## 1 Introduction  
Utilizing solar energy for water heating can reduce energy cost to a great extent. Serpentine type thermosyphon solar water heater (SWH) is type of passive heater that can be built easily and can be used with ease without any complexity. Such a system was constructed and tested on the premises of Dhaka University at Dhaka, Bangladesh [1]. The performance of SWH depends on several physical dimensions, operating parameters and meteorological parameters. While evaluating the performance, attention is often on the thermal efficiency only. However, the energy equation does not consider many internal and conversion loss factors. High thermal efficiency does not ensure high exergy efficiency [2,3]. Performance analysis considering exergy analysis by using the Second Law of thermodynamics can lead to the optimized performance of a system to a greater extent while tracking the entropy generation [4,5]. This paper mainly concentrates on dimensional, operating and meteorological parameter of the SWH for maximum exergy efficiency and how various parameters affect exergy efficiency. Serpentine tube has been widely used in heat exchangers and solar water heaters for supplying heat and hot water. Physical dimensional parameters have great effect on the performance of the system. Serpentine tubes need to be carefully designed and placed in the system. A Computation Fluid Dynamics (CFD) analysis of serpentine heat exchanger showed that the optimum position lies in contact with the walls of water tank with a certain tube diameter and length [6]. [PERSON] et al. [7] showed that the exergy efficiency varies inversely with width of the collector, using the \"Three Procedure Theory\" [8]. Thermal conductivity of absorber plate has significant effect on solar fraction and configuration. However, this effect can be minimized if the thermal conductivity is above 50 W/m-k [9]. [PERSON] et al. [10] analyzed the dependency of output exergy of air with respect to aspect ratio of collector, mass flow rate per unit absorber area and duct depth, and commented that there is an optimum inlet temperature of air and mentioned parameters for a particular system. However, they neglected the bluer work during the analysis. [PERSON] et al. [11] provided exergy analysis model of flat plate collector (FPC) and reported 5.96% exergy efficiency where major portion of the exergy destruction is caused by energy conversion from high sun temperature to low absorber plate temperature. [PERSON] et al. [12] recommended the optimum operation conditions of FPC by exergy analysis. However, they assumed constant water inlet temperature equal to ambient temperature. They did not take into account the pressure drop exergy destruction. [PERSON] et al. suggested FPC design method based on minimum entropy generation but they also neglected the exergy destruction due to pressure drop [13]. [PERSON] et al. [14] provided a detailed approach of exergy analysis and optimization of FPC and showed that the exergy efficiency is 3.898% at the optimum condition. Exergy analysis of numerous types of solar thermal collectors and processes was reviewed in detail by many researchers [15; 16; 17; 18; 19; 20; 21; 22]. Steps for exergy balance [23] and exergetic optimization of thermosyphon SWH for dimensional and operating conditions and optimum results is shown in this paper. These steps and optimum result can be used in determining the dimensions and selecting the operating conditions for similar other systems. MATLAB optimization toolbox [24] is used to determine the parameters for the maximum exergy efficiency. The values of parameters at optimum condition are then used for examining the effect of a certain parameter on the system performance. A summary of exergy destruction and loss [14; 25], dimensionless exergy [26] and exergy destruction ratio [27] is provided here, which can be used for predicting the performance of this type of systems. Various components of thermosyphon SWH are shown in Section 2. Section 3 presents the exergy model and optimization result.