FEATURE: Twisting fibres could untangle the secret to energy efficient cooling

Andrew Williams

As the world gets hotter, engineers are racing to develop efficient cooling technologies that won't make global warming worse
As the world gets hotter, engineers are racing to develop efficient cooling technologies that won't make global warming worse

Despite decades of incremental improvements in refrigeration technology, the cooling and air-conditioning sector still accounts for around a fifth of global electricity consumption and remains a significant contributor to global warming.

Higher temperatures around the world are also prompting increased demand for refrigeration and cooling, particularly in developing countries. 

In an effort to solve these problems, a number of organisations around the world are focused on the development of more energy-efficient refrigeration technologies – and there are signs of a coordinated global effort to deliver more efficient cooling systems in a more sustainable and equitable manner.

Twistocaloric cooling

One of the most interesting recent developments is a breakthrough at the University of Texas in Dallas and Nankai University in China, where researchers have discovered a technology for refrigeration based on twisting and untwisting fibres. The method, known as twistocaloric cooling, works by twisting a yarn or fibre, causing an increase in its temperature, which is used to heat a surrounding flowing liquid such as water. Untwisting causes the temperature of the yarn or fibre to decrease, in the process cooling a surrounding flowing liquid. The team has already successfully demonstrated twist-based refrigeration with materials ranging from natural rubber and ordinary fishing line to nickel titanium wire – and published the results in the journal Science. Notably, rapid release of the twist in rubber fibres has resulted in surface temperature cooling of 15.5°C – and release of both the twist and the stretch from rubber resulted in even higher cooling of 16.4°C.

Dr Ray Baughman, director of the Alan G MacDiarmid NanoTech Institute at the University of Texas in Dallas, explains that all methods of cooling use a process that results in an entropy increase, meaning an increase in what he describes as the “degree of disorder”. A common example of this is the entropy increase caused by the evaporation of a liquid as part of the cooling process used in conventional refrigerators. In an effort to overcome these limitations, twistocaloric cooling uses the entropy increase resulting from untwisting a yarn or fibre for refrigeration – with cooling resulting from the entropy increase owing to twist release.

Baughman describes the high material cooling efficiencies demonstrated for these novel twist fridges as “an important advance” – especially in view of the fact that conventional refrigeration “consumes about 17% of global electrical energy and releases gases that are a major contributor to global warming.

“Even when upscaled, our twist fridges are predicted to be smaller and lighter in weight than the cooling units of conventional refrigerators,” he says.

Cooling applications

In Baughman’s view, there are a number of key benefits to using twistocaloric cooling as a refrigeration technology. To begin with, he points out that, while commercially available refrigerators have an energy conversion efficiency of around 60% – after over a century of investigation – he and his team have already demonstrated a high material cooling efficiency of 67% for twist coolers (if the unused mechanical energy were recovered). 

“Our twist fridges are also expected to be much lighter and smaller than conventional liquid-compression-based refrigeration units,” he says.

“Since we only recently invented our twist fridges, they are not yet commercially applied. While they are potentially upscaleable to meet any refrigeration need, early applications are expected for small-scale device needs, like cooling electronic circuits,” he adds.

Baughman (left) describes the novel twist fridges as 'an important advance'

Baughman (left) describes the novel twist fridges as 'an important advance'

On a more cautious note, Baughman is keen to stress that, despite the clear potential of the new technology, a number of challenges and opportunities “exist on the path from these initial discoveries to commercialisation of twist fridges for diverse large- and small-scale applications. 

“Among the challenges are the need to demonstrate refined devices and materials that provide application-targeted cycle lifetimes, and even higher efficiencies,” he says.

“The opportunities include using performance-optimised twistocaloric materials, rather than the few presently studied commercially available candidate fibres and yarns.”

International collaboration

Another interesting recent development is the establishment of the Centre for Sustainable Cooling (CSC), which brings together a collaboration of international academic institutions to work with governments, industry, development agencies and NGOs to solve the challenge of how best to facilitate access to sustainable cooling for all who need it. As CSC member Toby Peters at Birmingham University explains, through this global collaborative coalition, the centre will develop new systems approaches integrating technological, policy, social, economic, energy, finance and business pathways to “better manage cooling demand and deliver sustainable solutions to help the most vulnerable in our society”.

