Solar Design, Quality, and Performance Part 2

Solar Design and System Performance

Solar Energy is becoming more common in the United States and especially in the central valley of California. If you are considering a solar energy system for your home, there are some design considerations that will help make your system cost effective, reliable and safe.

Cost Effective Solar Energy
The first and most important consideration for a solar array is to make sure it has sun on it. Although this seems obvious at first glance, there are many examples of installation of solar panels behind a chimney or under a tree. There will always be trade offs, as you can’t cut down all the trees or remove chimneys just for the sake of a little more energy.


IV Curve for Maximum Power Point Tracking
Solar cells are extremely sensitive to shading. If a single cell is shaded, it will affect all of the cells wired in series in the solar panel by acting as a resistor. Solar panels have a number of bypass diodes which will bypass a series of shaded cells and will mitigate the problem to a certain extent. The easiest way to illustrate this is by wiring in a 12 volt RV fan directly to a lower voltage panel. When placed directly in the sun, the fan runs extremely fast. When casting a shadow over a number of cells, the fan speed drops significantly. Another more expensive method is to use a clamp on DC current meter. When casting a shadow, the DC current will drop off quickly.


Shade Analysis Tool
There are design methods that can reduce the impact of shading. One of the more important tools is a Solmetric Suneye. The Suneye is a great tool for Solar Companies because it provides an indication of how much shading will occur on an array at different times of the day and different times of the year. It also predicts, however crudely, what affect the shading will have on energy generation. As the shade tool only predicts gross reductions in sunlight striking the surface of the module and doesn’t model individual solar cell characteristics or array IV curves, a little interpretation by by good solar designer is still required. Minimizing shading or eliminating it are the still best courses of action.

Array Orientation for Best Energy Production

Generally, if you want the best energy production out of a solar array, you would orient it to the South. A 10 degree tilt would give you the best summer production and, in California, a 30-35 degree tilt would give you the best year round production if the array is facing South. The further East or West the array is oriented, in general, the flatter the array should be for best production. The Azimuth and Roof pitch chart illustrates this concept. Note that, in general, most residential roofs are 4:12 pitch to 6:12 pitch. Anything steeper than this is pretty tough to walk around on.

PVWatts is another tool that will output the solar energy predicted per day, based on 30 years of flat plat data and simulates the predicted energy output per month and year for your solar energy system.

Many solar designers, sales persons and even solar energy companies do not fully understand how IV curves and Maximum Power Point Tracking work nor do they understand the ramifications of poor design on the solar energy output of a system. A good designer can squeeze the last few kWhs out of a system. System design is almost always a trade off between the space available, shading from objects including inter-row spacing, and the budget for the system.