe, exporting food has no detrimental impact on rivers, and has even helped to reduce environmental degradation of rivers in the export countries.
International trade has reduced the pressure on rivers globally by 11%, compared to if all food was grown locally, the researchers found.
“The fact that some countries can import food goods from countries that have less vulnerable river systems has a positive effect on efficiency,” says Soligno.
International trade is currently driven by socio-economic drivers, but understanding the impact that trade has on rivers could help to reduce the pressure on these environments further.
“We highlight hotspots of food-related river-environment degradation to target priority agricultural patterns that put surface water systems at risk,” says Soligno. “In order to support policy-makers, these initial results need to be further supported by local surveys, which should consider more case-specific data and incorporate additional socio-economic, cultural and environmental aspects.”
The theoretical particle physicist Murray Gell-Mann, who was awarded the 1969 Nobel Prize for Physics, has died aged 89. In the 1960s Gell-Mann developed a method to categorize the huge number of particles that were being created at particle accelerators worldwide. Gell-Mann’s model also predicted the existence of another fundamental type of particle that makes up particles including protons and neutrons. Gell-Mann dubbed them “quarks” and they were later discovered experimentally at the Stanford Linear Accelerator Center (SLAC) in the US.
Born on 15 September 1929 in New York City, Gell-Mann obtained a BSc in physics at Yale University in 1948. Three years later, he was awarded a PhD from the Massachusetts Institute of Technology before he joined the Institute for Advanced Study at Princeton University in 1952. After a stint at the University of Chicago, he joined the California Institute of Technology in 1955, where he remained for the rest of his career.
From the 1950s through to the 1970s, Gell-Mann played a leading role in particle physics. In 1961 Gell-Mann – together with the Israeli physicist Yuval Ne’eman – independently came up a with scheme that brought order out of the chaos of the “particle zoo” that was created by the discovery of some 100 kinds of particles in collisions involving atomic nuclei.
Dubbed the “eightfold way”, it involved ordering these subatomic particles onto clusters of eight and 10 based on a mathematical symmetry known as “SU(3)”. While the model was successful at describing existing particles and their interactions, it also opened the door for predicting the existence of new particle states, which were subsequently discovered. It was for this work that he won the 1969 Nobel Prize for Physics.
Using the eightfold way, in 1964 Gell-Mann and George Zweig independently proposed the existence of a new type of particle that made up particles such as neutrons and protons. Gell-Mann’s decision to call them quarks came from his interest in language, which was evident at an early age. Indeed, he was only 10 years old when he first leafed through a copy of Finnegans Wake – the novel by James Joyce that later provided him with the word quark in the phrase “Three quarks for Muster Mark”.
Back in the early 1960s, Gell-Mann wanted his new name for the components of protons and neutrons to sound like “kwork”, but he did not know how to spell it. However, since quark – which also describes the cry of a gull – was clearly intended to rhyme with Mark, Gell-Mann had to find an excuse to pronounce it “kwork”. But as Finnegans Wake is about the dreams of a publican and many phrases in the book are derived from calls for drinks at the bar, Gell-Mann argued that “Three quarks for Muster Mark” might actually mean “Three quarts for Mister Mark”. In 2002 he was able to study Joyce’s original manuscript for the novel during a visit to Dublin.
The existence of the quark was confirmed by deep inelastic scattering experiments at SLAC in 1968 and experiments have since provided evidence for all six flavours of quark — up, down, strange, charm, bottom and top. Quarks are permanently confined by forces coming from the exchange of gluons in the nucleus and Gell-Mann and others later constructed the quantum-field theory of quarks and gluons, known as quantum chromodynamics.
A love of language
In later life Gell-Mann was able to spend more time in other fields, indulging his interest in language. In 1984 Gell-Mann co-founded the Santa Fe Institute in New Mexico – an independent multidisciplinary centre that brings together researchers from different fields. In the 2000s Gell-Mann received funding to organize an international group of linguists, archaeologists, physical anthropologists and geneticists to explore distant relationships among human languages. This led to the establishment of a research project at the Santa Fe Institute that focuses on the origins, evolution, and diversity of human languages.
