Coupled radiative and fluid flow modelling for a direct absorption solar receiver

The application of nanofluids has the potential to solve technical issues in many solar thermal engineering systems. Recent literature indicates that nanofluids offer unique advantages over conventional fluids. Nanofluids are solid nanoparticles suspended in a liquid: the average dimensions of these particles may vary from 1 to 100 nm. The addition of particles in a base fluid can enhance its optical properties, in particular its absorption properties. Thus nanofluids can be successfully used in direct absorption solar collectors to directly absorb the solar radiation in their volume. In this kind of devices, it is possible to surpass the constraints of conventional collectors due to the absence of the absorber plate. An important advantage of direct absorption of solar radiation is to avoid the thermal resistance between the absorber surface and the heat transfer fluid. This paper investigates the application of water based nanofluids as volumetric absorber in a direct absorption solar collector: a suspension of single wall carbon nanohorns in water is chosen as nanofluid. A new model of a solar receiver with a planar geometry is developed: radiative transfer equation in participating medium and energy equation are numerically solved to predict the performance of the receiver; the optical and thermal behaviors of the nanofluid are modelled according to the properties available in the current scientific literature. Monte Carlo ray tracing is used to determine the directional and spatial distribution of the concentrated solar radiation coming from a parabolic trough concentrator. This distribution is then applied to the receiver geometry using a commercial computational fluid dynamic software to simulate the incoming solar flux. The developed model is capable to predict temperature distribution, heat transfer rate and penetration distance of the concentrated solar radiation inside the nanofluid volume. Two different configurations of the bottom surface are considered: transparent wall and reflecting wall. The effects of inlet temperature, flow rate and nanoparticle concentration on the energy efficiency of the receiver are studied. Finally, the model is applied to compare the performance of a direct absorption receiver to a conventional surface receiver under the same operating conditions.