publications

2021

1. Proc Combust Inst

   P-DRGEP: a novel methodology for the reduction of kinetics mechanisms for plasma-assisted combustion applications

   Aurélie Bellemans, Nicholas Kincaid, Nicholas Deak, and 2 more authors

   Proceedings of the Combustion Institute, Jan 2021

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   Detailed kinetics mechanisms for plasma-assisted combustion contain numerous species and reactions that model the interplay between non-equilibrium plasma processes and hydrocarbon oxidation. While physically accurate and comprehensive, such detailed mechanisms are impractical for simulations of unsteady multi-dimensional plasma discharges and their effect on reactive mixtures in practical devices. In this work, we develop and apply a novel methodology for the reduction of large detailed plasma-assisted combustion mechanisms to smaller skeletal ones. The methodology extends the Directed Relation Graph with Error Propagation (DRGEP) approach in order to consider the energy branching characteristics of plasma discharges during the reduction. Ensuring tight error tolerances on the relative proportions of energy lost by the electrons to the various classes of impact processes (i.e. vibrational and electronic excitation, ionization, and impact dissociation) is key to preserving the correct discharge physics in the skeletal mechanism. To this end, new targets that include energy transfers are defined and incorporated in DRGEP. The performance of the novel framework, called P-DRGEP, is assessed for the simulation of ethylene-air ignition by nanosecond repetitive pulsed discharges at conditions relevant to supersonic combustion and flame holding in scramjet cavities, i.e. from 600 K to 1000 K, 0.5 atm, and equivalence ratios between 0.75 and 1.5. P-DRGEP is found to be greatly superior to the traditional reduction approach applied to plasma-assisted ignition in that it generates a smaller skeletal mechanism with significantly lower errors. For ethylene-air ignition at the target conditions, P-DRGEP generates a skeletal mechanism with 54 species and 236 reactions, resulting in a 84% computational speed-up for ignition simulations, while guaranteeing errors below 10% on the time required for ignition following the first pulse, 1% on the mean electron energy, between 4 and 35% on electron energy losses depending on the process, and 5% on the laminar flame speed.

2. PNAS

   Valveless microliter combustion for densely packed arrays of powerful soft actuators

   Ronald H. Heisser, Cameron A. Aubin, Ofek Peretz, and 7 more authors

   Proceedings of the National Academy of Sciences, Sep 2021

   Publisher: Proceedings of the National Academy of Sciences

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   Existing tactile stimulation technologies powered by small actuators offer low-resolution stimuli compared to the enormous mechanoreceptor density of human skin. Arrays of soft pneumatic actuators initially show promise as small-resolution (1- to 3-mm diameter), highly conformable tactile display strategies yet ultimately fail because of their need for valves bulkier than the actuators themselves. In this paper, we demonstrate an array of individually addressable, soft fluidic actuators that operate without electromechanical valves. We achieve this by using microscale combustion and localized thermal flame quenching. Precisely, liquid metal electrodes produce sparks to ignite fuel lean methane–oxygen mixtures in a 5-mm diameter, 2-mm tall silicone cylinder. The exothermic reaction quickly pressurizes the cylinder, displacing a silicone membrane up to 6 mm in under 1 ms. This device has an estimated free-inflation instantaneous stroke power of 3 W. The maximum reported operational frequency of these cylinders is 1.2 kHz with average displacements of ∼100 µm. We demonstrate that, at these small scales, the wall-quenching flame behavior also allows operation of a 3 × 3 array of 3-mm diameter cylinders with 4-mm pitch. Though we primarily present our device as a tactile display technology, it is a platform microactuator technology with application beyond this one.

