The PTC Receivers Testing Laboratory aims at the exhaustive performance and durability characterization of the receiver tubes used in Concentrating Solar Power plants based in Parabolic Trough Collectors (PTC) technology. PTC Receivers are the most critical component in the performance of this kind of power plants and their behavior along the useful life of the plant is crucial for an appropriate economic return of the project. In the PTC Receivers Testing Laboratory the optical and thermal performance of the receivers is assessed by measuring optical transmittance and absorptance and by measuring the receiver heat losses at different temperatures. Durability is assessed by subjecting the receivers to extreme conditions and accelerated ageing and then measuring their performance again. Durability tests include high temperature and thermal cycling, bellows fatigue and hail impact tests to the complete receivers as well as several accelerated ageing tests to glass samples such as abrasion, condensation, humidity freeze and UV exposure. Most of the tests performed in PTC Receivers Testing Laboratory are based in the IEC 62862-3-3 standard draft.
The laboratory has the following main equipment:
Thermal characterization test bench:
Test bench for the thermal performance characterization of PTC Receivers based on the determination of the heat losses at different temperatures (typically 4 to 5 temperatures in the range 200 to 500 ºC). The thermal characterization test bench consists of a support frame, straight electrical heaters, helical electric heaters to ensure adiabatic receiver ends, electric power transducers and control system to ensure an stability of ± 1K. This test bench is adapted for receiver up to 4500 mm long and any absorber tube inner diameter.
Optical characterization test bench:
Test bench for the optical performance characterization of PTC Receivers based on measurements of glass envelope transmittance and absorber tube absorptance. The test bench counts on an specifically adapted spectrophotometer for the simultaneous and non-destructive measurement of the optical properties which moves along the receiver for assessing the optical properties homogeneity. The spectrophotometer measures the spectral optical properties from 300 nm to 2500 nm each 10 nm while solar transmittance and absorptance values are calculated by integration over the direct solar radiation spectrum at AM1.5 based on Standards ISO 9845-1 and ASTM G173.
Bellows fatigue test bench:
Accelerated ageing test bench for assessing the durability of the bellows between the absorber tube and the glass envelope. This test bench consists of a support frame, straight electrical heaters to raise the temperature up to the mean operating temperature of the tube, and a linear actuator to produce compression and expansion cycles reaching the maximum design stroke of the bellows. The linear actuator subjects the glass envelope and thus the bellows to a large number of compression and expansion cycles (typically 20.000) to simulate the complete useful life of the tube in about 5 hours. Receiver degradation is determined by performing the thermal characterization test before and after the fatigue test.
Overheating and thermal cycling test bench:
Accelerated ageing test bench for assessing the durability of the optical and thermal properties at high temperatures and thermal cycling for up to 6 simultaneous tubes. This test bench consists of straight electrical heaters to heat up the receivers up to about 100K over their maximum operating temperature and keep this temperature during a large time (typically 1000h at 500ºC). Also, this test bench is used to produce heating and cooling cycles (typically 100 cycles from 200ºC to 500ºC) in the receivers. The test bench also includes the required control system to ensure temperature stability as well as precise heating and cooling ramps. Receiver degradation is determined by performing the thermal characterization test before and after the overheating and thermal cycling test.
UV/Vis-NIR spectrophotometer Optronics Laboratories model OL-750 with integration sphere for the measurement of glass samples transmittance and absorber samples absorptance. This equipment measures the spectral optical properties from 300 nm to 2500 nm each 10 nm while solar transmittance and absorptance values are calculated by integration over the direct solar radiation spectrum at AM1.5 based on Standards ISO 9845-1 and ASTM G173.
Equipment for durability tests on glass samples:
The PTC Receivers Testing Laboratory counts on different climatic chambers with temperature control between -40 ºC and 85ºC and relative humidity control between 10% and 100% to subject glass samples to accelerated ageing processes such as the condensation test (480 hours at 40ºC relative humidity of 100%) and the humidity freeze test (40 cycles between -40ºC and 65ºC with relative humidity of 85%). Additionally, UV stability tests exposing the glass samples to high doses of UV radiation are also performed in the laboratory UV chambers (typically 15 kWh/m2 with 3-10% of UVB radiation between 280 nm and 320 nm and 90-97% UVA radiation between 320 nm and 400 nm). In all these tests optical transmittance of the glass samples are measured before and after the accelerated ageing. Furthermore, abrasion tests are performed by means of a Linear Taber equipment (image) with abrasive rubber according to the standard MIL-E12397. In these tests, optical transmittance of the glass samples is measured a number of times during the tests with increasing number of rubbing cycles (typically transmittance is measured before the test and after 5, 10, 20, 50 and 100 cycles).