Background: Solar water heating is a highly sustainable method of extracting thermal energy from the sun for domestic and industrial use. In residential buildings, thermal energy from a Solar Water Heater (SWH) can be used to heat spaces, shower, clean, or cook, either alone or in combination with conventional heating systems such as electricity- and fossil-fuel-based heaters. In the industrial sector, SWHs can be used in various high-temperature fluid processes, including chemical processing, manufacturing, power generation, and construction. Despite the technological advancements in water heating systems, there are still some significant technical and economic challenges that limit their widespread adoption and commercialization. Despite their potential to revolutionize the industry, these systems remain in the shadows of unsustainable water heating solutions. Main body of the This paper reviews recent selected publications on the technical and techno-economic aspects of solar water heating technology. The discussions include the effects of some parameters and components on the overall thermal efficiency of the SWHs as well as the techno-economic prospects of the technology. In addition, the paper provides the existing research gaps and recommendations for future research directions. Short conclusion: The present review paper is unique because it condenses the recent studies carried out on both the technical and techno-economic aspects of the SWHs. It provides a comprehensive framework for interested readers and researchers to gain insights into the technical and economic status of SWHs. However, it is not an exhaustive study. The information presented can aid researchers in conducting further research on the subject, as well as understanding the future of SWHs.

Air-to-water heat pump (AWHP) is a key technology for decarbonising building heating, yet their deployment in Alpine regions remains limited due to performance degradation at low source temperatures. These regions are climate-change hot-spots characterised by rapid warming and complex topography, raising the question of whether future climate change will alleviate or intensify the operational constraints affecting AWHPs. This study quantifies the combined impact of start-up and defrosting losses on AWHP performance under present and future climate conditions. An integrated TRNSYS-based model was developed, incorporating experimentally derived degradation correlations for start-up and defrosting cycles. A single-family house equipped with an inverter-driven AWHP was simulated for five Alpine municipalities in Northern Italy, across elevations from 83 to 652 m above sea level. In the base year, start-up operation accumulates up to 40 to 70 hours per month during transitional and winter periods, while defrosting reaches up to 30 to 40 cycles per month at higher elevations. These transient effects reduce the seasonal coefficient of performance (SCOP) by 4 to 6% due to start-up alone and up to 11% when combined with defrosting, leading to primary energy increases of 7 to 12%. By 2050, climate warming reduces defrosting frequency by up to 33 to 100% in transitional months, while start-up hours remain significant due to increased part-load operation. As a result, SCOP increases by about 2% in 2030 and 6% in 2050, with primary energy consumption decreasing by up to 10%. Explicit modelling of operational dynamics is essential for realistic AWHP assessment.