HEAT TRANSFER FLUIDS

The choice of heat transfer fluids has significant effects on the performance, cost, and reliability of solar thermal systems. In this chapter, we evaluate existing heat transfer fluids such as oils and molten salts based on a new figure of merit capturing the combined effects of thermal storage capacity, convective heat transfer characteristics, and hydraulic performance of the fluids. Thermal stability, freezing point, and safety issues are also discussed. Through a comparative analysis, we examine alternative options for solar thermal heat transfer fluids including water−steam mixtures (direct steam), ionic liquids/melts, and suspensions of nanoparticles (nanofluids), focusing on the benefits and technical challenges.Center for Clean Water and Clean Energy at MIT and KFUPM (Project 6918351)United States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center Award DE-SC0001299

Role of spectral non-idealities in the design of solar thermophotovoltaics

To bridge the gap between theoretically predicted and experimentally demonstrated efficiencies of solar thermophotovoltaics (STPVs), we consider the impact of spectral non-idealities on the efficiency and the optimal design of STPVs over a range of PV bandgaps (0.45-0.80 eV) and optical concentrations (1-3,000x). On the emitter side, we show that suppressing or recycling sub-bandgap radiation is critical. On the absorber side, the relative importance of high solar absorptance versus low thermal emittance depends on the energy balance. Both results are well-described using dimensionless parameters weighting the relative power density above and below the cutoff wavelength. This framework can be used as a guide for materials selection and targeted spectral engineering in STPVs.United States. Dept. of Energy. Office of Basic Energy Sciences (DE-FG02-09ER46577

Condensation on Superhydrophobic Copper Oxide Nanostructures

Condensation is an important process in both emerging and traditional power generation and water desalination technologies. Superhydrophobic nanostructures promise enhanced condensation heat transfer by reducing the characteristic size of departing droplets via a surface-tension-driven mechanism [1]. In this work, we investigated a scalable synthesis technique to produce oxide nanostructures on copper surfaces capable of sustaining superhydrophobic condensation and characterized the growth and departure behavior of condensed droplets. Nanostructured copper oxide (CuO) films were formed via chemical oxidation in an alkaline solution. A dense array of sharp CuO nanostructures with characteristic heights and widths of ~1 μm and ~300 nm, respectively, were formed. A gold film was deposited on the surface and functionalized with a self-assembled monolayer to make the surfaces hydrophobic. Condensation on these surfaces was then characterized using optical microscopy (OM) and environmental scanning electron microscopy (ESEM) to quantify the distribution of nucleation sites and elucidate the growth behavior of individual droplets with a characteristic size of ∼1 to 10 μm at low supersaturations. Comparison of the observed behavior to a recently developed model for condensation on superhydrophobic surfaces [2, 3] suggests a restricted regime of heat transfer enhancement compared to a corresponding smooth hydrophobic surface due to the large apparent contact angles demonstrated by the CuO surface.Massachusetts Institute of Technology. Undergraduate Research Opportunities ProgramUnited States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center)United States. Air Force Office of Scientific Research. Young Investigator ProgramNational Science Foundation (U.S.) (Award ECS-0335765

Jumping Droplet Dynamics on Scalable Nanostructured Superhydrophobic Surfaces

Environmental scanning electron microscope (ESEM) and high speed images of coalescence-induced droplet jumping on a nanostructured superhydrophobic copper oxide (CuO) surface are presented. Nanostructured CuO films were formed by immersing clean copper sheets into a hot (96 ± 3 °C) alkaline solution composed of NaClO2, NaOH, Na3PO4•12H2O, and DI water (3.75 : 5 : 10 : 100 wt.%). During the oxidation process, a thin (<200 nm) Cu2O layer was formed that then re-oxidized to form sharp, knife-like CuO oxide structures (Figure 1). Hydrophobic functionalization was obtained by depositing a fluorinated silane (trichloro(1H,1H,2H,2H-perfluorooctyl)silane) from a vapor phase. Individual droplet growth on the nanostructured CuO surfaces was characterized using an ESEM (Figure 2). The images were obtained with a beam potential of 20 kV and variable probe current. Droplets nucleated within the nanostructures and, while growing beyond the confines of the structures, their apparent contact angle increased as they developed a balloon-like shape with a liquid bridge at the base. Once droplets grew to diameters large enough to coalesce with neighboring droplets (R ≈ 7 μm), frequent out-of-plane jumping droplets were observed. To gain further understanding on jumping velocity, droplet jumping was studied in a pure saturated environment with a high speed camera. Recordings were taken at 2000 FPS. Figure 3 shows a time lapse of a coalescence event between two droplets. As the droplets coalesce, excess surface energy is converted into kinetic energy resulting in droplet jumping. The visualizations provide insight into these complex droplet-surface interactions, which are important for the development of enhanced phase change surfaces. In addition, these CuO surfaces offer ideal condensation behavior in terms of emergent droplet morphology and coalescence dynamics

Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces

When droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump from the surface due to the release of excess surface energy. If designed properly, these superhydrophobic nanostructured surfaces can not only allow for easy droplet removal at micrometric length scales during condensation but also promise to enhance heat transfer performance. However, the rationale for the design of an ideal nanostructured surface as well as heat transfer experiments demonstrating the advantage of this jumping behavior are lacking. Here, we show that silanized copper oxide surfaces created via a simple fabrication method can achieve highly efficient jumping-droplet condensation heat transfer. We experimentally demonstrated a 25% higher overall heat flux and 30% higher condensation heat transfer coefficient compared to state-of-the-art hydrophobic condensing surfaces at low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement but also promises a low cost and scalable approach to increase efficiency for applications such as atmospheric water harvesting and dehumidification. Furthermore, the results offer insights and an avenue to achieve high flux superhydrophobic condensation.United States. Dept. of Energy (Office of Basic Energy Sciences)Solid-State Solar-Thermal Energy Conversion CenterNational Science Foundation (U.S.) (NSF award no. ECS-033576)Irish Research Council for the Humanities and Social SciencesNational Research Foundation of KoreaKorea (South). Ministry of Education, Science and Technology (No. 2012R1A1A1014845