Thermal performance comparisons for solar hot water systems subjected to various collector and heat exchanger flow rates

Abstract

The thermal performance of solar domestic hot water (SDHW) systems is influenced by the rate at which heat transfer fluids within the system are circulated. An experimental investigation has been conducted at the National Bureau of Standards to quantitatively evaluate the influence of flow rates, both for SDHW systems that circulate potable water directly through the solar collector array and for systems that employ an external heat exchanger to transfer heat from the solar collector array to the potable water. This article presents data from side-by-side experiments that shows improvements in overall SDHW system performance as a result of lowering the collector fluid flow rate for direct systems utilizing conventional return tubes. Although they are limited to one location, specific system configurations, and time periods, these experimental results support the general conclusions reached in earlier experimental and simulation studies regarding the advantage of reduced collector flow rate. Side-by-side experiments were also performed for SDHW systems in which the tanks were fitted with return tubes designed to reduce internal tank fluid mixing. The results of these experiments show only a small difference in overall performance for the systems operated at conventional and reduced collector flow rates. Side-by-side tests of an indirect SDHW system that employs an external heat exchanger did not show improved performance at reduced tankside flow rates. A simulation study of an indirect SDHW for a range of heat exchanger designs and collector and tankside capacitance rates concluded that an optimum collector-side capacitance rate does not exist and an optimum tank-side capacitance rate occurs only for heat exchangers with overall heat transfer coefficients much larger than that used in the experiments.