The Modeling of a Natural Convection Heat Exchanger in a Solar Domestic Hot Water System

Abstract

The use of simulations can greatly aid in optimizing the design of natural convection heat exchangers (NCHEs) in solar domestic hot water (SDHW) systems. Fraser et al. (1992) presented a NCHE model that is used in the WATSUN solar simulation program, that requires experimental measurements of the heat exchanger thermal performance and shear pressure losses. Two TRNSYS models are here presented for a NCHE in a SDHW loop. The simple model, based on Fraser et al.'s work, requires experimental testing on the particular heat exchanger. The simple model can be used for optimizing SDHW system parameters (i.e. pipe lengths and diameters, collector areas, tank volume etc.) excluding the NCHE itself which is represented by the experimental curves. A detailed model, based upon cross flow correlations, requires geometric specifications of the NCHE being simulated and is applicable to shell and coil and counterflow configurations. By varying heat exchanger geometric parameters (such as the number of helices, diameters of helices, diameter and length of the heat exchanger shell) the detailed model can be used to design an optimum NCHE. Results comparing the detailed model with Fraser et al.'s experiments show reasonable agreement. Using the detailed model and the least cost savings economic analysis, simulations were performed to discover the optimal shell and coil NCHE geometry. It was found that considerably reducing the heat exchanger size led to enhanced economic performance over a 10 year period of economic analysis. Coil spacing and tube diameter had a lesser impact upon system performance than heat exchanger shell length and number of helices. Thermo Dynamics Inc. manufactures a shell and coil NCHE that contains 4 coils and is 0.635 m. The optimal heat exchanger design contains 2 helices and is 0.45 m long. For a given set of system parameters, a SDHW system containing the optimally designed heat exchanger would save the consumer an extra $110 in initial equipment cost, and $52 over a 10 year period. Heat exchanger designs were subject to variations in system parameters, such as collector area, hot water draw, location and glycol flow rate. Although each set of system parameters suggested a different optimal design, overall, the optimal design found for the initial set of system parameters remained adequate. As different economic assumptions will lead to differing optimal heat exchanger lengths, this work can serve as a guide for those who desire to optimize a shell and coil NCHE based upon a prevailing set of economic assumptions.