Thermal storage in packed beds: pressure drop & heat transfer K.G. Allen, T.W. von Backström, D.G. Kröger Solar Thermal Energy Research Group University of Stellenbosch K.G. Allen, T.W. von Backström, D.G. Kröger Solar Thermal Energy Research Group University of Stellenbosch Energy Postgraduate Conference 2013

Packed bed thermal storage for CSP Store thermal energy from gas turbine exhaust or central receiver Stored heat ( ºC) used for steam power cycle Example of packed bed use: SUNSPOT combined cycle

Packed bed characteristics Prediction of packed bed characteristics: Pressure drop (apparent friction factor) Heat transfer coefficient & thermocline (temperature) Influence of:  Particle shape?  Roughness?  Alignment? Why? Estimate cost …

Experimental work Sample materials and test section/wind tunnel

Friction factor – crushed rock Range of values obtained… 1.3 & 4.1 mm data from Allen et al., Powder Technology 246:

How does a packed bed store heat? Ideal bedActual bed Temperature profile? Heat transfer – h ? Thermocline Rock size dependent

Temperature prediction? (I) Glass spheres; Bi ≈ 0.18; Re ≈ 490

Temperature prediction? (II) Crushed rock (ΣV p /ΣA p = 1.3 mm); Bi ≈ 0.12; Re ≈ 280

Thermal decomposition Thermal cycling – cracking and disintegration  Availability of suitable rock? Other considerations

Conclusion Pressure drop  Importance of shape, roughness, alignment  No general correlation Temperature profile & heat transfer  Existing correlations work well up to ≈ 100 ºC  Thermal radiation at higher temperatures?