Diesel-electric hybrid bus conversion. Rooftop capacitors storing electrical power on a diesel bus being converted 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 these high-capacity capacitors (ultracaps). Electricity is also generated during braking. Photographed in 2012.