针对全球温室效应问题，全球大多数国家和地区制定了符合国情的碳达峰与碳中和技术路线。实现双碳目标的主要途径一方面是从根源上减少化石能源消耗，构建清洁能源体系；另一方面则是提高化石能源利用率。为助力双碳目标的实现，电力行业正在由火力发电向可再生能源发电转变。储能作为解决可再生能源发电不稳定性、时空不匹配的关键技术，显得尤为重要。

储能技术种类众多，各类储能技术的储能规模、发展状态、应用领域、技术成熟度、成本等均存在差异，各有优缺点，这决定了不同的储能技术有着不同的应用领域，考虑到储能容量、成本、充放电效率、运行费用、环保等原因，被公认的典型的大规模储能系统有抽水储能和压缩空气储能。结合抽水储能和压缩空气储能的性质特点，在一定程度上均受地域的限制，在可再生能源装机容量飞速增长时期，有必要寻求其它可灵活应用、便于规模化的新型储能技术。

以电能为输入能源，以冷、热能为存储能源，采用CO₂为工质的正、逆跨临界热力循环交替运行实现电力的存储和释放的储能系统被命名为CO₂跨临界热力循环储能系统。根据热力循环的性质和特征可知，该系统具备绿色发展、灵活应用、便于规模化、冷热电联供等特点，是双碳目标背景下促进可再生能源消纳的重要储能技术方案。

CO₂跨临界热力循环储能系统正逆循环相互耦合、系统复杂，耦合关系对性能显著有影响，系统热力特性尚不清楚，因此研究CO₂跨临界热力循环储能系统热力特性对系统的调控和管理是必要的。本文构建了以热水为储热介质、NaCl盐水为储冷介质的CO₂跨临界热力循环储能系统模型，对正循环高温端压力离散处理，并采用窄点分析方法对CO₂跨临界热力循环储能系统热力特性进行了分析。结果表明，换热器中窄点（最小温差点）和宽点（最大温差点）的位置变化会导致一些参数曲线不平滑；往返效率一般随正循环高温端的压力先升高后降低；为保证CO₂跨临界热力循环储能系统安全可靠运行，逆循环蒸发压力不宜过高（大于2.50 MPa），气冷器CO₂出口温度不宜过低（小于25°C）；在计算工况下，当正循环高温端压力为8.00-9.40 MPa时，高温端最佳换热分段数为4段，当正循环高温端压力为9.60-10.20 MPa时，高温端最佳换热分段数为5段。

CO₂跨临界热力循环储能系统的高温端多段换热性能尚不明确，针对多段换热性能问题，采用㶲分析法及窄点分析法研究了CO₂跨临界热力循环储能系统高温端的换热过程，并以㶲效率、热㶲损失下的温度匹配度和T-Q图中面积温度匹配度对多段换热与传统换热进行了对比分析。由于多段换热不仅可以满足用户的梯级温度利用，而且还可提高能源的利用率，因此假设了实际应用场景，并进了详细分析，为CO₂跨临界热力循环储能系统梯级供热提供了理论指导。结果表明，高温端的多段换热可显著提高换热㶲效率；当用户数为2时，随着切开处CO₂温度的升高，面积温度匹配度增大，表明较低的切开处CO₂温度更有利于提高系统的换热性能。

为进一步减小CO₂跨临界热力循环储能系统的低温端的换热平均温差，提出了以NaCl盐水为储能工质的直接接触式相变换热系统，分析了NaCl盐水与CO₂直接接触式换热的可行性。在此基础上，采用窄点分析法研究了高温盐水温度、低温盐水温度、逆循环蒸发器CO₂入口干度、逆循环蒸发器CO₂出口过热度、逆循环接触式换热温差、正循环冷凝器CO₂进口过热度、正循环冷凝器CO₂进口过热度、正循环接触式换热温差对系统换热性能的影响，并以正、逆循环CO₂平均温差为参数对系统的换热性能进行了评估。结果表明接触式相变换热一方面可明显减小正、逆循环CO₂平均温差，从而提高储能系统的储能往返效率；另一方面，储能介质相变换热，可大大减小储能介质的流量，从而使得系统更加紧凑，有利于降低系统成本。

In view of the global greenhouse effect, most countries and regions in the world have formulated the carbon peak and carbon neutral technology route in accordance with their national conditions. The main way to achieve the dual carbon goal is to reduce fossil energy consumption from the root and build a clean energy system. The other is to improve the efficiency of fossil fuels. To help achieve the dual carbon goal, the power sector is shifting from fossil fuels power generation to renewable power generation. Energy storage, as a key technology to solve the instability and spatio-temporal mismatch of renewable energy generation, is particularly important.

