Design and optimisation of linear Fresnel power plants based on the direct molten salt concept

Abstract

Abstract Thermal energy storage (TES) is an exclusive feature of solar thermal electric (STE) plants, allowing for solar energy to be efficiently stored and dispatched to the grid at a chosen point in time, thus providing an enhanced operation flexibility while potentially reducing the levelised cost of electricity generation (LCoE). The direct molten salt (DMS) concept, where a liquid nitrate/nitrite salt mixture is used both as heat transfer fluid in the solar field and as sensible heat storage medium in the TES represents currently the most promising technological option, in terms of LCoE optimisation and technology maturity, for STE plant with thermal energy storage. Here, different DMS plant configurations based on the linear Fresnel solar collector technology are investigated targeting to define optimal designs with regards to the LCoE. For this purpose, a simulation tool has been developed capable of describing the dynamic plant behaviour by means of best estimate methods to determine the annual yield and operational availability of linear focusing DMS STE plant, together with estimating the investment and annual operation costs of such plants. Simulations results from this tool have been validated against a well established thermo-hydraulic simulation tool (TRACE). The study concentrated on the following aspects: 1. Identification of optimal operational parameters of a STE plant with TES coupled to a power cycle 2. Economical evaluation of the chosen plant concepts and definition of optimal plant configurations 3. Investigation of LCoE reduction perspectives by means of higher operation temperatures These analyses exhibit for the given plant setup an optimum choosing a sub-critical reheat power cycle at 540 °C/165 bar and the so-called “Hitec” molten salt mixture as heat transfer fluid (HTF). However supercritical power cycles in a DMS STE plants are not considered and may bring further LCoE reductions. Additionally, in terms of LCoE, the study reveals an optimal plant capacity in the range of 100 MW el to 150 MW el . Finally, the study shows that a further increase in the temperature of both power cycle and solar field beyond the previously named optimal values leads to increased LCoE.