A Developed Solar Knapsack Sprayer for Sustainable Smallholder Farms: Performance, Biomechanics and Ergonomics Analyses

The present study proposes a configuration of a solar PV-battery-powered knapsack sprayer (SPKS) equipped with a rear-mounted multi-nozzle boom to serve as a sustainable, reliable and decentralized spraying system for smallholder farmers. This design aims to significantly reduce the physiological strain and inconsistent performance associated with conventional manual lever sprayers (MLSs). The SPKS was evaluated against the MLS during onion crop spraying in terms of hydraulic performance, field capacity, spray deposit uniformity, and operator ergonomics and biomechanics, alongside an economic and environmental sustainability assessment. Results of hydraulic tests revealed that the SPKS achieved the optimal spray distribution uniformity of C.V = 16.67% at an operating pressure of 350 kPa and a boom height of 40 cm. Field experiments demonstrated that the SPKS more than doubled the effective field capacity to 0.36 ha/h compared to 0.16 ha/h for the MLS, achieving a field efficiency of 67.55%. Moreover, the SPKS achieved high spray deposit coverage (88%) compared to the MLS (52.6%), while the integrated PV panel effectively doubled operational runtime by maintaining >50% battery state of charge under continuous load. Biomechanics and ergonomics pilot analyses indicated that the MLS operation imposed high musculoskeletal (RULA score = 7) and cardiac strain, whereas SPKS operation was classified as low-risk (RULA score = 3) with minimal cardiac strain. Economically, the SPKS saves approximately $8.70 USD per hectare in labor costs. Environmentally, it prevents ~17.0 kg of CO2 emissions annually and reduces pesticide application volume by 26.5%. By simultaneously addressing energy limitations, ergonomic hazards, and operational inefficiencies, the SPKS offers a holistic and superior solution for sustainable smallholder agriculture.