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

Single-Sided Digital Microfluidic (SDMF) Devices for Effective Coolant Delivery and Enhanced Two-Phase Cooling

Digital microfluidics (DMF) driven by electrowetting-on-dielectric (EWOD) has recently been attracting great attention as an effective liquid-handling platform for on-chip cooling. It enables rapid transportation of coolant liquid sandwiched between two parallel plates and drop-wise thermal rejection from a target heating source without additional mechanical components such as pumps, microchannels, and capillary wicks. However, a typical sandwiched configuration in DMF devices only allows sensible heat transfer, which seriously limits heat rejection capability, particularly for high-heat-flux thermal dissipation. In this paper, we present a single-sided digital microfluidic (SDMF) device that enables not only effective liquid handling on a single-sided surface, but also two-phase heat transfer to enhance thermal rejection performance. Several droplet manipulation functions required for two-phase cooling were demonstrated, including continuous droplet injection, rapid transportation as fast as 7.5 cm/s, and immobilization on the target hot spot where heat flux is locally concentrated. Using the SDMF platform, we experimentally demonstrated high-heat-flux cooling on the hydrophilic-coated hot spot. Coolant droplets were continuously transported to the target hot spot which was mitigated below 40 K of the superheat. The effective heat transfer coefficient was stably maintained even at a high heat flux regime over ~130 W/cm2, which will allow us to develop a reliable thermal management module. Our SDMF technology offers an effective on-chip cooling approach, particularly for high-heat-flux thermal management based on two-phase heat transfer

The effects of surface wettability on the fog and dew moisture harvesting performance on tubular surfaces

bird's-eye view, view on roof, looking west southwest to surrounding town, with prayer courtyard in foreground, June 198

The Effect of Surface Treatment on the Drag Characteristics in Laminar Flow

In this study, we explore the clear idea of drag change when the surface contains roughness. For that purpose, we performed a simple terminal velocity experiment using small solid spheres. The terminal velocity of Cu spheres is measured before and after they are surface treated to form superhydrophilic or superhydrophobic nanostructures and appreciable increase in the terminal velocity for both cases is observed. An analytic solution is derived to evaluate the corresponding slip length in an external flow and the result is comparable to values reported in previous studies of lithographically patterned superhydrophobic surfaces. To gain useful physical insight, incompressible Navier-Stokes equations are solved. From the solution, a simple explanation for the experimental observation is developed using the concept of friction drag and form drag. The total drag can be reduced when the reduction of the friction drag is larger than the increase of the form drag

Drag reduction in Stokes flows over spheres with nanostructured superhydrophilic surfaces

Nanostructured surfaces offer opportunities to modify flow induced drag on solid objects. Measurements of the terminal velocity reveal that the drag associated with laminar Stokes flows can be reduced for spheres with nanostructured superhydrophilic as well as superhydrophobic surfaces. Numerical simulations suggest that the formation of recirculating or nearly stagnant flow zones leads to significant reduction in the friction drag. Such reduction, however, is offset by an increase in the form drag that arises from nonuniform pressure distributions. Our work motivates further studies to optimally balance the friction and form drag and control resistance to laminar flows over objects with nanostructured surfaces.ope

Effect of geometrical parameters on rebound of impacting droplets on leaky superhydrophobic meshes

When a droplet impacts a superhydrophobic sieve, a part of the droplet penetrates through it when the dynamic pressure (rU2) of the impinging droplet exceeds the breakthrough pressure (gG/ A). At higher impact velocities, the ejected- jet breaks and separates from the main droplet. The remaining part of the droplet bounces off the surface showing different modes (normal bouncing as a vertically elongated drop or pancake bouncing). In this work, we have studied the effect of different geometrical parameters of superhydrophobic copper meshes on different modes of droplet rebound. We observe three different effects in our study. Firstly, we observe pancake like bouncing, which is attributed to the capillary energy of the rebounding interface formed after the breaking of the ejected- jet. Secondly, we observe leakage of the droplet volume and kinetic energy due to the breaking of the ejected- jet, which leads to reduction in the contact times. Finally, we observe that for flexible meshes, the transition to pancake type bouncing is induced at lower Weber numbers. Flexibility also leads to a reduction in the volume loss from the ejected- jet. This study will be helpful in the design of superhydrophobic meshes for use under impact scenarios

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

Fabrication and Characterization of the Capillary Performance of Superhydrophilic Cu Micropost Arrays

We report the fabrication of dense arrays of super-hydrophilic Cu microposts at solid fractions as high as 58% and aspect ratios as high as four using electrochemical deposition and chemical oxidation techniques. Oxygen surface plasma treatments of photoresist molds and a precise control of the initial electrodeposition current are found to be critical in creating arrays of nearly defect-free Cu posts. The capillary performance of the micropost arrays is characterized using capillary rate of rise experiments and numerical simulations that account for the finite curvatures of liquid menisci. For the given wick morphology, the capillary performance generally decreases with increasing solid fraction and is enhanced by almost an order of magnitude when thin nanostructured copper oxide layers are formed on the post surface. The present work provides a useful starting point to achieve optimal balance between the capillary performance and the effective thermal conductivity of advanced wicks for micro heat pipes.ope