Triethylenglycol as Novel Heat Transfer Fluid for CPVT Collectors with Spectral Splitting

Concentrating PVT (CPVT) collectors using the approach of Spectral Splitting are designed to provide thermal energy at high temperature level and electricity simultaneously with optimised efficiency. Among others, the Spectral Splitting effect can be achieved by a combination of a solid and a liquid absorption filter. In such a case, the utilised liquid serves both as optical filter and as heat transfer fluid and needs to fulfil several optical and thermodynamic requirements. Triethylenglycol (TEG) was investigated as a potential candidate fluid for utilisation in a Spectral Splitting CPVT receiver with a desired output temperature of 200°C. The performed analysis involves the mathematical description of temperature-dependent fluid properties, thermal expansion measurement, long-term thermal stress, and UV exposure tests. The main parameters for monitoring possible impacts of the stress tests are the spectral and the weighted transmittance values of the fluid, as these are essential properties for a stable long-term operation of the considered CPVT collector including Spectral Splitting. The investigation of TEG confirmed its suitability for the described application. The characteristic of the spectral transmittance in initial state shows the required downwards step at a wavelength of 1100 nm that is essentially necessary for the combination with a solid filter in the Spectral Splitting configuration. The UV exposure with a dosage of 50 kWh/m ² has negligible effect on the optical fluid properties. The long-term thermal stress tests caused an acceptable decrease in weighted transmittance of -3.2 percentage points within the spectral band from 780 nm to 1100 nm.