In the solar industry, the selection between thermopile and silicon sensors is a critical decision that impacts project precision and bankability. While silicon sensors, such as the EKO ML-01, (or reference cells) are suitable for specific localized applications, they differ fundamentally from the high-precision thermopile pyranometers required for utility-scale performance monitoring. Understanding these metrological differences is essential for securing reliable performance metrics and satisfying investor audits.
The Fundamental Difference in Measurement
The core distinction lies in how these sensors detect solar radiation. Silicon sensors operate on the photovoltaic principle, while thermopile sensors measure a thermal gradient.
- Spectral Range: A silicon sensor is sensitive only to a narrow band, typically between 300 nm and 1100 nm. In contrast, a Class A thermopile pyranometer measures the full broadband spectrum from 285 nm to 3000 nm.
- Spectral Flatness: Silicon sensors are not “spectrally flat,” meaning their accuracy fluctuates based on cloud cover or air mass. A spectrally flat thermopile sensor provides a uniform response, making it the primary choice for “ground-truth” total irradiance data and independent PR ratio monitoring.
- Long-term Stability: pyranometers (0,1 – 0,5% py) are inherently more stable over the long term, while reference cells drift faster (0.5 – 2% py) and need frequent recalibration.
Accuracy and Compliance Standards
For projects requiring IEC 61724-1 Class A compliance, international standards mandate the use of high-accuracy, spectrally flat sensors.
- ISO 9060:2018 Classification: Thermopile sensors are categorized into Class A, B, and C. Silicon sensors generally do not meet the ISO 9060:2018 “Spectrally Flat” Class A requirements for broadband irradiance.
- Thermal dependence: Silicon sensors often exhibit significant temperature dependence. Pyranometers are specified to have very low temperature dependence compared to reference cells.
Operational Stability and Maintenance
Long-term field deployment requires high stability, as silicon photodiodes are prone to sensitivity drift over time due to UV exposure.
- Recalibration: Most silicon sensors require frequent calibration to maintain basic accuracy. The MS-80SH is engineered for stability, allowing for a 5-year recalibration interval that significantly lowers the total cost of ownership (TCO).
- Response Time: While silicon sensors offer millisecond response, the MS-80SH features a fast-response thermopile reaching 95% in less than 0.5 seconds. This allows it to capture rapid cloud transients with the speed of a silicon cell but the accuracy of a broadband sensor.
Financial Impact on Performance Ratio (PR)
The Performance Ratio (PR) is the definitive metric for plant health. Utilizing a silicon sensor for a PR baseline essentially compares PV modules to a smaller, less efficient version of themselves. This can mask actual system degradation or environmental losses.
Measuring total available “solar fuel” with a Class A thermopile pyranometer provides an objective benchmark. This data is widely recognized by lenders and independent engineers (IEs) during financial audits.
Conclusion
Silicon sensors are suitable for rapid-response applications and general monitoring on smaller sites. However, for utility-scale assets where bankability and long-term asset valuation are primary goals, the EKO MS-80SH serves as the industry standard. It provides the spectral flatness, thermal stability, and international compliance necessary to ensure performance data remains accurate and defensible.
