Energy Performance Optimization of a Bus for Urban Public Transport

Nowadays many people use public transports in urban centres. Consequently, every day a lot of buses move within cities, trying to ensure the best service to citizens. During the year buses become crowded places and using an air conditioning system in constant operation, it tries to ensure a certain condition of comfort for the passengers on board. The aim of this study is to analyse and optimize the energy performance of a bus shell, identifying practical solutions that have not yet been adopted in order to reduce the impact of air conditioning on bus consumption and, therefore, on air pollution. For this reason it was decided to conduct a thermal analysis of a bus for public transport, in order to understand the behaviour of the bus shell and to deduce possible optimization measures that have not yet been made until now. The analysis was carried out considering the hottest day of July and the coldest day of January, considering the conditions of operation based on the most common graphics TGM able to define the concentration of traffic city during the 24 hours per day. The study was done using the dynamic simulation software TRNSYS. With this software it was possible to recreate faithfully the structure of the bus and the external environmental conditions, assessing the impact of different technical solutions for an improvement of internal conditions and a reduction of the cooling capacity required. Because the presence of passengers in public transport is considered like a "benefit" during the winter, the analysis started with the identification of a summer solution and the subsequent evaluation of this solution for the wintertime. The aim of this study was to optimize the bus shell and select the most appropriate solutions. Regarding the transparent surfaces it has been given importance to factors such as the thermal transmittance and the solar gain factor (g-value). Aware of the influence given by the solar radiation on the energy loads, we have simulated bus energy performance also considering different types of paintwork with high reflectance. I. INTRODUCTION Nowadays a lot of people spend many hours during the day using public transport. Traffic conditions in Rome necessarily require the frequent use of public buses. In Italy conditioning the public transport arises from the need to improve the comfort of people who daily use public services moving around the city. Internal temperature and humidity conditions are important factors in terms of passengers comfort and health. For this reason, the automobile industry has recently developed the thermal comfort issue which have to take care of both the health of the passengers and the driver. The problem connected with the comfort onboard is very complex because, inside, there are strong gradients of air velocity and temperature. Moreover the solar radiation inside the vehicle, especially during the summer, is very important (1). In this study it has been developed a model applying dynamic simulation software TRNSYS (already largely used to analyze the thermal exchanges (2)-(5)). In order to give a better understanding of energy behavior of the bus shell, the identification of practical solutions that have not been used yet, the improvement of the isolation from the outside and the limitation of the power required for cooling the cabin have been carried out. II. MODELLING The analysis was carried out considering a generic model of a bus and its geometrical characteristics are shown in Fig. 1. The vehicle stratigraphy is given in Table II and III. To provide the thermal analysis the dynamic simulation software TRNSYS has been used. Through this programme it was possible to faithfully reproduce both the geometry of the bus and the external environmental conditions: the bus modelling is done through the TRNSYS Build, which is the section of the software that allows to generate the vehicle's model only. The connection of the "build" with the outside world is possible by using the TRNSYS Studio, through which it is possible to connect specific types, generating a pattern of interconnection (Fig. 2). A. Trnsys Build