The global energy supply is transitioning to sustainable, low-carbon energy, as prompted by many policies from authorities [1]. Wind and photovoltaic power vary with time considerably, and the electricity demand fluctuates seasonally and daily due to the time-varying nature of residential production and life [2]. Therefore, the intensive penetration of renewable power into power grids poses a great challenge [3,4]. In some regions, the issue of renewable power abandonment is serious. The integration of energy storage systems is an effective means to match energy supply and energy consumption [5,6].

Many energy storage technologies have been developed in accordance with technological principles [7], and they can be classified as mechanical [8], electrochemical [9], electrical [10], thermal [11], and chemical [12] energy storage. Among them, thermal energy storage (TES) has the advantages of large energy storage capacity, long storage cycle, low cost, and so on. Thermal storage is suitable for the needs of large-scale energy storage [13,14]. Depending on the energy storage medium, TES technologies can be divided into three categories: sensible thermal energy storage, latent thermal energy storage, and thermochemical energy storage [15]. The selection of suitable materials is a key issue for TES applications. Many thermal energy storage (sensible heat storage or SHS) materials are screened as potential commercial energy storage materials to achieve low price, simple structure and good thermal stability [16,17]. Molten salt with a low melting point and high thermal stability can effectively reduce capital and operating costs and contribute to operational reliability [18]. Solar salt(60%NaNO₃ + 40%KNO₃) and Hitec salt (53%KNO₃ + 40%NaNO₂ + 7%NaNO₃) have been widely used as a thermal energy collection and storage fluid in different solar thermal plants [19]. In addition, many studies have been conducted on the development of new ternary- or quaternary-mixture molten salts with excellent physical and chemical properties, such as high specific heat capacity, low viscosity, and low corrosion rate [20,21].

The integration of TES can match energy supply and demand, and the feasibility and economic performance of integrating TES into different energy supply systems have been comprehensively studied. Carnot battery converts electric energy to heat energy for storage, using molten salt or water as the TES medium, and converts the heat energy back to electric energy as required. The Task 36 [22] of the IEA Energy Storage TCP, which is running currently, has developed the performance indicators of Carnot battery system and its key components to enhance the technology. TES integration makes concentrated solar power (CSP) technology more attractive than other renewable power technologies because of the dispatchable nature of TES. Achkari et al. [23] showed that 45.5% of operating CSP plants worldwide (i.e., 45.1% of the total installed capacity) are equipped with TES, and among them, 95.6% (i.e., 99.8% of the total installed capacity) use liquid SHS materials. Second, several studies have discussed the economic benefits of integrating TES systems into nuclear power plants. Aunedi et al. [24] proposed an upgraded 1610 MW nuclear plant option integrated with a TES system. The proposed configuration can increase the output power by 32% during the load-peaking period. In addition, TES is an effective solution to accommodate wind power. Yamaki et al. [25] developed an energy demand model that combines a molten salt TES with a wind-powered paper mill. Eighteen cases were studied, and the researchers found that greenhouse gas emissions can be reduced to around 7% of that of a conventional paper mill. The integration of TES systems into coal-fired plants can enhance operational flexibility. Li et al. [26] used Aspen Plus software and developed a dynamic model in which thermal energy is stored during off-peak hours and reused during peak demand hours. The simulation results showed a 13.3% reduction in output load during the charging process of TES and a 7.4% increase during the discharging process. Yong et al. [27] concluded that TES-based coal-fired power plants (CFPPs) have higher efficiency than traditional CFPPs, especially at low load ratios.

The determination of TES capacity is a key issue to be addressed during TES system design [28]. Yan et al. [29] discussed the impact of electrical energy storage and TES on load shaving and determined the TES storage capacity by developing a mathematical model that considers different coal and peak-valley prices. Benalcazar [30] proposed a method based on a mixed-integer linear programming approach that considers the specific investment costs of the storage technologies and annual operation scheduling of the CHP-TES system to determine the optimal capacity of TES units connected to a coal-fired CHP. He et al. [31] proposed a wind-PV-TES-electric heater cogeneration system and investigated the system capacity optimization problem by using the nondominated sorting genetic algorithm III coupled with principal component analysis. The results showed that the proposed CHP system has 6% and 21% lower levelized cost of electricity and life-cycle-equivalent CO₂ emissions, respectively, compared with a conventional purely electric system.

CHP units, which can supply power and heat to industrial and residential users, have to operate flexibly to accommodate renewable power, and integrating TES systems is a good solution. Gong et al. [32] considered a proposed system that integrates high-temperature TES into a biomass-fueled CHP plant. Their results showed that 53% of the renewable power that would otherwise be curtailed can be absorbed and used, and about 21% of the fuel in CHP plants can be saved. Wu et al. [33] integrated a solar TES into a CHP plant to preheat feedwater or reheat steam for reducing coal consumption. Lepiksaar et al. [34] integrated electric boilers and TES into CHP units to balance heat and power loads and thus improve the flexibility and energy efficiency of CHP. Their results showed that natural gas consumption can be reduced by 36%. Rong et al. [35] provided an auxiliary heat source by installing electric heating and heat storage devices to consume wind power during peak shaving; they found that the proposed approach can accommodate about 32% of wind power and expand the heating source to about 18% of the total heat demand.

Many comprehensive studies have been conducted on the design and optimization of systems integrated with TES. CHP systems based on TES that convert valley power to heat and store heat in TES may be an efficient way to accommodate renewable power and meet industrial and residential users’ demands for multiple kinds of energy. The performance of CHP systems based on TES is influenced by heat and power prices, heat storage medium, and system parameters. Therefore, performing thermodynamic and economic analyses of CHP systems based on TES is necessary to enhance the systems’ operational flexibility and economic efficiency.

In this study, a CHP system based on molten salt TES is developed. The system uses valley power from the power grid as the energy input and converts power to thermal energy stored in TES by an electric heater. The main parameters (molten salt type, heat transfer temperature difference, main steam temperature and feedwater temperature) that affect the design cost or system operating efficiency are analyzed and optimized. Moreover, exergy analysis is performed under different operation modes of the coupled system to reveal the energy flow utilization and losses of the system. The novelty and contributions of this study are as follows: (1) A system that uses electricity as input energy and outputs electricity and thermal energy with different parameters by using an electric heat production-thermal storage device is proposed. (2) The impact of peak-valley electric price difference, thermal load, electrical load, TES scale, and cogeneration layout on the cost is analyzed, and the system’s economic evaluation index is defined. (3) The main parameters that affect economy are optimized, and the optimal system configuration is obtained. (4) The exergy destruction rate for each equipment in the system is given to reveal the potential for improving energy utilization.