The computer simulation of automobile use patterns for defining battery requirements for electric cars

The study of a complex system is usually accomplished through analytical models which permit the direct calculation and optimization of the key parameters. In some cases parameters of interest can only be expressed as probability distributions, which complicates the modeling process. Here simulation methods are appropriate for developing a usable if not fully optimal solution to the problem. Since driving patterns vary from individual to individual, and from day to day for any one person, it is difficult to determine the daily driving range required for an urban automobile. This is a critical parameter for the analysis of electric vehicles because it fixes the energy density which the battery must deliver. A Monte Carlo simulation process was used to develop the United States daily range requirements for an electric vehicle from probability distributions of trip lengths and frequencies and average annual mileage data. The analysis shows that a car in the United States with a practical daily range of 82 mi (132 km) can meet the needs of the owner on 95 percent of the days of the year, or at all times other than his long vacation trips. Increasing the range of the vehicle beyond this point will not make it more useful to the owner because it will still not provide intercity transportation. A daily range of 82 mi can be provided by an intermediate battery technology level characterized by an energy density of 30 to 50 Wh/lb (66 to 110 Wh/kg). Candidate batteries in this class are nidkel-zinc, nickel-iron, and iron-air. The implication of these results for the research goals of far-term battery systems suggests a shift in emphasis toward lower cost and greater life and away from high energy density. In addition, if the implimentation of electric vehicles follows the "S-shaped" diffusion model typical of new technologies, the optimum strategy from the standpoint of saving petroleum is to introduce near-term, intermediate, and far-term battery technologies in vehicles at the earliest date at which each battery system can be developed to the point of commercialization.