From petrol, diesel and EV prototypes, it seems now Audi is turning its attention to fuel cell powered vehicles once again - after a brief affair with the Q5 HFC.

Presumably starting off a whole new range of fuel-cell powered 'tron', the Audi A7 Sportback is slated to start at the end of August, with trials.

Fuel cells generate electricity and heat through an electro-chemical reaction between a fuel and oxygen. When this theory is applied in the automotive world, the fuel will typically be hydrogen and when mixed with oxygen from the ambient air - water is produced as the only by-product.

The electricity produced from fuel cells can charge a battery or directly drive electric motors for propulsion, while the waste heat can be captured for thermal systems or additional electricity generation via thermocouples.

Unlike electric vehicles, fuel cell powered vehicles can be refuelled quickly, with the only setback being a lack of an established hydrogen infrastructure which may be a hindrance to the technology.