Similar value for tower and parabolic trough CSP technologies, according to NREL

Published on: Wednesday, 12 February 2014
Similar value for tower and parabolic trough CSP technologies, according to NREL

The findings of a new report just released by NREL may be a bit surprising as it finds similar value for two very diferent Concentrated Solar Power (CSP) technologies. Tower and torugh plants are based on the same principle, but the working process is really diferente from each other.

The report, “Estimating the Performance and Economic Value of Multiple Concentrating Solar Power Technologies in a Production Cost Model,” found that the value of delivered energy of dry-cooled tower and parabolic trough CSP plants, integrated with thermal energy storage, are quite similar.

CSP with thermal energy storage is a unique source of renewable energy in that the solar thermal energy can be dispatched in a similar manner as conventional thermal generation to respond to changes in supply or demand.

CSP uses the thermal energy of sunlight to generate electricity. Parabolic troughs and power towers both concentrate sunlight onto a heat-transfer liquid, which is used to drive a steam turbine. CSP can generate electricity not just when the sun is shining, but also after sundown, because a CSP plant can be built with thermal energy storage, such as molten salt.

“In our study, we analyzed various plant configurations and identified specific ones that provide significantly more value than has been found in previous analyses,” said NREL Analyst Jennie Jorgenson, the lead author of the report. “For example, we explored the potential benefits of extending thermal storage at CSP plants beyond six hours, a typically modelled amount. In this analysis we found additional benefits for six to nine hours of storage, but rapidly diminishing benefits for greater than nine hours of storage.”

The NREL report, funded through the Energy Department’s Office of Energy Efficiency and Renewable Energy in support of its SunShot Initiative, provides valuable quantitative results in a Colorado test system, comparing the two CSP technologies with thermal energy storage and evaluating how the operational and capacity value varies with plant configuration. The report also demonstrates that multiple CSP technologies and plant configurations can be analyzed using traditional planning tools such as production cost models.

NREL is currently undertaking a similar analysis looking at the value of multiple CSP configurations in California under an assumed 40% penetration of renewables within that state.

“For both conventional and renewable energy systems, low levelized cost of energy does not necessarily reflect these systems’ total value to the grid,” Jorgenson added. “So, providing tools that utilities and grid operators are familiar with can lead to more informed decision-making as greater levels of renewable energy penetrate the market.”

Relative Performance of Dry-Cooled Troughs and Towers

Both trough and tower CSP plants convert sunlight to electricity via steam turbines and can make use of TES. Although the governing steam turbine parameters are largely the same for trough and tower configurations (though tower turbine efficiencies are typically greater than the efficiency for troughs due to higher operating temperatures for towers), the hourly electrical energy from each system may be substantially different due to differences in the solar collection method. Figure 6 depicts the seasonal and daily variations of solar resource availability for trough and tower plants providing an equal amount of energy on an annual basis. The solar resource availability for the tower field is relatively constant throughout the year, while the trough resource exhibits a strong seasonal dependence. This is due primarily to the seasonal “cosine” effect due to low sun elevation angle in winter, which is more pronounced for single- axis tracking parabolic trough collectors versus two-axis tracking heliostats used for tower configurations.

NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.


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