Design, space optimization and modelling of solar-cum-biomass hybrid greenhouse crop dryer using flue gas heat transfer pipe network

•A hybrid crop dryer is presented.•Thermal model is developed and validated.•Heat transfer model predicts the heating load.•Economic analysis is presented. A solar-cum-biomass hybrid greenhouse crop dryer (HGCD) is presented to work on solar energy and on biomass heat for 24 h continuous operation at constant drying temperature of 62 °C. Vertical gap (clearance) between two consecutive trays is optimized for selected latitudes of 30°, 35°, 40°, 45° and 50°N. Global solar radiation and thermal models are developed to predict the solar radiation availability and HGCD chamber air temperature (Thgcd). Forced draft paddy straw bale combustor (FDPSBC) is used to generate flue gas above 500 °C temperature as supplemental heat source and coupled with flue gas heat transfer pipe network (FGHTPN) laid inside the HGCD to maintain Thgcd at constant temperature. Biomass heating load requirements (forced convection and radiation heats) were predicted through developed heat transfer model. The developed thermal model predicted the Thgcd of 26 °C to 38 °C equivalent to heating load of 4–6.5 kW when ambient air temperature remained between 10 °C and 18 °C at Ludhiana climate (30°N) India. Heat transfer model predicted about 26.2 kW and 13.4 kW of supplemental heat during severe and moderate night temperatures of −5 °C and 10 °C which can be met through complete combustion of 80 kg and 40 kg of paddy straw biomass per hour respectively to maintain Thgcd between 60 and 62 °C. Logarithmic drying model is the best fitting drying model for fenugreek drying. Economic analysis shows that the proposed technology can recover its cost within five years and has great adaptability potential in terms of paddy straw management in India.