Assessment of the performance, payback and preferability of residential solar water heaters (SWHs)—A case study in Windhoek, Namibia

The utilisation of solar water heaters (SWHs) even in sunny locations remains low. Most previous studies used assumptions or modelling and simulation software to assess the techno-economic performance of SWHs and unravel the reasons. In this study, real-life data on domestic thermosiphon SWHs, operating in the large solar resource city of Windhoek, Namibia, were used to assess the performance. A novel payback model, based on non-financial variables, was proposed. With this model, the impact on payback time of solar fraction (SF), temporal mismatch between energy supply and energy demand, and other operational factors was quantified. The results show that despite the good thermal performance, the energy savings of the SWHs were limited by the temporal mismatch between energy supply and demand.

The payback periods of the SWHs, obtained with the net-present-value (NPV) financial approach, ranged between 11.6 and 13.5 years. The predictions of the developed model agreed well with the payback periods obtained with the financial approach. Fully paid upfront SWHs provided high return on investment (8 - 11%) while loan financed SWHs had negative return on investment and their payback periods stretched beyond the lifetime of the systems.

The decision making process in the choice of domestic water heating option was also analysed. The results showed that the choice was governed by the initial cost, disadvantaging the pricey SWHs, despite their potential for high returns, but favouring the cheap to acquire yet costly to operate electric water heaters (EWHs). The preferability of SWHs increased notably when their purchase price dropped to the price which guarantees a payback time of five years.