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System Design Page. A Solar Power Primer

 

Remote Home Power System

 

This section is an introduction to designing a solar electric system for a "typical" single family home. The concepts apply to any size system, however. The calculations are oriented toward "off-grid" or battery-based systems. "Grid-tie" systems are connected to the utility electric service grid, sometimes provide only a fraction of the electricity used, and therefore require different analysis to determine if the system is a success.

How do I Aim / Point my solar panels?

Should I use a Tracker?

The angle of sun hitting the panel is extremely important. Ideally, it is nearly perpendicular. A panel mounted horizontally will have good performance in the summer when the sun is straight overhead, but poor performance in the winter when the sun is low in the sky. For small systems at 40-50 degrees latitude the panel is often fixed at 45 degrees. A good solution for larger systems is to adjust the angle twice a year. For summer a relatively flat angle (latitude minus approximately 15 degrees) is good. For winter a relatively steep angle is good (latitude plus approximately 15 degrees) and also helps to shed snow. Keep in mind that mounting an array absolutely flat will catch excessive water and dirt will accumulate.

In the Northern Hemisphere, if the panels are tilted they should face nearly South, within 15 degrees or so, to get maximum solar energy in a day.

Trackers work well to maximize energy output in the summer, if there is a wide horizon. The benefit is reduced in winter due to a shorter arc across the sky. Therefore trackers are especially beneficial for water pumping systems since maximum output is usually desired in the summer.

For off-grid systems, the challenge is to make sure the system can supply the necessary surge power and keep within the desired amount of generator run time. For grid-tie systems, design is simpler if the system has no batteries and the grid acts as a backup. There is also a more complex economic aspect to grid-tie systems. Electric rates and utility interconnection policies vary dramatically from state to state, even from town to town. Finally there is a hybrid type of grid-tied system where the grid acts as a backup for the solar panels and batteries act as a backup for the grid. These type of system inverters are improving rapidly. A grid-tie system with batteries, unless very large, usually requires household loads to be separated into two groups with a subpanel: loads to be backed up by battery and loads which only use grid power. This is necessary because often some home electrical loads are not practically powered by batteries.

Currently the grid-tie market is the fastest growing photovoltaic market. This has been facilitated by Public Utility Commission buydowns in many areas and the availability of large, high-quality modules to reduce labor costs. Whatever the economics of specific systems and programs, the growth of the market has indeed benefited all photovoltaic users by encouraging new products and furthering understanding of system design. If you're still interested in how to design a solar/battery power system, read on.


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