Penn State Prement
Donahoe solar

Monitoring Site:
Donohoe Center - Solar

Greensburg, PA

Donohoe Center in Westmoreland County has a solar photovoltaic (PV) system installed by Penn State and is located adjacent to a wind turbine.  

Visitors are welcome to stop by and see the facility.  The array is designed to provide up to 2 kW of power to the electrical grid, although the average power production over the course of a year is much lower, due to clouds, nighttime, and the changing angle of the sun.  ;kgkgk

Photovoltaic arrays produce low voltage, direct current (DC) electricity, which must be converted to alternating current (AC) by an “inverter” if one is to use the power on the electrical grid.

Energy Production: Donohoe Solar

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Donohoe Solar

Donahoe Solar

by Brandee Scheffey, student research assistant

[note: click here for a kids solar PV coloring page]

Solar energy has the potential to be the most popular type of alternative energy for the future. The sun already provides energy for most living things, and now it is possible to use solar radiation more directly by converting it into useful electricity. Photovoltaic systems convert solar radiation to electricity through a variety of methods. The most common approach is to use silicon panels, which generate an electrical current when light shines. Commercial solar panels cab be up to about 20% efficient, which means that for every 100 watts of solar radiation shining on the panel, only 20 watts of electricity are produced. As a reference, on a sunny summer day at noon, you can expect about 1000 watts of solar radiation per square meter of solar panel, although the average over the course of the year is much lower.

The initial cost of panels could possibly seem intimidating. Keep in mind that some electric companies will pay homeowners for the extra power they don't use every month. You also may get tax credits and other government incentives for "going green". Even so, the economics of solar photovoltaics are not always very positive in Pennsylvania.

At Penn State Extension's renewable energy demo site in Greensburg, PA, the Donahoe Center uses the energy of the sun to help power their own facilities. Penn State has installed a solar photovoltaic system to provide up to 2 kW of power to the electrical grid. (The electrical grid is an interconnected system for delivering electricity from suppliers to consumers. It consists of three main components: generating plants that produce electricity, transmission lines that carry electricity to demand centers, and transformers that reduce voltage so distribution lines can deliver power to consumers.)

Photovoltaic arrays produce low voltage, direct current (DC) electricity, which must be converted to alternating current (AC) by an "inverter" if one is to use the power on the electrical grid. Solar photovoltaics are especially helpful for remote locations where it would be too expensive to supply electricity through a wire. The main advantages of solar energy are that it is clean, able to operate independently or in conjunction with traditional energy sources, and it is very low maintenance. The main disadvantages are that it is currently more expensive than other forms of electricity, and the availability of solar radiation varies from day to day, and from season to season. In fact, some parts of Pennsylvania are among the cloudiest spots in the United States. Before installing solar panels, consider the amount of sunlight that reaches the location for the solar panels (1 or 2 hours per day isn't worth it). Ideally, find a spot that faces south with no shade cover at all, research thoroughly which company is best for you, and use insulation to reduce energy loss. Another factor limiting the amount of energy that can be produced is the changing angle of the sun, especially in the winter months. If you are curious about how much sun is available in your area or how much energy you will be able to produce, you can find out by going to

The graph below shows the trend that is associated with cloudy versus sunny days, or the brightness of the sun on a given day versus how much energy can be produced on that day. Unsurprisingly, more energy is produced on the bright and sunny days, while much less is created on cloudy days.




Take a look at the following table of measurements taken from the Donahoe Center on May 1, 2011. Graph the points to see the relationship between irradiance and energy. Use the slope of the graph to calculate the efficiency of the solar panel. The photovoltaic array consists of fourteen panels, each one 0.81 meters by 1.62 meters.

Time Irradiance (w/m2) Energy Produced (kW)
8:00 AM 5.25 0
9:00 AM 5.25 0
10:00 AM 5.25 0
11:00 AM 10 0
12:00 PM 40.25 0.04
1:00 PM 98 0.12
2:00 PM 66.75 0.12
3:00 PM 145.25 0.24
4:00 PM 190 0.36
5:00 PM 197 0.36
6:00 PM 138.5 0.24
7:00 PM 75 0.12

Need a few Hints? Here are some ideas to help you if you're stuck.

Keep in mind that the percent efficiency of the photovoltaic array is equal to 100 times the power going out divided by the power going in. The "Energy Produced" column in the table above is the "power going out". The power going in is equal to the irradiance times the size of the solar panel.

One way to approach this problem is to first multiply the irradiance values by the panel size to find out the total solar "power in" - in units of watts. Divide by 1000 to change it from watts (w) to kilowatts (kw). Now, plot the data on a graph, with "power in" as the x-axis, and "energy produced" as the y axis.

Add a "best fit line" to the data (you can use a rigorous line-fitting procedure or just eyeball it), and measure the slope of the line. Multiply the slope times 100 to find the efficiency of the panel.

Note: to be strictly accurate, the irradiance values should be measured at the solar panel, with the sensor paralell to the tilt of the panel. These readings were actually measured at the building next to the panel, with the sensor mounted in a horizontal position. It is possible to adjust the irradiance values to equal the irradiance on the tilted array, based on the position of the sun and the reflectance of the ground. However, this involves some pretty complicated geometry and we won't get into that right now.