Considerations for the next generation of concentrating solar power systems

May 22nd, 2019, Published in Articles: Energize

Unlike PV systems, concentrating solar power (CSP) technology captures and stores the sun’s energy in the form of heat, using materials which are low cost and materially stable for decades. This allows CSP with thermal energy storage (TES) to deliver renewable energy while providing important capacity, reliability and stability attributes to the grid, thereby enabling increased penetration of variable renewable electricity technologies.

Today’s most advanced CSP systems are towers integrated with 2-tank TES, delivering thermal energy at 565°C for integration with conventional steam-Rankine power cycles. These power towers trace their lineage to the 10 MWe pilot demonstration of Solar Two in the 1990s. This design has lowered the cost of CSP electricity by approximately 50% over the prior generation of parabolic trough systems; however, the decrease in cost of CSP technologies has not kept pace with the falling cost of PV systems.

Since the 2011 introduction of SunShot, the US’s Department of Energy’s (DoE’s) CSP sub-programme has funded research in solar collector field, receiver, TES, and power cycle sub-systems to improve the performance and lower the cost of CSP systems. In August 2016, the DoE hosted a workshop of CSP stakeholders which defined three potential pathways for the next generation CSP plant (CSP Gen3) based on the form of the thermal carrier in the receiver: molten salt, particle, or gaseous. Prior analysis by the DoE had selected the supercritical carbon dioxide (sCO2) Brayton cycle as the best-fit power cycle for increasing CSP system thermo-electric conversion efficiency. The research is designed to enable a CSP system that offers the potential to achieve the overall CSP SunShot goals – yet no single approach exists without at least one significant technical, economic, or reliability risk (see Fig. 1).

Fig. 1: Various pathways for CSP Gen3 technology, all of which have at least one technical, economic or reliability risk.

This roadmap addresses and prioritises research and development (R&D) gaps and lays out the pathway for a “Gen3 CSP Roadmap.” Throughout the roadmap process, the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (Sandia) engaged appropriate stakeholders, including the CSP industry and developers, utilities, and the laboratory and university research and development (R&D) community. An industry-led Technical Review Committee (TRC) was established to guide the roadmap activity. Technology gaps for each of the technology pathways were identified, together with research priorities designed to address them. This information will be used by the DoE to inform and prioritise R&D activities leading to one or more technology pathways to be successfully demonstrated at a scale appropriate for future commercialisation of the technology.

The three proposed pathways

Molten-salt pathway

Of the three pathways presented in this roadmap, molten-salt systems represent the most familiar approach. Conceptually there is no change from current state-of-the-art power tower design; however, the increase in hot-salt system temperature from 565°C to approximately 720°C brings significant material challenges. Although the engineering challenges associated with achieving the high receiver outlet temperature required to drive a sCO2 turbine at >700°C are relatively well understood, knowledge around the selection of a high-temperature molten salt is needed, especially with regard to its impact on containment materials that can achieve acceptable strength, durability, and cost targets at these high temperatures. Chloride and carbonate salt blends have been proposed and tested, but each brings new challenges. The corrosion mechanism differs among candidate salts and information is needed for component designers.

Falling-particle pathway

Within the falling-particle pathway, although many of the components are mature and have been developed by industry – for example, particle heat exchangers, particle storage bins, particle feeders and hoppers, and particle lifts – the unique application for solarised sCO2 systems at high temperatures and high sCO2 pressures offers unique challenges that need to be addressed. In addition, heating the particles with concentrated sunlight poses additional challenges with efficient particle heating, flow control and containment, erosion and attrition, and conveyance.

Gas-phase pathway

The gas-phase technology pathway relies on an inert, stable gas-phase heat transfer fluid (HTF), such as carbon dioxide or helium, operating within a high-pressure receiver. This pathway also describes a heat-pipe concept whereby liquid HTF is evaporated in the receiver, transported as a saturated gas to the TES, and condensed back into liquid form. Unlike the other two pathways, this pathway relies on indirect TES options such as a phase-change material or particle storage. Significant progress has been made on receiver designs for high-pressure operation under the SunShot program, and multiple institutions have put forward designs that demonstrate viability by way of modelling, lab-scale, and on-sun testing activities.

All three approaches have existing challenges to be solved but retain the potential to achieve the SunShot goal of US$0,06/kWh. Further development, modelling, and testing are now required to bring the technologies to a stage where integrated system tests and pilot demonstrations are feasible.

Recommended research would also focus on confirming the ability of each technology to address the market requirements defined by the Technical Review Committee, such as ramp rates, reliability, availability, and other market-driven criteria. For any of these technologies to successfully compete in the future marketplace, the needs of the evolving market must be understood, and changes must be incorporated into the technology development process.

More information on this study can be found on NREL’s website: www.nrel.gov/publications

Contact Antony Bruno, NREL, antony.bruno@nrel.gov

 

 

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