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Costs can be saved with a hiss

Energy recovery systems for vehicles are a familiar theme in green technology from Formula One cars to family saloons from major manufacturers but there are significant cost issues with storing that energy.

F1 uses fantastically expensive flywheels but most commonly, large and extremely heavy and costly battery packs are used. And neither can be fitted easily to existing vehicles, especially commercial ones like lorries and buses.

Recovery systems work by capturing some of the energy used in braking. So the stop/start nature of inner city bus routes makes them ideal candidates for the technology. But building a bus with a diesel/battery hybrid engine can double the cost compared with a conventional bus even though it will be much more fuel efficient.

Engineering professor Hua Zhao and his team looked at possible storage solutions that would be much cheaper and could easily be retrofitted – city buses regularly have on the road lives of up to 40 years going through multiple engine changes.

His solution was to use compressed air for energy storage, which means his technology can be fitted to both new and existing engines.

Prof Zhao’s air hybrid prototypes have been reliably taking passengers around Yulin City in China, home to Yuchai which makes 80% of all the country’s bus engines, since 2011. The company is now re-tooling to incorporate the patented system into all their engines and UK trials with an industry partner, PTech began early in 2014.

The system can cut fuel consumption by up to 10 per cent per annum, saving seven tonnes of carbon per bus each year. And because compressed air systems are a relatively cheap technology the pay-back period is measured in months not many years.