Prototyping and modelling a photovoltaic–thermal electrochemical stripping system for distributed urine nitrogen recovery

Distributed solar-enabled nitrogen capture from urine helps to manage the nitrogen cycle and increases fertilizer, sanitation and electricity access. Here we provide proof of concept for a photovoltaic–thermal electrochemical stripping (ECS) system, known as solar-ECS, that recovers ammonium sulfate fertilizer from real urine independently of the electricity grid. Constant control of photovoltaic currents and extracting waste heat to cool the solar panel while heating ECS enabled 59.3 ± 3.6% more power production and improved ammonia recovery efficiency by 22.4 ± 7.4% relative to prototypes with no heat transfer and uncontrolled currents. The added heat accelerated ammonia volatilization (the rate-limiting step of ECS), while preventing excessive current via charge controllers reduced energy use by 2.24 ± 0.25 kJ g−1 N per excess milliampere per square centimetre. A new process model for ECS operation at different currents and temperatures was proposed and applied to estimate possible net fertilizer revenues of up to US$2.18 kg−1 N in US markets and US$4.13 kg−1 N in African markets. By advancing the recovery of high-purity commodity chemicals from underused wastewaters, this work supports United Nations Sustainable Development Goals for zero hunger, clean water and sanitation, clean energy and responsible production. Recovering fertilizers from wastewater has the potential to make intensive agriculture more sustainable and reduce aqueous pollution, but energy requirements could be prohibitive. A prototype photovoltaic–thermal electrochemical stripping system shows how distributed ammonia manufacturing can be achieved through solar energy in off-grid locations, thus reducing energy and environmental costs.