Development of an off-grid polygeneration system utilizing multi-waste heat recovery from low-grade heat sources for sustainable production of e-methanol, potable water, liquefied CO2, and utilities

The multi-waste heat recovery approach represents an innovative solution for harnessing waste heat in industrial sectors. Harnessing and employing waste heat leads to a decrease in adverse environmental effects and improves overall performance. In this study, a near-zero polygeneration system is developed by utilizing cascade heat recovery from geothermal sources and flue gases, and integrating it with key subunits including an absorption chiller, carbon capture and storage, reverse osmosis, water electrolysis, methanol synthesis, and power generation cycles. In the proposed integrated off-grid system, multi-products are generated, including e-methanol, oxygen, drinking water, electricity, natural gas, chilled water, and liquefied CO₂. Energy, exergy, sustainability, and enviro-exergo-economic analyses were conducted to evaluate the proposed system. The thermodynamic evaluation revealed an energy efficiency of 67.71 % and an exergy efficiency of 42.79  %. Implementation of the designed system led to annual fuel savings of 6,885.30 L and a reduction in CO₂ emissions by 26,396.81 tonnes, corresponding to an annual carbon tax avoidance of $346,326.09. The sustainability index was calculated to be 1.748, while the LCOP and TUCP were determined to be $0.0926/kWh and $3.4385/GJ, respectively. Additionally, the effects of key operational variables- including seawater flow rate, geofluid flow rate and temperature, methanol synthesis reactor pressure, H₂/CO₂ ratio, electrolyzer power consumption, and LNG flow rate - on proposed system performance were thoroughly investigated. Finally, based on the results of the parametric study, a sensitivity assessment was performed using the SHAP approach to quantify the influence of key operational parameters on thermodynamic and economic performance indicators.