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A novel approach to thermal storage of direct steam generation solar power systems through two-step heat discharge

Li, Jing, Gao, Guangtao, Kutlu, Cagri, Liu, Keliang, Pei, Gang, Su, Yuehong, Ji, Jie, Riffat, Saffa
Applied energy 2019 v.236 pp. 81-100
cost effectiveness, heat, heat exchangers, power plants, solar thermal energy, steam, volatilization, water temperature
Steam accumulators are the only commercial solution for heat storage of direct steam generation (DSG) solar thermal power plants. Current accumulators have low storage capacity as the turbine suffers from inefficient off-design operation during heat discharge, thereby restricting the development of DSG technology. This work presents a novel approach to solving this problem by using two-stage accumulators and steam-organic Rankine cycles (RC-ORC). The system involves unique two-step heat discharge. Heat is initially released via water vaporization in a high temperature accumulator (HTA) to drive the RC-ORC, leading to an HTA temperature drop of approximately 30 °C. Water at a reduced temperature then flows from the HTA to a low temperature accumulator through a heat exchanger and the heat is used only to drive the ORC. Water temperature further drops by 130–190 °C. The fundamentals of the system are illustrated. A comparison with the conventional DSG system is conducted at a nominal power of 10 MW with an accumulator volume of 2500 m3. Thermodynamic performance of the system is investigated. The equivalent payback period (EPP) regarding the use of the second step heat discharge is estimated. Results indicate that the second step heat discharge can increase the storage capacity by 460%, with an EPP of less than 5 years in most cases. Overall, the proposed solution improves the cost-effectiveness of the DSG system.