Thermodynamic limits of using fertilizer osmosis to produce mechanical work via pressure retarded osmosis

The potential for concentrated fertilizer to drive water treatment, nutrient recovery, and/or power generation has received increased attention. Recently the concept of fertilizer-driven pressure retarded osmosis (or “Green PRO”) was introduced to the literature and experimentally validated. The limits of power from fertilizer osmosis however have not yet been established, and therefore the potential for this energy source to supply meaningful farm loads is uncertain. In this paper, a combination of analytical, numerical, and experimental methods are used to establish the thermodynamic and process limits of fertilizer energy conversion via PRO. The results indicate that up to 125 Wh/kg of energy is released from fertilizer when it is diluted in water. PRO process dynamics including operation at constant applied pressure, and the need to maintain high power levels throughout the batch process, limit conversion to work to up to 14.9 Wh/kg. Further, experimentally-calibrated simulation results suggest that only up to 6.8 Wh/kg can be expected when considering non-ideal transport dynamics such as reverse solute flux and concentration polarization. The effect of feed source concentration and recovery ratio on batch process dynamics are investigated, and it is found that using wastewater as feed may be comparable to scenarios where clean irrigation feed with high recovery is used. The potential of common hydroponic plant crops is evaluated, and results indicate that advances in membrane technology may allow energy recovery to approach 5% of typical greenhouse electricity consumption. [Display omitted] •Up to 125 Wh of energy is released per kg of fertilizer when it is diluted in water.•This energy approaches 5% of typical greenhouse electricity consumption.•PRO at constant applied pressure limits energy conversion to up to 14.9 Wh/kg.•Non-ideal transport dynamics further limit energy conversion to up to 6.8 Wh/kg.