Do high-efficiency solar modules mean greater energy yield?

High-efficiency modules offer a ton of benefits. Most solar arrays are space constrained and using higher-efficiency modules means that the system size will be larger and generate more total energy. Because the modules are high in efficiency, the costs of the rest of the system (including racking, labor and electrical components) are often lower on a per-watt basis.

However, developers who build and install high-efficiency solar modules often expect that the array is going to have a greater productivity (as measured by specific energy yield, or kWh/kWp) versus an array with traditional modules. When this doesn’t happen, it may appear that there is a flaw in the calculations. But the yield (kWh/kWp) of a high-efficiency module should actually be the same as a standard-efficiency module. Here’s why.

Defining “efficiency”
A big reason for the confusion is that the word “efficiency” isn’t clear. There are actually two aspects of efficiency.

1. Module efficiency: What proportion of a watt of sunlight is converted into a watt of DC power by the module?
2. System yield: How well does the system do at producing near its peak over the course of the year?

Both matter; the overall annual energy production is basically the product of both of these factors. High-efficiency modules will have a higher module efficiency, but will generally have the same system yield.

Also, higher-efficiency modules will generate more power per unit area than a standard module. But the energy generated over time per watt of peak power will generally be the same. So while the word efficiency seems like the modules would have greater productivity over time, it’s more accurate to think of those modules as “higher-power” rather than greater yield.

High efficiency in practice
We can also just look at a real-world example to see these differences in practice. On this simple residential system in Phoenix, the south-facing rooftop can hold 24, 60-cell modules. With low-efficiency modules (240-W Centrosolar), the rooftop can hold a 5.8-kW array. With high-efficiency modules (360-W SunPower), the roof has 8.6 kW of solar. With the same size roof area, the SunPower system is 50% larger than the Centrosolar system. This is the main benefit of a high-efficiency module.

folsom labs solar

Figure 1: this 24-module array is a 5.8-kW nameplate with 240-W Centrosolar modules, and 8.6-kW nameplate with 360-W SunPower modules

If we then simulate the arrays and look at the productivity of both, we can see that the metrics are within 0.5%. The kWh/kWp (also known as “specific yield”) for the SunPower array is 1,767, compared with 1,759 for the Centrosolar system. The performance ratios are also nearly identical: 73.7% for the Centrosolar design, and 74.0% for the SunPower design (details below).

Ultimately, the annual energy production favors SunPower. The SunPower array produces 15,300 kWh per year, while the Centrosolar system produces 10,100 kWh per year. But this is almost entirely driven by having a “bigger” system (8.6 kW versus 5.8 kW), not by squeezing more kWh from each kW of installed capacity.

Energy production comparison
To better understand why the productivity metrics are similar, we can look at the energy calculations for the two systems step-by step.

folsom labs solar

Figure 2: Energy calculation comparison between standard-efficiency and high-efficiency modules (click to enlarge).

The first half of the calculations are primarily based on the location of the array (the annual global horizontal irradiance), and the orientation of the array. The “nameplate” step is where the system size comes into play. With a 50% larger nameplate size, the SunPower system gets a significant boost here, 19,400 versus 12,900 kWh.

For the yield calculations, note the modules are fairly similar across the board. The SunPower modules are slightly better in low-light performance (the “output at irradiance” step of the calculation) and temperature losses (since the modules have slightly different temperature coefficients), and slightly worse in mismatch losses. But each of these differences is less than 1%. Because they offset, the net difference is less than 0.5%.

What does cause differences in yield?
This doesn’t mean that all modules will have similar energy yield characteristics; some may have better or worse low-light characteristics, others will have better or worse thermal losses. But importantly, those attributes are independent from module efficiency.

For example, CdTe modules have lower efficiency than crystalline silicon modules, but across the board have better temperature coefficients and lower temperature losses. So First Solar modules will generally have higher kWh/kWp values than higher-efficient crystalline silicon modules.

Finally, the analysis above assumes a space-constrained array, where the higher-efficiency modules result in a larger system size and greater energy production. But the comparison feels different if you are comparing systems with the same nameplate power. A 10-kW SunPower system will produce about as much energy as a 10kW Centrosolar system. The main difference is that the 10-kW SunPower system will take up less space.



Solar Power World


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s