Submitted to: Solar Energy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2012
Publication Date: 5/20/2012
Citation: Vick, B.D., Neal, B. 2012. Analysis of off grid hybrid wind turbine/solar PV water pumping systems. Solar Energy. 86(5):1197-1207.
Interpretive Summary: Thousands of windmills currently pump water from underground lakes (aquifers) that exist all across the Great Plains. Settlers in the late 1800’s would not have been able to survive the dry semi-arid climate of the Southern Great Plains without the invention and improvement of the windmill. In addition to the windmill, solar powered water pumping systems are becoming more common in remote areas of the United States. At the USDA-ARS Research Lab near Bushland, Texas, we conducted an experiment on the use of hybrid wind/solar systems to pump water from a depth representative of the aquifer. In the experiment we combined different size solar photovoltaic (PV) arrays with a single small wind turbine to pump water with a single pump. A controller was also needed to convert the AC power from the wind turbine to DC power before it was combined with the solar PV array DC power. It was found that combining a 640 Watt PV array with a 900 Watt wind turbine had the highest efficiency (55%). However, it was also found that the water pumped by this hybrid averaged 15% less than if the wind turbine and solar PV array pumped water individually. We also combined a 320 Watt solar PV array with the same 900 Watt wind turbine. For this hybrid the average amount of water pumped by the hybrid was 5% greater than if the wind turbine and solar PV array pumped water individually. A third case was tested where a 480 W PV array was combined with the 900 Watt wind turbine, and it was found that the water pumped with this hybrid was about 6% less (e.g. in between that of 320 W and 640 W PV array). We feel the hybrid with the 320 Watt PV array performed best because the voltage of this PV array was closest to that of the wind turbine. Therefore, we plan to use some additional electrical devices to improve the voltage match between the wind turbine and the solar PV array. Since most of the small wind turbines manufactured in the world are made in the U.S., increasing the sale of these wind turbines for water pumping should increase the number of jobs in U.S. Also, the hybrid wind turbine/solar PV array will be more reliable since it is not dependent on one resource like windmills and solar PV systems. This increased reliability should mean more production from the rancher and farmer.
Technical Abstract: While many remote water pumping systems exist (e.g. mechanical windmills, solar photovoltaic , wind-electric, diesel powered), very few combine both the wind and solar energy resources to possibly improve the reliability and the performance of the system. In this paper, off-grid wind turbine (WT) and solar photovoltaic (PV) array water pumping systems were analyzed individually and combined as a hybrid system. The objectives were to determine: 1) advantages or disadvantages of using a hybrid system over using a WT or a solar PV array alone; 2) if the WT or solar PV array interfered with the output of the other; and 3) which hybrid system was the most efficient for the location. The WT used in the analysis was rated at 900 W alternating current (AC). There were three different solar PV arrays analyzed, and they were rated at 320, 480, and 640 W direct current (DC). A rectifier converted the 3-phase variable voltage AC output from the WT to DC before combining it with the solar PV array DC output. The combined renewable energies powered a single helical pump. The independent variable used in the hybrid WT/PV array analysis was in units of W/m2. The peak pump efficiency of the hybrid systems at Bushland, TX occurred for the 900 W WT combined with the 640 W PV array. The peak pump efficiencies at a 75 m pumping depth of the hybrid systems were: 47% (WT/320 W PV array), 51% (WT/480 W PV array), and 55% (WT/640 W PV array). Interference occurred between the WT and the different PV arrays (likely due to voltage mismatch between WT and PV array), but the least interference occurred for the WT/320 W PV array. This hybrid system pumped 28% more water during the highest water demand month than the WT and PV systems pumping water individually.