These clean technologies could tackle cooling’s catch-22 problem

Cooling technologies are vital for human survival in a warming world. They do not just keep us comfortable – they protect food and vaccines, keep digital infrastructure running, and ultimately limit deaths when the mercury rises. Cooling has a ‘catch-22’ problem, however – as air conditioning, refrigerators and other technologies lower local temperatures they cause huge amounts of emissions, ultimately contributing to a long-term problem with a short-term solution.

‘Clean cooling’ could offer a better way of doing things. Here, Dr Tim Fox, chair of IMechE’s working group on climate change adaptation, sets out some technologies that could improve energy efficiency, sustainability and cooling performance in the built environment.

• Radiative cooling: This involves using specialised materials to emit thermal radiation and dissipate heat from objects. These materials (such as panels developed by SkyCool Systems) can radiate heat to the colder atmosphere, even in the presence of sunlight. Radiative cooling has potential applications in passive cooling systems for buildings.


• Phase change materials (PCMs): PCMs are substances that can store and release thermal energy during the process of melting and solidifying. They could revolutionise cooling systems by providing efficient and passive cooling solutions. PCMs can be integrated into building materials or thermal storage units to absorb excess heat during the day and release it at night (when it is typically cooler), reducing the need for active cooling.


• Thermoelectric cooling: This includes using the Peltier effect to transfer heat when an electric current flows across two different conductive materials. This technology is already in use in certain applications, but ongoing research aims to improve its efficiency and make it more suitable for broader cooling needs, such as air conditioning and refrigeration.


• Electrocaloric cooling: This is a solid-state cooling technology that utilises an electric field to induce changes in the temperature of certain materials. When an electric field is applied, the material heats up, and when the field is removed, it cools down. This process can be cycled to provide cooling.


• Artificial intelligence (AI): AI can be integrated into cooling systems to optimise their performance by predicting cooling needs, adjusting settings in real time and identifying areas where efficiency can be improved.


• Desiccant-based systems: Desiccant-based systems are also being developed as one alternative to overcome the shortcomings of using vapour compression refrigeration to control humidity as well as temperature (particularly important in regions with high humidity). Advantages include more accurate humidity control, better indoor air quality and higher coefficient of performance.

   

  System-level modifications are being researched to enhance the performance and efficiency of both liquid and solid desiccant-based technologies. In the case of the former, a shift from traditional ‘packed bed’ configurations to membrane-based designs is underway. Packed bed systems face limitations, such as a high pressure drop, channelling, and difficulties in heat and mass transfer. The alternative approach uses specially designed membranes that selectively allow the passage of water vapour, while retaining the desiccant solution. This advancement avoids liquid desiccant carryover with air, reduces pressure drop losses and provides better control.

  For solid desiccant systems, the focus is on moving from desiccant wheel technology towards using coated heat exchanger designs. This approach allows for efficient moisture adsorption and regeneration, offering advantages such as compact size, improved control over the dehumidification process, reduced maintenance and greater operational flexibility.

Several options are available for adapting refrigeration equipment and systems to increased seasonal ambient temperatures and more frequent and prolonged extreme heat events. These include the following:

• Reducing the cooling load. This can include using more effective insulation, exploiting sources of ‘free cooling’ for chilled products with higher temperature tolerances, reducing heat ingress though storeroom doors and minimising other heat load gains. In some cases, refrigeration set-point (target) temperatures could be increased in frozen stores. Processes such as ‘superchilling’ might also provide some opportunities, extending product shelf life compared with traditional chilling but operating at much higher temperatures than those applied for frozen foods.


• Correct system design. Options here can include maximising the suction pressure and, if appropriate, splitting cooling loads into different suction pressure levels. Other considerations include reducing high pressures with precise location of condensers, ensuring that heat exchangers are accessible and can easily be cleaned, and considering two-stage or economised cycles.


• Designing for higher temperatures. Higher temperatures can be tackled by using larger heat exchangers (though this adds costs and increases the onsite space requirements); applying evaporative or adiabatic condensers (if benefits are apparent); deploying passive cooling techniques such as installing condensers in shaded areas or locations with good air flow as well as using windcatchers for space cooling; and ensuring that defrosting systems can cope with higher defrost loads.


• Correct control philosophy. This includes optimising defrosts and lighting, and avoiding fixed head pressure control and fixed speed auxiliaries.


• Correct operation and maintenance. Considerations include repairing refrigerant leaks so that correct charge level is maintained; repairing door gaskets and seals; and ensuring heat exchange surfaces are clean. Dirt and debris build-up on external heat exchanger surfaces can have a dramatic effect on the equipment’s heat transfer performance. Poor maintenance can easily increase the condensing temperature or reduce evaporating temperature by several degrees, increasing energy use by 2-10%. There may also be opportunities to move electricity consumption to periods of low grid demand.

Find out more in The Hot Reality: Living in a +50°C World, a report authored by Dr Fox, Dr Leyla Sayin and Professor Toby Peters, featuring contributions from more than 35 subject matter experts around the world.

Want the best engineering stories delivered straight to your inbox? The Professional Engineering newsletter gives you vital updates on the most cutting-edge engineering and exciting new job opportunities. To sign up, click here.

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