Currently, power system operations planning practices are undergoing various transformations in an attempt to integrate efficiently significant amounts of sustainable low-carbon power generation technologies. At the heart of this efficient integration lies the need to plan for and exploit the available flexibility in power systems. Flexibility is a relatively new concept addressing the issue of increased variability and uncertainty in power systems admitting high penetration levels of renewable energy sources. To that end, we previously introduced the concept of flexible power envelopes to capture flexibility requirements and flexibility availability from power system resources in terms of time-constrained power deployment. Here, we extend this concept to flexible energy envelopes, thus giving rise to the notion of flexibility assessment in terms of time-constrained energy deployment. This approach is more appropriate for considering flexibility resources like energy storage and demand response. We demonstrate the advantages of planning with flexible energy envelopes with a receding-horizon economic dispatch example integrating energy storage assets.