Enhancing the Thermo-Economic Performance of a Direct Steam Generation Solar Tower Power Plant through the Implementation of Steam Flow Control Strategies for Flexible Operation

Abstract: Above 90% of the current installed concentrating solar power plants are based on conventional steam-turbine cycles. The operation of steam turbines in these plants is distinctive when compared to traditional base-load power plants. The reason goes back to the intermittent nature of solar power which, in the absence of thermal energy storage or a back-up combustion boiler, forces plant operators to shut down the turbines during night time or at times of low solar radiation. Furthermore, such intermittency often leads to undesirable off-design turbine operating circumstances, either by load variations or changes on live-steam conditions.The present study examines the influence of implementing two operating strategies dealing with steam flow control as a function of incoming solar power for enhancing the thermo-economic performance of a direct steam generation solar tower power plant. The first one consists of a simultaneous high pressure turbine stage- and feed-water preheater bypass. This strategy is used during periods in which the solar radiation is higher than nominal. On these occasions, the plant is capable of generating a larger flow of steam, which allows for an increase in the power production when inserting the additional steam in the turbine bypass. On the other hand, the second operating strategy consists of using an additional feed-water preheater when the power from the field is lower than nominal. In this way, the feed water can reach a higher temperature prior entering the boiler, which is not only beneficial during times of cloud-passages, but also during the start-up process.A dynamic model of a direct steam generation solar tower power plant has been developed following design and operation specifications of an existing reference plant. The two proposed strategies were implemented to the reference model, then a whole year worth simulation was performed for both the reference and the modified models. Lastly, the thermodynamic and economic performance of both systems was measured for the purpose of comparison, by means of using KTH in-house tool DYESOPT. Results show that the implementation of the proposed strategies can enhance the economic viability of the systems by yielding a reduction of 8.7% on the levelized cost of electricity, mainly due to allowing achieving a 12% increase in the net electricity production.