Direct-expansion solar-assisted heat pump water heaters (DX-SAHPWHs) are conducive to the environment due to the use of ambient energy and solar radiation. This paper aims to design and develop a thermodynamic model and a multi-criteria optimization of thermal performance for a DX-SAHPWH using R-134a as the working refrigerant that supplies domestic hot water for a typical building throughout the whole year. The thermodynamic model of the DX-SAHPWH is developed so that the performance of the system is solved through the try and error technique with the least initial data of the thermodynamic cycle of DX-SAHPWH. After the validation of the thermodynamic model, the performance of the DX-SAHPWH is analyzed for a typical building located at the temperate climate of Iran. Then, a parametric study is conducted to identify how various design parameters may affect the performance of the DX-SAHPWH system. In the current investigation, the effects of design parameters including solar radiation intensity, ambient air temperature, outlet water temperature of the condenser, solar collector area, compressor speed, length of tube in the condenser, external diameter of the tube in collector plate, fin thickness, and thermal conductivity of collector plate on the performance of the DX-SAHPWH are investigated. Besides, single and bi-criteria optimizations are carried out using genetic algorithm (GA) to obtain the optimal solutions of design parameters, where the coefficient of performance (COP) and the solar collector efficiency (SCE) are selected as two fitness functions. The optimal solutions achieved from the bi-criteria optimization process will be given as Pareto frontier. The final optimum solution from the available solutions on the Pareto optimal frontier is selected using decision-making methods, such as LINMAP, TOPSIS, and Shannon’s Entropy. The comparison of single and bi-criteria optimization results illustrate that the bi-criteria optimization method yields more proper results than single ones, mainly because of the lower deviation index from the ideal solution. The results of bi-criteria optimization show that SCE decreases 1.6% compared to the initial model while the COP increases close to 20% that makes the optimum solution more desirable compared to the single ones. Moreover, the performance of the optimized DX-SAHPWH system is compared with the initial model in each month of the year. The results indicate that the performance of the optimized DX-SAHPWH is highly improved so that the working hours of the system decrease close to 109 h compared to the initial model during the whole year.

直接膨胀太阳能辅助热泵热水器（DX-SAHPWH）由于使用了环境能量和太阳辐射，因此对环境有利。本文旨在设计和开发使用R-134a作为工作制冷剂的DX-SAHPWH的热力学模型和热性能的多准则优化，全年为典型建筑提供生活热水。开发了DX-SAHPWH的热力学模型，从而通过尝试和错误技术以DX-SAHPWH的热力学周期的初始数据最少的方法来解决系统性能。在验证了热力学模型之后，针对位于伊朗温带气候的典型建筑，分析了DX-SAHPWH的性能。然后，进行了参数研究，以确定各种设计参数如何影响DX-SAHPWH系统的性能。在目前的调查中，设计参数的影响包括太阳辐射强度，环境空气温度，冷凝器的出水温度，太阳能收集器面积，压缩机速度，冷凝器中的管长，集热板上的管的外径，散热片研究了DX-SAHPWH的厚度，集热板的导热系数。此外，使用遗传算法（GA）进行单准则和双准则优化，以获得设计参数的最优解，其中选择性能系数（COP）和太阳能集热效率（SCE）作为两个适应度函数。从双标准优化过程中获得的最佳解决方案将作为Pareto前沿给出。使用决策方法（例如LINMAP，TOPSIS和Shannon熵）从帕累托最优边界上可用的解决方案中选择最终的最优解决方案。单准则和双准则优化结果的比较表明，双准则优化方法比单准则优化方法产生的结果更合适，这主要是由于与理想解决方案的偏差指数较低。双标准优化的结果表明，与初始模型相比，SCE降低了1.6％，而COP增加了接近20％，这使得与单模型相比，最优解决方案更为可取。此外，还将优化的DX-SAHPWH系统的性能与一年中每个月的初始模型进行比较。