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Rather than converting heat energy into mechanical energy in an internal combustion engine and then converting mechanical energy into electrical energy in a magnetic field, thermophotovoltaic (TPV) generators work by directly converting heat energy in the form of infrared radiation into electricity using photovoltaic cells. A series hybrid vehicle uses this electrical energy to keep the batteries at a constant state of charge. These batteries then provide the energy for the electric motor, which propels the vehicle down the road.

Western Washington University has built two ground-up hybrid electric vehicles. The first vehicle, Viking 21, was utilized to test various systems before incorporating them into Viking 23 which is the vehicle discussed in this paper. The front wheels are driven by two brushless D.C. motors through a purpose built 4 speed transaxle incorporating a limited slip differential. The rear wheels are driven with a natural gas, fuel injected, four cylinder, 16 valve engine through a six speed gearbox and mechanical diodes at the rear wheel hubs. The body-chassis is made from sandwich construction carbon fiber composite. The NiCd battery is charged from a 720 Watt solar array covering the entire surface of the upper body's aerodynamic shell. The low weight (900 Kg), combined with a Cd of .275, yield a gasoline equivalent efficiency of more than 200 miles per gallon.