A self-described perfectionist when it came to language, Gell-Mann, was not satisfied with the written version of his Nobel lecture and did not submit it for publication. “I tried to write a better one, including an adequate discussion of quarks, and agonized over it for months,” he told Physics World in an interview in 2003. “But in the end, I did not finish it in time for it to be included in the volume.” Indeed, the publication of that Physics World interview was itself delayed by many months as Gell-Mann agonized over the proofs.
In the same interview, Gell-Mann admitted that he often had trouble with writing projects. “On many occasions it has delayed my writing up research, often by a year or more. By that time, it has sometimes happened that another theorist has had a similar idea and has written it up more quickly.” Indeed, in 1994, Gell-Mann published The Quark and the Jaguar – his only popular science book – in which he wrote about his ideas on simplicity and complexity. It too was apparently hit by delays, and failed to sell well.
Gell-Mann was also highly critical of how others wrote about him including his Caltech colleague Richard Feynman – who failed to mention Gell-Mann’s contributions to the development of the theory of the weak nuclear force in the first edition of his best-selling book “Surely You’re Joking, Mr. Feynman!”. Gell-Mann was also upset at how he was portrayed by the science writer George Johnson in his widely-acclaimed biography of Gell-Mann entitled Strange Beauty. “He got many things wrong about physics, about my family and my personal life, and about my motivations in writing papers the way I did. I could so easily have set him straight,” said Gell-Mann.
“As with most any book of such length and complexity, there were some errors — most very minor and all corrected in later printings and in an errata sheet on my website,” Johnson told Physics World. “You can be sure that Murray made me suffer over every one.”
Gell-Mann won many other prizes during his career including sharing the Ettore Majorana “science for peace” Prize in 1989 and the Albert Einstein Medal in 2005 that is awarded by the Albert Einstein Society in Bern. He also served on the US President’s science advisory committee between 1969 and 1972 and on the US President’s committee of advisors on science and technology from 1994 to 2001.
My first encounter with lithium was in chemistry class, watching a lump of dull-grey metal gently fizz and disappear in a beaker of water. My second encounter was in the atomic physics laboratory, building a machine that made light, zippy atoms of lithium vapour slow down enough so that they could be cooled and trapped with lasers. Lithium, as I quickly learned, is an atomic physicist’s dream, amenable to the tricks of the atom-trapper’s trade and usefully abundant in both fermionic (lithium-6) and bosonic (lithium-7) isotopes. I have spent many happy hours contemplating its properties, and many more contemplating those of its alkali cousins, rubidium and caesium, which I studied during my PhD.
Even so, when I told a friend of mine that I planned to write about lithium in Physics World’s IYPT contest, she immediately replied, “That’s my element.” Like millions of people worldwide, my friend has bipolar disorder. Without treatment, she is prone to cycling between manic highs and depressive lows. But for reasons that are still mysterious despite decades of research, a daily dose of lithium has an almost magical effect on many individuals with bipolar, stabilizing their moods and making it possible for them to live healthy, happy lives. In my friend’s case, lithium means that she spends time in mental institutions as a clinician, not a patient, and that she is “crazy” only in the fun, positive sense of the word. It is no exaggeration to say that she owes her health, and possibly even her life, to this unassuming little element.
The power of lithium also manifests itself in other ways. Even if you’ve never been prescribed it by a doctor, chances are that you, too, rely on lithium in your daily life. Some of the same properties that make lithium fizz in a beaker of water and zoom about in a vacuum chamber also make it well-suited for storing energy. Smartphones, portable electronic devices and most electric vehicles run on rechargeable lithium-ion batteries, which use the ions as fast, mobile charge carriers and at least one lithium compound in their electrodes. Solid lithium-metal batteries are also replacing batteries containing lead, cadmium or mercury in many applications, reducing hazardous waste. In short, this light, reactive, relatively non-toxic element is behind two of the most important trends in modern history: the increase in portable computing power and the shift away from energy sources that poison our land, air and water while playing merry hell with our climate.
Lithium isn’t perfect. Poorly-designed or poorly-maintained lithium-ion batteries are prone to exploding (and are banned in airline hold luggage for that reason). As a drug, lithium carries an increased risk of birth defects and kidney damage. But in a flawed world where neither patients nor the environment can afford to wait for something better, lithium is an absolute godsend. As a playground for atomic physicists, a progenitor of better mental health, and a bridge to a cleaner, greener planet, therefore, lithium gets my vote.
Which is your favourite element? Contact us at pwld@ioppublishing.org with your pick – and the reason why – or via Twitter using the hashtag #battleofelements.
