CEI News

CEI Research Highlights: January 2018

January 24, 2018

Scalable Planning for Energy Storage in Energy and Reserve Markets

As renewable resources are integrated into the grid, energy storage must be planned and deployed in order to harness and balance the natural fluctuations in electricity generation. Electrical engineering professor Daniel Kirschen and his students have continued to develop a model for optimal siting and sizing of storage utilities by adding a profit constraint for proposed systems. The Kirschen group utilized the Western Electricity Coordinating Council (WECC) testbed for their simulations. The 240-bus (power transmission node) system represents the Western Interconnection grid that spans from the Pacific past the Rockies and includes British Columbia, Alberta, and part of Baja California. The results of the case study, as published in IEEE Transactions on Power Systems, show that increased rate-of-return requirements result in decreased deployment of energy storage. A comparison between lithium-ion batteries and above-ground advanced adiabatic compressed air energy storage (AA-CAES) shows that AA-CAES has the higher potential for reducing system-wide costs. However, AA-CAES technology is still at the pilot stage, with the first-ever utility-scale installation scheduled to open in Germany later in 2018. Lithium-ion storage systems are not deployed in any of the model market scenarios at current investment costs, but those costs are expected to continue to decrease for the next 10 years. At a 30% decrease in investment cost, lithium-ion installations are projected to be profitable investments within the Western Interconnection energy market.

Self-Cleaning Surfaces for Solar Panels

Solar photovoltaic cells can experience drops in power generation efficiency when dust accumulates on the cover glass. In lieu of regular manual cleaning, engineers have modeled self-cleaning technology after the leaves of the lotus plant, using a microstructured surface to bead up water droplets that collect environmental contaminants as they roll off. However, this movement is random, so these technologies cannot clean the whole surface without an additional tilting mechanism. CEI Graduate Fellow Di Sun, advised by electrical engineering professor and Institute for Nano-engineered Systems (NanoES) director Karl Böhringer, has created a prototype that methodically cleans an entire surface using a new technology called Anisotropic Ratchet Conveyors (ARCs). As presented this June at the 19th International Conference on Solid-State Sensors, Actuators and Microsystems, in Kaohsiung, Taiwan, a self-assembled monolayer is deposited onto a silicon surface and etched into a rung pattern, which induces the water droplet to travel on a predetermined path when vibrated. The droplets can even be made to move uphill, up to a 15° incline. The ARC layer is optically flat and transparent, resulting in less than 1% degradation of light relative to bare glass. This November, Sun shared an update to his research at the International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, in Kanazawa, Japan. The self-cleaning technology has successfully been applied to a photovoltaic module, and the ARCs can now be fabricated using a polymer-based stencil instead of the etching technique, helping to maintain the hydrophobicity of the underlying surface. Sun’s work has recently received support from the Amazon Catalyst program, a collaboration between Amazon and UW’s CoMotion to help fund “bold, globally-impactful, disruptive projects.”

Estimating Chemical Potential in Oxide-Supported Metal Nanoparticles

Metal nanoparticles (NPs) supported on oxide surfaces are often used in industrial catalysis for energy and environmental technologies, such as in the production of clean fuels and the cleanup of exhaust emissions. The catalytic activity and deactivation rates of metal atoms within NPs have been shown to depend upon NP size and the properties of the supporting oxide, but catalysis chemists have yet to be able to quantitatively predict these effects. In this paper, published in ACS Catalysis, Professor Charles T. Campbell (chemistry) and graduate student Zhongtian Mao present a method of estimating the chemical potential of these metal atoms. This represents a significant step toward predictions of catalytic activity and deactivation rates, as those characteristics are correlated with chemical potential in known ways. For late transition metals, the chemical potential of the atoms in a particle of a chosen size can be modeled as a function of the surface energy of the metal, the adhesion energy at the NP/oxide interface, and the molar volume of the metal. Campbell also presents a linear estimation of that adhesion energy, based on oxygen density on the surface of the oxide and known thermodynamic properties of the metal and the oxide. The model is the first to allow for predictions of chemical potential vs. NP size for different metals on different oxides, with relative errors better than ~20%.

Helical Luminescence in a Two-Dimensional Magnet

This June, a team led by Professor Xiaodong Xu (physics; materials science & engineering) discovered the first two-dimensional (2D) material with intrinsic magnetism: chromium triiodide (CrI3). This December, Xu, physics professor David Cobden, CEI Graduate Fellow Kyle Seyler, and graduate students Ding Zhong and Bevin Huang observed photoluminescence in monolayer CrI3 crystals, as detailed in a paper published in Nature Physics. This observation is a first for a magnetic monolayer, and curiously, the photoluminescence exhibits spontaneous circular polarization. The emitted light waves trace out a helix, where the clockwise or counterclockwise orientation is determined by the magnetization direction of CrI3. This circularly polarized photoluminescence does not require an external magnetic field, in contrast to similar phenomena in other materials. The UW team also studied bilayer CrI3, which did not exhibit circular polarization in its photoluminescence. This observation supports their previous finding: that CrI3 bilayers have zero net magnetization. These discoveries are promising for further magneto-optical studies, as well as novel magneto-optoelectronic devices and van der Waals heterostructures. Described as “atomic Lego,” these devices consist of artificially stacked monolayers held together by van der Waals intermolecular forces. Van der Waals heterostructures already show promise in next-generation solar cells, LEDs, and transistors, and the UW team’s investigations of CrI3 could expand these applications to include energy-efficient magnetic information processing and storage.