The increasing energy production from variable renewable energy sources such as wind and solar energy significantly affects the way energy systems are managed. The residual load, which represents the difference between the load and the total production from privileged sources, can have different properties than the load alone. Additional flexibility is required from the remaining power plants and the other resources in the system to maintain the generation-demand balance under the conditions of higher variability. This thesis gives an overview and description of different flexibility options in energy systems, including both supply and demand side. The energy storage technologies potential application in energy system with high share of variable energy sources and a overview of the energy storage technologies is given. The importance of frequency and active power regulation in an electrical systems is also described, as well as the impact of an increase in the production from variable energy sources on the required regulation power reserves. A mathematical model of a microgrid, comprising cogeneration plants, photovoltaic module, thermal energy storage system and controllable loads, has been developed. Given the important role of the thermal energy storage system in the flexible microgrid operation, a one-dimensional mathematical model of the thermal storage tank has been developed, and the temperature profile in the tank is simulated under certain conditions of the microgrid operation. Finally, a simulation of microgrid operation during a winter week, summer week, and a week in the transitional period is performed on the basis of several proposed strategies of the system operation. The proposed strategies are compared with respect to cost effectiveness.