Performance evaluation and multi-objective optimization of a solar-thermal-assisted energy system: Supercritical CO2 Brayton cycle and solid oxide electrolysis/fuel cells
Zhicong Fang, Zhichao Liu, Shuhao Zhang, Zekun Yang, Xiaomei Huang
Abstract
Typical solar power tower (SPT) systems employ molten salt as the heat transfer and thermal energy storage medium to facilitate stable energy output. However, these systems are constrained by their limited operating temperature, which is insufficient to supply heat for high-temperature electrolysis. In this paper, a small-scale (2.5 MW) solar-thermal-assisted energy system with a SPT, a supercritical CO2 (SCO2) Brayton cycle, and solid oxide electrolysis/fuel cells (SOEC/SOFC) is proposed. With heat supply via air, the SOEC subsystem can operate at 800 ℃ to reach high energy efficiency and reduce electric demand, which replaces conventional waste heat from flue gas produced by fossil fuel combustion. Such a system can convert excess electricity into hydrogen for storage or sale and provides stable electricity 24 h a day. The key factors determining the system performance are investigated, including the turbine inlet parameters, main compressor inlet parameters, the recompression fraction of the SCO2 subsystem, and the operating temperature and current density of the SOEC/SOFC subsystems. The case study shows that the net hydrogen output can reach 13365.4 kWh/d in summer with the SOEC operating at 800 ℃, which is 86.7 % higher compared to 7157.6 kWh/d when operating at 600 ℃. To recoup the investment costs by the 20th year, hydrogen must be priced at 6.5 $/kg. After multi-objective optimization, the optimal exergy efficiency and capital investment for the SPT-SCO2-SOEC subsystem are determined to be 29.6 % and 3.65 M$, respectively. For the SOFC subsystem, the corresponding figures are 56.1 % for exergy efficiency and 0.23 M$ for capital investment. This study benefits the solar power generation and hydrogen production by high-temperature electrolysis.