Engineering of Balance of Plant for High-Temperature Systems


National Renewable Energy Laboratory (NREL)

Capability Expert

Zhiwen Ma, Janna Martinek


System Integration

Node Readiness Category

2: High-Temperature Electrolysis (HTE)
2: Solar Thermochemical (STCH)
1: Hybrid Thermochemical (HT)


This capability serves as interface engineering for integration of the balance of plant (e.g. solar field, receiver, operations) into high-temperature solar fuel systems. Thermal energy in the temperature range of 600°–800°C is necessary for high-temperature electrolysis process using solid oxide electrolytic cell (SOEC) and hybrid solar thermochemical hydrogen (STCH) production. The low end of this temperature range can be supported by existing molten nitrate salt power tower systems while the high end temperatures will require advanced molten-salt or particle-based concentrating solar thermal (CST) systems currently under development. When integrated with the appropriate storage technology, CST systems can provide non-intermittent hydrogen production at high capacity factors, reducing the levelized cost of hydrogen production.

Capability Bounds‎

NREL uses an integrated suite of solar field, receiver, and thermal storage design tools (e.g. SolarPILOT, SolTrace, Aspen, ANSYS Fluent) to maximize the performance of CST systems. NREL's high-temperature optical and heat transfer materials laboratories can be used to develop and characterize advanced materials (high temperature salts, particles, absorber materials) envisioned for high-temperature BOP components and systems.

Unique Aspects‎

Under direction and funding through DOE SETO's CSP program office, NREL has developed a unique portfolio of integrated capabilities dedicated to supporting the design of CST BOP components and systems. NREL has developed relationships with public and private entities engaged in R&D focused on delivering concentrated solar flux to a centralized receiver and transporting this energy through high-temperature materials. Current and past research partners include SolarReserve and BrightSource (solar field design and integration), Babcock & Wilcox (particle-based thermal system), and Savanah River National Laboratory and University of Wisconsin (characterization and containment of advanced high-temperature molten salt fluids).


NREL's CST BOP design capability can integrate the hydrogen production with the solar thermal collection and storage for plant overall heat and mass balance. The modeling tools include SolTrace, SolarPILOT (both are freely downloadable), energy balance and component design analysis, and need support to maintain the capability. The Thermal Energy Storage laboratory in NREL ESIF is user facility for characterizing the CST heat transfer media for high-temperature application.


Proper design and configuration of the solar field and other BOP design is often ignored, yet is a critical element for delivering high-temperature thermal energy in the range of 600°–800°C required for SOEC or hybrid STCH, and for delivering solar energy to two-step STCH processes for temperatures above 1,000°C.


Figure 1. Rendering of integrated particle-based CSP system including BOP components (top), SolarPILOT solar field optimization tool (bottom left) and SolTrace solar field and receiver ray trace software (bottom right).

Figure 2. Previous raytracing with all 5 tubes in place.

Figure 3. Side view of SolTrace ray trace of Secondary/Reactor


1. Zhiwen Ma, Mark Mehos, Greg Glatzmaier, and Bartev B. Sakadjian, "Development of a Concentrating Solar Power System Using Fluidized-Bed Technology for Thermal Energy Conversion and Solid Particles for Thermal Energy Storage", doi:10.1016/j.egypro.2015.03.136, Energy Procedia, Volume 69, May 2015, Pages 1349–1359. 2. Zhiwen Ma, Methods and Systems for Concentrated Solar Power, U.S. Patent No. 9,347,690 B2, filed: Apr. 2, 2013; awarded: May 24, 2016. 3. Janna Martinek, Zhiwen Ma, "Granular Flow and Heat Transfer Study in a Near-Blackbody Enclosed Particle Receiver," doi: 10.1115/1.4030970, J. Sol. Energy Eng. 2015; 137(5):051008-051008-9.