When the physicist Steve Haake began his career as a “sports engineer” in the early 1990s, he was greeted by “an embarrassed silence”. His PhD on the impact of golf balls on golf greens was sneered upon by his academic colleagues, for whom the physics and technology of sport was deemed a trivial and niche activity. But as Haake admits in Advantage Play: Technologies that Changed Sporting History, the “real world” was delighted, and he began giving popular lectures to schools and professional societies.
Now based at Sheffield Hallam University in the UK, where he has built up what he claims is the world’s biggest academic sports-engineering research group, Haake’s work has stretched far beyond golf, to include the mechanics of footballs, the aerodynamics of sledges, the traction of sports boots and more besides. A regular in the media, he once even made a series of videos with Physics World on the physics of running, swimming and cycling. But as Haake emphasizes, people have been using technology to boost sporting performance for centuries; it’s only in recent years that sports engineering as a field of research has emerged.
In Advantage Play, Haake seeks to take a chronological approach to describing technological changes to sport. He starts with ancient Greek athletes, who used “halteres” – special hand-held weights – to jump further, before exploring everything from bike designs and swimsuits to prosthetic limbs and wheelchairs. Haake’s intention is to introduce new sports as they were created over time, but that narrative ploy gets rather lost – the book often switches gear unexpectedly between history, sports trivia and the author’s own work. It’s almost as if Haake has too much to say; stories start promisingly before being abruptly abandoned for other lines of thought. Haake’s message is that although people are often ambivalent about new technology – sometimes dismissing it as “cheating” – it drives sporting progress. Starting blocks for sprinters, for example, which were introduced in the 1930s, are today an essential part of athletics and no-one would dream of banning them. Indeed, the blocks now include mechanical strain gauges to check no-one’s started running too early. So despite being light on basic scientific principles, I hasten to add that Advantage Play will appeal to sports-mad physicists, who relish seeing how physics underpins much of sport.
Researchers from the Mayo Clinic have described methods for using particle therapies, such as carbon-ion therapy, to treat abnormal heart rhythms by targeting and ablating their source (WO/2019/046732). The technique can be used to treat atrial fibrillation and ventricular tachycardia, for instance, as well as hypertension, seizures and gastrointestinal maladies. The accelerated particles can create tissue lesions sufficient to eliminate abnormal heart rhythms, and could also be safer for patients by avoiding organ damage that occurs, for example, with catheter-based ablation. Contouring and gating may be used to account for cardiac and respiratory motion, helping to reduce collateral damage.
A team at the University of Wisconsin–Madison has developed a method for treating one or more malignant solid tumours using targeted radiotherapy (TRT) to drive anti-tumour immune response to immunotherapies (WO/2019/094657). The treatment first involves systemically administering an immunomodulatory dose of a TRT agent, such as a radioactive metal chelate compound, radiohalogenated compound, radiolabelled antibody or a radioisotope that’s differentially taken up by and retained within solid tumour tissue. Next, immunotherapy is performed via systemic administration of an immunostimulatory agent, such as one or more immune checkpoint inhibitors.
Laser-based design enhances ion beam generation
HIL Applied Medical has published details of methods for generating ion beams, including proton beams (WO/2019/074497). Such systems may incorporate a beam of electromagnetic radiation directed onto an ion-generating target to create a proton beam. A detector measures at least one laser–target interaction property, which is used by a processor to produce feedback to adjust the proton beam. The system may also include an electromagnet and an automated switch to filter the energy of a pulsed ion beam and/or provide pulsed ion radiation at desired times. According to the filing, such systems reduce the size, complexity and cost of proton beam generation, while improving speed, precision and configurability. Within proton therapy, the systems enable shorter treatment times, higher patient throughput, more precise treatment of the desired areas and less damage to healthy tissue.
Biofeedback enables self-positioning during radiotherapy
Opus Medical has invented a patient-guided stereoscopic surface imaging and biofeedback system for self-positioning during radiotherapy (WO/2019/072950). The set-up includes a patient couch mounting system, an array of at least two imaging sensors (a camera, or infra-red or ultrasound imager) for imaging the patient at multiple viewing angles and a viewing screen that displays images from the sensors. The sensors, viewing screen and controller are configured to output 3D surface information of the patient under test, extrapolated 2D patient position information and 1D patient position information. The screen, which displays patient position boundary markers overlaid on the images from the sensors, is positioned to be viewable by the subject on the patient couch, to provide biofeedback during radiotherapy. This biofeedback informs the patient of a correct position to adjust to and to maintain during treatment.
