Solar energy conversion in CO2–particle solar receivers: A single-particle study

In counter–flow particle solar receiver (PSR), the falling particles absorb solar irradiation and heat up the counter flowing air simultaneously, generating high–temperature air as product. To improve its performance, CO₂–PSR is proposed by replacing air with CO₂. The radiative participating CO₂ cannot only absorb the convective heat dissipation from particles, but can also directly absorb its radiative dissipation in preferred band of CO₂ (2.5–4.5, 13–17 μm). The sum of the two dissipations is the usable energy for CO₂ (UE–CO₂). To examine particle’s performance on energy conversion from solar irradiation to UE–CO₂. This study focused on a single particle in CO₂–PSR, set up a calculation model at spectral level according to the zone method, and carried out numerical simulation. Results suggested that, particle’s radiative and convective dissipations were tightly coupled. The radiative proportion depended mainly on spectral absorptivity of particle, according to which the Z number was introduced. The particle with Z > 1 was suitable for CO₂–PSR. It could supply UE–CO₂ either primarily through radiation (radiation–control mode, RC) or primarily through convection (convection–control mode, CC). Particle working in RC provided UE–CO₂ with higher temperature and intensity; but with higher efficiency in CC. The two modes could be switched by altering solar irradiation flux or convective heat transfer coefficient at particle surface. The ratio of particle radiative heat dissipation in CO₂′s preferred band to its total received irradiation was defined as the efficiency of spectral modulation (ESM). The state ESM reached the maximum value in RC was the optimum working state, for which a series of empirical formulae for ESM, equilibrium temperature and corresponding solar irradiation flux were proposed. These results provide particle selection criterion and also technical support for design/operation of CO₂–PSR.