Greenhouse solar drying as a circular economy strategy: Thermal, economic, and environmental advantages over electric drying

An experimental evaluation was carried out on two variants of a greenhouse solar dryer: a Direct-Mode configuration (DMGD) and a Mixed-Mode configuration, the latter equipped with four solar air heaters arranged in series (MMGD). A conventional Electric Dryer (ED) served as a benchmark for economic and embodied-energy comparisons. Over six-hour tests, MMGD evaporated 14.38 L of water, 33.7% more than DMGD (10.76 L) despite slightly lower drying efficiency (23.87% vs. 24.12%). Initial investment for MMGD was $8478, 2.4 times that of DMGD ($3558). Embodied energy analysis showed that DMGD required 3930 kWh, whereas ED required 3187 kWh but covered only 4.5% of the drying area. Energy Payback Time (EPBT) for DMGD was 0.74 years, among the shortest reported for solar dryers. MMGD achieved an EPBT of 1.18 years, supported by its annual solar thermal yield of 1830.7 kWh. Over a 10-year lifespan, MMGD mitigates 30.94 tons of CO₂, outperforming DMGD (24.92 tons of CO₂). The DMGD excels in capital efficiency and rapid energy return, whereas the MMGD delivers superior drying capacity and decarbonization. Both configurations are viable alternatives to electric drying, with the choice depending on whether the priority is minimizing upfront cost or maximizing environmental impact reduction.