Smart solar greenhouse heating via hybrid PTC–FPC integration: A thermo-hydraulic and exergy assessment

Meeting the growing energy demands of controlled-environment agriculture requires efficient and sustainable thermal-management solutions. This study experimentally and analytically investigates a novel hybrid solar-thermal greenhouse heating system integrating a linear parabolic trough collector, an unglazed flat-plate solar collector, and a variable-speed storage circulation unit. Energy, exergy, and hydraulic analyses were performed to characterize the effects of mass flow rate on the system performance. To comparatively assess the tradeoff between thermal benefit and hydraulic penalty, a dimensionless thermohydraulic Performance Index (PI) was introduced, defined as the ratio of the normalized useful thermal output to the normalized pressure-drop penalty. Experiments conducted at mass flow rates ranging from 0.5 to 1.5 kg min −1 showed good agreement with the analytical model, with correlation coefficients of 0.96–0.98. The results demonstrate that increasing the mass flow rate enhances heat-transfer performance but also results in a disproportionately greater pressure-drop penalty. The PI consequently decreases monotonically from 1.0 to 0.27 over the investigated flow-rate range. Among the tested operating conditions, 0.5 kg min −1 provides the highest PI and is therefore identified as the most favorable operating condition according to the proposed thermohydraulic screening criterion. This result does not imply simultaneous maximization of the individual thermal and exergy performance metrics. The proposed PI provides a comparative framework for evaluating the balance between relative useful thermal output and hydraulic penalty in hybrid solar-thermal greenhouse heating systems. The findings can support the design and operation of efficient solar-assisted thermal-storage systems for controlled-environment agriculture under climatic conditions comparable to those investigated in this study.