Keeping track of cavitation during focused ultrasound
INSIGHTEC has devised methods for accurate detection of cavitation signals from a target during a focused ultrasound procedure, without requiring large numbers of cavitation detectors. Such monitoring can help minimize any undesired effects of microbubble cavitation (WO/2019/058171). The approach involves placing a limited number of cavitation detection devices in contact with the patient’s scalp at regions with high transmission efficiency for cavitation signals. The transmission efficiency for each scalp region is computed based on the predicted beam path from the target through the skull and scalp, and anatomical characteristics of the scalp and/or skull in the intercepted regions, which may be acquired via imaging. The detector locations may also be selected by considering characteristics such as the geometry of the scalp and skull.
Superconducting cyclotron enables compact proton therapy system
Hefei CAS Ion Medical and Technical Devices of China has designed a compact proton therapy system comprising a superconducting cyclotron, an energy selection system, a beam transport system, a dedicated treatment room and a rotatable machine support frame (WO/2019/071977). The superconducting cyclotron delivers a fixed-energy proton beam that is converted by the energy selection system to adjust the beam energy from 70 to 200 MeV, thereby achieving longitudinal beam range adjustment during tumour treatments. The superconducting cyclotron, energy selection and beam transport systems, and the treatment head cooperate to control and achieve transverse expansion of the proton beam, enabling conformal and intensity-modulated radiation therapy.
A cheap, safe, and effective potassium-ion battery system with promising characteristics has been described in a Nature Energy article by Yaxiang Lu, Yong-Sheng Hu and co-workers in Beijing, bringing the renewable energy grids of the future closer to the realization.
Green energy storage
One of the obstacles to total green energy reliance is that many sources of carbon-neutral energy, such as solar and wind power, are unpredictable and intermittent. A possible solution is to build energy storage facilities that can charge up while excess energy is being generated, then discharge when demand overtakes supply.
Battery-based grid-storage facilities using a range of battery types have already been built for this reason, but there is still a need to develop an economical, safe, and long-term solution. Li-ion systems such as the 129 MWh Hornsdale power reserve are the current state of the art, benefiting from high energy density of Li-ion batteries. Na-S cells are a close competitor (e.g. the 300 MWh Buzen substation), having excellent energy density and low cost. Both of these battery types use highly flammable parts which increases their cost due to safety considerations. The main goal is optimizing the cost per MWh over the whole lifetime of the battery, for which non-flammable aqueous battery systems are a tempting prospect. They may also be manufactured more cheaply on larger scales than other batteries, which require rigorously dry conditions.
Novel materials
The cathodes in the cells Lu and Hu et al. demonstrated are more stable than in other systems, retaining 90% of their energy storage capacity after 10,000 cycles. One reason for their stability is that part of the distortion prone manganese in the Prussian Blue (KxFeyMn1 − y[Fe(CN)6]) material is substituted with iron. Another reason is the use of an electrolyte containing more potassium salt than water, to inhibit the dissolution of the cathode material over its long life. The anode used in the cells, an organic paint pigment (PTCDI), also has the potential to be manufactured cheaply.
The most remarkable feature of the cells was their tolerance of high rates of charging and discharging, comparable to Li-ion battery performance, without losing much of their capacity. Although the overall energy density of the cells is moderate, due to their comparatively low voltage of 1.3 V, there is potential for optimization in this system, both in increasing the voltage by adjusting the cathode metal and anode composition, and by lowering material costs to produce a very low cost per MWh system.
A new passive radiative “cooling wood” that reflects infrared radiation could reduce the energy costs associated with cooling buildings by between 20 and 60%. The material, which is more than eight times stronger than natural wood, is made by removing the lignin from wood and then compressing the delignified structure.
“Buildings account for more than 40% of the total energy demand and 70% of electricity use in the US, leading to an annual energy bill of more than $430 billion,” explain Liangbing Hu of the University of Maryland and colleagues. “Heating and cooling accounts for roughly 48% of this energy use, making it the largest individual energy expense.”
