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Making a case for organic Rankine cycles in waste heat recovery

A team from City, University of London’s Department of Engineering believes that a new approach to generating energy through waste heat could yield important insights into delivering environmentally-friendly power.

In this recent paper, Making the case for cascaded organic Rankine cycles for waste-heat recovery, published in the Energy journal, Dr Martin White has identified optimal single-stage and cascaded organic Rankine cycle systems (ORC) to maximise performance, and has designed accompanying heat exchangers.

The ORC is based on the principle of heating a liquid which causes it to evaporate, and the resulting gas can then expand in a turbine, which is connected to a generator, thus creating power. Waste heat to power organic Rankine cycle systems can utilise waste heat from a range of industrial processes in addition to existing power generation systems.

A cascaded ORC system is essentially two ORC systems coupled together, with the heat that is rejected from the first ORC being used as the input heat for the second.

However, in developing his model of a cascaded ORC system, Dr White hastens to add that there is a trade-off between performance and cost — in the case of the heat exchangers deployed, the general rule is that the better the performance, the larger and more costly the heat exchangers.

He says the trade-off can be explored through optimisation and the generation of what is called a ‘Pareto front’ — a collection of optimal solutions that considers the trade-off between two things.

If quite large heat exchangers (in this specific case, greater than around 200m2), were affordable, then for that amount of area, it is possible to generate more power with a cascaded system than a single-stage system.

However, if the size of the heat exchangers was restricted, one would probably be better off with a single-stage system.

Dr White’s results suggest that in applications where maximising performance is not the primary objective, single-stage ORC systems remain the best option. However, in applications where maximised performance is the goal, cascaded systems can produce more power for the same size heat exchangers.

His paper emerged out of his work on the NextORC project, funded by the Engineering and Physical Sciences Research Council (EPSRC).

New green materials could power smart devices using ambient light

We are increasingly using more smart devices like smartphones, smart speakers, and wearable health and wellness sensors in our homes, offices, and public buildings. However, the batteries they use can deplete quickly and contain toxic and rare environmentally damaging chemicals, so researchers are looking for better ways to power the devices.

One way to power them is by converting indoor light from ordinary bulbs into energy, in a similar way to how solar panels harvest energy from sunlight, known as solar photovoltaics. However, due to the different properties of the light sources, the materials used for solar panels are not suitable for harvesting indoor light.

Now, researchers from Imperial College London, Soochow University in China, and the University of Cambridge have discovered that new green materials currently being developed for next-generation solar panels could be useful for indoor light harvesting. They report their findings today in Advanced Energy Materials.

Co-author Dr Robert Hoye, from the Department of Materials at Imperial, said: “By efficiently absorbing the light coming from lamps commonly found in homes and buildings, the materials we investigated can turn light into electricity with an efficiency already in the range of commercial technologies. We have also already identified several possible improvements, which would allow these materials to surpass the performance of current indoor photovoltaic technologies in the near future.”

The team investigated ‘perovskite-inspired materials’, which were created to circumvent problems with materials called perovskites, which were developed for next-generation solar cells. Although perovskites are cheaper to make than traditional silicon-based solar panels and deliver similar efficiency, perovskites contain toxic lead substances. This drove the development of perovskite-inspired materials, which are instead based on safer elements like bismuth and antimony.

Despite being more environmentally friendly, these perovskite-inspired materials are not as efficient at absorbing sunlight. However, the team found that the materials are much more effective at absorbing indoor light, with efficiencies that are promising for commercial applications. Crucially, the researchers demonstrated that the power provided by these materials under indoor illumination is already sufficient to operate electronic circuits.

Co-author Professor Vincenzo Pecunia, from Soochow University, said: “Our discovery opens up a whole new direction in the search for green, easy-to-make materials to sustainably power our smart devices.

“In addition to their eco-friendly nature, these materials could potentially be processed onto unconventional substrates such as plastics and fabric, which are incompatible with conventional technologies. Therefore, lead-free perovskite-inspired materials could soon enable battery-free devices for wearables, healthcare monitoring, smart homes, and smart cities.”