Experimental Investigation and Optimisation of Granular Flow Dynamics in Gravity-Driven Moving Bed Electric Heaters for CSP and Energy Storage Applications

Background

Next generation Concentrated Solar Power (CSP) systems are moving towards solid particle technologies to overcome the cost, stability, and temperature limitations of molten salts. Solid particles such as olivine can operate beyond 700 °C, enabling coupling with high-efficiency cycles (e.g., sCO₂ Brayton cycle) and hybridisation with photovoltaics (PV).

A critical component of such systems is the Moving Bed Electric Heater (MBEH), which uses surplus PV electricity to provide temperature uplift in the particle loop. Gravity-driven flow across horizontally arranged tubular heating elements offers mechanical simplicity and low parasitic losses. However, granular flows present unique challenges: particles form stagnant zones above tubes and void zones below, reducing effective heat transfer and creating a risk of local overheating.

The Horizon Europe Powder2Power  (P2P) project develops and demonstrates a MW-scale particle-driven CSP prototype. Within this project, KTH investigates MBEH concepts using transparent cold-flow rigs with olivine to characterise flow dynamics, optimise design parameters, and establish performance limits.

Thesis objectives

The thesis aims to experimentally investigate granular flow dynamics in a moving bed electric heater, quantify particle velocity fields, and estimate theoretical heat transfer coefficients to guide optimisation of the P2P superheater design.

Specific Objectives

1. Flow characterisation – Quantify bulk and near-wall particle velocities using Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV).

2. Geometry optimisation – Investigate the influence of tube spacing, number of rows, and alternative geometries (cylindrical, airfoil, hexagonal, elliptical) on flow uniformity, stagnation, and void formation.

3. Heat transfer estimation – Estimate theoretical heat transfer coefficients (HTC) from measured flow data using empirical/semi-empirical granular heat transfer correlations. Benchmark HTC values against literature data.

4. Design recommendations – Provide guidelines for optimal tube geometry and spacing in MBEHs to reduce overheating risk and improve efficiency for CSP integration.

Research Questions

1. How do tube spacing and geometry affect particle flow dynamics, stagnation, and void formation in MBEHs?

2. What is the relationship between bulk velocity, near-wall velocity, and effective residence time for granular flows around tubular elements?

3. What are the estimated HTC values for different configurations, and how do they compare with theoretical models and literature data?

4. Which design trade-offs balance flow uniformity, heat transfer performance, and mechanical simplicity in electrically heated MBHEs?

Methodology

1. Literature review: Granular heat transfer, moving bed heat exchangers, and experimental methods (PIV/PTV).

2. Experimental campaign:

   • Use of KTH’s transparent cold-flow MBEH rig.

   • Particle Image Velocimetry and Tracking Velocimetry (PIV) with high-speed imaging.

   • Interchangeable 3D-printed tube arrays for varying spacing and geometries.

   • Test with olivine fractions (various).

3. Data analysis:

   • Velocity fields and near-wall velocities using PIV.

   • Quantification of stagnant and void zones.

   • Estimation of HTC values from flow data and comparison with granular flow correlations.

4. Scale-up discussion: Application of findings to pilot-scale superheater design in P2P.

Expected Outcomes

• Experimental database of velocity fields, stagnation and void zone characteristics.

• Demonstration of suitable tracer methods for granular PIV/PTV.

• Estimated HTC values for varying geometries and spacings.

• Recommendations for industrial-scale MBEH optimisation in CSP applications.

Duration

The project should start in January 2026, and should not extend for more than 6 months. Specific earlier starting date could be discussed.

Location

KTH, Department of Energy Technology

Main Supervisors

Examiner