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Title[re][re]Solar Car(t) Building Project2017-08-21 10:41:23

Solar Cart Project

By Connie Lee, Bella Lee, KyoungA Song, JinWoo Lee, and Thomas J. Gentz

The great solar eclipse on Aug 21, 2017, has swept throughout all America. The shadow of the total solar eclipse cut a path 70 miles wide, which was 1000 miles away from Tucson, AZ. The total eclipse lasted a few minutes along the path of the totality, but a partial solar eclipse here in Arizona lasted nearly three hours long.

Over the summer, a group of BASIS students gathered to develop a special project called the “solar cart project”. It took two months to assemble the solar cart during the steamy summer inside the RV garage. The idea was to use the solar cart to measure the shading effect during the solar eclipse.

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Figure 1. Team photo in front of the solar cart. From the left, Connie Lee, Bella Lee, JinWoo Lee, and Thomas J. Gentz.

Solar cart consists of 4 major components, two 100 Watt mono-crystalline Si solar panels, 40 A charge controller, 200 Ah deep cycle batteries, and 3 kW inverter. Framework of the solar cart is based on T-slotted aluminum framing system that is easy to build and can be configured into the simple projects and solutions. Single axis solar tracking capability has been demonstrated by mounting the solar panels using variable angle pivots. Solar cart system is wired around the 40 A Grape Solar charge controller, which regulates the photogenerated current flow from the solar panel at maximum power point to charge the batteries efficiently. Since the charge controller has a Bluetooth datalogging capability, we were able to take a screenshot of the system status regularly. Two deep cycle batteries are connected in parallel by AWG 10 wires. Emergency circuit braker and 15 A fuse between the charge controller and the batteries are installed on the solar cart system for safety. Xantrex Freedom SW inverter is tied to the batteries and there is another DC 300 A fuse installed between the batteries and the inverter. 

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Figure 2. Control panel (at the center) with LED indicator (upper left corner) and charge controller (upper right) for solar power generation on the solar eclipse day of Aug 21, 2017. Inverter system control panel is at the center and circuit braker at the bottom inside the control panel.


At first, we thought about traveling with the solar cart to the nearest path of totality. We could not only see the total eclipse, but also monitor the complete solar eclipse by the solar cart. However, making a field trip to Oregon or Idaho would take more than a day and we didn’t want to skip too many school days. Rather we decided to measure the partial solar eclipse here in Tucson, AZ. Since the peak of the solar eclipse happened to be early in the morning, we anticipated the only half day excuse from the school. One big advantage of watching the partial solar eclipse is that we can enjoy the solar eclipse in the back yard at home. The weather was really good and Sunny in Tucson, AZ, on 8/21/17.

One of the questions we had with regards to the solar eclipse is how much it will impact the solar power generation. About 60% partial shading was predicted at Tucson, AZ. Our hypothesis was that the solar power generation would be affected by the solar eclipse shading accordingly. Therefore we were trying to capture the images of the Sun by the pinhole viewer during the solar eclipse. The pinhole viewer made out of a shoebox worked fine to project the image of the Sun in normal day. However, we were not able to take a clear image of the eclipsed Sun because the smart-phone camera was not sensitive enough to capture the glimpse of the eclipse. Rather we found the NASA’s eye simulation program useful. NASA's eye simulation program is an interactive, web-based 3-D simulation, that can show you your view of the eclipse of August 21st, 2017 from anywhere on the planet. Once we entered the GPS location of Tucson, 32.1839 N, 110.5713 W, NASA’s eye simulation showed what the eclipse would look like throughout the eclipse period.


Figure 3. Screenshot of NASA’s eye simulation program. Red color circles and triangle were drawn to estimate the shaded area of the solar eclipses at Tucson location.

Since NASA’s eye simulation doesn't tell us about how much area of the Sun becomes eclipsed by the Moon, we developed an algorithm to estimate the area of solar eclipse. Assuming the apparent size of the Moon is same as the area of Sun, the area of the eclipse is equal to the twice of the difference between the circular sector and the triangle. Ratio of eclipses could be calculated by repeating this procedure. On eclipse day of 8/21/17, we measured the power generation by the solar cart and repeated the measurement once again on the other Sunny day on 8/27/17, which can be compared with the eclipse data as a control. Solar cart power data was acquired every 5 minutes during the solar eclipse in order to study the variations and test if the eclipse area predicted by the NASA’s eye simulation would agree with the actual solar power generation.

Figure 4. Comparison of the solar cart powers generated on eclipse day on 8/21/17 (red rectangle) and control on 8/27/17 (black line). Simulation data from the NASA’s eye (blue line) is fitted against the eclipse results. 

Figure 5. Solar power loss during the eclipse (red rectangle) and the estimates from the NASA’s eye simulation (blue line).

Figures above show the solar power profiles measured by the solar cart on 8/21/17 and 8/27/17, and the result of the NASA’s eye simulation. This figure shows how solar energy production goes down as the shadow of the Moon eclipses the Sun. It confirms the hypothesis that the amount of the solar power loss would be proportional to the shaded area during the solar eclipse. Simulation results matched very well with the actual solar power data generated by the solar cart. Especially the peak of the eclipse ratio data and its magnitude matched really well with the NASA’s eye prediction. Note that simulation results seem to underestimate the solar power loss in the solar eclipse results. Solar panel may have an intrinsic characteristic at different climate condition during the measurements at the eclipse and control. Indeed, Si panel may perform worse at low light intensity condition. As a result, we can conclude the solar power loss is linearly proportional to the eclipsed area of Sun during the solar eclipse.

With solar energy now having greater significance for national power generation, the solar eclipse in 2017 reminded us of the importance of solar energy in our lives. Last but not least, we become to appreciate the science and engineering by building the solar cart and really enjoyed watching the solar eclipse in 2017!


Figure 6. Blueberry “solar” smoothies made by the solar cart project team to celebrate the great solar eclipse in 2017.


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