A Photovoltaic/Thermal Electrochemical Stripping System for Urine Nitrogen Recovery
Authors: Coombs Orisa Z , Barbosa Botelho Amilton , Tarpeh William Abraham 
Published: August 2024

Introduction

Distributed production of urine-derived fertilizers offers a sustainable and equitable alternative to Haber-Bosch fertilizer. Lab-scale electrochemical stripping (ECS) recovers nitrogen as ammonium sulfate fertilizer from many complex wastewaters, including urine.1 Developing nations in Africa pay higher prices than the global average for Haber-Bosch-based ammonia due to accessibility issues.2 Using solar panels as a power source, ECS can be deployed for low-carbon, on-site ammonia production which can minimize transport, optimize energy, and lower fertilizer prices. This study aimed to (1) understand the ideal ranges of ECS operating parameters (current density and temperature) for efficient nitrogen recovery, and (2) provide proof-of-concept for a photovoltaic/thermal ECS system.

Methods

Experiments were performed in a three-chamber electrochemical reactor. Electrodialysis was used to remove nitrogen from urine in the anode chamber to the catholyte, while membrane stripping recovered nitrogen from the catholyte as ammonium sulfate in the trap chamber.
Solar panels face diminishing efficiency as they heat up.3 On the other hand, membrane stripping becomes more efficient with increased catholyte temperature, increasing the nitrogen recovery rate.4 Thus, experiments varying the temperature of catholyte from 30°C to 60°C were performed to quantify the extent to which heat improves ECS efficiency. Current densities ranging from 5mA/cm2 to 20mA/cm2 were evaluated for their effect on nitrogen recovery rate and energy efficiency to inform the sizing of photovoltaic components.
A photovoltaic/thermal ECS prototype was constructed and operated indoors using a solar simulator, and outdoors in real sunlight. Heat was transferred from the solar panel to the catholyte using a cold plate fixed to the backside of the solar panel. Energy production and usage of the solar-ECS system were compared across several conditions: with or without heat transfer, and with or without batteries and charge controllers (Figure 1).

Results

Higher temperatures led to higher nitrogen recovery rates, with a maximum rate of 5.4 mg N/min at 60°C as compared to 4.2 mg N/min at room temperature. Heat also decreased electrical resistance inside the reactor, resulting in lower energy consumption. However, higher temperatures increased the movement of water vapor from the catholyte to the trap chamber, causing a more dilute ammonia sulfate solution. Additionally, experiments were operated as an open system. As a result, ammonia evaporation out of the reactor was observed as the temperature increased.
The primary effect of current density is on ammonia migration from the anode chamber to the cathode chamber (removal). Nitrogen removal occurs from the influent faster with higher current densities at a rate of 0.09 g N per mA/cm2 over 7 hours. However, the system experiences lower current efficiency and lower overall energy efficiencies, using an additional 2.2 kJ/g N per additional mA/cm2. Removal speed and energy efficiency have implications for the cost of ECS. For a single reactor to extract a set amount of nitrogen in a given period, a larger reactor can be built and operated at a low current density for a higher capital cost with lower operating costs, or a smaller reactor can be operated at a high current density for a converse cost scheme.
A functional photovoltaic/thermal ECS prototype was built. The simple cold plate attachment effectively lowered the steady state temperature of the solar panel surface by 17°C and raised the temperature of the catholyte by 13°C. Integration of a battery, maximum power point tracker (MPPT), and DC/DC converters allowed for a 53% increase in the production of solar electricity and control over current input to the ECS reactor, although they increased the cost of the system. Using the outputs of this solar-ECS prototype as a benchmark, a techno-economic analysis will be completed to elucidate competitiveness.

Conclusion

Understanding relationships between electrical energy, heat, and ECS performance informs the scale-up of ECS for distributed ammonia production. Capturing nitrogen for reuse reduces reliance on Haber-Bosch ammonia, prevents aqueous pollution, and supports a circular nitrogen economy. Broad application of this technology would promote equitable access to fertilizer and support local food production. Integration with solar panels improves the applicability of ECS to remote and developing regions by avoiding the need for grid-based electricity. The joint solar-ECS system advances the United Nations’ Sustainable development goals for responsible production and consumption, clean energy, and clean water and sanitation access.

References

Tarpeh, W.A. et al. Environ. Science Technol. 2018

Comer et al., Joule (2019)

Fragassa, C. et al., Appl. Comput. Mech. 2021

Liu, M. J. et al. Water Research 2020

Figure 1