June 4, 2015 Bernardsville, NJ
The AP Physics Class at Bernards High School became interested in the 92.51 kW solar array that sits on the school’s rooftop. The students, led by Alec S., came up with a list of questions in order to better understand the science behind their clean-energy electrical system.
1. What angle or angles do the solar panels make with the ground?
In order to harness as much solar energy as possible, the panels are tilted 10 degrees towards the south. In the summer, the sun’s path travels much higher overhead, but is still generally south. At solar noon (the time that the sun is highest in the sky) the sun is at an angle of about 73 degrees in the sky (90 degrees would be perpendicular, so 73 degrees is about 4/5ths the way to being directly overhead, but not quite there). Conversely, on December 21st the sun has a much lower position in the sky at solar noon and reaches a peak of about 26 degrees. The ten degree tilt on the panels helps provide better solar exposure to the system. The tilt also allows rain water to wash dirt and pollen off of the modules. Now, we do not go any higher than a 10 degree tilt on the roof because we cannot allow the panels to shade each other. So, as the panels are tilted, they are also spaced apart so that the sun is not blocked. Too high of a tilt would require too much spacing between the panels and the system size would have to be decreased. This is a critical aspect to the design of each system.
2. What compass direction(s) do the panels face?
There are actually two separate solar arrays on the HS roof. One (28.71kW) faces 139 degrees relative to TRUE south (not magnetic) and one (63.8kW) faces 197 degrees relative to true south. Since the panels face different directions, there is a separate inverter for each in order for the equipment to properly manage and produce the power. Panels that face different directions on a single inverter cause issues and can lead to poor performance since the inverter is trying to manage a group of panels that have different electrical characteristics as a result of being exposed to different levels of irradiance (intensity of the sun’s energy on the panels).
3. Do any panels swivel or are they all stationary?
The panels are fixed and do not move. Tracker systems are quite common in many areas, but not so much in the northeastern US and especially not on rooftop systems. Trackers are always working to follow the sun, regardless if it is raining, cloudy or snowing, making them a great fit for a system in the dessert or other area where there is a lot of sun. Since we have so many cloudy and rainy days here in the northeast, common design practice is to deploy fixed systems to harness the suns energy when it is available. The weather in the northeast also causes more maintenance requirements for tracking systems, since they have many moving parts.
4. What was the upfront cost of the system?
Under the Power Purchase Agreement, the most important aspect for the school is that they did not have to pay anything. The school simply purchases the power that the system generates. With that said, roof mounted systems, such as is located at the Bernards HS, are the most cost effective installations for solar. Ground mount systems cost a little more to install and canopy systems over the parking lots can be much more expensive. Also, for projects that are done on public schools and government sites, the State of NJ specifies the rates the workers are paid, which is called prevailing wage. Prevailing wage can lead to more skilled workers and better quality of the work, but does increase the cost of the projects. Lastly, the time the project was built is somewhat important. The cost of solar panels and other equipment needed to build the systems has been decreasing over the years as more companies compete and improve their products. The materials needed for the Bernards HS project were purchased a couple years ago in the middle of this price decline. So, in solar, we speak in terms of dollars per watt, which means for every watt(a single solar panel on the school’s roof is 290 watts, but then there are all of the other component costs to add) that is installed, you will spend those dollars. In 2011 when the Bernards HS system was nearing construction, prevailing wage solar industry costs were around $3.50 a watt here in NJ. Had the system been constructed in 2009, the cost would have been closer to $5 to $6 a watt and if it were installed today, would probably be closer to $2.50 a watt.
5. What kind of upkeep is typically required? Have repairs been necessary thus far with the system at BHS? What is the average price over time for a solar panel system such as BHS’s?
We perform preventative maintenance inspections on every system at least twice a year. The system at Bernards HS is operating quite well and has not required any major work. We also spend a lot of time looking at the performance data so that we can track how the systems are working even when we are not on site. We do this for all of the county project sites that we maintain and can achieve some economies of scale in terms of cost.
6. How long do solar modules typically last?
The solar panels are warrantied to provide 80% of their original power (in this case 290 watts each) in year 25 of operation. That means that in 25 years, the system should still be producing at least 80% of what it did when it was turned on in day one. Solar panels degrade over time at a rate of about 0.5% per year. So, over the course of 25 years, they will lose about 20% of their initial power rating. This is not so bad. There are other components such as system inverters, monitoring and racking components that also require attention over the years, but in terms of the solar panels, they have a very long life and would be conceivably still producing power 100+ years from now, just less power.
7. What is the surface area of the BHS system?
The roof mounted system used on the roof of the HS tilts the panels so that they have a better exposure to the south, where the sun’s path travels year-round. Since the panels are tilted, they have to be spaced apart so they do not cast shadows on the panels to the north. Shading, even just a small amount, is a bad thing for a solar system. So even though the array covers around 17,000 SF, actually only about 75-80% of that has solar panels directly over the roof. The other areas make up the row to row spacing. My estimate in square footage is approximate, and I encourage you to look at the school in Google Earth and use the measuring tool where you can also see for yourself!
8. What percentage of the school’s power comes from the solar system?
That is a very good question, and actually depends on the current usage at the school. When designing a solar system for a building, the expected amount of energy to be produced by the system cannot exceed the 12-month historical usage based on recent utility bills. So I can guarantee that it is not greater than 100% of what the school uses. Bernards HS is also a nice and large building that has at least three utility meters serving the building. However, whatever energy that is produced by the solar system provides a cost savings to the school, since it is cheaper to buy power from the solar system than it is from the utility.