Hadi Ghasemi

Solar steam generation by heat localization

Currently, steam generation using solar energy is based on heating bulk liquid to high temperatures. This approach requires either costly high optical concentrations leading to heat loss by the hot bulk liquid and heated surfaces or vacuum. New solar receiver concepts such as porous volumetric receivers or nanofluids have been proposed to decrease these losses. Here we report development of an approach and corresponding material structure for solar steam generation while maintaining low optical concentration and keeping the bulk liquid at low temperature with no vacuum. We achieve solar thermal efficiency up to 85% at only 10 kW m(-2). This high performance results from four structure characteristics: absorbing in the solar spectrum, thermally insulating, hydrophilic and interconnected pores. The structure concentrates thermal energy and fluid flow where needed for phase change and minimizes dissipated energy. This new structure provides a novel approach to harvesting solar energy for a broad range of phase-change applications.

Charging-free electrochemical system for harvesting low-grade thermal energy

Significance Tremendous low-grade heat is stored in industrial processes and the environment. Efficient and low-cost utilization of the low-grade heat is critical to imminent energy and environmental challenges. Here, a rechargeable electrochemical cell (battery) is used to harvest such thermal energy because its voltage changes significantly with temperature. Moreover, by carefully tuning the composition of electrodes, the charging process is purely powered by thermal energy and no electricity is required to charge it. A high heat-to-electricity conversion efficiency of 2.0% can be reached when it is operated between 20 and 60 °C. Such charging-free characteristic may have potential application for harvesting low-grade heat from the environment, especially in remote areas. Efficient and low-cost systems are needed to harvest the tremendous amount of energy stored in low-grade heat sources (<100 °C). Thermally regenerative electrochemical cycle (TREC) is an attractive approach which uses the temperature dependence of electrochemical cell voltage to construct a thermodynamic cycle for direct heat-to-electricity conversion. By varying temperature, an electrochemical cell is charged at a lower voltage than discharge, converting thermal energy to electricity. Most TREC systems still require external electricity for charging, which complicates system designs and limits their applications. Here, we demonstrate a charging-free TREC consisting of an inexpensive soluble Fe(CN)63−/4− redox pair and solid Prussian blue particles as active materials for the two electrodes. In this system, the spontaneous directions of the full-cell reaction are opposite at low and high temperatures. Therefore, the two electrochemical processes at both low and high temperatures in a cycle are discharge. Heat-to-electricity conversion efficiency of 2.0% can be reached for the TREC operating between 20 and 60 °C. This charging-free TREC system may have potential application for harvesting low-grade heat from the environment, especially in remote areas.

Flexible artificially-networked structure for ambient/high pressure solar steam generation

The heat localization approach has promised a new route to solar steam generation with a higher efficiency than that of the current bulk heating approaches. In this approach, the material structure localizes the absorbed solar energy, forms a hot spot, and wicks the fluid to the hot spot for steam generation. The non-equilibrium nature of this approach minimizes energy losses leading to its superior performance compared to equilibrium approaches. However, to date, the generated steam is only at ambient pressure and not suitable for high pressure applications. Herein, we report development of a flexible artificially-networked material structure that is highly efficient for ambient and high pressure steam generation with integrity for large-scale and varied geometrical implementations. This flexible material is composed of a porous polymer skeleton coated with exfoliated graphite and artificially-networked 3D veins. The porous skeleton and the interconnected veins are developed in two distinct fabrication steps. The structure generates steam in the temperature range of 100–156 °C and at the pressure range of 100–525 kPa under solar irradiation. This material structure promises a robust and highly efficient approach for solar steam generation for both ambient pressure applications (e.g. solar ponds and desalination) and high pressure applications (e.g. power generation, solar cooling technologies, and hygiene systems).

Membrane-Free Battery for Harvesting Low-Grade Thermal Energy

Efficient and low-cost systems are desired to harvest the tremendous amount of energy stored in low-grade heat sources (<100 °C). An attractive approach is the thermally regenerative electrochemical cycle (TREC), which uses the dependence of electrode potential on temperature to construct a thermodynamic cycle for direct heat-to-electricity conversion. By varying the temperature, an electrochemical cell is charged at a lower voltage than discharged; thus, thermal energy is converted to electricity. Recently, a Prussian blue analog-based system with high efficiency has been demonstrated. However, the use of an ion-selective membrane in this system raises concerns about the overall cost, which is crucial for waste heat harvesting. Here, we report on a new membrane-free battery with a nickel hexacyanoferrate (NiHCF) cathode and a silver/silver chloride anode. The system has a temperature coefficient of -0.74 mV K(-1). When the battery is discharged at 15 °C and recharged at 55 °C, thermal-to-electricity conversion efficiencies of 2.6% and 3.5% are achieved with assumed heat recuperation of 50% and 70%, respctively. This work opens new opportunities for using membrane-free electrochemical systems to harvest waste heat.

