Performance analysis and optimization study of a new supercritical CO2 solar tower power generation system integrated with steam Rankine cycle

Concentrating solar thermal power generation refers to gathering solar radiation to obtain thermal energy and convert it into the high-temperature working medium to drive the turbine to do work [1]. Compared with other concentrating solar power generation modes, solar tower power generation (STPG) has higher operating temperature, larger system capacity, and higher energy conversion efficiency [2]. In the STPG system, as the concentration ratio increases, the receiver surface temperature can be more than 1000 ℃ [3]. When the working temperature is higher than 750 ℃, the steam seriously corrodes the metal [4], so the steam Rankine (SR) cycle is not suitable for the STPG. As an inert gas [5], CO₂ is not easy to react with metal. When the maximum temperature is more than 600 ℃, the thermal efficiency for the SR cycle is lower than that for supercritical CO₂ (S-CO₂) closed Brayton cycle [6].

Based on above advantages, S-CO₂ STPG system has become a research focus. Turchi et al. [7] compared the S-CO₂ closed Brayton cycle performance with the SR cycle performance. The S-CO₂ closed cycle efficiency is higher, the S-CO₂ closed cycle with dry-cooling achieved more than 50% thermal efficiency. For the S-CO₂ STPG system, Al-Sulaiman et al. [8] comparatively analyzed five different cycle layouts. The thermal efficiency for the recompression cycle was the highest. The regenerative cycle performance was similar with the recompression cycle. Khatoon et al. [9] compared the above two cycles in detail. The recompression cycle power output was 1.34 MW more than the regenerative cycle power output. Atif et al. [10] conducted system exergy calculation. The solar field exergy loss was the largest. Liang et al. [11] designed the system coupled with organic Rankine cycle to increase thermal efficiency. The organic Rankine cycle absorbed top cycle waste heat. The system efficiency was increased by up to 4.4%.

For the steady operation, the STPG systems equipped with heat storage were proposed [12]. Based on conventional solar salt (NaNO₃-KNO₃), Yang et al. [13] designed a new optimization method for the S-CO₂ STPG system. The solar multiple (SM), maximum heat storage and other parameters were optimized. The studies showed that in most cases, increasing the SM and the heat storage capacity is the most economical way. Liang et al. [14] optimized the working medium parameters, component efficiency and thermal energy storage (TES) capacity of a 50 MW S-CO₂ STPG system. The results showed that the economic performance inevitably decreased when the environmental performance was improved, and vice versa.

The highest working temperature of the conventional solar salt is 565 ℃ [15], which limits the S-CO₂ STPG system efficiency improvement. Trevisan et al. [16] proposed to use the air to solve the problem of conventional solar salt temperature limitation and conducted economic research. The levelized costs of electricity (LCOE) were 100 $/MWh under 10 MW power output and 65 $/MWh under 50 MW power output, respectively. Chen et al. [17] studied the influences of receiver parameters on system performance with particle-based model. The optimal incident irradiance range for the receiver was 1200–1500 W/m². With the decrease of the receiver outlet temperature, the system design efficiency decreased, and both the annual power output and annual efficiency increased. Wang et al. [18] built S-CO₂ STPG system with molten salt and explored the influence of the molten salt temperature. The molten salt with the higher maximum temperature was more suitable for S-CO₂ STPG system. Wang et al. [19] proposed several kinds of new salts for the S-CO₂ STPG station and studied the properties of MgCl₂-KCl. The results showed that, without reheat process was more suitable for the S-CO₂ STPG system with MgCl₂-KCl. When the operating temperature was lower than 565 ℃, MgCl₂-KCl had no advantages over conventional solar salt. Liu et al. [20] compared and analyzed compressed CO₂ and conventional molten salt energy storage. The studies showed that compressed CO₂ energy storage efficiency was greater than conventional molten salt energy storage efficiency.

In the past, the studies of the S-CO₂ STPG system mainly focused on optimizing cycle layout and system parameters. The problem of low solar energy utilization rate due to high inlet temperature of S-CO₂ cycle has not been solved. The S-CO₂ closed Brayton cycle inlet temperature is too high [21] (the superheated working medium temperature is higher than 450 ℃, and the reheated working medium temperature is higher than 500 ℃), resulting in the high heat transfer fluid (HTF) temperature of cold storage tank (CST), small temperature difference between the CST and hot storage tank (HST). Under the same HTF mass condition, compared with the SR cycle, the maximum heat storage of the S-CO₂ closed Brayton cycle is too small, resulting in the reductions of solar energy utilization rate. To sum up, for the S-CO₂ STPG system, it is imperative to increase the temperature difference between the HST and the CST, and increase the maximum heat storage of the TES subsystem to the improve solar energy utilization rate. Therefore, a new S-CO₂ STPG system integrated with SR cycle is proposed. The system model is developed by Ebsilon, and system operation calculation is developed by MATLAB. The main originalities are as follows:

(1) Based on the traditional S-CO₂ closed Brayton cycle, a new S-CO₂ STPG system coupled with the SR cycle is proposed. The introduction of SR cycle can reduce the HTF temperature of the CST and increase the maximum heat storage of the TES subsystem.

(2) For the conventional system and new system, the daily and monthly performances are compared in detail.

(3) The influences of the SR cycle power output on the performances of the combined cycle, TES subsystem, solar field (SF) subsystem and the whole system are analyzed. For the low-direct normal irradiance (DNI), medium-DNI and high-DNI districts, the SR cycle power outputs corresponding to the optimal thermal performance and optimal economic performance are obtained, respectively.