There are a variety of energy storage technologies. There are differences in energy storage scale, development status, application field, technology maturity and cost of various energy storage technologies. Each has its own advantages and disadvantages, which determines that different energy storage technologies have different application fields. Considering the energy storage capacity, cost, charge and discharge efficiency, operating cost, environmental protection and other reasons, the typical large-scale energy storage systems are pumped hydro energy storage and compressed air energy storage. Combined with the characteristics of pumped hydro energy storage and compressed air energy storage, both are limited by region to a certain extent. In the period of rapid growth of renewable energy installed capacity, it is necessary to seek other new energy storage technologies that can be flexibly applied and convenient for large-scale applications.

The energy storage system with electric energy as the input energy, cold and heat energy as the storage energy, and the alternating operation of positive and inverse transcritical thermodynamic cycles with CO₂ as the working medium to realize the storage and release of electric power is named CO₂ transcritical thermodynamic cycle energy storage system. According to the properties and characteristics of the thermodynamic cycle, the system has the characteristics of green development, flexible application and easy to scale, cold, heat and electricity energy supply. It is an important energy storage technology scheme to promote the absorption of renewable energy under the dual carbon background.

The positive and reverse cycles of the CO₂ transcritical thermodynamic cycle energy storage system are coupled and complex, and the coupling relations have a significant impact on performance. The thermodynamic characteristics of the system are not clear. Therefore, it is necessary to investigate the dependent relationship between operating parameters of the CO₂ transcritical thermodynamic cycle energy storage system for the regulation and management of the system. In this paper, a CO₂ transcritical thermodynamic cycle energy storage system model with hot water as the heat storage medium and NaCl brine as the cold storage medium was constructed. The pressure at the high-temperature side of the positive cycle was discretized, and the thermodynamic characteristics of the CO₂ transcritical thermodynamic cycle energy storage system were analyzed by pinch point analysis method. The results show that the change of position of pinch point (minimum temperature difference point) and wide point (maximum temperature difference point) in a heat exchanger will cause some parameter curves not smooth. The round trip efficiency generally increases and then decreases with the pressure at the high-temperature end of the positive cycle. In order to ensure the safe and reliable operation of the CO₂ transcritical thermodynamic cycle energy storage system, the reverse cycle evaporating pressure should not be too high (more than 2.50 MPa), and the CO₂ temperature of the air cooler outlet should not be too low (less than 25°C). Under the calculation condition, when the pressure at the high-temperature side of the positive cycle is 8.00-9.40 MPa, the optimal number of heat transfer segments at the high-temperature side is 4 segments. When the pressure at the high-temperature end of the positive cycle is 9.60-10.20 MPa, the optimal number of heat transfer segments at the high-temperature side is 5 segments.

The performance of segmented heat transfer at the high-temperature side of the CO₂ transcritical thermodynamic cycle energy storage system is not clear yet. In view of the segmented heat transfer problem, exergy analysis and pinch point analysis were used to investigate the heat transfer process at the high-temperature side of the CO₂ trans-critical thermodynamic cycle energy storage system. The segmented heat transfer and traditional heat transfer were compared and analyzed based on the temperature matching degree of exergy efficiency, thermodynamic exergy loss and area-type temperature matching degree in T-Q diagram. Because segmented heat transfer can not only meet the user's step temperature utilization, but also improve the energy utilization rate, the actual application scenario is assumed and a detailed analysis is carried out, which provides theoretical guidance for the step heating of CO₂ trans-critical thermodynamic cycle energy storage system. The results show that segmented heat transfer at high temperatures can significantly improve the exergy efficiency of heat transfer. When the number of users is 2, the area-type temperature matching degree increases with the increase of CO₂ temperature at the cutting point, indicating that the lower CO₂ temperature at the cutting point is more conducive to improving the heat transfer performance of the system.

In order to further reduce the average temperature difference of heat transfer at the low-temperature side of the CO₂ transcritical thermodynamic cycle energy storage system, a direct contact phase transfer heat system using NaCl brine as the working medium of energy storage was proposed, and the feasibility of direct contact heat transfer between NaCl brine and CO₂ was analyzed. On this basis, pinch point analysis was used to investigate the high-temperature brine temperature, low-temperature brine temperature, reverse cycle CO₂ inlet dryness, reverse cycle evaporator CO₂ outlet superheat degree, reverse cycle contact heat transfer temperature difference, positive cycle condenser CO₂ inlet superheat degree, positive cycle condenser CO₂ outlet superheat degree, positive cycle contact heat transfer temperature difference of heat transfer performance of the system. The heat transfer performance of the system was evaluated with the average temperature difference between positive and reverse cycle CO₂. The results show that the contact phase transformation heat can obviously reduce the average temperature difference between positive and reverse cycle CO₂ on the one hand, so as to improve the round-trip efficiency of an energy storage system. On the other hand, the phase conversion heat of the energy storage medium can greatly reduce the flow of the energy storage medium, thus making the system more compact and beneficial to reducing the system cost.