Diesel-electric hybrid bus conversion. Chief engineer Fernando Gamez Coca overseeing work to convert diesel buses to a hybrid diesel-electric technology in Barcelona, Spain. This retrofitting project is being carried out by TMB (Transports Metropolitans de Barcelona). The conversion makes existing diesel buses cleaner, quieter and more efficient. Excess heat energy is converted and stored in high-capacity capacitors (ultracaps) on the roof of the bus. Electricity is also generated and stored during sudden braking. Photographed in 2012.