Solar Energy Design Considerations for your Home

Poor Design Reduces Power Output

Solar Energy is becoming more common in the United States and especially in Southern California including Orange County and San Diego County.  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

Click Here to Enlarge

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.

Coastal areas and even up to 5 miles inland, will often have morning haze or “June gloom”.  The Azimuth chart shows that SE and SW are equal, when in fact, depending on the micro-climate, a West facing roof will sometimes outperform an East facing roof.  On occasion, we have seen a West facing roof outperform a South facing roof in areas with a lot of morning fog and haze.

PVWatts first screen for entering zip code

There are a number of simulation tools available to determine power output for Solar arrays. One of the better ones is called PVWatts V2.  The first page that comes up allows you to input a zip code.  For this, I used 92660. 

PVWatts Input Screen

 The PVWatts input screen is where you can input parameters in order to simulate your array. 

In the DC rating cell, you can put in the size of the system.  If you have a (20) 200 watt panels, the DC Rating would be 20 x 200 divided by 1000 = 4.0kW.

Here are a few assumptions.  If you keep the array clean, you don’t have to derate the array by 5%.  If you buy a -zero tolerance module, you don’t have to derate the module by 5%.  Most Grid Tie Inverters are also in the 96% range.  With good design, you can easily reduce the voltage drops for AC and DC wiring.  I suggest using a derate in the DC to AC derate factor of around .88 or 88%.

Click Here for a larger view

In the Array tilt, you can simulate different tilts and in the Array Azimuth, you can simulate orienting your array at different compass directions.  Using a 4.0kW system, .88 derate, 33.793 array tilt and 180 degrees South, you will get the the output shown to the left.

PVWatts will output the solar energy predicted per day, based on 30 years of flat plate 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.   

Popularity: 7% [?]