NREL Forges Foundation for Advanced Concentrating Solar Power Receivers

Sept. 16, 2014 | By Karen Atkison | Contact media relations

As part of DOE's SunShot effort, NREL's Thermal Systems Group is performing research and development on components for high-temperature concentrating solar power (CSP) receivers. DOE supports R&D of CSP technologies in order to achieve SunShot Initiative cost targets with systems that can supply solar power on demand through the use of thermal energy storage. The thermal energy from the receiver can be stored and subsequently dispatched to produce electricity via a turbine driving a generator.

NREL is currently developing two receiver types: a direct supercritical carbon dioxide (s CO₂) receiver and a near-blackbody enclosed particle receiver. Both receivers are capable of driving higher-temperature power cycles than are possible using today’s receivers.

Direct Supercritical Carbon Dioxide Receiver

An NREL project team is developing a thermal receiver for the high-temperature s CO₂) power cycle technology that is currently under development at NREL and elsewhere. The direct receiver concept uses s-CO₂) from the power cycle as the heat-absorbing fluid in the thermal receiver in a similar fashion to existing direct steam Rankine cycles or air-Brayton systems. Direct systems have several advantages and challenges, which are explored and quantified in this project.

The ultimate goals of the project are to:

Develop and experimentally demonstrate a new receiver technology capable of high-temperature operation, enabling increased power conversion efficiency.
Validate predictive performance models that can be used to develop an optimal commercial product with a set of lab-scale experiments and a small-scale prototype.

Phase 1 work involved developing conceptual and detailed receiver designs within a robust and detailed modeling framework, using both in-house and standard engineering tools. Phase 2 focused on refining the receiver design further and investigating design details to substantiate the promising results from Phase 1.

During Phase 2, the research team was able to:

Develop a project-value-based optimization objective function model, which, combined with simulation software, is used to optimize the receiver geometry and optical properties. Quantify thermal stress in various situations on the lifetime of receiver materials. Develop and improve modeling tools and methodologies that NREL researchers and the CSP industry can use to design better power tower systems. Identify a set of test procedures that will characterize performance within the targeted limits of experimental uncertainty and at relatively low cost. Work with the engineering firm Optimation Technology, Inc. to design a high-pressure s-CO₂) supply loop for use in prototype testing. Publish several peer-reviewed papers, generate a record of invention, and submit a patent application on the receiver technology.

Phase 3 will focus on execution of further proposed experiments and additional optimization of the receiver design with models validated by experimental results.

Near-Blackbody Enclosed Particle Receiver

A separate NREL team is developing the near-blackbody (NBB) particle receiver. All the team's research is groundwork for the development of a prototype receiver that will be tested at the National Solar Thermal Test Facility located at Sandia National Laboratories in Albuquerque, New Mexico.

The NBB receiver consists of a module with an array of NBB absorber tubes. Solid particles flow around the outside of the absorbers, which transfer heat from the incoming solar flux to the particles. The absorber-cavity flux absorption reduces thermal losses, resulting in high receiver thermal efficiency (>90%) and operating temperatures (>800°C) that can drive a variety of high-efficiency power cycles.

As with the direct s-CO₂) receiver effort, the team is aiming for an installed cost of less than $150 per kilowatt thermal, with a 30-year service life. In Phase 1, the team estimated the receiver's performance. Phase 2 tasks focused on realizing those performance estimates using a design based on available materials and manufacturing methods, and designing a prototype particle-receiver that will be verified through a field experiment in Phase 3.

Some of the key accomplishments from budget period 2 included:

Developing a detailed model to assess overall receiver performance and to support the commercial receiver design.
Characterizing the high-temperature stability (greater than 500°C) of specular and diffusely reflecting metal and ceramic coating materials used in various receiver configurations.
Performing flow visualization and heat transfer experiments on absorber-tubes under configurations envisioned for future prototype and commercial particle receivers.
Developing receiver periphery equipment designs and overall receiver cost estimates.
Analyzing heat exchanger designs for use with steam Rankine, s-CO₂), and air-Brayton power cycles.
Generation of considerable intellectual property and publications comprising four papers; six inventions, including one NREL provisional patent; two NREL records of invention; and three Babcock & Wilcox patents.