Publications

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Concentrating solar power (CSP) technologies continue to mature and are being deployed worldwide. Power towers will likely play an essential role in the future development of CSP due to their potential to provide dispatchable solar electricity at a low cost. This Power Tower Technology Roadmap has been developed by the U.S. Department of Energy (DOE) to describe the current technology, the improvement opportunities that exist for the technology, and the specific activities needed to reach the DOE programmatic target of providing competitively-priced electricity in the intermediate and baseload power markets by 2020. As a first step in developing this roadmap, a Power Tower Roadmap Workshop that included the tower industry, national laboratories, and DOE was held in March 2010. A number of technology improvement opportunities (TIOs) were identified at this workshop and separated into four categories associated with power tower subsystems: solar collector field, solar receiver, thermal energy storage, and power block/balance of plant. In this roadmap, the TIOs associated with power tower technologies are identified along with their respective impacts on the cost of delivered electricity. In addition, development timelines and estimated budgets to achieve cost reduction goals are presented. The roadmap does not present a single path for achieving these goals, but rather provides a process for evaluating a set of options from which DOE and industry can select to accelerate power tower R&D, cost reductions, and commercial deployment.

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The Solar Two Project was a United States Department of Energy sponsored project operated from 1996 to 1999 to demonstrate the coupling of a solar power tower with a molten nitrate salt as a heat transfer media and for thermal storage. Over all, the Solar Two Project was very successful; however many operational challenges were encountered. In this work, the major problems encountered in operation of the Solar Two facility were evaluated and alternative technologies identified for use in a future solar power tower operating with a steam Rankine power cycle. Many of the major problems encountered can be addressed with new technologies that were not available a decade ago. These new technologies include better thermal insulation, analytical equipment, pumps and values specifically designed for molten nitrate salts, and gaskets resistant to thermal cycling and advanced equipment designs.

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The authors have founded a temporary international core team to prepare a SolarPACES activity aimed at the standardization of a methodology for electricity yield analysis of CSP plants. This core team has drafted a structural framework for a standardized methodology and the standardization process itself. The structural framework has to assure that the standardized methodology is applicable to all conceivable CSP systems, can be used on all levels of the project development process and covers all aspects affecting the electricity yield of CSP plants. Since the development of the standardized methodology is a complex task, the standardization process has been structured in work packages, and numerous international experts covering all aspects of CSP yield analysis have been asked to contribute to this process. These experts have teamed up in an international working group with the objective to develop, document and publish standardized methodologies for CSP yield analysis. This paper summarizes the intended standardization process and presents the structural framework of the methodology for CSP yield analysis.

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A method for applying probabilistic models to concentrating solar-thermal power plants is described in this paper. The benefits of using probabilistic models include quantification of uncertainties inherent in the system and characterization of their impact on system performance and economics. Sensitivity studies using stepwise regression analysis can identify and rank the most important parameters and processes as a means to prioritize future research and activities. The probabilistic method begins with the identification of uncertain variables and the assignment of appropriate distributions for those variables. Those parameters are then sampled using a stratified method (Latin hypercube sampling) to ensure complete and representative sampling from each distribution. Models of performance, reliability, and cost are then simulated multiple times using the sampled set of parameters. The results yield a cumulative distribution function that can be used to quantify the probability of exceeding (or being less than) a particular value. Two examples, a simple cost model and a more detailed performance model of a hypothetical 100-MW e power tower, are provided to illustrate the methods. Copyright © 2010 by ASME.

NREL's Solar Advisor Model (SAM) is employed to estimate the current and future costs for parabolic trough and molten salt power towers in the US market. Future troughs are assumed to achieve higher field temperatures via the successful deployment of low melting-point, molten-salt heat transfer fluids by 2015-2020. Similarly, it is assumed that molten salt power towers are successfully deployed at 100MW scale over the same time period, increasing to 200MW by 2025. The levelized cost of electricity for both technologies is predicted to drop below 11 cents/kWh (assuming a 10% investment tax credit and other financial inputs outlined in the paper), making the technologies competitive in the marketplace as benchmarked by the California MPR. Both technologies can be deployed with large amounts of thermal energy storage, yielding capacity factors as high as 65% while maintaining an optimum LCOE.

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