FEATURE AUTOMOTIVE
Harvesting waste heat from the surface of a car exhaust pipe
David Wood from Microsystems Technology Group, School of Engineering and Computing Sciences at Durham University, explores energy harvesting in automotive applications using antenna/diode arrays, to draw reusable power from radiant heat
T
he intense development of the internal combustion engine has produced
considerable gains in fuel economy over the last 20 years, but the industry acknowledges that it is now against the limits of fundamental science. Energy recovery systems such as regenerative braking and engine stop/start work well, but are only functional for very small portions of a journey. In response, Microsystems Technology Group at the School of Engineering and Computing Sciences at Durham University are working on developing an idea that promises continual energy recovery. A petrol internal combustion engine is
~25-30% efficient in turning chemical energy into vehicle mobility, with ~40- 45% being lost down the exhaust pipe. Many attempts have been made to access this waste energy, but none have been successful at overcoming the associated problems of disrupting the airflow to make the technology worth the effort. Instead, the research group have looked at the more accessible radiant heat from the exhaust pipe surface, where any scavenging technology will not disrupt the main function of the pipe. Although only 10-20% of the exhaust heat (i.e. 4-9% of the overall combustion energy) is lost this way, this still represents a large source (well into the kW range) of recoverable energy. The engineers have produced world-
leading results in using thin film metal- oxide-metal (MOM) diodes with an associated antenna, known as rectennas. These promise to be simple to install via a flexible substrate wrapped around the exhaust. A cross-section of the implementation is shown in Figure 1, with the research groups’ vision of a large scale array in Figure 2. A MOM diode consists of two dissimilar
metals with an intermediate oxide layer that is thin enough (<5nm) to allow electron tunneling as the current transport mechanism. In practice, an operational diode is best achieved using one metal on which to grow a native oxide, and another metal that is oxidation resistant. It is also beneficial to maximise the work function difference between the two metals. Harvesting waste heat from the surface of a car exhaust pipe is a challenging
28 SUMMER 2015 | MICROMATTERS
environment for electronics, and other solid state technologies such as thermoelectric (TE), microthermophotovoltaic (μTPV) and self-switching diode (SSD) devices have all been investigated. As can be seen from Figure 2, many rectennas will be needed to cover an exhaust – the number turns out to be ~1012, which can be a little disconcerting. However, the devices are produced using only four mask levels, are repetitive and can be arranged in series or parallel to prevent failed individual devices from affecting the overall performance.
MOM DIODES IN THE FIELD Most work in MOM diodes has concentrated on making individual devices, and where electron-beam lithography (EBL) is well suited to produce optimum performance. However, when faced with making a large number of devices, EBL quickly becomes impractical. The team are pursuing the route of Nanoimprint Lithography (NIL), which is far more suited to the mass
Figure 1:
A cross-section of the implementation of using thin film metal-oxide-metal (MOM) diodes with an associated antenna, known as rectennas
Figure 2:
Many rectennas will be needed to cover an exhaust
production of these diodes. This is particularly important where ~1012 devices per exhaust will be needed. NIL uses a master and stamp technique to make multiple pattern copies, which makes it a highly suitable process to incorporate in a Roll to Roll (RtR) production of an exhaust wrap. The EBL process takes over a day, whereas the RtR process is completed in less than six minutes. Finally, RtR is a more recent technology, and is improving at a faster rate, with the latest installations capable of printing at up to 200m/min. The researchers have found that MOM device technology bears favourable comparison with all other solid state competitors. It was proven that only NIL with RtR has a realistic chance of the necessary production speed and volumes. With ~1012 devices per wrap, a current integrated circuit (IC) manufacturing cost of $10-8 per bit would mean $10k per wrap – is clearly not acceptable. However the IC industry has achieved a 102 reduction in unit price in the last 10 years, giving a more realistic $100 per wrap. With a device that is much simpler than a transistor, made using fewer mask levels and in a cheaper factory, the scope for further cost reduction in the same timescale means the manufacturing cost will become acceptable. The UK carbon reduction strategy means a 34% reduction in CO2 output by 2020, with an 80% reduction by 2050. Transport is a ~26% contributor to UK-based CO2 emissions: a technology that reduces fuel consumption by up to 9% will give a 2.1% reduction in emissions on its own. Moreover, the technology is intended to be implemented in an exhaust wrap, which is reusable. With many kiloWatts of power currently lost from exhaust pipe radiation, this represents probably the last major opportunity for energy harvesting in an automotive environment. And, of course, the devices would work equally well in other environments where a large amount of heat is generated.
Microsystems Technology Group
www.dur.ac.uk/microsystems.technology/ david.wood@durham.ac.uk
/ MICROMATTERS
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