Magnetic slippery extreme icephobic surfaces.

Anti-icing surfaces have a critical footprint on daily lives of humans ranging from transportation systems and infrastructure to energy systems, but creation of these surfaces for low temperatures remains elusive. Non-wetting surfaces and liquid-infused surfaces have inspired routes for the development of icephobic surfaces. However, high freezing temperature, high ice adhesion strength, and high cost have restricted their practical applications. Here we report new magnetic slippery surfaces outperforming state-of-the-art icephobic surfaces with a ice formation temperature of −34 °C, 2–3 orders of magnitude higher delay time in ice formation, extremely low ice adhesion strength (≈2 Pa) and stability in shear flows up to Reynolds number of 105. In these surfaces, we exploit the magnetic volumetric force to exclude the role of solid–liquid interface in ice formation. We show that these inexpensive surfaces are universal and can be applied to all types of solids (no required micro/nano structuring) with no compromise to their unprecedented properties.

A flexible anti-clogging graphite film for scalable solar desalination by heat localization

Solar–thermal energy conversion is an economically promising route for power generation, desalination, and distillation. With the recent introduction of the heat localization concept, highly efficient solar harvesting has garnered more attention with accelerated research efforts. In this concept, the material structure localizes solar energy at the desired interface minimizing the heat loss by the bulk phase occurring in conventional solar bulk heating approaches. However, the development of materials for long-term solar desalination through heat localization remains an open challenge due to clogging of the structure after a short period of time. Herein, we report a new efficient and flexible material structure for solar desalination with anti-clogging characteristics. The material structure has a porous polymer skeleton with embedded graphite flakes and carbon fibers. The geometry of pores in this structure and anti-clogging coating prevent any salt accumulation in the material structure. We have demonstrated five orders of desalination of highly salty brine (1.52 × 105 mg L−1) in a long-term performance with no change in its efficiency. The performance of this structure in the laboratory and outside environment is assessed. This cost-effective and durable material along with its easy fabrication procedure provides a path towards large-scale efficient solar desalination.

Surface Tension of Solids in the Absence of Adsorption

A method has been recently proposed for determining the value of the surface tension of a solid in the absence of adsorption, γS0, using material properties determined from vapor adsorption experiments. If valid, the value obtained for γS0 must be independent of the vapor used. We apply the proposed method to determine the value of γS0 for four solids using at least two vapors for each solid and find results that support the proposed method for determining γS0.

Continuous fabrication platform for highly aligned polymer films

Superior mechanical and thermal properties in bulk polymers can be achieved by aligning the molecular chains through drawing-induced plastic deformation. Although highly aligned polymer films (HAPF...

Dispensing nano-pico droplets of ferrofluids

Dispensing miniature volumes of a ferrofluid is of fundamental and practical importance for diverse applications ranging from biomedical devices, optics, and self-assembly of materials. Current dispensing systems are based on microfluidics flow-focusing approaches or acoustic actuation requiring complicated structures. A simple method is presented to continuously dispense the miniature droplets from a ferrofluid reservoir. Once a jet of the ferrofluid is subjected to a constrained flux through a membrane and an inhomogeneous magnetic field, the jet experiences a curvature-driven instability and transforms to a droplet. Ferrofluid droplets in the range of 0.1–1000 nl are dispensed with tunable dispensing frequencies. A model is developed that predicts the dispensed volume of the ferrofluid droplets with an excellent agreement with the measurements.

Decoupled Hierarchical Structures for Suppression of Leidenfrost Phenomenon

Thermal management of high temperature systems through cooling droplets is limited by the existence of the Leidenfrost point (LFP), at which the formation of a continuous vapor film between a hot solid and a cooling droplet diminishes the heat transfer rate. This limit results in a bottleneck for the advancement of the wide spectrum of systems including high-temperature power generation, electronics/photonics, reactors, and spacecraft. Despite a long time effort on development of surfaces for suppression of this phenomenon, this limit has only shifted to higher temperatures, but still exists. Here, we report a new multiscale decoupled hierarchical structure that suppress the Leidenfrost state and provide efficient heat dissipation at high temperatures. The architecture of these structures is composed of a nanomembrane assembled on top of a deep micropillar structure. This architecture allows to independently tune the involved forces and to suppress LFP. Once a cooling droplet contacts these surfaces, by rerouting the path of vapor flow, the cooling droplet remains attached to the hot solid substrates even at high temperatures (up to 570 °C) for heat dissipation with no existence of Leidenfrost phenomenon. These new surfaces offer unprecedented heat dissipation capacity at high temperatures (2 orders of magnitude higher than the other state-of-the-art surfaces). We envision that these surfaces open a new avenue in thermal management of high-temperature systems through spray cooling.