Abstract The greenhouse dryers consist opaque and insulated north wall with solar air heating collector at the bottom. It has been used for bitter gourd flakes drying under natural and forced modes. Thermal models were developed for the both modes of drying. Experiments were performed concurrently for open sun, natural and forced modes to compare their thermal modeling and drying kinetics results. Energy analysis includes; embodied energy, CO2 emission, CO2 mitigation and economic evaluation were done for both modes of drying. The predicted values of bitter gourd flakes and room air temperature, and moisture evaporation rate showed the fair agreement with the experimental observations within the root mean square of percentage deviation ranged from 3.25% to 5.82% and 2.83% to 5.26%, and the coefficient of correlation ranged from 0.98 to 0.99 and 0.99 to 1 under natural and forced modes, respectively. The final moisture ratio was achieved 0.12 after 15 h, 0.14 after 16 h and 0.25 after 21 h under forced, natural and open sun modes respectively. Crop drying under forced mode is found significantly faster than other modes of drying. The Logarithmic model was selected as best curve fitting technique for non-linear regression analysis for forced mode. The energy payback time of developed dryers under natural and forced modes is 1.68 and 2.35 years, respectively. The net CO2 mitigation of bitter gourd flakes drying is 33.04 and 36.34 t under natural and forced modes, respectively.