Energy requirements and productivity of machinery used to harvest herbaceous energy crops
Herbaceous energy crops (miscanthus, switchgrass, and tallgrass prairie) are promising biomass feedstocks, but are difficult to harvest with traditional machinery. Energy required for harvesting novel herbaceous crops was evaluated in both field and laboratory settings. Previous work on harvesting concluded that crop flow and parasitic losses accounted for the majority of harvesting power requirements. In mowing, for example, research has shown that cutting of plant material accounted for only 3% energy usage. Investigations of single stem cutting of traditional forages (timothy, alfalfa, wheat straw, maize) have identified a critical cutting speed at which energy requirements are minimized and ideal cut quality occurs. This critical speed is believed to occur when blade velocity is high enough to sever the stem without moving it, which occurs when inertia forces equal or outweigh the external force imposed by cutting. Results of high speed cutting experiments in this study confirm the low influence of crop cutting on total harvest energy. Energy requirements of single stem cutting in miscanthus were 9.30 ± 2.60 J per stem, which represented only 2.1% of in-field mowing requirements. A critical cutting speed for miscanthus was not found and may occur above the range considered (10-20 m/s). Mowing and baling power demands ranged from 25-63 kW. Parasitic energy, required to power the machine without crop input, was high by multiple estimates, and averaged 56% of power usage. Due to these losses, harvest efficiency is favored by maximizing machine groundspeed and throughput, which increases instantaneous power requirements but accomplishes more useful work per unit of energy lost. Mowing productivity (23.6 Mg/h) was higher than baling (16 Mg/h), which may complicate the design of single-pass harvesting machinery. Due to the influence of ground speed on energy usage, modern hay and forage equipment will benefit from tractor-baler integration and self-controlled groundspeed. Overall, energy requirements of baled crop on a dry basis were 18.2, 10.9, 19.4, and 13.4 MJ/Mg for the large square baler, pull-type rotary mower, self-propelled rotary, and sickle, respectively. Based on these results, electronic throughput control shows promise as means of reducing energy usage. Conversely, advances in single stem cutting efficiency are not likely to reduce harvest power, since cutting represents a small percentage of overall power demands. In-field power measurements would benefit from additional work aimed at decreasing uncertainty using higher resolution yield measurements and considerations of draft power.
Biomass, Herbaceous Energy Crops, Miscanthus, Switchgrass, Hay and Forage, controller area network (CAN), Harvest
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign