18Apr

Solar Installers Panel Orientation Squeeze Every Watt of Electricity From the Sun That You Can

Monday | Filed in: Solar Commentary

If you’ve read any do-it-yourself books on solar installation, you probably know a little about how solar installers should orient panels. Keep them facing true south. Make sure you’ve got the correct tilt, or adjustable seasonal tilt, and no shading, not even partial shading. With This is all good general advice. But sometimes, true south orientation may not be quite as important as once believed.

I’m not advocating anything drastic, like pointing solar panels north. Under certain conditions, the orientation can be flexible without drastically reducing the energy produced. But this depends a lot on exactly what kind of solar system you have. Wait a minute, you say. We always want the greatest amount of energy collection from our expensive solar electric panels, right? Well, not always. Depending on your specific load, climate, location and other factors, computer simulations of solar systems with various orientations and configurations show that the ideal orientation for your solar array is not necessarily the standard formula.

Solar orientation is defined as a combination of two independent variables.

• Tilt is the angle of the PV array from horizontal.
• Azimuth is the angle between the PV array and true south.

Typically, tilt is the only variable adjusted, and azimuth is kept at zero (pointing directly south). However, as more roof-integrated, solar grid-tied arrays are installed, solar installers and users are increasingly choosing or accepting a nonsouth azimuth.

Off-grid solar systems will usually produce much less usable energy per installed watt than grid-tied solar systems. Aside from battery losses and older, non-MPPT controllers, this is because they are usually sized for less than ideal sun conditions in the winter months. During the summer, the batteries may be completely charged by noon, so the solar-electric array is turned off by the series charge controller. The potential energy of those solar is wasted by not being captured all afternoon.

Off-grid, we try to tilt the solar array at the ideal tilt, and directly south. Often, tilt angles are changed throughout the year. Take a look at the monthly and annual KWH results for different tilt angles in the two graphs. These graphs are from a computer simulation of the potential production from a 100 watt solar array in Sacramento, California, latitude 38 N.

According to the simulation, if you or your solar installers are going for a fixed array, the 30 degree tilt angle gives the best production of the two fixed regimes. This simulation also indicates that if you are willing to adjust your array twice a year, you’ll get the maximum energy with angles of about 23 degrees for summer and 53 degrees for winter. Annual energy will be 3.3 percent higher than with a fixed array. If you are willing to adjust once a month, annual energy will increase by 4.7 percent over a fixed array. This may be enough to justify the added cost of an adjustable rack.

If you have a custom stand-alone solar system, you want the energy when you need it, not just sometime during the year. Depending on the appliance usage patterns of its occupants, the electrical load of off-grid homes may be higher in the winter, the summer, or fairly constant from season to season. Since there’s significantly more sun hours during the summer months, optimizing the tilt angle of a fixed array for winter makes sense in some cases. But some off-grid systems have larger summer loads such as irrigation or air conditioning, so optimizing the array to catch winter sun is not always the best choice. Maximum annual energy production is not the holy grail of off-grid solar systems. What you want is maximum energy production when you need it.

Nowadays, more and more solar systems are grid-tied. Investing in solar electricity on the grid is cost effective for more and more places in the U.S. Grid-tied systems with annualized net billing have the benefit of essentially unlimited energy “storage.” Any surplus put into the grid in the summer is immediately used by another utility customer, and provides energy credits to the system owner. If more energy is needed in the winter (or at nighttime), it can be purchased back from Mr. Utility. With this unlimited “battery‚” return on investment is maximized by putting the solar panels where they generate the most annual energy.

Depending on your exact location, your array could be up to 75 degrees off from solar south, or 10 or 15 degrees too steep or too shallow of a tilt, and still get 90 percent of the benefit. We’ve all heard of goofs when someone didn’t know the difference between true and magnetic south. In most locations in the U.S., this is less than 20 degrees, so in reality, it doesn’t have a significant impact on solar performance. (It is worth noting that the exception to this thinking might be solar thermal panel orientation. Solar thermal should point as close to true south as possible.)

In general, the sunnier the climate, the more forgiving it will be of off-ideal orientation. The higher the attitude, the less sensitive it will be to off-azimuth error. Used with permission. © Home Power, Inc.

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