热力系统是能源系统中最重要的部分之一，它的性能分析及优化对于提高能源利用效率具有重要意义。本文首先借鉴经典力学和传热学的能量法提出了平衡态热力学的能量法，建立了变分形式的热力过程和循环的性能判据，可用以导出卡诺循环，具有最小作用量原理的性质；将(火积)分析应用于可逆热力过程及循环，建立其(火积)守恒关系，并以热机—热泵联合循环为例研究(火积)守恒关系的应用，发现将等量的热量从低温提升至高温不需要消耗净能量，而需消耗净(火积)。从对称性的角度考察了平衡态热力学，发现正、逆可逆热力循环的能量转换规律、性能判据及参数存在对称性。通过对称性引入了逆循环功效率及正循环性能系数两个物理量，后者可高于卡诺效率，但由于计入了系统从外界吸收的功量，并不违反热力学第二定律。发现在可逆循环的条件下热量与体积（膨胀）功量的地位是等同的，热与功的不等价性是不可逆过程的结果。为实现热系统中能量转换与输运特性的整体分析，以吸收式储能系统为例，建立它的热量流模型，得到系统中运行条件与设计参数间的约束关系，并实现了以储能功率最大为目标的系统整体优化，发现总投资成本一定时可通过分配流量及传热面积达到最大的储能功率，并给出了系统的设计优化准则。将热量流法扩展至溶液浓缩和吸收过程，建立了吸收式制冷机的热量流模型，实现了制冷机性能整体优化，考察边界条件变化对系统性能的影响。基于得到的热量流模型结合部件特性建立了制冷机的精细模型，进行了制冷机功耗优化，发现温度与浓度一定时系统的最优热量输运阻抗特性随总面积变化很小，引入阻抗概念可简化系统的分析及优化。针对双效吸收式制冷机，比较了三种构型双效吸收式制冷机的热量流模型，发现溶液回热流程不同不影响系统的能量流动拓扑结构，但将改变系统元件特性，影响系统的性能。以双效吸收式建筑制冷系统为例，分别以最大和平均制冷负荷进行了系统的运行成本设计优化及稳态变工况模拟，发现以平均制冷负荷为边界条件的优化结果可比最大负荷优化结果降低1.11%总驱动热量和1.8%总流量。最后，构建了储热—储电一体化储能系统的热量流模型，实现了以弃风量最小为目标的系统运行优化，发现储能设备可同时降低火电及热电联产机组的出力，考虑传热约束的优化结果更为准确，并通过双时间尺度建模考虑了电能与热能的不同时间特性，降低系统调节成本。

Thermodynamic system is one of the most important energy utilization systems, and its optimization research is of great importance for energy efficiency improvement. In this thesis, the energy method for classical thermodynamics is proposed based on energy methods in classical mechanics and heat transfer. First, the variational performance criteria for reversible thermodynamic processes and cycles is constructed, which can be regarded as the least action principle for reversible thermodynamic processes. Besides, the entransy conservation relation in thermodynamic processes and cycles is constructed based on the entransy theory, and it can be found that the net entransy is required to move a certain amount of heat from low temperature to a higher temperature. The symmetry of the thermodynamics is then studied from three aspects: the energy conversion principle, the performance criteria and parameters of the thermodynamic cycle and reversed cycle. Based on the symmetry, two physical quantities, i.e. the work efficiency of the reversed cycle and the coefficient of performance of the thermodynamic cycle are proposed. The coefficient of performance of the thermodynamic cycle can be greater than the Carnot efficiency since the work absorbed from outside is included due to its definition, and it does not contradict with the second law of thermodynamics. For the absorption energy storage system, the heat current method is employed to construct the heat current model, which offers the global system constraint, which is described by Kirchhoff's laws in circuitous philosophy instead of complex component analysis. Besides, the entransy theory is applied to present the physical interpretation of the system. Finally, the system is optimized using the Lagrange multiplier method to directly obtain the optimal structural and operating parameters which maximize the energy storage power. For the single-effect absorption chiller, the heat current is first moderated and expanded to describe the solution concentrating and diluting processes, then the heat current model of the system is derived. Furthermore, the detailed model of the chiller is constructed based on the effectiveness constraint of the recuperator and heat transfer correlations of components. Optimization results present that the thermodynamic status and heat transfer resistances of the system does not change much as the heat transfer parameter changes, and the mass flow rates and heat transfer areas can be adequately allocated to keep the system has optimal performance. For the double-effect absorption chiller, the heat current model is established based on the single-effect case. It can be found that the different solution flow arrangements do not change the heat current topology of the system. Besides, the building refrigeration system is studied and optimized using the heat current model. Optimization results based on the average cooling load could save 1.11% total driving heat amount and 1.8% total mass flow rates in the case. At last, the integrated energy storage system including the large-scale battery and thermal energy storage device is studied using the heat current method, where the waste heat of the battery is recovered to provide heat load. The system is then optimized to minimize the wind curtailments, and the optimal operation strategy for the battery is explored. Optimal results present that the introduction of the heat recovery and TES subsystem could decrease the coal consumption of the thermal power plant and the combined heat and power plant simultaneously. Meanwhile, the mass flow rate of the heat conducting oil keeps high in most time and drops to near zero when the battery switches between charge and discharge. Furthermore, the alternation frequency of the mass flow rate of the heat conducting oil has a lower bound for normal operation.