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Modeling of high temperature volumetric solar receivers with supercritical CO2 and nanoparticles

A new method for capturing solar energy is proposed for a Concentrated Solar Power (CSP) plant with supercritical CO2 (s-CO2) as a working fluid. The idea is to disperse a low volume fraction of nanoparticles in s-CO2, that functions as a heat transfer fluid in a volumetric solar receiver and as a working fluid in a power cycle. The advantage of this new proposed method is that it incorporates two technological applications that are complementary: s-CO2 Brayton power cycles for CSP provide the possibility for high receiver temperatures, and the volumetric solar receiver with nanofluids, are expected to have high efficiencies at high receiver temperatures. To quantify the benefits of this method, simulations have been performed where a traditional surface receiver with s-CO2 as heat transfer fluid is compared with the proposed method, where nanoparticles are dispersed in s-CO2 and heated volumetrically. The radiative properties of s-CO2 are simulated to overcome the unavailability of experimental data. The HITRAN database is used to obtain absorption line parameters at standard pressure and temperature and the absorption lines are consequently broadened by means of a modified version of open-source software. Furthermore, an empirically fitted model is included to account for high pressure effects. The broadening model is validated for reduced pressures by means of experimental results available in literature. The optical properties of the supercritical nanofluid are determined for a broad wavelength spectrum by means of the simplified medium theory. Rayleigh scattering theory is used to calculate the absorption coefficient for a cloud of particles, where the complex index of refraction of the particle material is taken from literature. The receiver simulations are done using an in-house developed CFD-code coupled with a radiative heat transfer solver that is capable of incorporating non-gray radiative properties by means of theWeighted-Sum-of-Gray-Gasses (WSGG) model. The constants of the WSGGmodel are calculated by transforming the absorption spectrum of the heat transfer fluid into Kdistributions and consequently fitting these by means of a Genetic-Algorithm (GA) algorithm. Both of the receivers are modelled as a 2-D turbulent channel flow (using Reynolds-Averaged Navier-Stokes (RANS) and a k-? model) with different boundary conditions as to account for the different receiver methods. The comparison between the volumetric- and the surface receiver shows that the former one has the potential to be more efficient at a high solar concentration factor because of the relatively low surface temperature caused by the change of heating mechanism. Radiative losses for the volumetric receiver with an optimized volume fraction of nanoparticles are contained in a thin layer close to the transparent surface due to the high mean Planck absorption coefficient of s-CO2. Furthermore, it is shown that the radiative heat transfer for a surface receiver with s-CO2 is not negligible for a relatively long length of the channel. Further optimization of the receivers should be done for higher concentration factors and larger Reynolds numbers as this receiver method proofs to be more optimal for these conditions. Furthermore, engineering questions related to the construction of a volumetric receiver (operating at high pressure and -temperature conditions), and stability of the supercritical nanofluid should be investigated, so that the idea can realized in practice.