Integrated thermal-hydraulic and exergy-based optimisation of volumetric flow rate in parabolic trough collectors using commercial heat transfer fluids
Annotatsiya
The optimization of volumetric flow rate in parabolic trough collectors (PTCs) represents a complex thermo-hydraulic challenge, since an increase in flow rate improves convective heat transfer inside the receiver while also causing greater pressure losses and higher pumping power requirements. Traditional assessment approaches generally emphasize useful heat gain or thermal efficiency; however, such criteria alone may not identify the most suitable operating conditions when the hydraulic energy demand becomes considerable. This study develops an integrated thermal-hydraulic and exergy-based optimisation framework for determining the optimum volumetric flow rate of commercial heat transfer fluids in an LS-2 parabolic trough collector. Eight fluids Syltherm 800, Therminol 55, Therminol 59, Therminol 66, Therminol 68, Therminol XP, Therminol VP1 and Therminol D-12 were analysed under identical collector geometry and operating conditions over an inlet-temperature range of 273.15-523.15 K. The model combines optical energy absorption, receiver heat losses, internal convective heat transfer, pressure drop, pumping power, net useful power, exergy efficiency and entropy-generation indicators. The thermal model was validated against the SEGS LS-2 experimental dataset, yielding a root mean square error of 0.79 K and a coefficient of determination of R² = 0.99992 for outlet-temperature prediction. The results show that the optimum volumetric flow rate generally increases with inlet temperature because viscosity reduction improves hydraulic behaviour and allows stronger internal convection. However, the optimum does not correspond to the maximum flow rate; it occurs where the marginal thermal benefit is balanced by the additional pumping-power penalty. Therminol D-12 and Therminol VP-1 provide the most favourable overall behaviour under the investigated conditions because they combine comparatively high net heat gain with low pumping-power demand. Therminol 55 exhibits the strongest performance deterioration at elevated temperatures, indicating limited suitability for high-temperature operation. The entropy-generation interpretation confirms that flow-rate optimisation cannot be based on thermal efficiency alone, because hydraulic irreversibility and pumping-power penalty can offset the thermal benefit of stronger circulation. The proposed framework provides a physically consistent basis for temperature-dependent flow-rate control, heat-transfer-fluid selection and industrial PTC loop design in concentrated solar power and solar process-heat systems.