Modelling the Electric Bus Charging Network Requirements for HVAC Purposes

Abstract

Wider use of electric buses is hindered by necessity to provide cabin heating for the passengers during winter months, which in cold climates uses up as much energy as driving, thus significantly reducing bus driving range and requiring larger traction batteries, which are expensive. When the lifetime operational costs are included, unless they are heavily subsidized, the battery electric buses have troubles competing with internal combustion engine (ICE) buses in cold weather conditions. For typical ICE buses, the power capacity for heaters is in the range of 50 kW. It would be possible to increase the driving range by using heat accumulators to provide energy for vehicle thermal requirements. These heat accumulators could be recharged in bus stops using wireless charging to minimize battery size. This article describes the results of a mathematical model developed to determine optimum wireless charging power requirements for heat accumulators. The model is created for public transportation system in Riga city based on weather data in years 2017–2022 to provide passenger thermal comfort. The model simulates energy flows for bus movement and heating purposes using worst-case scenario approach. The model analysed 198 916 bus stop data, which were pertinent for weekday travels in Riga city. The analysis covered 1716 individual bus stops and 448 routes. The results showed that the charging availability during the day varied from 3 % to 55 % and on average the total energy needed for heating would be 75.6 kWh. However, in the worst-case scenario, the number rises to 237 kWh making the system too expensive for practical applications. During the research there is a very limited availability of research regarding bus heating requirements. Most of the research on thermal comfort in busses are done in hot climate and consequently mostly concerns air conditioning. The thermal comfort research about cold weather is predominantly about buildings and indoor comfort and could not directly apply, as clothing should be considered. Also, the research shows that the temperatures for comfort are lower for short haul vehicles than for the long-haul ones. All these factors strongly indicate that current conventions on temperature requirements for buses are outdated and coming from ICE buses where the heat came from the engine and was free. Therefore, the next studies should include field research analysing passenger thermal comfort levels in buses in winter to explore if there are additional opportunities for energy savings in bus HVAC systems.