We explore the impact of renewable energy under free market conditions on the security of electricity supply using data for the German electricity market. We design a fundamental electricity market model, where renewable energy capacity is not driven by expansion goals, but is dynamically modelled as an economically-driven investment option. Furthermore, we analyse the economics of five policy scenarios designed to secure both electricity supply and renewable energy expansion. Our analysis demonstrates that renewable energy expansion leads to conventional power plant shut-downs (due to economic losses) and, as a result, to energy shortages. We find that the application of a fixed feed-in tariff mechanism for renewable energy (i.e. a fixed payment for the provided energy) is an appropriate instrument to simultaneously achieve renewable energy expansion and uninterrupted electricity supply. However, when internalizing the external costs of electricity generation, the scenario of a free market for renewable energy together with subsidies for conventional power plants becomes the most cost efficient option.