Separating dehumidification and cooling loads in air-conditioning systems has been proven to be a potential strategy to reduce electricity consumption if the dehumidification of air is mostly performed by heat-powered system components. Referring to experimental experiences in Singapore, this paper presents a novel electricity-efficient air-conditioning system consisting of a membrane unit, an evaporatively cooled sorptive dehumidification system (called ECOS system) and a high-efficient conventional cooling unit. The dehumidification of air is performed by a combination of the membrane unit and the ECOS system, and the sensible cooling of air is accommodated by a high-efficiency conventional chiller and in part by the membrane device. In order to find an optimized balance of the three air-conditioning components, an optimization-based simulation approach using a genetic algorithm is developed. The optimization is based on a simple objective function that comprises operating and investment costs. The optimization results reveal that an integration of a relatively large membrane unit, a small ECOS unit and a chiller operating at an elevated evaporation temperature is the most cost effective combination meeting comfort criteria. The resulting optimized combination has potential to save more than 50% of the system's lifetime operating cost compared to conventional systems supplying 100% fresh air. (C) 2016 Elsevier Ltd. All rights reserved.