This paper presents a tool for an aggregator of thermostatically controlled loads (TCLs) to optimally combine their flexibilities into a few representative bids to be submitted to flexibility markets. The tool employs a “bottom-up” approach based on physical end-use load models, being the individual flexibility of each individual TCL simulated with a second-order thermal model describing the dynamics of the house. The approach is based on a direct load control (DLC) of thermostat temperature set-point by the aggregator. End-users receive an economic compensation in exchange for the loss of comfort. The applicability of the proposed model is demonstrated in a simulation case study based on an actual power system in Spain.The research leading to this publication has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 691405
