Large-scale energy storage systems typically withdraw electricity from the grid and transform it into another form for storage. When the grid is unable to meet demand, the process is reversed and the stored energy is transformed back into electricity. Instead of this traditional approach, the following thesis explores the concept of ‘generation-integrated energy storage’, in which a generator’s existing energy conversion pathway is used to store energy in an intermediate form. This has two benefits: (i) the hardware used for generation can be exploited to reduce storage costs and (ii) fewer energy transformations are required when compared to traditional ‘electricity-in-electricity-out’ forms of storage. This means a high effective (exergetic) round-trip efficiency can be achieved at low cost. Specifically, this thesis focuses on the integration of thermal energy storage with the feedwater heating system of steam plant. (In modern energy systems this is likely to be nuclear-powered.) In the proposed system, the plant’s electrical output is flexed whilst maintaining constant reactor power. During charge, the plant’s electrical power output is reduced below its normal full-capacity level, and during discharge, it exceeds this level. This approach provides the equivalent of an electricity storage system and facilitates the adoption of a load-following role for nuclear plant. By allowing the reactor to operate constantly at maximum power output, the system also avoids the economic constraints and practical problems of part-load operation, which currently favour the use of nuclear plant for baseload only. An important feature of the proposed system is that the wet steam turbine bleed flows automatically provide good thermal matching with the feedwater temperature profile. This means that heat can ultimately be transferred to and from sensible-heat thermal-storage media with high exergetic efficiency. Various options are discussed for the thermal stores, including pressurised water tanks, thermal oils, and packed beds. This thesis is focused on the engineering research and development of the feedheat- integrated energy storage system and how this technology would be valuable in a modern energy system. The following contributions have been made: (i) Thermodynamic analysis – Detailed thermodynamic analysis is presented for an elec- tricity storage system in which thermal stores are integrated with the feedwater heating system of steam plant. The findings indicate that a round-trip efficiency greater than 80% is likely and that the plant’s power output can be varied between 85–113%. The analysis is also extended for heat cogeneration applications, for which the effective COP is estimated to be approximately 8 for modern district heating and 4 for industrial process heat. (ii) Off-design steam plant operation – A detailed off-design steam plant model is created. It is shown that the plant performs sufficiently well when operated off-design, and is able to efficiently transfer work to heat and then heat back to work. (iii) Capital cost estimation – A comprehensive cost analysis of the proposed system is undertaken, with an emphasis on the marginal cost of oversizing existing compo- nents. Costs for a well-designed system are approximately 250–1000 $/kWe and 15–20 $/kWhe. (iv) Thermo-economic optimisation – Parametric studies and a genetic algorithm optimisa- tion method are used to determine the optimal trade-off between efficiency and cost, and inform best design practices. (v) Steam turbine operation – A streamline equilibrium throughflow method is used to numerically validate Stodola’s ellipse law, and to explore the unusual off-design conditions caused by the storage system. Throughout this thesis, these contributions are routinely placed in the context of the modern energy system. It is demonstrated that integrated systems which perform multiple roles – electricity generation, energy storage, and possibly heat cogeneration – will be highly valuable for the transition to a low-cost, secure, and decarbonised energy system.