Dissecting the Differences between Pyranometer and Reference Cell

Irradiance Measurements

Anton DriessePV Performance Labs, Freiburg, Germany

October 24, 2016

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Acknowledgements

European Joint Research Center (JRC)

Sandia National Laboratories (Sandia)

National Renewable Energy Laboratory (NREL)

Fraunhofer Institute for Solar Energy Systems (ISE)

ηη Introduction

Output: ± 1% or better Input: ± 3% or worse

ηη Introduction

Output: ± 1% or better

± 0.5%

Input: ± 3% or worse

± 5.0%

ηη 42 Sensors

1. Secondary standard thermopile pyranometers

2. Second class thermopile pyranometers

3. Photodiode pyranometers

4. Reference cells

ηη Test Facilities

ηη 42 Sensors

ηη 42 Irradiances

ηη “Recalibration” at AM1.5

ηη Similarities

Pyranometer

• output signal mainly proportional to irradiance

Reference cell

• output signal mainly proportional to irradiance

ηη Differences

Pyranometer

• circular

• dome (usually)

• thermoelectric principle

Reference cell

• square

• flat (usually)

• photovoltaic principle

ηη Angular Response

ηη Spectral Response

ηη Temperature Response

ηη Today’s Question

• How much do each of these effects contribute to differences in reported irradiance (and by extension to reported PV system performance)?

Procedure• Using measured spectra, component irradiances and

other weather parameters, for one year, calculate the expected output signal of each type of sensor.

• Calculate and illustrate the differences from a reference value both in absolute and relative terms.

ηη Locations Details

• Irradiance measurements from NREL-Solar Radiation Research Laboratory

– DNI, GHI, DHI, global tilted at 40°

– ambient temperature

• Spectral measurements from NREL Outdoor Test Facility

– Eko MS-700

– global tilted at 40°

• “Latitude tilt”, therefore high irradiance levels in winter as well as summer

ηη Results

ηη Daily Profile for a Reference Cell

ηη Daily Profile for a Reference Cell

ηη Daily Profile for a Pyranometer

ηη Daily Profile for a Pyranometer

ηη Annual Profile for a Reference Cell

ηη Annual Profile for a Reference Cell

ηη Annual Profile for a Pyranometer

ηη Annual Profile for a Pyranometer

ηη Reference Cell vs Pyranometer

ηη Additional Factors

Pyranometer

• delayed response

• thermal offsets

• alignment accuracy

• non-linearity

Reference cell

• spectral resp. vs temperture

• spectral resp. vs. angle

• alignment accuracy

• non-linearity

ηη Conclusions

• Understanding of the systematic difference between sensor types gives insight into when and why different instruments readings are obtained

• Precise quantification of those systematic differences allows instrument readings to be corrected, or adapted to the analysis needs

• Temperature and angular effects can be measured and processed with reasonable effort

• Spectral effects are challenging because spectral measurements have limited wavelength range

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