Feasibility of active solar water heating systems with evacuated tube collector at different operational water temperatures

Abstract

With rapid advancements in society, higher water temperatures are needed in a number of applications. The demand for hot water presents a great variability with water required at different temperatures. In this study, the design, installation, and evaluation of a solar water heating system with evacuated tube collector and active circulation has been carried out. The main objective is to analyze how the required tank water temperature affects the useful energy that the system is capable of delivering, and consequently its profitability. The results show how the energy that is collected and delivered to the tank decreases with increasing the required temperature due to a lower performance of the collector and losses in the pipes. The annual system efficiency reaches average values of 66%, 64%, 61%, 56%, and 55% for required temperatures of 40°C, 50°C, 60°C, 70°C, and 80°C. As a result, profitability decreases as temperature increases. The useful energy, and therefore the profitability, will decrease if the demand is not distributed throughout the day or focused on the end of the day. The system’s profitability was determined in two cases: considering maximum profitability of the system, assuming 100% utilization of useful energy (scenario 1); assuming a particular demand, considering that on many days all the useful energy the system can supply is not used (scenario 2). The analysis shows that through proper sizing of the system, optimizing the number of solar collectors, the investment in the solar system can be profitable with similar profitability values in the two contemplated scenarios. In scenario 2, a combined-delivery system (solar and diesel boiler) generates savings of between 23% and 15% compared to a single-delivery system of diesel, with a reduction in consumption of diesel close to 70%. The number of collectors that maximizes the profitability depends on the required temperature; therefore, in designing this kind of installation water temperature requirements must always be taken into account. From an environmental point of view, CO2 emissions can be reduced between 392 and 325kg CO2 per m2 of collector, depending on the required temperature. The results of this study can be very useful in determining the feasibility of using such systems to supply a part of demand for hot water.