The following is a guest post by Dr. Joshua Rhodes, a Postdoctoral Research Fellow in The Webber Energy Group and the Energy Institute at the University of Texas at Austin.

There has recently been a lot of buzz about which direction solar panels should face. It seems the initial conversation was prompted by a Pecan Street Inc. report that looked at output from real solar arrays and found those facing west tended to have better power output alignment with summer peak power demand – something different than maximum total annual energy production. Not long after, Greentech media ran an article titled “Are Solar Panels Facing the Wrong Direction?” that was reblogged/run/wired through other outlets including National Geographic, USA Today and others. Recently, the New York Times and Popular Science even purported that solar panels should face west, rather than south, to avoid unintended consequences. However, each of the above only considers the simplified case of west- or south-facing panels. Just like the amount of sun that a place receives, the answer is not the same everywhere.

A recent research paper published (by me! et al.) in the journal Solar Energy took a comprehensive look at this exact question. We built a model of a generic solar panel and calculated its output for every possible placement, including orientation (like the face of a clock with 12 as north and 6 as south) and tilt (from 0 degrees, or flat on the ground, to 45 degrees). Then, we repeated the calculation for over 1000 different locations across the United States. This analysis allowed us to calculate the total energy produced for any given location (using Typical Meteorological Year data) and any given placement. Because the model calculates solar power output for every hour of the year, that power output could also be multiplied by various different time-varying electricity prices. Figure 1 uses an example of the Texas electricity grid (ERCOT – the Electricity Reliability Council of Texas) to show how changing solar array placement can not only change the time that the array generates electricity, but how those changes relate to average wholesale electricity market prices (ERCOT SPP – ERCOT Settlement Point Prices).

The effect of south-facing (red line) and west-facing (blue line) solar array placements on the shape of the full-year and summer only generation curves in relation to the shape of the average wholesale electricity market price (ERCOT SPP) for the same time period.

The graph shows the yearly (top) and summer (bottom) average hourly output for a square meter (m2) of solar panel facing south (red line) and west (blue line). The difference between the red line and the blue line shows how facing a solar panel in a different direction shifts the time that it is producing solar power – an east facing panel would shift the curve left of the south (red) curve. The graph also shows the average ERCOT wholesale electricity market price (black line). Over the entire year, the west-facing array produces about 14% less energy than the south-facing array. However, during summer months, the difference is just 1% less. If, over the entire year, we multiply the hourly output by the hourly price (black line), and look for the best placement, we get the orange line, which is at about 219 degrees, or 39 degrees west of south, almost right between south and west. If we consider an extreme case where we only care about the energy produced during peak times (3—7 p.m.), then a western orientation is the most preferred.

In the end, it all comes down to incentives. In Texas, the current structure of compensating for the energy produced by solar panels incentivizes south-facing panels. However, what utilities really want are west-facing panels to hit peak demand as hard as possible. Given the limited daily solar resource, the market calls for something in the middle. All of this is just for Austin, Texas. The answer changes depending on the local solar resource, local prices, and local incentive structure. As solar penetration increases, local grid authorities might want to reconsider how structuring solar rates and rebates can reduce, to a certain extent, solar panels’ impact on the grid.

 

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Joshua D. Rhodes, Ph.D. is a Postdoctoral Research Fellow in The Webber Energy Group and the Energy Institute at the University of Texas at Austin. His current research is in the area of residential smart grid applications, including system-level applications of energy efficiency and distributed generation. He enjoys CrossFit, mountain biking, rock climbing, and living it up in ATX. His research webpage is: https://sites.google.com/site/joshdr83/ Follow on Twitter @joshdr83.