Optimization of Collector Area and Storage Volume in Domestic Solar Water Heating Systems with On-Off Control—A Thermal Energy Approach

Numerical simulations of solar water heating systems using on-off control were performed for four locations in the Portuguese territory, two collector types, and a wide range of parameters. Two main design parameters were considered: the collector area and the storage volume. The relationship between system performance and system parameters was investigated by computing the optimal design parameters needed to achieve a predefined yearly solar fraction. The optimum collector area is higher (an increase of 5–15 %) for a double-glazed than for a single-glazed collector, suggesting that in Portugal, the single-glazed type is more effective for solar water heating applications. This fact is more significant for lower solar fractions and warmer climates. A small increase in the optimum collector area leads to a very strong decrease in the optimum storage volume (an area increase of 1% leads to a volume decrease of 27 %). Collector flow rate and storage volume increments result in lower collector areas. However, high flow rates lead to high pumping losses and lower collector efficiencies, and when the storage tank losses are significant, higher tank sizes may lead to higher collector areas. The solar fraction has a very strong impact on the collector area and storage volume (when the solar fraction increases from 0.5 to 0.99, the optimum collector area and storage volume increase about 17 and 56 times, respectively)