Power Check FAQ#

SunPeek is an open-source software tool that provides a transparent, high-quality implementation of the ISO 24194 Power Check. The software offers functionalities for plant configuration, data upload and inspection, automated data quality checks, calculation of performance metrics, and generation of reports. SunPeek also incorporates a database of Solar Keymark certified collectors, facilitating easy access to collector parameters required for the Power Check.

ISO 24194 is a standard that defines multiple methods for verifying the performance of solar thermal collector fields. SunPeek currently provides an implementation of the ISO 24194 Power Check method.

The Power Check is a method used to verify the power performance of a solar thermal collector field. It involves a procedure to calculate the expected power output of the field based on parameters from ISO 9806, a safety factor, and measured operating conditions. The estimated power output is then compared to the actual measured power output to assess the collector field’s performance. In doing so, the Power Check accounts for all major factors that influence the power performance of a collector field: the collectors used (ISO 9806 parameters), measured operating temperatures and weather data. Hence, Power Check performance results can be compared among plants in different geographical locations, using different collector technologies, running on different temperature levels and exposed to different climates.

The Guide document (to be published) serves to supplement and clarify the ISO 24194 standard. It provides explanations, practical examples, and implementation guidance for applying the Power Check effectively. The guide also addresses limitations of the standard, suggests enhancements, and highlights areas requiring further discussion and development within the solar thermal community.

ISO 24194 outlines three formulas or equations for calculating the estimated thermal power output of a solar thermal collector field.

• Formula 1 is based on the collector’s optical efficiency and heat loss coefficient at normal incidence.

• Formula 2 considers the incidence angle modifier for beam irradiance and is generally the preferred formula to use.

• Formula 3 is for concentrating collectors with a high concentration ratio and does not consider the heat loss coefficient.

Note: This is based on ISO 24194:2022, and is subject to change in revisions of the ISO 24194 standard.

The required measurements for a Power Check include:

• Solar irradiance: Global, beam, and / or diffuse irradiance in the collector plane.

• Ambient temperature: Measured near the collector field. Wind measurement is optional.

• Collector temperatures: Inlet and outlet temperatures at the collector field or at the heat exchanger.

• Thermal power output: Volume or mass flow rate of the heat transfer fluid.

The required or recommended accuracy levels for these measurements are defined in ISO 24194 and influence the overall uncertainty of the Power Check results.

The safety factor in the Power Check is used to account for various uncertainties and unmodeled effects that can influence the difference between the estimated and measured power output of a collector field. Such effects can include piping losses, measurement uncertainties, and other factors.

Accuracy levels (I, II, and III) in the Power Check are defined in ISO 24194. They define the allowable uncertainties for different measured quantities. Each accuracy level specifies maximum permissible errors or standard uncertainties for measurements like temperature, irradiance, and flow rate. For example, Level I typically represents the highest accuracy level with the strictest limits on measurement uncertainties. The standard requires stating the accuracy level when providing a performance estimate.

Stagnation occurs when the heat transfer fluid in the collector overheats because there’s no flow to remove the absorbed solar energy. The standard recommends excluding stagnation periods from the Power Check analysis. However, specific criteria for defining and identifying stagnation events are not explicitly provided in ISO 24194. There are different approaches for handling stagnation, including the definition of stagnation based on temperature changes, the use of a minimum specific power output criterion, and potential challenges related to interpretation of stagnation events.

1. Data Acquisition: Gather the necessary measurement data, including solar irradiance, ambient temperature, collector temperatures, and fluid flow rates.

2. Data Processing: Prepare the data by checking for errors, converting units, and applying any necessary corrections.

3. Power Estimation: Calculate the estimated power output using appropriate formulae from ISO 24194, considering collector parameters, incidence angle modifiers, and safety factors.

4. Performance Comparison: Compare the estimated power output with the actual measured power output to assess the performance of the collector field.

5. Analysis and Reporting: Analyze the results to identify potential performance issues and generate a report summarizing the findings.

• Data Availability: Obtaining all the necessary measurement data with the required accuracy can be challenging, especially for existing installations.

• Data Quality: Measurement errors, missing data, and inconsistencies can affect the reliability of the Power Check results.

• Interpreting Results: Understanding the implications of the performance comparison and identifying the root causes of performance issues require expertise.

We encourage community-based development for future revisions of the Power Check and SunPeek. Users, researchers, and developers are invited to contribute in various ways: by sharing real-world use cases, providing open datasets of plant operation, engaging in discussions and pro-posing improvements, contributing code to SunPeek, or developing educational materials. For details, see the SunPeek Contribution Guide.