Systemutformning av autonom batteridriven traktor inom lantbruket
Abstract: In this study, data from different studies were used to find how much energy that a conventional tractor required per hectare when it was performing different operations on a field. Data for operation speed and power demand were taken from an earlier study for each task of the tractor model Valtra 6600. In this report, the operation speed for each task on the field were simulated in Excel together with a given electrical motor, a battery and a charger. This was done in order to find what affected the working time for an autonomous battery powered tractor. Two scenarios were studied in Excel; how the distance to the charger, and different charging speeds affected the working hours in order to operate a 200-hectare farm. The size of the battery affected the results most when the distance between the field and the charger was studied. The system design was in need of a larger battery size when the distance to the charger was 10 km or more. With a smaller distance, a smaller battery was required to reach valid results. Larger power of the charger means not only lower charging time of the battery. The results also showed that if the system configuration had a larger charging power, the working days of the tractor did not decrease significantly by larger battery size. In other words, the working hours of the tractor were more affected by larger battery size for a low-powered charger than for a high-powered charger. In this study, a high-powered charger was set to 100 kW to decrease the purchasing cost. Considering the high cost and weight of the battery, the smallest battery size as possible was chosen, without increasing the working time on the field too much. A battery of 100 kWh was chosen since the results did not improve generally by installing a larger battery size. The electric synchronous permanent magnet motor was chosen to the system configuration, due to its normally high-power density and high efficiency. The power of the motor was set to 70 kW, since a smaller motor has lower weight and can be powered by less energy per hour compared to a larger motor. With a system configuration of a high-powered charger of 100 kW, a high energy density battery of 100 kWh and one synchronous permanent magnet motor of 70 kW, the working time for the autonomous battery-powered tractor to perform different operations on a field was calculated to 63 days. This included field operation, transport between the charger and the field and charging time. Assuming the tractor was working 24 h a day at a 200-hectare farm and that the speed of the tractor on the road between the field and the charger was 30 km/h. The cost and the weight of the system configuration was calculated to 587 000 SEK and 646 kg. Considering maintenance and whether variations, in reality the autonomous tractor would have needed more time than 63 days of working the farm.
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