Investigation of the effect of non-uniform heat flux variations as a result of the sun's hourly position on parabolic trough collectors using two-phase saturated R245fa

• R245fa flow boiling was studied for non-uniform heat flux distributions. • Solar times from 12:00 to 18:00 were considered for a parabolic trough collector. • Local and average heat transfer coefficients were obtained via an inverse problem. • The solar time heat flux shape significantly affected the thermal performance. Direct steam generation in concentrated solar power systems enhances thermal efficiency by directly producing steam in receiver pipes, eliminating intermediate heat transfer fluids. Parabolic trough concentrators can play a critical role in this regard by focusing solar radiation in a non-uniform fashion onto receiver pipes, where the working fluid is heated. In this experimental investigation, flow boiling of R245fa was considered for such relevant non-uniform heat flux distributions in a 17.11 mm inner diameter pipe at a mass flux of 200kg/m 2 ·s, a saturation temperature of 40 °C and at vapour qualities ranging between 0.05 and 0.55. Low and high heat flux intensities, and different focal positions (associated with different solar times) were studied. Coupled with a finite difference wall conduction numerical scheme, the data was processed with an inverse problem using a Genetic Algorithm to obtain local flow boiling heat transfer coefficients. Large variation in the heat transfer coefficients, depending on the heat flux distribution was observed. For uniform heat flux cases (reference), the heat transfer coefficient depended strongly on vapour quality, while for parabolic trough (non-uniform) heat flux distributions only a weak correlation existed. For parabolic trough heat flux distributions, the focal point placement (as dependent on the solar time) did however, have a very significant influence on the heat transfer performance. In general, the best heat transfer performance was exhibited when the focal point was directly at the bottom of the tube, corresponding with midday. As the solar time progresses to 18:00, the heat transfer coefficient dipped at 13:00, then exhibited a steady increase until 16:00 (predominantly heating from the side), and then a relatively steep decline to 18:00.