Direct steam generation (DSG) in parabolic trough solar collectors is a feasible option for economic improvement in solar thermal power generation. Three-dimensional Eulerian two-fluid simulations are performed under OpenFOAM to study the turbulent flow in the evaporation section of the parabolic trough receiver and investigate the phase change, and pressure drop of water as a heat transfer fluid. First, the model's validity has been tested by comparing the numerical results of a laboratory scale boiler with the available correlations and semi-correlations of boiling flows from the literature. Simulations agreed well with Rouhani–Axelsson correlation for horizontal tubes, with a mean relative error of less than 7.1% for all studied cases. However, despite a mean relative error of less than 13.19% compared to the experimental data in the literature, the reported pressure drop factor remains valid; overprediction remains tolerable for most engineering applications. Second, the scaling effect on the mathematical model, from laboratory to commercial-scale configuration, was tested with experimental data of the DISS test loop in Platforma Solar de Almeria, Spain. The Monte Carlo Ray Tracing method under the Tonatiuh package allowed for obtaining the nonuniform heat flux distribution. Due to the large size of the evaporation section in the DISS loop (eight collectors), each collector is considered independently in the simulations. Thus, simulations follow each other, taking the numerical results of each collector output as input data in the next collector and so on until the last. The numerical results showed an excellent agreement for the void fraction with 3.53% against the Rouhani–Axelsson correlation. Frictional pressure losses are within a 17.06% error of the Friedel correlation, in the range of previous work in the literature, and the heat loss is less than 4.69% error versus experimental correlation.

Closed greenhouse systems optimize internal climatic conditions for both reducing energy loss and high-quality yields. Nevertheless, careful monitoring of the parameters of the microclimate requires a better understanding of the thermal phenomena that coexist at the same time inside the greenhouses. In the present study, the surface radiation effect on the natural convection in the greenhouse was investigated numerically based a turbulent unsteady model. The k-ε model was adopted for the turbulent flow and the discrete ordinate (DO) method for the radiation heat transfer. Assuming a no isotherm conditions at the floor and roof faces of the greenhouse and for a Rayleigh number ranges from 0.6×10¹⁰ to 2.3×10¹⁰. The results showed a strong radiation effect on the thermal behavior near the walls and considerably reduces the flow dynamics within the greenhouse. The contribution of the radiation heat transfer on the total Nusselt number at least 50% greater than that without. The results obtained for the selected values of Rayleigh numbers are in good agreement with the experimental data of the literature.

New receiver designs have been proposed to boost the heat transfer rate in the parabolic trough receiver (PTR) absorber tube, which can further improve the overall performances of the parabolic trough module. The objective of this study is to investigate the thermal performance and heat losses of the newly designed PTR wavy tube by means of a 1-D thermal model taking into account the change of the kinetic energy and pressure drop, and establishing a comparison with the conventional straight and smooth PTR absorber tube. According to the final results, the use of the S-curved absorber leads to higher thermal performance and simultaneously to lower heat losses. According to the thermal efficiency results, it is found an increase of 0.16% in the case of 450 K of the heat transfer fluid (HTF) inlet temperature and a straight length of 7.8 m of the receiver, while the heat losses decrease by a maximum of 7.72%.

This study proposes the replacement of the conventional straight absorber by the newly designed longitudinally undulated. Numerical results revealed that the new absorber could dethrone the former for several reasons: Among others, it allows a more homogeneous distribution of concentrated solar radiations on its outer surface (Monte Carlo Ray Tracking results); unlike other techniques which improve the inner heat transfer by increasing simultaneously the load of the absorber and the pressure drop within it, the proposed curved absorber is going to generate in a natural way, without any additional mechanical components, vortices within the main streaming which allowed to increase the heat transfer coefficient of about 63 % with an increase of the pressure drop penalty of about 60 %. On the other hand, it allows a drastic reduction of the size of the solar collector field. All these facts lead to decreasing the wall temperature gradient bellow 40 K. Results are obtained for the Syltherm 800 Reynolds number range 2.5×104 to 12.3×104 and a fluid inlet temperature of 450 K.

One of the main options to drive the cost of parabolic trough collectors (PTCs) technology down is to reduce the size of the solar field. This work proposes a novel receiver longitudinally undulated as a replacement for the conventional straight tube and investigates the effects on the size of absorbers, PTC modules and entire solar field. For this purpose, the developed method based on the similitude analysis should provide tools for drawing a comparison between the various designs of the absorber and should give useful measures of the scenario of their commissioning. Undulated absorber in service and without added supplementary mechanical components; the size of a solar collector field should reduce about ~29.5% consequence of the reduced size of the solar collector module and the absorber. The increase of the pressure drops through the novel absorber pipe should be re-balanced by the reduction in its size

One of the main options to drive the cost of parabolic trough collectors (PTCs) technology down is to reduce the size of the solar field. This work proposes a novel receiver longitudinally undulated as a replacement for the conventional straight tube and investigates the effects on the size of absorbers, PTC modules and entire solar field. For this purpose, the developed method based on the similitude analysis should provide tools for drawing a comparison between the various designs of the absorber and should give useful measures of the scenario of their commissioning. Undulated absorber in service and without added supplementary mechanical components; the size of a solar collector field should reduce about ~29.5% consequence of the reduced size of the solar collector module and the absorber. The increase of the pressure drops through the novel absorber pipe should be re-balanced by the reduction in its size.