The receiver is one of the key components in a solar parabolic trough collector and its properties are crucial for the efficiency of a CSP plant. A heat transfer medium flows through the receiver tubes and absorbs the irradiated energy as heat. Afterward, it is led to a heat exchanger where steam is produced in order to generate electricity. At present, the most common medium is synthetic thermal oil.
To determine the receiver characteristics, the DLR QUARZ Center uses measurements in steady-state of the heat loss power over the relevant operating temperature range and absorbed power. The measured samples of the parabolic trough receivers are 4060 mm long at room temperature and the absorber tube has an outer diameter of 70 mm. These receivers are equipped with a selective absorption coating and a glass envelope with an antireflective coating. Receivers with other sizes of up to 100 mm diameter and up to 4900 mm length may be measured on request.
At the DLR QUARZ Center, heat loss tests are performed with a method developed at DLR. This method is also applied at two other research centres; at the US American NREL (National Renewable Energy Laboratory) and at the Spanish CENER (Centro Nacional de Energías Renovables). Electrical heaters are used to heat up the absorber tube to various temperature levels up to 550°C. The test result is the characteristic curve of the heat loss in W and W/m over the absorber temperature at ambient conditions of 20°C to 25°C temperature and still air. In addition to the quality evaluation of receiver models, the results may also be used in solar power plant models, for example in software such as Greenius and SAM.
The QUARZ Center uses an ElliRec – a linear focus solar simulator test bench – for making absorption tests. In this test, the measured enthalpy flow of a cold water stream through an irradiated receiver tube under solar simulator light is compared to the optical efficiency of a reference receiver (DLR 70-1). Since the full length of the receiver is irradiated, the results include properties of the glass, the absorber tube, and the effects of the bellow geometry.
The durability of parabolic trough receivers is determined by accelerated ageing tests. In order to analyse the entire receiver tubes, the DLR QUARZ Center offers an absorber overheating test and a thermal cycling test. In a bellow fatigue test, the heat losses are determined by repeating expansion and compression of the bellows at the end of a receiver tube. We also offer durability tests of the antireflective coating of receivers and the effect of various environmental influences. More information about the accelerated ageing tests can be found under Durability of components.
Together with its Spanish cooperation partner CIEMAT, DLR operates the outdoor test facility KONTAS at Plataforma Solar de Almería, where parabolic trough receivers can be tested under realistic operational conditions. A heating and cooling unit enables tests with thermal oil from 20°C to 390°C and the azimuth track enables testing of the receiver at all relevant incidence angles. The testing objectives may be defined at the KONTAS test facility by the customer.