Inverted Brayton Cycle Waste Heat Recovery System

HiETA have developed and bench tested a novel waste heat recovery system, aimed primarily at automotive and truck internal combustion engines but with applications in energy, marine, rail and off-highway sectors. Together with collaborative partners Axes Design, the University of Bath and Equipmake, HiETA have developed a prototype system, built and tested it, and validated our thermodynamic model.

The system is based around an Inverted Brayton Cycle, and in its basic form takes exhaust gas (post after treatment) at high temperatures and ambient pressure, expands through a turbine, rejects heat and then compresses back to ambient to exhaust. The high-temperature expansion generates more work than the low-temperature compression, and as such, the system generates net power. HiETA and partners are now developing an advanced system that also integrates a partial Rankine Cycle element and significantly boosts power and efficiency.

The system offers a number of unique benefits. In its basic form, it comprises a turbocharger, a heat exchanger, and an electric machine, and as such is easily available, scalable and low cost. It imposes no back pressure on the engine, unlike competing technologies such as turbocompounding and Organic Rankine Cycles, and can even be configured to run with a depressed inlet pressure, offering unique advantages to vehicle manufacturers. It is also very light – the prototype system for a 2.0L gasoline engine weighed less than 10kg.

The prototype system has run on a hot gas stand at Bath University and has generated over a kilowatt of power. Fuel saving/emission reduction benefits across a whole automotive drive cycle have been calculated at between 5-15%, and for stationary power or similar applications can be as high as 20%.

HiETA have developed and tested their highest ever effectiveness heat exchanger for this application (over 96%) and the compact heat exchanger with many packaging possibilities and low-pressure drop is one of the key factors in making this technology successful.

The consortium are continuing to develop this technology and hope to demonstrate on a vehicle soon.