A completely new concept for harnessing waste heat and solar energy
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Researchers from the Faculty of Mechanical Engineering at the University of Ljubljana, led by Assistant Professor Dr. Primož Poredoš, in collaboration with researchers from the ITEWA Innovation Team at Shanghai University, have developed a new concept for utilizing ultra-low temperature (waste) heat by simultaneously introducing solar energy and low-temperature heat into the system. They are the first in the world to present an entirely new concept for producing distilled water from saline, untreated water using thermally driven membrane distillation (MD).
Multi-stage solar-driven membrane distillation units (Figure 1A) are based on the evaporation of saline water and the transfer of water vapor molecules through a membrane positioned between the feed water (saline, untreated) and the distilled water. Unlike traditional MD units (Figure 1A), where heat is supplied to the first stage, the authors explored the effects of simultaneously heating both the first and lower stages of the multi-stage MD device (Figure 1B). Paradoxically, the authors heated, rather than cooled, the condensers in the lower stages with ultra-low-temperature heat (Figure 1B), which enabled almost a doubling of the distillate flux per stage compared to existing literature. This groundbreaking research was published in the prestigious multidisciplinary journal Nature Communications (IF = 14.7) by the publisher Springer Nature.
In the article, the authors first presented the role of basic physical principles in increasing the distillate flux within the air gap of the MD device (Figure 1C) and, based on initial studies, later built three devices with integrated heaters for utilizing ultra-low-temperature heat (Figure 1D, configurations K1, K2, and K3). The experimental data demonstrated a groundbreaking distillate flux of over 9.0 L m⁻² h⁻¹ and an energy conversion efficiency of 407% when heating the condensers with a heat flux of 250 W m⁻² in the 7th and 8th stages, and with solar radiation intensity of 1000 W m⁻² in the 1st stage. Calculated per stage, the authors achieved a record-breaking distillate flux of 1.13 L m⁻² h⁻¹ S⁻¹, which exceeds the current benchmarks in the literature by approximately 88% (Figure 1E).
Globally, at least 16% of the world’s primary energy use is released as waste heat, with more than 63% of this heat occurring at very low temperatures below 100°C. Consequently, through this study, the authors highlight the critical need for more efficient use of waste heat, directly supporting several United Nations Sustainable Development Goals (SDGs).
Based on experimentally validated simulations in the COMSOL Multiphysics numerical simulation software, the authors found that a 16-stage system based on MD+STB has the potential to reduce capital costs by more than 70% per liter of produced water, making it more competitive with other modern solutions such as photovoltaics with reverse osmosis (PV+RO).
The research is extremely important, as water scarcity is becoming an increasingly serious global challenge, already affecting the lives of more than four billion people worldwide. Climate change further exacerbates water security, threatening both natural ecosystems and living conditions as well as economic development. Using renewable and sustainable energy sources, such as solar energy and waste heat, thus represents an effective solution for producing fresh water from existing terrestrial water sources.