MEASURING DEGRADATION RATES WITHOUT IRRADIANCE DATA

MEASURING DEGRADATION RATES WITHOUT IRRADIANCE DATA

Steve Pulver Daniel Cormode

Alex CroninU of A

Dirk JordanSarah KurtzRyan Smith

NREL

Outline• Goal: Measure degradation rates for

systems that don’t have irradiance data• Approach: Use 20 PV systems from TEP• Step 1: Quantify relative degradation

rates• Challenge: Relative vs. absolute rates• Step 2: Bayesian statistics to derive

absolute degradation rates

TEP solar test yard

Missing POA irradiance sensors

Daily Final Yield, 2005-2009

Daily and annual insolation can vary by more than the degradation rate

Variation in daily insolation

Removing irradiance variation

Define Daily Relative Final Yield as:Daily Final Yield divided by field average final yield

Daily Final Yield Daily Relative Final Yield

Relative rates of change

Provides an easy comparison between systems, and can be calculated with low uncertainty

Rate of change (percent per year)2 systems

Example: only two systems…

relative

absolute

2 systems

5 systems

10 systems

There appears to be a higher probability of low degradation rates

Adding more systems…

Rate of change (percent per year)17 systems

Translating to absolute…

relative

absolute

In a large dataset, possibly there is a system that changes very little

Assume the best system doesn’t degrade?

Challenging scenarios

Best performing system degraded by 0.5 (percent / year)

Outlier at 2.0 (percent / year)

Normalizing by the best system may not be the best solution

Our analysis

Starting point: a PDF (Probability Distribution Function) to describe the absolute degradation rate.

Histogram of our dataset(relative rate of change)

Histogram of a much larger data set(absolute rate of change)

Comparison to histograms

Exponential PDF appears to be capable of handling both our dataset and the larger dataset

Results depend on the function used

Amount to shift relative rates

Consistent results between rates measured w/ and w/o irradiance data, and similar uncertainties between the two methods.

Absolute rates of change

Conclusions

• We determined degradation rates without irradiance data.

• Relative degradation rates provide a useful comparison between systems

• Absolute degradation rates can be obtained from relative rates