In a groundbreaking advancement at the nexus of sustainable energy, waste management, and agriculture, researchers have unveiled an innovative system that harnesses sunlight and thermal energy to convert human urine into valuable ammonium sulfate fertilizer. This pioneering technology, known as the photovoltaic–thermal electrochemical stripping (solar-ECS) system, epitomizes a novel approach to managing the global nitrogen cycle, offering multifaceted benefits ranging from enhanced fertilizer production and improved sanitation, to increased access to electricity in resource-limited settings.
The solar-ECS prototype represents a fusion of photovoltaic and thermal technologies, designed to operate independently of conventional grid electricity. By integrating photovoltaic panels with an electrochemical stripping apparatus, the system efficiently extracts ammonium ions from real urine, a notoriously underutilized waste stream, converting them into a concentrated, commercially valuable fertilizer form. This innovation not only alleviates the nitrogen management challenge but also redefines waste as a resource, contributing to a circular economy.
Key to the solar-ECS’s improved performance is the precise regulation of photovoltaic current and the strategic management of waste heat. The system actively extracts thermal energy generated by the solar panels, redirecting it to heat the electrochemical stripping cell. This deliberate thermal coupling enhances ammonia volatilization, identified as the rate-limiting step in nitrogen recovery. Simultaneously, the extracted heat cools the photovoltaic panels, thereby boosting their electrical output. The result is a synergistic gain: 59.3% more electricity generation and a 22.4% enhancement in ammonia recovery compared to prior designs lacking such heat transfer and current control.
This dual thermal management strategy underlines the sophistication of the solar-ECS design. By preventing excessive electrical current through charge controllers, the system curtails unnecessary energy consumption, quantified as 2.24 kilojoules per gram of nitrogen per unit of current density beyond optimal levels. Consequently, this fine control minimizes wastage and maximizes energy efficiency—critical attributes for off-grid applications where resources are often scarce and energy conservation is paramount.
Beyond technical refinement, the team advanced a comprehensive mathematical model describing electrochemical stripping under varying current densities and temperature conditions. This process model permits accurate prediction of system behavior across operational parameters, facilitating optimization tailored to site-specific solar irradiance, urine composition, and ambient conditions. Such modeling is indispensable for scaling the technology, enabling informed decisions regarding design adjustments and deployment strategies to match local needs.
Economically, the solar-ECS holds transformative potential. Model-based assessments estimate possible net revenues from recovered fertilizer at up to $2.18 per kilogram of nitrogen in United States markets and as high as $4.13 per kilogram in African contexts. These figures underscore the system’s capacity to generate substantial value from waste, potentially enabling decentralized fertilizer production that strengthens agricultural productivity and farmer livelihoods, particularly in regions where supply chains are fragile or costly.
Underpinning this innovation is a broader vision aligned with the United Nations Sustainable Development Goals (SDGs). By facilitating zero hunger through sustainable fertilizer provision, advancing clean water and sanitation via nutrient recovery from wastewater, and promoting clean energy through photovoltaic integration, solar-ECS exemplifies a multi-benefit technology addressing intertwined global challenges. Its distributed nature lends itself to deployment in remote or marginalized communities, where traditional infrastructure is often lacking.
The use of real urine in experimentation is a critical step toward real-world applicability. Urine, which contains approximately 80% of the nitrogen excreted by humans, typically ends up diluted in wastewater systems or untreated, contributing to environmental pollution. Solar-ECS enables its direct valorization, circumventing the inefficiencies and environmental costs associated with centralized wastewater treatment and synthetic fertilizer manufacturing. This approach could dramatically reduce nitrogen runoff, eutrophication, and greenhouse gas emissions associated with the conventional nitrogen cycle.
Technically, the electrochemical stripping process operates by inducing ammonia volatilization from urine through controlled heating and electrochemical reactions. The volatilized ammonia is then captured and converted into ammonium sulfate, a high-purity fertilizer compatible with existing agricultural practices. By combining photovoltaics for electricity generation with thermal management to accelerate ammonia release, the solar-ECS system reflects an elegant integration of fundamental principles across disciplines including electrochemistry, solar energy engineering, and environmental science.
Notably, the system’s modular design lends itself to scalable, distributed installation. Unlike large centralized plants, solar-ECS units can be deployed at community or household scales, bringing fertilizer production directly to users. This reduces dependency on long supply chains and mitigates logistical hurdles prevalent in many developing regions, enhancing resilience and local autonomy. Furthermore, the electricity produced can support auxiliary needs such as lighting or device charging, contributing to improved quality of life beyond nutrient recovery.
The research team meticulously demonstrated that the interplay between solar panel cooling and ECS heating was critical to the performance uplift. By extracting the waste heat to warm the ammonia stripping phase, the system achieves higher reaction rates without compromising photovoltaic efficiency. This thermal synergy is a clever exploitation of otherwise wasted energy flows, emblematic of sustainable design thinking aimed at maximizing output while minimizing inputs.
The development of charge controllers to maintain optimal current densities is another highlight. Without this regulation, excessive current leads to energy losses and potential deterioration of system components. By implementing precise electronic control, the solar-ECS ensures stable and efficient operation, extending system longevity and reliability. These engineering details are crucial for practical adoption, especially in decentralized contexts where maintenance capabilities may be limited.
Looking ahead, the authors anticipate further enhancements in material selection, reactor geometry, and control algorithms to push system efficiencies even higher. Integrating advanced sensors and IoT connectivity could enable remote monitoring and adaptive operation, which are attractive features for deployment in resource-poor settings. Moreover, adapting the technology to recover other nutrients and contaminants from various wastewaters broadens its applicability and impact.
Importantly, the solar-ECS system stands as a compelling example of circular resource management and renewable energy utilization converging to address pressing environmental and social challenges. By transforming an anthropogenic waste stream into valuable commodities using only sunlight and smart engineering, this innovation encapsulates the promise of technology-driven sustainability. It paves the way toward more resilient and self-sufficient communities while mitigating the adverse impacts of conventional nitrogen management.
In sum, this research marks a significant leap toward sustainable nutrient recovery, coupling electrochemical innovation with renewable energy integration in a practical, usable form. The solar-ECS system’s demonstrated improvements in power production and ammonia recovery efficiency, grounded in rigorous modeling and experimental validation, highlight its potential as a game-changer in global efforts to balance nitrogen cycles, improve sanitation, enhance clean energy access, and foster sustainable agriculture worldwide.
Subject of Research: Photovoltaic–thermal electrochemical stripping system for decentralized recovery of nitrogen fertilizer from human urine.
Article Title: Prototyping and modelling a photovoltaic–thermal electrochemical stripping system for distributed urine nitrogen recovery.
Article References:
Coombs, O.Z., Joo, T., Botelho Junior, A.B. et al. Prototyping and modelling a photovoltaic–thermal electrochemical stripping system for distributed urine nitrogen recovery. Nat Water 3, 913–926 (2025). https://doi.org/10.1038/s44221-025-00477-w
Image Credits: AI Generated
