Absorber glass tube reference temperature measurement Procedure

Owned by Anne Schlierbach
Last updated: Oct 21, 2024 by Christoph Prahl
5 min read

1. Scope 

The present document defines the measurement procedure for temperature at HCE glass tubes as a reference to perform the calibration of IR camera of the drone. 

2. Background 

To ensure the reliability and precision of the glass tube temperature results, independent reference data are collected during the data acquisition process with the drone. This data is used to validate the results obtained with the drone. The goal is that airborne temperature values acquired by Volateq are within an uncertainty range of < 8°C (1 sigma). 

3. Infrared Camera model 

The infrared camera must contain a long wave infrared spectral band (8 to 14 μm) so that the measurements discard incident solar radiation (< 4 μm) and the measurement can be made without defocusing the tubes.  

Detector resolution, min 160 x 120, optimal: 640 x 480 

Examples: 

Fluke Ti480 PRO Infrared Camera   

https://www.ht-instruments.co.uk/en-gb/products/infrared-cameras/electrical-mechanical-application/tht200/   (or similar) 

 3.1 Camera Settings

The emissivity must be set according to manufacturer and/or literature values  

Epsilon = 0.88 

4. Conditions for taking measurements and mandatory requirements  

  • To be carried out the same day as the data acquisition flights for HCE glass tube temperatures at solar noon during operation (online). 

  • Within a time-window of +/- 30 minutes before or after the drone flight.  

  • The ambient conditions may only change insignificantly during this time.  

  • Wind speed variation between drone flight and reference measurement:
    < 2 m/s 

  • Ambient temperature variation between drone flight and reference measurement:
    < 5°C 

  • Distance to Camera to HCE:
    Recommendation: 4 - 6 meters 

  • HTF flow rate through the collectors:
    within the same range as during plant operation. 

  • HTF temperature >270 C:
    The normal operating temperature of the plant on days with radiation. 

  • Number of HCE samples to be measured to ensure a reliable reference data set:  

  • At least 20 HCEs with glass tube temperature higher than 120°C  

  • At least 30 HCEs with glass tube temperatures below 75°C  

5. Procedure 

The fastest and easiest way is to walk near the header piping under the collector inlets and outlets and measure some receivers on the first 2 - 4 modules (SCEs) in each SCA row (cold and hot). At the hot ends there will be some HCEs with different heat losses, as well as non-defective ones at the cold inlet. This way the data can be recorded in a very short time. 

 

image-20241021-150151.png

 

From the ground, use the hand-held IR camera to measure the glass tube temperature through the gap between two SCEs. 

At the time of measurement, the collectors are between 75° and 110° elevation and the absorber can only be measured from the gap between the modules.  

 

The absorbers should not be measured from diagonally below but when standing vertically in front of them. 

Using this method, only the absorber that overlaps the gap can be measured.  

Correct 

Incorrect 

 

image-20241021-150214.png

 

 

 

  Table 1: Measurement of receiver temperatures through gap between modules 

5.1 Avoid measuring at sun reflex

Please note that for plants located in the Northern Hemisphere, reference measurements must be taken at the northern end of the SCA. For plants located in the Southern Hemisphere, these measurements must be taken at the southern end of the SCA. This is necessary to avoid any overlap between the reference measurement and the sun reflex.

Do not measure at the reflex from the round Pizza shied installed at the shield covering the glass to metal connection at the bellow on the support. Measure plain tube out of sight of the shield (check out temperature at correct picture = 78,3°C).

This leads to falsely high temperatures (check out temperaute at pitcture = 189°C)that do not represent the correct value:

Correct 

Incorrect 

 

 

 

 

If the collector has a torque box structure, it is possible to measure from the rear side through the gap of the upper and lower mirror rows. 

 

 

Make sure that the selected receiver really corresponds to the one you have measured. 

The assignment of the correct absorber tubes to the measured temperature is very important, so that the drone data precisely matches the measured tubes. 

When entering the measurement data into the app, please perform a plausibility check: glass envelope temperatures cannot be below ambient temperature, for example. The "cold" sky was measured if such low temperatures are shown  

 

 

 

 

The temperature values may vary depending on ambient temperature, HTF temperature, module position in the loop (cold inlet or hot outlet) and manufacturer. 

6. Data Documentation 

To learn how to put the reference data in the web application please go to: Add HCE Temperature Reference Measurements to the Web App.