Solar PV

We will experience some solar PV installations and technology while traveling, so I provide some more details about it below.

• In order to maximize panel output, the panel should be as close to perpendicular to the sun's rays as possible, which allows it to capture the most solar energy possible at a given time. (Here is a really good interactive animation of why a perpendicular panel captures the most sunlight.) Most solar panels are fixed, i.e., they don't move. This means that you need to choose your location carefully. For fixed panels, there are two factors to consider in this regard:
  • Azimuth/orientation is the compass direction that the panel faces. Typically, 0º is due north, 90º is east, 180º is south, and 270º is west.
  • Tilt is the angle above the horizon to which the panel is tilted.
• Shading is a very important consideration as well. All else being equal, more shading means less output.
• In the Northern Hemisphere, the rule of thumb to maximize the output of a fixed panel is that the panel should be faced due south (180º orientation) and at "latitude tilt." You may recall from a Geography class that latitude is how far north or south a location is from the equator. The equator is 0º, the tropics are at 23.5º, and the North and South Poles are at 90º north and south, respectively.
  • For example, State College, PA is at just over 40º north, so the ideal array tilt is about 40º, and the ideal azimuth is 180º (south).
  • Anchorage, Alaska is at about 61º north, so the ideal tilt and azimuth are about 61º and 180º (south), respectively.

Figure 6.3: Rooftop solar array. There are 270 panels in this array. You can see some of them in the left image. The right image shows the side view of a panel row. The panels are tilted at about 13º. You cannot see it in the image, but the panels have an orientation of about 222º, which means they are faced southwest.

Credit: D. Kasper.

A few more terms that are important to know:

• A solar cell is the smallest current-generating part of an array. They can be any size but are normally around 6 inches by 6 inches. It's difficult to see, but in the image above there are 72 cells in each panel (12 rows, each with 6 cells).
• A solar panel (aka module) is a number of cells wired together in a single panel. They can be any size but are usually about 5 feet by 3.25 feet.
• A solar array is a number of panels wired together. The image above shows part of a solar array, which as I noted has 270 total panels.
• Capacity refers to the rated maximum output of a panel or array. Under optimal conditions, a 250 W panel will output about 250 W of electricity. Note that this is only the immediate output. By the time the electricity goes to the building or the grid, some of it is lost due to a variety of factors (usually 10% - 15%).
  • The array above has 270 panels, and each panel has a capacity of 305 W. Thus, the capacity of the array is (305 W x 270 = ) 82,350 W, which is 82.35 kW.
• A solar panel outputs DC (direct current) electricity. DC means that the electricity flows in one direction. Household outlets (and the electric grid) use AC (alternating current) electricity, which rapidly alternates direction of flow (60 times per second in the U.S., in case you are interested), so electricity from an array must be converted to AC if it is to be used in a building or distributed to the grid. An inverter is a piece of equipment that converts DC to AC and is a standard part of most solar arrays. 
• A grid-tied array is one that is connected to the grid. A standalone or off-grid system is not tied to the grid.

The Impact of Tilt and Orientation 

Recall that the rule of thumb is that the optimal tilt is "latitude tilt," and the ideal orientation in the Northern Hemisphere is due south (180º). This begs the question: what happens if the tilt and orientation are not optimal? The answer, as you might guess, is "it depends." This impact can be quantified by something that is called tilt and orientation factor (TOF). The tilt and orientation factor is a decimal that indicates what percent of the maximum solar output you would receive throughout the year at said tilt and orientation. So if you install an array and it has a TOF of 0.85, that means that it will only be able to output about 85% of the energy it would output if it were at the ideal tilt and orientation. 

Wilmington, Delaware is at about 40º north. As it turns out, the ideal tilt is closer to 35º (rules of thumb are only rules of thumb, after all!). The tables below show the TOF at different tilts and azimuths. The first table illustrates the TOFs of different tilts, all with an orientation of 180º. The second table shows the TOFs at different azimuths, all at a tilt of 35º (the ideal tilt). (You can investigate the TOF for locations throughout the U.S. by going to the Solmetric website.)

Calculating Solar Output

Okay, now we're ready to calculate the solar output. There are a number of software programs and a formula or two that can do this, but the National Renewable Energy Laboratory (NREL) provides a free one that is well-regarded in the energy industry called PVWatts. In the video below, I demonstrate how to calculate the annual output of the array in the images above, which has the following specs:

• address: 400 Stanton Christiana Rd, Newark, DE
• 82.35 kW capacity
• 13º tilt
• 222º azimuth

Figure 6.4: Chart depicting the average monthly output in a typical year for the system specs indicated above. This chart was generated using the output from the PVWatts calculation detailed in the video above.

Credit: D. Kasper.

A Few More Notes

Hopefully, by now you have a relatively good grasp on some of the considerations that go into designing and calculating the output of solar PV. Solar PV really took off in the early- to mid-2000s, led by residential array installations, which generally had capacities of a few kW. The solar industry in the U.S. is not dominated by utility-scale solar, which is much cheaper per W to construct because of economies of scale. Utility-scale arrays can be thousands of watts (multiple MWs) in capacity!

Finally, there are a few ways that people can use and pay for solar PV:

• Systems can be purchased and owned by individuals. This can be done out-of-pocket or using loans. Out-of-pocket purchases usually generate the highest return.
• Power purchase agreements allow people or businesses to pay a third party to build an installation on their own roof. The individual or business then pays the third party for the electricity generated at a contractually agreed-upon rate. This is the model that SolarCity (now Tesla Energy) used to rise to prominence. This is a popular model because it requires no up-front cost (among other reasons).
• Community-owned solar or just community solar is used increasingly in the U.S. and elsewhere. Community solar allows individuals to "buy into" a solar array that is installed somewhere else, or perhaps on a shared roof or field space. This provides access to individuals and businesses that would not otherwise be able to use solar due to factors such as not having a suitable site (e.g., a heavily shaded roof) or living in rented space. (The SEIA provides a good overview of community solar here, and NREL provides a more robust explanation here.) 
• Most utility-scale installations effectively act as power plants, and the electricity is sold to the grid.