Cooling a building is generally more difficult than heating it. Passive radiative cooling materials, which cool a structure by deflecting incoming solar radiation and dissipating heat with no external energy input, have come along in leaps and bounds in recent years, but these can be difficult to manufacture on a large scale.
Wood has been used in construction for thousands of years and has recently emerged as an important sustainable building material that could replace steel and concrete. Hu and colleagues engineered their cooling wood by completely removing the element that absorbs solar radiation, lignin, from it.
Cellulose nanofibres strongly emit light in the infrared
Scanning electron microscopy images reveal that the wood contains cellulose nanofibres that do not absorb in the visible part of the electromagnetic spectrum. These multiscale fibres and the channels between them in fact act as random and disordered light scattering elements at all visible wavelengths. At the same time, the molecular vibration and stretching of cellulose molecules in the wood strongly emit light in the infrared at all angles from the surface of the material in the so-called first and second atmospheric transparency windows of 8 to 13 μm and 16 to 25 μm respectively. The result is that the heat flux emitted by the material is greater than the absorbed solar irradiance, which leads to passive cooling.
A nanoscale view of the wood’s natural nanostructures. Credit: University of Maryland
The researchers measured the Fourier transform infrared absorbance of their wood and found that the strong infrared emission at wavelengths of 8 to 13 μm comes mainly from the complex infrared emission of OH groups on the cellulose together with C-H, C-O and C-O-C stretching vibrations between 770 and 1250 cm-1. The strongest IR absorbance by OH and C-O centres is at around 1050 cm-1 (9 μm).
Good radiative cooling powers
The team, which includes scientists from the University of Colorado Boulder, the University of California Meced and Huazhong University of Science and Technology in Wuhan, tested the performance of the wood in Cave Creek, Arizona, using continuous thermal measurements on samples measuring 200 x 200 mm placed 12 metres above ground (to avoid heat conduction from the ground to the samples).
The researchers found that the wood had radiative cooling powers of 63 and 16 W/m2 during the night and daytime (between 11 o’clock in the morning to 2 pm) respectively. This leads to an average cooling power of 53 W/m2 over the 24-hour period.
Continuous sub-ambient cooling
“Our wood is capable of continuous sub-ambient cooling – that is, keep its surface cooler than ambient air temperature,” says Hu. “It also features an exceptional tensile mechanical strength of as high as 404.3 MPa and a toughness of 3.7 MJ/m3, values that are, respectively, eight and 10 times tougher higher than those of natural wood.”
The material also has a specific tensile strength (the ratio of mechanical strength to weight) of 334.2 MPa cm3, which is higher than that of most structural materials, including Fe-Mn, Al-C steel, magnesium, aluminium alloys and titanium alloys, he adds. It requires no energy input either and could thus be a sustainable material for improving the energy efficiency of buildings.
The new building material that rejects heat. Credit: University of Maryland
Modelling potential energy savings
The researchers, reporting their work in Science 10.1126/science.aau9101, modelled the potential energy savings of using their cooling wood on the exterior surfaces of buildings using the whole building energy simulation program EnergyPlus version 8 and parameters listed in their paper. This model accounts for a total heat balance on both the internal and external building enclosure surface, the heat transfer through the building enclosures and heat sources and sinks, they explain.
The building models employed in this study are midrise apartments in 16 cities in the US and include both old (built before 1980) and new (built after 2004) buildings. The energy modelling process established a baseline energy consumption for these buildings and then recalculated this by supposing that the building materials had, in part, been replaced by the cooling wood.
We determined the total cooling energy-saving patterns for the selected 16 cities and the percent savings relative to the baseline and found that found that an average of ~35% and ~20% in cooling energy savings can be obtained for old and new midrise apartment buildings, respectively, say the researchers.
The energy savings that come from installing the cooling wood on the exterior surfaces of these buildings show that Austin (22.9 MJ/m2), Honolulu (28.2 MJ/m2), Las Vegas (21.1 MJ/m2), Atlanta (17.1 MJ/m2), and Phoenix (32.1 MJ/m2) would have the highest energy savings among the selected 16 cities. Phoenix had the highest because of its hot and dry climate.
“We conclude that the Southwest may be the area to benefit the most from this material to reduce energy consumption for cooling,” says Hu. “Indeed, UMD spinoff company Inventwood LLC in Maryland is now commercializing this technology,” he tells Physics World.
