SOLAR SPLASH® "Handbook"
The three Handbook Chapters contain information from various Solar Splash newsletters and form a "handbook" for teams preparing to enter SOLAR SPLASH for the first time or for those getting ready for their next SPLASH.
Handbook Chapter 1 - So you want to enter the SOLAR SPLASH
Handbook Chapter 2 - Managing the sun
Chapter 3 - Getting the Most From the Sun
During the 1999 Solar Splash, David Luneau of the University of Arkansas at Little Rock and Technical Manager of the Splash gave two talks on the subject of maximizing the power from the sun. This chapter is centered on the content of those talks and expands the principles to include some of the system tradeoffs that are relevant.
As part of the inspection process at the Event, the output of the solar array is tested for conformance with the 480 watt rule. In the course of doing this, the short circuit current and open circuit voltages are measured, as well as many other I-V data points. Solar panel characteristic curves, and their resulting Peak Power Point (PPP), are calculated and made available to each team. It is not uncommon to find that the voltage of the PPP is not the same as the system voltage of the boat. Thus, the output power of the array is not maximized.
In analyzing the competitors' systems for the World Solar Challenge, Chester Kyle explains, "Somewhere in between the open-circuit voltage and the short-circuit voltage, the cell will develop its maximum power output. This peak power point will change according to the illumination, the temperature, the cell design and its load or power demand." Since the Endurance Event is run on a closed course, the sun angle is constantly changing, as are other factors such as the variability of clouds and their effects on the amount of energy reaching the Endurance course. "For this reason, peak power point trackers that will automatically operate each part of the panel at its maximum power level can significantly improve performance. Power trackers oscillate about the peak power point, locating it by taking frequent samples of the cell's transient properties."
From figure 1, the PPP can easily be seen. If this were a 24 volt system, the value of trackers would be significant. How important is it in the overall design considerations of the craft?
If we go back to the late 80's when solar cars were being raced in Europe, the most successful designs tended to be aerodynamically clean, with the arrays mounted on tilting or adjustable structures usually on the top of the car. It is important to understand that these races were relatively short sprints from one city to another in relatively complex terrain. When the first World Solar Challenge was announced in 1987, AeroVironment did extensive evaluations of various design concepts and concluded that the increase in incoming energy resulting from tilting the array was more than offset by the significant aerodynamic drag. In that event, the course was essentially a North-South line of 3000 km with the sun changing position as the day passed. Sunraycer was designed by AeroVironment to minimize aerodynamic drag and, with the utilization of power trackers, to optimize the power from the array as one side tended to be shaded in the morning and the other in the afternoon.
The applicability of these lessons to the Solar Splash is interesting to examine. Speeds during the Endurance event are sufficiently low that one does not need to be very concerned about the aerodynamic drag of the array, but the effect of wind on this structure may have a significant effect on the stability and handling of the craft. It is also important to note that the Endurance heats are run within approximately 2 ½ hours of solar noon. This, combined with the relatively low sensitivity of solar cells to sun angle, makes one stop to consider the various tradeoffs. Weight, simplicity of fabrication, and the stability of the boat often lead to the conclusion that a flat, horizontal array has many advantages.
It is also interesting to review the applicable meteorology. In the absence of clouds, the geometry is rather straightforward, but the Endurance heats are run over a period of about 6 hours and in that time, conditions can be highly variable. Although several unusual conditions had been anticipated for the race across Australia, the circumstances under which the maximum incoming solar radiation was measured were unforeseen. The actual peak, well above clear conditions, was a text book altocumulus cloud formation in which the individual cells and spaces created lens-like quality and thus scattered more radiation from a larger area of the sky than essentially one point. At another time, the sun reflected off a large cumulo-nimbus, and thus the strongest radiation was coming from two rather different directions.
Whether the array is fixed or tiltable, there seems to be no doubt that the addition of power trackers would be a valuable addition to any system design. But, are they mandatory? No. It can be argued that a manual system will work. In the 1990 World Solar Challenge, the winner, Honda, chose a manual system, even though the next 9 finishers all had trackers.
Certainly, over the years, we have seen significant success from a number of the Splash participants who have stuck with the KISS principal (keep it simple, stupid) and those who have built a more reliable system. Another important lesson learned is that one must start somewhere and build on success. Re-designing the entry from scratch each year only leads to new problems. Do as many things as well as possible but do not hesitate to set realistic goals and leave some things for future teams.
As always, we are here to be of assistance to anyone. We will not make decisions for you, but perhaps our many years of experience can provide some help.
Remember, if you have questions, do not hesitate to contact Headquarters at hq@solarsplash.com.
Handbook Chapter 1 - So you want to enter the SOLAR SPLASH
Handbook Chapter 2 - Managing the sun