The energy system is rapidly changing in all aspects, and also renewable energy is integrated on a large scale to the grid in recent years. Renewable energy provides advantages in decarbonising energy system, energy security and also improving energy access. It is certain that renewable energy brings benefits, but also brings operational challenges to the energy system. Variability in the renewable energy resource asks for larger flexibility to compensate the losses and balance the system as a whole. In future, some of the conventional generators which have characteristics like highly efficient, low specific emissions are highly desirable in providing flexibility at large scale. In this study, a combined cycle cogeneration power plant is chosen, and flexibility improvements using heat-electricity decoupling methods are analyzed. A techno-economic analysis of a combined cycle cogeneration power plant is conducted using DYESOPT (DYnamic System OPTimizer) tool, to compare power plant performance in different operational conditions. The öresundverket power plant located in Malmö, Sweden is considered as a reference power plant to develop a simulation model. Annual performance of the power plant is analyzed at three different modes of operation namely full condensation mode, design alpha value with constant DH demand and a varying DH demand modeled using statistical data. Technical, economic and environmental performance indicators are used to analyze the performance. One of the drawbacks is that power plant does not have provision to take advantage of energy prices in both electricity and district heating markets, i.e. excess electricity is produced at decreased heat generation in the cogeneration power plant. A hot water accumulator of 500 MWh capacity is considered as a heat-electricity decoupling strategy to overcome the drawback and also to improve flexibility. Two weeks of power plant operation with storage system improved economics by 12,800 USD. It is found that combined operation of the power plant with thermal storage has provided better operational flexibility for power plant operation in electricity and district heating market.