Design and modelling of an air conditioning system for Kayoola electric buses.

Abstract

The study aimed to design and model an air conditioning system for the 12 m Kayoola Electric Vehicle Solution (EVS) bus operating under Uganda’s tropical climatic conditions. The main objective was to develop, simulate, and validate an optimized AC system that enhances energy efficiency, passenger comfort, and driving range. The specific objectives were: (i) to design the AC system; (ii) to model and simulate the system thermodynamically to obtain optimum performance parameters; (iii) to evaluate field performance in terms of thermal comfort and energy consumption; and (iv) to assess the system’s financial viability. The research applied the HBM to compute the total cooling load, yielding 46.245 kW, with motor heat, radiation, and metabolic load contributing the largest share of cabin heat gains. The system was then modelled and simulated in Engineering Equation Solver (EES) software to analyse the vapour compression cycle performance, resulting in an optimum cooling capacity of 41.79 kW at an evaporator temperature of 11 °C. A multi-criteria decision analysis (MCDA) guided the selection of a variable-speed scroll compressor, brushless fans, an electronic expansion valve, and R134a refrigerant, which were integrated into a roof-mounted commercial variable-frequency AC unit. Field tests conducted under sunny and rainy conditions showed that the proposed system reduced specific energy consumption to 0.926 kWh/km, compared to 1.2916 kWh/km for the existing system, extending the driving range from 232 km to 382 km per charge. Cabin temperatures averaged 22.6 °C, maintaining comfort within ISO 7730 standards, with 96% passenger satisfaction recorded. Financial evaluation using the Federal Energy Management Program (FEMP) life cycle cost model revealed long-term savings of USD 18,929.25 over 20 years despite a slightly higher initial cost. The results confirm that a variable-capacity AC system, optimized for local climatic conditions, can significantly improve energy efficiency, passenger comfort, and operational economy of electric buses, supporting the advancement of sustainable urban mobility in Sub-Saharan Africa.