Mitigating the losses in nanofluid-based direct solar absorption receivers

Solar thermal receivers are designed to absorb as much sunlight as possible (e.g., to increase energy inputs) while minimising heat losses and parasitic pumping energy requirements (e.g., to reduce energy losses). The field of direct absorption solar collectors has devised numerous elegant solutions for increasing energy inputs. Mitigating the energy losses, however, represents the other—generally overlooked—factor. To address this research gap, this study investigates the impact of nanofluids, superhydrophobic walls, an anti-reflective coating, and a transparent selective on the thermal efficiency of a microchannel-based solar receiver. The proposed modifications were ranked by their impact on the overall collector efficiency. It was found that compared to a pure water reference case, using a nanofluid working fluid provides the biggest reduction in optical losses (validating the literature's focus on this aspect). However, a micro-patterned surface achieved nearly the same improvement in optical efficiency. Adding a nanofluid, an anti-reflective cover, and a superhydrophobic surface (in concert) can boost the efficiency by ∼7% (compared to the reference case). Overall, this study provides a systematic approach to investigating features to mitigate energy loss mechanisms in direct absorption solar collectors via detailed component-level experimental testing and collector-level theoretical analysis.