2020

1. Appl. Therm. Eng.

   Assessing geothermal/solar hybridization – Integrating a solar thermal topping cycle into a geothermal bottoming cycle with energy storage

   Joshua D. McTigue, Daniel Wendt, Kevin Kitz, and 3 more authors

   Applied Thermal Engineering, May 2020

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   Geothermal and concentrating solar power (CSP) technologies typically use heat at different temperatures in their commercial deployments. This enables a technically viable hybridization of a solar topping cycle and a geothermal bottoming cycle at locations where both resources are available. In this article, an underperforming geothermal power system based at Burley, Idaho is used as a baseline to investigate the technical and economic potential of such a hybrid cycle. A direct thermal energy storage (TES) system is also integrated to overcome the intermittency of the solar resource. Design and off-design behavior of key components are modelled to simulate the annual performance of the hybrid system on an hourly basis. The sizing of the solar field and thermal storage is investigated and is seen to significantly impact the annual electricity generation and efficiency. Various cost scenarios for the solar field and thermal energy storage are investigated. An economic metric – levelized cost of electricity (LCOE) – is used to optimize the solar field sizing and TES capacity. The hybrid plant converts the additional solar heat input into additional work with an efficiency of 32.9%. Retrofitting a geothermal plant with solar and eight hours of energy storage can achieve an LCOE of 0.136 \/kWhe in current cost scenarios and 0.081 \/kWhe in a future cost reduction scenario. Although the hybrid system cannot directly compete with current PV systems without batteries, it has an LCOE 32% lower than that of a PV-battery system. This paper provides insights into the research and development of the future grid with a high renewable energy penetration and encourages further study of energy hybridization for improved efficiencies and economics.

2019

1. Sol Energy

   Sensitivity analysis on optical performance of a novel linear Fresnel concentrating solar power collector

   Nicholas Kincaid, Greg Mungas, Nick Kramer, and 1 more author

   Solar Energy, Mar 2019

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   A novel, low-cost linear Fresnel collector for concentrating solar power applications is being developed by Hyperlight Energy (Hyperlight) and the National Renewable Energy Laboratory (NREL). Hyperlight is currently deploying their linear Fresnel collector technology in a half-acre pilot loop near Brawley, California. The optical performance of a concentrating solar power collector represents the largest loss in the efficiency of the overall system. The ability to accurately model the complex optical interactions of a collector becomes essential in successfully implementing a new collector technology. This study presents a detailed sensitivity analysis of the optical performance of Hyperlight linear Fresnel technology, characterizing the effects of potential optomechanical error sources on collector performance. Optical models are implemented in SolTrace, a Monte-Carlo ray-tracing software developed at NREL. First, SolTrace is used to analyze collector sensitivity to individual optomechanical error perturbations in both the primary reflectors and the receiver assembly. Then, a high-fidelity optomechanical error model is adopted to capture the realistic performance of the installed Hyperlight linear Fresnel collector. The sensitivity analysis can provide insightful guidance to inform the tightening and relaxation of tolerances during manufacturing and implementation of a new collector technology.

2. Energy Build

   Analysis and comparison of two vegetative roof heat and mass transfer models in three different climates

   Sergio Vera, Camilo Pinto, Paulo Cesar Tabares-Velasco, and 5 more authors

   Energy and Buildings, Nov 2019

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   This study analyzed and compared in detail two existing heat and mass transfer models for vegetative roofs, which were developed by Sailor (2008) and Tabares-Velasco and Srebric (2012). The main equations governing the heat transfer through a vegetative roof were compared side by side. Similarities and differences were highlighted. Both models were programmed in MATLAB, and thermal capacitance of the substrate was implemented by using the finite difference method. The accuracy of both models was evaluated by comparing their results with experimental data obtained on six extensive vegetative roofs, located in three different climate zones, during different seasons of the year. Overall, results showed that both models provide similar predictions of the substrate temperatures. Furthermore, these are in close agreement with the experimental data: for five out of six investigated vegetative roofs, the root mean square deviation is limited to the range of 1.2 °C to 2.5 °C for the Sailor model and of 0.6 °C to 3.0 °C for the Tabares-Velasco and Srebric model. Despite the close agreement between both models, the study also revealed that they greatly differ in the way they evaluate the latent (evaporation) and sensible (convective) heat fluxes, suggesting that at least one of them is miscalculating these heat fluxes. Further research on this topic is recommended.

