Because solar fields represent a large portion of capital investment in concentrating solar power (CSP) plants, NREL is working to improve their cost and performance.
Optical Materials Properties and Weatherability
NREL's advanced optical materials laboratory and outdoor weathering facilities provide analytical and measurement capabilities for developing new coating materials and testing optical properties, performance, and weatherability of materials used in CSP systems.
Mirror and Receiver Laboratory Characterization
NREL uses the Solar Optical Fringe Alignment Slope Technique (SOFAST), an automated optical characterization system that assesses the quality of mirrors for solar applications. Developed by Sandia National Laboratories, SOFAST can be used in conjunction with large footprint environmental chambers to investigate the performance of advanced mirror facets subject to extreme environmental conditions. NREL's Receiver Test Laboratory houses a parabolic trough receiver test stand used to analyze steady-state, off-sun thermal losses of receivers used in solar parabolic trough power plants.
In-Situ Opto-Mechanical Characterization
NREL developed a parabolic trough solar field characterization tool, Thermal Scout and Distant Observer, to efficiently assess the performance of collectors and receivers. These tools have been licensed to our partner Solar Dynamics. Partnering with Tietronix and Sandia National Laboratory, NREL is developing a nonintrusive optics tool to characterize the optical performance of heliostat fields during routine operation.
Modeling and Analysis
NREL develops and maintains modeling tools to analyze and overcome technical barriers to accelerate CSP technologies. NREL has developed open-source tools such as SolarPILOT and SolTrace to model and analyze the performance of CSP systems and components. NREL has developed the Heliostat Aimpoint and Layout Optimization Software (HALOS) tool that interfaces directly with SolarPILOT to obtain optimized solar field layouts and aimpoints when provided a receiver, location, and heliostat characteristics as input. In addition, NREL has CSP-specific modes as part of System Advisor Model (SAM), which allows a direct techno-economic analysis of a CSP system for electricity generation and process heat generation under various financial scenarios.
Heliostat field operation and maintenance is a particularly challenging issue because of the large number of individually tracking heliostats for power tower plants and future Gen3 plants. A degradation in optical precision at a level of a couple of milliradians can significantly reduce energy production (up to 24% based on initial analysis). In addition, plant operators do not have efficient and accurate tools to effectively assess and correct the field optical errors. NREL is developing and demonstrating a nonintrusive optics tool for in-situ heliostat characterization, which will consist of an imaging system mounted on an unmanned aerial system.
Sandia National Laboratories, TieTronix Inc., and Crescent Dunes Solar Plant
The proposed technology has two major impacts on the heliostat field: reduced operation and maintenance costs and increased field performance—both of which lead to reduced levelized cost of electricity. In addition, a properly monitored and well-maintained field will prevent unintentional hot spots resulting from problematic heliostats, which could lead to a plant shutdown and millions of dollars in lost revenue.
Optical errors (e.g., slope error and receiver misalignment) are notoriously difficult to measure and directly affect parabolic trough CSP plant performance. NREL has developed a prototype of a tool to measure these errors and adapted it for fast, aerial data collection by unmanned aerial vehicle. This tool, called Distant Observer (DO), requires substantial human labor to post-process massive amounts of data when applied to a commercial-scale solar field. By using computer vision and novel deep learning methods, we aim to automate the most labor-intensive DO processes.
Solar Dynamics LLC
Unmanned aerial system-driven DO with the assistance of machine learning technology will make operating strategy suggestions, and plant performance can be improved at little cost to the operators beyond the relatively low cost of a DO study. Technical improvements, such as adding support for sagging receivers and twisting solar collector assemblies, can increase plant performance and inform next-generation trough designs. Improving plant efficiency will lower the total levelized cost of electricity and provide useful information for the advancement of this technology.
Department of Energy goals for CSP require aggressive cost and performance improvements across the plant. The solar field represents 30%–40% of initial capital investment of CSP plants and is an important factor in reaching the 2030 cost target of $0.05/kWhe for baseload CSP plants. Heliostats must be high performing (<1.53 mrad optical error), low cost (≤$75 m-2), and durable (lifetime >30 years). Unfortunately, there are no standardized methods to evaluate the long-term performance of solar mirrors. This project will develop guidelines for a xenon arc lamp accelerated aging method. This project will also create a publicly accessible database of the results of NREL's efforts in the evaluation of solar mirror materials.
An accelerated aging method guideline would benefit the CSP community by providing validation of an xenon arc lamp exposure accelerated aging method as the first step in developing a CSP-specific durability assessment standard. Such a standard is necessary to ensure equivalent and accepted reporting of CSP mirror performance, which CSP materials manufacturers may use to demonstrate value and enable financing.
Because wind loads affect the sizing of all major components of CSP parabolic trough and heliostat collectors, even modest wind loading reductions can have important system-level cost impacts. NREL performed high-fidelity computational-fluid-dynamics analysis of wind loading on single- and multi-row CSP parabolic-trough collector devices using the Nalu-Wind code and validated the results against wind-tunnel test data. The project has been extended to include a field campaign at Acciona's Nevada Solar One CSP power plant to characterize the turbulent wind flow conditions and resulting loads experienced by the parabolic trough collector structures in their full-scale operational environment.
Acciona Solar Power Inc.
Detailed at-scale wind measurements and high-fidelity computational models will be released publicly to stakeholders and are expected to drive improvements in the way the collector structures are designed and built, thereby improving reliability and offering pathways for cost reductions.
NREL developed an innovative methodology to optimize secondary reflectors for linear Fresnel collectors that is shown to greatly improve optical efficiency. Hyperlight Energy has adopted NREL's secondary reflector technology in their linear Fresnel collector design and received a $5.5 million grant to develop a commercial solar field to provide process heat for a cheese factory in California.
The project will enable a direct collaboration with a U.S. industrial partner to execute a commercial deployment of a low-cost CSP collector for process-heat application. The success of the project will lead to the commercialization of NREL's technology and the increasing penetration of CSP technology in California's energy market.
A Non-Intrusive Optical Approach To Characterize Heliostats in Utility-Scale Power Tower Plants: Flight Path Generation/Optimization of Unmanned Aerial Systems,” Solar Energy (2021)
A Non-Intrusive Optical (NIO) Approach To Characterize Heliostats in Utility-Scale Power Tower Plants: Sensitivity Study: Methodology and In-Situ Validation, Solar Energy (2020)
Controllable Solar Flux Heating for Freeze Recovery in Molten-Salt Parabolic Trough Collectors, Journal of Energy Resource Technology (2020)