Did You Know?

Stelios Mores looks at the latest developments in refrigeration and lithium batteries plus a Young Einstein experiment

Twisted-fridge.jpg

Cool Twist

Fridges and freezers have been around since ancient times. Apart from the direct use of naturally found ice, usually held in snow and ice pits and typically insulated by grasses and branches, the ancient Egyptians placed boiling water in shallow earthen pots on the roofs of buildings at night. The pot exteriors were moistened throughout the night such that the evaporation cooled the water held within. Ancient societies appear to have used such primitive refrigeration techniques for mostly cooling drinks, whilst its use for preserving food began a lot later in the early 1800s.  

The first true refrigeration machine was developed in 1755 by William Cullen whilst he was at Edinburgh University. The technique applied a vacuum to a flask of ether which boiled as a result, removing heat from itself and the flask. Since those early days, the concept of removing heat through evaporation has been the main principle that is still used in today’s modern fridges and freezers. Most of today's appliances are based on the refrigeration cycle, which uses a compressor, a condenser, an expansion valve and an evaporator. This was first developed by the American inventor Oliver Evans in 1805.

This technique for cooling is not the only one. Recently the use of thermoelectric cooling has become prevalent in cooling electronic components within our ever more complex electronic devices. Unfortunately, this technology is extremely inefficient as a means of refrigeration and is only used where the refrigeration cycle approach is impractical.

Lately, however, a team of scientists led by Zunfeng Liu at Nankal University in China developed a totally new method for cooling. Unlike the refrigeration cycle or thermoelectric methods, this approach is entirely mechanical and relies on the expansion and contraction of solids. Based on the fact that rubber heats up when stretched and cools down when relaxed, they have developed Supercoiled Fibres composed of different materials including nickel-titanium wires and polythene.  

These are capable of extremely efficient cooling by being twisted and untwisted repeatedly, bringing down temperatures by over 20°C. In time they expect to improve on this, making this technique extremely attractive for the development of fridges and freezers which will be both highly efficient and, most importantly, free of the refrigerants which are greenhouse gases. So further work on these materials is expected to provide a cool twist to the centuries-old story of refrigeration. 

Hot Batteries

Lithium Ion Battery - Schematic.jpg

The first ever lithium-based batteries were developed by Stanley Whittingham in the 1970s whilst he was working for Exxon. These initial attempts relied on the use of titanium (IV) sulphide and lithium metal, which, despite being both impractical and particularly hazardous, spearheaded the development of lithium ion (Li-ion) batteries and the mobile communications revolution which depended heavily upon them.

Today the use of Li-ion batteries in electric and hybrid vehicles is transforming our personal transportation choices, with the intent of ultimately providing us with cars which might one day rely entirely on renewable energy. As a result Stanley Whittingham, John Goodenough and Akira Yoshino won the 2019 Nobel Prize in Chemistry for their contributions to the development of the Li-ion batteries. Today, efforts in developing lighter and more powerful Li-ion batteries involve tens of thousands of scientists and engineers throughout the world.

Since those initial efforts, Li-ion battery chemistries have evolved immensely, but the basic principles remain much the same. As such Li-ion batteries consist of three basic components, these being the positive and negative electrodes and the electrolyte. At present there are numerous competitive Li-ion battery technologies based on variations of the original chemistry.  Most mobile devices, for instance, use lithium polymer gels and lithium cobalt oxide (LiCoO2) as the cathode. These are capable of storing large amounts of energy in the smallest volumes (high energy density) but present significant safety risks as they can potentially burst into flames if their integrity is compromised. Other chemistries include lithium manganese oxide (LiMn2O3/LiMn2O4), lithium nickel manganese cobalt oxide (LiNiMnCoO3) and lithium iron phosphate (LiFePO4). These are used in applications that require greater safety such as electric vehicles and medical devices but have lower energy densities and are therefore likely to require recharging more often. So as always in most applications a compromise has to be reached.

One aspect of lithium battery usage which has been researched extensively is the tendency for the batteries to generate heat when being charged and discharged. This has been one of the technology’s main constraints, since this can lead to lithium batteries heating up so much as to become unsafe. Thankfully, most lithium batteries are designed to shut down if their temperature gets too high when used normally, and so failures are usually the result of damage through abuse and misuse. Consequently, efforts are always made to ensure that batteries are kept at room temperatures when being used. This can be achieved by controlling their temperature through careful product design and low current use, or by actively removing the heat generated through additional cooling.

With the growth of the electric vehicle industry, the problems of safety facing chemists and technologists sit at odds with the demands of the technology which requires extremely rapid charging and discharging. In fact, it is considered that the success of the electric vehicle industry will depend significantly on allowing drivers to rapidly charge their vehicles (within a few minutes) as well as be able to sustain fast discharge (such as when overtaking) without overheating.  Evidently maintaining a battery at an ambient temperature can be extremely challenging, such that this aspect is a major consideration in the design of electric vehicles. 

Recently, however, researchers at Pennsylvania State University discovered that perhaps a rise in temperature is not all bad. It seems that a consequence of a raised battery temperature is to enable it to charge and discharge more rapidly, as this speeds up the battery chemistries, as well as increase battery longevity by almost thirtyfold. So it seems that fifty years on from the discovery of the lithium battery and the Nobel prize won by its protagonists, it is still a hot topic of research and development.

Einstein Cartoon - Beige.jpgLittle Einstein’s Corner - Invisible Water
Water can evaporate so that it changes from a liquid to a vapour or a gas. It can also condense and change from a vapour or gas back to a liquid. In this experiment we will see how water evaporates. You will need the following:

1. Two transparent plastic cups
2. A marker
3. Cold water
4. Hot water
5. A clock or watch
6. A notepad and pencil

First mark one cup with the letter ‘C’ and the other with an ‘H’.  Take cup ‘C’, fill it ¾ full with cold water and mark the water level with the marker. Do the same with cup ‘H’, but this time use hot water from a hot tap. Place the cups side-by-side in sunlight (if you can) on a window sill. Use your clock to note the time in your notepad and every five minutes check the cups, marking the water level on each every time. As time passes you will notice that the water level drops as it evaporates. Do you notice any difference between the cold and hot water?
 

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