2018

1. Appl. Energy

   An optical performance comparison of three concentrating solar power collector designs in linear Fresnel, parabolic trough, and central receiver

   Nicholas Kincaid, Greg Mungas, Nicholas Kramer, and 2 more authors

   Applied Energy, Dec 2018

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   The optical performance of a concentrating solar power (CSP) collector is critical to the overall efficiency of the system. This study presents a detailed optical comparison between three representative CSP collector designs including linear Fresnel, parabolic trough, and central-receiver technologies. Optical models are implemented in SolTrace, which is ray-tracing software developed at the National Renewable Energy Laboratory. The ray-tracing algorithm is used to calculate a collector’s design-point performance as well as its incidence-angle modifiers to evaluate the collector performance at any sun position during a typical meteorological year. The efficiency over a one-year period is then analyzed based on ray-tracing results. Using China Lake (California) as an example, the annual optical efficiency is 60% for the selected parabolic trough collector, 52% for the selected central-receiver technology, and 40% for the selected linear Fresnel collector. The parabolic trough has the highest optical performance among all. The selected central-receiver technology provides the most consistent seasonal production profile over the course of the year due to its two-axis-tracking ability but would suffer most from the increasing solar collector optical error. It is also shown that a dramatic cost reduction is required for the selected linear Fresnel technology to be competitive in the future energy market. Sensitivity of three CSP technologies to the deployment locations and the overall optical-error magnitude is also examined through annual performance analysis. The results will provide insights into a better understanding on inherent technical aspects of different CSP technologies.

2. Feasibility Analysis of Nanostructured Planar Focusing Collectors for Concentrating Solar Power Applications

   Qing Ding, Shama F. Barna, Kyle Jacobs, and 11 more authors

   ACS Applied Energy Materials, Dec 2018

   Publisher: American Chemical Society

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   Concentrating solar power (CSP) technology is an attractive approach to harvesting solar energy. Unlike photovoltaic (PV) technology, thermal storage is used in lieu of batteries for electricity generation. However, the cost of current collection optics in a CSP plant hampers commercial competitiveness with PVs and natural gas. The use of a planar focusing collector (PFC) could help reduce cost of materials, installation, and maintenance. We present two candidate PFC designs, one based on metasurfaces and the other a Fresnel-like model. We feed each design through the entire system process (design, fabrication, scalability, and techno-economic feasibility) and discuss the challenges met at each stage. Two-photon and nanoimprint lithography are used to make PFC molds and replicas, respectively. We find that the annual optical efficiency for the Fresnel-based PFC is ∼40%, higher than the current 30% target for natural gas, thereby suggesting potential economic advantages in the market of industrial process heat.

3. Solar Thermal and Geothermal Hybrid Power Plant Study

   Kevin Kitz, Joshua Dominic P. McTigue, Dan Wendt, and 3 more authors

   GRC Bulletin, May 2018

   Institution: National Renewable Energy Lab. (NREL), Golden, CO (United States) Number: NREL/JA-5500-75565

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   Over the past couple of decades, the concept of integrating solar thermal energy into a geothermal power plant has been studied and documented in various publications. A new concept, to use concentrated solar thermal energy first in a high pressure steam topping turbine, and then as an additional heat source to the Raft River geothermal binary power plant (GBPP) in Idaho, USA was investigated. The approach yielded an economically attractive design through lower capital cost, higher net output and higher efficiency. An unexpected result was the option to incorporate solar thermal storage into the hybrid cycle for 4 or 8 hours at a cost far below battery storage of electrical energy. U.S. Geothermal Inc. (USGeo), the National Renewable Energy Laboratory (NREL), the Idaho National Laboratory (INL), and POWER Engineers (PEI) worked together to investigate the concept. The study was made possible through a 2017 Small Business Ventures Pilot Program grant, funded by the U.S. Department of Energy (DOE) Geothermal Technologies Office.This article is a high-level summary of a detailed study that is pending publication by INL and NREL, and of a geothermal paper that will be presented at the forthcoming October 2018 GRC Annual Meeting in Reno, Nevada.