According to Peters, conventional cooling technologies such as refrigeration, air conditioning and fans account for more than 10% of all global greenhouse gas emissions and more than 13 new cooling devices are deployed per second. In addition, as the world gets hotter, demand for cooling will increase – resulting in what he describes as a vicious circle of rising emissions. Despite this growth, he points out that over one billion people face immediate risks from lack of access to cooling, which threatens sustainable development goals relating to health (including deaths from heat extremes and ineffective vaccines), food security (including food waste owing to lack of cold chains), clean energy, labour productivity, sustainable cities, and gender equality. A recent study also suggests that, if global warming continues unchecked, the heat levels predicted later this century in some parts of the world will bring “nearly unliveable” conditions for up to 3bn people.

Soaring demand

“Without radical intervention, if we are to deliver access to cooling for all who need it, we will potentially see four times as many appliances deployed using five times as much energy as today. How do we meet this challenge and provide cooling for all without overheating the planet?” asks Peters.

“The Global Cooling Prize is an outstanding example of what can be achieved – an 80% lower climate impact than the mainstream devices currently being deployed in countries like India. Now we need to see regulations and standards drive these breakthrough technologies through to market,” he adds.

In Peters’ view, an equally fundamental challenge is that “when people talk about energy, they often mean electricity, and when they talk about energy storage they mean batteries”. For him, this blurring of concepts matters because it fails to recognise some basic energy facts of life – that a large slice of our consumption comes in the form of thermal energy; that one of the fastest-growing sources of energy demand over the next 20 years, if not this century, will be for cooling; and that cooling would often be “better served by energy carriers other than electricity and batteries”.

“We are finally seeing a change in the question to ‘what is the service we require, and back-casting how can we provide it in the least damaging way’ – rather than ‘how much electricity do I need to generate?’ Within this we are seeing recognition of the value of thermal energy storage including such a simple technology as using ice,” he says.

Reshaping cooling provision

Looking ahead, Peters believes that clean cooling starts with what we can do today to reduce demand and deliver immediate efficiency improvements – including more effective use of shade and natural ventilation, painting roofs white and putting doors on chillers in supermarkets “through to installing best-in-class refrigeration and air-conditioning equipment”. However, although these interventions are important, he stresses that, given the anticipated growth in cooling demand, they will “not deliver the required reductions in energy usage, emissions and pollution, nor will they ensure access to cooling for all”.

In recognition of this fact, he points out that the delivery of sustainable, energy-efficient and equitable cooling is about “investing in a radical reshaping of cooling provision thinking about how we use, make, store, move and finance cooling”. It also requires a deep understanding of the nature and extent of multiple cooling needs and the locations of what he describes as “free, waste-based and wrong-time energy resources available to help meet these needs” – as well as defining the right mix of “design, novel energy vectors, thermal stores, cooling technologies underpinned with fit-for-market business models and policy interventions to optimally integrate the available resources through self-organising systems”.

“In short, thinking systems and thinking thermally,” he says.

Ultimately, Peters argues that, in order for a government to ensure that the cooling needs of its population are met equitably and sustainably, including for the most vulnerable, it first needs to understand what these needs are – namely health, food productivity and safety. 

“For example, how much cold-chain would be required to enable farmers’ incomes to double by 2025, or to end hunger and malnutrition? How much cold-chain is needed to provide access to vaccines? And not just for newborns, children or at-risk groups, such as the elderly, but broader demand in the event of pandemics and epidemics. Or how many degrees of cool comfort are required to avoid heat stress at home or in the workplace?” he says.

Sustainable solutions

“In short, what needs must be met in a country to meet sustainable development goals with no one left behind? An underestimation of the scale of the cooling demand and its impact on energy demand risks contributing to a lack of ambition in policy, infrastructure and technology development, and could ultimately have far-reaching social, economic and environmental consequences,” he adds.

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.


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